We are using sulfuric acid to maintain the pH of cooling water at our plant. We are experiencing severe damage to the SS-316 material piping at the outlet of the sulfuric acid storage vessel. The damage is pronounced at the weld and at the HAZ area. My question: Why are welds and HAZ areas specifically targeted? My second question is based on the finding that when the damaged spool of pipe was cut in half, a continuous ferrous sulfate (brown colored) acid level line was observed just above the centerline of the pipe. The extent of damage was far worse above this line in the horizontal portion. However the damage to the vertical weld joint is uniform throughout the circumference. Wow is this failure phenomenon explained?
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Brian Dalder Forum Moderator 169 Posts
Re: Why is there corrosion damage at the weld and HAZ areas?16 September 2010 at 1:29pmPreferential corrosion of a weldment and/or a heat-affected zone is a widely known phenomenon. High-temperature exposure and welding can significantly affect the microstructure and properties of the weldment and/or heat-affected zone that makes them more susceptible to corrosion. The microstructural changes that occur in weldments or heat-affected zones include carbide precipitation and intermetallic phase formation. Rough welds, especially those associated with mill-scale and slag deposits, may suffer accelerated corrosion. These changes can be minimized by following proper welding procedures and using low carbon versions of stainless steel alloys like 316L. You also mentioned a case of preferential corrosion above the liquid line. Corrosion depends on many factors including the specific nature of the environment so corrosivity can be different for areas in the liquid space, at the interface, and in the vapor space. Sulfuric acid is a tricky substance to deal with as its corrosive properties change dramatically with velocity, concentration, impurities, and temperature. So you may find it beneficial to consult some specific references on the design and construction of sulfuric acid storage tanks and distribution systems. A good place to start is the NACE standard SP0294 "Design, Fabrication, and Inspection of Storage Tank Systems for Concentrated Fresh and Process Sulfuric Acid and Oleum at Ambient Temperatures."