Bubbles under the finish - zinc/copy cad kit

[phk1967]

On a few engine parts/accessories I plated I noticed some small bubbles appearing under the fresh finish, how can I prevent this from happening and should I replate the parts or will the finish be durable even with the bubbles.
Thanks.

[XK120DHC]

On a few engine parts/accessories I plated I noticed some small bubbles appearing under the fresh finish, how can I prevent this from happening and should I replate the parts or will the finish be durable even with the bubbles.
Thanks.

It might help to know what "few parts/accessories" you are trying to plate are made of, their total surface area, the amps you were running, for how long as well as your pre-plate cleaning routine.. Otherwise, the goldfish in your plating tank might have farted and caused the bubbles.. ;-}

[JHudgens]

I would also like to know what causes bubbles. I have been plating for several years with the old Copy Cad bath and occasionally I get bubbles also. The parts are of course steel and I always sand blast and dip in Pickle #1 before plating (among other cleaning steps). I am not sure but it may be related to old zinc or cadmium plating on the part. Sometimes its difficult to know if you have all of the old plating off a part since when sandblasted bare steel and fresh zinc have a similar experience. I know my problem is not directly related to cleaning since I go to great extremes to get my parts clean before plating.

Jerry Hudgens

[woodjames]

The majority of the time blisters arise from areas that still contain oxidation. Depending on how much scale is built up in a pit, pickel 1 usually isn't strong enough to remove it. The other related portion of the equation is that rust on steel tends to burrow, rather than spreading out across the surface as a passive film - such as in the case of zinc or aluminum for example. The problem with the "burrowing" is that the deepest areas become so fine that capillary action retains cleaners, acids, plating solutions. They just get sucked up into there - with such a strong force that immersion in a rinse tank, or pressure assisted rinsing like a spray bottle will not remove it. That's where ultrasonics come into play. But even then, if the acid isn't strong enough to remove all of it, there's still oxidation there. It is impossible to adequately plate over oxidation. It's too loosely bonded, that even if you managed to get plate to take over it, it'll break free. In the case of capillary action, the plate can build/bridge across the oxidation if it's small/fine enough; essentially trapping a corrosive solution underneath the plate (contained in the pit itself), which continues to eat at the base metal until it becomes too saturated with metal, which can increase with heat.

This really doesn't come into play with new parts, mainly where the elements have created a pin hole in existing plate, which allows attack on the base. Looking at original blistering/rusting out plate - Capillary action again retains the corrosive agent, which just makes things worse. Zinc baths aren't acidic enough to be a strong indicator of existing corrosion. Acid copper for example, with a ph in the range of 0-1 will really attack the oxidized portion, and it's indicated by black streaking/bleeding out around pin head sized black dots (the corrosion); which usually shows up about 5 minutes after pulling the item from the bath.

How to get past it? More aggressive pickling helps (usually cathodic pickling in a weaker concentration), along with simply drilling out any black specks that remain. Another part of the equation with small deep pits is that the area is simply too low of a current density area within the pit itself, and really won't develop any plate. By opening the area up you increase the current density of the area.

Another way to help identify remaining corrosion is to use a strong pickel, and rather than quickly rinsing things off, hold it suspended above the pickel, still covered in the acid. If you see black streaks develop - look close enough and they'll radiate from a small black dot - get rid of that dot!

See ya.

I would also like to know what causes bubbles. I have been plating for several years with the old Copy Cad bath and occasionally I get bubbles also. The parts are of course steel and I always sand blast and dip in Pickle #1 before plating (among other cleaning steps). I am not sure but it may be related to old zinc or cadmium plating on the part. Sometimes its difficult to know if you have all of the old plating off a part since when sandblasted bare steel and fresh zinc have a similar experience. I know my problem is not directly related to cleaning since I go to great extremes to get my parts clean before plating.

Jerry Hudgens