Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Randy Taylor, Advanced Tooling Corporation
Plated Surface Integrity
Q. We are specifying 0.5 mil hard chrome on 316 SS valve balls for metal-to-metal seating. The balls are from forged bar stock, machined, ground and polished prior to plating. Occasionally the finished surface exhibits clusters of porosity that are deemed excessive and are rejected. What may be the likely causes and appropriate remedial actions?
A. Steve, Review your pre-plate steps. Check to determine if:
- Polishing residue is being completely removed before plating.
- Good handling practices are being followed, clean cotton gloves or suitable rubber gloves, free from lint, oil and other contaminants. Shop carts with clean paper trays, etc.
- Part(s) were given a warm up period in the chrome tank before plating.
- Did part(s) receive reverse etch in the chrome tank prior to plating? If so how long and at what current/voltage? If not chrome, what then?
- What is the type, the approximate size and shape of the anode? Conforming or stick?
- After reverse etch, how was forward plating current applied? Voltage/Current brought up gradually in increments or some other technique?
If the defects are isolated in a small area, "clustered" as you put it, surrounded by large areas of good plating, suggests the part surface may have come into contact with something resulting in a scuff. Porosity in chrome could be caused by a contact burn.
Try analyzing the affected area under magnification. With a 30X viewer, you may be able to determine a cause.
If you get back to me with more info, I may be able to help you further .
Thanks - RT