Permeability of iron to hydrogen cathodically generated in 0.1 M NaoH

Scripta METALLURGICA

Vol. 19, pp. 521-524, 1985 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

PERMEABILITY OF IRON TO HYDROGENCATHODICALLY GENERATED IN 0.1M NaOH T. Zakroczymski* Institute of Physical Chemistry, Polish Academy of Sciences Kasprzaka 44/52, 01-224 Warsaw, POLAND {REceived December 19, 1984) {Revised January 17, 1985) The majority of available experimental data shows that the entry rate of hydrogen into iron is significantly lower when the gas is cathodically generated in alkaline rather than in acid solutions, equally in the absence and presence of hydrogen entry promoters. For example, whilst in pure 0.1M NaOHthe observed values of the product of the steady state permeation rate and the thickness of the iron membrane (i~ " L) are of the order of 1013-1014 H atoms/m's(1-5), in dilute sulfuric acid solutions the value of i~ " L is about 1015 H atoms/m's(2,5). presence of promoters the typical values of ":Ip

In the

L are about 1014-1015 H atoms/m's in alkaline

solutions (1,5,6) and about 1015-1016 H atoms/m's in acid ones (2,5). A l i s t of l i t e r a t u r e data compiled by Kumnick and Johnson (Table I I in Ref. 7) appears to suggest that these differences are related to the very large variation of the input fugacity of hydrogen as affected by changes in electrolyte composition and cathodic current density. In contrast with acids in which hydrated protons are discharged on the cathode, in alkaline solutions water molecules are discharged and hydroxyl ions are produced at the electrolyte/cathode interface. I t has been proffered, therefore, that OH anions are co-adsorbed with H atoms on the cathode surface, inhibiting entry of H into iron (5). Many other attempts have also been made to relate the H permeation rate to the mechanism of the hydrogen evolution reaction and/or to the coverage of the cathode surface by adsorbed H atoms (8,9). Unfortunately, the exact mechanism of the hydrogen evolution reaction on iron in alkaline solutions has not yet been sufficiently explored and there is no reliable data on H coverage. In this paper, experimental evidence is given showing that the r e l a t i v e l y low permeability of iron to hydrogen evolved from alkaline solutions is caused by the formation of a barrier layer which hampers the entry of hydrogen into iron. This barrier can be removed by a s u f f i c i e n t l y long and uninterrupted cathodic charging of iron in the given alkaline solution. Experimental The permeation rate of hydrogen through Armco iron sheet specimens, 0.5 mmthick, was measured by the well known electrochemical technique (10). The output side of each test specimen was coated with a thin layer of Pd and anodically polarized in 0.1M NaOH solution to a constant potential of 0.150 V against a Hg/HgO reference electrode. The input side was chemically polished for 30s in a mixture of hydrogen peroxide with hydrofluoric acid in water. Charging with hydrogen occurred galvanostatically in a pure, deaerated, aqueous 0.1M sodium hdyroxide solution at 25±0.1°C. Results Permeability Curves The permeability versus time curves obtained are plotted in three gradually enlarged scales in Fig. la, b, and c. Under the experimental conditions employed, hydrogen began to appear on the output side of the membrane in about 10 s after switching on the cathodic current (Fig. la). During the f i r s t hour, the permeation current attained about O. 03 A/ m2. Its further increase was very slow (Fig. lb) but after about 20 h, a substantial acceleration of the hydrogen flux *Present address: Fontana Corrosion Center, Department of Metallurgical Engineering, The Ohio State University, 116 West lgth Avenue, Columbus, Ohio 43210.

521 0036-9748/85 $3.00 + .00 Copyright (c) 1985 Pergamon Press Ltd.

522

HYDROGEN PERMEABILITY

was observed (Fig. ic).

OF IRON

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19, No. 4

In 8 days, permeation reached a very high value, about 2 A/m2 (which

corresponds to ip'L = 6.2x1015 H atoms/m's), even greater than that usually measured under similar experimental conditions in acid solutions. As depicted in Fig. lc, where the 90% confidence l i m i t s are marked, the reproducibility of experimental results was not good for intermediate charging periods but improved with time. H D i f f u s i v i t y Determination After a series of preliminary experiments, the following procedure was chosen for H d i f f u s i v i t y measurements taken at three stages of uninterrupted cathodic charging, namely at ti<360 s, t2=15 h, and t3=200 h. The respective d i f f u s i v i t y values were marked D1, D2, and D3. Their determination occurred by analyzing the build up and/or decay permeation transients (11,12). A typical example of the f i r s t build up permeation transient is shown in Fig. la, and i t s analysis is given in Fig. 2.

The i p ' t 1/2 versus I / t relationship is not linear; the resulting

D1 values change with time as shown in Fig. 2b.

The highest D1 is about 1 x 10-9 m2/s.

For D2 and D3 determination, decay transients followed by build up transients were used. The cathodic charging current density was rapidly reduced from 90 A/m2 to 12 A/m2, after 60 s the current was shifted back to i t s previous value. Typical permeation transients and the results of their analysis for t2=15 h are plotted in Figs 3a and b, respectively. The d i f f u s i v i t y values, D2~ and D2~ for decay and buildup, respectively, were computed from the slopes of linear ralationships log (i~l-i)tl/2~ versus 1/t and log (ip-i~l)tl/2~ versus l / t , where m

ipl is the steady state permeation rate before change of the input current, and ip is the actual permeation rate at the chosen time t . In Fig. 3, one sees that the recovery of permeability to i t s previous steady state is slow, and Fig. 3b indicates that D2~ differs from D2f; their ratio D2~/D2~ is about 1.4. The results of the same measurements taken at t3=200 h are shown in Fig. 4. In this case the recovery of permeability occurred rapidly. The reproducibility of both the decay and build up transients was excellent; the procedure used could be repeated several times giving the same runs. D3~ was close to D3f, so that D3~/D3W ~ 1, Fig. 4b. Effect of Input Current Interruption Let us consider an iron membrane subjected to prolonged cathodic charging in 0.1M NaOHat a constant current density until an approximately steady state permeation rate is reached. I f next the input current is switched off for a time period of At, after which the same current as before is applied, a significant drop of permeability relative to the previous rate is observed. The longer At, the greater the drop that results, and the longer the charging time that is needed to restore the previous steady state permeation, Fig. 5. Discussion The above results suggest that when iron is immersed into 0.1M NaOH, some barrier layer forms at the metal/electrolyte interface, hindering the entry of hydrogen into the metal bulk. As a result, in early stages of the cathodic charging, the concentration of H beneath the input surface is low, and very low H permeation rates are observed. Moreover, an abrupt increase of the input current does not lead to a rapid increase of the concentration of H. Therefore, H d i f f u s i v i t y measurements undertaken in the early stages of charging result in low and unsteady d i f f u s i v i t y values such as D1 in Fig. 2. On the other hand, long, uninterrupted cathodic charging gives rise to a gradual removal of the surface barrier. As shown in Fig. 4, after 200 h, the H permeation reaches a surprisingly high value, close to that observed in sulfuric acid solutions, and the measured H d i f f u s i v i t y is D3=8.2x10-g m2/s, consistent with recent l i t e r a t u r e data for the l a t t i c e diffusion of hydrogen in iron. The nature of the barrier layer is not clear. Effects of input current interruption depicted in Fig. 5 suggest that the exposure of iron to 0.1M naOH solution under open c i r c u i t conditions, i . e . at corrosion potential, results in the formation of a passivating film which hinders the entry of H into Fe, most probably by blocking active sites for the adsorption of H

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HYDROGEN

PERMEABILITY

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523

atoms on the metal surface. However, the passive film can be removed by a long and uninterrupted cathodic reduction accompanied by hydrogen evolution. Presumably, as reduction progresses, more and more active sites are exposed, leading to increased adsorption of H atoms and their entry into the metal bulk. Finally, when the film is completely removed, the whole iron surface in contact with the alkaline electrolyte becomes able to adsorb and absorb hydrogen. The results of this study indicate that the input fugacity of hydrogen cathodically generated in alkaline solutions does not substantially d i f f e r from that generated in acid solutions, provided that the surface constraints, effective at high pH values, are eliminated. Acknowledgments The author wishes to express his deep gratitude to Prof. M. Smialowski for his stimulating discussions and valuable comments, and for his help in preparing the manuscript. References 1. 2. 3. 4. 5. 6. 7. 8.

M.A.V. Devanathan, Z. Stachurski, and W. Beck, J. Electrochem. Soc., 110, 886 (1963). J.O'M. Bockris, J. McBreen, and L. Nanis, J. Electrochem. Soc., 112, ]-0-~5 (1965). A.J. Kumnick and H.H. Johnson, Met. Trans., 5, 1199 (1974). H.M. Shih, M.S. Thesis, Cornell University, T974. T. Zakroczymski, Z. Szklarska-Smialowska, and M. Smialowski, Werkstoffe u. Korrosion, 26, 617 (1975); 27, 625 (1976). J. McBreen, ]~TD Thesis, University of Pennsylvania, 1965. A.J. Kumnick and H.H. Johnson, Met. Trans., 6A, 1087 (1975). J.O'M. Bockris and A.K.N. Reedy, Modern Elec~ochemistry, Vol. 2, Plenum/Rosetta Edition,

19/8. 9.

P.K. Subramanyan, Electrochemical Aspects of Hydrogen in Metals, in Comprehensive Treatise of Electrochemistry, Vol. 4, Electrochemical Materials Science, Edited by J. O'M. Bockris, et a l . , Plenum Press, 1981. 10. M.A.V. Devanathan and Z. Stachurski, Proc. Roy. Soc., A270, 90 (1962). 11. J. McBreen, L. Nanis, and W. Beck, J. Electrochem. S o c ~ 1 3 , 1219 (1966). 12. T. Zakroczymski, Corrosion, 1984, in press.

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524

HYDROGEN PERMEABILITY

OF IRON

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