652541 nut used on Continental TSIO-520M engine broke because of hydrogen embrittlement.
(applicable to carbon and alloy steel with a strength >145,000 psi)
Hydrogen embrittlement causes high-strength carbon and alloy steel to break well below its rated strength. The part may break immediately, or years later without warning. The aircraft mechanic may accidentally cause hydrogen embrittlement during aircraft maintenance.
Hydrogen embrittlement may be caused by having parts electro-plated, or parts that have corroded. An example of how electro-plating can lead to hydrogen embrittlement failure is when the mechanic has a worn aircraft crankshaft journal chrome plated back to standard size. There is a long history in aviation of aircraft crankshafts breaking.
Another example is when old, used aircraft hardware is reclaimed and re-plated. At one time this was a very common practice among engine overhaul shops to re-plate aircraft AN bolts. Some aircraft bolts sold today as "new" is in fact reclaimed. Fasteners above 145,000 psi tensile strength, such as most of the NAS bolts and Grade 8, should not be reclaimed.
Corrosion causes hydrogen embrittlement and failure and is described as Stress Corrosion Cracking (SCC). Fasteners above 145,000 psi need to be absolutely protected from corrosion. Those fasteners that have any signs of corrosion require replacement. Bolt strength out of the box is far greater than bolt strength after it has pitted and corroded.
The strength of a grade 8 bolt is considerably reduced by hydrogen embrittlement and Stress corrosion cracking. Much more than grade 5. A non-aircraft example, is the installation of a tow motor on a pickup truck using Grade 8 bolts because they are stronger than Grade 5 bolts. A tow motor on a pickup truck is corrosion waiting to happen. Grade 8 is susceptible to hydrogen embrittlement and SCC, whereas Grade 5 bolts are not.
In an application where we expect some corrosion on the bolts, the same joint strength can be achieved by selecting a larger diameter Grade 5 bolt. There is far less danger of Grade 5 bolts failing due to hydrogen. This is an example of designing for strength AND endurance.
If the grade 8 bolt torque value is reduced, this would also reduce Stress Corrosion Cracking and hydrogen embrittlement. But why not just buy the grade 5?
Another concern when working with high-strength steels is the application of chemical cleaners and strippers. Chemicals should pass standard industry hydrogen embrittlement tests before being used.
