Hydrogen Embrittlement

The need to prevent hydrogen embrittlement was one of the major reasons for the creation and use of mechanical plating. A critical concern in electroplating and other coating processes used on ferrous parts is the embrittling effects of hydrogen absorbed by the part. The current used in electroplating acts to enhance the possibility of hydrogen embrittlement--both because most electroplating generates hydrogen at the cathode and because the negative charge acts to pull hydrogen into the part. Hydrogen embrittlement can cause sudden development of breaks or cracks in highly stressed areas, with subsequent total rupture of the part or the assembly. The risk increases for parts that have high hardness from cold working or heat treating, especially those made of high-carbon steels.

In the normal electroplating process, an important source of hydrogen gas is the reaction between acids and metals present in the plating solution. The hydrogen migrates through the metallic substrate and concentrates at high stress points and grain boundaries. The trapped hydrogen builds internal pressures that can lower the tolerance to stresses applied in actual use. Dangerous failures in critical applications can result.

Other atmospheres that can potentially cause or contribute to hydrogen embrittlement include heat-treating furnaces, cleaning solutions, and pickling baths.

Advantage of Mechanical Plating in Avoiding Embrittlement. Mechanical plating deposits metals while eliminating or at least minimizing the risk of embrittlement caused by the coating process itself. A hydrogen-producing reaction does occur in mechanical plating:

However, this reaction occurs essentially on the surface of the powdered zinc particles, which are approximately 5 to 10 pm in diameter. The reaction proceeds at a relatively slow rate and within a more porous, less oriented metallic grain structure than that produced by electroplating. Therefore, the hydrogen gas is not likely to be trapped or occluded within or under the metal particles in the coating. The effusion or escape of hydrogen gas through the deposit and away from the substrate is more likely than diffusion into the base metal. A substantial portion of the mechanical plating performed today is done on parts and metals that are predisposed to hydrogen embrittlement. These include spring steels, parts heat treated to 42 HRC or higher, cold-headed parts, or any part for which service application or structural integrity is highly critical.

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