chrome oxide coating failure - 24" dia. steel roll

[rconnor]

We are having premature roll failures in our cleaner bath on our tin mill. We use suppliers to apply a chrome finish on our rolls. We build up the roll based on drawing specs to a O.D of about 24", we send the roll to one supplier for chrome plating and another supplier for chrome oxide coating.

We are seeing blistering in the middle of the roll followed by chrome flaking. Oddly, only the center of the roll has the thin band (2"-3" wide) where the chrome plating is failing. Roll width is a maximum 45".

Our colleagues in another tin mill claim this is a sign of poor roll surface preparation. However, I have not had feedback from this colleague on what "poor surface preparation" means. What standard should I hold my suppliers to if I want a chrome oxide surface coating for this roll, in a caustic cleaner bath? Has anyone had a better experience with another surface coating in this application? The chrome oxide coated rolls are lasting anywhere's from less than a month to about 3 months, but not more.

[kschewe]

If i had to guess I would say the surface was contaminated with something. Is the acid bath being checked for contamination and the right concentration?

[shantanu]

I understand that chrome oxide coating is being done over hard chrome plating. Hard chrome plating has a hardness of 55 HRC, so probably a decent surface profile is not being created prior to coating it with chrome oxide...this is what your colleagues may mean by " poor surface preparation"
Also, is the chrome oxide coating being plasma sprayed or spayed using Rokide?

Shantanu

[loriolo]

We use to coat zincroll with HVOF and chrome oxide, without any problem, but it's important to apply a sealcoat on the coating.