Surface wave studies of H entry into steel

Scripta

METALLURGICA

V o l . 15, pp. 5 1 3 - 5 1 6 , 1981 Printed in t h e U . S . A .

Pergamon P r e s s Ltd. All r i g h t s r e s e r v e d

SURFACE WAVE STUDIES OF H ENTRY INTO STEEL

Ellina Lunarska I and N. F. Fiore 2 i) Institute of Physical Chemistry Polish Academy of Sciences Warsaw 42, Poland 2) Dept. of Met. Eng. and Mat. Sci. University of Notre Dame Notre Dame, IN 46556 (Received

February

23,

1981)

Introduction Internal friction is extremely sensitive to the presence of point defects in solids, so it has been used [i] in the study of H solubility and H embrittlement (HE) in steels. A limitation of this technique is that when H is entering a sample via an electrolytic process, e.g., corrosion, plating, cathodic charging, the sample is immersed in a liquid electrolyte. Since internal friction or damping experiments usually require that the sample be vibrated, in situ studies are difficult since the electrolyte causes large extraneous damping effects. In an effort to overcome this difficulty, an ultrasonic technique has been developed [2] in which Rayleigh surface waves (RW) are propagated on a material from a transmitting transducer to a receiving transducer while the material is undergoing cathodic charging with H (Figure i). The change of amplitude of the wave is inversely related to its attenuation (or internal friction) change d~ring charging, Changes in transit time of the wave between the two transducers of fixed distance of separation are related to changes in the RW velocity. The RW penetrates approximately one wavelength ~ of the surface, but its intensity decays exponentially with depth [ 3 ] . Consequently the wave samples the near-surface region preferentially and is sensitive to changes in attenuation ~ and velocity v R occurring at depths as small as 10-1to i0 -~ ~ [4]. For a typical ferrous material, v R ; 3 x 103 m/s [5], so that at a typical test frequency f of 3 MH 3, % - vR/f 4 lO-3m; thus the technique can yield information about processes occurring within the first i0 -~ m (one Dm) of the sample surface. This high resolution along with the capability for in situ measureR F Pulse Genera?or and F ments were the basis for the charging studies on ~t,enuot. . . . . d ,r'J -j.7 Receiver Fe described in this note. Veloclt y Recorder L •

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Experimental The studies were conducted on strips of hotrolled, low-carbon steel (0.08C, 0.53 Mn, 0.OIIP, 0.008S). The strips were 170 mm x 25 mm x 1.8 mm thick, bent to an L shape to be accommodated in the fixture. The surfaces of the strips were polished to a 6Nm diamond finish to eliminate surface roughness as a variable. The details of the experimental system are described in Reference 2. In brief, RW transducers (Automation Industries, Type SMZ) were bonded to the strips with stopcock grease (Fisher Nomaq). The ultrasonic signals at frequencies

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513 0036-9748/81/050513-04502.00/0 Copyright (c) 1 9 8 1 P e r g a m o n Press

Ltd.

514

H ENTRY

from 2.5 to 7 M]]z, were generated, Inc. ) .

INTO

STEEL

Vol.

15,

No.

5

received and recorded by means of a commercial system (MATEC,

The system was tuned, with the sample in air, to maximum received signal amplitude. The electrolyte, 1% H2S04, was then poured into the cell, and the sample was activated cathodically at 2 mA/cm 2 for 3 min. to clean the surface. Then a and v R were determined as a function of cathodic current density to 5.5 mA/cm 2 and charging time to 3h. Solutions containing As +3 were injected into the electrolyte to evaluate this ionic species' effectiveness as a charging promoter (H recombination poison). In addition, Cu +2 was added to determine the influence of an extraneous metal ion on ~ and v R. Results and Discussion When a sample was placed into the pure H2SO 4 solution and polarized, attenuation and velocity decreased a few percent over a period of hours, an effect observed for a Ni-base alloy [2]. When however, 2 mg/l of As203 was added at any polarization, a marked decrease in attentuation instantly took place, (Figure 2a). In case of a Ni alloy, the decrease appeared to be [2] associated with the H-recombination poisoning effects of As, which promoted the entry of H atoms into the sample surface. The possibility exists that the decrease in ~ and v R in the presence of As is not related to H. It has been shown [6] that ionic species change the capacitance of the electrolyte at a sample surface during charging, and that such changes may influence RW behavior. Moreover, As can plate out on Fe, although predominantly at the corrosion potential not at cathodic potentials [7], and the metallic film could influence RW behavior. For these reasons the possibility that extraneous ions may influence ~ and v was examined by injecting a fairly concentrated solution (0.5 g/l) of CuS04 into the electrolyte. No change in ~ was observed (Figure 2b) although Cu deposition on the sample surface was visible to the naked eye. This is strong evidence that the decreases in ~ and v are related to the entry of II atoms into the sample rather than to any capacitance or plating effects.

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FIG. 2b Surface Wave Attenuation is Independent of Presence of Metallic Ions in Electrolyte.

Since the changes in ~ and v R in the steel at moderate current densities were so marked and appeared to be H-related, a series of experiments to better document the effects of charging time, current density and surface coating in the poisoned electrolyte were undertaken. The results are summarized in Figure 3. Figure 3a shows the decrease in v as a function of square root of charging time t at 4.4 mA/cm 2 . The velocity decreases quite ~apidly initially, reaches a plateau and then gradually decreases further. At lower current densities the velocity changes are smaller. Figure 3b summarizes the attenuation data. Curve i, for an unpolarized sample, shows that after an incubation time of about 16 min, ~ decreases very slowly in the electrolyte containing 2 mg/l As203. The role of polarization is to accelerate this effect. The incubation time decreases as current density increases; moreover the linear decrease of ~ with t I/2 is steeper at the higher current density (Curves 2 and 3). The marked effect of current density on ~ is further evidence that the technique is sensitive to the presence of electrolytically generated H.

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15,

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5

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FIG. 3a Change of Surface Wave Velocity During H Ingress.

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FIG. 3b Change of Surface Wave Attenuation During H Ingress. (Primed Curves are for Cu-Coated Samples).

The sensitivity of ~ and v R to H-related effects is borne out by the second portions of Curves 2 and 3, which show an increase in ~ beyond some critical charging time. Microscopic examination of the samples shows that sub-surface blisters are forming when ~ begins to increase. The plateau and gradual decrease regions in the relative velocity plot correspond to the onset of blister formation. When the polarization is removed, G and velocity tend to return to their {)riginal values, but because of the irreversible H damage, they remain at higher levels than in the uncharged samples. Chatterjee et al [8] have shown that a layer (even a semi-continuous one) of Cu decreases H permeation rate through Fe. In an effort to relate their results to the attenuation measurements, steel samples were covered by electrolytic displacement with a 2 pm-thick layer of Cu and then charged at 2.2 and 4.4 mA/cm 2. As may be seen from Figure 3 (Curves 2' and 3') Cu-coating increases the incubation time for the linear decrease in ~ at a given current density. In addition, the time required to form blisters at a given current density is lengthened by Cu-coating. These data are consistent with the interpretation that a Cu layer decreases H permeation rate and that changes in ~ and v R are indeed caused by H atoms in solid solution at the surface region of the sample.

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density equal to 5.5 m A / ~ . The decrease in ~, its increase upon blistering, and the blistering itself (Figure 5b) are evident. The greater the current density (H fugacity), the nearer to the surface damage occurs. The RW wavelength % is about 500 Nm at 6 MHz, yet most of the blistering occurs in the first 50 ~m of the surface region. This illustrates again that the RW technique is extremely sensitive to effects which occur at a fraction of %.

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Figure 4 gives evidence of the H damage at various current densities. At 3Mll3 the initial decrease in :z is less marked at 2.7 mA/cm 2 (Curve i) than at 4.4 mA/cm 2 (Curve 2). The blistering which marks the minimum in ~ is apparent in cross sections of the sample (Figure 5a). Curve 3 shows ~ versus t i/2 at 6 MHz with the current

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Surface Wave Attenuation as a Function of Frequency, Charging Time and Current Density.

516

H ENTRY

INTO

STEEL

Vol.

FIG. 5a

15,

No.

5

FIG. 5b

Blistering in H-charged Samples at Minimum of ~ vs t Plots (Figure 4). a) Charged at 2.7 mA/cm 2. Tested at 3 MHz. b) Charged at 5.-5 mA/cm 2. Tested at 6 MHz. The magnification of the attenuation changes at the higher frequency is in keeping with the generally observed increase in internal friction losses with increasing f [9]. The exact nature of the damping losses, especially those associated with the rapid decrease in ~ before blistering, is difficult to determine. Decreases in ~ during charging may be caused by decreases in the dislocation damping [i0]. If the changes in ~ are caused by the pinning of dislocations by H atoms and the subsequent decrease in dislocation damping, then a plot of ~_I~ versus t ]~ should be linear. In the present work ~, not ~-I~ varies as t 1~ so the classic pinning model is inapplicable. On the other hand, the present work involved H-charging at high fugacities. The high concentration of H dissolved near the surface may produce high internal stresses and a strained surface region of high dislocation density [11,12]. This process, equivalent to work hardening at the surface, may account for a decrease in dislocation mobility and therefore dislocation damping. A further observation in support of this hypothesis is the observed decrease in v R during charging, since the decrease in ultrasonic wave velocity in the presence of applied stress has been documented in the literature [13,14]. Although the details of the kinetics of the changes in ~ and v are not clear at this time, it is clear that the surface wave technique allows monitoring of th~ entry of H atoms into the surface region of Fe during electrolytic charging. The changes are magnified by the presence of As ions, an H recombination poison, and are not affected by Cu ions, which do not influence H recombination. They are decreased by the presence of a deposited Cu film on the sample surface, which is a barrier to H permeation. Finally, the changes in ~ and v R clearly reflect the behavior of H dissolved in the outer 50 ~m of the sample surface, a distance which is a fraction of the RW wavelength. Acknowledgments This work was supported by the Division of Materials

Research,

National

Science Foundation.

References 1 2 3 4 5 6 7 8. 9. i0. Ii. 12. 13. 14.

E. Lunarska, A. Zielinski and M. Smialowski, Acta Met 25, 305-309 (1977). E. Lunarska and N. F. Fiore, J. of Applied Physics (in press). H. Kolsky, Stress Waves in Solids (Dover Publications, New York, 1963), p. 16. T . L . Szabo, J. Appl. Phys. 46, 1448-1454 (1975). J. de Klerk, Physics Today 25, 32-40 (1972). L . H . Hall, J. Acoust. Soc. Am. 24, 704-709 (1953). E. Lunarska, S. Sklarska-Smialowska and M. Smialowski, Werkstoffe and Korrosion, 26, 624-628 (1975). S. S. Chatterjee, B. G. Aleya and H. W. Pickering, Met. Trans. 9a, 389-95 (1978). R. Truell, C. Elbaum and B. Chick, Ultrasonic Methods in Solid State Physics, (Academic Press, New York, 1969). Ch. 3. A. Granato, A. Hikata and K. Lucke, Acta Met 6, 470-480 (1968). M. Smialowski, Scripta Met 13, 393-395 (1979). E. Lunarska and M. Smialowski, Soviet Mater. Sci. 4, 3-6 (1973). N. Noronha, J. R. Chapman and J. J. Wert, J. Testing and Evaluation I, 209-214, (1973). J. Szilard and R. Haynes, Brit. J. of NDT 8, 128-136 (1980).