Metallurgical Effects on the Diffusion of Hydrogen through Alloy 718

Cathodic protection is widely utilized for corrosion prevention. However, hydrogen generation and incorporation at the cathodically protected metal surface, followed by diffusion through the metal, may cause hydrogen embrittlement and hydrogen induced stress corrosion cracking. Thus, a materials susceptibility to hydrogen-induced corrosion depends upon a balance between hydrogen uptake, permeation, diffusion and trapping.

Alloy 718, a high performance alloy, often finds its application in extreme environments. Electrochemical hydrogen permeation studies were performed through Alloy 718 foils at 50 °C in three different metallurgical conditions: cold rolled, solutionized, and precipitation hardened. The effective hydrogen diffusion coefficient is considerably higher (5.3-6.8x10^-11 cm²/sec) for the solutionized Alloy 718 than for either the cold rolled (3.3-4.2x10^-11 cm²/sec) or precipitation hardened (2.1-2.9x10^-11 cm²/sec) specimens. Microstructural studies indicate that the reduced hydrogen diffusion coefficients in the latter specimens arise from hydrogen trapping at dislocations and precipitates that are present at much lower concentrations in the solutionized specimens. Repeated hydrogen permeation transients were conducted in three different metallurgical conditions to identify the role of irreversible trap sites. These results indicate that differences between hydrogen transport during the first and subsequent permeation transients provide evidence for irreversible hydrogen trapping in the cold rolled and precipitation hardened specimens, but not in the solutionized specimens. Mathematical studies indicate constant concentration to be the appropriate boundary condition for rise and decay transients.