Significance of protection potential in pitting, intergranular corrosion and stress-corrosion cracking

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Generally speaking, the solutions existing within living pits and within active stress-corrosion cracks of many passive metals and alloys are locally acid, as a result of hydrolysis phenomena, and, if chloride exists in the corroding solution or in the metal, this acid is hydrochloric acid, which is particularly aggressive. This also occurs in other cases where corrosion proceeds under conditions of restricted diffusion, such as intergranular corrosion and crevice corrosion. In all these cases, a full knowledge of the electrochemical data (of a thermodynamic and kinetic nature) concerning both the behaviour of the active cavities existing in pits or cracks (anodes), which are generally acid, and the behaviour of passive areas outside these cavities (cathodes), which are generally alkaline, helps to aquire a full knowledge of the mechanism of the corrosion; it thus helps to overcome the corrosion. As a general rule, pitting corrosion, stress-corrosion cracking, intergranular corrosion and crevice corrosion may be annihilated by keeping the electrode potential below a given critical value, called the “protection potential”. This protection potential (below which pre-existing pits, cracks and crevices may not grow) should not be mistaken for the “rupture” or “pitting potential” (above which pits may be formed). This protection potential is approximately equal to the potential existing inside acid pits or cracks. Attention is given to the case of copper, iron, stainless steel, high strength steels, titanium and aluminium alloys. Emphasis is given to the usefulness of a long range research programme for the systematic electrochemical study of the corrosion phenomena occurring under conditions of restricted diffusion: pitting corrosion, intergranular corrosion, stress-corrosion cracking and crevice corrosion.