Plating Systems & Technologies Inc.
Hydrogen Embrittlement
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 sealingthe 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 brittlement.
How much Baking Do Electroplated Parts need?
(ASTM B 850-94)
Tensile Tensile Rockwell Post-Plate Bake
Strength Strength Hardness (@1900 - 2200 C.
(MPa) (000 psi) HRc or 3740 - 4280 F.)
1700 - 1800 247 -261 49 - 51 22+
1600 - 1700 232 -247 47 - 49 20+
1500 - 1600 218 - 232 45 - 47 18+
1400 - 1500 203 - 218 43 - 45 16+
1300 - 1400 189 - 203 39 - 43 14+
1200 - 1300 174 - 189 36 - 39 12+
1100 - 1200 160 - 174 33 - 36 10+
1000 - 1100 145 - 160 31 - 33 8+
Per ASTM B 850-94 "For Steels of actual tensile strength 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 plating 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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