| Sulfuric acid is the most productive inorganic acid at present,and it is also an important raw material for many inorganic and organic chemical products.Sulfuric acid has a very wide range of applications in the chemical,national defense and other important fields.In the actual industrial production,the transportation of concentrated sulfuric acid is completed by pipelines.The corrosion of the welded joints in the concentrated sulfuric acid pipe is the most obvious,because the material will be heated during the welding process which will lead to the elements segregation and precipitation.The corrosion failure phenomenon of the concentrated sulfuric acid pipeline seriously hinders the development of the sulfuric acid industry.The corrosive leakage problem in the position of the welded joints of a company’s concentrated sulfuric acid pipeline is taked as its starting point to carry on relevant study.In this thesis and the trend for corrosion behavior changing with the concentrated sulfuric acid temperature was studied by surface analysis technology and electrochemical methods at different positions on the commonly used materials(304 stainless steel,316L stainless steel and 20#carbon steel)in industrial concentrated sulfuric acid pipeline,the main conclusions are as follows:(1)The welded joints of pipline on industrial site were studied from the aspect of surface corrosion morphology analysis,material chemical composition analysis,and electrochemical analysis.The results are as follows:Pitting is the main corrosion forms in the area of welded joints of both 304 stainless steel and 20#carbon steel samples.There is no pitting phenomenon in base metal,the corrosion in the heat affected zone of the 304 stainless steel material is the most serious,and there is a clear corrosion perforation phenomenon.The corrosion groove appears obviously in the welded metal position of the 20#carbon steel samples.(2)The reaction characteristics of 304,316L stainless steel and 20#carbon steel samples were studied through microscopic metallography and electrochemical experiments strategies in static concentrated sulfuric acid under different temperature conditions.Microscopic metallographic experiments show that the corrosion in base metal is lighter while the corrosion in the heat affected zone is obvious after the polarization of 304 and 316L samples.For the 20#carbon steel samples,after the polarization,there are obvious pitting phenomenon in welded metal and the main corrosion forms of the base metal is uniform corrosion.When the temperature of the medium is constant,the corrosion resistance in different zone of both 304 and 316L stainless steel samples from strong to weak is the base metal>welded metal>heat affected zone,and the corrosion resistance in different zone of the 20#carbon steel from strong to weak is base metal>heat affected zone>welded metal.As the temperature of the corrosion medium increase,the radius of the capacitive arc radius and the peak value of the phase angle decrease,and the electrochemical reaction resistance value will also show a decreasing trend,but the reaction speed increase.(3)After the heat treatment for 304,316L stainless steel,20#carbon steel samples in concentrated sulfuric acid reaction process,the following conclusions are drawn:After heat treatment,the grain size of 304 and 316L stainless steel samples become larger significantly,the depth and number of pit etching on the surface of the samples were both decreased.The corrosion resistance of the samples both from strong to weak is base metal>welded metal>heat affected zone.For 20#carbon steel,the grain size become smaller and the.uniformity of the samples material was improved after heat treatment.The degree of corrosion at different position of the 20#carbon steel sample is close after polarization,and the distribution of pit etching at the sample surface is relatively uniform.After heat treatment,the reaction resistance value of the base metal of the 20#carbon steel decreased,but the reaction resistance in the welded metal and the heat affected zone of the 20#carbon steel significantly increased. |
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