Study On Phosphating Process And Corrosion Resistance Of Fasteners Surface

The application of phosphating technique is the widest in chemical conversion technology, therefore, phosphating technology become an effective method to improve the wear-corrosion resistance and extend life span of the substrate. The types of phosphate coatings are various, and the best quality film can be obtained in manganese salt phosphating solution. In this work, Different phosphating processings were tried to prepare high-temperature manganese phosphate coating on 40NiCrMo7 steel as a corrosion protection layer for fastener surface of nuclear power pump, aiming at meeting fastener practical application in industry. The crystal structure, phase constitution and corrosion resistance of the thin films were also studied and evaluated by scanning electron microscope( SEM), energy dispersive X ray spectroscope( EDS), X-ray diffacation( XRD), electrochemical measurement system, salt-spray tests machine and copper sulfate spot test, respectivety.Experimental results showed that the phosphate coating are mainly composed of Mn3(PO4)2·3H2O and(Mn, Fe)5H2(PO4)4·4H2O. In this experiment, the properties of the phosphate coatings mainly depent on phosphating temperature, phosphating time and phosphating solution concentration. Surface morphology of phosphating coatings was changed and the phosphate crystal grain size increased evidently with the increase of phosphating solution concentration. Corrosion resistance can be improved along with the thickening of the phosphating film, as the phosphate coatings thicken, corrosion resistant performance descended.The appropriate increase of phosphating temperature and phosphating time can obtain good quality of the phosphating film, but with the continuous increase of phosphating temperature and time, on the contrary make phosphating film aging, phosphate crystal grain excessive growth and become porous and loose, reduce the corrosion resistance of phosphating coatings. The best phosphate coating can be obtained by concolling the ratio of acidity of phosphate liquid be 7.5; phosphate temprature and time be 95℃ and 30 min.The uniform and dense phosphating film could be obtained in the best phosphate process,and he phosphate crystal grains were distributed in stacked type. Surface coverage of the phosphating film was completed, with a smooth surface. Meanwhile, the surface roughnessand porosity of the phosphating film were low. Through the influence of phosphating process on electrochemical corrosion, salt spray corrosion performance, copper sulfate spot corrosion test results, compared with other process, the corrosion resistance of the phosphating film obtained in this process was significantly improved. The corrosion potential of the phosphating film was 113 m V nobler than that of the 40NiCrMo7 steel. The corrosion current density of the phosphating film was one order of magnitude lower than that of the 40NiCrMo7 steel. Salt spray test show that no detective corrosion could be observed on the phosphating film even after 168 h salt spray corrosion. The phosphating film obtained in this process exhibited the highest corrosion resistance, fully meet the practical application requirement of fasteners in nuclear power pump.