On an assembly line, high-strength screws are precisely tightened using DC drivers. Once installed, each screw holds more than 6,000lb of pre-load. After coming off the assembly line, the assembled products are placed in boxes for shipping. Suddenly, inside the shipping boxes, the heads of the screws begin to pop off, one by one. When the boxes are opened, about 10 percent of the screws are found to have failed in this way. The rest of the fasteners are intact. What happened?
The scenario described is most likely an example of hydrogen embrittlement. This failure mode can affect fasteners, as well as other metal components. Usually, but not always, the components involved are plated. While most often associated with high-strength carbon and alloy steels, it can also affect other materials, including stainless steels, aluminum, titanium, and copper alloys. Typically, failure occurs without warning, under stresses well below the yield strength of the material, and within a relatively short period (anywhere from a few hours to a week or more). The risk of hydrogen embrittlement can be mitigated by switching to a lower-hardness material, minimizing exposure to hydrogen, or by heating to a temperature that allows the hydrogen to diffuse out.
Hydrogen embrittlement can occur when a metal absorbs atomic hydrogen. Hydrogen atoms are small enough to squeeze in between the atoms of the metal. The presence of hydrogen reduces the cohesive force between the metal atoms. This allows the metal to fracture under stresses it would normally be able to withstand. (Exactly what happens at an atomic level continues to be a topic of research and a source of debate among metallurgists, but the decohesion theory I have just explained is one of the most widely held views, as well as being the easiest to explain.)
The most common source of hydrogen in metals is electroplating. Acid cleaning processes performed prior to plating, as well as the electroplating process itself, can cause metals to absorb hydrogen. Hydrogen can also be produced during the arc welding process. Galvanic corrosion is another possible source of hydrogen; hydrogen is produced at the cathode (i.e. the more noble of the two metals). Service exposure to strong acids or to hydrogen gas can also introduce hydrogen into the metal.
The materials most commonly affected by hydrogen embrittlement are carbon and alloy steels, with hardness above around 35 Rockwell C. Steels with hardness below this value are rarely affected. One widely repeated myth about hydrogen embrittlement is that stainless steels are immune. Actually, precipitation-hardenable and 400-series stainless steels are very susceptible to hydrogen embrittlement. It's rare, but not impossible, for hydrogen embrittlement to occur in 300-series stainless steels.
The fact that hydrogen embrittlement is not often seen in 300-series stainless steels is partly due to the fact that these steels are seldom plated, and seldom have a hardness above 35 Rockwell C. However, severe cold work (such as cold heading) can produce a hardness this high, which will make the steel subject to embrittlement if exposed to hydrogen. Aluminum alloys, titanium alloys, and copper alloys can also be affected by hydrogen embrittlement. Again, the highest-strength alloys are the most susceptible.