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topic 45039

Hydrogen Embrittlement During Stripping Process



A discussion started in 2007 & continuing through 2017

(2007)

Q. My company has low carbon (1022) heat treated fasteners that have been electroplated with yellow zinc dichromate. Our customer wishes to a different coating in place of the electroplated yellow zinc dichromate. If the fasteners are stripped using a low temperature hydrochloric acid bath to remove the electroplate, what potential is there to induce hydrogen embrittlement if the fasteners are not baked after the stripping process? If hydrogen embrittlement can be induced in the stripping process, what processes are available that would eliminate this hydrogen embrittlement potential?

Greg LeCompte
Product Designer - Bryan, Ohio, USA


simultaneous (2007)

A. As soon as you say fasteners, you are in another world from conventional plating.
1022 is normally not heat treat hardened to a high hardness. Different industries have different hardnesses where they demand de-embrittlement. Strip will certainly induce embrittlement. Typically, if it is below Rc 36 , it is not de-embrittle treated. The Rc will depend on your customer's call out.
The treatment is typically 2-4 hours at temperature in an oven at 375 °F.

James Watts
- Navarre, Florida


(2007)

A. Yes, stripping can induce hydrogen embrittlement. However if done quickly and removed from the acid as soon as stripped, there may be no problem. Hydrogen embrittlement can be relieved by baking @ 375F for four hours, but is usually necessary only if the parts are RC 39 or harder, which 1022 probably isn't.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg,
      South Carolina



(2007)

A. Exposure to HCl acid will create potential for hydrogen embrittlement. Baking is the required method for embrittlement relief. Standards such as ASTM B 850 or SAE/USCAR-5 should be referenced regarding embrittlement relief procedures.

Toby Padfield
Automotive module supplier - Michigan


(2007)

A. Have you considered an ammonium nitrate stripping process instead of acid? Seems that should not induce hydrogen embrittlement therefore you need not worry about bake?

Douglas A. Hahn
- Saint Charles, Missouri


(2007)

A. Good afternoon:

Just wanted to let you know that there are many heat treated fasteners made from 1022 that require baking. The two types of fasteners that come to mind are self drilling screws per SAE J 78 and tapping screws per SAE J 933. These screws, in addition to having a core hardness typically mid 30s HRC are also given a shallow case.

Steve Bizub
- St Louis, Missouri


(2007)

Q. Mr. Bizub,
I didn't know the carbon level in a 1022 steel was enough to reach that hardness. More so inside, for there is also the cooling rate issue. Could it be the case treatment you mentioned alters its chemical analysis rendering something that is not 1022 anymore? Or could it be due to some work hardening effect?

Guillermo Marrufo
Monterrey, NL, Mexico


(2007)

A. Good afternoon, Mr. Marrufo:

Many of these fasteners have a small major diameter. A fairly fast oil quench will harden out 1022 Si killed fine grained fasteners if they are #12 or smaller. Above that diameter, 1022 Si killed coarse grained steel is often used to give added hardenability. The self drilling, tapping, and thread rolling screws typically have a case only about 0.004-0.010" deep, depending on the major diameter of the screw. There is no work hardening effect; they are fully austenitized when they drop into the oil quench.

Steve Bizub
- St Louis, Missouri


December 2, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi,

I have tried to use HCl to strip zinc from my motorcycle bolts so I could Nickel plate them. I may have done something very wrong and I need some help to understand what is salvageable.

As I blasted the parts prior to stripping the zinc from them I was getting bad nickel plating results as I imagine I had got zinc into the metal by blasting it.


For that reason I have tried different HCl approaches to strip the zinc:

Approach A:
Some of the bolts have been dipped into muriatic acid (25% HCl 75% distilled water) for a COUPLE HOURS hours. Visually they seem fine.

Approach B:
Some of the bolts have been dip into muriatic acid (25% HCl 75% distilled water) for 5 DAYS. Visually they seem fine.

Approach C:
Some of the bolts have been dip into 100% HCl for a COUPLE HOURS. Visually they seem corroded, brittle and light in weight. Completely ruined.

Based on that I have TWO questions:

Question 1 - As these fasteners are a structural part of the motorcycle and C seems to have completely ruined them. I would like to know if the approach A & B although the parts look fine may have made them weaken by hydrogen embrittlement or something else?

Question 2 - And if I did cause some weakening is there a way that I as a Hobbyist could test or detect it?

Gui Maia
Hobbyist - Wellington-New Zealand


December 2017

A. Hi Gui. Are you sure that the original coating was zinc plating? That should have come off in about a minute, 100X less time than your most conservative approach and 7000X times less than your second approach :-)

All of your hardware is probably dangerously compromised, and needs to be replaced. I think that what a hobbyist should probably do instead of impact testing is to always immediately bake the fasteners after rinsing. Don't use your kitchen oven though, because only food should go into a kitchen oven.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



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