NOTE ON EMBRITTLEMENT FROM ALKALINE TIN PLATING

C. A. ZAPFFE AND M. E. HASLEM
Baltimore, Md.

 

In the June, 1950 issue of PLATING, a research was described in which hydrogen absorption by a steel cathode during electroplating was measured in terms of the embrittlement manifesting itself in a special bend test(1). Figs. 3 and 4 of that paper carried data for acid and alkaline ti’ baths, respectively. The data for the alkaline bath have since been called in question by Hedges and Lewis of the Head Office and Laboratories of the Tin Research Institute in Middlesex, England(2), and by Nekervis at that Institute’s Laboratories in Columbus, Ohio(3). The point of the objection is that the procedure used in the research did not take advantage of recent recommendations of the Institute, particularly with regard to temperature and current density.

Because the embrittlement from the alkaline bath was of serious proportions, any advantages offered by altered plating conditions warrant attention. The research was, therefore, extended to include the recommendations of the Tin Research Institute. The results comprise the present communication.

REMARKS ON TESTING
Details of the testing procedure can be foud in previous publications of the authors. The bend machine produced a constant rate of bend around a fixed pin; the specimen was 16-gauge wire; angles of fracture between 0° and 180° of bend were measured, a 180° bend representing a lack of embrittlement so far as the sensitivity of the test is concerned.

Table I contains a description of the specimens, which included both a stainless steel and a carbon steel of representative compositions. The stainless steel was from the same lot as used in the preceding research; but the SAE 1060 steel replaced the carbon steel employed in the previous research, specimens of which were no longer available.

Table II lists two plating baths, both of the alkaline sodium stannate type. The one identified as “ME” was used in the foregoing research, its formulation coming from Oplinger and Bauch’s article in the 1942 edition of “Modern Electroplating”(4). The bath identified as “TRI” is the one recommended by the Tin Research Institute(6).

In comparing the two baths, one will note that they have identical tin concentrations and that ‘the TRI bath is more alkaline and lacks the sodium acetate and hydrogen peroxide present in the ME bath. The H2O2 is omitted in the present experiments because the earlier work(1) had shown it to have no measurable effect upon embrittlement.

RESULTS FOR STAINLESS STEEL
In addition to these differences in bath composition, both current density and temperature in the previous work differed considerably from the TRI recommendation, principally because of arbitrary selections on the low side of the Oplinger-Bauch temperature range and the high side of their current-density range.

Accordingly, three principal variables were to be explored, namely, (1) bath type, (2) temperature, and (3) current density.
In Table III, the ME and TRI baths are compared with respect to all three variables. Four plating periods were used throughout, of which the first represented a very brief treatment for disclosing immediate effects of the plating, and the three longer periods serv-ed to outline the general limits which the embrittlement might reach.

In the first column of Table III, the current density (37 asf; 4 amp/dm2) is the same as that used in the previous research, but the temperature is 80°C (176° F) instead of 60°C (140°F). The data are virtually identical with those shown on the graph of Fig. 4 in the prior study.

In the second column, the data refer to the same ME bath, but at the temperature and current density recommended for the TRI bath. The temperature of 80°C (176°F) is the upper limit of the range recommended in Modern Electroplating; the current density is toward the lower end of the recommended range. Bend data’ for the longer periods of plating show no significant difference from those of the other tests, although the 30-second treatment indicates less embrittlement in early stages. This is consistent with the know loss of plating efficiency at higher current densities. Nevertheless, numerous researches now conducted on hydrogen behavior during electroplating make it clear that current efficiency is a far less important factor in determining hydrogen absorption than is commonly supposed.

In the third column of Table III, the TRI bath is compared directly with the ME bath in the preceding column, being operated at identical temperatures and current densities. The data correspond closely, but they somewhat favor the ME bath, particularly in the early plating period. This possibly reflects the higher alkalinity of the TRI bath, although the characteristically increased hydrogenizing power of alkaline baths(1) has not yet been identified with an alkalinity factor. As for a comparison between the third and first columns, the data for the’ embrittlement from the TRI bath at the recommended: 80°C (176°F) and 15 asf (1.6 amp/dm2) are identical in all but :one reading with those of the ME bath operating at the same temperature but at the higher current density of 37. asf (4 amp/dm2).

On the last column of Table III, the TRI bath is tested at a temperature of 60°C (140°F) merely for purposes of completing the comparison with the ME bath and of determining the effect of temperature in the TRI bath. There is a slight indication of improvement, particularly in the early period, which is consistent with known effects of temperature on hydrogen absorption(6).

*Treeing

RESULTS FOR CARBON STEEL
Because significant differences have been found between the behaviors of stainless and carbon steels during hydrogenizing(7), a concluding series of experiments was conducted specifically to make certain that the foregoing results are valid for carbon steel as well and to disclose the general scope of the hydrogenizing to be expected from the TRI bath. Table IV lists the bend angles for plating periods again chosen in logarithmic progression, but with a 1-minute period substituted for the 30-second period of the previous tables. The specimens were tempered to a hardness causing breakage of the blank at an angle of bend of 115 ± 10°.

Embrittlement clearly manifests itself within the first minute of plating. This effect continues on extended plating, with the bend angles attaining a considerably lower limit than shown for carbon steel in Fig. 4 of the previous paper(1); this, however, is a more sensitive steel, having a lower blank bend value. No data for the ME bath are included, because extensive previous studies make it obvious that the behavior of the TRI bath is characteristic of alkaline baths in general and that the data of Table II suffice for establishing the desired comparison.

CONCLUSIONS
On the basis of these data, one can draw the following conclusions:
(1) There is no significant difference between the ME and TRI baths so far as hydrogen embrittlement of the basis metal is concerned;
(2) There are indications of very slight advantages for (a) the lower bath temperatures, (b) the lower current densities, (c) the ME bath as compared to the TRI bath at identical temperatures and current densities, particularly during brief plating periods;
(3) Whereas the two preceding conclusions are drawn on the basis of experiments with Type 440-C stainless-steel wire, tests with the TRI bath on SAE 1060 carbon-steel wire confirm a similarity in behavior, consistent with that observed in other grades of steel and comparable to that exhibited by other alkaline tin plating solutions.

BIBLIOGRAPHY
(1). C. A. Zapffe and M. E. Haslem, “Hydrogen Embrittlement in Nickel, Tin, and Lead Electroplating”, Plating 37, No. 6, 61013 (1950).
(2). Personal communications 1950-1951.
(3). R. J. Nekervis, “Tin and Its Alloys”, Ind. Eng. Chem. 43, No. 10, 2272-2275 (1951).
(4). “Modern Electroplating”, Electrochem. Soc., Columbia University, N. Y. (1942), p. 334.
(5). “Instructions for Electrodepositing Tin”, Tin Research Inst., Middlesex, England (1950), 24 pp.
(6). C. A. Zapffe and M. E. Haslem, “Acid Composition, Concentration, Temperature, and Pickling Time as Factors in the Hydrogen Embrittlement of Mild Steel and Stainless Steel Wire”, Trans. Am. Soc. Metals 39, 213-237; disc. 237-240 (1947?.
(7). C A. Zapffe and M. E. Haslem, “Evaluation of Pickling Inhibitors from the Standpoint of Hydrogen Embrittlement. I. Acid Pickling of Stainless Steel”, Wire and Wire Products 23,- No. 10, 933-939 (1948), “II. Acid Picklg of Carbon Steel”. Ibid. 23, No. 11, 1048-1053, 1080-1082(1948); “III. Conditions of Cathodic Pickling”, Ibid. 23, No. 12, 1126-1130, 1172-1175 (1948), disc. 24, No. 1, 56, 90-92 (1949).