Understanding and simulating crevice corrosion
Stainless steel structures exposed to seawater are susceptible to crevice corrosion. A positive feedback is created between aerobic bacteria and sulfate reducing bacteria (SRB) colonising cracks and crevices of the stainless steel. Over time, crevice corrosion can weaken the steel to the point of failure, raising issues of safety as well as maintenance and repair costs. A consortium of nine organisations was funded through the GROWTH Programme to tackle the problem of crevice corrosion. CEA-Saclay in France was charged with the difficult task of recreating the complex seawater chemistry in the laboratory. First CEA-Saclay's scientists sought to better understand the mechanism by which crevice corrosion takes place. It was learned that aerobic and anaerobic biofilms created by the bacteria promote corrosion by accelerating reduction and limiting passivation respectively. CEA-Saclay managed to simulate the aerobic reactions in the laboratory environment by adding glucose oxidase and glucose to sterile seawater. Conversely, the anaerobic chemistry was recreated by adding sodium sulphide (Na2S) and hydrogen chloride (HCl) to sterile de-aerated seawater. They recommend maintaining the pH between 6.7 and 7 and the total sulphide concentration at around 400 parts per million (ppm). CEA-Saclay and the CREVCORR consortium are sharing their findings with the academic and industrial communities. A paper summarising the results was presented at the Stainless Steel World Conference in 2003 in the Netherlands. The two synthetic seawater mixtures will be useful to engineers looking to test the resistance of new types of stainless steel to crevice corrosion.