Hydrogen is reactive so it pairs up into molecules of 2 atoms of hydrogen H-H.
Plating, such as zinc plating, causes a reaction that releases hydrogen atoms (atomar hydrogen). Some of the hydrogen atoms pair up and form hydrogen molecules (H-H) and bubble to the surface as hydrogen gas. Other hydrogen atoms bond to the iron atoms on our part being plated.
It is these hydrogen atoms that cause the problem. Allowed to remain weakly bonded to iron, they will diffuse deeper into the part. Absorbed hydrogen atoms concentrate at minute faults in the iron and create tension which can lead to an explosion-like breaking of the part. This is hydrogen embrittlement.
One method of reducing the amount of absorbed hydrogen - and hydrogen embrittlement, is to drive the hydrogen out of the metal by heating. Heating has to be done as quickly as possible after plating before the hydrogen atoms have had a chance to diffuse into the metal. The time and temperature required is highly dependent upon the type and method of plating. Some plating, especially zinc plating, can create an impermeable barrier that the hydrogen has trouble diffusing through. Additionally, the plating process and thickness also determine how much hydrogen is absorbed into the part and how much baking is required to remove it.
Hydrogen embrittlement created by plating is so unpredictable that often a test piece is plated together with the parts. These test pieces can then be destroyed during tests or kept as control parts for future testing.
Hydrogen embrittlement resulted in several aircraft accidents after the engine manufacturer switched from cadmium to zinc plating on a crankshaft bolt. Read more...
