Understanding Ferrite and Stress-Corrosion Cracking in Stainless Steels

The presence of ferrite in stainless steels can significantly influence their resistance to stress-corrosion cracking (SCC). While nominally austenitic stainless steels may contain some ferrite that helps block cracks, this effect is often negligible for worked steels, which typically lack sufficient ferrite. In contrast, castings of equivalent steels may retain non-equilibrium structures with up to 13% ferrite due to incomplete peritectic reactions. This ferritic phase can provide a useful degree of protection against SCC, a critical consideration for engineers and manufacturers.

Duplex stainless steels are specifically designed to contain both austenite and ferrite, regardless of whether they are cast or wrought. This unique composition grants them enhanced resistance to SCC compared to their purely austenitic counterparts. However, the relationship between the composition of austenitic stainless steels and their susceptibility to SCC is not entirely clear. Although high levels of nickel and chromium are known to improve resistance, the impact of other alloying elements, such as molybdenum and carbon, remains inconsistent, possibly due to complex interactions among various alloy components.

The mechanisms behind stress-corrosion cracking involve both electrochemical and mechanical contributions. The process typically unfolds in two stages: induction and crack propagation. During the induction phase, the passive film on the metal surface undergoes local breakdown, establishing active/passive cells that facilitate electrochemical reactions. The initiation of cracks is often associated with transgranular features of the austenite grains rather than their boundaries, which is a common consideration in aluminum alloys.

As cracks propagate, they can follow crystallographic planes but do so inconsistently. Two main perspectives exist regarding the propagation mechanism. One viewpoint suggests that cracks are driven by electrochemical dissolution, while the mechanical aspect maintains the crack in an active state, preventing re-passivation of the metal. Conversely, another perspective posits that mechanical tension separates the metal, while electrochemical activity sharpens the crack tip when it becomes blunted. Both explanations may hold validity, highlighting the complex nature of SCC.

Beyond stainless steels, plain carbon steels also exhibit susceptibility to stress-corrosion cracking under specific conditions, often influenced by agents like nitrates, bicarbonates, and hydrogen sulfide. Such cracking can occur in alkaline environments, sometimes referred to as caustic cracking, and typically follows intergranular paths. Understanding the conditions that promote SCC is crucial for industries that rely on steel, especially in environments with high temperatures and pressures, where the risk of cracking is exacerbated.