Potential Adjustment Protection

Normally passive metals when exposed to oxidizing halide environments can suffer from pitting and crevice corrosion. By the application of a direct current the structure potential can be shifted back to the passive region from the transpassive region.

Stainless Steel - Corrosion Resistance: Pitting Corrosion

Pitting Corrosion

Under certain conditions, particularly involving high concentrations of chlorides (such as sodium chloride in sea water), moderately high temperatures and exacerbated by low pH (ie acidic conditions), very localised corrosion can occur leading to perforation of pipes and fittings etc. This is not related to published corrosion data as it is an extremely localised and severe corrosion which can penetrate right through the cross section of the component. Grades high in chromium, and particularly molybdenum and nitrogen, are more resistant to pitting corrosion.

Pitting corrosion mechanism of crude oil tanker's cargo tank bottom plate

Source: http://www.jfe-steel.co.jp/en/release/2008/080129.html

Pitting Corrosion: Mechanism

For a defect-free "perfect" material, pitting corrosion is caused by the ENVIRONMENT (chemistry) that may contain aggressive chemical species such as chloride. Chloride is particularly damaging to the passive film (oxide) so pitting can initiate at oxide breaks.

The environment may also set up a differential aeration cell (a water droplet on the surface of a steel, for example) and pitting can initiate at the anodic site (centre of the water droplet).

Pitting Corrosion (2)

ASTM-G46 has a standard visual chart for rating of pitting corrosion

The shape of pitting corrosion can only be identified through metallography where a pitted sample is cross-sectioned and the shape the size and the depth of penetration can be determined.

Source: http://www.corrosionclinic.com/types_of_corrosion/pitting_corrosion.htm

Pitting Corrosion (1)

Pitting Corrosion is the localized corrosion of a metal surface confined to a point or small area, that takes the form of cavities. Pitting is one of the most damaging forms of corrosion. Pitting factor is the ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss. This following photo show pitting corrosion of SAF2304 duplex stainless steel exposed to 3.5% NaCl solution.