Alkaline Zinc Nickel Adhesion to Hardened Steels
I am having difficulty get good adhesion of my rack plated alkaline zinc nickel to a 1566 steel thru hardened 55-62 HRC. My pretreatment consists of alkaline soak, alkaline electrocleaner ( reverse only) muriatic acid activation ( 6N) with adequate rinsing between each step. The parts are not blistering but when they are tapped together, the plating is chipped off easily. I have also tries a woods nickel strike with no success.
I don't have knowledge of this particular problem, but alkaline zinc-nickel is presumably similar in operation to alkaline zinc, notoriously difficult to plate onto hardened steel due to the hydrogen overvoltage situation. I suspect that you are going to need an alkaline de-ruster, and even then, you may need to put the Wood's nickel or perhaps a flash of cad or acid zinc on first. But you might seek advice from those who offer alkaline zinc-nickel, like Canning-Gumm, rather than relying on my guess.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
As Ted indicated, hydrogen evolution is a vital factor affecting the adhesion of Zn-Ni plating on the substrate, in addition to cleaning processes. Does the chip-off occur on the parts from high CCD region or the projecting portion on the parts? It is very important to control temperature, concentrations, Ni to Zn ratio, additive, power and plating time. If temperature is too low (<70F), caustic concentration is under the specified range, organic additive concentrations are too high, CCD is too high, plating time is too long (over 0.7mil), Ni/Zn ratio is over the specified range, it is common to have chip-off problem. Ted provide you with a mwthode, i.e. zinc-flash. I believe this process should help you a lots in solving this problem if your customer has no restriction on it.
- Grand Rapids, Michigan
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