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Pickling inhibitor, and hydrogen embrittlement of screws
Q. We are doing ammonium chloride based acid zinc plating to screws/fasteners. Our process, is get wire, then wire drawing, annealing, phosphating, skin pass, heat treatment & then zinc-plating.
We have got complaints from customers for hydrogen embrittlement. Cracking of screws, etc. I have strong feeling that this hydrogen embrittlement must be taking place in plating line. Please explain: what is hydrogen embrittlement & how can I avoid it. Also how to test for hydrogen embrittlement (any simple test).
Regards,
SANTOSH ZOPE- Dubai, UAE
2002
A. Hi,
Hydrogen embrittlement has been known for decades to affect high strength steel, not commercial low carbon steel, by sudden rupture. It happens when Hydrogen ions penetrate steel microstructure during pickling, acid treatments, plating, etc. They interfere with the order of molecules and crystals. To avoid, a bake at 200 °C for 1-2 hours is effective for small parts.
Guillermo MarrufoMonterrey, NL, Mexico
2002
A. Hi,
Please check the pickling process; the solution must contain inhibitor. Then check the current density (the current per area of work piece); this is an important factor that affects hydrogen embrittlement (at the surface of work the bubbles of hydrogen are seen and the efficiency of your solution is decreased). If this problem occurs you must decrease your applied current. And finally you can sustain the screw at 200 °C for 1-2 hr.
Best wishes,
Shapour abd sharif abadi- Tehran, Iran
2002
Q. Hello,
This is further to my question on Hydrogen Embrittlement. I have observed in our process that, as our processing screws are heat treated & have burnt oil on the surface, when we process this in acid zinc plating (ammonia -ammonium chloride) & if surface is not properly cleaned then there is reaction (vigorous bubbling) observed in barrels. But same screws when thoroughly cleaned, then there is no such reaction occurs.
As there must be formation of hydrogen gas in plating bath occurs simultaneous to coating formation, there must be lot of chances of hydrogen getting diffused in metal structure of screws. Many people says that it is due to acid pickling, but I strongly feels that it is due to plating chemical bubbles.
Can anybody advise me for the same.
Regards,
SANTOSH ZOPEwire - DUBAI, UAE
2002
A. Dear Santosh.
Hardened, heat treated material tend to absorb Hydrogen during several wet processes employed in metal finishing field. The processes of acid pickling ,Electroplating are prone to generation of hydrogen and its absorption in metal surface.
Electroplating is cathodic phenomenon and during electrolysis hydrogen is liberated at the cathodic surface.The plating solutions with lower efficiency like Chromium plating and cyanide zinc plating etc are worst for hydrogen absorption and subsequent embrittlement.
It is safer to minimise the exposure of component to such situations like acid pickling. Use of more efficient options for plating e.g. acid zinc plating are to be preferred.
In your case, improper cleaning has resulted delayed covering of screws with zinc deposit and till screws are covered with zinc, hydrogen generated can play havoc with uncovered portion.Once the surface is covered with zinc intensity of absorption greatly reduces. Even though you are using acid zinc delayed coating creates situation similar to that of low efficiency cyanide zinc plating.
If hardened material is to be processed De-embrittlement by giving suitable heat treatment can be tried. Best Of luck, Santosh.
- Mumbai, INDIA
2007
Q. Is inhibitor effective to avoid Hydrogen de-embrittlement process in Zinc plating?
Jayakumar Selvaraj
metal finisher - Chennai, Tamilnadu, India
February 28, 2012
A. Hi, Jayakumar. Inhibitors in the pickling step may be of some effectiveness in reducing embrittlement in that step and should be used, and anodic electrocleaning may cause less problem than cathodic -- but you still need to bake hardened parts after zinc plating. Good luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 28, 2012
Thank You Mr. TED MOONEY....
Jayakumar Selvaraj
metal finisher - Chennai, Tamilnadu, India
February 29, 2012
Q. Hello.
I am writing to you to get some information about pickling inhibitor and also know that hydrochloric acid pickling will almost certainly cause hydrogen embrittlement. I need to obtain some documentation about them to avoid hydrogen EMBRITTLEMENT during plating. I was surfing about this problem on the internet and I have seen about these materials in plating industry. Could you help me to find a proper pickling inhibitor?
And I want to know important factors to choose one of the proper inhibitors?
- saveh, Iran
July 11, 2012
Q. What's the effect of inhibitor for pickling on tension Spring, and how much proportion of inhibitor in acid?
Also I tried baking the spring at 200 °C for 2 hours but hydrogen de-embrittlement test (i.e., bend test) was failed.
Please suggest. And also please send me the plating process for springs.
Sachin Industries - Jalgaon, Maharashtra(India)
October 5, 2014
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