Hydrogen Embrittlement Relief

Hydrogen embrittlement is the failure that results from the absorption of hydrogen into metals. Hydrogen embrittlement usually occurs in combination with residual or applied stresses in a part, happening most frequently in high-strength steels and occasionally in other high-strength alloys.

Hydrogen can be introduced into a metal by processes such as pickling, electrocleaning, acid activation, electroplating, or electroless deposition. Although the hydrogen produced by electroless nickel plating is much less than that produced by an electrolytic process, such as cadmium or hard chrome plating, it can be enough to cause cracking of high-strength steels. To prevent this, components are baked at 200 ± 10 °C (390 ± 18 °F) to diffuse the absorbed hydrogen out of the steel. This usually restores the mechanical properties of the steel almost completely, helping to ensure against failure.

The time required to remove hydrogen from a steel and avoid embrittlement depends on the strength of the steel. Longer relief treatment periods or higher temperatures are needed as the strength of the steel increases. Recommendations for embrittlement relief of steels on different strength levels are summarized in Table 10. Longer times may be require for parts with deposit thickness greater than 1 mil. Deposits are amorphous, thus there are no grain boundaries for the hydrogen to follow. Shorter times may be used if unplated areas are present. Temperature ramp-up times should be longer than for hydrogen relief of other metal deposits. Hydrogen embrittlement relief treatment should begin within 4 h of the completion of electroless nickel plating (Ref 2, 49, 50).

Table 10 Heat treatment of steels to relieve hydrogen embrittlement

Maximum specified tensile strength

Heat treatment at 190 to 210 °C (375 to 410 °F), h

MPa

ksi

< 1050

< 152

Not required

1051-1450

152-210

2

1451-1800

210-260

18

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