Nickel-phosphorus Alloy Coatings Preparation And Corrosion

The loss of steel caused by corrosion is 1 ton every 90 seconds all over the word. It is estimated that the corrosion loss in 2005 amounts to at least 500 billion RMB, and around 17.50 million tons of standard coal was consumed due to corrosive encrustation. Under this circumstance, it is imperative that effective measures of anti-corrosion and energy saving should be taken. Surface treatment by electroless plating of low-grade materials has important economic and practical implications for reducing the initial investment in equipment and prolonging the service life of equipment.Thus it can be used as an alternative to high-grade materials. On the other hand, electroless plating increases the surface energy of heat exchanger unit and enables drop-wise condensation of vapor, which may lead to higher efficiency of heat exchange rate. With these two objectives, an in-depth study on electroless Ni-P plating of heat exchanger unit, together with condensation test analysis, is elaborated on in this paper.Taking the characteristics of electroless plating, alloy deposition mechanism, experiment conditions, technological requirements, and coating performance into consideration, the author obtains the major compositions of solution used for electroless Ni-P plating from repeated experiments. More effort has been made to optimize the mol ratio of major compositions to reducing agent. The study on proper selection of various complexing agents, which have a vital impact on the performance of plating solution , service life and coating quality, solves the key problem of electroless plating. Meanwhile, the impact of stabilizer and other agents on the performance of plating solution and coating quality is also investigated. Compositions that meet the design requirements are determined on the basis of a great number of experiments.Basic processes of electroless Ni-P plating and major procedures that may affect the Ni-P coating are investigated. The result indicates that the quality of coating has a intimately relationship with pre- and post-treatment and plating process. Therefore, quality of coatings can only be achieved by implementing process of electroless Ni-P plating properly. Pre-treatment is a very important portion of electroless plating. Process including test piece preparation, rust and oil removal, and activating treatment, are required to be done according to relevant standards. The competence of consecutive experiments depends on the pre-treatment quality. A careful study is made on the control of major compositions, including preparation and application, measuring, analysis, and replenishing of plating solution. With the consumption of ion Ni²⁺ contained in solution during the process of plating, sodium hypophosphite needs to be measured and replenished. The effect of PH value on deposition velocity, content of Ni-P contained in plating, and performance of plating solution is observed through experiments. Some diagram showing the fluctuation of PH value, and the relations between PH value and deposition velocity, content of Ni-P, and performance of plating solution are presented. More study are made to understand the impact of deposition velocity on plating quality. Meanwhile, the effect of temperature on the performance of plating solution and coatings is explored. It is verified that the solution is apt to decompose under high temperature and is reluctant to react under low temperature because of the low deposition velocity. Therefore, the desirable and optimal temperature of plating is recommended to be controlled within±2℃around 83℃. Post-treatment include de-hydrogen, deactivation, and hole sealing. A new kind of technology of collosol-gel hole sealing is attained.Over 300 samples have been tested from the initial stage to successful determination of electroless Ni-P plating processes. With the assistance of scanning electron microscope, metallographic microscope and hardness meter, the author analyzes the test pieces, proposes experiments plans using orthogonal test methodology.Corrosion resistance, a key index of plating quality, is studied in this paper. In the corrosion tests, test pieces treated under different plating conditions are put into the same corrosion environment, and pieces treated under the same plating conditions are put into different corrosion environment. Mechanism of corrosion resistance of electroless Ni-P plating is examined. The data obtained from corrosion behavior experiment may serve as references for improving compositions in solution, optimizing plating processes, evaluating pre- and post-treatment of plating. Based on corrosion experiment, a new, feasible plating process is determined. Using this new plating process, coatings of Ni-P is prepared and corrosion resistance of the coating is proved to be competent.