There are a lot of factors here. The chlorides are the big enemy of stainless steel and can cause corrosion of the welds quite easily. Was the unit passivated or electropolished before use (or both)? The weld area in the heat affected zone is a totally different kind of steel from the rest of it and is very prone to corrosion unless passivated or polished and passivated. Even then it will still be the weakest link on the 316L part. The worst condition is if it is exposed to sea water sporatically. In other words, if you alternatively expose it to sea water and air.

mickjubb: People do use 316LSS in seawater service, but successful use is tricky. Several corrosive mechanisms can come into play, including chloride pitting and MIC. Deposits also can contribute to localized corrosion. If the pitting is confined to welds, it is possible that heat tint from the welding is reducing the corrosion resistance in those areas. Heat tint will reduce the effective chromium content of the base metal, even with low-grade carbon content SS, and will lower the corrosion resistance. All heat tint should be removed from the welds and HAZ. The introduction of oxygen increases the aggressiveness of seawater and should be eliminated. The successful use of the 300-series grades of stainless steel in seawater also depends on maintaining adequate seawater velocities (>3 ft/sec.) and no stagnent environements (from shut-ins, etc.). P.S. The chloride content of seawater is %7E30,000 ppm, consisting primarily of sodium chloride with less quantities of calcium and magnesium chlorides. Hope this helps! David Hendrix The Hendrix Group Inc.