Intercrystalline corrosion

Grain disintegration that may occur at non-corroding steels in a critical temperature range (400 to 900 °C).

Intercrystalline corrosion refers to the mainly corrosive disintegration at the grain boundaries of the micro-structure of stainless steels (Fig. 1).

It occurs mainly at and in the range of grain boundaries that are depleted due to heat treatment at chromium. In this process, the carbon that is dissolved in the stainless steel diffuses particularly quickly at the grain boundaries. It bonds with the reactive chromium and diffuses as chromium carbide. The free chromium depletes in this range (chrome depletion) and is no longer available to form a protective layer against corrosion. For austenitic steels, this process runs particularly quickly at temperatures around 475 °C (formation of the Chi phase) and in the range between 600 and 850 °C (formation of the Sigma phase).

In general, the formation of instercrystalline corrosion can be lowered by the reduction of the carbon content in the chemical composition of stainless steel. This type of stainless steel is marked with an L (Low Carbon). As an alternative, however, reactive elements (titanium, niobium) can be added to bind the carbon by their high tendency to do so to prevent further reaction with the chromium.

Additional references:
Internal oxidation
Corrosion
Spongiosis

  • Fig. 1: Blade wheel made of 1.4408 steel, destroyed by intercrystalline corrosion after three months of operation in 30 % nitric acid (source:  Consulting engineer Holger Döbert, www.korros.de)
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