Study On Preparation Of Hard Anodic Oxide Coating On 15-5PH Stainless Steel Hot-dip Aluminum

Stainless steel is used widely, because it not only has excellent comprehensive mechanical properties, but also anti-corrosion, high temperature and other special properties. But how to further improve the surface hardness of stainless steel, reduce the friction coefficient and increase the self-lubricating stainless steel effect have been the hot topics of improving the surface properties. Combination of aluminum and aluminum alloy hard anodizing technology, in this paper, 15-5PH stainless steel is used as the research object. After hot-dip aluminum on the surface firstly, hard anodizing in the electrolyte which contain polytetrafluoroethylene (PTFE). Then a good self-lubricating property of PTFE-Al₂O₃ composite membrane is formed. At the same time, the film has high hardness and good corrosion-resistance.This topic is divided into two experiment steps to complete. The first step is to dip aluminum on 15-5PH stainless steel surface, get the suit dipping aluminum layer, which is prepared for hard anodizing. After trial and error comparison, the main parameters of the best leaching aluminum technology for 15-5PH stainless steel are alkaline cleaning, pickling and help plating. The dipping temperature is between 630 degree and 720 degree, dipping time is 4min, the hoisting speed is about 80 mm/s, and NaCl, KCl, Na₃AlF₆ and a small amount of carbon residue are used as covering agent. The dipping layer is consisted by intermediate alloy coatings and the outermost layer of aluminum. The technological parameters of hard anodizing are 18 20% concentration of sulfuric acid, minus 4 degree zeros degree, 2 4A/dm², 40 ⁶0 min and PTFE content of 10 to 25%. Then PTFE-Al₂O₃ composite membrane is formed.In the experiment, dipping aluminum layer and PTFE-Al₂O₃ composite membrane are analyzed by SEM and EDS, at the same time, the effect of parameters on the properties of dipping aluminum layer and PTFE-Al₂O₃ composite membrane is studied. When other conditions are the same and the temperature is less than 720 degree, with the dipping temperature increase, the thickness of leaching coating increase. But when the temperature is more than 740 degree, the thickness of leaching coating decrease. In order to get a thicker dipping aluminum layer, the dipping time must be 4 min. By the SEM analysis of hard anodizing film, composite membrane is much more compact than oxidation film. By the result of EDS, the film main contains aluminum and oxygen. Besides, there is a small amount of fluorine. So it can be fully showed that the PTFE particles in the electrolyte have been gone into the micro holes, which make the film more compact.PTFE-Al₂O₃ composite membrane and oxidation film are dipped into NaCl aqueous solution for corrosion resistance test. The results show that corrosion-resistance of PTFE-Al₂O₃ composite membrane is much better. By the results of abrasion resistance experiment on friction testing machine, it shows that friction factor of composite membrane is closely relevant to the concentration of PTFE in the electrolyte. With the concentration of PTFE increase, friction factor will decrease. But when the concentration of PTFE is 25 percent, the change of friction factor is no longer obvious.