Effect of surface modification, microstructure, and trapping on hydrogen diffusion coefficients in high strength alloys

Cathodic protection is widely used for corrosion prevention. However, this process generates hydrogen at the protected metal surface, and diffusion of hydrogen through the metal may cause hydrogen embrittlement or hydrogen induced stress corrosion cracking. Thus the choice of a metal for use as fasteners depends upon its hydrogen uptake, permeation, diffusivity and trapping. The diffusivity of hydrogen through four high strength alloys (AISI 4340, alloy 718, alloy 686, and alloy 59) was analyzed by an electrochemical method developed by Devanathan and Stachurski.;The effect of plasma nitriding and microstructure on hydrogen permeation through AISI 4340 was studied on six different specimens: as-received (AR) AISI 4340, nitrided samples with and without compound layer, samples quenched and tempered (Q&T) at 320° and 520°C, and nitrided samples Q&T 520°C. Studies on various nitrided specimens demonstrate that both the gamma'-Fe 4N rich compound surface layer and the deeper N diffusion layer that forms during plasma nitriding reduce the effective hydrogen diffusion coefficient, although the gamma'-Fe4N rich compound layer has a larger effect. Multiple permeation transients yield evidence for the presence of only reversible trap sites in as-received, Q&T 320 and 520 AISI 4340 specimens, and the presence of both reversible and irreversible trap sites in nitrided specimens. Moreover, the changes in microstructure during the quenching and tempering process result in a significant decrease in the diffusion coefficient of hydrogen compared to as-received specimens. In addition, density functional theory-based molecular dynamics simulations yield hydrogen diffusion coefficients through gamma'- Fe4N one order of magnitude lower than through &agr;-Fe, which supports the experimental measurements of hydrogen permeation.;The effect of microstructure and trapping was also studied in cold rolled, solutionized, and precipitation hardened Inconel 718 foils. The effective hydrogen diffusion coefficient is considerably higher for the solutionized Inconel 718 than for either the cold rolled or precipitation hardened 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 permeation transients provide evidence for irreversible hydrogen trapping in the cold rolled and precipitation hardened specimens, but such effects are insignificant in the solutionized specimens.;The effect of trapping in determining the hydrogen diffusion coefficients was also studied in alloy 686 and 59 specimens. Microstructural studies indicate the presence of bcc-Mo rich inclusions concentrated along the grain boundaries in alloy 686 specimens, but randomly distributed in alloy 59 specimens. Multiple permeation transients show an increase in diffusion coefficient values for the decay transients compared to rise transients in alloy 686 specimens. On the other hand, the first rise transient had a lower diffusion coefficient compared to successive rise and decay transients in alloy 59 specimens. Effective diffusion coefficient (Deff) values of hydrogen in multiple permeation transients suggest that hydrogen trapping sites are predominantly reversible in alloy 686, but mixed reversible and irreversible in alloy 59.