Comparison of Critical Pitting Temperatures of Stainless Steels in different Salt Solutions

摘要

Localized corrosion, such as pitting and crevice corrosion, is one of the most common reasons to failure in stainless steels. The standard method ASTM G150 measures the critical pitting temperature (CPT) electrochemically, and is used to rank the pitting corrosion resistance in stainless steels. However, for high-alloyed materials such as hyper-duplex and super-austenitic grades with a Pitting Resistance Equivalent (PRE) above 48, the resistance to pitting corrosion is too high to be measured in 1M NaCI, mainly because of limitations in the boiling point of NaCI. For such alloys, the critical crevice temperature (CCT) may be used for ranking these materials, but it is more difficult to rank and compare high-alloyed stainless steels with respect to their pitting resistance. This paper attempts to compare the CPT for alloys when ASTM G150 is not applicable. By using MgCl_2 as electrolyte, the boiling point is increased and the measuring zone for CPT is broader. The CPT values of austenitic and duplex stainless steels have been measured in 1M NaCI and in 3M MgCl_2. A linear relationship between the CPT values was found. The chloride concentrations and the pH-values were measured and the solubility concept in the salt solutions has been considered.