ABSTRACT
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 NaCl, mainly because of limitations in the boiling point of NaCl. 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 MgCh 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 NaCl and in 3M MgCl2. 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.
INTRODUCTION
Localized corrosion is a major issue in industry today as it may have devastating consequences if a failure occurs during operation. The challenge for stainless steel industry is to identify if an alloy will be susceptible to localized corrosion or not. Reliable methods to measure the corrosion resistance with respect to e.g. pitting or crevice corrosion are needed, also for high alloyed stainless steel. There are several standard methods used for ranking stainless steels in terms of their pitting or crevice corrosion resistance, although methods to determine CPT (critical pitting temperature) for high-alloyed stainless steels, with PRE-values (pitting resistance equivalent) above 48, are lacking.
