ABSTRACT

The effect of applied potential and chloride concentration on localized corrosion and stress corrosion cracking (SCC) of austenitic stainless steels (SSs) and Ni-base alloys was studied. The repassivation potential Erp, for deep pits was the same as that for crevice corrosion. Laboratory tests over a two-year period and field experience have shown that the ET is a conservative p meter to predict the occurrence of both these fail modes. For type 316L SS, the U-bend SCC test was found to be more conservative than the slow strain rates tests under controlled potential conditions for a variety of chloride concentrations. No SCC was observed below Erp. The Erp was also found to correlate with boiling nitric acid test results and the degree of sensitization of alloy 825. The difficulties in using cyclic potentiodynamic polarization tests for assessing localized corrosion of highly alloyed materials such as alloy C-22 are pointed out.

INTRODUCTION

The use and limitations of various corrosion arid stress corrosion cracking (SCC) test techniques have been of great interest to the process industries for a number of years. Relatively rapid tests such as cyclic potentiodynamic polarization (CPP) test and slow strain rate tests (SSRT) have been particularly valuable beta of the need to rapidly screen materials for new process, prevent product degradation during testing as in case of the pharmaceutical industry,2 or to map out a broad environmental range in which corrosion or SCC occurs for further intensive investigation. An additional benefit of the electrochemical tests is the possibility of monitoring the performance of components through the measurement of corrosion potentials. In some case protection of equipment through the application of potentials (anodic protection) has been successfully attempted Most alloy manufacturers use parameters such as critical pitting/crevice corrosion temperature, depassivation pH, and pitting resistance equivalence (PRE) to compare the performance of various alloys. While these parameters are useful in comparing performance of materials some of which have already been tested in the field parameters that can be used in predicting the long-term behavior of materials in new environments are needed

Nowhere is the need for long-term prediction as critical as in the case of high-level nuclear waste disposed programs. The performance periods, ranging from hundreds to thousands of years, are beyond the realm of current engineering experience and there is no operating high-level waste disposal facility to provide guidance for even shorter term performance. Prediction of the occurrence of localized corrosion has been attempted by a combination of short-term experimental data, numerical modeling, and longer-term testing under controlled conditions to gain confidence in the model predictions, The results of experimental investigations on austenitic stainless steels (SSs) and nickel-based alloys for the high-level radioactive waste (HLW) disposal program are presented here with the objective of examining the applicability of similar methodologies to the needs of the process industries. Most of the discussion in this paper will focus on a nickel-base alloy, alloy 825. However, more highly alloyed materials and lower grade SSs wil also be examined for comparison.

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