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"How to remove rust from zinc plated steel without removing or damaging the zinc plating"
February 10, 2016
I'd like to find an efficient method for removing rust from zinc plated steel i.e. (bolts, screws brackets etc) without removing or damaging the zinc plating. I've tried using different kinds acids (phosphoric, hydrochloric, vinegar, CLR, in varying dilution ratios but the zinc is always destroyed. I was wondering about the process of chelation, where, if my understanding is correct, doesn't use acid to remove the rust. If chelation is a viable option, what is an efficient way to use this process.
car restorer - rochester, New York USA
February 10, 2016
A. The zinc was originally plated on the article to prevent it from rusting by sacrificial protection; in other words, zinc, being more active, will dissolve preferentially to protect the steel. And that's what happens when you put the article in any acid. If you're seeing red rust it means most of the zinc is already gone, having performed its' purpose. One option is to have the part replated; another is to strip the part of all coatings and apply zinc-rich paint, and a third option (often the most effective and least expensive) is to buy a replacement part.
CTO - Jackson, Michigan, USA
February 10, 2016
Q. Tom, thank you for responding to my question. I have a follow up question. I'm in the process of doing an experiment to see how quickly zinc plating will dissolve using three different diluted mild acids. Here are the acids I used: 1 pt white vinegar to 1 pt water, 1 part CLR to 1 pt water and 1 pt molasses to 10 pts water. I placed a zinc plated fastener (all the same) in each container. Here are the results so far:
I just realized I didn't ask the follow up question that I referred to in my previous post. Here it is. I would like to be able to clean and remove any signs of red/brown rust from small zinc plated parts i.e. fasteners, brackets, clips, etc. and leave as much as possible of the zinc plating intact. Based on my experiment (last post) it appears that the molasses/water will accomplish this. Do you agree or would you recommend something different. Thank you for you assistance. It's much appreciated.mary ross [returning]
- rochester, New York USA
February 11, 2016
A. Try 5 % ammonium citrate,ph 3,5 (dissolve 50 gms citric acid in 1 liter water,add ammonia until pH is 3,5!!!).
February 19, 2016
Q. What about rust removal from the I.D. of tubing that has been zinc plated. The integrity of the plating itself on the O.D of the tube is fine. It is rusted on the I.D. because of the heavy stripping process in preparation for plating has attacked the raw I.D of the tube and thus rust. How do you remove that type of rust and not attack the plating on the exterior?Russell Ahrens
- Sturgis, Michigan USA
A. Hi Russell. I don't think you'll find any chemical or electrochemical way to remove rust without attacking the zinc plating. I think you will be limited to some sort of masking operation whereby whatever you do (sandblasting, acid dip, alkaline derusting) must be restricted to only the rusty bare steel inside of the tubing area.
In the future it may be possible to phosphatize and oil the inside of the tubing immediately after plating. That would be because the aggressive rust-removing acids would be applied before the zinc plating process. That is, the potential process might be: soak clean, electroclean, acid dip, zinc plate, chromate, phosphatize, oil (with rinses between each step).
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey