20 Onstream Corrosion case studies Three case studies are outlined below to illustrate the different types of corrosion described on the previous page.     1. Sewer main A cast iron sewer main had experienced pitting corrosion resulting in the release of raw sewage into the sea. It was important to determine whether corrosion occurred due to chemicals present in the sewage, sulphur reducing bacteria or due to sea water entering the sewer through the small crack (pictured left). Following microscopic inspection the corrosion product was found to consist of a dense matrix overlying voids in the pitting which had occurred. Elemental scans were used to determine what was present in the voids and it was discovered that the voids were rich in sulphur, indicating that Sulphur Reducing Bacteria (‘SRBs’) may be present (pictured left). Bacterial culture tests confirmed the presence of SRBs under the corrosion product and failure was attributed to their presence under the layer of dirt and debris at the bottom of the pipe creating an anaerobic environment. This was determined to be due to the lower than expected flow in the pipe from an uncompleted housing development (pictured left). 1. Pitting corrosion in sewer main     1. Corrosion product on the internal surface of the sewer main (black) found to be rich in sulphur (yellow)     1. Bacterial cultures of SRBS © 2013 Xchanging