The effect of phosphorus on the mechanism of intergranular stress corrosion cracking of mild steels in nitrate solutions

Abstract

The corrosion behavior of iron-phosphorus alloys with 0.003–2 wt% phosphorus has been studied in nitrate and sulfate solutions. The high P alloys can be considered as a model for grain boundaries with segregated phosphorus. Polarization tests indicated that phosphorus impedes the formation of protective oxide layers on the metal surface in nitrate solutions. According to surface analysis of the corrosion layers by X-ray photo-electron spectroscopy (XPS) it is supposed that upon dissolution phosphorus passes the metal/electrolyte phase boundary in a negatively charged state (corresponding to phosphide) and that the impeding action on the layer formation is due to this state. Constant strain rate tests on mild steels with phosphorus contents in the range 0.003–0.05 P showed a detrimental effect of phosphorus in NH 4NO 3 solution in the potential range −300 mV to −100 mV (NHE). The effect can be correlated to phosphorus segregated at the grain boundaries and can be predicted from the behavior of the Fe-P alloys. However, at potentials −50 mV(NHE) the susceptibility of the steels to intergranular stress corrosion cracking (IGSCC) is even higher and independent of the P content. Mild steels are susceptible to IGSCC even with a very low P content.