۱- Introduction

۲- Materials and Methods

۳- Results and Discussion

۴- Summary of Passive Behaviour

۵- Conclusions

References

Abstract

The corrosion behaviour of type 316L stainless steel in aqueous 30–۵۰ wt%. NaOH at temperatures up to 90 °C has been elucidated. Exposure to room temperature environment showed parabolic weight loss behaviour, with corrosion rates of up to 0.4 mm/year. Higher NaOH concentrations and exposure temperatures resulted in a reduced stability of the electrochemical passivity domain, associated with higher corrosion rates. Exposure to de-aerated 50 wt%. NaOH presented corrosion rates of up to 0.5 mm/year at open circuit potential, with maximum corrosion rates under polarisation of up to ≈ ۱۸ mm/year. The formation of a dark iron-oxy-hydroxide and nickel-oxide was observed, with exposure to temperatures in excess of 50 °C. The behaviour of type 316L stainless steel in hot caustic environment is compared to types 204, 304, 2205 stainless steel, and nickel alloy 200.

Introduction

The corrosion of stainless steel in hot caustic environment has been the subject of a number of investigations, typically with a focus on Kraft pulping liquors [1–۱۱]. The material performance in these environments is governed by the stability of the passive surface flm, which develops as a function of alloy microstructure, chemical composition, and associated environmental interactions. Austenitic stainless steels generally show high corrosion rates when exposed to concentrated, deaerated alkaline aqueous environment at temperatures above 60 °C, with typical corrosion rates of 0.05–۲ mm/year. Very dilute caustic solutions show reduced corrosion susceptibility, for example, with exposure to mildly caustic environment up to pH 12, resulting in corrosion rates of 0.01 mm/year [12–۱۴]. The latter investigation also concluded that temperature is key for understanding material behaviour in these environments, directly afecting corrosion rates of stainless steel in NaOH containing environment [6, 7]. In NaOH environment at room temperature, rates as low as a few μm/year have been reported [6, 15–۲۲]. Based on the Pourbaix diagrams of Iron, Chromium and Nickel, the possibility of forming a passive flm with exposure to caustic pH supports the argument of using stainless steel in these media. A summary of the corrosion behaviour of stainless steels with exposure to diferent caustic environments is provided in [12]. The aim of this paper is to provide a comprehensive overview of the corrosion performance of type 316L in hot caustic aqueous environments. The project was centered on identifying alloy composition and environment-related parameters, including temperature, de-aeration, and NaOH concentration, to better understand the passivation behavior of stainless steels microstructure.