Study On The Corrosion Mechanism Of 904L In High Temperature Concentrated Sulfuric Acid Solution

904L is the super austenitic stainless steel with high content of chromium, nickel and molybdenum element. It is often used in harsh environments for its excellent corrosion resistance. In this paper, 904 L super austenitic stainless steel, as the primary material of HF industrial heat exchanger, is treated by using laboratory simulation methods and electrochemical methods in concentrated sulfuric acid to study its corrosion behavior with different content of fluorine ion, temperature and concentration of sulfuric acid. Furthermore, Corrosion resistant nitriding of 904 L is explored. The results are as follows:(1) Laboratory simulation methods and electrochemical tests show that in high-temperature concentrated sulfuric acid 904 L exhibits excellent corrosion resistance, 304 follows and 2507 exhibits poor corrosion resistance. In working condition the corrosion resistance of this three stainless steel increase for the fluoride ion added. The corrosion resistance of 2507 changes a lot. While, the corrosion resistance of 904 L is more stable and reliable. The corrosion resistance of stainless steel in high-temperature concentrated sulphuric acid with fluorine ion increase because nickel ions and fluorine ions form more stable compound replace nickel sulfide.(2) The effect of fluoride ion concentration on 904 L corrosion resistance in concentrated sulfuric acid has been studied. The results show that in concentrated sulfuric acid, the lower concentration of F- promote corrosion, while the higher concentration of F- can inhibit corrosion. When F- concentration increases, metastable pitting turns to be steady pitting on the electrode surface. When the concentration increases to 2%, the pitting phenomenon disappears. At this time polarization resistance increases, and EIS curve shows two time constants due to a layer of adsorption layer form on the electrode surface.(3) The temperature and the concentration of sulfuric acid on the electrochemical properties of 904 L stainless steel ha ve been researched. The results show that With temperature increasing, the corrosion current density increases gradually, and the passive region narrows down, and pitting breakdown potential decreases, and the resistance value decreases. The feature of nyquist plot changes from the single capacitive loop to the high frequency of capacitive loop and low frequency Warburg resistance. F inally negative resistance appears, and active–passive transition occurs. With the concentration of sulfuric acid increasing, the corrosion rate decrease at the first, then it increases, and finally it decreases. There is a minimal value in 60wt% sulfuric acid solution while there is a maximum value in 80wt% sulfuric acid solution. 904 L tend to be self-passivation in a high concentration of sulfuric acid.(4) The corrosion resistance and nitriding layer organization of the 904 L after two kinds of nitriding treatment have been researched. The results show that the surface hardness of the samples improves obviously after two kinds o f nitriding treatment, the surface hardness of the sample after plasma nitriding is higher than the one after tufftride process. The nitrided layer after two kinds of nitriding treatment composed of compound layer and diffusion layer, the corrosion resista nce of the 904 L decreases after two kinds of nitriding treatment, but the corrosion resistance of the sample after plasma nitriding is higher than the one after tufftride process.