Preparation Of Palladium Alloy Films On316l Stainless Steel And Their Corrosion Behavior In Reductive Mediums

Due to the passive film on the surface of stainless steel, stainlesssteel have excellent corrosion resistance in lots of corrosive mediums.However, in reductive corrosion mediums such as boiling dilute sulfuricacid solutions or boiling acetic and formic acids, passive film can notremain stable and active corrosion may happen. The corrosion behaviourof stainless steel in acetic acid with or without halide ions has beenstudied by many authors. However, few researches were focused on thecorrosion behaviour of stainless steel in purified terephthalic acid （PTA）slurry which is important to industrial production. Different techniqueswere reported for palladium deposition on316L stainless steel, and Pdand Pd alloy films show good corrosion resistant in boiling dilute sulfuricacid solutions or boiling acetic and formic acids. However, to improve thecorrosion resistance of the film, and understand the mechanism of theanti-corrosion rensistance in reductive corrosion mediums, it is necessaryto carry on further reaserch on Pd alloy film deposition technology andcorrosion behavior of Pd alloy film. In this research, the corrosion behaviour of316L stainless steel insimulated drying process of PTA slurry was studied. Pd film, Cr-Pd film,PdCuNiMo film and Pd-Ni/Pd-Cu double coating were deposited onstainless steel, and corrosion behavior of the films were investigated. Themechanism of anti-corrosion resistance for Pd film in0.5mol L-1H₂SO₄+2ppm F-solution at80℃was studied.The corrosion behaviour of316L stainless steel in acetic-10%formicacid mixture containing Br-and in simulated PTA slurry was investigated,to understand the corrosion mechanism of316L stainless steel in dryingprocess of PTA slurry. The results show that in acetic-10%formic acidmixture containing Br-, corrosion rate of316L stainless steel increaseswith the increase of Br-concentration and temperature. In PTA slurry, thecorrosion rate of316L stainless steel changes in a parabolic form withincrease of the solid to liquid ratio of the slurry. The slurry has thestrongest corrosivity when the solid to liquid ratio of the slurry reachesabout6.25. In a half dry-half wet PTA slurry, when the316L steel surfaceis remained at a suitable temperature to avoid continuous liquid film onthe surface, corrosion rate of the steel will be greatly reduced.PdCuNiMo alloy film can be deposited on304L stainless steel byelectroplating. The property of film closely related to deposition current.With smaller deposition current, the film is more smooth and dense, andthe hardness of film is higher. So correspondingly, the property of corrosion resistance is better. As the deposite current density decreases, Inboling acetic and formic acids containing200ppm Br-, the corrosion rateof the films prepared with1.2A/dm²current density is0.61g·m^-2·h^-1. Butthe corrosion rate of film drops to0.05g·m^-2·h^-1once the current densityreduce to0.8A/dm². The influence of Cu content was also investigated.When the content of CuSO₄is1.5g/L, the film prepared with0.8A/dm²current density has the best corrosion resintance, and the corrosion rate isonly0.02g·m^-2·h^-1.The codeposition hydrogen in the film can be removed by heattreatment, hence the electrochemical performance can be greatlyrecovered, which leads to higher corrosion potential and better corrosionresistance. After heat treatment, the micro-hardness is obviouslyimproved, in addition, the lattice structure and element content remain thesame. XPS analysis indicated that Pd is metallic state after200℃treatment, However, after treating at400℃，oxidation reaction happenedto Pd.A Pd-Ni/Pd-Cu double coating with a thickness of about3μm wasdeposited on stainless steel surface by electroplating. The double coatingshows obviously higher hardness, modulus of elasticity and adhesivestrength to the substrate in contrast to single Pd-Cu coating. The porosityof the deposited coating also decreased evidently. The lower porosity ofthe double coating may reduce chloride migration through the deposited film. The higher hardness, modulus and adhesive strength would increasethe erosion resistance. All these factors result in better corrosionresistance. The Pd-Ni/Pd-Cu double coating significantly improved thecorrosion resistance of316L stainless steel in strong corrosive medium.Particularly, the double coating exhibits better resistance in complicatedcorrosive environment including erosion and chloride ions. In boilingacetic and formic acids mixture containing0.005mol/L Cl^-with900r/min stir, the corrosion rate of the double coating coated316L stainlesssteel is about one order of magnitude lower than that of Pd-Cu coatedsamples.A thin layer of Pd deposition on the surface significantly improves thecorrosion resistance of316L stainless steel in0.5mol L-1H₂SO₄+2ppmF-solution at80℃. The Pd film promotes spontaneous passivition andthe corrosion rate of the Pd plated specimen decreases by three orders ofmagnitude in contrast to that of the original316L specimens. When316Lstainless steel specimen is coupled with Pd electrode, the passive filmformed on stainless steel shows much better corrosion resistance than theair-formed passive film. After the disconnection of the stainless/Pd couple,the stainless steel specimen still remains passivated for a relatively longtime. The Mott-Schottky measurements show that compared with theair-formed passive films, there are less point defects in the passive filmon the specimens coupled with Pd. XPS measurements show that the main difference between the compositions of the two films is theenrichment of Cr and the presence of Cr（OH）₃and Fe₃O₄in the passivefilms coupled with palladium. These differences lead to the bettercorrosion resistance of the passive film formed under the effect ofpalladium film.Last but important, As a potential material for bipolar plates in protonexchange membrane （PEM） fuel cells, Pd and Cr-Pd film was depositedon316L stainless steel. Corrosion resistance and interface contactresistance were studued. Comared with original316L stainless steel, thecorrosion rate of specimen with Pd or Cr-Pd film decrease by three ordersof magnitude, and the interface contact resistance is also obviouslydecreased. Adding Cr in Pd film can further improve the corrosionresistance for Cr（OH）₃exsits in Cr-Pd film as a beneficial component. Inaddition, adding Cr can lower the cost of the films.