ASPECTS OF TESTING AND SELECTING STAINLESS-STEELS FOR SEAWATER APPLICATIONS

Relatively recent failures in seawater systems of highly alloyed (i.e. , pitting resistance equivalent [PREN] > 40) stainless steels (SS) hav e highlighted the issue of corrosion testing and safe-use limits. Thre e 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) temper atures was analyzed with respect to its ability to generate data relev ant to chlorinated seawater systems. CPT seemed valid as safe-use limi ts, but the CCT determined were too high. A modified test approach was proposed. Generally, significant scatter in CCT and CPT values was re gistered, focusing on the importance of statistical experimental appro ach 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 b e ranked according to the indices, the effect of heal treatment and pr oduct 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 corr osion control.