Aspects of Testing and Selecting Stainless Steels for Seawater Applications

Abstract

Relatively recent failures in seawater systems of highly alloyed (i.e., pitting resistance equivalent [PREN] > 40) stainless steels (SS) have highlighted the issue of corrosion testing and safe-use limits. Three aspects of testing and selection of SS for seawater application were reviewed: relevancy of the electrochemical test methods, the quality control system, and the importance of repassivation. A potentiostatic test for assessment of critical pitting (CPT) and crevice (CCT) temperatures was analyzed with respect to its ability to generate data relevant to chlorinated seawater systems. CPT seemed valid as safe-use limits, but the CCT determined were too high. A modified test approach was proposed. Generally, significant scatter in CCT and CPT values was registered, focusing on the importance of statistical experimental approach and data analyses. Recommendations were made on use of the ASTM G 48 test as a quality control method. The validity of critical pitting and crevice indices was analyzed. Even though rolled materials could be ranked according to the indices, the effect of heat treatment and product form may be far more important than minor variations in chemical composition. Repassivation phenomena were studied, and repassivation properties were suggested as the basis for material selection and corrosion control.