Research Of The Relationship Between Microstructure Evolution And Corrosion Behavior Of Typical Stainless Steel

In this thesis, two main aspects of the work have been carried out:High temperature oxidation of ferritic stainless steel and localized corrosion of austenitic and duplex stainless steel.Ferritic stainless steel is widely used for a lot of applications because of their good high temperature and stress corrosion resistance, low coefficient of thermal expansion and low cost. During high temperature, there is a compact oxide film formed to increase its high temperature oxidation resistance on the surface of ferritic stainless steel. However, the mechanism and oxidation rate of materials will be changed during water vapor environment, which will lead to protective oxide film breakaway. So it’s necessary to study of the oxidation mechanism of materials working in water vapor, so that we can provide theoretical support for practical application and development of new steel species. The specific research contents and innovations are as follows:Successfully built the water isotopic labeling technique the first time in China, and studied the oxidation mechanism of ferritic in water vapor by combining the isotopic labeling technique and secondary ion mass spectrometry (SIMS). The results showed that oxidation in water vapor proceeded by outward chromium transport, especially, the oxidation involved inward transport of water molecules. On the other hand, we did further research about the influence of Nb and Ti to oxidation of 430SS. The results showed that the oxidation rate of 430SS will increase if only added Nb element, on the contrary, the oxidation rate will decrease if added both Nb and Ti.In usually, because of heat treatment and improper welding secondary phase will precipitate in austenitic and duplex stainless steel, such as:carbides, nitrides,σ,χand R etc. which will lead to corrosion happened easier and material failure. Thus, it is extremely important to clarify the alloy element distribution and heat treatment (high temperature solution and sensitization temperature) influence on microstructure evolution and localization corrosion resistance of duplex stainless steel. At last, the rule for the chemical composition design and microstructure control should be put forward, which provide important science theoretical for development of new duplex steel species.Duplex stainless steel has a more complex structure, which contains ferrite and austenite two-phases. So, Single phase austenitic stainless steel was studied firstly. The specific research contents and innovations are as follows:Studying of the intergranular corrosion of austenitic AISI 301B and 301S stainless steel after sensitization treatment during 500-900℃by double-loop electrochemical potentiokinetic reactivation (DL-EPR). Moreover, produce time temperature sensitization (TTS) diagram for 301B/S. The relationship between carbide morphology, precipitation position and the associated IGC has been determined by microstructure analysis. At last, we put forward a model for IGC evolution of 301B/S stainless steel and appropriate measures to prevent intergranular corrosion. The results demonstrate that the root reason of intergranular corrosion happening is Cr depleted zones due to carbides precipitation. Meanwhile,301B SS has better intergranular corrosion resistance than 301S SS, as 301B contains less carbon. Tip temperature of 301S and 301B is 800℃, carbide precipitation is the domination cause on low temperature, and chromium elements re-distribution in the grain boundary is the domination cause on high temperature. Finally, proposed three measures for prevention of intergranular corrosion, that is reduce the content of carbon, adding useful alloying elements such as:Nb, Ti, etc. and control the location and precipitation rate of carbides.Our further researches focus on duplex stainless steel, which is base on understanding the intergranular corrosion of single-phase austenitic. The specific research contents and innovations are as follows:We successfully built the electrochemical method for evaluating the selective corrosion susceptibility of duplex stainless steel. The method is an optimized double loop electrochemical potentiokinetic reactivation method (DL-EPR). Using of the method to evaluate the interactions between precipitation, chromium depletion and selective corrosion sensitization. Finally, the scanning electrochemical microscopy (SECM) test was applied to study of the electrochemical characteristic of duplex stainless steel’s microstructure firstly. The results showed that the optimized condition is measurement in a solution of 2MH2SO4+1MHCl at 30℃with a scan rate of 1.66mVs-1can. The root reason of selective corrosion happening is chromium depleted zones due toσphase precipitation. The results indicated healing due to re-diffusion of Cr and Mo from theσphase to theγ2 phase. Finally, the charge transfer kinetics at the corrosion precursor/electrolyte interface was studied further by SECM approach curves and fitting to theoretical models successfully. It was shown that the local electrochemical activity of the sample surface is quite heterogeneous, moreover the dynamics of charge transfer at austenite phases are higher than the ferrite andσ phases, K0 was 0.056 and 0.009 cm s-1, respectively.