The area near welds and welds in the standard grades of stainless steels are occasionally subject to microbiological influenced corrosion (MIC) in stagnant and slowly moving waters.

One example of MIC attack occurred on Type 409 (11% chromium) stainless steel frames of experimental rotating biological contractors (RBC's) in one WWTP. Figure 1 shows an RBC unit being disassembled. Figure 2 shows a MIC mound more or less centered on the weld of the frame. The RBC's, with their stainless steel frames slowly rotating in and out of the sludge at about 2 RPM, create conditions almost ideal for MIC. Other instances of MIC in Stainless steel piping has served as the standard material of construction for municipal waste water treatment plants have not been encountered.

The unusually low incidence of crevice corrosion and MIC in municipal waste water treatment plants may well be due to several of the following factors favouring good performance:

• Removal of heat tint scale by pickling or mechanical cleaning on the exterior of the aeration piping exposed to the sludge in aeration basins.
• Smooth contoured and crevices free circumferential butt welds of piping exposed to sludge. (Note: Removal of heat tint and the absence of crevices have been found to significantly enhance resistance to MIC and other forms of localized corrosion)
• Aeration and agitation of the sludge reduces the tendency of the sludge to adhere to the surface of the piping.
• Through wash down cleaning of the sludge from the outside diameter of the piping when the aeration basin is taken out of service.