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Attack upon substrate from chrome stripping

I have recently had 4 injection molds stripped of their chrome coating in anticipation of mold repairs. Our plan was to perform the repairs and then rechrome. Three of the molds are made of semi-prehard machinable steel (NAK 55). One of the molds is a 420 stainless. The molds were originally polished to a #1-#2 finish before the chrome was applied which is considered to be of an optical type polish (very reflective). When the molds returned from stripping the chrome was gone and the remaining steel polish looked original except for tiny pits that were not instantly evident. Upon further inspection we found that all four molds had the same pits. The surface appears as if it was glass beaded from 5 feet away. The pits almost look as if they were applied on purpose because they cover the surface so consistently. I am curious if this type of pitting is unusual. Our business is also involved in nickel plating so I am surprised that the shop that stripped the molds was not prepared to prevent such a problem. Any comments concerning the likelihood of this type of pitting will be appreciated. Thanks.

Dan Friel, Jr
Plating shop - Avondale, Pennsylvania
2007

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2007

Dan,

Have you got any pictures of the problem? I must say that if the pattern of pitting is consistent over the whole surface then it sounds like something odd it going on.
I would also suggest that you talk to the company that didd the stripping for you and find out what their exact process was. This may give you an idea of potential causes.

Brian Terry
Aerospace - Yeovil, Somerset, UK

The situation doesn't sound terrible and cetainly not unusual. This possibility must always be anticipated when stripping optically polished surfaces. Remember A-1 is around 1 microinch rough. Defects or pits as shallow as one tenth of an inch deep (100 microinches) appear like craters. Re-polish the mold begining with fine sandpaper depending on the severity of the problem and down to the required finish. It'll take time and someone will have to pay for it. If you are running out of time, consider an electroless nickel plate instead of chrome. Though a little softer, EN levels many imperfections which means less pre and post-polishing. With chrome, you absolutely need to go down to A-1 on the steel to get an A-1 chrome.

Guillermo Marrufo
Monterrey, NL, Mexico
2007