| It is well known that stainless steels owe their corrosion resistance largely to the formation of passive films on the alloy surface. Stainless steels show good corrosion resistance in oxidizing corrosive mediums where the passive films formed on the surface are stable. However, in many reductive corrosion mediums such as boiling dilute sulfuric acid solutions or boiling acetic plus formic acids, passivity can not be steadily established on the surface and active corrosion happens for stainless steels. For the alloys with passive ability such as titanium alloys or stainless steels, if corrosion potential of the alloy is raised from active potential into passive region by applied anodic current or by alloying with elements with higher oxidation/reduction potentials, corrosion resistance would be improved. Different techniques were reported for palladium deposition on titanium, including vacuum evaporation, ion beam mixing, etc.. The excellent corrosion resistant Pd and Pd alloy films were prepared on 316L stainless steel by electroless plating, electroplating and electronic brush plating in this paper. The palladium plated stainless steel obtained very good corrosion resistant in boiling dilute sulfuric acid solutions and boiling acetic plus formic acids. These processes provided the possibilities of these films used in many industry conditions. (1) An uniform palladium film on 304 stainless steel was obtained by electroless plating. Scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), weight loss tests and electrochemical measurements were used to character the properties of the film. The palladium plated stainless steel samples showed excellent corrosion resistance in strong reductive corrosion mediums. In boiling dilute sulfuric acid solutions and boiling acetic/formic acids, corrosion rates of palladium plated 316L stainless steel samples were 4 and 2 orders of magnitude lower than the original 316 stainless steel samples. In these acid solutions with Cl- or Br- concentration less than 0.1mol/L, the palladium plated samples also showed better corrosion resistance. The Pd plated samples also showed very good corrosion resistance in boiling mixtureof 90% acetic acid + 10% formic acid + Br- solution. However, the corrosion resistance of Pd plated samples decreased with the increase of halogen ions concentration.(2) Palladium films with good adhesive strength were deposited on 316L stainless steel by electroplating. SEM, EDS, XPS, weight loss tests and electrochemical methods were used to study the properties of the films. The electroplated palladium film was almost pure palladium. XPS analysis indicated that palladium was present in the films as metal state. The palladium plated stainless steel samples showed excellent corrosion resistance in strong reductive corrosion mediums. In boiling 20% dilute sulfuric acid solution, the corrosion rates of the palladium plated 316L stainless steel samples were 4 orders of magnitude lower than that of the original 316L stainless steel samples. In the solution with 0.1mol/L Cl- or Br-, the palladium plated samples also showed better corrosion resistance. The Pd plated samples also showed very good corrosion resistance in boiling mixtureof 90% acetic acid + 10% formic acid + Br- solution. However, the corrosion resistance of Pd plated samples decreased with the increase of halogen ions concentration. In comparison, the electroplated samples showed slightly better corrosion resistance than electroless plated samples, which may be attributed to less impurities and thereby higher corrosion potential for the former.(3) Palladium films with good adhesive strength were deposited on 316L stainless steel by brush plating. SEM, EDS, XPS, weight loss tests and electrochemical methods were used to study the properties of the films. The brush plated palladium film was mainly consisted of palladium. XPS analysis indicated that palladium was present in the films as metal state. The palladium plated stainless steel samples showed excellent corrosion resistance in boiling 20% H2SO4 solution and boiling acetic/formic acids with 0.005mol/L Br- ions added. The corrosion rates of the palladium plated 316L stainless steel samples were about two orders of magnitude lower than that of the original 316L stainless steel samples. This method provides a possibility to prepare protective palladium films on stainless steel facilities with large areas in industrial sites. The brush plating parameters have obvious influences on electrochemical properties of the palladium films. As the brush plating voltage and the brushing speed increasing, the open circuit potential of the plated samples decreased. The effects of brush plating parameters on corrosion behaviors of the films are mainly attributed to the effect of co-deposited hydrogen in the films. When hydrogen is removed from the Pd films by heat treatment, corrosion resistance of the films turns better.(4) Pd-Ni alloy film was plated on stainless steel by electroplating and Pd-Cu alloy film was plated by brush plating. Immersion tests show that the film Pd-Ni alloy films have good corrosion resistance performance in boiling sulfuric acid, but they are not corrosion resistant in boiling acetic/formic acids. Pd-Cu alloy film does not have corrosion resistance in boiling sulfuric acid, but they showed excellent corrosion resistance in boiling acetic/formic acids with 0.005mol/L Br- ions added. SEM, EDS, XPS, X-ray diffraction and other methods were used for studying these films. The crystal lattice of Pd-Ni alloy film is face centered cubic structure and the Pd-Ni alloy film is complete solid solution. The Ni content can be controled from 30 to 50 at.% by plating parameters. The surface of the film is almost constituted of Pd element. The corrosion resistance of film decreases with the increasing of Ni content. The grain size of brush Pd-Cu alloy plating film decreased with increase of brush voltage, but the relative content of Pd and Cu was not affected by the brush voltage. The Cu element is enrichment on surface of Pd-Cu alloy film.(5) The corrosion mechanism and corrosion failure mechanism of stainless steel and Pd composite system in high-temperature sulfuric acid were studied in detail. The potentiodynamic scanning technique, potential measurement, galvanic corrosion test, EIS, XPS are used in research. Due to the galvanic effect, stainless steel substrate shifts to the passive zone by the covering of Pd, and stable passive film is produced on stainless steel surface where exposed to acid solution. Thus, the stainless steel obtained excellent corrosion resistance. The corrosion potential of galvanic Pd/stainless steel is closely related to Cathode/anode area ratio. There is a corresponding relationship between the area ratio and galvanic potential. Only the galvanic potential reached or exceeded the passive potential of stainless steel, the system can be stable passive. While the larger of the cathode/anode area ratio, the system is more stable. The corrosion failure process of Pd/stainless steel system can be basically divided into three stages:stabilization stage, the slow erosion stage and rapid corrosion stage. The more of the area exposed by slow corrosion of the substrate and hydrogen reduction are the two most fundamental reasons of corrosion failure of the Pd plated stainless steel. The EIS measurement results fit the physical model well. The Mott-Schottky curves and XPS analysis show that the surface of Pd-plated sample is p-type oxide semiconductor at open circuit potential. It indicated the surface passivation film is composed of the low-state oxides of Cr2O3 and PdO.
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