It is convenient to classify the corrosion of metals in terms of (a) the metals and (b) the environment.
The reduction potential is the most important characteristic of a metal that determines its susceptibility to corrosion. This has been illustrated byTable 9.4. Thus, the noble metals, gold and platinum, are resistant to corrosion and will only dissolve in strong oxidizing solutions which also contain complexing halides or other ions, for example, (CN–). For metals in seawater, the relative order of the reduction potential of metals and alloys has been established. This is illustrated in Table10.1where distinction is made between active and passive surfaces for some metals. Magne- sium is a most active metal, whereas platinum and graphite are the least active materials. The voltages are given with respect to the saturated calomel electrode (SCE).1 The oxidation reaction (10.1) represents corrosion which must be accompanied by a reduction reaction (10.2), (10.3), or (10.4) as well as reactions such as
Fe3þþe! Fe2þ (10.5)
and
3Hþþ NO3þ2e! HNO2þH2O (10.6)
The reaction which occurs depends on the solution in which the metal corrodes, but in most cases the cathodic reaction involves O2.
The corrosion rate will thus depend on the partial pressure of oxygen. This is shown in Table10.2.
Hence, the removal of oxygen from water in steam boilers is one method of reducing corrosion.
If hydrogen evolution is the cathodic reaction (10.4), then it can be reduced by increasing the overvoltage. The overvoltage of H2on mercury is very high (seeTable 9.2), and reaction (10.4) can be inhibited if mercury is used to coat the metal surface and to form an amalgam (see the zinc–air cell, Sect.9.6). The overvoltage is dependent on current density which is determined by the area of the metals. Hence, as the cathode area decreases, the polarization can be expected to increase resulting in a decrease in rate of corrosion. In the case of iron (anode) on a large copper sheet (cathode), the large cathode/anode ratio favors corrosion of the iron. This is shown in Fig.10.1.
1The saturated calomel electrode is a convenient reference electrode often used instead of the standard hydrogen electrode: 12Hg2Cl2þe! Hg þ Cl,ℰ¼0.2224 (25C).
The type and amount of impurities in a metal will affect the rate of corrosion. For example, a zinc sample which is 99.99 % pure (referred to as 4n zinc) would corrode about 2,000 times faster than a 5n sample. Even improperly annealed metals will show excessive corrosion rates.
Another factor which controls the rate of corrosion is the relative volume of the corrosion product (oxide) to the metal as well as the porosity of the oxide layer. For example, the volume ratio of oxide/
metal for Al, Ni, Cr, and W is 1.24, 1.6, 2.0, and 3.6, respectively. The oxide layer on a metal can Table 10.1 Galvanic metal
and alloy potential V (vs.
SCE) in seawater
V (V)
Mg 1.60.02
Zn 1.000.02
Be 0.990.01
Al alloys 0.890.11
Cd 0.710.01
Mild steel 0.650.05
Cast iron 0.610.05
Low alloy steel 0.600.02
Austenite Ni 0.500.03
Bronze 0.360.05
Brass 0.350.04
Cu 0.340.04
Sn 0.320.03
Solder Pb–Sn 0.310.03
Al brass 0.310.03
Manganese bronze 0.310.02
410,416 stainless steel 0.310.03
Active potential 0.510.04
Silicon bronze 0.290.02
Tin bronze 0.290.03
Nickel silver 0.280.02
Cu/Ni 90/10 0.260.04
Cu/Ni 80/20 0.260.04
430 stainless steel 0.240.04
Active potential 0.520.06
Pb 0.230.03
Cu/Ni, 70130 0.210.02
Ni/Al bronze 0.200.05
Ni/Co 600 alloy 0.170.02
Active potential 0.410.06
Ag bronze alloys 0.150.05
Ni 200 0.150.05
Ag 0.130.03
302,304,321,347, SS 0.080.02
Active 0.510.05
Alloy 2C, stainless steel 0.000.06
Ni/Fe/Cr/Alloy 825 0.080.04
Ni/Cr/Mo/Cu/Si alloy 0.070.03
Ta 0.090.06
Ni/Cr/Mo alloy C 0.070.07
Pt 0.130.10
Graphite 0.140.16
10.2 Factors Affecting the Rate of Corrosion 177
convert a metal from one that corrodes to one that is inert. Aluminum can react with water to form hydrogen by the reaction
2A1 þ6H2O ! 2Al OHð Þ3þ3H2 (10.7)
followed by
2Al OHð Þ3! Al2O3þ3H2O (10.8)
However, the oxide layer which forms prevents the water from contacting the aluminum surface.
Only in acid or alkali is the Al2O3solubilized, and the aluminum reacts to liberate hydrogen.
An oxide layer is readily formed on many metals when they are made anodic in aqueous solutions.
In the case of aluminum, this process is called anodization. It is also referred to as a passive film which reduces the corrosion rate. Such passive films can be thin, from 0.01mm, and fragile and easily broken. Thus, when steel is immersed in nitric acid or chromic acid and then washed, the steel does not immediately tarnish nor will it displace copper from aqueous CuSO4. The steel has become passive due to the formation of an adhering oxide film which can be readily destroyed by HCl which forms the strong acid H+FeCl4.
The factors influencing the rusting of iron can be illustrated by the electrochemical treatment of the overall reaction.
2Fe þO2þ4Hþ! 2Fe2þþ2H2O Fe ! Fe2þþ 2e E ¼0:440 V O2þ4Hþþ4e!2H2O E ¼1:67 V Table 10.2 Effect of O2
pressure on corrosion of iron in seawater
P(O2) (atm) Rate of corrosion (mm/year)
0.2 2.2
1 9.3
10 86.4
61 300
Fig. 10.1 The corrosion of an iron rivet in a copper plate. The large copper surface results in a low O2overvoltage, allowing the corrosion to proceed at a rate controlled by O2diffusion
From the Nernst equation (9.12)
Ecell¼ E0cell ðRT 4F= Þln ½Fe2þ2
Po2½Hþ4 (10.9)
The corrosion reaction (10.9) ceases whenℰcell0
0¼1:67 ð0:0591 4= Þln ½Fe2þ2
Po2½Hþ4 (10.10)
Hence,ℰcell 0 when
log ½Fe2þ2
Po2½Hþ4113 (10.11)
Let us consider extreme conditions where Po2¼106atm; Fe2þ
¼10 M; ½Hþ ¼1014 M log 102=ð1061056Þ ¼log 1064¼64
Since 64<113, corrosion will continue to occur. In strong NaOH solution, rusting is reduced because the Fe2O3forms a protective layer over the metal.