Evaluation Of Intergranular Corrosion Of 316L Stainless Steel Diffusion Bonded Structures By EPR Method

As an advanced materials joining technology, diffusion bonding has been widely used in aerospace and and nuclear power industries. In recent years, diffusion bonding has attracted extensive attention in package in Micro-chemical Mechanical Systems. Austenitic stainless steels are preferably selected for manufacturing micro heat exchangers and micro-reactors due to its excellent corrosion resistance and good mechanical properties. Because of corrosive medium in Micro-chemical mechanical systems and effect of vacuum diffusion bonding process on corrosion properties of austenitic stainless steels, it is therefore essential to investigate corrosion behavior of the diffusion bonded joints of austenitic stainless steels for both optimization of bonding parameters and reliable package of engineering structures.Electrochemical Potentiokinetic Reactivation (EPR) is generally used for evaluation of corrosion austenitic stainless steel. It is non-destructive, rapid and quantitative features and can be used for industrial on-site inspection of material sensitivity to intergranular corrosion.The main content and conclusions of the paper are as follows:(1)Sensitizzation treatment for both 316L stainless steel base materials and diffusion bonded joints were carried out. Metallographic inspections were conducted to investigate the precipitation of carbides. The results showed that:grains of diffusion bonded joints are much larger than thosed of base material. Base material sensitized for 2 hours showed no precipitations at grain boundaries, while after 8 hours'sensitization treatment, a few precipitates were observed. As the holding time increases, that is, sensitization time becomes longer, the amount of precipitates gradually increased. Base material sensitized for 100 hours already has produced a large number of precipitates, and even mesh formed. However, diffusion bonded specimens sensitized for 100 hours showed no carbide precipitation. This is because diffusion bonding is a thermal mechanical process in which large quantities of twin boundaries produced. Crystal boundary network increases the proportion of low-energy crystal boundaries, effectively blocking large-angle crystal boundaries depleted of chromium continuous distribution and reducing the carbonization precipitated material possibilities.(2) Using double-loop EPR, diffusion bonding of 316L stainless steel specimens in solution of H2SO4 + Na2S4O6 corrosion sensitivity was investigated. Sensitivity of base materials and diffusion bonded structures were compared under the same condition. Metallographic inspections were conducted after the test. The results showed that reactivation ratio of diffusion bonded joint is less than that of base material. That is, diffusion bonded specimen has more excellent intergranular corrosion resistance compared to base material.(3) Double loop electrochemical potentiokinetic reactivation (EPR) testing method is employed to assess intergranular corrosion for 316L diffusion bonded joints in H2SO4 + Na2S4O6 solution. Taking reactivation ratio Rr as a evaluation index, orthogonal experiment is conducted to find out how parameters affect intergranular corrosion of 316L stainless steel. The results show that diffusion bonding technology will not reduce but enhance intergranular anti-corrosion of 316L stainless steel. The regression potential has the biggest effect on reactivation ratio, then solution temperature, sensitization time, concentration of H2SO4, scan velocity and concentration of Na2S4O6.