Study On Technology Of Electroless Ni-W-P And Mechanism Of The Corrosion Resistance Of Coating In Sulfuric Acid

Electroless Ni-W-P alloy coating with strong corrosion resistance and high hardness has a wide range of applications and is easy to form amorphous structure. In order to enhance the tungsten content,corrosion resistance and hardness of Ni-W-P alloy coating, the approach of single-factor experiment is used to investigated the effects of the various concentration of the bath components and technology conditions on the coating quality using coating deposition rate and corrosion resistance as an index and the final batch formula of the process is firmed up. Effect of heat treatment on the surface morphology and organization structure of electroless Ni-W-P alloy coating and effect of the sodium tungstate content in the bath on the composition and surface morphology of the Ni-W-P alloy coating are studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy spectrum analysis. The hardness of the coating of different tungsten content and heat treatment is tested by microhardness instrument. The corrosion resistance of different coatings is researched by the method of total immersion method.The results show that with the increase of sodium tungstate content in the bath , W content in the coating increases first, and then reduces. This illustrates that the W content in the coating can't be very well raised if only the sodium content of tungsten increased. The corrosion resistance of Ni-W-P alloy coating is investigated by total immersion method. The results show that the corrosion resistance of the coating enhanced with the increase of W content. The results of scanning electron microscope show that with the increase of W content in the coating, the blow-up of granular cellular become more dense and Ni-W-P alloy coating gets more uniform. With the increase of temperature, coating grains grows up first, and then become blurred. X-ray diffraction results show that the structure is amorphous in the state of as-plated. After 300℃heat treatment, the structure of the coatings become mixed crystal. When the temperature rises to 400℃, the structure become crystalline state. With the increase of the temperature, Ni-Fe phase appears in the coating. This illustrates coating and matrix interdiffuse. The hardness of the coating increases with the increase of the W content. With the increase of heat treatment temperature, the hardness of the coating first increases and then decreases. At 400℃, it reaches the largest value 1120HV. The corrosion resistance of the coating enhances first and then decreases with the increase of heat treatment temperature. With the increase of temperature of corrosive medium, the corrosion resistance reduces. In the same state, with the increase of the acid concentration, corrosion resistance reduces, but when the acid concentration increases to a certain degree, the corrosion resistance is strengthened.