Refinishing Wheel Hubs

 Q. Our supplier plates steel wheel hubs with zinc. If the final plated part does not meet specifications, the parts are stripped and sent through the electroplating process again. One of our engineers suggested that this can cause problems with hydrogen embrittlement. We currently require that if the parts are stripped and replated, a baking heat treating process must be performed after the plating step. What are your thoughts on this? W.T. 

1. How many times has the part been rejected and replated? I would air on the caution side and say that after the second redo the parts should be scrapped.
2. What are the parameters of the bake cycle being used by your plater? The number of hours and temperature can make a big difference in terms of hydrogen embrittlement.
3. Are the parts replated immediately after stripping or do they sit for a number of days prior to the replating step? In the best of all possible worlds, the part should be baked after the stripping step and after the plating step. If the parts are immediately replated after stripping, you probably are okay with not doing the bake after the stripping step.
4. What is the stripping step that your plater is using? There are different methods for stripping zinc from a steel substrate with acid formulations being the most common. Placing a zinc plated part in an acid containing solution will generate some hydrogen. The hydrogen can cause hydrogen embrittlement.
5. One other factor is the hardness of the steel. The harder the steel, the more likely you are going to have problems with hydrogen embrittlement and the baking/heat treatment process must take this into account.