Hydrogen embrittlement
Hydrogen embrittlement turns up in 3 fields: corrosion, heat treating, fasteners. Here is what it means to the people in each one.
Hydrogen embrittlement in corrosion
Loss of ductility when atomic hydrogen gets into steel, leading to sudden cracking under stress.
Damage in which atomic hydrogen, generated by corrosion, cathodic protection, pickling or electroplating, diffuses into steel and cuts its ductility and toughness. High-strength steels, roughly above 1000 MPa tensile strength, are the most vulnerable, and fasteners can crack hours or days after they are loaded. Excessive CP potentials on high-strength steel and plated bolts that were never baked are the classic causes.
"Those plated bolts snapped two days after torquing, smells like hydrogen embrittlement, ask if they were baked."
Hydrogen embrittlement in heat treating
Loss of ductility in high-strength steel when absorbed hydrogen causes delayed cracking under stress.
Damage mechanism in which atomic hydrogen picked up during pickling, electroplating, welding or corrosion collects at stressed regions and causes brittle cracking, often hours or days after assembly. Steels above about 39 HRC are most at risk. Plated high-strength fasteners and springs get a relief bake, commonly 190 to 220 °C for 4 to 24 hours, started within a few hours of plating.
"Grade 8 bolts are back from zinc, they go in the oven for the hydrogen embrittlement bake within the hour."
Hydrogen embrittlement in fasteners
Delayed brittle cracking of hard steel fasteners caused by hydrogen absorbed from plating, cleaning or corrosion.
Atomic hydrogen enters steel during acid pickling and electroplating, or later from corrosion in service, and collects at grain boundaries under stress. In fasteners harder than about 39 HRC it can cause sudden brittle fracture hours or days after tightening, usually at the head fillet or the first engaged thread, with a grainy intergranular fracture face. Prevention relies on avoiding electroplating on hard parts, relief baking and coatings such as zinc flake.
"Three of the 12.9 screws popped their heads overnight with nobody touching them, that's hydrogen embrittlement until proven otherwise."