Microbially influenced corrosion of austenitic stainless steel weldments

This dissertation investigated microbially influenced corrosion of austenitic stainless steel weldments in fresh water systems. The corrosion studies were carried out in the laboratory using a simulated Tennessee River water and in the field using a ground water which contained a high chloride concentration and a low dissolved oxygen content. The weldments used in the studies were Type 304L and 316L base materials, and 308L and 316L filler materials, in which a variety of welds was produced using gas tungsten arc welding. The corrosion behavior of the weldments was studied using electrochemical techniques and visual surface examinations in the fresh waters. Crevice and pitting corrosion resistance of the 304L/308L weldments was evaluated in the fresh water and a ferric chloride solution. The oxides produced on the 304L/308L weldments during welding and on 304L plates using the Gleeble simulation technique were examined by x-ray photoelectron spectroscopy. Auger electron spectroscopy, and scanning electron microscopy techniques. It was found that when the weld oxide was removed chemically or electrochemically, the weldment specimen was corrosion resistant regardless of the presence of an imposed crevice and bacteria. When the weld-oxide was not removed, and the bacteria were not present, the weldment specimens showed corrosion resistance in the absence of an imposed crevice. However, if a crevice was present in the weld oxide area, crevice corrosion developed in the form of corrosion nodules. When the weld oxide was not removed, and the bacteria were present, the weldment specimen showed reduced corrosion resistance. Anodic polarization tests revealed that the weldment specimens were not passivated under these conditions, which indicates that both the as-welded surface and the bacteria simultaneously contributed to the enhanced corrosion. It also was found that the low temperature oxide remote from the weld bead was susceptible to pitting corrosion in a ferric chloride solution whereas the high temperature oxide adjacent to the weld bead was not susceptible. Explanations for the various corrosion behavior of the stainless steel weldments are presented based on corrosion potential, oxide characteristics, aspect ratio of oxide defects, IR drop, and the presence or absence of imposed crevices.