Stainless steels hydrogen cracking

M. J. Morgan and M. H. Tosten, "Microstructure and Yield Strength Effects on Hydrogen and Tritium Induced Cracking in HERF Stainless Steel", Hydrogen Effects on Material Behavior, ed. N. R. Moody and A. W. Thompson, (Warrendale, PA TMS, 1990), 447-457. 

Although hydrogen cyanide is a weak acid and is normally not corrosive, it has a corrosive effect under two special conditions (/) water solutions of hydrogen cyanide cause transcrystalline stress cracking of carbon steels under stress even at room temperature and in dilute solution and (2) water solutions of hydrogen cyanide containing sulfuric acid as a stabilizer severely corrode steel (qv) above 40°C and stainless steels above 80°C. 

Some of the most obvious examples of problems with gas and materials are frequently found in refining or petrochemical applications. One is the presence of hydrogen sulfide. Austenitic stainless steel, normally a premium material, cannot be used if chlorides are present due to intergranular corrosion and subsequent cracking problems. The material choice is influenced by hardness limitations as well as operating stresses that may limit certain perfonnance parameters. 

Wilde, B. E. and Kim, C. D., The R61e of Hydrogen in the Mechanism of Stress-corrosion Cracking of Austenitic Stainless Steel in Hot Chloride Media , Corrosion, 28, 350 (1972) Lin, F. and Hochman, R. F., Electrochemical Study of Stress-corrosion Cracking of Ti 8-1-1 Alloy and NaCl Solutions , Corrosion, 28, 182 (1972)... 

The above simple concepts need to be modified and expanded to allow discussion of stress-corrosion cracking of stainless steels in general, especially as the role of hydrogen in assisting crack growth must be accounted for. 

Similarly it seems that retained austenite may be beneficial in certain circumstances , probably because the austenite acts as a barrier to the diffusion of hydrogen, although in high concentrations (such as those obtained in duplex stainless steels) the austenite can also act as a crack stopper (i.e. a ductile region in the microstructure which blunts and stops the brittle crack). 

Tritium and its decay product, helium, change the structural properties of stainless steels and make them more susceptible to cracking. Tritium embrittlement is an enhanced form of hydrogen embrittlement because of the presence of He from tritium decay which nucleates as nanometer-sized bubbles on dislocations, grain boundaries, and other microstructural defects. Steels with decay helium bubble microstructures are hardened and less able to deform plastically and become more susceptible to embrittlement by hydrogen and its isotopes (1-7). 

Figure 10. Effect of Hydrogen, Tritium and Decay Helium on the J-Integral (J) vs. Change-in-Crack-Length (da) Behavior for Conventionally Forged Type 21-6-9 Stainless Steels.

Brittle Failure (8). Brittleness is a principal consideration in selecting construction materials for liquid hydrogen service. Brittle fracture can result in the essentially instantaneous release of a vessel s contents, the hazard being a combined one of PV energy release and the possibility of fire and/or explosion. Three conditions must exist for a brittle fracture to occur 1) a stress riser, a crack, notch, or other discontinuity, 2) a section where the actual stress exceeds the yield stress of the material, and 3) a temperature below which failure occurs without appreciable plastic deformation. Metals that are satisfactory for liquid hydrogen service include aluminum, stainless steels, brass, and copper. Carbon steel is not suitable. 

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