Corrosion Evaluation On Brazed Joint Of Stainless Steel

The stainless steel plate-pin heating exchanger is mainly produced by thetechnology of vacuum brazing. With the extensive use of plate-pin heating exchanger inthe areas of petrochemical, metallurgy, refrigeration, air-conditioning and etc, thecorrosive failures of vacuum brazed joint have been presented. It is necessary to studythe corrosive principles and corrosion resistance of the vacuum brazed joints of stainlesssteel deeply and systematically. Based on the former researches on the brazed joint, firstly the techniques ofmicroscopic test and SEM test are used to analyze the forming principle of stainlesssteel vacuum brazed joint, and the phases and the chemical composition of the brittlechemical compound in the joint are confirmed. Moreover, the influencing factors ofbrazing operations on the structure of brazed joint are analyzed in the paper. The annual corrosion depth of every brazed joint in the solution of 80℃3.5%NaClis calculated with the method of weight loss. Its anticorrosion level is Ⅱgrade, and theresult shows that the joint has better uniform corrosion resistance in the solution. According to the analysis of the brazed joint, the corrosive behavior of stainlessbrazed joint filled with BNi-2 filler metal is studied by the immersion test, microscopictest and SEM test. In the solution of 80℃3.5%NaCl, pitting corrosion, intergranularcorrosion and galvanic corrosion are found in the brazed joint. Through analyzing thecorrosion characteristics, cracking patterns and corrosion process of the joint, thecorrosive model is presented in the paper. By employing experiment of 10% oxalic acid electrolyze test, the intergranularcorrosion susceptibility of brazed joint is evaluated and the effects of brazing operationsare investigated. In the middle of brazed joint, because there are some carbide particlesand TCP phase in the grain boundary, the microstructure of brazed line has theintergranular corrosion susceptibility. In the interface layer and diffusion layer of thebrazed joint, the boron formed the chemical compound due to the boron diffusing from iiiABSTACTthe filler metal, so the intergranular corrosion is serious in the area, and the corrosiondegree is mainly affected by the rate of cooling of brazing. Using electrochemical method, the electrochemical characters of SS304 and BNi-2filler metal in 3.5%NaCl solution and the galvanic corrosion between the two metals arestudied. The results indicate that corrosion potential of SS304 is reduced with thetemperature rise, because the passivated film of SS304 is affected by temperature, whilethe filler metal is passivated in the solution, so its corrosion potential changed little.When the two metals contacted in the solution with the same area, the galvanicpotentiality located in the passivated area of filler metal, so the galvanic current densityis low, and the galvanic corrosion is light. While in the brazed joint, the area of fillermetal is far lower than the stainless steel, so the galvanic corrosion in the interface of thejoint area is very serious.