Surface Electroplating Treatment Process for Powder Metallurgical Parts

Powder metallurgy parts, especially iron based parts, are coated with a layer of zinc, nickel, chromium, or copper on the surface, which is both aesthetically pleasing, corrosion-resistant, and rust resistant. Therefore, electroplating methods are increasingly being adopted. However, the electroplating of powder metallurgy parts has certain difficulties, mainly due to the presence of pores in the parts. Direct electroplating of powder metallurgy parts results in poor surface quality, with excessive pores that cannot be plated, and the electroplating solution that penetrates into the pores cannot be cleaned, causing internal corrosion. Therefore, powder metallurgy parts require sealing treatment before electroplating, that is, sealing the pores on the surface of the parts.

There are several methods for sealing holes on the surface of powder metallurgy parts:

1. Mechanical sealing method. The parts have a large finishing allowance and surface pores are closed through finishing. Other mechanical methods can also be used to deform and seal pores on the surface, such as sandblasting, grinding, and polishing.

2. Solid substance blockage method. Immerse the parts in molten organic compounds such as stearic acid and paraffin wax, allowing them to penetrate into the pores for sealing.

3. Steam treatment method. Steam treatment can form an oxide film on the surface of the part, thereby sealing the surface pores.

4. Melting infiltration method. Using low melting point metals for melt infiltration sealing, such as copper infiltration.

5. Passivation liquid filling method. Immerse the parts in a solution of 2g/L baking soda and residual water to fill the pores with passivation solution.

6. Organic silicide sealing method. Immerse the parts in a carbon tetrachloride solution containing 4% organic silicides. The parts are preheated to 200 ℃ and then dried at 200 ℃. This can form an organic silicide film on the surface of the part, which can prevent the electroplating solution from immersing.

After sealing the holes of powder metallurgy parts, electroplating is carried out. The electroplating process flow is as follows: powder metallurgy parts (dried at 180 ℃) - chemical degreasing (2min) - electrolytic degreasing (2min) - hydrochloric acid solution corrosion - (1min) - sodium cyanide (10%) - cyanide copper plating (2h as needed) - hydrochloric acid solution corrosion (20%) - zinc plating (as needed) - nickel plating (as needed) - chromium plating (as needed) - hot water flushing - drying.

In recent years, Dacromet surface spraying technology has emerged for powder metallurgy parts. Dacromet surface spraying originated in European and American countries in the 1950s and initially belonged to military classified technology. Later, it gradually became popular worldwide and domestically. There are various types of Dacromet spraying fluids, including chromium containing and chromium free according to environmental requirements. The most important advantage of Dacromet compared to electroplating process is its high temperature resistance and strong corrosion resistance (7-10 times that of zinc plating).