This Month's Tech Tip: Hydrogen Embrittlement

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Tech Tip Archives:

Hydrogen Embrittlement

Todays theme is about hydrogen embrittlement. This is a problem associated with high strength fasteners. The fasteners or parts under stress can fail suddenly without any warning. I have read a great deal on this subject and there are many theories on the exact cause. I would only like to give you a brief overview of the subject. I am not a scientist or an engineer so I cannot claim to fully comprehend the pheneomen.

Hydrogen embrittlment can occur any time atomic hydrogen is absorbed in the fastener and retained. The most common way this happens is through the eletroplating process. The hydrogen can be absorbed during the cleaning or descaling process and then is trapped by the plating. The common way to relieve the hydrogen is through baking. This is thought to relieve the hydrogen trapped inside the part.

Recently I have read that the hydrogen is not "relieved" it is trapped so it cannot migrate and cause further cracks. It is thought that the quicker the parts are baked after the plating the more relief they will experience. The window seems to be four hours. If you were to bake the parts after four hours the absorbed hydrogen may have already started cracks. The baking will not remove cracks already started, the fastener would most likely fail.

How, as end users of fasteners can you fight hydrogen embrittlement? I would say the easiest way is to avoid electroplating high strength fasteners. There are now many companies offering different chemical and organic coatings. Many of these coatings offer superior corrosion resistance. You could also choose mechanically plating the parts. Of course for many of us this is not an option. Electroplating is very efficient and economical. We then must be very careful how we plate these products and always be sure that we are dealing with responsible plating companies. The fasteners must be baked properly within a correct time frame. Another important process is to be sure you are testing the parts when they returned from the plater. This will hopefully guarantee the parts are safe and ready for use.

There is a tremendous amount of information available on this subject. For sources I have used the Industrial Fasteners Institute, Fastener Standards book and a wonderful technical book written by John Bickford. The title is: An Introduction To The Design And Behavior Of Bolted Joints. Both of these are great sources for information and are very helpful in explaining difficult theories.