Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Randy Taylor, Advanced Tooling Corporation
Hard Chrome Plating Problem
Q. When I machine near the surface of the hard chrome plating area the plating is peeling off. Is this problem related to machining or plating? If plating what could be the reasons?
A. Poor adhesion due to pre plate problems may be the cause, but aggressive grinding on hard chrome can also lead to trouble.
1.)review all pre-plating steps to ensure that base material is prepared correctly to ensure it is clean, free from paint, grease, rust, scale, masking residue and surface defects. If allowable, parts should be polished or grit blasted before plating. Keep in mind there is a time limit between polish/ grit blasting and plating, rule of thumb not to exceed 1 hour. Keep parts covered and clean. Use clean gloves when handling parts before plating.
2.) Verify that the anodes and tooling, (i.e., plating rack frame, buss bar hook ups, cables and other set up hardware) are all cleaned and in good condition. Many plating problems come from bad connections and poor current flow to the part and anodes. Use a hand held DC amp meter to gage connections to ensure that the DC current is flowing properly to all components. Look for hot spots and poor connections.
3.) In hard chrome plating, the initial reverse etch is the most important part of a successful plating process. Many times platers go through the routine steps for reverse, but in actuality may not get a thorough reverse. For most ferrous alloys, the reverse etch process should occur at 4.5 to 5.0 volts for 1-2.5 minutes depending on part size and anode spacing. A good reverse etch should impart a dull gray mat appearance, with no water break. A skilled plater may raise the part from the bath momentarily to verify this. He may also observe the gassing bubbles and plating action at the surface of the solution to verify success. Timing the reverse is very important. Large parts need a little more time for the current to reach all areas of the part. Bath chemistry is also crucial for a good reverse etch. What ever the reason, if the pre-plate etch is too short, uneven or incomplete, this can lead to adhesion problems and pitting.
There are of course, many other causes for poor adhesion, such as chrome over exotic alloys and high heat treated metals, but we'll start with the fundamentals and see where it takes us.