cathodic charging during hydrogen embrittlement...

File: zzCathodic Charging_Qian_Atrens_140619.doc 1 of 6 Thursday, June 19, 2014

Cathodic charging during hydrogen embrittlement testing

Qian Liu, Andrej Atrens

The University of Queensland, Materials Engin, St Lucia, Qld, Australia 4072

[email protected]

Summary Cathodic charging is a common method by which to introduce hydrogen during hydrogen embrittlement research. This work reviews our recent research at UQ into the evaluation of the equivalent hydrogen pressure under conditions of constant applied electrochemical potential. Included are the following: (i) theoretical evaluation of the thermodynamic relationship between pressure and cathodic charging parameters, (ii) measurement of the key quantities, (iii) influence of surface condition, and (iv) influence of solution composition. This review includes typical evaluations from our recent research.

1. Introduction Hydrogen in steels can lead to catastrophic failures. Consequently, there have been many studies into the influences of hydrogen on the properties of steels, such as those of our group [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]. There are typically two methods by which to introduce the hydrogen into the steel. Gas phase charging typically charges the steel at high pressures and high temperatures, so that there is a substantial amount of hydrogen in the steel when it is back at room temperature. The alternative method is to introduce the hydrogen using cathodic charging. The steel is made a cathode in an electrochemical cell, hydrogen is liberated at the steel surface by applied electrochemical potential or applied current, and some of the hydrogen at the steel surface enters the steel.

Cathodic charging is easy to use. Cathodic charging suffers from the disadvantage, however, that it is not easy to provide a meaningful characterization of the activity or pressure of hydrogen that is in the steel, although it is possible to measure the amount of hydrogen in the steel, see e.g. [2,6,7].

Liu et al. [1] developed a method to characterize the equivalent hydrogen pressure during such cathodic charging, based on the approach suggested by Atrens et al. [23], based on [24,25,26]. The modified Nernst equation provides a relationship between the hydrogen activity (or pressure, or fugacity) at a steel surface during cathodic charging and the overpotential, η, which can be written as:

fH2 = Aexp−ηFζRT"

#$

%

&' , (1)

where F is the Faraday, R the gas constant, T the absolute temperature, and A and ζ


are constants, related to the mechanism of the hydrogen evolution reaction (HER), which was discussed in detail by Liu et al [1]. The overpotential,η, is given by

η = Ec – EH0 (2)

where EH0 is the equilibrium potential at the steel surface in the charging solution of

the hydrogen evolution reaction (which is by definition at a hydrogen fugacity of one atmosphere pressure), and Ec is the applied potential. EH

0 can be determined experimentally [23], or can be also calculated using the Nernst equation.

Hydrogen, at a hydrogen fugacity of, fH2 , causes a hydrogen concentration, CH, in the steel in equilibrium with the gaseous hydrogen atmosphere, which is given by

CH = S √ fH2 (3)

This hydrogen concentration can be measured as the hydrogen concentration at steady state for an ideal permeability experiment, and is given by [27]:

CH =i∞LFD

(4)

where i∞ is the steady state permeation current density, and L is the permeability specimen thickness.

A rearrangement of these equations, gives the following expression:

i∞=FDSL

fH2

e( )1/2

=FDSL

Aexp −ηFζRT

#

$%

&

'(

#

$%%

&

'((

1/2

(5)

where fH2e is the fugacity during cathodic charging.

This means that it is possible to use permeability experiments to determine the critical parameters in Eq.(1).

2. Experimental Method Liu et al. [1] provided full experimental details of how the permeation experiments were conducted using an annealed low interstitial steel, which was essentially pure iron, after 48 h pre-charging in the solution to condition the surface to a reproducible condition.

3. Results Fig. 1 presents a typical set of hydrogen permeation transients which were measured using different cathodic potentials at the input side of the low interstitial steel specimen in the 0.1 M NaOH solution after 48 h charging at -1.500 VAg/AgCl. Each permeability transient was fitted to the theoretical expression for an ideal transient, and the operative value of the diffusion coefficient, D, was obtained. These were in good agreement for pure iron.


Fig. 1 Hydrogen permeation transients [1] measured at different cathodic potentials at the input side of the low interstitial steel specimen in the 0.1M NaOH solution after 48 h charging at -1.500 VAg/AgCl: P1: -1.200 VAg/AgCl; P2: -1.300 VAg/AgCl; P3: -1.400 VAg/AgCl; P4: -1.500 VAg/AgCl; P5: -1.600 VAg/AgCl; P6: -1.700VAg/AgCl; P7: -1.800 VAg/AgCl.

Fig. 2 presents the steady-state hydrogen permeation current density, i∞ (µA cm-2), vs. the overpotential, η (V), for the low interstitial steel (triangles and squares), measured using the 0.1 M NaOH solution in the input side of the permeation cell. The circles represent data from Bockris et al. [73] in the same solution. The unknown constants in Eq(1), were evaluated by fitting the data in Fig. 2, and the resultant fugacity is presented in Fig. 3.

4. Discussion The hydrogen evolution reaction at a metal surface has the following three steps in an alkaline solution [24,25,26]:

H2O + M + e- → MHads + OH- (6)

2MHads → H2 + 2M (7)

MHads + H2O + e- → H2 + OH- + M (8)

Eq. (6) represents electrochemical discharge of hydrogen from a water molecule. This predominates for a small coverage of hydrogen atoms on the metal surface at small values of the overpotential. The adsorbed hydrogen atoms can move around the surface, until two such atoms meet to form a hydrogen molecule via Eq. (7), which can leave the metal surface as a hydrogen bubble together with other hydrogen molecules.


Fig. 2 The steady-state hydrogen permeation current density [1], i∞ (µA cm-2), vs. the overpotential, η (V), for the low interstitial steel (triangles and squares), obtained using the 0.1 M NaOH solution in the input side of the permeation cell, for specimens of the stated thickness, L. The circles represent data from Bockris et al. [73] in the same solution.

Fig. 3 Hydrogen fugacity, fH2, versus overpotential, η (V), for the low interstitial steel in (i) 0.1 M NaOH solution, and (ii) acidified pH 2 0.1 M Na2SO4 solution. The open circles were evaluated from permeation data in 0.1 M NaOH from Bockris et al. [26].


At high values of surface coverage of hydrogen, electrochemical desorption via Eq(8) becomes more likely, so that then the hydrogen production can occur via the two reactions, Eq. (6) and (8).

The hydrogen adsorbed on the surface is in local equilibrium with the hydrogen dissolved in the steel, by the following reaction:

MHads ⇔ MHabs (9)

The knees in the relations shown in Figs. 2 and 3 were attributed to an increasing amount of hydrogen liberated by electrochemical desorption for higher over-potentials

The hydrogen fugacity in the 0.1 M NaOH solution evaluated in the work of Liu et al [1] were significantly greater than those evaluated from the data of Bockris et al [26] for the same solution. This difference is attributed to a greater quantity of hydrogen liberated and easier Hydrogen ingress, associated with less surface coverage by oxides, because the data of Liu et al [1] pertain to measurements after cathodic precharging for 48 h, which was not carried out by Bockris et al [26]. Similarly, the higher fugacity in the 0.1 M NaOH solution is attributed to differences in surface oxide coverage in the two solutions.

5. Conclusions 1. A method has been developed to evaluate the hydrogen fugacity during cathodic

charging.

2. The method has been applied to 0.1 M NaOH solution, and the acidified pH 2 0.1M Na2SO4 solution.

3. The differences in hydrogen fugacity values were attributed to differences in the hydrogen evolution reaction, and differences in surface state, including surface oxide coverage.

6. Acknowledgements This work is supported by an Australian Research Council linkage grant and Alstom (Switzerland) Ltd.

7. References

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[9] S Ramamurthy, A Atrens, The influence of applied stress rate on the stress corrosion cracking of 4340 and 3.5NiCrMoV steels in distilled water at 30 °C, Corrosion Science, 52 (2010) 1042–1051

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[23] A. Atrens, D. Mezzanotte, N.F. Fiore, M.A. Genshaw, Electrochemical studies of hydrogen diffusion and permeability in Ni, Corrosion Science 20 (1980) 673-684

[24] J.O.M. Bockris, J. McBreen, L. Nanis, The Hydrogen Evolution Kinetics and Hydrogen Entry into a-Iron, Journal of The Electrochemical Society 112 (1965) 1025-1031.

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within metals in terms of the overpotential developed during the evolution of hydrogen, Electrochimica Acta 16 (1971) 2169-2179

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cathodic charging during hydrogen embrittlement...

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