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#007 Hydrogen Embrittlement - Dehydrogenation prior to Plating

Category : Hydrogen embrittlement
August14, 2009

The occluded hydrogen during pickling process is thought to be retained near the steel surface, and is recommended that the hydrogen be released prior to plating layer is deposited. The hydrogen is slowly released by simply letting the part sit out in the air, as shown in [Fig.1] below.

[Fig.1] Hydrogen embrittlement rate and sitting-out time duration

Diffusion movement of hydrogen accelerates at higher temperatures, so the dissipation time of occluded hydrogen can be shortened by heating. Normally, an alkaline wash process follows a pickling process, and relatively high temperature of 60~70 deg.C for this wash process is advantageous for the hydrogen dissipation. A test result is shown in [Fig.2] below.

[Fig. 2] Alkaline wash after pickling and hydrogen embrittlement

In actual plating work practices, it is difficult to setup the alkaline wash stage for the purpose of dehydrogenation due to variations of rust and scales on the subject metal and wash process time.

For steel parts that are subjected to long pickling processes, a separate dehydrogenation process (baking) is required. Although it is more effective to bake for long durations at high temperatures, this will cause oxide layers to form on the steel surface, requiring a pickling again. So the baking takes place at i.e. 200 deg.C for 30 minutes, as shown in [Fig.1] above.

For example, baking is typically performed after zinc plating, because this is intended to release the hydrogen occluded during the zinc plating. But, the hydrogen occluded during pickling needs to be released before plating or else the zinc coating will prevent sufficient emission.