Sintered Metals

Q. Many of our steel parts are zinc plated after manufacturing. Our zinc plater is having a heck of a time with a part that is made from sintered steel. The shop does a good job with all of our non-sintered parts. Do you have any advice for solving this problem? K.M.V.

 

A. Sintered parts can be very difficult to plate. Here’s why: a sintered part, most commonly copper or steel, is made from powdered metal that is placed in a furnace and heated to a temperature below its melting point. Pressure is applied, and the metal particles adhere to each. The finished product is porous and has what I like to call “nooks and crannies.” If the part is plated, it goes through a process using various water-based solutions, and some of these liquids are trapped in these nooks and crannies. The part looks beautiful after being plated, but shortly after the part starts showing dark spots and ultimately is a reject. Why? The trapped liquids are reacting with the plated deposit and are reacting with the plated layer.
How can this problem be minimized? The general thing to remember is good rinsing is required between each step of the plating process. Since I don’t know the exact process used by your plater, I can’t comment in detail. Using alternate warm and cold rinses can lesson this pitting. This “pumps” the pores to a certain extent and helps reduce spotting. The first rinse after the plating step should be at the same temperature or warmer than the temperature of the plating bath.
In cases where the parts are of high value, the part may be subjected to a process in which the pores are filled with an inert material using a vacuum process. If the surface pores are filled, liquid will not be trapped on the surface and the part should plate in a satisfactory manner.