8
Fundamentals of Corrosion
2.2 Types of Local Cell Formations
Three main types of local cell formations leading to corrosion are encountered in practice:
1. Dissimilar electrode cells
2. Concentration cells:
a. Salt concentration cell
b. Differential aeration cell
3. Differential temperature cells
TabLE 2.1
Standard Oxidation–Reduction Potentials
25°C, Volts vs. Hydrogen Electrode
Redox Reaction
Oxidation–
Reduction Potential
Au = Au 3+ + 3e −
+1.498
O 2 + 4H + + 4e − = 2H 2 O
+1.229
Pt = Pt 2+ + 2e −
+1.2
Pd = Pd 2+ + 2e −
+0.987
Ag = Ag + = e −
+0.799
2Hg = Hg + 2e
2
2+
—
+0.788
Fe 3+ + e − = Fe 2+
+0.771
O 2 + 2H 2 O + 4e – = 4OH −
+0.401
Cu = Cu 2+ + 2e –
+0.337
Sn 4+ + 2e − = Sn 2+
+0.15
2H + + 2e − = H 2
0.0000
Pb = Pb 2+ + 2e −
−0.126
Sn = Sn 2+ + 2e −
−0.136
Ni = Ni 2+ + 2e −
−0.250
Co = Co 2+ + 2e −
−0.277
Cd = Cd 2+ + 2e −
−0.403
Fe = Fe 2+ + 2e −
−0.440
Cr = Cr 3+ + 3e −
−0.744
Zn = Zn 2+ = 2e −
−0.763
Al = Al 3+ + 3e −
−1.662
Mg = Mg 2+ + 2e −
−2.363
Na = Na + + e −
−2.714
Κ = K + + e −
−2.925