Chapter 17: Corrosion and Degradation of Materials ISSUES TO ADDRESS... • How does corrosion occur? • Which metals are most likely to corrode? • What environmental parameters affect corrosion rate? • How do we prevent or control corrosion?
Chapter 17 - 1
THE COST OF CORROSION • Corrosion: -- the destructive electrochemical attack of a material. -- Ex: Al Capone's ship, Sapona, off the coast of Bimini. Photos courtesy L.M. Maestas, Sandia National Labs. Used with permission.
• Cost: -- 4 to 5% of the Gross National Product (GNP)* -- in the U.S. this amounts to just over $400 billion/yr** * H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc., 1985. **Economic Report of the President (1998). Chapter 17 - 2
ELECTROCHEMICAL CORROSION Ex: consider the corrosion of zinc in an acid solution • Two reactions are necessary: Zn Zn2 2e -- oxidation reaction: -- reduction reaction:
2H 2e H2 (gas)
H+ Oxidation reaction Zn Zn2+ H+
Zinc
flow of e 2ein the metal
H+
H+ + H H+ H2(gas) H+ reduction reaction
Acid solution
Adapted from Fig. 17.1, Callister & Rethwisch 8e. (Fig. 17.1 is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGrawHill Book Company, 1986.)
• Other reduction reactions in solutions with dissolved oxygen: -- acidic solution O2 4H 4e 2H2O
-- neutral or basic solution O2 2H2O 4e 4(OH) Chapter 17 - 3
STANDARD HYDROGEN ELECTRODE • Two outcomes: -- Electrodeposition
H2(gas) Mn+ H+ ions H+
e-
25ºC
e-
ne -
2e -
metal, M
metal, M
ne -
e-
Platinum
e-
Mn+ ions
H+ 2e H+
H2(gas)
Platinum
-- Corrosion
25ºC
1M Mn+ sol’n 1M H + sol’n
1M Mn+ sol’n 1M H+ sol’n
-- Metal is the anode (-)
-- Metal is the cathode (+)
o Vmetal 0 (relative to Pt)
o Vmetal 0 (relative to Pt)
Standard Electrode Potential
Adapted from Fig. 17.2, Callister & Rethwisch 8e. Chapter 17 - 4
STANDARD EMF SERIES
more anodic
more cathodic
• EMF series metal Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K
o Vmetal
• Metal with smaller o Vmetal corrodes.
+1.420 V • Ex: Cd-Ni cell +0.340 o o V < V Cd corrodes Ni Cd - 0.126 - 0.136 + - 0.250 DV o = - 0.277 0.153V - 0.403 - 0.440 Cd Ni 25ºC - 0.744 - 0.763 - 1.662 1.0 M 1.0 M - 2.363 Cd 2+ solution Ni 2+ solution - 2.714 Adapted from Fig. 17.2, Data based on Table 17.1, Callister & Rethwisch 8e. - 2.924 Callister 8e. Chapter 17 - 5
CORROSION IN A GRAPEFRUIT Cu (cathode)
Zn (anode)
+
H+
Zn 2+
2e -
reduction reactions
2H 2e H2 (gas) O2 4H 4e 2H2O
H+
-
H+ H+
oxidation reaction
H+ Acid H+
Zn Zn2+ 2e
H+
Chapter 17 - 6
EFFECT OF SOLUTION CONCENTRATION AND TEMPERATURE • Ex: Cd-Ni cell with standard 1 M solutions
V V 0.153 V o Ni
o Cd
-
Cd
+
25ºC
Ni
1.0 M 1.0 M Cd 2+ solution Ni 2+ solution
• Ex: Cd-Ni cell with non-standard solutions
VNi VCd
-
Cd
RT X V V ln nF Y o Ni
o Cd
+
T
Ni
XM YM Cd 2+ solution Ni 2+ solution
• Reduce VNi - VCd by -- increasing X -- decreasing Y -- increasing T
n = #eper unit oxid/red reaction (= 2 here) F= Faraday's constant = 96,500 C/mol.
Chapter 17 - 7
GALVANIC SERIES
more anodic (active)
more cathodic (inert)
• Ranking of the reactivity of metals/alloys in seawater Platinum Gold Graphite Titanium Silver 316 Stainless Steel (passive) Nickel (passive) Copper Nickel (active) Tin Lead 316 Stainless Steel (active) Iron/Steel Aluminum Alloys Cadmium Zinc Magnesium
Based on Table 17.2, Callister & Rethwisch 8e. (Source of Table 17.2 is M.G. Fontana, Corrosion Engineering, 3rd ed., McGrawHill Book Company, 1986.)
Chapter 17 - 8
FORMS OF CORROSION • Stress corrosion Corrosion at crack tips • Uniform Attack when a tensile stress • Erosion-corrosion Oxidation & reduction Combined chemical attack and is present. reactions occur uniformly mechanical wear (e.g., pipe over surfaces. elbows).
• Selective Leaching
• Pitting
Preferred corrosion of one element/constituent [e.g., Zn from brass (Cu-Zn)].
• Intergranular Corrosion along grain boundaries, often where precip. particles form. g.b. prec. attacked zones Fig. 17.18, Callister & Rethwisch 8e.
Forms of corrosion
• Galvanic
Downward propagation of small pits and holes. Fig. 17.17, Callister & Rethwisch 8e. (Fig. 17.17 from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)
• Crevice Narrow and
Dissimilar metals are confined spaces. Rivet holes physically joined in the presence of an electrolyte. The Fig. 17.15, Callister & Rethwisch 8e. (Fig. 17.15 more anodic metal is courtesy LaQue Center for Corrosion Technology, Inc.) corrodes. Chapter 17 - 9
CORROSION PREVENTION (i) • Materials Selection -- Use metals that are relatively unreactive in the corrosion environment -- e.g., Ni in basic solutions -- Use metals that passivate - These metals form a thin, adhering oxide layer that slows corrosion.
Metal oxide Metal (e.g., Al, stainless steel)
• Lower the temperature (reduces rates of oxidation and reduction) • Apply physical barriers -- e.g., films and coatings
Chapter 17 - 10
CORROSION PREVENTION (ii) • Add inhibitors (substances added to solution that decrease its reactivity) -- Slow oxidation/reduction reactions by removing reactants (e.g., remove O2 gas by reacting it w/an inhibitor). -- Slow oxidation reaction by attaching species to the surface.
• Cathodic (or sacrificial) protection -- Attach a more anodic material to the one to be protected. Galvanized Steel Adapted from Fig. 17.23, Callister & Rethwisch 8e.
Using a sacrificial anode
Zn 2+
zinc
zinc 2e - 2e steel
e.g., zinc-coated nail
steel pipe
e-
Cu wire Mg Mg 2+ anode Earth
e.g., Mg Anode
Adapted from Fig. 17.22(a), Callister & Rethwisch 8e.
Chapter 17 - 11
SUMMARY • Metallic corrosion involves electrochemical reactions -- electrons are given up by metals in an oxidation reaction -- these electrons are consumed in a reduction reaction
• Metals and alloys are ranked according to their corrosiveness in standard emf and galvanic series. • Temperature and solution composition affect corrosion rates. • Forms of corrosion are classified according to mechanism • Corrosion may be prevented or controlled by: -- materials selection -- reducing the temperature -- applying physical barriers -- adding inhibitors -- cathodic protection Chapter 17 - 12
ANNOUNCEMENTS Reading:
Core Problems:
Self-help Problems:
Chapter 17 - 13
