Role of surface finish and post-fabrication cleaning on localised corrosion of type 316L austenitic stainless steel flash chambers

Abstract

Abstract Several carbon steel flash chambers clad with type 316L austenitic stainless steel and operating in seawater at 80°C exhibited severe localised corrosion on the inner wall after three months of continuous service. Visual investigation, carried out on the corroded wall surfaces revealed that the corrosion attack was confined to mechanically damaged and iron-contaminated areas. Failure to keep the oxygen level within the allowable limit of 0·026 ppm had promoted the corrosion. Electrochemical studies confirmed the influence of surface condition apparent from the visual inspections. Various specimens of type 316L austenitic stainless steel with different surface finishes (as received, ground, mechanically scratched, and iron contaminated) were exposed to natural sea water at 80°C Metallographic examination revealed that pits which formed on the as received specimens were uniform in shape, while those on specimens with mechanically damaged surfaces were larger and irregular in shape. The results suggest that surface finish exerted a significant influence on the rapid corrosion of the flash chambers. Post-fabrication cleaning by acid pickling markedly improved the resistance of the type 316L specimens to localised corrosion (pitting). However, the results of acid pickling were dependent on the surface condition and, to a large extent, the temperature of the acid.