Factors And Reduction Strategy For The Porosity Of Electroless Nickel

Hydraulic pillars are commonly used for mechanical support equipment under mine, to improve the corrosion resistance of hydraulic props, electroless nickel plating surface treatment are often used on the surface of pillars. However, in actual production, because of pre-treatment not fully(or over), improper operation in process of electroless plating or other reasons, it often results in relatively high porosity on the surface of electroless nickel-phosphorus alloy, and then leads to formation of corrosion micro-batteries with iron substrate as its anode and nickel plating as its cathode, results in corrosion accelerated. Therefore, it is of great significance to research the factors which affects the porosity of electroless nickel-phosphorus alloy and reduction measures.First of all, scanning electron microscopy (SEM) test was used to prove that the microporous of electroless nickel-phosphorus alloy coating could not be accurately and quantitatively characterize in the way of posting filter, iron solubility value was first used to quantitatively characterize the porosity of electroless nickel plating in this paper. The composition of the iron solubility test solution was determined, a simple iron solubility experimental tank was designed, and the test practices were determined, iron absorbance standard curve was tested, and the formula between transmission rate and iron solubility value was derived, quantitative method was used to prove that nickel(Ni ion) did not affect the absorbance of iron ions. The iron solubility value method was compared with the posting filter method and was proved to be reliable.Impacts of matrix and pre-treatment on porosity of coating were researched, the same thick coatings on different matrix of materials, different porosity; the lower surface roughness was, the lower porosity was. Pre-treatment had a great influence on the porosity, no degreasing and acid washed wiould lead to very high porosity. The effect of process of chemical plating on the porosity were researched, with the increase of plating cycle or the aging of the bath, the porosity increased. 400r/min magnetic stirring, strong ultrasound, hot start-plated, intermittent lifting and immersed in the bath and timely additional helped to reduce porosity.Three representative anionic, cationic, amphoteric surfactants, sodium lauryl sulfate(SDS), OP-10 cetyltrimethylammonium bromide(CTAB), were selected to be studied, the conclusion was that their respective best concentration in electroless nickel plating bath ware 20ppm, 1ppm and 20ppm, they could significantly reduce the porosity. Also sealing effect of three surfactants were compared when they were in their respective best concentration in electroless nickel plating baths, and found that OP-10 was best, followed by sodium dodecyl sulfate, cetyltrimethylammonium bromide worst, but the difference was little.The impacts of passivation treatment to the porosity were researched, it was found that it could reduce porosity of Ni-P alloy coating after passivated in 30g/L K₂Cr₂O₇ for 10min at 70℃. Tafel and EIS tests were used to found that compared to unpassivated, the corrosion resistance decreased significantly for by-passivated Ni-P alloy coating, consistent with the porosity significantly reduction. In addition, compared with a single passivation, several passive and air parked reduced the porosity.