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Author: Subject: Reactive solvent for zinc galvanized coating
chloric1
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[*] posted on 9-6-2019 at 08:16
Reactive solvent for zinc galvanized coating


I was considering neutralizing muriatic acid with excess strong ammonia to dissolve galvanized coating off of steel object while not attacking steel too heavily. Not sure about the ammonium chloride effect on steel in alkaline solutions. Anybody ever tried this?



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Ubya
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[*] posted on 9-6-2019 at 13:58


Use a solution of sodium hydroxide, it will dissolve zinc forming sodium zincate. Steel is going to be untouched if the sodium hydroxide solution is not really concentrated




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WGTR
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[*] posted on 9-6-2019 at 14:18


Quote: Originally posted by chloric1  
I was considering neutralizing muriatic acid with excess strong ammonia to dissolve galvanized coating off of steel object while not attacking steel too heavily. Not sure about the ammonium chloride effect on steel in alkaline solutions. Anybody ever tried this?


Have you tried a small sample with just muriatic acid to see what happens? The acid might not attack the steel very much at all until all of the zinc is reacted away. That’s the whole point of the zinc being there in the first place.




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chloric1
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[*] posted on 9-6-2019 at 18:22


Quote: Originally posted by WGTR  
Quote: Originally posted by chloric1  
I was considering neutralizing muriatic acid with excess strong ammonia to dissolve galvanized coating off of steel object while not attacking steel too heavily. Not sure about the ammonium chloride effect on steel in alkaline solutions. Anybody ever tried this?


Have you tried a small sample with just muriatic acid to see what happens? The acid might not attack the steel very much at all until all of the zinc is reacted away. That’s the whole point of the zinc being there in the first place.


Of coarse, been doing that for decades. Diluting the muriatic acid to 10-15% hydrogen chloride does keep iron dissolution slow enough.

As far as sodium hydroxide goes, I find it’s reaction with zinc slow and incomplete. That’s why I wanted to go the amine complex route. Zinc ammonium chloride is a good flux for steel as well. I will have to go ahead and run a test ASAP.




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chloric1
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[*] posted on 15-6-2019 at 16:20
Results...


Got a scrap screw and washer and tried my experiment. I tried with just 10% ammonia and it had little action or was exceedingly slow. I added a small amount of concentrated muriatic acid then followed with additional ammonia to keep everything alkaline. That’s when it got interesting. Ammonia does not attack zinc without and oxidizer. Even when minute amounts of hydrogen chloride are added a reaction commenced at once. It was considerably slower than diluted HCl but it removed the galvanized coating none the less. Did not experience flash rusting on fasteners when rinsing and drying. Was able to use butane torch and get characteristic bronze, blue and grey/blue heat colors normally seen on steel.



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[*] posted on 15-6-2019 at 19:18


Caution: Assuming the reaction of NH3 and O2 acts similarly with Zn as it does with Cu, the electrochemistry (which is part of the reaction) results in is a current, which can lead to hydrogen embrittlement. My take:

NH4+ = H+ + NH3

The cell generates e- which reacts with H+ as follows:

H+ + e- → •H

The so called hydrogen atom radical which will result in, to quote:

"The phenomenon of delayed failure in steel has been correlated with the presence of electrolytically introduced hydrogen." See https://apps.dtic.mil/dtic/tr/fulltext/u2/038142.pdf .

In my opinion, not likely a major concern here, but in general should be mentioned.
----------------------------------------

Related copper chemistry:

Cited half-reaction:

1/2 O2 + H2O + 2 e- ---> 2 OH- (cathodic reduction of O2 at surface of the Copper)

And, at the Copper anode, the formation of the complex:

Cu + 4 NH3 + 2 H2O --) [Cu(NH3)4(H2O)2]2+ + 2 e- (anodic dissolution of Cu by a complexing agent)

With an overall reaction:

Cu + 4 NH3 + 1/2 O2 + 3 H2O ---> [Cu(NH3)4(H2O)2]2+ + 2 OH-

Also, some interesting cited standard chemical reactions, where copper oscillates between cuprous and cupric states as noted by the source below:

2 Cu + 4 NH3 + 1/2 O2 + H2O --> 2 [Cu(NH3)2]OH

2 [Cu(NH3)2]OH + 4 NH3 (aq) + 1/2 O2 + H2O --> 2 [Cu(NH3)4](OH)2

Cu + [Cu(NH3)4](OH)2 <---> 2 [Cu(NH3)2]OH

Reference: "Kinetics and Mechanism of Copper Dissolution In Aqueous Ammonia", at https://www.google.com/url?sa=t&source=web&rct=j&...

[Edited on 16-6-2019 by AJKOER]
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draculic acid69
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[*] posted on 15-6-2019 at 22:24


NH4cl sounds a lot milder than using HCl which causes rust which if you're using for chemistry is a problem.nh4cl is used as a flux for ss isn't it?
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