13.5.1 Introduction
Pitting corrosion can be thought of as a form of localised corrosion that occurs on one part of a metal surface at a much higher rate than over the rest of the surface. Pitting corrosion is associated with the breakdown of a surface film and often occurs on completely flat surfaces. If the surface film is cathodic, an area without the film can act as a small anode, thus suffering intense corrosion that leads to pitting.
Pits can also be nucleated at points determined not only by faults in the surface film, but also at sites determined by the underlying metal. Such sites can arise from alloy heterogeneities in the surface, or be associated with grain or phase boundaries. Solid non-metallic inclusions from processing procedures or from impurities, e.g. sulphides in stainless steels, can also provide initiation sites.
Figure description:
- Text: E CSS
- Visuals: Italicized serif font; "E" in purple on a white background; "CSS" in white on a purple background; slanted rectangular/parallelogram frame.
13.5.2 Alloy Susceptibility
13.5.2.1 Alloy Steels
Stainless steels possess excellent resistance to pitting corrosion in oxidising atmospheres. However, in the absence of oxygen the protective oxide film can break down and pitting corrosion can occur very rapidly.
13.5.2.2 Aluminium
In most natural environments, aluminium and its alloys give satisfactory resistance to pitting corrosion. However, at high and low pH values, in the absence of oxygen, or in the presence of film destructive ions, the protecting oxide film can breakdown with a resultant pitting attack. In Al-Zn-Mg alloys this can lead to intergranular corrosion and a significant loss of strength.
13.5.2.3 Titanium
This metal and its alloys are extremely resistant to pitting corrosion.
13.5.3 Prevention of Pitting Corrosion
The incidence of pitting corrosion can be reduced by selection of materials that exhibit a high resistance to this form of corrosion.