Measurement and modelling of hydrogen uptake and transport Alan Turnbull
Hydrogen gas (+ H 2 O vapour, H 2 S) dissociation General and localised corrosion, cathodic protection, galvanic coupling; electroplating Material processing Welding
Gas impurities such as O 2, O, can block hydrogen entry to an extent A surface oxide will tend to reduce effective solubility 0 ad 1 ad k k abs des k. k o m RAs in aqueous solutions H H H H H Refilming occurs in aqueous media but not in pure H 2 gas
H 2 (g) H + +e - H 2 O + e - H 2 (aq) H ads +OH - H ads +H ads H ads +H + + e- H abs
YES - In most practical applications, hydrogen can be reduced and be absorbed at potentials above that associated with the nominal reversible electrode potential Rate of generation may become transport limited - dependent on rate of transfer of dissolved molecular hydrogen away from the reacting surface and rate of absorption of H atoms
Hydrogen permeation Thermal desorption ocal distribution: micro autoradiography, hydrogen microprint technique, SIMS, atom probe tomography..
Electrochemical hydrogen permeation measurement For field measurements, preferable not to disturb surface at which measurements are made so techniques that simply analyse evolved hydrogen gas are best
k = k 0 exp[(-v 0 )/RT] p = p 0 exp[-(v 0 - E)/RT] 1 1 1 1 1 1 p ) (1 k N = t p ) (1 k N = t 2 2 2 2 2 2 t - t - σ.v RT ωd - + ωd = t 2 1 h H 2 (V 0 equivalent to E in figure)
Trap occupancy: / N exp[ E / RT) =. 1 / N exp[ E / RT) Binding Energy / kj/mole (ev) Trap occupancy 40 (0.41) 0.045 45 (0.47) 0.26 50 (0.52) 0.73 55 (0.57) 0.95 60 (0.62) 0.99 65 (0.67) 1.00 70 (0.73) 1.00 75 (0.78) 1.00 80 (0.83) 1.00 Trap occupancy assuming low of 2x10 15 atoms cm -3 (4 10-4 ppm by mass)
1.0 disch / init 0.8 0.6 0.4 0.2 0.0 80 kj/mole 70 kj/mole 65 kj/mole 60 kj/mole 55 kj/mole 50 kj/mole 45 kj/mole 40 kj/mole 0 20 40 60 80 100 120 140 Time / d alculated leakage rate from uniformly precharged 13r steel (2 mm thick) with varying binding energy
Not really effective for continuous exposure Very deep traps could be beneficial when hydrogen generation is short lived as in electroplating, welding temperature of subsequent application in service needs to be considered key parameter is release rate of hydrogen from deep traps Fixing hydrogen safely upon cooling (from hydro-reactors) - cooling rate a factor? Possible benefit if exposure involves short transients? - trap density and depth has to be managed?
J in D n DV RT H h n ic F i r F chem i elect r F V H kabs exp h ad kdes0(1 ad ) RT predicts limiting cases of diffusion control of sub-surface hydrogen concentration, 0, and constant flux control predicts limit to 0 with respect to charging conditions allows assessment of net effect of complex and spatially variable source of hydrogen charging (e.g. local vs bulk charging at crack tip)
Diffusion flux small compared to surface kinetics (e.g. thick specimens) Diffusion flux very large compared to surface kinetics (e.g. very thin specimens) J in D n DV RT H h n i c F
Normalised permeation current density 1.0 0.8 0.6 0.4 0.2 0.0 i c =4.8 A/m 2 thick i c =4.8 A/m 2 thin i c =0.48 A/m 2 thick i c =0.48 A/m 2 thin i c =0.048 A /m 2 thick i c =0.048 A/m 2 thin 0 500 1000 1500 Dimensionless time (Dt/ 2 )
For validity: fast exchange between traps and lattice (after Oriani) t = D eff 2 + - D eff R T. V H m 0 (no deep traps and lattice concentration low) gives Fick s law with: D eff 1 N N r D exp E / RT) 1 D N k r p r r In practice, D eff often used when Fick s law is not applicable and based on time-lag or time-tobreakthrough. It is then sometimes used erroneously to derive total diffusible hydrogen concentration, 0R (reversible and lattice hydrogen), from steady-state permeation data only valid for 0 i ss F = D 0 F = D eff 0R F
alculated D eff (time-lag) as function of lattice sub-surface H concentration: 23
Activation energy based on time-lag D eff is function of trap occupancy/charging conditions e.g. DSS 1E-13 22 r (1 ma/cm 2 ) 22 r zero occupancy 25 r (1 ma/cm 2 ) 25 r zero occupancy D eff / m 2 s -1 1E-14 Ea = 39.6 kj/mole 1E-15 52.7 kj/mole 45.6 kj/mole 55.8 kj/mole 0.0028 0.0029 0.0030 0.0031 0.0032 0.0033 0.0034 0.0035 1/T / K -1
Temperature dependence of time-lag D eff for RAs under P conditions D eff / m 2 s -1 10-11 10-12 10-13 super 13 r Ni 21,22 22 r 18 Alloy G 6 245 SMO 17 82T SS 25 Alloy 600 23 Alloy 600 24 Alloy 750 SA 23 Alloy 750 DirA 23 Alloy 750 SA 24 Alloy 718 SA 23 Alloy 718 DirA 23 10-14 AISI 300 7,8 10-15 -276 6 25 r 18 10-16 0.0024 0.0028 0.0032 0.0036 1/T / K -1
H 2 (g) H + +e - H 2 O + e - H 2 (aq) H ads +OH - H ads +H ads H ads +H + + e- H abs
0 /ppm 10-2 k chem r = 212 A/cm 2 (k elect r =0.048 A/cm 2 ) k chem r = 21 A/cm 2 (k elect r =0.048 A/cm 2 ) k r elect = 0.0048 A/cm 2 (k r chem =212 A/cm 2 ) 10-3 10-4 10-6 10-4 10-2 10 0 harging urrent Density/A cm -2
Bulk charging acidic solutions, H 2 S (other recombination poisons), flow accelerated corrosion (e.g. ANDU reactor feedpipe), overprotection favours bulk uptake; oating favours local uptake? Precharging and coating (e.g. d) and test in air coating will fail as soon as crack propagates and hydrogen will be effusing out of the crack tip ocation of maximum in hydrogen concentration ahead of crack tip affected by source?
Permeation results do not correlate with cracking suggesting local uptake is critical Reliable work of Johnson et al (NTNU) suggests conclusion cannot be generalised function of charging conditions and temperature
i p (Acm -2 ) i c (Acm -2 ) alcareous scale from seawater exposure under P kinetics of H 2 O reduction not so affected uptake? - results tend to be variable; best to assume no benefit 0.7 0.6 150 SMSS in seawater under P 0.5 0.4 i p 125 100 0.3 75 0.2 0.1 i c 50 25 FeS 0.0 0 5 10 15 20 25 30 Time (days) enhanced cathodic kinetics barrier to H uptake - film cracking and delamination a factor 0 FeO 3?
hallenges in modelling hydrogen diffusion
Relative solubilities, diffusivities, as well as interfacial trapping For DSS; Turnbull et al. neglected diffusion in austenite at ambient T but accounted for trapping at interface, tortuosity and orientation (model will be limited at elevated T) Olden et al, Kalmin et al (Maxwell-Garnet D eff ) neglected trapping at interface but consider diffusion in both phases, with partitioning No coherent model that embraces all factors is it feasible? Theoretical permeation curves for parallel phase model accounting for higher solubility in austenite to show minimal effect of diffusion in austenite 22 r DSS - longitudinal
Issue of dynamic traps, e.g. voids, and cyclic stress Models of stress/strain distribution? Does hydrogen change the local material properties: non-linear problem? Boundary conditions at crack tip and crack walls tend to be over simplified - challenging to quantify Models often invoke D eff without accounting for concentration dependence or trap density after Gangloff t N N R p p r r r r h t - V. T D - + D = t H 2
IHE, HEE and H-assisted S and fatigue after ynch H-stabilised excess vacancies from deformation facilitating nano-void development? HI (H pressure mechanism ) HI is not a mechanism of crack advance per se but a different way of generating the stress through internal hydrogen pressure at voids
Measurement and modelling of hydrogen diffusivity is critical in guiding test duration, evaluating conditions for controlled effusion of hydrogen, delayed hydrogen cracking, understanding effect of microstructural traps, supporting predictive models of cracking The threshold for cracking in aqueous media may be critically linked to resistance to hydrogen entry o e.g. higher alloyed high strength MSS steel may be more resistant to crack initiation than a lower alloyed medium strength MSS because of faster refilming or resistant to pitting