The Commercial Application of CHROMIUM PLATING
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MANY industrialists are familiar with chromium plating as an ornamental process, without appreciating to the full the extent to which it is being applied to the reclamation of worn tools and parts. There is a number of applications for hard chromium plating that have still to be fully developed.
One method of using chromium plating in industry is to produce parts such as crankshafts, plug gauges, etc., by machining them undersize to the extent of .002 in. to .006 in. The part or tool is then built up to its proper dimensions by a chromium deposit of the requisite thickness. The advantage of this process is that when in due course, wear • has again made the parts undersize, they are not scrapped. All that is necessary is to remove the original plating, and replate. . There is no need for any grinding.
Grinding After Plating
Occasionally it may be necessary to deposit a much greater thickness of plating on a badly worn part, and this may be as much as 0.02 in. to 0.05 in. Because the part is not worn to the same depth all over, it may be necessary to grind after plating to ensure that the areas that are too thick are restored to the correct dimension. Such parts can then be replated on top of the plating first given, and ground down again to the correct dimension, a practice which represents a marked economy in time.
This process of grinding-down a second plating which has been put on top of an original one calls, however, for special care. The correct technique must be adopted, otherwise there is a danger that the new plating may peel off when ground. The first step is to remove all dirt, grease, etc., from the sprface to be plated. While it is still hot, it should be immersed in a concentrated solution of hydrochloric acid. After gas has been freely given off for approximately 10 secs., the part must be washed in clear, cold water.
Electrolytic Pickling .
Thus cleansed, the component is immersed in a 5 per cent, solution of sodium cyanide in which solution it is allowed to remain for 15 mins. This gives it an electrolytic pickling, which should be followed by a further washing in cold water. Etching, in the usual manner, follows, and, finally, the part is chromium-plated.
There is no insuperable difficulty in carrying out chromium plating efficiently so long as certain points be borne carefully in Mind. The plating solution adopted must produce hard, bright coatings of chromium without too great an expenditure of time. It is advisable to plate the part to the final dimensions, if this can be done.
The next point to be considered is the type of solution to be used for plating. The three that are most useful commercially are:— (i) A solution of chromic acid, sulphuric acid and water. The chromic acid is in the proportion of 33 oz. per gallon; and is used at a temperature of 60 degrees C. The sulphuric acid amounts to 0.33 oz. per gallon, and one gallon of water is required. The current density of the bath is 500 amps. per sq. ft., and the rate of chromium deposition is .0015 in. per hour.
(ii) The next solution is also of chromic acid, sulphuric acid and water, but here the chromic acid is in the proportion of 48 oz_ per gallon, the sulphuric acid 0.48' oz. per gallon, and the current density 400 amps. per sq. ft. The amount of water is the same. The rate of deposition is .006 in. per hour.
(iii) The third solution has a similar basic composition, but the proportions are: Chromic acid, 53 oz. per gallon; sulphuric acid, 0.53 oz. per gallon. Water and current density remain as in the second solution, but the rate of deposition is 0.001 in. per hour.
Of these three solutions, the second produces a much thicker coating in a given period, and is, therefore, more economical in time. It has, in fact, a cathodic efficiency of 17.5 per cent., against 13.5 per cent. for the third solution.
As the proportion of chromium concentration in the bath increases, the resistance declines, so that, assuming the current density in both instances to be the same, the voltage needed is less. In the first solution, nevertheless, so long as a distanci, of 3 ins, between cathode and anode be retained, and working conditions be not altered, this factor is not of great importance. It suffices, therefore, to employ 6 volts The third solution offers some economic advantage in reducing power expense. This particular solution is much more conductive than the first, and, in consequence, expenditure of power is relatively lower. Nevertheless, the first solution appears to be the most economical, because of its superior allround efficiency.
Although the current density of the first solution has been given as 500 amps. per sq. ft., this is not necessarily the maximum, It is quite possible to employ a higher current density if the operator proportionately increases the temperature A current density as high as 2,500 amps, per sq. ft, has been employed at a temperature of 80 degrees C., and under such conditions plating has been carried out at the remarkable rate of 0.004 in. per hour. For industrial plating to be done at such high temperatures and densities, a very small area of cathode is, however, essential.
The expense of grinding can be cut out by plating to a specified limit, so long as the thickness of plating be not more than .002 in. If the thickness required exceeds this limit, it is important to choose the right type of anode, and it is advisable to employ metallic strips to transfer concentrations of current from sharp corners—one of the most thorny problems of chromium plating.
Forethought is important in chromium. plating. The plating bath must be kept constantly in use, and if a proper schedule of work has been drawn up, so that the bath is never idle, the last portion of the shift can be devoted to flash plating, which takes about 20 minutes. It is also a good plan to ensure that all work is waxed in readi
ness forplating. • Immersion Overnight
For some types of work the deposit must be extremely thick—about 0.012 in. Such parts can best be given the required coating by immersion overnight. The bath must be kept at the right concentration, and it is possible to introduce fresh supplies either of water or solution automatically. The method used is to maintain water in a suitable reservoir from which it can run into the bath so soon as the level falls. When the level of the bath has risen sufficiently to cover a glass tube, the water ceases to flow
The plating solution will, yield the most effective results if the quantity of chromic acid be held to within plus or minus 3 oz. per gallon of the original proportions. The sulphuric acid must be held within plus or minus 0.1 oz. per
When a part has been subjected to • chromium plating, there is always a danger that its plating and the parent metal will become brittle as a result of the presence of hydrogen. The remedy is to heat the part to a temperature in the region of 200 degrees C. for a period of about two hours.