Page 375 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION DAMAGE, DEFECTS, AND FAILURES 353

(a) (b)

(c) (d)

Figure 5.35 (a) Pitting pattern with several pits surrounding a large pit. (b) Pinhole size pit
inside the tube. (c) Close-up of pit. (d) Close-up of inside of pit. (Figure originally published
in Reference 26. Reproduced with permission of the Canadian Institute of Mining, Metallurgy
and Petroleum. www.cim.org.)

Neutral solutions (pH ∼7.0) (anodic):

−
Cu–2e + H O → Cu(OH) + 2H +
2
2
−
Ni–2e + H O → Ni(OH) + 2H +
2
2
Cathodic reactions:
−
1 ∕ 2O + H O + 2e → 2OH −
2
2
−
Cu 2+ + 2e → Cu (redeposit)
Low pH reactions lead to dissolution of Cu and Ni. At the beginning of pit initiation
copper and nickel hydrolyze leading to low pH.
Cathodic reaction involving oxygen produces a hydroxyl ion.