Hydrogen cracking and metal finishing...causes and cures

Recently, a customer was experiencing base metal cracking after chrome plating.  The cracking was almost invisible to the unaided eye, and was only accruing in a bend area on a cold stamped, formed, and machined part. (See accompanying photo of similar part). A lengthy investigation into this problem was launched, examining the customers' base material, heat treating, cold stamping, forming, and machining procedures. The advice of several experts in hydrogen damage and metallurgy was enlist­ed. At the completion of this investi­gation it was determined that the hydrogen embritllement cracking was caused by the stress that was induced into the part during cold stamping, forming and machining before plating. The plating was only acting as a catalyst for the stress that had previously been induced into the base material. It was recommended to the customer to have their base material annealed to reduce the hardness of the material before it was cold worked. This annealing operation solved the problem, eliminated rejected parts for this defect, and saved the customer money.

Hydrogen embrittlement has been a problem for metal finishers, design engineers, and metallurgists since the beginning of the industrial revolution.  Hydrogen is one of the most widely distributed, lightest, and atomically smallest elements.  Hydrogen also effects high strength steel the most.  Stainless steels, nickel alloys, copper alloys, titanium, and aluminum are also susceptible to the effects of hydrogen.  The higher the strength of steel the more susceptible it is to hydrogen embrittlement damage.  Metal finishing can be a catalyst for hydrogen stress that has already been induced into a part’s processing life cycle.  Stress can also enter from casting, forging, grinding, and cold working. 

Often times the hydrogen, which has been induced into a part during previous processing operations does not reveal itself as damage until during or after metal finishing, because metal finishing acts as a catalyst for hydrogen stress.

Relieving and minimizing before and during metal finishing

Due to the nature of a plating operation, eliminating the exposure to hydrogen is impossible. But engineering metal finishers can reduce this exposure by taking several effective steps before and during processing. Stress relief baking or shot peening performed before metal finishing can prevent potential hydrogen damage that results from the previous processing operations in the life cycle of a part. Most aircraft specifications call for a stress relief shot peen before plating. These pre-plating stress relief operations are critical to parts 40 Rockwell C and higher. 

Cleaning and de-scaling of the parts prior to plating can be performed by mechanical means instead of hydrogen induced chemical means. During electroplating the best strategy for reducing exposure to hydrogen is to use the highest practical current density for plating. This maximizes plating efficiency and minimizes the generation of hydrogen in the process. Metal finishers can give their customers reliable advice on ways to minimize the hydrogen that is induced into base materials during processing operations before plating.