Mechanisms

Severe local attack such as pitting, (see figure 1) and crevice corrosion can be a particular problem in stainless steels and other alloys which depend on the presence of self healing (when oxygen is available), adherent and relatively defect-free oxide films for their resistance to corrosion.

Figure 1. Pitting attack (a) possible defect site and (b) growth from defects.
Pitting

Pitting results from local breakdown of the barrier film which allows an anodic reaction to begin at point sites on the exposed metal. The cause of the point site may be local mechanical damage or chemical breakdown of the surface film, oxygen depletion beneath debris particles on the surface or chemical (galvanic effect) differences between second-phase particles or inclusions and the metal matrix.

Pitting is quite often self-accelerating due to local rises in acidity in the pits and is usually associated with the presence of certain ions, for example chloride and sulphide, in the corrosive environment. Such ions contribute to film breakdown and prevent film repair.
Stainless Steel Design

In stainless steels a minimum of 12% chromium is required in solid solution in the matrix to provide passivation by protective oxide film formation. Nickel additions (normally 6-10%), molybdenum and nitrogen are also used to improve general corrosion and pitting resistance. They also help to control matrix structure, for example to produce austenitic or duplex grades. In these steels, carbon content is kept as low as possible during alloy production. Subsequent processing, welding and heat treatment variables are carefully controlled to avoid the formation of Cr-rich carbide precipitates and other damaging second phases which can not only reduce toughness but also cause severe pitting and intergranular attack due to micro-galvanic effects.

A row of pits can form along deep scratches as the oxide film will not be impervious and the underlying metal will contain additional internal stress. Larger pits can form at dross and sand inclusions, highlighting the damaging effect of inadequate casting cleanliness. Casting defects such as shrinkage pores and inclusions can also result in severe local attack.

To minimise pitting, stainless steels are solution treated and quenched to dissolve second phase precipitates and to prevent their reformation on cooling. Due to the presence of chloride ions, conventional austenitic and duplex grades are prone to both pitting and crevice corrosion in seawaters but their resistance can be improved by additions of up to 6% Mo and 0.2-0.5% N.
Predicting Pitting Resistance

The relative behaviour of various grades can be compared by an empirical relationship for Pitting Resistance Equivalent (PREN), based upon laboratory corrosion tests in chloride containing solutions:



PREN = %Cr + 3.3%Mo X%N



where X = 16 for duplex and X = 30 for austenitic steels. The higher the PREN value the better the pitting resistance.

Hence super-duplex and super-austenitic grades give PREN values of 40-50, lean alloy duplex 27-30 and 18/8 (type 304) austenitic a value of about 20. For duplex structures, alloys are designed to produce ferrite and austenite phases with the same pitting resistance to avoid preferential attack of either phase.

Research into pitting and the growth of corrosion fatigue and stress corrosion cracks which are believed to originate at pits can now be aided by the use of special techniques which allow real-time mapping of local corrosion activity and the determination of localised corrosion rates. Such information is essential in the accurate prediction of safe working life.