Letter 40088

Hydrogen Bakeout  

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I would like to have a better understanding of the term "Hydrogen Bakeout". I work in the petrochemical industry and have heard this term quite often. I understand how its is done but not why. Can any one shed some light on this for me.

Gary Tibbetts
- Saint John N.B.

Hydrogen bakeout is something of a misnomer. The hydrogen that accumulates in the coating may be baked "out" of the part, but the hydrogen that actually diffuses into the steel is only redistributed, not removed completely. This process is correctly called embrittlement relief. Steels that have a hardness > 32 HRC (320 HV) or a tensile strength > 1000 MPa are required to be baked after electroplating because the hydrogen that diffuses into the steel can embrittle it, causing delayed fracture. If a high strength steel that has a high hydrogen concentration is stressed, the hydrogen diffuses to areas of high tensile stress, and promotes early fracture. Steels with a microstructure of as-quenched martensite or tempered martensite are most susceptible, with the failure mechanism being intergranular fracture along prior austenite grain boundaries.

Toby Padfield
Automotive module supplier - MI, USA