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topic 1748

Hydrogen while stripping?



1998

To all our finishing community.

This is my second attempt to attract your attention to an issue of interest to everyone working with high strength steels:
DANGER OF EMBRITTLEMENT WHILE STRIPPING METALLIC COATINGS.

Basically there are 2 opinions:

1) Stripping is a chemical or electrochemical process, therefore it may embrittle the parts - bake is obligatory.

2) Considering the chemical nature of the stripping operation, you may or may not produce any hydrogen, therefore you may not need to bake after the stripping.

Several aerospace plating specifications insist on bake after any stripping operation (these belong to Boeing, McDonnell Douglas, Aerospatiale, Bombardier Aerospace, GE Aerospace etc.,). In contrast, Bell Helicopter, Menasco Aerospace, Pratt & Whitney, etc., do not specify it. There is also a MIL-STD-871 -- a very good and comprehensive standard (despite one bad technical mistake!), unfortunately cancelled already, which clearly indicates several stripping procedures that do not require bake after stripping.

Who is right here?

My personal view is for approach 2:

Most of the chemical/electrochemical methods used for stripping coating from high strength steels do not emit any hydrogen while the stripping takes place: these are based on oxidation reactions versus simply dissolving in acid where the metal being stripped (and the base metal too!) substitutes the hydrogen in the acid...

Here are the examples of stripping methods that I consider as not causing any additional hydrogen:

1. Stripping of Chrome plate anodically in alkaline solution.

2. Stripping of Cadmium in ammonium nitrate solution.

3. Stripping of Cadmium, Nickel, Zinc, Tin, Copper, etc. from steels in alkaline salt of nitro-sulphonic acid mixed with cyanide (there are several proprietary and generic variations of this).

4. Stripping of silver plating from steels by anodic treatment in cyanide solutions.

5. Stripping of silver, nickel, cadmium, copper, etc. from stainless steels (in case you wander, there are several high strength stainless steels susceptible to embrittlement - mostly PH series) by immersing in ~50% nitric acid.

In fact, I am not sure if I can name a stripping method that anyone concerned with embrittlement would use on high strength materials which will emit hydrogen while stripping.

Although I am not prepared to give a detail explanation of chemistry involved in some of these methods (serious research needed for method 2), I think that methods 1, 4 and 5 being anodic will not be a problem at all to explain why there is no hydrogen. For method 2, I think the resulting chemical reaction is as follows:

12NH4NO3 + 5Cd = 5Cd(NO3)2 + N2 + 2NH3 + 6H2O

Most probably there the mechanism is such that N2O is formed due to the decomposition of NH4NO3 at the surface of Cd (see below) which is very unstable and disintegrates rapidly oxidizing the metallic Cd. When steel surface is reached, the passivating action of NO3 prevents further reaction and steel is not corroding.

Decomposition of ammonia nitrate:

2NH4NO3 = 2N2O + 4H2O

Keeping the above in mind, why does opinion 1 prevail in the industry? May be there is much more to it than hydrogen?

Max

P.S. of course, we must follow the specification, but is good to know the reasons behind these...

Max Stein
captive metal finisher - Montreal, Québec, Canada

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