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Hydrogen Embrittlement

Hydrogen embrittlement is a major cause of fastener failure. The 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 mechanisms have been proposed, and most have at least some merit. The current thinking is that the susceptibility to hydrogen embrittlement is related directly to the trap population. Generally, hydrogen embrittlement can be described as the adsorption of hydrogen promoting enhanced decohesion of the steel, primarily as an intergranular phenomenon.

The Problem with Electroplating

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, issued a specification for baking cycles, Standard Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement (B850), as shown to right. For the production plater, having to remove the parts from the production line to bake — followed by a separate chromating or passivating process — is a laborious undertaking.
Zinc Deposit
Steel Substrate

Why Mechanical Plating?

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 percent density) deposit can therefore effuse the hydrogen that would otherwise produce hydrogen embrittlement.

How Much Baking Do Electroplated Parts Need? (ASTM B850)

Tensile Strength (MPa)
Tensile Strength (000 psi)
Rockwell Hardness HRc
Post-Plate Bake (@190°-220°C or 374°-428°F)
1,700-1,800
247-261
49-51
22+
1,600-1,700
232-247
47-49
20+
1,500-1,600
218-232
45-47
18+
1,400-1,500
203-218
43-45
16+
1,300-1,400
189-203
39-43
14+
1,200-1,300
174-189
36-39
12+
1,100-1,200
160-174
33-36
10+
1,000-1,100
145-160
31-33
8+
Source: ASTM B850, accordingto Section 6.2 - Steels of actual tensile strength below 1000 MPa, Heat treatment after plating is not essential.

A Proven Solution

For nearly 50 years, mechanical plating has been accepted as a means of eliminating hydrogen embrittlement. Today, many specifications reflect the 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.
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Hydrogen Embrittlement