View Full Version : Removing zinc plating
02-04-2008, 01:20 AM
So after reading the replies to “what makes a weld hard thread” thread I headed to the store to pick up some nuts & bolts to practice on. I could only find zinc coated (the bright silver zinc) in the size I need so I picked them up and decided I would try and chemically remove the coating.
I tried naval jelly & industrial strength drain cleaner (lye), I soaked the same bolt in each for an hour and all I accomplished was slightly dulling the gloss.
I thought the naval jelly would have done a good job, but apparently naught. Should I pick up some stronger phosphoric acid, or do I have to use muratic?
02-04-2008, 01:41 AM
Sulfuric acid. AKA "battery acid" will strip zinc. You need to neutralize it after with a soda wash. It will effervesce so wear protective gear (eys, gloves, clothing). When it stops fizzing, the zinc is gone. Watch the area you do it in too. A plastic drop cloth or bag under the cup or whatever to protect surfaces.
02-04-2008, 01:50 AM
Forgot to mention: It works fast! So don't leave it in there or walk away. You'll end up with a Willey Coyote burnt stick look for a rod. :eek:
It's late and I gotta quit editing. Night.
02-04-2008, 01:58 AM
where do you get Sulfuric acid locally?
02-04-2008, 02:05 AM
NAPA auto parts should have it or they can get it pretty quick. I get mine at a local family auto parts. I went in the morning, ordered it and went back at lunch to pick up. Five gallons worth! But you can get it in quart jugs. I think the quarts are about $2. Any motorcycle shop should have the quarts too.
02-04-2008, 03:42 AM
Any acid will do to strp zinc even vinegar but weak acids work slower. I used to use muratic but its fumes traveled everywhere to rust anything it could get at.
In the last few years I've used Jasco metal prep for paint prepping metal,as a pickle for descaling black iron, and to etch off zinc and galvanize. It contains phosphoroc acid that attacks scale, zinc, and to a certain extent the base metal. In doing so it somehow passivates steel so it doesn't rust - provided you scrub off the residues with a fiber brush.
Adding: a phosphoric matal prep solution needs no post-pickle neutralizing. The dingy film not only prevents rust but enhances paint adhesion. Good all around stuff for the metal shop that H2PO4.
I suggest any home shop metalworker steer clear of sulfuric and muratic acid unless you have a well ventillated space to safely use it and inform yourself of the various commercial buffering agents that prevent base metal attack. Then you have to neutralize the whole artical in hot soda solution scrubbing the metal clean of smut.
By the way, the earlier post on welding and about how it produces hard spots? All you have to do is post heat the weld and the immediate area to 1000 F or so and let it cool slowly. This partly anneals the metal to a readily machinable condition. I imagine others already made this suggestion but I thought I'd be, yet again, repetitious, some more, etc.
Your Old Dog
02-04-2008, 05:41 AM
By the way, your earlier post on welding producing hard spots? All you have to do is post heat the weld and the immediate area to 1000 F or so and let it cool slowly. This partly anneals the metal to a readily machinable condition. I imagine others already made this suggestion but I thought I'd be, yet again, repetitious, some more, etc.
Thanks for the repeat Forrest, I hadn't caught that and I've avoided using the welder when it may have served well had I known the tip.
It for that same reason that 4130 tubing is gas welded when used in aircraft. Electric welding doesn't give that built in normalizing effect.
02-04-2008, 07:21 AM
Any acid will do to strip zinc even vinegar but weak acids work slower. I used to use muratic but its fumes traveled everywhere to rust anything it could get at.
I use muratic acid too and it works very fast and does a good job. Yes, the fumes are a problem, but I have done it outside of the shop and had no problems. When done with the stripping I neutralize the acid with baking soda.
02-04-2008, 11:47 AM
I had to remove zinc from a hot dipped structural angle, 4", and used lye in a strong solution to do it. It was slow, but worked well. Took 4-5 days to remove it, had to scrub it a few times to remove a black sludge that formed on the metal. Used SS brush to scrub with.
Are you sure the bolts you are using are ZINC plated? Most hardware store bolts are Cadmium plated.
02-04-2008, 12:22 PM
Dan could the nut,s be cadmium plated .
02-04-2008, 12:44 PM
Hi guys, I've been mostly lurking here, but being trained as a chem tech, I thought I should comment.
Please be very careful with acids! Sulfuric, even diluted to battery electrolyte levels is dangerous. You do NOT want to get any in your eyes. It will eat away at flesh, and it seems to disintigrate cotton (read jeans) faster than anything else.
Hydrochloric acid (muriatic acid) is MUCH safer to work with, and in this case I think it will actually work faster.
Remember if you're diluting acid "Do like you oughter, add acid to water". This is especially true with sulfuric. It gets very hot when diluting, and can suddenly boil, spurting all over you. If it spurts mostly water it's not such a problem. Hydrochloric will also heat up, but not nearly to the same extent, and it doesn't have the same effect on flesh.
Something that is not intuitive is that acids seldom dissolve something at the fastest rate when they are concentrated. The ideal concentration depends on what you're dissolving. I don't have a reference, but I'd try about 25% first.
Metalprep, with phosphoric acid in it, is probably even safer than hydrochloric acid. And it leaves a somewhat protected phosphated surface on steel.
If you're trying to get rid of cadmium, then I don't know of anything off hand that will dissolve it. I do know that it's quite a bit more toxic than zinc.
02-04-2008, 02:18 PM
Zinc is one of those metals that will dissolve in either acid or alkali. Cadmium dissolves in nitric acid. I agree that hydrochloric fumes will get everywhere. If you have to keep it around, I suggest that you transfer it into smaller, clearly labelled, PET plastic bottles. These seem to keep pop from going flat after it is opened, so they should do a good job of confining HCl fumes. To be extra safe, put the bottle in a zip-loc bag. To use HCl for rust removal, dissolve some sodium sulfite in it to scavenge the released oxygen. That way it will not attack the base metal. There used to be a product called Chloroclen that was used by Water Utilities to clean water meters before they were overhauled-it was essentially hydrochloric acid and sodium sulfite. It works on either copper alloys or iron and steel. Another, friendlier acid is sulfamic. It is sold as a swimming pool chemical "pH Minus." It is a dry powder and dissolves readily in water. Nitric, sulfuric and hydrochloric acid are all MEAN chemicals, and nitric is the hands down worst. They are safe to use if you are careful and respectful. Duffy
02-04-2008, 02:23 PM
I'm pretty sure its zinc, as the drain cleaner (lye) dulled bolts (read started to etch the zinc). I want to try and avoid muriatic if possible, because of the corrosive gas it creates.
02-04-2008, 02:37 PM
Hey, drop'em in your tank of hot bluing salts. That'll do'er.
It sure does make ya' mad tho when ya' gotta' piss-away a whole new tank of salts............:mad:
Hold a torch to them. It will burn off. Do it outside and stand upwind. Metal fume fever won't kill you but it will make you wish it did.
02-04-2008, 08:54 PM
Well, I did not even think about citric acid. That would certainly be the safest option. You may have to heat it to get a reasonable removal rate.
Sulfamic acid is classed as a weak acid, so it should be reasonably safe. I may be mistaken, but I think it's the base for Sparex pickle for gold and silver. It's normally heated for that application, and would probably need to be heated for zinc removal at a reasonable rate.
A crock pot is probably the safest way to heat either of these, because it will not boil. You do not want it to splash. Please don't heat anything classified as a strong acid - that means Hydrochloric, Nitric, and Sulfuric to name the most common. Nitric is nasty, but Sulfuric is worse in some ways, especially when diluting it. They are both quite good at dissolving flesh and you don't want the tiniest drop of either in your eyes, so use safety glasses if you get near either.
02-04-2008, 11:14 PM
My Gawd! The Nannies are at it again. I'm surprised you guys are allowed to work around machinery. Geez, ya might lose an eye, hand, arm, leg, or some private part with blood spirting everywhere.
Send all your tools to me--They're way too dangerous for you to handle. :rolleyes:
02-05-2008, 12:40 AM
Nitric? Sulfuric? Muratic? These strong mineral acids work great but they eat holes in your jeans and they require safe use procedures and ideally some safety equipment. Also when you're done with them, what do you do with the residues? So why even bring them up? Zinc is a very active metal: any acid (even vitamin C) and most bases will etch it; zinc plating is dead easy to strip.
You guys are getting 'way too elaborate. You want a simple, safe, and cheap solution for a plain old zinc plating strip? Use an over the counter product like a proprietary metal prep solution containing phosphoric acid or white vinegar. Metal prep is quicker and vinegar is slightly less hassle. Both are down the drain disposable even un-neutralized.
02-05-2008, 09:55 AM
I picked up some concrete etch (phosphoric) last night, and it removed the zinc in about 20 minutes.
I was going to lay some test welds to turn down, but when i went over to the lathe it was covered in sweet. I spent the next hour or so cleaning everything up. It was a good thing i had soaked it with way oil after using it on Friday.
I don't know about the rest of you, but i'm getting really tired of this weather. in the last week we have had two days of 40 degree plus temperature swings, snow, rain, and yesterday it looked like this out side (97% humidity).