Bigpond, Cracks seen are SCC in carbon steel - Intergranular cracks (along grain boundaries) will come in welds /HAZ/parent plate where hardness is high or varied from other locations. This develops due to reaction of liq. ammonia (with whatever contaminants like Oil carry over from compressor discharges, CO, CO2, mercaptans from ammonia cracking etc) on high residual stress (tensile stress mainly) to develop cracks after some years in service in such storage tanks. Elimination is possible : By keeping 0.2% water and with no aeration in the tank. Still wherever high stresses are present (due to hardness increase) minor SCC will appear after some years. Repairing (surface cracks) is not a major problem for the 19 mm plate thickness. In future, care has to be taken with liq. ammonia with lesser contaminants and by maintaining 0.2% water in tank and with aeration avoided. Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India Sept 06, 2008 E-mail: nishi@vsnl.com >Dear all, >we are inspecting a ammonia storage tank which was in >service for more than 30 years. We have finding some cracks >along the horizontal weld seams joining the plates (from >inside) these cracks are straight line cracks (could be >construction related given the inspection techniques in that >era) The hardness measured in these areas are vaying but >there are some areas with higher hardness as well. My >question is that is SCC in carbon steel look similar to what >it looks like in stainless steel i.e. along the grain >boundries. How can we eliminate these defects / cracks from >SCC or service related defects. >Secondly repairing these crack will increase hardness of the >plates (these are 19 mm thk max) and invite SCC in futur, if >yes how these can be avoided. >cheers