Irregular zinc coverage

[gregsolis]

I'm getting some areas of black streaks. Some areas I don't think are getting any zinc at all (when I try to do the Yellow chromate it is very patchy).

I've been using Aluminum oxide to blast with, I don't have beads. The surfaces are pitted a little, but I'm not really worried about the appearance so much as rust protection.

Is it the aluminum oxide blasting that is causing the problem?

I'm also having a tought time estimating square inches, but I'm trying to get close to the 1 amp/40 sq. inches the manual recommends.

Do I need to re-do a dummy?

[seanc]

Re: Irregular zinc coverage

Greg:

Blasting with AlOxide should not be a problem by itself. Did you thoroughly degrease the parts afterwards?

Is your picture before of after chromating?

I'm guessing (based on the Sharpie pen) that those long parts are about 10-11" long, by 3/4-1" wide? If so, that puts them @ 15-20 sq-in. each. How much current did you use? How long did you plate it for?

Your hanger wire looks thin, what gauge is it? It might not have enough contact area to pass the required currrent.

Are you plating all 3 pieces at the same time? How much anode area do you have? If you have the normal Caswell anode (8x4) cut in half, then you might have about 28" of immersed anode area?

If that's about right, then you can't do all 3 parts at the same time. Your parts should not exceed the immersed area of the anodes.

Try just one piece at a time. Plate it at .8-1 amp/10 sq-in (80-100 mA/sq-in), for 10-15 minutes minimum. I know, that's 4 x what the manual says, but it works better. That's what I plate at, check my samples:

Zinc plating samples

Sean

[gregsolis]

Re: Irregular zinc coverage

Sean, thanks. Your pics look nice, I'll take your advice

I did all three of those parts at once.

I used about 4 amps for 60 min (roughly 60). I did the 60 min since the manual says chromate needs 0.001" to work.

The wire is 24g, I also have 16g, would that be better?

I degreased it before I blasted it.

If I try to re-do it, can I just degrease again and then put it back in the tank?

Oh, I'm using the electrodes that came with the kit (3 gal kit), there are two of them that look to be about 4x8" (using both, one on either side of the tank).

[seanc]

Re: Irregular zinc coverage

Greg:

I used about 4 amps for 60 min (roughly 60).

How many sq-inches did you estimate the parts at? If my guess (based on your pictures) was close, this should have been about right, perhaps a bit high. What are the actual dimensions of your long pieces?

I did the 60 min since the manual says chromate needs 0.001" to work.

60 min. at 100 mA/sq-in would give you about .001" plate, but I never go that long. .0005 is about the thickest you find on commercially plated stuff, and often thinner than that.

I plate .0005 on brackets and such, but only .0002-.0003 on nuts and bolts, since heavier plate tends to make threads interfere.

Chromate works fine even on these thin coatings.

The wire is 24g, I also have 16g, would that be better?

Probably, but not for the reason you might think. 24 ga. is good for 2-3 amps, but loosely looped like you have, results in a very small contact area with the part. Using larger ga. wire gives you more contact area able to carry the needed current.

If I try to re-do it, can I just degrease again and then put it back in the tank?

Not if it's been chromated already. You need to strip at least the chromate off in a mild acid (5% muriatic). I've tried a few times to put fresh zinc over old zinc, and it hasn't worked very well. If I have to redo a part, I'll strip it all the way back to raw steel and start over again. If you strip in acid, you don't need to degrease again. Just rinse the acid off and into the zinc tank.

I'm using the electrodes that came with the kit (3 gal kit), there are two of them that look to be about 4x8" (using both, one on either side of the tank).

Each one is 4 x 8? So you have about 7" of each in the electrolyte? That would give you about 56 sq-in of anode. Should be more than enough to do 40-50 sq-in. of parts

Sean

[gregsolis]

Re: Irregular zinc coverage

Been estimating the area of each part at 20 sq. inches (40 sq. inches total). They are almost exactly 10" long and 1" wide in the wide part. I guess with the holes and the narrow section they are probably more like 15 sq. inches each, so I may have been using too much current.

I just tried them again at about 2 amps, and I'm still getting spotty coverage and black streaks.

I just put a dummy in (4"x4" square, sanded down and degreased). So that will be 32 sq. inches, 16 per side. I have that in now at about 1 amp. I'm using the Caswell 25 amp recitifier, and it makes it hard to guage small amperages I got the 25 amp one since I also am going to start chrome plating, once I get the hang of this.

I hope my dummy will plate out any impurities in the tank (?).

I actually never did a true dummy when I first got the tank set up, so that may be my problem. I figured I'd use the first couple pieces as "dummies" since I didn't really care about appearance. Perhaps I was the "dummy" for not following the instruction......

Also, I just noticed that my anodes have some black discoloration on the surface that is facing my work pieces (not on the back side). Is this contamination on the anodes?

[seanc]

Re: Irregular zinc coverage

Greg:

Your estimations should have been close enough. I came up with the same estimate based on your pictures. Although you have the holes and narrow portion to account for, if the part is thick enough, the edges can also make up a significant surface area, and they will add back to the total. I would have used @ 1.7-1.8 amps for one long piece, but 2 should not have caused any problem.

I have to ask the usual dumb questions. Did you use distilled or deionized water to mix your electrolyte? Are the parts absolutely squeaky clean & pass the water-break test?

I'm not familiar with the 25 A rectifier. Is it a constant current unit? When you're plating a part, what voltage does it indicate?

I didn't have to do much of a dummy plate before my electrolyte came clean (1 12" piece), but I think it would depend on your particular batch.

I also get the dark smut on my anodes. I don't know exactly what it is, but it doesn't affect my plating. If I'm not going to be plating again anytime soon, I rinse the anodes in water, and most of this smut rinses off. It will come right back the next time the anodes are used though, so I don't know that rinsing accomplishes anything.

Sean

[-ego-]

Re: Irregular zinc coverage

I haven't done any zinc plating using a rectifier, but I have dipped parts in zincate in order to flash copper and then nickel and then decorative chrome. Looking at your parts in that picture I couldn't stress the connection theory more. I have had all sorts of wierd things happen to me with bad connections, brass looking parts coming out of the copper tank, etc. I used to use copper wire until I switched and made myself a really nice rack out of aluminum. You can make a rack out of copper as well or some other metal, but with the amperage you say you are using that wire is way too small plus the tiny point of the wire that it is touching your part. I wish I could give you some better ideas and ways to rack that part but i don't have any. If it has thread in it some where then your in luck. Try threading a bolt in a thread and attatching that to a metel rod. If you have no threads then its up to you, but I can almost garuantee that is your problem.

Edit: Actually I see you are only using 4 amps so that wire might be able to handle that, but I still think you need a better connection... more contact... etc.