The on -the -spot research and observation of corrosive shape w ere made for mesh dehiscence of water hat screen cloth in 904L under the working conditions of 6 MPa , 155 ℃, pH 37 using H type resin catalyst for producing me thyl-ethyl-ketone .The mesh crack w as found to be belonged to stress corrosion cracking through analyzing and observing alloy agent mass fraction, corrosive media test, inner material impurity , mesh residual stress resource , and crack shape of 904L.And also the expatiating was made for the main reason causing SCC ;lack of Ni, Mo and Cu content in the materials, excess of C and inner material impurities, much residual stress, and the damage to the inactivited surface during transportantion and installation.The process of SCC is that catalyst and mesh surface form the gap , and it is quickly acidificated.When pH falls to activating value of 904L , passivating film is destroyed in the gap because of reducing reaction .The macro corrosive battery is formed among metal inter surface , electrolyte solution and outer surface.When the corrosive depth develops to a certain value , it causes stress concentration resulting in cracking .The crack is vertical to the mesh circle and develops along with the mesh radius.When it reaches to the neutral surface of the mesh, the cracking disappears.There is no crack in the internal pressure area of neutral surface .Therefore , the crack can' t be found in the deeper corrosive pit of mesh surface .