Effects of Sensitization Temperatures on the Microstructure, Ferrite Count and Corrosion of Austenitic and Ferritic Stainless Steel in Marine Environment

Abstract

The study investigated the effect of the sensitisation temperatures on the microstructure, ferrite count and corrosion of austenitic and ferritic stainless steel in a marine environment. Austenitic stainless steel with 16.43% Cr, 9.72% Ni and 2.10% Mo and ferritic stainless steel with 16.51% Cr and 0.12% Ni were used for the study. An X-ray Fluorescent, Oxford Instrument XMET 7000, was used to identify the metals. A total of ten specimens were prepared for the study, five ASS 316 and five FSS 430, with one sample of both specimens set aside as the as-received sample. The Leeb hardness, ferrite count and tensile strength of the specimens were determined before they were subjected to sensitisation and accelerated linear polarisation resistance (LPR) corrosion test. Specimens were also subjected to a surface scan to determine their microstructure. The ASS 316 was observed to have experienced major weight loss when sensitised to 700°C, while the FSS 430 specimen only experienced slight weight loss. The ferrite count for ASS 316 specimen reduced greatly when sensitised to 700°C, and for FSS 430 at 600°C. The Leeb hardness test revealed very hard specimens at 600°C, and decreases with an increase in temperature. Worthy of note is the increase in corrosion rate as the immersion time increases. The rate of corrosion is higher for FSS 430 as compared to the ASS 316 specimen. The micrographs revealed chromium-depleted zones for the ASS 316 are higher for sensitisation temperatures of 500°C and 600°C. While the FSS 430 specimens revealed chromium, depleted zones increased with an increase in the sensitisation temperature.