Hydrogen embrittlement is a major cause of fastener failure. Prevailing thought is that steels with Rockwell hardness above C30 are vulnerable. The phenomenon is well-known although the precise mechanism has eluded extensive research. A number of proposed mechanisms have been proposed, and most have at least some merit. Current thinking is that the susceptibility to hydrogen embrittlement is related directly to the trap population. Generally, hydrogen embrittlement can be described as absorption and adsorption of hydrogen promoting enhanced decohesion of the steel, primarily as an intergranular phenomenon.
Electroplating is a major cause of hydrogen embrittlement. Some hydrogen is generated during the cleaning and pickling cycles, but by far the most significant source is cathodic inefficiency, which is followed by sealing the hydrogen in the parts. Baking is often performed on high strength parts to reduce this risk, and the ASTM, in 1994, issued a specification for baking cycles, as shown below. For the production plater, having to remove the parts from the production line to bake - followed by a separate chromating process - is a laborious process.
Mechanically deposited zinc coatings consist of small platelets formed from the mechanical action of glass beads on fine (3 to 7 microns) zinc dust particles. The platelets thus formed are "cold-welded" to the substrate and to each other. The porous (approximately 80% dense) deposit can therefore effuse the hydrogen that would otherwise produce hydrogen embrittlement.
How much Baking Do Electroplated Parts need? (ASTM B 850-94)
49 - 51
Per ASTM B 850-94 "For Steels of
actual tensile strenth below
1000 MPa, Heat treatment after plating is not essential."
For Nearly fifty
years mechanical Plating has been accepted
as a means of
eliminating hydrogen embrittlement. Today, many specifications reflect industry's
confidence in this unique process. While it is true that mechanical plating uses
inhibited acids which generate less hydrogen, PS&T believes that mechanical
platinng as a process is inherently free from hydrogen embrittlement because the
deposit is porous (as are phosphate coatings), allowing hydrogen to escape through
the coating; in electroplating, by way of contrast, hydrogen is sealed in the part.
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