Chapter 11 Corrosion and Degradation of Materials

CHAPTER 11: CORROSION AND DEGRADATION OF MATERIALS

Issue to address…. • What is corrosion and why is it important? • How does corrosion occur? • What are the forms of corrosion? • How can we suppress or prevent corrosion? • How does degradation occur in ceramics, polymers and composites?

1 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

Corrosion of metals 1.0 Introduction 1.1 Definition of Corrosion • Corrosion is the degradation/deterioration of a material, usually a metal, by chemical or electrochemical reaction with its environment. • Deterioration by purely physical causes is not corrosion but is described as erosion or wear. • Corrosion is a spontaneous process. • Free energy is released in the process. • Metals returned to its stable state. Corrosion is extractive metallurgy in reverse. • The driving force for corrosion reaction: chemical energy - energy stored in chemical bonds of substances -internal energy.

Rust is a term for deterioration processes only applied for iron and its alloy


Extractives metallurgy in reverse

+ ΔH - ΔH

Metals extracted from their ores and are in high energy states, thermodynamically, they are not stable. Corrosion is a natural occurring process, predicted by thermodynamics laws. It is this tendency of metals to recombine with elements presents in the environments that leads to the phenomenon known as Corrosion.

Metals corrode because they are not in their natural state 3 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

1.2 The Cost of Corrosion Direct Cost: ~ 4.2% GNP Replacement of corroded components Costs of maintenance/servicing Costs of rebuilding/construction Equipment capital Use of corrosion resistant alloys Use of coatings or inhibitors Electrochemical protection measures Chemical expenses Environmental regulation Safety considerations

Indirect Cost Loss of production during downtime Loss of products due to leakage Loss of efficiency due to corrosion Contamination Loss of human lives due to explosion, fire etc

Mitigation corrosion is necessary to save costs, time and environment 4 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

1.3 Corrosion can be controlled Materials selection Proper design Electrochemical protection Environment control Inhibitors Paints/Coatings

1.4 Significance of Corrosion Control Economics Safety Environmental concerns


2.0 The Corrosion Process Metal corrosion is a chemical reaction between a metal surface and its environment. Metals react with the environment, producing corrosion products similar to the original ore from which the metal was obtained. Corrosion can occur in a gaseous (dry) environment or a damp (wet) environment. Dry Corrosion At room temperature, most metals carry a thin oxide layer as a result of the reaction of metals with oxygen in the atmosphere. Increase of temperature may cause formation of a heavier layer, or the layer may detach. Zinc and zinc coatings carry a fairly protective zinc hydroxide or carbonate layer (zinc patina) which increases in thickness very slowly. Aluminium carries a thin, highly protective oxide layer. Corrosion in a gaseous environment produces a surface layer of converted metal. For example atmospheric corrosion of zinc produces the dull, gray zinc oxide layer seen on galvanised street light poles. Unoxidised zinc coating fresh from the hot dip galvanisers is bright and shiny.


Wet corrosion Wet corrosion takes place in environments where the relative humidity exceeds 60 %. Wet corrosion is an electro-chemical phenomenon. When two metals are in contact with water solution containing salts, an electric potential is formed between two different metals or the surfaces of the same metal with different surface conditions. This causes the dissolution of the less noble metal. The more noble metal remains protected but the less noble metal corrodes. Wet corrosion is most efficient in waters containing salts, such as NaCl (e.g. marine conditions), due to the high conductivity of the solution. Chlorides also may increase the corrosion rate of metals. Corrosion in a wet environment attacks the metal by removing the atoms on the metal surface. The metal atoms at the surface lose electrons and become actively charged ions that leave the metal and enter the ‘wet’ electrolyte. The metal ions join with/to oppositely charged ions from another chemical and form a new, stable compound. Most metal corrosion occurs via electrochemical reactions at the interface between the metal and an electrolyte solution.


2.1 Corrosion Reaction Most metal corrosion occurs via electrochemical reactions at the interface between the metal and an electrolyte solution. A thin film of moisture on a metal surface forms the electrolyte for atmospheric corrosion. Electrochemical corrosion involves two half-cell reactions; an oxidation reaction at the anode and a reduction reaction at the cathode. For iron corroding in water with a near neutral pH, these half cell reactions can be represented as: Fe2+ + 2e-

Anode reaction:

Fe

Cathode reaction:

1/2O2 + H2O + 2e-

Corrosion reaction: Fe + 1/2O2 + H2O

2OHFe(OH)2

There are obviously different anodic and cathodic reactions for different alloys exposed to various environments. These half cell reactions are thought to occur (at least initially) at microscopic anodes and cathodes covering a corroding surface. 8 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

From the above reaction it should be apparent that there are four fundamental components in an electrochemical corrosion cell: • An anode. • A cathode. • A conducting environment for ionic movement (electrolyte). • An electrical connection between the anode and cathode for the flow of electron curr ent.


Electrochemical nature of corrosion


Corrosion Chemistry of Iron Corrosion of iron consists of the formation of hydrated oxide, Fe(OH)3, FeO(OH), or even Fe2O3.H2O. It is an electrochemical process which requires the presence of water, oxygen and an electrolyte. In the absence of any one of these rusting does not occur to any significant extent. In air, a relative humidity of over 50% provides the necessary amount of water and at 80% or above corrosion of bare steel is worse. When a droplet of water containing a little dissolved oxygen falls on an steel pipe, the solid iron or Fe(s) under the droplet oxidizes: Fe(s)

Fe2+(aq) + 2e

The electrons are quickly consumed by hydrogen ions from water (H2O) and dissolved oxygen or O2(aq) at the edge of the droplet to produce water: 2H2O(l) 4e + 4H+ (aq) + O2(aq) More acidic water increases corrosion. If the pH is very low the hydrogen ions will consume the electrons anyway, making hydrogen gas instead of water: 2H+(aq) + 2e-

H2(g) 11

Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

The equations above tell only a small part of the story. Hydrogen ions are being consumed by the process. As the iron corrodes, the pH in the droplet rises. Hydroxide ions (OH-) appear in water as the hydrogen ion concentration falls. They react with the iron(II) ions to produce insoluble iron(II) hydroxides or green rust: Fe2+(aq) + 2OH-(aq)

Fe(OH)2(s)

The iron(II) ions also react with hydrogen ions and oxygen to produce iron(III) ions: 4Fe2+(aq) + 4H+(aq) + O2(aq)

4Fe3+(aq) + 2H2O(l)

The iron(III) ions react with hydroxide ions to produce hydrated iron(III) oxides (also known as iron(III) hydroxides): Fe3+(aq) + 3OH-(aq)

Fe(OH)3(s)

The loose porous rust or Fe(OH)3 can slowly transform into a crystallized form written as Fe2O3.H2O the familiar red-brown stuff that is called "rust". Since these processes involve hydrogen ions or hydroxide ions, they will be affected by changes in pH. With limited O2, magnetite is formed (Fe3O4). 12 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

Corrosion Reaction of Iron Fe + ½ O2 + H2O

Fe(OH)2

O2

Fe(OH)3 • oxidation • hydrolysis

α - FeOOH β - FeOOH δ - FeOOH ∂ - FeOOH

Fe3O4

Fe2O3

Fe2O3.nH2O

Etc.

Corrosion products of Iron Fe2+

O2

• O2 • hydrolysis

• O2 • hydrolysis Fe(OH)2

Fe2+

Fe(OH)3 Fe

Fe(OH)2

FeOOH

Fe(OH)2 Fe(OH)3


Fe3O4

FeOOH 13

2.2 Reaction feasibility

anode : cathode :

free energy change

is negative meaning that reaction is feasible and spontaneous must be positive


2.3 Nernst equation

For hydrogen ion reduction :

For oxygen reduction :


2.4 CORROSION RATE DETERMINATION


CORROSION RATE

Faraday Law :


Conversion to other unit The following charts provide a simple way to convert data between the most common corrosion units in usage, i.e. corrosion current (mA cm-2 ) , mass loss (g m-2 day-1 ) and penetration rates (mm y-1 or mpy) for all metals or for steel. mA cm-2

mm year-1

mpy

1

3.28 M/nd

129 M/nd

8.95 M/n

0.306 nd/M

1

39.4

2.74 d

0.00777 nd/M

0.0254

1

0.0694 d

0.112 n/M

0.365 /d

14.4 /d

mA cm-2 mm year-1 mpy g m-2 day-1

g m-2 day-1

where: mpy = milli-inch per year n = number of electrons freed by the corrosion reaction M = atomic mass d = density Note: you should read the Table from left to right, i.e.: 1 mA cm-2 = (3.28 M/nd) mm y-1 = (129 M/nd) mpy = (8.95 M/n) g m-2 day-1 18 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

For example, if the metal is steel or iron (Fe), n =2, M = 55.85 g and d = 7.88 g cm-3 and the Table of conversion becomes:

mA cm-2

mm year-1

mpy

1

11.6

456

249

mm year-1

0.0863

1

39.4

21.6

mpy

0.00219

0.0254

1

0.547

g m-2 day-1

0.00401

0.0463

1.83

1

mA cm-2

g m-2 day-1

Note: you should read the Table from left to right, i.e.: 1 mA cm-2 = 11.6 mm y-1 = 456 mpy = 249 g m-2 day-1


Example 11.2 Suppose that in a corrosion cell composed of copper and zinc, the current density at the copper cathode is 0.05 A/cm2. The area of both the copper and zinc electrodes is 100 cm2. Calculate (1) the corrosion current, (2) the current density at the zinc anode, and (3) the zinc loss per hour.

Solution 1. The corrosion current is: 2. The current in the cell is the same everywhere. Thus:

3. The atomic mass of zinc is 65.38 g/mol. From Faraday’s equation:


STANDARD EMF SERIES


GALVANIC SERIES


Example 11.1 An iron container 10 cm × 10 cm at its base is filled to a height of 20 cm with a corrosive liquid. A current is produced as a result of an electrolytic cell, and after 4 weeks, the container has decreased in weight by 70 g. Calculate (1) the current and (2) the current density involved in the corrosion of the iron.

Solution 1. The total exposure time is:

From Faraday’s equation, using n = 2 and M = 55.847 g/mol:

2. The total surface area of iron in contact with the corrosive liquid and the current density are:


3.0 Factors Affecting Corrosion Material Properties Chemical composition determines the energy considerations necessary for corrosion to occur

Metallurgical Factors In pure metals, anode areas are grain boundaries or small microheterogeneities. In alloys and metals with impurities: Chemical segregation Presence of multiple phases Inclusions Defects Nonuniform stresses

Surface condition that inhibits electrochemical action between the metal and its environment. Formation of an uniform oxide film Film of adsorbed gas Surface defects


Cont…. Environment Chemical nature: Acids (oxidizing and reducing) Bases Salts (acid, neutral, alkaline) Gases Solvents Operating conditions: Intended service life Temperature Velocity Concentration Impurities Aeration


4.0 TYPES OF CORROSION Corrosion can be divided into different groups according to their form of occurrence.

4.1. General/ Uniform corrosion Uniform attack is a form of electrochemical corrosion that occurs with equal intensity of the entire surface of the metal. Metal corrodes uniformly all over the surface.

Metal

4.2. Localized corrosion Part of the structure corrodes at a considerably higher than average rate. The categories of local corrosion are: Pitting corrosion Crevice corrosion Galvanic corrosion Intergranular corrosion Selective corrosion Filliform corrosion


Pitting corrosion The formation of small pits on the surface of a metal or alloy. The corrosion effect is concentrated on localised areas and leads to pitting. Downward propagation of small pits & holes where passivating layer fails.

Crevice Corrosion Crevice corrosion is a consequence of concentration differences of ions or dissolved gases in an electrolytic solution. proceeds at locations covered by a corrosion product and other deposits (dirt or trash). Crevice corrosion typically occurs in small cavities, gaps, recession, etc


Galvanic Corrosion Galvanic corrosion occurs when two metals having different composition are electrically coupled in the presence of an electrolyte. The more reactive metal will experience severe corrosion while the more noble metal will be quite well protected. • 2 different metals • Electrically connected • In same electrolyte

Fe

Cu

(less noble)

Intergranular Corrosion Corrosion along grain boundaries, often where special phases exist.

precipitation of chromium carbide makes the low Cr austenite in the grain boundaries anodic


Selective corrosion When one element or constituent of a metal is selectively corroded out of a material it is referred to as selective leaching. The most common example is the dezincification of brass. After leaching has occurred, the mechanical properties of the metal are obviously impaired and some metal will begin to crack.

Filiform corrosion Surface corrosion where the paint film remains intact but adhesion to the substrate is destroyed causing trails or underfilm corrosion to form, like worm tracks.


4.3. Combined effect of mechanical factors and corrosion Mechanical wearing as well as static or dynamic stresses often act in combination with corrosion. The main categories for combined effect of mechanical factors and corrosion are: Stress corrosion Corrosion fatigue Erosion corrosion Fretting corrosion

Stress Corrosion Stress corrosion can result from the combination of an applied tensile stress and a corrosive environment. In fact, some materials only become susceptible to corrosion in a given environment once a tensile stress is applied. stress corrosion occurs when a metal in a corrosive environment is exposed to static stress that results in fracture


Corrosion fatigue Corrosion fatigue is caused by the combined effect of corrosive environment and varying state of stress. The fatigue fracture is brittle and the cracks are most often transgranular, as in stress-corrosion cracking, but not branched

Fretting corrosion Caused by the rubbing of two contacting surfaces under load, which leads to a steady erosion of material.


Erosion-Corrosion Erosion-corrosion arises from a combination of chemical attack and the physical abrasion as a consequence of the fluid motion.

Cavitation corrosion Cavitation corrosion is erosion caused by the combined effect of corrosion and the pressure caused by the breaking of gas bubbles formed in liquid (cavitation).

Micro-biological corrosion is caused by an accumulation of micro- organisms (eg. bacteria) which are found in contaminated fluids and is normally confined to the internal surfaces of integral fuel tanks.


5.0 CORROSION PROTECTION Controlling Corrosion Corrosion control involves hindering the natural chemical reactions that occur between the metal and its environment. The common methods used are to: Materials selection (modify the properties of a metal) Modify the environment Install a protective coating over the metals Impose an electric current to supply electrons Proper design


5.1 Materials selection Corrosion of metals in service environment are dependent on : • the types of metal • chemical composition of the metal. A. Marine environment Fe/ Fe alloys Cu Cu –Zn alloy [ 70% Cu – 30% Zn] Titanium

poor/unsuitable good better much better

B. High pressure gas Stainless steel(SS] [18% Cr, 8% Ni, Fe] [18% Cr, 8% Ni, 3% Mo, Fe]

good better

C. Damp environment Fe, Zn Aloi Fe-Zn Fe-Ni aloy

unsuitable good much better


D. Modify the Properties of the Metal Alloying the metal to produce a more corrosion resistant alloy, e.g. stainless steel, in which ordinary steel is alloyed with chromium and nickel. Stainless steel is protected by an invisibly thin, naturally formed film of chromium sesquioxide Cr2O3. From the galvanic series we can see that the more noble metals are less likely to corrode. When these metals are metallurgically combined with those from lower in the series, the resulting alloy takes on corrosion resistant properties. The resistance can come from the development of a protective oxide film on the outside surface or because the new alloy has a different voltage potential which acts to make it behave more noble. PASSIVITY: ALLOYING EFFECTS Passive oxide film can inhibit aqueous corrosion as well! E.g. Stainless steel • Alloying Fe with >12% Cr results in formation of protective Cr-rich passive film, large increase in corrosion resistance • 304 SS: 18% Cr + 8% Ni • 316 SS: further addition to 18-8 SS of about 2% Mo, increases corrosion resistance yet more 35 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

5. 2 Modify the environment Controlling the environment Use of inhibitors A. Controlling the environment • Controlling the relative humidity – %RH < 50% most metal such as Al, Cr and Ni will less corrode. • Removal of oxygen - By the removal of oxygen from water systems in the pH range 6.5-8.5 one of the components required for corrosion would be absent. The removal of oxygen could be achieved by the use of strong reducing agents e.g. sulphite and hydrazine N2H4 + O2

N2 + H2O

• Controlling pH, pressure, ionic contaminants and etc. Iron would be corroded at pH > 13.00. The presence of 5 ppm Cl-1 may induced corrosion in many metals.


B. Use of inhibitor A corrosion inhibitor may be defined, in general terms as a substance which, when added in a small concentration to an environment, effectively reduces the corrosion rate of a metal exposed to that environment. Corrosion inhibitors may combine with the corroding metal (anode) or the protected metal (cathode) to form a barrier layer that reduces the flow of ions and electrons across it to very low values and virtually stops the corrosion. Some of the mechanisms of its effect are formation of a passivation layer (a thin film on the surface of the material that stops access of the corrosive substance to the metal), inhibiting either the oxidation or reduction part of the redox corrosion system (anodic and cathodic inhibitors), or scavenging the dissolved oxygen. Anodic inhibitor - forms a passivation layer on aluminium and steel surfaces which prevents the oxidation of the metal. e.g. Chromate, nitrit , borate , molybdates, etc.

2Fe + NaNO2 + 2H2O

γ-Fe2O3 + NH3 + NaOH

Cathodic inhibitor - retards the corrosion by inhibiting the reduction of water to hydrogen gas (inhibit the reduction processes). e.g.zinc salts, Magnesium salts, amine etc. Mixed inhibitor - An inhibitor that acts both in a cathodic and anodic manner.e.g. chromate/ polyphosphate 37 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

The role of corrosion inhibitors • React with corrosive agents such as Na2SO3 dan N2H4 • forms a passivation layer • forms of adsorption film /protection layer on the metal surfaces


5.3 Protective coating To isolate the metals from the corrosive environments • coating/covering with more noble metal or better corrosion resistant proprties • non-metallic coating (inorganic and organic coatings) A. Metallic coating • coating with more noble metals eg. Iron coated with Cr, Ni and Sn for decoration purposes Coating of Zn in galvanized steel Coating of Sn on iron for containers


B. Non metallic coatings Non-metallic coatings put over a metal can be of two types. They can act as a physical barrier and bar access to the metal surface or they can introduce a very high resistance into the corrosion cell circuit and drastically reduce the flow of electrons. The barrier type coatings protect the metal as long as there are no cracks. If a crack occurs corrosion becomes intense at the metal surface. Resistance type coatings include additives that breakdown in the presence of water and oxygen into inhibiting agents. • Inorganic coatings - concrete, silica and ceramics • Organic coatings - tar, paint, plastics and etc. Paint coatings are considered the most practical and economical means for the corrosion protection. There are several mechanisms by which paint coating systems provide corrosion protection: Barrier: limiting the access of chemical species involved in electrochemical corrosion reactions, notably oxygen in the cathodic reaction. Barrier: maintaining a high electrical resistance at the substrate interface, restricting the access of ionic species. Inhibition: corrosion inhibitors in the primer, passivating the underlying metal surface. Cathodic protection: Sacrificial protection of the underlying substrate, a galvanic effect (for example, steel protection by zinc). 40 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

Binder (resin – PV, PU, epoxy, alkyd etc.) PAINT

Solvent (organic , water) Pigment/colouring Additives (anti-corrosive, anti-fungal etc.) Decorative / top-coat Primer / anti-corrosive paint

Metal / alloy

Metal

top coats intermediate Anti-rust / anti-corrosive Wash primer

Wash primer : adhesion Anti-corrosive paint : corrosion protection Intermediate : thicken the paint layer Decorative : decoration and barrier to the environment. For immersed structure in seawater must be replaced by anti-fouling paint which prevent the attachment of marine fouling such as barnacles, algae and etc. 41 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

5. 4 Impose an Electric Current A. Cathodic Protection B. Anodic Protection A. Basic Theory of Cathodic Prevention Corrosion occurs at areas on a metal surface where current leaves the metal and passes into the adjacent electrolyte. Such current discharge areas are termed "anodic", where anodic chemical reactions, Fe = Fe++ + 2e proceed, and iron is oxidised to an ionic state. To prevent such corrosion occurring we need to reverse the direction of electric current flow, such that the above reaction cannot proceed and the iron remains in its metallic state. Such a reversal of current flow at anodic sites can be achieved by superimposing a reverse flow to overpower any corrosion current. When there is a net current flow on to all points on the metal surface, the entire surface is cathodic and protection from corrosion is complete. This technique of corrosion protection is termed "cathodic protection". Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface become cathodic to the environment or the cathode of the corrosion cell. 42 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

Cathodic protection can be achieved in two ways: • By connecting a more anodic metal into the corrosion circuit than the metal to be protected. (galvanic protection or sacrificial anodes) • By passing of an electrical dc current so that all areas of the metal surface become cathodic and therefore do not corrode The principle of cathodic protection is in connecting an external anode to the metal to be protected and the passing of an electrical dc current so that all areas of the metal surface become cathodic and therefore do not corrode. The external anode may be a galvanic anode, where the current is a result of the potential difference between the two metals, or it may be an impressed current anode, where the current is impressed from an external dc power source.

Galvanic protection or Sacrificial anodes Galvanic anode systems employ reactive metals as auxiliary anodes that are directly electrically connected to the steel to be protected. The difference in natural potentials between the anode and the steel, as indicated by their relative positions in the electrochemical series, causes a positive current to flow in the electrolyte, from the anode to the steel. Thus, the whole surface of the steel becomes more negatively charged and becomes the cathode. The metals commonly used, as sacrificial anodes are aluminium, zinc and magnesium. 43 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

GALVANIC PROTECTION


Galvanic protection / sacrificial anode


Impressed-current systems For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection. Impressed Current Cathodic Protection (ICCP) systems use anodes connected to a DC power source (a cathodic protection rectifier). Impressed-current systems employ inert (zero or low dissolution) anodes and use an external source of dc power (rectified ac) to impress a current from an external anode onto the cathode surface. Anodes for ICCP systems are tubular and solid rod shapes or continuous ribbons of various specialized materials. These include high silicon cast iron, graphite, mixed metal oxide, platinum and niobium coated wire and others.


B. ANODIC PROTECTION In circumstances where cathodic protection is not practical, such as in strongly alkaline or acidic environments, anodic protection is a useful corrosion control technique. Specifically, in metal-environment conditions where active-passive behaviour is demonstrated, anodic protection is usually effective. In practise, the metal-environment potential is held in the passive region by polarizing the structure in the electropositive direction (where the passive layer is most stable). Historically, anodic protection has the widest application in the process industries and in particular on mild or stainless steel equipment use for concentrated sulphuric acid storage. Equipment, such as pulp and paper mill digesters and recausticizing (white, green & black) liquor clarifiers and storage tanks have also been effectively protected. Here the metal potential is shifted to the passive zone from the active region by the application of a direct current.


Cathodic Protection Advantages:

Limitation:

can be used for all metals installation cost is low corrosion rate can be reduced to zero can only be used in weak to moderate environment operating conditions must be determined by empirical testing

Anodic Protection Advantages: can be used in extremely aggressive environment applied current is direct measure of corrosion rate operating conditions can be easily controlled operation cost low Limitation

only applicable to active-passive metals installation cost is high 48


5.5 Proper Design Design awareness and the life-cycle Good control of corrosion requires the awareness and co-operation of the entire design team, including engineers and designers not only in each specialised discipline but in project management and cost control. Adequate means for collecting, reporting and recording corrosion information from operational situations must also be planned.


MINIMIZING OR PREVENTING CORROSION • Use more corrosion-resistant material, e.g. metal that passivated • Remove aggressive species from environment • Add chemical to environment that inhibits corrosion • Separate the metal from the environment with a barrier layer, e.g. paint • Remove crevices from design • Prevent galvanic corrosion by using compatible materials or electrically insulating dissimilar materials • Reduce T (slow kinetics of oxidation-reduction reactions) • Use cathodic / sacrificial protection or anodic protection


6.0 Deterioration of Ceramics • Generally thought to be immune from most corrosion and deterioration mechanisms • Not totally true although they are as good as anything in resisting environmental changes in the material with aging • Corrosion of fillers, impurities within a ceramic • If corrosion and loss, flaw initiation points from the formed void • The primary means by which ceramics and glass objects deteriorate is through accidental cracking and breaking due to improper handling, shipping, storage, or display. • Other sources of deterioration for ceramics and glass can include deterioration of the clay body or the glass as result of poor manufacturing methods or materials. • Porous ceramics can also deteriorate due to the presence of. soluble salts deep within the ceramic body itself The salts dissolve and re-crystallize as the relative humidity fluctuates. When the salts re-crystallize they expand in size and crush the surrounding ceramic structures. For example, a flowerpot that has become saturated with fertilizer salts over time may exhibit this effect. • Leaving liquids inside vessels for long periods of time can damage glass. Some constituents of the glass dissolve into the liquid, making the interior of the vessel appear cloudy or to have residue inside


7.0 Deterioration of Polymers • Things that make polymers more brittle: UV Aging (crosslinking) High Temperatures (crosslinking and volatilization of plasticizers) Oxidation environments: bond scission due to oxygen attack

• Things that make polymers softer: solvent absorption ( Armor All) water absorption and attack (hydrolysis) enhanced by acids and bases

• Degradation can change MW of the polymer • Consider Primary Bond Fracture vs Secondary Bond Fracture • Other factors: Creep and Dimensional Stability Orientation relaxation (shrinkage

Remember the link between molecular architecture and environmental sensitivity (heterochain backbones are more susceptible) 52 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

Deterioration of Polymers - Chemical Effects • Linear polymers can be attacked through: • Random scission - breaking of random molecular bonds • Depolymerization - shortening of average molecule length • Both act to reduce the average molecular weight

• Cross-linking can either be induced or damaged by chemical attack Induced cross-linking occurs in low density polyethylene makes stiffer material. Damage of cross-links in polyisoprene rubber results in a brittle material

• Change in bond structure can result in different compound with very different properties. e.g: Polyvinyl acetate + water --> water soluble polyvinyl alcohol + acetic acid


Deterioration of Polymers - Sterilization Effects Dry sterilization - occurs at temperatures between 160 and 190°C Affects microscopic and geometric structure of material Oxidation can also occur for some polymers (ex. nylon) during dry sterilization, even though it is below the melting temperature Only teflon and silicon rubber should be dry sterilized Steam sterilization (autoclaving): Occurs at high pressure under relatively low temperatures (120 - 135°C) If a polymer is susceptible to attack by water, the water vapor will cause degradation during sterilization Polyvinyl chloride, polyacetals, low density polyethylenes, and polyamides should not be steam sterilized Chemical sterilization: At low temperatures, uses Ethylene or propylene oxide gas, phenolic or hypochloride solution Chemical agents can sometimes cause deterioration even at room temperature Due to overnight time required (relatively short exposure to chemicals), can be used for most polymers Radiation sterilization: Uses 60Co isotope Can cause deterioration of high polymers at high doses Polymer chains can be broken and recombined Polyethylene becomes hard and brittle 54 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

Deterioration of Composites The environment can affect each component of a composite material separately If the properties of a single component are degraded, the properties of the whole composite will also be affected The bond between the matrix and the reinforcing elements may be subject to attack in addition to the materials themselves


SUMMARY Corrosion occurs due to: • the natural tendency of metals to give up electrons. • electrons are given up by an oxidation reaction. • these electrons then are part of a reduction reaction. • Metals with a more negative Standard Electrode Potential are more likely to corrode relative to other metals. • The Galvanic Series ranks the reactivity of metals in seawater. • Increasing T speeds up oxidation/reduction reactions. • Corrosion may be controlled by: • using metals which form a protective oxide layer • reducing T • adding inhibitors • painting • using cathodic protections. 56 Dr. Mohd. Hazwan Hussin, Materials Chemistry (KIT 257) , [email protected]

Chapter 11 Corrosion and Degradation of Materials

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