Evaluation of techniques for measuring corrosion activity of carbon steel welds

Abstract

Three test methods, namely immersion tests, galvanostatic anodic dissolution tests and the scanning vibrating probe technique, have been evaluated as candidate methods for ascertaining the susceptibility of welds in carbon steels to preferential corrosion. The use of hardness measurements by profiling and mapping was also evaluated. Three welds were tested for which the production had been controlled to produce different types of corrosion behaviour. In addition, a weld that had failed 'in service' was used to validate the findings of this assessment. In the immersion tests, lasting 9 months, the three controlled weld specimens ex posed to ASTM D1141 sea water behaved as expected, in terms of which region of the weld was most susceptible to corrosion. The results of the rapid galvanostatic anodic dissolution tests compared well with those of the immersion tests, with the same regions of the controlled welds showing susceptibility to corrosion. The galvanostatic anodic dissolution test on the failed weld produced corrosion in the regions that had been susceptible to corrosion in service. However, at very high current densities the differential corrosion loss between different regions of the weld was not maintained and careful control of this parameter is therefore necessary in testing. The scanning vibrating probe technique successfully identified the areas susceptible to preferential corrosion in both the controlled weld specimens and the in service failure. The electrolyte used was ASTM D1141 sea water diluted 10 : 1 with distilled water. However, these trends were not apparent when tests were conducted in distilled water owing to the lower solution conductivity. Hardness mapping produced a two-dimensional visual representation of the specimen hardness. This was very effective in locating small regions of high hardness, in contrast to conventional hardness profiling, and would be effective in locating hot spots for hydrogen embrittlement. However, no correlation between hardness and corrosion susceptibility was observed in the three weld specimens that were examined. The galvanic anodic dissolution test is recommended as a rapid and reliable test to determine the relative corrosion susceptibility of carbon steel welds. The scanning vibrating probe technique can also be effective but at considerably greater expense.