Synergy between molybdenum and nitrogen on the pitting corrosion and passive film resistance of austenitic stainless steels as a pHdependent effect

摘要

Stainless steels are extensively used in aggressive environments because of their good mechanical properties and corrosion resistance. Since stainless steels can be employed both in new reinforced concrete structures, several interesting studies focus on the corrosion behavior of the passive film formed in solutions simulating concrete. However, few studies report the effect of the pH on the passive behavior of these films [1]. Nitrogen is an alloying element that is known to improve the resistance of stainless steels to localized corrosion, especially pitting [2; 3]. As such, it participates in the empirical pitting resistance equivalent numbers (PREN) equation with a large weight [3]. The effects of Mo and N seem to synergistically interact when both of them are present in the same austenitic stainless steel grade, as in 316LN [4], but this issue has not been clearly understood yet.This paper brings up some insights upon the pH dependence of the synergistic effect of Mo and N on the localized corrosion resistance of austenitic stainless steels. The objective of this work is to study the synergitic effect of Mo and N additions on corrosion and passive film properties of austenitic grades. A comparison between Mo containing (3 wt% Mo); Mo and N containing (3 wt% Mo and 0.1 % N) and free Mo or free Mo and N grades of highly controlled laboratory heats was done considering their localized corrosion resistance and oxide film formation in different aggressive conditions, from neutral to alkaline pH. The passive layer was characterized by EIS and XPS analyses. The combined effect of Mo and N on the pitting potential was confirmed to be synergistic, and not just the addition of their individual effects [5]. Moreover, this effect was found to be pH-dependent, being very positive in acid to neutral conditions whereas it was almost inexistent in high pH. In terms of mechanism, while Mo plays a role in pit initiation and propagation, N helps in the repassivation process, consequently driving the Epit to more noble values. This was accompanied by a thicker passive film as observed with EIS and XPS [5].