Complete Galvanizing Process

Hot-Dip Galvanizing Process Guide

HOT-DIP GALVANIZING PROCESS GUIDE

Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223

Hot-Dip Galvanizing Process Guide Introduction: Galvanizing :- The Protection of steel from corrosion by hot dip galvanizing has been in practice for about 200 years, and the technique is still preferred for the long term protection of structural articles, which may be subjected to physical abuse during shipping and installation. This is generally due to the nature of the alloy coating developed when chemically clean steel is immersed in molten zinc. In current use, galvanizing is typically referred to hot-dip galvanizing, which is a metallurgical process used to coat steel or iron with zinc it is basically done to prevent corrosion of the ferrous item. While galvanizing is accomplished using non-electrochemical means. It serves as an electrochemical purpose. Hot-dip galvanized steel is in effective use for more than 150 year. The importance of hot-dip galvanizing stems from the relative corrosion resistance zinc that under most service conditions, is substantially better than steel and iron. In addition to developing a physical barrier against corrosion, zinc (applied as a hot-dip galvanized coating) cathodically protects the exposed steel. Moreover, galvanizing for protection of steel and iron is preferred because of its ease of application, low cost, and the extended maintenance-free service Which provides hot-dip galvanizing deposits a thick robust layer, which may be more than essential for the protection of the underlying metal in some applications. This has found to be true in the case of automobile bodies, where additional rust proofing paint will be used. In such cases, a thinner form of galvanization is applied by electroplating, known as “electro galvanization”. However, the protection that this process provides is insufficient for products, which are continuously exposed to corrosive materials, such as salt water, Even so, most of the nails made today are electrogalvanized. Galvanic protection can be attained by connecting zinc both electronically as well as ionically. In such a configuration, zinc is absorbed into the electrolyte in preference to the metal, which it protects, and maintains the structure of metal by inducing an electric current. The Galvanizing Process: The Process of galvanizing consists of coating metals, such as iron and steel, with a thin protective layer of zinc. The layer of zinc provides protection to the metal from corrosion. The result of galvanizing is a corrosion resistant product to be used for many outdoor applications, where the weather is a factor in the life expectancy of the material. The protective layer of zinc prevents the ferrous material from coming in contact with the oxygen causing oxidation. Oxidation is the combination of a substance with oxygen, in iron and steel the main combination result is rust. Without this contact the ferrous material does not corrode. An example of how this works is when iron reacts with oxygen it forms iron oxide ( the main component in corrosion), however if the iron is galvanized the zinc reacts with oxygen causing zinc oxide prior to reaching the iron. There are two types of oxide caused by iron, ferric oxide and ferrous oxide. Ferric oxide (Fe2O3) is a dark red compound occurring naturally as a hematite ore and rust. This material is used in pigments, metal polishes and on magnetic strips. Ferrous oxide (FeO) is a chemically formed powder used in the manufacturing process of steel, heat absorbing glass, and enamels. The layer of zinc already has a naturally occurring zinc oxide film which protects the zinc layer against corrosion making it corrosion resistant. If there is a small damaged area or bare spot that exposes the ferrous material, the zinc will sacrifice itself for oxidation. This is result of the zinc being more electrochemical, attracting oxygen, than the ferrous material causing zinc to act as a anode protecting the ferrous material. This is the method used in the protection of galvanized products . There are several ways to galvanizing steel products, such as hot dip galvanize, electroplating galvanize, mechanical coating, zinc spraying, and zinc dust painting. In hot dip galvanize there are two different processes that are used. One is the process of continuous hot dip galvanizing which consists of a start point and a finish point. In between these two points the material never stops going from a raw material (iron/steel) to a finished good ready to be shipped. The other is stationary hot dip galvanizing in which as individual item is dipped into a pot of molten metal and removed. The following steps are taken in hot dip galvanizing in both hot dip and continuous operations, cleaning, pickling, pre fluxing, galvanizing, cooling, finishing, inspected, and throughout the process the product is rinsed and air blown. The main difference between the two ( continues hot dip and stationary ) is that the material on a continuous operation is finished when the cycle is complete versus just dipping the material in most cases is hand finished to remove excess burs, runs, and bumps that are result of no external wipe.


Hot-Dip Galvanizing Process Guide Typical Steps in the manufacturing process of Galvanizing: The hot-dip galvanizing process consists of following basic steps : 1). Cleaning :- The step in which the oil and grease removal, wherein a hot alkaline cleaner is used to remove dirt, grease, oil, and soluble markings, Also known as caustic cleaning, this process, however, do not remove paint, epoxies, and welding slag. These contaminants can be removed using sandblasting. 2). Rinsing & Pickling:- A bath of cooled water to rinse any of the cleaning residue off and air knives to aid in the removing of any additional residue and water that were used on the material before entering the pickling system. The diluted hydrochloric or sulfuric acid solution that serve to remove surface rust and mill scale to provide a chemically clean metallic surface. 3). Rinsing:- Another water reservoir to clean the pickled material or job that may be left on the material before entering the flux stage. 4). Pre fluxing:- The steel is immersed into a liquid flux to remove oxides and to prevent oxidation prior to entering the molten metal. The liquid flux is a concentration that consists usually of zinc ammonium chloride solution. The flux is the key ingredient in aiding in the ability of the zinc to adhere to the iron or steel. 5). Preheating/Air flow:- Between the pre flux area and the molten metal the material is blown dry using a air knife to remove in moisture that consists prior to entering the liquid metal. 6). Galvanizing:- The item is immersed in a bath of molten zinc, with a temperature range between 450 oC to 460oC. The temperature needs to be constantly in this range, if too cold, the zinc freezes, if too hot, the coating consistency is not symmetrical causing low coating, bare spots, or a burnt gray color to exist on the material. With the temperature within range the zinc metallurgic alloy bonds to the steel, creating a highly resistant zinc iron alloy layer. The zinc iron alloy is formed in the galvanizing process with pure zinc and the ferrous material being galvanized. This alloy layered is a bond between the iron that is diffused from the steel product being galvanized and the zinc in the molten stage, The layer of coating has several different phases of alloy layers with names such as Alpha, Gamma, Delta, Zeta, and the free Zinc layer. Each of these layers have different amount iron, zinc ratio’s. This alloy layer is dependent on the type of steel being used for the galvanizing process. These different alloy layers are what make up the weight of coating that the material receives, determining the finished gauge of the product. The layer gauge to be put on the material is determined by the time in which the material is actually in the zinc bath. 7) Quenching:- In this step, the galvanized steel is dipped into a dilute chromate solution to give a long lasting luster and to reduce temperature for prompt handling and inspection Inspection :- Inspection is the final step in the galvanization process that is done to ensure compliance with the specification it is done to evaluate coating thickness, uniformity, adherence and appearance. Electronic testing gauges are used to ensure quick, convenient and accurate measurements of the coating or Analog gauge is also used to determine the coating thickness. In addition to this certificate of tested results issued by company to ensure the proven quality standard.



Zinc and Sustainable Development 7 Facts You Should Know 1.Zinc is Natural Zinc is a gift of nature for the benefit of mankind. It exists naturally in air, water and soil. Zinc is present everywhere in the environment and is continuously mobilized and transported by natural Processes such as erosion, forest fires, aerosol formation above the sea and volcanic eruptions. 2.Zinc is Essential in the Environment All life on earth has evolved in the presence of zinc, which is used by nature for many biological processes. All living organisms - including man, animals, fish, plants and micro-organisms - need zinc for growth and development. Zinc intake is regulated by each organism’s natural processes. 3.Zinc Protects Steel One of zinc’s most remarkable characteristics is its ability to protect steel against corrosion. Corrosion is a drain on the economy, estimated to cost at least 4% of GDP in industrialized countries. The life and durability of steel is greatly improved when coated with zinc. No other material known to man can provide such efficient and cost-effective protection for steel. 4.Zinc Helps Save Resources By protecting steel against corrosion and prolonging its useful life, zinc helps save the energy that would otherwise be needed to frequently replace corroded steel structures and manufactured goods. It has been estimated, for example, that the use of zinc coated steel in Sweden saves the country the energy equivalent of one nuclear power plant each year. Thus, by contributing to lower energy consumption, the use of zinc to protect steel impacts positively on the problem of climate change. Life-cycle costing of public


Hot-Dip Galvanizing Process Guide infrastructure shows how significant the savings are: longer service life, lower maintenance costs, longer payback on investment. 5.Zinc is Recyclable Today, over 30% of the global zinc supply comes from recycled zinc, recovered from both new and old scrap. 80% of all the zinc used today will be recycled sooner or later. Due to the long life span of many zinc products – over 100 years in some cases – much of the zinc used in the past is still in service. Zinc recycling technology is advancing and the supply of zinc available for recycling is growing too. Zinc can be recycled indefinitely, without loss of its physical or chemical properties, thus constituting a valuable and sustainable resource for future generations. 6.Zinc Protects Human Health Recent advances in medical science are revealing the importance of zinc for the proper functioning of the immune system, the transfer of nervous signals, the expression of genes and many other vital functions. Zinc supplementation is proving successful in the fight against major causes of child mortality such as diarrhea, pneumonia and malaria. Zinc is known to be vital to the functioning of more than 300 enzymes in the human body. Research is also showing, however, that as much as half the world’s population is at risk from zinc deficiency, with even greater numbers at risk in developing countries and among poor populations. Zinc supplementation is proving to be an effective and cheap intervention that can greatly improve the health status of groups at risk. 7.Zinc Contributes to Society’s Wellbeing Zinc is used to purify water, thus contributing a small solution to one of the great environmental problems of the planet. Recyclable zinc-air batteries successfully power electric vehicles, offering another solution to the problem of urban air quality. Zinc is a major constituent of brass, a health protective metal due to its bacteriostatic qualities. Zinc is an important pharmaceutical ingredient, providing daily skin care and protection against the harmful rays of the sun. Zinc is needed in fertilizers that boost crop yields and so help feed the world’s growing population. And zinc is present everyday, everywhere in our homes and household appliances, fittings, tools and toys, in our offices and computers, our cars, trains and planes. Important Design Factors:- Understanding the fundamentals of the important design factors for the galvanization of steel products help in getting the best protection for steel products. Hot-dip galvanizing is a inspecting process, which depends heavily on suitable design to attain a quality result . The primary difference between hot-dip galvanizing and paint coatings is that the hot dip galvanized coatings can only be used on perfectly prepared surfaces. Some of the important design factors to be considered are. * The zinc will not react with the steel to form the galvanized coating unless the steel surface is completely clean. * The hot-dip galvanized coating will not give results unless the zinc can closely contact the steel surface * The hot-dip coating will not form unless the steel is heated to 4450 – 4600oC * Products cannot be galvanized unless they fit in the preparation tanks and galvanizing bath. Design for Effective Galvanizing Falls into 4 major Categories – Advantages Grades

Details

Venting

Vent holes may be required to prevent air pockets and to allow ash to escape During the galvanizing process..

Draining

Drain may be required to permit zinc to flow freely as the item is immersed and Removed.

Dimensional

Steel is heated to a approx 455oC. In galvanizing and heat & stress distortion


Hot-Dip Galvanizing Process Guide Instability

Should be considered

Design

Basic design rules include steel selection, thickness, welding, cutting etc.,

Advantages: Hot-dip galvanizing offers several benefits and advantages to the specifies some of the important benefits include

10 GOOD REASONS FOR HOT DIP GALVANIZING 1. COMPETITIVE FIRST COST

For most applications the cost of galvanizing is lower than that of applying alternative coatings. Furthermore galvanizing has been getting steadily cheaper compared with painting. The reason is simple: alternatives - particularly painting- are very labour intensive compared with galvanizing which is highly mechanized, closely controlled factory process 2. LOWEST LIFETIME COST Low initial cost and long life make galvanizing the most versatile and economic way of protecting steel for long periods. There are bonuses from no maintenance or extended maintenance intervals; fewer problems of access in remote areas, difficult terrain or when buildings are closely packed together; also when there are safety restrictions e.g. electricity pylons. 3. LONG LIFE Well documented years of experience have proved the benefits of hot dip galvanized protection. At least 20 - 40 years are commonplace in industrial and marine environments, 50 - 100 years in less aggressive atmospheres. 4. THREE-WAY PROTECTION Galvanized coatings protect steel in three ways. Firstly, the coating weathers at a very slow rate giving a long and predictable life. Secondly, the coating corrodes preferentially to provide cathodic protection (sacrificial) to any small areas of steel exposed through drilling, cutting or accidental damage; scratches are sealed by weathering products from the zinc. Thirdly, if the damage area are one day. larger, the sacrificial protection prevents the sideways creep of rust which can undermine paint coatings. 5. SPEED AT APPLICATION A full protection coating can be applied in A 4-coat paint system requires a week 6. EASE OF INSPECTION Galvanized steel simplifies inspection of the protective finish. The nature of the process is such that if the coating looks continuous and sound, it is so. Thicknesses (weights) - simply specified through ENISO 1461 or regular, predictable and simply specified. Galva-national standards - can be easily checked with a magnetic probe or other simple non- destructive testing gauge 7. RELIABILITY The process is relatively simple, straightforward and closely controlled. The thicknesses of coatings formed galvanizing is one of the coatings which is completely defined by DS/ISO 1461, NS 1978, SFS 2765 or SS 3583. Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223

Hot-Dip Galvanizing Process Guide 8. FASTER CONSTRUCTION Galvanized steel is ready for use. No further site surface preparation, painting, touch up or inspection necessary. Once erected the galvanized steel is ready for use: and cladding can begin immediately, thus accelerating the construction programme. 9. COATING TOUGHNESS Galvanizing is unique: the hot dip process produces a coating which is bonded metall- urgically to the steel. No other coating process has this feature and as a result galvanized steel has by far the greatest resistance to mechanical damage during handling, storage, transport and erection - an important factor where steelwork is to be shipped across the world. Furthermore the toughness of this protection makes it particularly suitable for use in situations where abrasion could be a problem. 10. COMPLETE COVERAGE Because it is formed by dipping steel in molten zinc all parts of the surface of the steel are coated – inside, outside, awkward corners and narrow gaps which wood be impossible to protect in any other way. In addition the coating builds up at vital corners and edges – rather than thinning out as do brushed, sprayed and other dipped coatings.


Hot-Dip Galvanizing Process Guide The typical characteristics of appearance:- There are several characteristics to the appearance to include; dull gray color, rust stains, blisters, texture (smooth, rough), lumps, pimples, bare spots, bulky white deposit, and dark spots. The following information will tell more about these different appearance issues 1). Dull gray coating:- This type of coating is normally acceptable and is caused by growth of the zinc iron alloy layers through to the surface of the galvanized coating. 2). Rust stains:- The rust stains are acceptable when present as a surface stain, this is usually caused by contact with or drainage with other corroded steel surfaces. 3). Blisters:- Small intact blisters in the coating are acceptable and are usually due to absorption of hydrogen by the steel during the pickling process being expelled as a result of the heat put off by the galvanizing process. 4). General roughness and thick coatings:- General roughness of the outer material is normally acceptable as long as not otherwise agreed on with the consumer. The cause of rough galvanized coatings is usually a result from uneven growth of zinc iron alloys because of the composition or surface condition of the steel. 5). Lumpiness and runs:- Lumps are normally acceptable unless otherwise specified and are caused by uneven drainage. Lumps and runs have no impact on the coating life. 6). Pimples:- Pimples are grounds for rejection depending on size and extent. Pimples are caused by inclusions of dross in the coating. Dross, which is a zinc iron alloy particles has a similar corrosion rate to the galvanized coating and its presence as finely dispersed pimples is not objectionable. Gross dross inclusion may be grounds for rejection as they tend to embrittle the coating. 7). Bare spots:- Bare spots are generally acceptable if small in area and suitably repaired, depending on the nature of the product. Some reasons for bare spots are faulty processing, rolling defects, laminations and nonmetallic impurities rolled into the surface. 8). Wet storage stain or bulky white deposit:- A bulky white or gray deposit, known as wet storage stain may form on the surface of closely stacked freshly galvanized articles which become damp under poorly ventilated conditions during storage or were left damp as a result of the product not being dry before storing. One main reason for wet material being stored is an insufficient wax concentration, either to much was or to little


Hot-Dip Galvanizing Process Guide Comparative properties of various Zinc Metal Coating: Some of the comparative properties of various zinc metal coating are shown in table. Zinc coating can be applied to steel by different methods which vary from a zinc containing paint to hot dip galvanizing. All these methods can provide successful protection if you choose the right method corresponding to the environment. In general terms the life of a zinc coating is more or less proportional to its thickness. The table provides comparative properties of five well known zinc coating methods. Hot dip galvanizing

Electroplating 1

Zinc spraying

Coating thickness

70 - 215 µm

3- 12 µm )

40 - 200 µm

Standard

EN ISO1461

ISO 2081

2972

Adherence

Alloying with the steel

Electro-cemical

Mechanical

Application

Dipping in melt zinc

Electrolytic deposit

Spaying of molten zinc droplets

Appearance

Shiny and/or gray

Shiny

Light gray

Finish

Good

Very good 2

Spread in thickness

Low

Medium

Coating inside cavities

Yes

No

Very good (25 - 75 years)

Bad (1 - 6 years)

Outdoor protection

4)

Good Higher 3 No

5

Good (10 - 50 years) Easy 5

Quality control

Easy

Easy

Upper size limitation

Around 25 m in Norden

Around 6 m in Norden

No limits

Lower size limitation

3 mm wood screw

Approximately 2 mm

Heavy enough for the spray pressure.

Use of coated product

Utedoor when long lifte time is required

Indoor or outdoor during shorter periods

Utdoor

Life time cost

Very low

High

Low

Cathodic protection

Very good

Limitid

Good

No

No

Deformation during application May occur


6



Comparative Properties of various Zinc Metal Coating Characteristic Hot Dip Galvanizing of the coating Hot-Dip Galvanizing Process Guide Adhesion Coating is integral with the steel because the formation process produces zinc/iron alloy layers over coated with zinc.

Electro Coating

Mechanical Coating Good, comparable Good, comparable with other with electroplated electroplated coatings. coatings.

Zinc Spraying

Zinc Dust Painting

Good mechanical interlocking provided the abrasive grit blasting pretreatment is carried out correctly.

Good abrasive grit blasting of the steel gives best result

Continuity and Uniformity

Good – any discontinuities are readily visible as “black spots”. Some excess zinc at drainage points on products.

Thin at corners the opposite of hot-dip galvanized coatings.

Depends on operator skill Coating are porous but the pores soon fill with zinc corrosion products and are thereafter impermeable.

Good-any pores fill with reaction products. Thin at corners.

Thickness

Normally about 50-125 Microns on tube and products; thicker coating up to 250 Microns obtained by silicon killed steel or grit blasting before galvanizing. Coatings 10 – 30 Microns applied to continuous wire and sheet

Uniformity within limitations of “throwing power” of bath. Pores not a problem, as exposed steel protected by adjacent zinc Thickness variable at will; generally 3 -15 Microns Thicker layers are possible but generally uneconomical.

Variable at will, usually between 10 – 80 Microns.

Thickness variable at will generally 100 – 150 Microns but coatings of up to 500 Microns can be applied

Up to 40 Microns of paint (and more with special formulations can be applied in one coat

Characteristic of the coating Formability and Mechanical Properties

Hot Dip Galvanizing

Zinc Spraying

Zinc Dust Painting

When applied to finished articles, forming not required. Can weld through thin coating if necessary but preferable to mask edges to be welded and spray these afterwards

Abrasion resistance better than conventional paints, Painted sheet can be formed and resistance welded with minor damage.

Conventional coatings applied to finished articles, not formidable; alloy layer is abrasive but brittle Prepared By resistant M.Sreedhar Patnaikon bending. Email: [email protected] Special coating with Phone : 08985219223 little or no alloy layer readily formed (e.g. on sheet) and resistance

Electro Coating

Mechanical Coating Electroplated steel has Good. Does not excellent formability causes emand can be spot brittlement of high welded. Small strength steels. components are usually finished before plating.


What is Corrosion? Corrosion (rust) is the deterioration of a material that alters its mechanical properties. This is caused by a chemical reaction between the material and oxygen or some other type of chemical. Corrosion is one of the most important factors in the breakdown of materials causing a shorter life expectancy than what the product was set out to achieve. This chemical breakdown of the material causes many unfortunate accidents that might have been prevented with the use of a corrosion resistant product (HSC-1999)

Galvanized Products: The following list is some practical uses for galvanizing steel Products : Angles

Bridge super structure

Fence poles

Lattice towers

Amusement ride

conveyor systems

Garbage cans

Metal sculputures

Beams

Cooling tower parts

Gas turbine skids

Over head cranes

Bicycle racks

Channels

Gratings

Over head sign supports

Box rails

Dock hardware

Handrails

Picnic table frames

Bridge substructure

Dock levelers

High-way guardrail/posts ladders

Pipes

Platforms

Stair treads

Transmission Poles

Wheel barrow

Rebars

Sound barriers

Utility trailers

Wire mesh

Signal light poles

structural Steel

Water / Waste treatment structures

Summary: Galvanizing (to coat iron or steel with zinc) is today's best way of preserving materials for tomorrow without much environmental impact, due to the recyclable ability of zinc. In today's society the environmental implications of the use of industrial products and processes are essential to the life of all living organisms. With no space left for radioactive waste or other non-biogradeable products Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223

Hot-Dip Galvanizing Process Guide something needs to be done to preserve life for tomorrow. Galvanizing is a process that not only works to prevent present day failure but it also looks to the future. Zinc the main ingredient used in the galvanizing process has been found to be essential to humans as well as to the life of many other living organisms.



Design and fabrication of structures Design of structures to be hot-dip galvanized is no different from that for good structural, surface treatment and welding practice in general. However, certain components require special attention. •

It is important that larger structural components are matched to the size of the pickling tank and zinc bath (Fig 1). Sometimes “doubledipping”, which involves the handling of structural components longer than the zinc bath, is employed. (Fig.2).

•

Bolted joints are preferred instead of welding, since welding burns off the galvanized coating, which havs to be repaired.

•

The weight of the construction. The galvanizing plants can only handle constructions that the cranes can handle.

Fig. 1. Schematic diagram of hot dip galvanizing.

Fig. 2. Schematic diagram of doubledipping.

Safety requirements Hot-dip galvanizing involves dipping components and structures in both pretreatment baths and molten zinc. This means that hollow structures, such as pipes, tubing, and containers and also beam structures, which are welded round the connection point, must be provided with holes for drainage and venting. If a sufficient number of holes is not provided to enable complete venting, there is a significant risk that the structure will explode on immersion. Also, pickling acid is capable of penetrating pores in welds. Upon dipping in the zinc bath the residual pickling liquid is vaporized and the pressure can be so high that the object explodes. The risk of serious injury to personnel, or damage to materials, is great when such explosions occur.


Hot-Dip Galvanizing Process Guide Suitable dimensions for venting holes are given in table 1. The holes must be located to enable total venting and to enable pickling acid and molten zinc to run in and out easily. Illustrations show this point. Examples of the location of the holes are given in the illustrations. It is also advisable to contact the galvanizer to discuss alternative hole sizes and locations. Holes can be formed by drilling, grinding or gas cutting.

Table 1. Suitable hole sizes for venting of tubular structures.

Fig.3. Galvanizing of cable trays. Tilting the goods improves the run off of excess zinc when leaving the bath.

Design considerations The following guidelines are not necessarily preconditions for hot-dip galvanizing but, if they are observed, quality is likely to be considerably better and the galvanizing process easier. Provision for handling A structure should preferably be designed with easily handled components, which are assembled after hot-dip galvanizing. Threaded unions are a good means of achieving this. The structure or object should be equipped with lifting holes or lifting eyes, to which lifting gear can be connected. Avoid inwardprojecting pipe connecting stubs in containers. Pipe connecting stubs should be located in corners and diagonally, otherwise the container cannot be completely emptied of zinc (fig.4).


Hot-Dip Galvanizing Process Guide Fig.4. Inwardly projecting pipe con-nections prevent containers from being completely emptied of zinc and flux. Locate pipe connections near comers and diagonally. It will also be easier for the zinc to run out if the pipe con-nections are not too small. On large containers lifting eyes should be applied for easier handling.

Fig.5. Do not weld parts of widely differing thickness together. The object may be deformed in the zinc bath and during cooling after dipping due to differing heating and cooling rates.

Mobility Components that are to move on each other should be assembled after hot-dip galvanizing. If this is not possible, mobility can be retained by giving the components a clearance of 1.0 mm on each side, i.e 2.0 mm larger than the shaft. Avoid structures that can distort Materials with excessive variation in sectional thickness should not be combined on the same structure or object (fig. 5). Otherwise, heating in the zinc bath would be uneven, and the object could warp. Long, slim structures should be avoided. Large flat sheet surfaces, where the sheet thickness is less than 3—4 mm should be stiffened (fig.6), or they might warp.

Fig.6. Examples of stiffening of sheet surfaces to prevent distortions.


Fig 7. Objects with welded-on reinforcements and lap joints must be provided with holes if the contact area between the pieces is larger than 70 cm2. An acid trap is created, but must be accepted, since there is a great risk of explosion if a hole is not made.

Hot-Dip Galvanizing Process Guide If possible, structures should be designed so that welding can be performed symmetrically, around an axis of flexure. The welding sequence should be planned so that stresses are as low as possible. Avoid combining different materials and different surface conditions Newly rolled steel should not be welded to cast iron, or to steel that has been attacked by deep-seated rust, since the zinc coating will be uneven and the appearances of the surfaces will be different. Such materials must be galvanized in different ways, which is not possible if they have been welded together. Different types of steel, such as rimming or aluminium killed steel, should not be welded to silicon-killed steel. If this is not observed, both the appearance of the surface and the thickness of the zinc coatings will be different. Avoid acid traps Structures should be designed to avoid narrow crevices or gaps (fig. 9 and 11). As far as hot-dip galvanizing is concerned, butt welds are better than lap welds. If lap welds must be used, they should be welded around the entire joint. It is important to avoid the formation of pinholes. If the gap between contact surfaces is greater than 70 cm2, venting holes must be provided to avoid the risk of explosion (fig.7). If acid has penetrated a crevice, it cannot be removed later. Since molten zinc is more viscous than acid, it will seal the opening during hot-dip galvanizing. After a while, the acid causes holes in the coating over the opening. A rust-coloured liquid then runs on to the surface and spoils the appearance. Corrosion in such crevices can become so severe that the connection eventually fails. Castings should have an even, sealed and sand-free surface. Pores and cavities can act as acid traps and should therefore be avoided. Avoid pockets Structures should if possible be designed so that the zinc can run freely off all surfaces when the object is withdrawn from the zinc bath (fig.8). The coating will otherwise be uneven.



Fig.8. Corners on stiffening plates in e.g. beam Fig. 9. Seal the ends on a double fillet weld so that acid webs should be cropped. Drainage holes should be cannot penetrate into the gap. located as close to the web as possible. Diagonals in lattice structures should not run up against frame bars. Note that an acid trap is created in alternative 2. This should be avoided.

Fig.10. In welded structures venting holes must be inserted at all junctions. The holes must enable the zinc to run in and out freely. Closed tubes incur the risk of explosion! Threaded objects Male threads must be cut undersize to enable them to fit the respective nuts after galvanizing. Suitable reduction of the thread diameter is given in standards. Female threads are cut or cleaned to nominal size after



galvanizing. Male threads in assembled structures must be cleaned after galvanizing. The zinc coating on male threads also protects the steel in female threads. Marking Temporary marking of steel to be hot-dip galvanized should be carried out with water-soluble paint only. Permanent marking can be done by stamping letters or numbers on the object, or to an attached ID plate. Stamping should be to sufficient depth to enable the marking to be read even after galvanizing. Welding Welds should be free of pores, which could act as acid traps. In case of double-sided fillet welds, the weld must be continued over the ends of the sheet to prevent the penetration of acid into any conceivable crevice (fig.9). Welding methods which do not produce slag, such as MIG welding, are preferable. If coated welding electrodes are used, all slag deposits should be carefully removed from the weld. It is not possible to remove welding slag through normal pickling, and any residue causes the formation of black spots during hot-dip galvanizing.

Fig. 7-10. Examples of how weldments should be designed in order to avoid acid traps (narrow crevices). General A pre-requisite for good results in hot-dip galvanizing is clean steel surfaces. One should therefore try to keep the surfaces free of paint, grease, oil and tar. Impurities such as these cannot be removed by pickling and result in black, uncoated spots after galvanizing. Surface contaminations in the form of grease, oil, tar, paint and welding slag require removal by means of degreasing, sandblasting or grinding. Contamination may be difficult to detect. If it becomes apparent after galvanizing the article may need to be regalvanized, which increases costs. Laser or flame cutting heats the steel and produces surfaces that react differently when galvanized. The coating thickness may be lower than expected and the adherence to the steel surface may be poor. The cut surfaces should therefore be ground to remove the heat affected zone before galvanizing.



Hole Sizes

millimetres D

50

100

150

200

13 Hole 0 minimum

25

38

50

Note: 2 holes required- one each end

Larger Holes

Better Drainage

Higher Quality

Inadequately vented fabrications may explode when immersed in molten zinc. Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223


Venting

(1) Sealed Selections must be vented for reasons of safety and drainage

vent holes should be placed not more than 10mm from their respective sealed ends.

(1) On longer members, the vent and drain holes must diagonally

be positioned at

Opposite ends the, the holes can be located in the top or the side end. Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223


Hand Rails

 Handrails must be vented at each closed end and at each bend.  The Vents can consist of drilled holes or slots.



Fully Welded Plates

 Fully welded plates having an area in excess of 100 cm2 require relief holes.  The holes must be sealed with silicon in order to minimize the entry of pickling liquid which can vaporize in the hot zinc bath.



Holes or Notches

With hollow Section fabrications, provision must be made for venting and draining holes or notches must be placed within 10mm of the sealed end of the section. On vertical members a drilled hole ‘V’ notch should be provided at each end of the section. Each hole must be located a opposite of the section. Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223


Cut Outs


Hot-Dip Galvanizing Process Guide  It is preferable to have external venting holes for all sections, As an alternative internal cutouts which lead to other externally vented sections can be used. Whether circular or rectangular in shape, that cutouts should be nearly as big as the hollow section. For safety reasons, cutouts must be verified visually or by a photograph.

Plates



Drain holes located away from the edge of the plates lead to some pickling liquid and molten zinc remaining in the tube. Retained zinc adds to the weight of the Component Retained Pickling liquid can vaporize in the galvanising Bath which can damage the structure or cause an explosion.

For Complex shapes contact Visakha Steel Allieds.

Large Corner Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223


Stiffeners

Crops allow the

Dead pockets can lead to air locks pooling of Pickle liquid and Zinc build up, The high temperature In the galvanizing bath causes the pickle liquid to Vaporize resulting in misses and excess ash. ` eliminating dead

molten zinc to freely drain

away Pockets will lead to a cleaner and more blemish free Finish stiffeners should have the corners cropped in Order to eliminate dead pockets.

Base Plates Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223


Hanging Points



Mesh Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223


Spot welds Due to the expansion and contraction of the steel During the galvanizing process, there is a risk of Distortion. A weld should be placed only in every alternate Space or an every second wire, depending on the Construction.

Surface Contaminants & Distortion Paint , Oil and Grease Paint, Oil and Grease interferes with the galvanizing process. Surface Contaminants must be removed before galvanizing can proceed. Cleaning results in time delays and can incur extra charges. Distortion Distortion is an ever present risk during galvanizing. It results from the relieving of stresses which have built up during the steel manufacturing process and also during fabrication. Structures fabricated from sections having different thicknesses and large surface are particularly prone. For assistance in design, please contact Visakha Steel Allieds





Galvanizing Defect & Repair Methods Legend: A- Accept R-Reject N- Negotiate C- Clean REP- Repair Responsibility: G- Galvanized D- Designer B- Builder / Fabricator S- Steel Type / Surface


S.No.

Description

Cause

Effect / Remedy / Responsibility

Hot-Dip Galvanizing Process Guide 1 APPEARANCE OF Although the recommended quantity SODIUM of sodium dichromate is about 0.15 DICHROMATE. to 0.3 %, occasionally when topping A small amount of up, more is added. This often results sodium dichromate in a dark yellow to brown colour on is generally added the galvanized surface. The darker to the quench water colour will provide enhanced initial bath for passivation. corrosion protection. 2 ASH DEPOSITS. Zinc oxide deposits cool take place Ash deposits are when the component is dipped or grey, non-metallic when it is removed from the bath. deposits consisting of zinc oxide that have been deposited on the hot dip galvanized coating. S.No. 3

Description

Maintain concentration of Sodium dichromate at about 0.15 to 0.30 %. G

The coating is normally intact underneath the ash deposits. Ash must be removed and the coating thickness verified for conformance to the specification requirements. Remove ash from all liquid conveyance pipes

Cause

BARE SPOTS. There are several causes of bare spots. Although excluded These include: from ISo 1461, bare Over drying. If the time between fluxing and hot dip galvanizing is spots of about 5mm prolonged or the drying temperature is toe high, the barrier protection (2.2 x 2.2mm), due provided by the flux may be lost. This is indicated by a rusty to small localized appearance on the ungalvanized article, which can result in coating flows, ore discontinuities after hot dip galvanizing. G adequately protected by the Excess Aluminium. A condition some times referred to as black spots sacrificial properties may occur if the aluminium content of a bath becomes too high. No of zinc and will trouble should be experienced if flux concentration is correct and the have very little aluminium content of the bath is maintained below approximately effect on the service 0.007%. G life of the coaling. PreparedWhere By M.Sreedhar necessary, Patnaik Further causes are: Email: [email protected] such spots may be Blowouts: flux deposits: stains and inclusions; mechanical damage; Phone : 08985219223 repaired using one touch marks; uncoated surfaces caused by surface contaminants, scale of the specified or sand; welds and weld spatter. repair methods. Gross uncoated

A/R/N/C/ REP A

A/N

REP If necessary A/R/N/C/ REP A/R

REP If necessary


S.No.

Description

Cause


4

BLACK STEEL INSPECTION. Inspection prior to hot dip galvanizing is extremely important.

Components should be checked for distortion caused during rolling or fabrication. Check for appropriate vent, fill and drainage holes; removal of weld slag and spatter; venting of overlapping surfaces; temporary identification markings; clearance for moving ports and potential distortion due to the process.

Insufficient inspection prior to hot dip galvanizing can be the causes of dispute. G

5.

Incurred nozzle pressure; nozzle BLASTING angle; sweeping distance; size of DAMAGE. abrasive and recycling of grit. Sweep blasting, done correctly, substantially increases paint adhesion and final coating appearance hut done incorrectly can result in coating damage.


A hot dip galvanized coating wi8 be partially or fully destroyed by excessive blasting. Refer to the HDGASA Cede of practice. D/B

A/R/N/ C / REP A/R

R


S.No.

Description

Cause


6

BLOWOUTS. Staining and coating defects around unsealed weld areas and vent holes, similar to stains ceased by weeping.

Pre-treatment chemicals penetrating sealed overlap areas through the required vent holes and escaping during immersion in the molten zinc. This effect tends to damage the flux coating, causing localized uncoated areas.

Pre-heat item prior to immersion in zinc bath to dry out overlap area as much as possible.

Molten zinc has a high surface tension and will not easily drain from holes under 8mm in diameter.

Make holes as large as possible. Removal of molten zinc over the bath and utilization of vibrators will reduce the likelihood of clogging.

7

CLOGGED HOLES. Zinc film clogging or partly bridging holes.

A/R/N/ C / REP A

D/G C/ REP If Necessary

A

D/G C If Required



S.No.

Description

Cause


8.

CLOGGED THREADS. Threaded components or attachments have threads dogged with zinc.

Insufficient centrifuging or poor drainage of threaded attachments on withdrawal from the galvanizing bath.

The correct centrifuging equipment or post galvanizing thread cleaning by heating, wire brushing or oversize tapping of nuts, will generally remove clogging. If necessary specify delivery of bolts and nuts in nutted up farm. G

9.

10.

DESIGN STRUCTURES IN MODULAR LENGTHS AND OPTIMUM WIDTHS TO SUIT AVAILABLE BATH SIZES. DISCOLOURING AFTER HOT DIP GALVANIZING CAUSED BY GRINDING OR OTHER RESIDUES.

Double dipping can sometimes be used to hot dip galvanize fabrications that are too long or wide far single immersion. If the fabrication exceeds the bath size, members in the fabrication may require touching up.

Touch up and repair can be avoided, the cost of hot dip galvanizing reduced and the overall quality improved, if the design of the component is restricted to the length, width and depth of the galvanizing bath. D/B

Material stored in contact with rusty steel, or iron filings can cause surface rust staining.

Once the cause has been removed the stain will gradually disappear. B


A/R/N/ C / REP R

C / REP

A

C / REP

A C If Possible


S.No.

Description

Cause


11.

DISTORTION. Distortion is the unwanted warping that occasionally becomes evident after hot dip galvanizing.

The hot dip galvanizing process occurs all molten zinc temperature of 450deg C. This is at the lower end of the stress relieving temperature for treating steel. Thus, any inherent rolling or welding stresses in the fabrication, are likely to be released. This may result in a dimensional change, i.e. distortion.

12.

DRAINAGE SPIKES. Spikes and teardrops of zinc often appear along the edge of a component after hot dip galvanizing.

Use symmetrical designs; Use A/N sections of similar thickness; Stiffen un supported thin will sections; Use performed members with the correct minimum bend radii; Use balanced or staggered welding REP techniques; Make use of If temporary braces on thin walled possible sections such as troughs, cylinders and angle frames. Avoid quenching after galvanizing. Components can be straightened after hot dip galvanizing. D/G A/N Drainage spikes are easily removed at the bath while still molten but with complex C / REP fabrications, the solidified IF spikes will be removed by Possible fettling by the galvanizer prior to inspection. G

The edge most likely to have these spikes is the last to leave the bath on withdrawal. This applies particularly to complex fabrications.


A/R/N/ C / REP

S.No.

Description

Cause

Hot-Dip Galvanizing Process Guide 13. DROSS. Gross Dross deposit from the Dross particles are bottom of the zinc bath, trapped in iron/zinc alloy the corner of a fabrication. The crystals produced dross incorporated in the coating when hot dip prevents drainage of the zinc in the galvanizing steel. intermediate area and a build up These precipitation to occurs. the bottom of the bath and are removed by the galvanizer. 14.

15.

DULL GREY OR MOTTLED COATING APPEARANCE. Dull grey or mottled coatings can appear as a dark grey circular patter, a localized dull patch or, may extend over the entire surface of the component

FLAKING OR DELAMINATION OF COATING. No adhesion of zinc to steel surface. Thick, rough coating.


A/R/N/C/ REP

Dross consists of the same iron/zinc alloy as the coating, it will provide the same corrosion protection as a normal hot dip galvanized coating. G

R/N Mechanically remove if possible, repair or regalvanize C/REP IF necessary

This appearance indicates the presence of extensive iron/zinc alloy phase growth mused by steels with high reactive levels of silicon and phosphorous in steels.

Although not as aesthetically pleasing as a coating with free zinc on the surface, a dull grey coating provides similar or hotter corrosion protection. D/S

High phosphorous content (greater than 0.03% causes entire coating to delaminate from the steel.

Use a steel that has a phosphorous content of lower than 0.03% D/S

Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223 FLUX DEPOSITS, 16. STAINS AND Flux deposits or stains may occur INCLUSION. as a result of excessive “dusting” Flux deposits or stains with ammonium chloride on

Flux deposits or stains should be removed and the underl~4ng coating measured to determine

A

R/N

A


S.No.

Description

Cause


17.

DISCOLOURATION OF THE PAINT COATING OVER HOT DIP GALVANIGING AFTER EXPOSURE TO THE ENVIRONMENT.

Inadequate repair of a damaged surface on the hot dip galvanized coating prier to the application of a paint coating.

Make use of the correct repair materials and application procedures when touching up cut or welded hot dip galvanized components,

COATING THICKNESS PROVIDED ON FASTENERS USE TO ASSEMBLE HOT DIP GALVANIZED STRUCTURES

No matter how the zinc coating is applied, the coating life is proportional to its thickness in a given environment. Often electroplated fasteners with insufficient coating thickness are incorrectly used in external environments.

18.

B


Specify hot dip galvanized fasteners to ISO 1461, where required. Alternatively overcoat fastener with an approved zinc rich point or epoxy. D/B

A/R/N/ C / REP R

REP

R

C / REP If acceptable


S.No.

Description

Cause


19.

MECHANICAL DAMAGE. Mechanical handling or transport damage may occur, particularly with extremely thick coatings, which tend to be brittle in nature.

The use of chains, wire ropes, dragging or dropping of the component onto a hard surface, can cause mechanical damage. This is particularly relevant with thick brittle coatings.

Warming labels, highlighting a thick coating and possible damage if manhandled, should be attached by the galvanizer, before the component is transported. The use of nylon lifting slings is recommended. G/B

OXIDE LINES. Light aluminium oxide film lines on hot dip galvanized surface.

Due to the shape and / or drainage conditions of some components, the hoist crane has stopped and started upon withdrawal of the items from the molten zinc.

No effect on corrosion resistance. The overall appearance becomes uniform in time. G

20.


A/R/N/ C / REP A

REP If necessary

A


S.No.

Description

Cause


21.

PIMPLES OR BLISTERS. Pimples or blisters formed during hot dip galvanizing are usually associated with surface imperfections such as dross inclusions.

Dross pimples result from agitation of the dross layer at the bottom of the bath or from dragging material through the dross layer. They appear as small, hard lumps on an otherwise normal galvanized surface. Blisters may be formed by hydrogen, which is absorbed daring pickling and diffused at galvanizing temperatures.

The galvanizer should avoid disturbing the dross layer at the bottom of the bath by controlling immersion depths and dressing regularly. Since dross pimples represent minor disturbances in coating uniformity, they do not affect corrosion resistance. G

22.

REACTIVE AND This difference in coating thickness, is NON-REACTIVE brought about by a combination of a STEELS WELDED more reactive silicon killed steel, TOGETHER. and/or high phosphorous resulting in a Variations in coating thicker coating and a less reactive thickness can arise aluminium killed steel, resulting in a when reactive and coating thickness sometimes below non-reactive steels are that required in the specifications. welded together. should the galvanizer be asked to reEfforts to increase galvanize in accordance with the coating thickness on specification, the resultant coating the less reactive steel thickness on the reactive steel will be may result in an excessively thick, resulting in a brittle undesirably thick and coating more susceptible to damage brittle coating on the most reactive steel. Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223

A/R/N/ C / REP A

C If necessary




S.No.

Description

Cause


23.

REMOVAL OF ZINC COATING BY EXCESSIVE CLEANING. Unless otherwise agreed, the galvanizer will limit cleaning of the final coating by mechanical means to that required in the specification.

Excessive cleaning of the coating, particularly the edges, by mechanical methods, can result in uncoated areas.

The affected areas usually only appear after the component is installed. Care should be exercised by the galvanizer to avoid over cleaning. G/B

A/R/N/ C / REP R

REP At the galvanizer or alternatively at site

A 24.

ROLLING DEFECTS IN STEEL. These defects maybe broadly classified as surface discontinuities in the steel that have been elongated during rolling.

Steel may occasionally include laminations, laps, folds and nonmetallic impurities, which result in silvers rolled into the metal surface. Defects of this type are sometimes detected before or after pickling, but may only become apparent after hot dip galvanizing.


Surface flaws in the base material may be removed by local grinding after hot dip galvanizing followed by repair of the affected surface. Minor surface defects will not adversely influence coating life. S

REP If necessary




S.No.

Desciption

Cause


25.

ROUGH COATINGS, CAUSED BY STEEL SURFACE CONDITIONS.

Rough surfaces, typical of coatings on corroded steel surfaces, can be hot dip galvanized satisfactorily. The coating will, however, reflect the texture of the substrate. Other causes of rough surfaces include uneven cold working, over pickling, a high galvanizing temperature and / or extended immersion in the molten zinc.

The rougher surface will produce a thicker coating and result in a longer service life. S

26.

ROUGH HEAVY COATINGS, CAUSED BY A ROUGH SURFACE AND / OR THE CHEMICAL COMPOSITION OF THE STEEL “TREE BARK EFFECT”

Rough, heavy coatings refer to hot dip galvanized components showing markedly rough surfaces. This can include coatings that have a generally rough surface and, in some cases, groove type surface configurations, “tree bark effect” caused by variations in surface steel analysis.


A/R

A/R/N/ C / REP A




S.No.

Desciption

Cause


27.

ROUGH HEAVY COATINGS CAUSED BY INSUFFICIENT CENTRIFUGING

Efficient centrifuging, will generally remove excess zinc and provide a smooth and attractive exterior.

Provided the steel/casting surface is reasonably smooth, correctly centrifuged articles will provide an acceptable finish, G

STAINS CAUSED BY WEEPING

The Salts from acid or flex that have penetrated porous welding or between contact surfaces during pickling can weep after hot dip galvanizing and water quenching, producing a stained area.

The stain can be easily removed by means of bristle brushing. Should the component be destined for a corrosive area, the crevice should be sealed with a sealant after cleaning. D/B

Heavy walls and thick flanges used in the manufacture of piping can act as a heat sink when immersed in molten zinc. This effect considerably lengthens the immersion time. Occasionally the galvanizer will remove the pipes before all the zinc has melted from the inside of the pipe.

The galvanizer should ensure all zinc hen been removed from the aside of the pipe by longer immersion times. G

28.

29.

TIGHTLY ADHERENT LUMPS OF ZINC ON THE INSIDE OF HEAVY WALLED STEEL PIPING.


A/R/N/ C / REP R

C / REP If acceptable A

C / REP If necessary R

C / REP If acceptable




S.No.

Desciption

Cause


30

TOUCH MARKS. The zinc in the galvanizing bath should have free access to all component surfaces or small unmated or damaged areas can result.

Articles entering the galvanizing baths should not be in tight contact with each other. Jigging wire should also be loosely attached to eliminate wire marks. Where a component has been resting on jigging or dipping equipment, an uncoated area and touch mark could appear.

Minimize contact between components and jig connections. (Loosen jigging wire). Small components can be centrifuged.

31

TYPICAL SPANGLED HOT DIP GALVANIZED COATING. A typical hot dip galvanized surface is shown in the example. The surface is silver grey in color and not necessary but often has a spangled effect (Zinc Crystals) in a range of sizes.

Surface appearances may very according to the chemical composition of the steel. Cooling rate has a direct effect on the surface brightness and spangle size. Easter cooling usually results in a brighter coating with a smaller spangle size.


G

Small additions of aluminium to the molten zinc, brightness the coating. S/G

A/R/N/ C / REP A

REP If necessary and acceptable A




S.No.

Desciption

Cause


32.

UNEVEN DRAINAGE. Uneven drainage results in an uneven or lumpy area on which zinc build up has occurred.

The condition can occur over the entire surface grin isolated areas. Uneven drainage also includes drips and the ends of pots, runs near halos. The cause is withdrawal speed too high or low galvanizing temperature.

Although not particularly attractive, this condition dons not adversely affect coating performance. Protuberances and lumps, which interfere with mating surfaces are unacceptable. G

A/R/N/ C / REP A

C If necessary A/N/R

33.

UNCOATED SURFACES CAUSED BY STEEL SURFACE CONTAMINANTS OR ENTRAPPED AIR.

Residues, such as oil based paint, grease, oil or labels on the steel surface or incorrectly positioned vent holes, can result in localized un-galvanized areas in an otherwise continuous galvanized coating. Defects after galvanizing can very in color from grey black to brown while no galvanized coating has been formed.


Ensure all paint or grease is removed prior to hot dip galvanizing. Make usual suitable marking pens for temporary identification. Correctly position adequately sized vent hubs. B

REP If necessary




S.No.

Desciption

Cause


34.

UNGALVANIZED SURFACES CAUSED BY SCALE OR SAND.

Sand on cast iron or scale on the steel surface is generally caused by the process used to form or roll the product. A localized un-galvanized area in an otherwise continuous coating can occur if scale or sand from the moulding or rolling is not removed by acid pickling or abrasive blasting.

These un-galvanized areas may occur in a linear pattern on angles, channels or other rolled products. They can also appear on cast iron products. S/G

A localized un-galvanized area near a weld can be caused by weld slug deposit, weld porosity or weld undercut. Oxide deposits and residues from welding area resistant to normal pickling adds and mint be removed before the work is pickled and hot dip galvanized.

Weld slag deposits should be removed by the fabricator by means of abrasive blast cleaning. The deposit can also be removed by proper chipping or wire brushing. Shielded arc welding as opposed to slick welding is preferred for components which are to ho hot dip galvanized. B

35.

UNGALVANIZED AREA IN THE VICINITY OF A WELD.


A/R/N/ C / REP R/N

REP If acceptable

R/N

REP If necessary




S.No.

Desciption

Cause


36.

USE CONVENTIONAL FIXING METHODS SUCH AS BOLTS AND NUTS, OR REPAIR DAMAGED COATINGS CAUSED BY WELDING OR NON CONVENTIONAL FIXING METHODS.

Conventional drilling and bolting after hot dip galvanizing is preferred. Should welding or noon conventional method of fixing be used, resulting in damage to the coating, an approved repair method is necessary.

Coating repair can be done by zinc metal spraying, zinc rich painter epoxy, providing the product conforms to the requirements of the specification.

37.

WELD SPATTER. Weld spatter is oxidized, normally spherical expelled weld metal, that is fused or not onto the surrounding material during welding.

A/R/N/ C / REP A

REP

D/B

Weld spatter is caused by weld pool explosions when improper welding parameters are used, or if the material is dirty or contaminated.


Loosely adherent weld spatter should be removed prior to hot dip galvanizing. Although not acceptable in terms of the specification the presence of tightly adherent weld spatter after hot dip galvanizing will not affect the corrosion resistant properties of the coating. B

A/N




S.No.

Desciption

Cause


38..

WET STORAGE STAIN OR WHITE RUST. Wet storage stain or white rust as it is commonly called, is a white voluminous deposit that is occasionally found on the surface of a freshly galvanized coating.

Wet storage stain (Zinc hydroxide) is formed on freshly galvanized surfaces which are in closed contact in presence of moisture. Freshly galvanized coatings react with the environment until such time as a stable zinc carbonate film is formed on the cooling surface.

Wet storage stain ceases when the cause is eliminated. If the coating thickness at the affected area is equal to, or greater than the minimum required in the specification, it is not a cause for rejection, other than for aesthetic reasons. The latter is subject to discussion with the end user. Customer during transport and storage. G/B

ZINC METAL SPRAYED REPAIR APPLIED TO INADEQUATELY BLASTED SURFACES OR NOT WIRE BRUSHED AFTER APPLICATION.

In order for zinc metal spraying to adhere on application, the damaged surface must be adequately blasted. As it is difficult not to over spray, excess zinc metal. Spray loosely adheres to the surrounding coating.

39.


Prepare surface for repair by adequate blasting. Loosely applied zinc metal sprayed coating at the perimeter of the repair should removed by wire brushing if net removed, there is no compromise in the corrosion resistance, G/B

A/R/N/ C / REP A

C If necessary

A

C




S.No.

Desciption

Cause


40.

ZING SPLATTER. Splashes and flakes of loosely adherent /zinc, caused by moisture on the steel surface when hot dip galvanizing

When hot dip galvanizing an unusually deep fabrication by double dipping , moisture an the surface of the steel contacts with the molten zinc causing splashes of zinc to loosely adhere hot dip galvanized surface.

The loosely adherent zinc splashes are easily removed. An experienced galvanizer can ensure the coating overlap on double end dipped surface. Is not visible. G


A/R/N/ C / REP A

C




DEFECTS IN GALVANIZED COATINGS INTRODUCTION Unlike many manufacturing processes, the hot dip galvanizing of fabricated steelwork involves many variables that can impact on the appearance and characteristics of the finished product. Hot dip galvanizing is primarily an industrial protective coating process designed to provide a tough and durable protective coating for steel that will generally outlast the design life of the structure to which it is applied. The variables inherent in the process can each affect the appearance of the galvanized product. Many of these variables are classified as ‘coating defects’. Most are easthetic and do not affect the performance of the galvanizing as an anti-corrosion coating.

Distortion of thin checker plate welded to heavier supporting members can be reduced by fixing the checker plate after galvanizing

FACTORS AFFECTING HOT DIP GALVANIZED COATINGS The factors affecting hot dip the appearance and characteristics of hot dip galvanized coatings include: • The size and shape of the item • The steel chemistry • The steel surface condition • The design of the item with respect to • The metallurgy of the galvanizing process. The hot dip galvanizing process involves immersing steel items in molten zinc at 450oC after pretreatment to remove organic materials, rust and mill scale. This hot dipping process, where the steel is immersed in the molten zinc for several minutes, gives the galvanized coating its unique characteristics. SIZE & SHAPE OF THE ITEM Zinc freezes at 420oC, so there is very little superheat in the molten zinc as the item is withdrawn from the bath of molten zinc. Thicker sections will retain their heat longer and promote better drainage of the Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223

Drainage spikes will always occur where edges are parallel to the molten zinc surface. These are removed by grinding of filing during the inspection process.


molten zinc from their surfaces than thinner sections. The angle with which the work can be withdrawn from the bath will influence the drainage characteristics. One-dimensional items that are short enough to dip vertically will produce smoother and more uniform coatings than 2- or 3-dimensional items where some surfaces will be at shallow withdrawal angles from the molten zinc. THE STEEL CHEMISTRY The rate at which molten zinc reacts with steel to form the galvanized coating is dependent on the steel chemistry. Pure iron has a very low reaction rate and for this reason, galvanizing kettles are manufactured from steel of this type. Structural grades of steel always contains alloying elements, the most common of which are carbon, manganese and silicon. Sulfur and phosphorous are residual elements arising from the raw material used to make the steel, although some special steels have these elements added deliberately.

Blinding of small holes (6mm or under) will occur because of the surface tension of the molten zinc. This can be minimised by detailed attention to the item as it emerges from the molten zinc.

Of these elements, silicon and phosphorous have the most significant effect on the galvanized coating’s characteristics, with silicon being the most common steel alloying additive affecting coating appearance. High silicon levels 9obive 0.20%) may result in the galvanized coating have a duller or matt gray appearance, or a blotchy variable appearance. High phosphorous levels (not normally found in structural steels) can give rise to dark gray or browning coatings that are prone to delamination from the steel’s surface. STEEL SURFACE CONDITION The surface condition of the steel will be reflected in the galvanized coating in two ways. Rougher surfaces have a higher surface area per unit of surface and thus generate thicker galvanized coatings. Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223

Blinding of small holes (6mm or under) will occur because of the surface tension of the molten zinc. This can be minimised by detailed attention to the item as it emerges from the


Hot rolled sections typically produce hot dip galvanized coatings 30-50% thicker than smooth, cold rolled surfaces.

molten zinc.

Galvanized coatings on steel with gross surface defects such as pitting will conform to the defects and follow the surface profile of the steel. While the coating will provide the same degree of protection from corrosion, the appearance may not be acceptable to the end-user and thus steel surface quality is an issue where the highest standards of surface appearance are required. DESIGN OF THE ITEM FOR GALVANIZING The design of a fabrication is critical in determining the surface quality of the finished product. Unless the pre-treatment chemicals can penetrate to all surfaces of the item, the inaccessible surfaces will not be galvanized. Conversely, if the molten zinc cannot completely drain from the work, zinc puddles will freeze in pockets and cause problems. The techniques of designing for galvanizing are well understood by galvanizers and consultation in the design stage of fabrication will always result in a better quality outcome.

Threaded items will usually require post-galvanizing treatment to clean the threads of zinc. This can be done by reheating the threads to galvanizing temperature (450 oC), then removing the excess zinc by wire brushing.

The main issues to be addressed in designing for galvanizing to minimise defects are as follows: 1. The item needs to be suspended during galvanizing. The provision of lifting lugs to eliminate touch marks from wire or chains is an example of design improving quality. 2. The item needs to be have adequate venting and draining provisions to ensure the smooth flow of zinc into and out of the item. Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223

The fracture of Bisalloy wear plate is a result of


3. The dimensions of the item need to comfortably fit within the bath dimensions to allow adequate withdrawal angles. Double dipping of the item will frequently produce a better quality than single dipping an item that can be barely submerged in the bath. THE METALLURGY OF THE GALVANIZING PROCESS Many of the surface imperfections that are characteristic of hot dip galvanized coatings are a function of the metallurgy associated with the process.

Hydrogen embrittlement arising from the pickling process. High strength (over 800 MPa) steel is susceptible to this type of embrittlement.

Prior to galvanizing, the steel is chemically pre-treated and at the final stage, is immersed in a zinc ammonium chloride (ZAC) preflux solution. This process puts a thin film of ZAC crystals on the steel surface. When the steel is immersed in the molten zinc bath, a number of reactions take place. These are: 1. The preflux is activated by the molten zinc and performs a final cleaning action on the steel surface. Oxidation products are produced by this reaction that contain a combination of zinc chloride and zinc oxide. These oxidation products are lighter than zinc and float to the surface. This is called zinc ash. 2. When the steel reaches galvanizing bath temperature, the zinc reacts with the steel to form a series of zinc-iron alloys. Some of these zinc-iron alloy crystals float off the surface of the steel and enter the zinc bath. 3. The zinc bath becomes saturated with iron in solution, either from work entering the bath or from the steel galvanizing kettle. This saturation level is typically about 250 parts of iron per million at galvanizing temperature. 4. Over time, these zinc-iron crystals coalesce and sink to the bottom of the zinc bath. This forms a mushy layer called ‘dross’. The dross is periodically removed by scooping it from the bath with special equipment. Zinc dross is crystalline and has a much higher melting point than zinc (650oC versus Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223

These lateral striations are a result of surface stresses in the steel section cause by the rolling process reacting at a higher rate with the molten zinc than the surrounding steel.


420oC). 5. The molten zinc surface is constantly oxidising, and this oxide film is always present on the zinc surface to a greater or lesser degree. In the process of galvanizing a steel fabrication, the presence of ash and dross may have the following effects on the appearance of the hot dip galvanized coating: • If the item comes in contact with the dross layer in the bottom of the bath, the dross can get caught in or on the work and form a rough, lumpy deposit.

The `white rust’ on these galvanized flat bar sections has been caused by their being wet in the bundle. Galvanized coatings require good ventilation when newly applied to allow the passive oxide films to develop .

• If areas of the item are inaccessible for skimming, ash may stick to the surface of the galvanizing as it is withdrawn from the bath. • If the rate of withdrawal varies or stops during the extraction of the galvanized item from the bath, tide marks (oxide lines) may appear on the surface of the galvanizing causing variations in the shiny appearance of the coating. • With smooth-surfaced work such as tanks or large diameter pipes, dross crystals floating in the bath may be caught up on the galvanized coating. These ‘dross pimples’ give the coating a gritty appearance. STEEL DEFECTS AND THEIR EFFECTS ON GALVANIZED COATINGS Hot dip galvanized coatings, as described above, are not ‘coatings’ in the conventional sense, but alloys that form on the steel to make the galvanized steel item a composite material. As such, galvanized coatings, unlike paint coatings are not very effective at covering up defects in the steel, and are more likely to highlight steel surface quality problems. Steel, like most manufactured products, can be prone to a variety of common defects, most of which are Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223

Dross pimples on this pipe spool arise from dross crystals being stirred up from the bottom of the galvanizing bath by long or deep items. Appearance is unsightly but durability is unaffected.


associated with the casting, rolling and manufacturing processes. The surface area of the steel per unit of area has a significant effect on how the zinc reacts with the steel to form the galvanized coating. Rough surfaces will react more vigorously than smooth surfaces to form thicker galvanized coatings. Hot rolled surfaces with mill scale may have 30 to 50% more surface area per unit than smooth, cold rolled surfaces. Abrasive blasting will artificially increase surface roughness and is a way to deliberately increase coating thickness for heavy duty applications. Galvanized coatings are totally unlike paint films in the way in which they conform to sharp edges or corners. The surface tension effects with paint films tend to thin the paint film out over corners and edges. Hot dip galvanized coatings,, on the other hand, tend to be thicker on edges and corners because the crystals in the alloy layer can flare out and have more room to grow around the corner radius. For this reason, hot dip galvanized coatings may exaggerate the appearance of defects where rolling defects cause sharp ridges on the steel surface.

Hydrogen embrittlement failure of Hercalloy chain is caused by exposing this 1200 MPa steel to pickling acid. These types of steels should not be galvanized.

Small particles or slivers of steel can be rolled into the steel’s surface, and these may be almost invisible to the naked eye. However, when the steel is pickled and the surrounding oxide is removed from these shells and scales, as they are called, the galvanized coating can take on a pimply appearance as the zinc reacts more quickly with these small, loosely attached steel slivers. The manufacturing operations performed on some steel sections can also show up in the form of visual defects, the most common of these being striations. These are narrow (typically about 1-2 mm wide) longitudinal raised lines in the coating usually running the full length of the section. Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223

This miss in the galvanized coating is caused by surface contamination by residual


The most common cause of striations is stress lines on the surface of the steel caused by cold working of the section. Heavy gauge rod will sometimes show a spiral striation pattern arising from the rod or wire being passed through a roller/straightener prior to final manufacture.

oxides not removed in the pickling process.

Welded sections will frequently have much thicker coatings in the weld areas. While this is rarely a problem on structural steel sections, it can cause aesthetic problems on high quality fabrications such as wrought iron and security doors, where the welds have been ground flush and the item is to be painted after galvanizing. This phenomenon is caused by the differing chemistry of the weld metal compared to the parent metal. Most weld metals contain high levels of silicon, and the higher silicon in the weld metal will result in thicker (sometimes 3-4X as thick) galvanized coatings. A reverse of this phenomenon can occur where heavier steel sections (over 12mm) are flame-cut. The edges of flanges of welded beams are a common example The flame cut edges of these heavy plate sections will frequently show a wide variation in coating thickness from zone to zone in the flame-cut area.

Zinc run caused by zinc freezing while draining from flat RHS surface. Durability is enhanced but thick runs may interfere with connecting surfaces.

This is caused by irregular oxidation of some of the constituents in the steel on the surface, giving the flame cut surface a variable metallurgy compared to the parent metal. The zinc coating in these flame cut areas may appear as a series of sharply defined plateaus, with low thickness coatings where significant oxidation has taken place, and normal coating thickness, typically well over the minimum standard required, in less affected areas. EMBRITTLEMENT Embrittlement of steel as a result of the hot dip galvanizing process is rarely encountered with structural grades of steel. However, the use of higher-strength steel grades or inappropriate fabrication techniques can lead to embrittlement problems with galvanized steel. Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223

Flux staining has occurred on unsealed vent holes in overlaps on this fabrication. The stains are caused by


There are three types of embrittlement encountered in the hot dip galvanizing process. These are: 1. Hydrogen embrittlement 2. Strain-age embrittlement 3. Liquid metal embrittlement

iron salts, that have penetrated the overlap during pre-treatment, rehydrating with atmospheric moisture.

Hydrogen embrittlement is most commonly encountered and affects susceptible steel whole yield strength is above 800 MPa. Hydrogen atoms from the acid pickling process penetrate the grain boundaries of these high strength steels and can cause brittle fractures under certain conditions. Many high strength steels can be galvanized and standard pre- or post-galvanizing treatments are available to eliminate the risk of hydrogen embrittlement. Consultation with the galvanizer is essential where fabrications involving the use of high-strength steel are involved. Strain-age embrittlement is caused by excessive cold working of the steel prior to galvanizing. The heat of the galvanizing process accelerates the embrittlement , so susceptible steels present a long-term embrittlement risk, even if they were not galvanized. Localised heating of severely cold worked steels through welding may also precipitate stain-ageing failure. Bending steel through too tight a radius, re-bending already cold worked steel, or hole punching through thick sections are all manufacturing processes that can give rise to strain-age embrittlement problems. Many manufacturing standards for products such as re-bar address this issue within the standard, and fabricators and manufacturers who abide by established practices and use appropriate steel grades rarely experience problems. Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223

The gray coating on these pipes is a result of high silicon content of the steel, making it more reactive and converting all the coating to zinc-iron alloy. These gray coatings are typically thicker than shiny galvanized coatings on similar sections.


Liquid metal embrittlement is specific to certain types of steels that are infrequently encountered in the galvanizing process. The most susceptible are many grades of stainless steel that from time to time, are attached to mild steel assemblies as fittings or components. The molten zinc penetrates the grain boundaries of these steels and fracture under load may result. If the use of stainless steel components is required, mechanical attachments after galvanizing is strongly recommended. DISTORTION Distortion in galvanizing is one of the most common causes for concern, In practice, a small proportion of fabricated items are prone to distortion during the galvanizing process. Some items will always represent a high risk of distortion, and the galvanizer is well aware of the types of steel sections that are prone to distortion.

This miss in the galvanized coating is caused by surface contamination by an incorrect type of marking paint that is resistant to removal by pre-treatment chemicals.

There are three significant causes of distortion with steel sections. These are: 1. The thickness and shape of the section being galvanized. 2. The design of the fabrication with respect to location of welds, symmetry and fabrication stresses present prior to galvanizing. 3.The way the work is handled during the galvanizing process. It is the heat of the galvanizing process and the manner it is transferred and withdrawn from the steel during processing that determines the dimensional stability of an item being galvanized. The work has to be heated from around ambient temperature to 450oC almost instantaneously as it enters the molten zinc. Not all of the item is heated at the same time as it has to be lowered into the bath at a rate that is safe for Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223

The textured surface on this steel angle results from the rough as-rolled surface of the base steel, usually cause by excessive mill-scale on the steel section in the hot rolling process.


the operators and consistent with the ability of the molten zinc to flow over and into the item. At the galvanizing temperature, steel loses 50% of its yield strength, and regains it as it cools down. Much research has been done on the effect of the galvanizing process on steel strength, and this has revealed that there is no measurable loss, and in most cases, there is a slight increase, in yield strength after galvanizing. Thin plate sections are most prone to distortion, and this is caused by the rapid differential heating and cooling of the sections, and its relative weakness as a thin section. Thin steel that has been processed into hollow sections or cold-formed structural shapes is much more stable and is routinely galvanized without problems. Corrugating thin sheet will also largely eliminate the tendency to distort.

Fishbone striations on large diameter pipes arise from surface stresses in the steel causing different zinc-steel reaction rates on the pipe surface.

Fabrications that are asymmetrical, or have significant welding on one side of the section are in a highly stressed state due to the heat effects of the welding processes. Hot dip galvanizing will stress relieve these sections and distortions may result, typically cambering or sweeping of beam sections. Fabrications containing elements of significantly different section thickness are another potential problem area, and high stresses can be generated during the heating and cooling cycle that may cause distortion due to differential heating and colloing.. Platforms with checker plate welded to structural framing are a common problem area that can be avoided by galvanizing the thin and thick sections as separate elements. The galvanizer has the responsibility to handle the work in a way that will reduce the risk of distortion. Locating lifting points located at the optimum points to support the fabrication so that its own unsupported weight does not contribute to it bending or sagging during galvanizing is a galvanizers’ responsibility. Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223

Thin, flat sheet will always buckle in the galvanizing process. Corrugating or ribbing of these type of sections will significantly reduce the likelihood of distortion during galvanizing.


The way the item is immersed in the molten zinc and the post galvanizing quenching process may also have an influence on the dimensional stability of the finished product. Competent galvanizers should be aware of these issues and be prepared to advise the client on design details that can minimize distortion problems. Where distortion is unavoidable, particularly with products such as waste bins and trailers, clients are now prepared to accept the appearance of the thin metal plate sections because of the dramatic increase in service life provided by the coating. The acceptance of unavoidable distortion during hot dip galvanizing will thus always be a trade-off between appearance and performance. Only the client can decide. PICKLING CORROSION Hydrochloric acid is used for pickling steel prior to galvanizing. Some galvanizers add inhibitor to the acid but most operate at acid concentration levels of around 10%.

Where jigs are used to support large items in the galvanizing processes, touch marks will occur where the galvanized coating adheres to the jig at the point of cobntact.

At this level, the acid will efficiently attack and dissolve rust and mill scale but will not react with the base steel at a very high rate. The exception is with high sulfur steel. The main application for these types of steel is for small part manufacture of machined components. The sulfur is added to the steel to weaken its grain structure so that during high speed machining operations, the chips break off in short sections and facilitate the machining operations. These steels are used for non-critical components such as threaded fittings and repetition machined steel non-stressed components.


Weld splatter left on the steel will be galvanized along with the work, resulting in a rough and pimply appearance.


From time to time, threaded fitting such as sockets, spigots, nipples and bushes are welded into fabricated assemblies. Attack by the acid on these threaded components can quickly dissolve the threads and render the item unserviceable. The corrosion of the threaded area may also be caused by zinc attack in the galvanizing bath, but it is largely a function of acid attack during the pickling process. BLOWOUTS AND BLEEDING Molten zinc at galvanizing temperature is only about 30 degrees above its freezing point. The viscosity of the molten metal limits its ability to penetrate small cavities and crevices, and gaps less than 1 mm in width may not allow penetration by the molten zinc.

Delamination of this hot dip galvanized coating has occurred because of excessive coating thickness (500 microns) generated by highly reactive steel

Many fabricated items have overlapping surfaces, and during the pre-treatment process, pretreatment chemicals will penetrate these overlaps and may not drain out because of surface tension holding the liquor in place. When the item enters the galvanizing bath, any moisture trapped in these crevices will rapidly boil and eject steam and pretreatment resides onto the adjacent surface of the work. This contamination interferes with the zinc’s ability to react with the steel and uncoated areas adjacent to these `blowouts’ will result. Another defect arising from unsealed overlapping surfaces is the `bleeding’ which occurs out of the overlap after galvanizing.


This small RHS section has severe galvanized coating delamination arising from very high phosphorous levels in the steel, creating a thick, fragile coating almost 1000 microns in thickness.


This is commonly referred to as `acid staining’. Hydrochloric acid vaporises at below 200 degrees C so there is never any acid residues left in or on the item after galvanizing. The staining that occurs is caused by flux residues that have been desiccated by the heat of the galvanizing process, re-hydrating from contact with atmospheric moisture. The brown liquor bleeding from overlaps is not particularly aggressive to zinc but causes unsightly staining of the work and highlight the lack of a galvanized coating, and a potential for corrosion, in the uncoated overlap area. FLAKING Flaking of galvanized coatings can occur on reactive steels which produce thick alloy layers in the galvanizing process. On thick reactive steel, coating thicknesses in the order of 300-500 microns may be produced. These alloy layers are very hard and inflexible and differential expansion or contraction of the steel can cause areas of the alloy layers to shear off the steel surface. Localised flaking can occur with these types of coatings if there are subject to impact or point pressure from lifting slings or handling equipment. Flaking can be minimised by quick cooling of the work after it exits the galvanizing bath but may be aggravated by slow air-cooling. Modification of standard galvanizing practice in handling reactive steel sections prone to flaking will generally minimise the risk of delamination of heavy coatings after galvanizing, but careful design is required to ensure that the items can be immersed and withdrawn from the molten zinc as quickly as possible. WHITE RUST Zinc is a reactive metal, relying on the development of inert complex carbonate oxide films for its Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223


excellent atmospheric corrosion resistance. When freshly galvanized steel is exposed to pure water (rain, dew, condensation), the zinc will react with the water to form zinc hydroxide, a characteristically bulky white deposit. If the zinc surface remains wet and cannot get access to good air circulation (the main source of carbon dioxide to form the stable oxide films), the water will continue to react with the zinc and can severely damage the coating at worst and result in unsightly staining at best. Most galvanizers post-treat items with a chromate solution integral with the quenching process. This provides a short-term passivating film on the zinc surface that will prevent white rusting in the event of wet weather. The main white rusting problems arise where galvanized work is stacked or packed in tightly nested bundles, where water can be trapped and air circulation is poor. Additional post-treatments that provide more durable protection from white rust over time can be arranged in consultation with the galvanizer , usually at an additional small cost. Exposure to sea water will also rapidly promote white rust, and for this reason, it is recommended that galvanized products such as boat trailers be washed down with fresh water after each salt water immersion. Leaving salt water to dry on the surface will accelerate the consumption of the galvanized coating as the chlorides will strip the stable carbonate film from the surface. PINHOLING On some fabrications, particularly welded beams and heavier sections that have been assembled using submerged arc welding techniques, the weld areas may show evidence of a series of small pinholes Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223


along the weld bead. These pinholes are generally less than 1mm in diameter. The small size of the pinholes, combined with the thickness of the galvanized coating on the weld metal (typically 200 microns or more) means that they have no measurable effect on coating performance. The cause of this phenomenon has been attributed to small particles of sub-arc welding slag powder being fused into the weld surface. These refractory particles are not affected by the pre-treatment processes and remain on the surface throughout the galvanizing process and form small barriers to the galvanized coating’s formation. PUDDLING, DROSS & SPLATTER Puddling caused by poor drainage, dross caught in an item, and splatter from the zinc bath ending up in or on the surface of the galvanized item are processing issues. Some of these, such as removal of splatter, are dealt with the inspection/QA stage. Design issues are the main cause of pudding and can only be eliminated by good design. CONCLUSION With the exception of blowouts and bleeding, and where the galvanized coating is missing, most galvanizing defects have no effect on the coatings’ durability. In fact, many `defects’, because they result in thickening of the coating or deposition of much more zinc or zinc-rich residues, coating life may be increased significantly. For example, a dull or mottled galvanized coating may be 2X or 3X as thick as a standard shiny coating that complies with the relevant standard. The thicker gray coatings will thus have a coating life 2X or 3X as long as the standard coating. Where aesthetic issues are involved, hot dip galvanized coatings can only deliver a level of quality within the limitations of the process. However, an understanding of the issues of design, steel composition and the hot dip galvanizing process will go a long way to ensure that hot dip galvanized Prepared By M.Sreedhar Patnaik Email: [email protected] Phone : 08985219223


coatings on structural fabrications can be produced to a standard that will satisfy architectural applications.



THANQ


Complete Galvanizing Process

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