Influence of the Cation on the Mode of Stress-Corrosion Failure of Iron in Nitrate Solutionsc

Abstract

The nature of attack on unstressed iron and the mode of cracking in stressed iron during immersion in boiling Ca(NO₃)₂, NaNO₃, NH₄NO₃ solutions and various mixed Ca²⁺ and NH⁴⁺ solutions have been followed by microscopic examination of specimens removed from the solutions at various fractions of the mean failure time and by stress-corrosion tests.

Only NH₄NO₃ gives rise to intergranular trenching in the absence of stress, and this ability, together with severe pitting attack, leads to a mode of stress-corrosion failure that is markedly different from that in Ca(NO₃)₂ or NH₄NO₃ in which intergranular damage occurs only when the specimen is stressed.

Experiments are described which indicate that the roles played by stress during cracking in Ca(NO₃)₂ or NaNO₃ and NH₄NO₃ are different, concluding that the phenomenological term ‘stress-assisted intergranular corrosion’ is preferable to the generic term ‘stress-corrosion cracking’ in describing an NH₄NO₃ type failure in order to distinguish it from the type of nitrate cracking which occurs in the absence of the NH₄ ⁺ ion.