The thiourea was added in the electroless plating solution to analyze the influence of thiourea on the Ni-P coating deposition rate, surface morphology and porosity, etc.. The corrosion resistance was evaluated through polarization curves and electrochemical impedance spectroscopy. The results show that when 1 mg/L thiourea in the electroless plating solution was added, the deposition rate of coating accelerates and the corrosion rate increases. This is mainly reason after thiourea was added in the electroless plating solution, the porosity of coating increases, which promote the corrosive medium to seep deeply into the surface of the substrate resulting in increasing the number of corrosion micro cells and forming a corrosion cell of the large cathode(coating)-the small anode (substrate). The corrosion current density increases and the charge transfer resistance reduces. While thiourea was added more than 3 mg/L, the electroless plating solution is poisoned and unable to plating.

Through SEM observation, EDS analyses, the effect of dual thermal cycle on microstructure of 2.25Cr1Mo steel was analyzed. The results show that dual thermal cycle results in the formation of bainite. However, grain boundary of coarse prior austenite resulted from first thermal cycle does not become finer and still appears to maintain the original forms of it even though subjected second thermal cycle. EDS analyses indicate that micro zone composition are uniformly distributed which means that austernitizing homogenization is completed more sufficiently after subjected second thermal cycle. Micro zone Cr content of granular structure increases compared with matrix. It is assumed that carbides mainly composed of Fe, Cr element are formed among granular structure.

The effect of ternary thermal cycle on microstructure of 2.25Cr1Mo steel was analyzed by SEM and EDS. The results show that ternary thermal cycle results in the formation of bainite. Grain size does not grow larger obviously after subjected ternary thermal cycle of maximum peak temperature 1 195 ℃, total t8/5 more than 97 s of ternary thermal cycle. Because phase transformation does not occur upon cooling of first thermal cycle, effect of peak temperature of second thermal cycle on grain size, which may result in grain finer does happen. EDS analyses of specimen after thermal cycle indicate that micro zone compositions of precipitated phase are similar and Fe element in the majority. It is assumed that precipitated phases formed are Fe, Cr, Mo carbides.

Ni-Cu-P ternary alloy plating was made by electroless plating technology in the surface of 20R steel for improving corrosion resistance of 20R steel in hydrofluoric acid.The surface morphology and structure of the coating were observed and tested through metallography and X-ray diffraction technique.The corrosion behavior of 20R steel and the coating was tested by immersion method and electrochemical system in hydrofluoric acid solution.The results show that:Ni- Cu-P coating can deposit smoothly in the surface of 20R steel.The coating is smooth,compact and crack free and has prevented 20R steel from contacting hydrofluoric acid directly.The coating is much easier to passivate in hydrofluoric acid solution.Thus,the coating has corrosion resistance to some extent.

To study the corrosion of Ni-Cu-P electroless coating in different concentrations HF solution,surface morphology of Ni-Cu-Pcoating was analyzed by metalloscope graphic.The corrosion resistance of Ni-Cu-Pcoating was tested by soaking method and electrochemical impedance spectroscopy testing in different concentrations HF solution.The results show that Ni-Cu-P coating is uniform,dense and has good corrosion resistance in HF solution.With the increase of HF concentration,the corrosion rate ofthe coating appears to increases then decreases in different concentrations of HF solution.

The critical transformation temperature Ac1 and Ac3 of steel 2.25CrlMo0.25V steel under different heat treatment process conditions are studied by thermal expansion method. The results show that the critical transformation temperature of the steel annealed is 797.1 ℃ and 886.2 ℃, the critical transformation [KG*4]temperature [KG*4]of [KG*4]the [KG*4]steel [KG*4]quenched [KG*4]is 801.3 ℃ and 891.4 ℃ and the critical transformation temperature of the steel quenched and tempered is 809.2 ℃ and 898.7 ℃. Different structures affect the critical transformation temperature. So the influence of heat-treating process on the performance and structure should be considered.