2008

Carpén, Leena

Traditionally, many firewater mains are made of carbon steel or cast iron. At present, more and more stainless steel is used as material for firewater pipes due to usually better corrosion resistance in natural waters than iron or carbon steel. In some few cases, however, unexpected corrosion failures in systems using austenitic stainless steels have been observed only after a relatively short time of usage ranging from a few weeks to a few years. Most of these corrosion damages are associated on the weld nuggets or in the heat-affected zones of girth site<br/>welds. One of the most important factors affecting the corrosion resistance of stainless steel at welds and in heat-affected zones are the surface oxide films originating from the welding heat in the presence of oxygen. Therefore, proper root shielding is important especially when welding pipes for fire protection systems where the water is taken from natural sources (e.g. from rivers, lakes or tap water), as microbially induced corrosion (MIC) can increase the risk for<br/>corrosion damages significantly. Two failure cases in fire protection systems (FPS), one in stainless steel factory and one in power plant have been studied and are described in this study. A proposal for the corrosion/failure mechanism is suggested and general recommendations how to avoid or diminish the risk for unexpected localized corrosion failures in FPS are given.