Cast iron in zinc

[kenrinc]

I'm trying to plate a cast iron brake caliper. I see a note in the new manual stating 20amps per sq ft with cast iron. Got that. I'm just having some issues getting a "ballpark" surface area calculation. How do most of you measure weird parts like this. There are a lot of knooks and crannies. I've masked off the brake cylinder bores with paint but intend to plate the rest. I thought about taking a sheet of paper and wrapping it around the thing to get an idea of sq in?

The last time I tried this the electrolyte just frothed like crazy at 16amp. I could never see the part the entire time. After I was done I took the part out and the zinc had only covered the higher areas and left grainy deposits elsewhere. The tank I used didn't allow for much space so the annodes were very close to the part. That may have had something to do with it? After an hour the platingin began to bubble and crack. I thought I had prepped the part well by sandblasting, pickle, SP degreaser and then rinse.

Ken-

[seanc]

Ken:

I've never seen a "new" manual, so can't comment on why they would recommend a higher current density for cast iron. However, it's more realistic than the old manual. 20 amps/sq-ft comes out to 140 mA/sq-in, while the old manual said 25 mA/sq-in. I've done a few cast iron parts at 80-100 mA/sq-in and they came out fine. When I ran tests on steel at 150 mA/sq-in, it was worse.

Figuring out EXACT surface area of something like a caliper is near impossible, but here's what I've done for carburetor bodies (also got lots of nooks & crannies & weird shapes):

Take dimensions of each of the 3 major sides, like you'd see in a 3-view mechanical drawing. Calculate each side, add the 3 together, then double it. Plate it at the high end, 100 mA/sq-in. Any extra surface area added by the nooks & crannies should still keep your actual current in the 70-100 range.

Keep in mind that the amount of "frothing" you see depends on both the current density AND the size of the part.

For instance, a 1" piece will hardly produce any frothing at all, but a 100" piece will produce LOTS of it, even at the SAME current dentsity, simply because it's giving off 100 times as many bubbles.

If your anodes are too close, you won't get good coverage. You might try rotating the part, so different sides get better exposed to the anodes.

So just how big is the caliper, once you get a good estimate? And then how big are your anodes? If the anodes aren't at least as big as the part, you won't get good coverage either.

If your plating is cracking, then it's a good bet that it's being overpowered & burnt. Electricity will take the path of least resistance, and if the anodes are too close (how close are they?), then all the plating is going to the nearest portions of the caliper, leaving other parts uncovered.

You need a few inches of separation even for small parts. Larger parts need more for even coverage.

Sean

[kenrinc]

Thanks Sean. Caliper is roughly 8" long, 6" wide and about 5" tall on one side. Sort of complicated by the mounting flange. I've lacquer masked the piston bores. The only reason I ask about the frothing and current density is that I believe I completely "wasted" my last zinc bath doing one of these things. I had calculated roughly 130sq in for the caliper and the foam was just so intense that after I was done I was left with this pile of yellow like shavings at the bottom of the tank. After that, the bath pretty much died. If I try to plate any part now, the part just turns black or dark grey regardless of current.

I sandblast my zinc annodes and assume they last until they are dust? I have about 6 of them but I calculate that if each side of the annode is in the bath then I have more than enough surface area. I actually used 4 when I tried the caliper. The annodes were probably 1/2 " away in some areas. The reason is just logistics; all I had is the small round 2 gallon bucket. This time around I'm going to try a 5 gallon bucket. The problem is the 1.5 gallon kit only works within certain containers. If you use a larger container, that means the bath will be shallow. I bought 3 gallons of bath this time so we shall see what happens. Thanks for your help.

Ken-

[seanc]

Ken:

If your dimensions are close, I come up w/236 sq-in. Adding in for nooks & crannies, and center cavity, might bring it closer to 280-300. You'll need 15-20 amps at least.

Do your anodes add up to 250-300 sq-in? They may be too small even though you've got 4 of them. And 1/2" separation is WAY too small.

Don't know for sure what your yellow shavings would be. After a number of plating runs, I'll build up a yellow sediment in the tank, which doesn't seem to affect plating, but I filter it occasionally anyway, just to be able to see the parts better.

A couple of options:

1) Don't try to plate the whole thing at once. You can plate one half, rinse it, flip it over and plate the other half. It shows a parting line where the 2 halves plated, but most of that will disappear if you acid bright dip afterwards. Don't touch the raw zinc on the first half with your fingers! It will etch your fingerprints into the zinc. Wear rubber gloves if you have to handle to part while flipping it.

2) Put all your anodes on one side of the tank, and your part as far away on the other side as possible. This gets you maximum separation. You will have to rotate the part manually, so that each side gets even coverage. I've done this with larger parts.

3 gallons will certainly be easier to work with, and a rectangular tank helps a lot too.

I don't see any need to blast the anodes. All I do is rinse them off when I'm done, and hang to dry. They do turn dark where they've been immersed in the electrolyte, but it has never affected the plating action. I used a scotchbrite pad to clean them once, but it didn't make any difference.

Sean

[kenrinc]

Sean

I'm not opposed to doing each side separately although I had tried it before. I wasn't too succesful because as I plated the 1st side the 2nd side would rust and turn green as the 1st side plated. I'd then have to re blast and clean before I could get it plated. Because I was hanging the part vertically, the side that needed good plate was toward the bottom (opposite the annodes) and the bubbles would load up and I just could not keep them off with the setup that I had. Now that I have the larger tank and better air stone setup I may have more breathing room.

What do you use for the zinc bright dip? I haven't done that before. I yellow chromate everything.

Ken-

[jrow]

Brake Caliper.
I don't know if this will work but its just my thoughts.

If you take a round container with parallel sides, say 18" tall and 12" in diameter. Put a measured amount of liquid in it to cover the caliper and figure your square inches that way.

IE; Lets say you have 10" of liquid in your container and when you add the caliper it moves the liquid up to 13". Figure the volume of the CYLINDER at 10" and then figure the volume of the CYLINDER at 13". This will give you two answers in CUBIC INCHES. Subtract the two answers (big from little) and that will give you the CUBIC INCH size of your caliper. Divide that answer by 9 that will give you square inches.

Do a Google for that formula for a cylinder.

Hope this helps.

John

[jrow]

Originally Posted by jrow

Brake Caliper.
I don't know if this will work but its just my thoughts.

If you take a round container with parallel sides, say 18" tall and 12" in diameter. Put a measured amount of liquid in it to cover the caliper and figure your square inches that way.

IE; Lets say you have 10" of liquid in your container and when you add the caliper it moves the liquid up to 13". Figure the volume of the CYLINDER at 10" and then figure the volume of the CYLINDER at 13". This will give you two answers in CUBIC INCHES. Subtract the two answers (big from little) and that will give you the CUBIC INCH size of your caliper. Divide that answer by 9 that will give you square inches.

Do a Google for that formula for a cylinder.

Hope this helps.

John

The above post is incorrect I apologize for the error.


John

John

[seanc]

Ken:

I never ran into the 2nd half corrosion problem. Then again, I've only done it twice, on steel parts. Is your caliper cast iron? That's probably more susceptible to flash corrosion.

What do you use for the zinc bright dip? I haven't done that before. I yellow chromate everything.

Initially I used muriatic or sulphuric acid at @ 3-5% solution. You can see the effect on these parts:

Spring plate and U-Bolts

I'm now using nitric acid at @ .5-1%, and it's MUCH better. It will help some even w/yellow chromate, but not as much as with clear chromate. Problem is, nitric is difficult to come by, and expensive to mail order.

Sean

[kenrinc]

Sean,

I gave up on the caliper. It was too much work and I spent way too much money and in the end, each time the failed attempt killed the bath. A 1 ohm 150w power resistor was not big enough to handle the 10amps I was drawing and I blew that up. It's almost as if the bath get's contaminated after trying. The caliper is dead clean, passes the water break test add infinitum but same result everytime. I have no other problems with any other parts, I think these are just too big for what I'm capable of doing at home. I've tried doing just one side at a time and even then it doesn't work well. I'm taking them to a local plater in the morning.

Ken-

[eprigge]

My 2 cents. It sounds like your current density was way too high. Even with the right current for the part area calculated, if the anodes are 1/2 in. from the part the current density in that vicinity will be much higher.

Really high current density will do a few things: The zinc plated will be 'burnt', dull, grainy, dark, etc. Zinc can't dissolve fast enough from the anodes so some oxygen is formed instead, it will oxidize the brightener chemicals resulting in some unwanted organics in the bath. The pH of the bath will change, you may need to adjust it back into the right range.

You may be able to save the bath if you carbon treat and then adjust pH back to normal. Caswell tech supp will know best.