Chromium and Cadmium Plating
These comparatively young members of the group of electro-deposited finishes have been the subject of much discussion and experiment since their large scale application in recent years. Chromium and Cadmium plate each hold a firmly established position in industry, now that their properties and limitations are more widely known. The former is the decorative stainless finish par-excellence, while the latter is held in high esteem for the protection of ironwork where prevention of rust is the predominating function required.
|Chromium and Cadmium plating equipment. Note the Cadmium ball anodes in cage hung at end of vat|
The attractive and lustrous appearance of chromium plate is familiar to all. Extreme hardness combined with non-tarnishing properties under severe conditions are the main characteristics of the deposited metal.
It was first deposited, as far as is known, by Bunsen in 1854. Since then, the perfection of the process and methods used are examples of painstaking attention to minute detail. Only during recent years have developments resulted in processes being available to industry which are fairly simple and reliable in operation. From this time, however, the rate of expansion has been phenomenal.
Chromium plating practice differs in several respects from that used for metals such as nickel, copper, zinc, etc. It has not been found possible to obtain anodes of chromium or chromium alloy which will maintain the strength of the electrolyte to a reasonable degree. Consequently, inert plates, usually of lead, are used for this purpose. The electrolyte consists of a strong solution of chromic acid which contains a small proportion of sulphate. This may be introduced as sulphuric acid, and an accurately maintained proportion is necessary to yield the best deposits.
The solution is worked warm, at 900 Degrees Fah. to 1000 Degrees Fah. Here again conditions are critical and close control must be maintained. High current densities in the order of 60 to 80 amperes per square foot at 2.5 volts are required for plating. This means that provision must be made for an ample supply of current with heavy connections. in practice parts to be plated are not suspended with the usual thin copper wires, special jigs of heavy gauge brass strip being designed to suit the various individual parts. The chromium deposited is drawn entirely from the chromic acid solution which needs replenishing with this chemical from time to time.
Chromic acid is a dangerous and highly corrosive chemical. in addition, the liquid is evolved as a fine spray during plating because of the large amount of gas liberated from the cathode. Consequently, vats are specially constructed, being fitted with fans and exhaust ducts which remove and condense the dangerous fumes.
It is necessary to comply with stringent Home Office regulations which ensure that the operators are adequately protected from contact with the liquid.
While chromium itself is quite stainless, a chromium deposit alone gives little or no protection to base metals against corrosion. This is because the deposits can be penetrated by corrosive atmospheres, and rust develops below the plated surface. To overcome this, the articles are first heavily nickel plated, and finally treated in the chromium vat. Most of the original failures were due to lack of knowledge on this point.
When bright deposits are desired, the nickel plate is polished prior to chromium plating. The latter deposit, under these circumstances, is bright when deposited and needs little or no final treatment.
Lack of throwing power is at present the chief difficulty which confronts the chromium plater. By this is meant, that,
it is very difficult to deposit the metal in narrow recesses or in the interior of deep articles. Improvement is this respect is being gradually obtained as the technique of the process is more widely appreciated. In some instances, parts are being altered in design, so that shapes are simpler and therefore more suitable for the deposit.
In the telephone industry, the wide application of moulded instruments has decreased the demand for decorative plated parts. Were it not for this fact, chromium plate would undoubtedly be one of the most important of finishes. Nevertheless, the Ericsson Company has taken every opportunity to utilise the finish wherever plated external instrument parts are demanded. These include intercommunication set fittings such as plates, push buttons, etc. certain auto dial parts, and a variety of equipment used on time recording clocks or similar apparatus. Users will no doubt appreciate the refinement.
This element is a member of the group of metals which are anodic or electro-positive to iron. Extremely thin deposited films give excellent protection against rust even under severe conditions. This is because chemical corrosion is diverted by the minute potential which develops at the point of attack, between the cadmium and the exposed basic metal.
The generally accepted standards for deposit thicknesses to give good rust resistance to iron under adverse conditions are approximately:-
For this reason the last named is finding its most important application in the finishing of accurately machined parts of mechanisms where minimum alteration to dimensions is desirable.
The high cost of Cadmium is one of the main reasons for the limited use of the electro deposit. As far as the actual plating process is concerned, little difficulty is presented. Cyanide solutions are generally used with anodes of the metal itself. These usually take the form of cast balls, suspended in iron cages, so that they may be used up entirely with practically no waste. The deposit is silver in colour, possessing an attractive appearance when polished or scratch-brushed. The appearance of parts as they leave the vat is, however, inclined to be rather greyish. This is removed by a rapid dip in dilute acid.
In practice it has generally been found that deposits of cadmium tend to discolour, unsightly dark stains developing after extended periods. While this is no actual disadvantage from the point of view of rust protection, the effect has been to retard the utilisation of the finish. The effect can be largely overcome, however, by coating the surface with a thin film of mineral wax, from a suitable solution.
A number of concerns are employing cadmium plate as an undercoat, prior to the nickel plating of iron. It is claimed that the rust-proof quality of this finish is superior to that of nickel alone. Recent extensive tests by the American Electroplaters Society and other authorities tend to discount this, and evidently it is desirable that further investigations should be carried out regarding this point.
Other users have found that even when cadmium deposits are removed by wear, a definitely improved resistance to rust remains. This is explained by the theory that the cadmium alloys slightly with the iron surface. Designers are taking advantage of this feature by using the finish for cams and gears which are subject to wear.
At the present time, cadmium plate is not used to any great extent in the telephone industry. Where the properties outlined are found to be of advantage, however, the finish is being used with good results. Continued experiment with this deposit and patient observation of results will no doubt amply repay those who continue to investigate the possibilities in this field.
|BACK||Home page||BT/GPO Telephones||Ericsson Telephones||Ericsson Home Page||Ericsson PABX||All Telephone Systems|