In the first three parts of this series we discussed the physical and metallurgical aspects of hydrogen embrittlement. An understanding of how the phenomenon occurs is the foundation of the ultimate question – how can hydrogen embrittlement be prevented.
The two critical points at which most hydrogen embrittlement failures can be prevented are at the design stage and during the manufacturing process. Designers with limited materials engineering exposure may not realize the implications of the materials and manufacturing processes they call for in their drawing specifications. As with other failure types, hydrogen embrittlement failures can inadvertently be “designed into” a part.
On the manufacturing side, avoiding the use of reducing acids where possible removes an abundant source of hydrogen from potential exposure to a part. Pickling, etching and electroplating are common manufacturing processes in which acid exposure occurs. Because it is so widely used, electroplating requires particular attention. Minimizing plating time and maximizing current density will generally reduce the volume of absorbed hydrogen. However, consideration of electroless plating or vapor deposition as an alternative eliminates the possibility of hydrogen absorption from the coating process altogether.
Protecting components that will be heat treated or welded from corrosion, or cleaning them prior to these processes will avoid hydrogen introduction by these routes. It is also critical that welding rods are stored in a manner which prevents the absorption of moisture into their flux coating. Any process associated with elevated temperatures will also increase the mobility and absorption of hydrogen, increasing the potential for hydrogen embrittlement.
Despite the most stringent precautions, processing requirements will sometimes introduce hydrogen into parts that are at or above the tensile strength and hardness thresholds at which hydrogen embrittlement can occur. Fortunately, there is a procedure that will effectively remove absorbed hydrogen. This is an oven heating process, referred to as “baking”, that is performed within the following parameters:
- Parts must be baked within 4 hours of hydrogen exposure. The sooner, the better.
- Parts must be baked at a minimum of 400º F.
- Parts must be held at 400º F for a minimum of 4 hours. Longer may be required depending on part size.
To be effective, these time and temperature parameters must be strictly followed. Short-cuts or delays will dramatically reduce the effectiveness of baking in removing absorbed hydrogen. For example, twice as much hydrogen will be baked out at 400º F versus 350ºF, and doubling the bake time doubles the amount of hydrogen that is baked out.
The sooner baking begins after exposure to hydrogen, the better. The 4 hour “window” is a maximum. Note that baking must be performed within 4 hours of each hydrogen exposure. For example, within 4 hours of pickling, AND within 4 hours of subsequent plating. No amount of baking will salvage embrittled parts if these time and temperature parameters have not been met.
A final word of caution. A 30 year analysis of hydrogen embrittlement failures in the aircraft industry found that over 70% resulted from improper baking procedures.
In the next, and final, part of the series we’ll talk briefly about the “history” of hydrogen embrittlement, and why it’s a relatively recent phenomenon.