Anti-Corrosion Methods and MaterialsCORROSION MYSTERY WITH CADMIUM PLATING

Article information:To cite this document:, (1962),"CORROSION MYSTERY WITH CADMIUM PLATING", Anti-Corrosion Methods and Materials, Vol. 9 Iss 10 pp. 266- 266Permanent link to this document:http://dx.doi.org/10.1108/eb020006

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CORROSION

CT CASE

HISTORY

CORROSION MYSTERY WITH CADMIUM PLATING

ONE of the more interesting cor-rosion problems investigated during the current year occurred at a manufactory of electronic equip-ment. Located in the north of Eng-land, this company specialised in the production of audio frequency ampli-fiers of a very high quality, and the management were disturbed, over a period of five years, by sporadic com-plaints of corrosion on a small pro-portion of their chassis units. It took the form of dark grey or black patches, up to about 2 sq.in. in area, which did not develop until the units had been built into specially made wooden cabinets at a sub-contractor's premises. An interval of several months could pass between manufacture and instal-lation, and units inspected after stor-age and immediately prior to dispatch to the sub-contractor were sub-sequently returned in the condition described.

The company concerned had no laboratory, no metallurgist and no chemist; in short, there was no scape-goat at all. The small plating shop, which deposited zinc, cadmium or nickel as required, had in some way penetrated into the responsibility orbit of the chief designer and was kept under remote technical control by the plating materials supply house.

The first reaction was, of course, to blame the plating, but the supply house extricated themselves from this dangerous position with considerable adroitness and under the cover of heavy technical artilleryan open verdict was returned at each of the inquests. The cabinet makers were then investigated, it being thought possible that the effect was initiated by contact with hardwoods, some of which are known, in the presence of moisture, to display malevolent ten-dencies to adjacent metallic materials. This promising theory was promptly repudiated by the cabinet makers, who were able to demonstrate that corrosion occurred least at areas which should have been affected most had this mechanism been operative.

From this point, the hypotheses advanced grew more ingenious until, the phenomenon being adjudged super-natural in origin, no loss in dignity was involved in co-opting professional assistance.

The localised nature and random distribution of corroded areas were strongly suggestive of some form of contamination. Inspection of the plating facilities and the storage con-ditionsthese, in particular, were ideal, being warm and dryoffered no possible explanation. Qualitative examination of corrosion product yiel-ded little, only the smallest traces of chloride being detectable, and the cause of the contamination was obscure until the chassis was examined by artificial light. It was then possible to discern a fingerprint in one of the corroded areas, the corrosion product reproduced, in considerable detail, the epidermal convolutions. This was much less apparent in normal day-light, but on continued examination it was found that the greater majority of corroded areas contained at least identifiable portions of fingerprints. Fig. 1 is a photograph of one of the more complete specimens.

From this point the situation could be readily resolved. The chassis itself was cadmium plated. Zinc, copper and nickel surfaces were completely free from corrosion. The most heavily corroded areas on the cadmium surface were precisely those most handled during the assembly of components

into the chassis. At areas where it was physically impossible for the finger to penetrate, there was no corrosion, and the limit of such areas was quite frequently delineated by a corroded zone.

Body fluid variation Body fluids conform to quite narrow

ranges of composition, so much so, in fact, that minor variations can often indicate a serious pathological con-dition. Perspiration normally tends to be corrosive, being mildly acidic and saline in character.

In the assembly of these amplifiers, female labour was employed and, during menstruation, there is not only a much greater tendency for the hands to perspire but there is also a change in the nature and composition of the perspiration. Certain individuals, at such times, can become potent sources of corrosion, and during the war years dangers of this type were eliminated by the removal of operatives during this active period to work of a different character. When the war passed, and long-term storage was no longer a requirement of the product, the rather tiresome organisational procedures were gratefully forgotten. There are, of course, many references to this characteristic of cadmium in the literature: U. R. Evans, in Metallic Corrosion, Passivity, and Protection, is explicit on this point'Cadmium plating . . . being somewhat sensitive to perspiration, should not be applied to articles which have to be handled'.

An interesting feature of this case was the dormant nature of the pheno-menon. A chassis could be assembled, inspected, put into storage for a con-siderable period, withdrawn, again inspected and passed, issued to the sub-contractor, and corrode within a few days. The residues were clearly insufficient to initiate corrosion alone and were inactive under the warm, dry storage conditions. During instal-lation into the cabinets, conditions were less ideal and there was no longer a suspension of the corrosive processes.

266 CORROSION TECHNOLOGY October 1962

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