In Finland stainless steel is rarely applied for drinking water installations, but in a number of water distribution systems and fire protection systems stainless steel is being used replacing older installations of carbon steel or cast iron. Corrosion resistance of stainless steel in cold, flowing, low-chloride containing water is excellent, but diminishes with increasing temperature, chloride content or stagnant conditions. In welded structures, corrosion is often concentrated in the weld nugget itself or in the heat-affected zone [3, 4]. On-site welds are often welded from the outside of the pipe using shielding gas inside. Normally, it is difficult to clean the root of the weld after welding properly and heat tint layers (coloured oxides) will be formed close to the weld, if the shielding gas is not optimum. These heat tint layers promote initiation and growth of corrosion pits in seemingly harmless environments and the pitting resistance is reduced in both the heat affected zone and in the weld itself. The pitting resistance in chloride media with sulphide produced by microorganism has also been shown to be lowered for the specific microstructure formed in the heat affected zone, which is also affecting the morphology of pitting attacks