Erfahrungsaustausch und Schadensfälle. Grain boundary attack of stainless in ammoniacal atmospheres

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Abstract
Grain boundary attack of austenitic stainless steels in media containing ammonia in high concentrations cannot be attributed to chromium depletion. In the particular conditions existing in urea manufacture reactors the solubility of metal oxides in the reaction medium is particularly important; this effect comes to bear in particular at high nickel contents. On the base of an evaluation of the behaviour under industrial conditions it may be concluded that the corrosion manifests itself in particular at the grain boundaries, while the grain as such is not affected. In order to combat this type of corrosion the ammonia; water ratios should not exceed 6:1; an alternative solution would consist in increasing the temperature at the points where intercrystalline attack had been found.

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