Question

What can you tell me about stress corrosion cracking (SCC) and hydrogen embrittlement of nickel alloys?

Figure 1. Stress corrosion cracking (SCC) factors

Answer

One of the most important criteria (for material selection) is resistance to stress corrosion cracking and hydrogen embrittlement. The risk of these occurring increases at higher temperatures and pressures and is mitigated by low stress levels. Therefore, material selection, fabrication processes and material thickness should be chosen with a view toward achieving low residual stresses. For example, in piping, typical methods of lowering stresses once the operating temperature and pressure have been decided are closer pipe support spacing, thicker pipe walls and thermal relieving of residual welding stresses.

Test methods to differentiate between hydrogen embrittlement and stress corrosion cracking include:

1. Tensile and notched tensile property evaluation (per ASTM G 142-98)
2. K1H fracture test (to evaluate the threshold stress intensity factor for hydrogen stress cracking)
3. Slow strain rate (SSR) test
4. Disc pressure test (to measure the susceptibility to hydrogen embrittlement of metallic material under high-pressure hydrogen (ASTM F1459)

Figure 2. Effect of baking at 300°F (150°C) on the time-to-failure at a given level of applied stress and stress ratio of notched specimens - 4340 steel heat treated to a strength level of 230 ksi (1586 MPa)

Hydrogen Bake-Out
With respect to plating, specifications now state that a plated carbon-steel fastener "shall be baked for not less than 23 hours at 375+25°F within 2 hours after plating to provide hydrogen embrittlement relief" (per MIL-N-25027D). In the past, the plating specifications required baking at 375+25°F for only 3 hours within 4 hours after plating. This treatment was found to be inadequate, and most plating specifications were revised in 1981-82 to reflect the longer baking time. Hydrogen embrittlement problems also increase as the fastener strength increases.