| Stainless steels show good corrosion resistance in many corrosive environments due to the compact and stable passive film on the surface. But in reductive mediums such as high temperature sulfuric solutions or boiling acetic and formic acids, the passive film cannot be established stably and thus severely corrosion may happen. In our previous studies, Pd or Pd alloy films were prepared on stainless steels and good corrosion resistance was observed in such reductive mediums. However, flowing and wear are frequently present in practical industrial environments, which will cause severe erosion and corrosion damages. So it is necessary to do further studies on how to improve the erosion-corrosion resistance of the film in reductive mediums. In this paper, Pd-Co alloy films were prepared on 316L stainless steel by electrodeposition and the erosion-corrosion resistance was stuied.The influence of process parameters such as surface morphology, content, hardness and porosity on the properties of Pd-Co film has been studied. With the increase of current density, Co content increases and the film become coarser. The hardness increases but then decreases. The film gets the lowest porosity and crystalline size when the pH is 7-8. With the temperature increases, the Co content and hardness decrease. The crystalline size decreases and then increases. The best corrosion resistance of the film in boiling acetic and formic acids with 0.005 M Br’is obtained when the current density is 1 A/dm2, pH is 7-8, and temperature is 35℃.The corrosion resistance of Pd-Co film in boiling acetic and formic acids is studied. The Co content in the film can be controlled in a large range from 21.9 at.% to 57.42 at.%. In the bromine-free solution, both the Pd and Pd-Co plated sample show low weight loss rate. But in the bromine-containing solution plus stirring, the Pd-Co plated sample exhibits obviously better corrosion resistance than the Pd plated sample. This is because that Pd-Co films show finer grain size, lower porosity and obviously higher micro-hardness compared with Pd film, which will reduce bromine migration through the deposited film, increase the resistance to erosion and retard the development of micro-pores in the film.The erosion-corrosion resistance of the Pd-Co and Pd-Cu films plated SS samples in a simulated PTA slurry environment is studied. Under static state condition both the Pd-Cu and Pd-Co plated samples exhibit good corrosion resistance and the Pd-Cu film behaves slightly better. With increasing stirring speed, the erosion-corrosion rate of the Pd-Co plated samples increase obviously while that of the Pd-Co plated samples show only slight increase. As a result, the Pd-Co plated stainless steel samples show better erosion-corrosion resistance.The erosion-corrosion behavior of the Pd-Co and Pd plated samples in hot sulfuric acid solution with SiO2 particles was investigated. Under static state condition both the Pd-Co and Pd plated samples exhibit good corrosion resistance and the Pd film behaves slightly better. At low stirring speed, the erosion-corrosion processes of both Pd and Pd-Co plated samples were controlled mainly by the corrosion process. At high stirring speed, the erosion-corrosion resistance of Pd plated sample decreased rapidly, whereas that of the Pd-Co plated sample kept almost stable under the testing conditions, showing better erosion-corrosion resistance. The reason is that there was a significant synergistic effect between erosion and corrosion. The low erosion-corrosion resistance of the Pd plated sample is attributed to the mechanical damage of the Pd film by the impingement of sand particles and then further corrosion and erosion happened in the slurry environment. By increasing hardness of the film, the erosion-corrosion resistance of the Pd-Co film may be increased obviously.Finally, the Pd-Co gradient film was prepared by controlling the electrodeposition current density. The results show that the Co content increased from the inner surface to outer surface of the gradient film. Compared with Pd-Co film with uniform content, gradient film had better corrosion and erosion-corrosion resistance. |
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