Hydrogen Embrittlement in Plating

A. Hydrogen embrittlement is a phenomenon that affects high-strength steel. Hydrogen--often from pickling or plating--invades the grain structure of a high strength steel, making it brittle and subject to catastrophic failure.

The best approach is to avoid or minimize processes that cause hydrogen embrittlement. The second best approach is to bake the parts to drive the hydrogen out, preferably very soon after plating. Some people feel that some degree of permanent damage occurs during the time between plating and baking, and that it is thus vital to bake immediately. Others feel that the old standard "bake within 24 hours" is sufficient. ASTM Committee B-8 spent a lot of time on this problem, so I think their specs are highly trustable.

A. Michael: We have had some experience with the phenomena of hydrogen embrittlement. A good reference book is "Hydrogen Embrittlement and Stress Corrosion Cracking" [affil link on Amazon or on AbeBooks ] by Gibala & Hehemann (ASM Publication). It is quite detailed and informative.

A. Over the years, the Airline Plating Forum has had several papers given on the subject. Available thru the AESF bookstore. There is a stray article in the plating magazines. Seems to me that the big book ASM library had some information on it. You might check the index for the various Sur-Fin abstracts. These probably will be more technical and less practical.

A. Yes and no. Since the edges (corners) are high current density areas, they plate much faster, so more H2 is generated there, so it will be a little more prone to problems there. In high stress deposits, the additional thickness puts the parent material under a lot more stress, so will tend to have more problems there.

A stress riser like a tiny cut from a razor blade in masking on a high tensile strength part will be much more prone to fail. Inadequate radius at flanges are very prone to failure. The end result is a failed part, but the root causes are wildly different.

A. The amount of Hydrogen Embrittlement absorbed during electroplating is related to the current density on the cathode surface of the article being plated. Higher current density areas will evolve more Hydrogen gas and therefore the potential for embrittlement is higher. Edges will normally be higher current density than flat surfaces. Edges that are on bottom rows of racks will most likely have higher levels of absorbed Hydrogen. In most cases of electroplating, edges will have higher preferential sites for Hydrogen Embrittlement. Maintaining optimum electroplating efficiencies and additions of wetting agents are variables that can help reduce embrittlement by Hydrogen gas.

A. Higher current densities will definitely produce more hydrogen per metal deposited, but...

Some processes (for sure low embrittlement cadmium, but maybe some others too?) use high current densities on purpose in order to produce porous plating that allows for easy hydrogen molecule escape during the subsequent bake.

In general I do not think that areas where most of the hydrogen is absorbed would suffer higher danger of embrittlement: metal is a "transparent" media for protons (hydrogen atom's nucleus), therefore the bulk of the plated part (even the non-plated part of it, if masked) will be equally prone to embrittlement.

The question remaining - where it may break? - in the point of maximum stress, even if that portion of the part is not plated.

This is just an opinion...

A. If you had access to a Scanning Electron Microscope (SEM) and someone with experience in analyzing for Hydrogen Contamination it could be possible to show breakage was caused by Hydrogen Embrittlement.

However, since you say the parts are not breaking before plating but are breaking after, the possibility is strong that hydrogen embrittlement is occurring in your parts. Have you baked the parts for hydrogen embrittlement relief immediately after plating and before bending.

A. You have about as close to 100% probability as you will ever get that you have a classic H2 embrittlement problem.

Bake as soon as possible after plating. The longer the H2 is trapped between the plate and the base metal, the further it penetrates the steel and the harder it is to get out.

Do not flex the parts even a tiny bit taking them off the rack!

If you are baking, get there sooner and bake longer or at a higher temp.

After bake a very quick dip in a mild (weak) nitric solution will activate it enough that it will take a chromate. Consult your chromate vendor and do some trial and error testing to see how weak you can get by with.

A. A considerable amount of work on hydrogen embrittlement has been conducted and published in the literature. Some important work was done and published by scientists at the Naval Air Warfare Center. The bottom line is: it is nearly impossible to remove all of the hydrogen that has been introduced, using common baking methods. Once the damage is done, it can be reduced to some extent. The amount of damage and reduction will depend on dozens of variables such as the part, its material, coating, process, amount of hydrogen generated and absorbed and combinations thereof. Relief is likely to be inconsistent. This inconsistency could be reduced by porosity in the coatings.

A. Hydrogen embrittlement is typically manifested as "delayed fracture" at static stresses, which would be contrary to immediate failure during bending. Your parts, however, are quite susceptible to hydrogen damage and may be cracked during plating if the parts have sufficient residual stress from heat treating. A static load test with less deflection than will cause immediate cracking could be definitive.

Another alternative to consider is possible embrittlement due to the baking process. This material is susceptible to tempered martensite embrittlement, usually from exposure to temperatures from 400 °F to 700 °F. I would avoid baking at temperatures above 375 °F.

I agree with Mr. Sunthankar that baking is not 100% effective. The extreme susceptibility of this material to cracking by hydrogen embrittlement would suggest to me that an alternative corrosion protection would be prudent.

Alternatives to Plating to Minimize Hydrogen Embrittlement

adv.

A. If hydrogen embrittlement is of concern, you may want to consider mechanical plating and/or galvanizing. I have been involved in many studies comparing zinc electroplating, hot dip galvanizing and mechanical -- and mechanical does not impart hydrogen.

A. One of the best systems for coating that do not have any hydrogen embrittlement is the Thermal Diffusion zinc coating system. Google it and you'll learn quite a bit.

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Ed. note: Thermal Diffusion is an upgrade to the older process called Sherardizing, which is discussed in many threads on this site.

A. I really do not want to get in the middle of an argument, but will express my opinion.

I have never heard of hydrogen embrittlement being caused by bake.

Uncalibrated ovens probably did not contribute much to hydrogen embrittlement of your parts because bake relief is really wide open as a function of time and temperature. 375 °F for 4 hours is + or - 25 °F as a common relief.

Two things cause more grief than a few degrees difference.

1. The time between plating and into the oven. For spring material is should be as soon as possible. The 4 hour book value is really too long. The longer you wait, the harder it is to get out and the more damage has been done.

2. The acid activation or etch contributes more to embrittlement than the plating process does. For spring steel I would use periodic reverse or anodic electroclean followed by a very short, say 10 second dip in a conventional hydrochloric or sulfuric acid tank. I have done cyanide and acid cad without a failure of notch bar testing at some very high tensile strengths.

If these are not critical parts to safety, I would bake them for 24 hours, possibly 48 for better reliability and use them. These are tiny parts that can be nicely run in a lab oven at a very low cost with a decent recovery of the unbroken parts.

A. To answer part of Mr. Rubin's question (can austempering cause hydrogen embrittlement?):

I would say no, especially if the austenitizing (not the quenching) is done in salt. If the austenitizing is done in an atmosphere furnace (typically endothermic gas at 40% H2), then some hydrogen is absorbed by the metal. I suppose one could argue that this might cause HE. I know it can cause delayed quench cracking in parts quenched in media such as water or oil, and although hydrogen levels are probably much lower than in "classical" HE, it could be called hydrogen embrittlement. I doubt very much that cracking would occur in an austempered part because the residual stresses and residual hydrogen levels are typically much lower due to the gentler quench rates and the residence time at the austempering temperature.

In other words, it's hard to imagine austempering causing HE, and I think most metallurgists and heat treaters would agree they've never identified such an occurrence or even suspected the possibility.

A. Bake- You have to conform to what the spec calls out.

Many specs would not call for a bake at an Rc of 40. I do not remember doing one at Rc48, so cannot be of any help there.

Many people will not refer to phosphate as plating, since you are not depositing a metal or alloy.

Acid normally will be as strong as you need to do the job, but no stronger. Time is a tradeoff with acid strength. You have to specify the acid when you talk %'s. Sulfuric acid has two H's, so is basically twice as strong as hydrochloric (muriatic). Some people use mixed acids.

Acid etch is a major contributor to hydrogen embrittlement, so any etch should be the minimum to get by on hard parts.

If you use an acid etch, You really need to use a proprietary Ti compound additive to the last rinse prior to phosphate. Normally, you will get a lousy phosphate if you do not. I have heard of a very few that have avoided it.

A. What kind of steel and what is the hardness? Usually embrittlement is not considered a problem below Rockwell C-40 or 180 ksi, although some callouts go as low as C-30. How hot and how long is the bake cycle for your E-coat? Have you run any test specimens? We have found tempered Belleville washers to be simple test pieces. Easily compressed and you can run a large number for statistical reliability,

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Ed. note: Thanks, C.A.! That Belleville washer idea sounds like a great suggestion.