Techniques in Spray Coating Surfaces Utilising Various Metals

This information was originally published in Practical Engineering 1940 Vol1 No17. Information within this article is therefore correct as of 1940. The publication of this material aims to provide historical insight on the developments in engineering during this time period.

The spraying of metals or other surfaces with metal is a skilled operation. Aluminium, gold, lead, nickel, silver, tin and zinc have been successfully sprayed onto surfaces, and because of the rapidity of the process, there is hardly any oxidation. It is probable that electroplating on iron, of brass, bronze, copper, silver and gold will hold the field for some years to come. For these metals, when sprayed, do not seem to produce a sufficiently homogeneous coating to prevent the iron or steel corroding and rusting when brought into an acid atmosphere, or in contact with electrolytes.

Preparation of the Surface

Prior to metal-spraying it is necessary to roughen all surfaces; this is done either by sand-blasting or by blasting with steel grit so that a rough mesh-like effect is produced on the metal. Such a fine structure in a way matches the size of the particles of the finely divided metal in the molten spray.

Cleanliness is essential, of course; metals in the finest state of division simply will not adhere to any surface unless it is absolutely free from dirt and the same precautions which have to be taken in electroplating are likewise important in hot-metal spraying. A deposit appears to be made up of countless granulations formed as each particle rapidly cools. These particles, before solidifying, become coated with a minute film of oxide, but this does not prevent the entire deposit from holding firmly together by the partial fusion of each semi-fluid particle, the whole coating adhering to the actual metal surface through the minute net-like surface effect of sandblasting.

The metal, in its transit from the blowpipe feed, consists of many millions of isolated globules of molten metal, which partially flatten themselves against the surface to be coated as the result of an intense collision or, if we like, through a heavy bombardment somewhere in the neighborhood of a speed of 12 miles per minute. Each particle has a diameter approximating to a hundredth of a millimeter.

Method of Melting

The fact that a fine wire when placed in the hottest zone of a blowpipe flame becomes white-hot and then molten forming globules is the basis of the metal-spray process. If the globules of fluid metal fall into the path of a high-velocity current of gas, they are immediately broken up into a continuous stream or spray of molten metal. A blowpipe, incorporating a suitable high-pressure gas supply nozzle, forms the spray-metal tool.

According to the metal used for spraying so is the structure of the surface produced. The essential difference between the structure of a casting lies in the fact that in the latter there is an almost continuous layer (upon layer) of fine metal particles separated by the inclusion of imprisoned gases, whereas in a sprayed-metal surface, instead of such gas spaces there are minute films of oxide separating each particle. A considerable difference attaches to the close structure produced by the metals of lower melting-point. In this case, the  particles   are   much   smaller   and form far finer spray than metals like nickel, gold, etc. This fact renders it necessary to give a thicker coating of the higher-melting-point metals.

It has been stated that there is slight oxidation of the molten particles on reaching the metal surface, but this does not apply to the metal wire itself in its passage from the blowpipe flame to the surface. Proof of this is witnessed in the complete freedom from burning of magnesium, of all metals. As in the case of metal-hardening by rapidly bombarding it with blows (or the impingement of steel balls as in the later methods) so in metal-spraying, the film of deposited metal is slightly harder than the same metal when cast, while its density is a little less.

These slight variations make for certain alterations in the working of sprayed-metal articles, such as higher speed and lighter cut when machining, chiefly perhaps to prevent peeling of the particles by cutting directly through them, instead of " cut-shearing " them. Polishing offers no difficulty whatever, the usual methods being used.

Aluminium Spraying

Of all metals, aluminium probably offers greatest scope in industry, both on account of its durability on exposure and fairly good conductivity. It is notoriously difficult to electro-deposit aluminium—results are so uncertain. Some years ago when experimenting with chromium-plating, the writer made use of an aluminium anode and, with an outrageously high voltage around 200, current density of 5 amperes per square foot of- article to be plated (cathode), and a concentrated chromic acid bath, obtained a very satisfactory deposit of aluminium. A spoon so treated is still in existence and has been fairly well used for five years or so.

Additionally, the writer still has some specimens of aluminium-plated iron screws and nails, also a small sheet of aluminium-plated brass which were not quite so successful as the spoon, but nevertheless somewhat of a curiosity in the way of accidental plating. The same thing, too, was the fact that almost as good results were obtained with a voltage of six, although the film was more or less oxidised and matt. Aluminium, as far as its electro-deposition is concerned, is a most evasive sort of metal; far and away better results apparently have been obtained by the spraying method.? The aluminium film or oxide is remarkably strong and the metal .adapts itself admirably for spraying even for large areas and bulky structures. Temperature of working is rather critical; above a certain limit rapid absorption takes place, and as a protective coat the deposit becomes useless. This temperature limit is 970 deg. C.—this being about 310 deg. C. above the melting-point of aluminium. Iron particles which are spray-treated with aluminium form an exceedingly hard alloy with the latter, which then oxidises to form a scale-free coating. Alternate spraying of aluminium and zinc is most advantageous in many instances, successive layers of the two metals' producing a most adherent protection.

Tin and Lead Spraying

The two metals tin and lead adapt themselves to the spraying process, for the former, on account of its feeble reactivity, is ideally suited to the preservation of foodstuffs and is one of the most useful of the common metals. Unfortunately, it seems that sprayed tin requires to be of fairly good thickness considering the rather porous nature of the metal. Most pots, pans, and other culinary utensils need only a fight deposit of tin, which is ordinarily too porous, so to overcome this the film is coated with a layer of varnish. The general thickness of tin deposits is about V64 in., which is quite considerable and just shows what may be achieved with the metal spray. Deposits of 10/1000in. of the tin coating are also usual and are very protective. It should be remembered that tin is a really beautiful metal and it is almost remarkable that more use is not made of it for ornamental purposes and motor-car fittings, etc. Its price is fairly high, of course; about £250 per ton, as compared with copper at £60 per ton.

Lead, principally associated with the definite protection of metals against acid fumes, owing to its great acid resistance, requires a limit of deposit thickness for the latter purpose of x/16in.

Nickel and Silver Spraying

The chief advantage in the spraying of nickel lies in the fact that great areas may be covered; such would be impossible in the electroplating of the metal. In this manner huge rolls of sheet iron are easily treated, the size of which would absolutely preclude them from entering the largest plating-vat made. The use of the nickel spray, however, is decidedly limited, far better results being obtained by the electro method on account of the-brilliance of the latter deposit. Thus far, therefore, such spraying appeals only to the commercial and bulk handling of large and unwieldy articles and machinery.

In the case of silver, no particular advantage can be claimed over the electro-deposition method, since any thickness may be obtained by the latter process coupled with many pleasing variations from a matt to a bright finish. Some intricate filigree work may, perhaps, be more effectively treated with the molten-spray method than by electro-depositing, but it is doubtful. Generally speaking, the size and nature of articles for silvering adapt them to the plating-vat rather than the spray. One advantage perhaps of the latter might be in the direct application of silver to iron and steel, which do not require to be coppered beforehand as in the case of electro-silvering.

Copper, Brass and Zinc Spraying

The remarks which apply to copper also apply to brass and bronze, etc. As a protective coat, brass is often applied to iron, as it also is for ornamental purposes, but a certain thickness is essential. Since steel and iron are by far the commonest bases to which copper and brass are applied, the porosity of the latter metals is important to consider; this is partly due to the largeness of the particles of molten metal—repeated layers of brass and copper are needed to give a perfectly homogeneous and close skin. What happens if this is not provided is this: when brought into an acid alkaline or damp atmosphere, the under-surface of steel or iron becomes corroded and gradually oxidised, so that the otherwise protective coating of copper or brass peels off. Thickness of deposit alone is not all that must be considered, for the same peeling effect is liable to occur if the article is damaged in any way so that the coat is scratched. Indeed, this is the most important thing to guard against in all coated metals.

Zinc is chiefly used as a coating to iron, and the incorrectly so-termed "galvanising" usually consists of dipping the iron articles in molten zinc. Electro-galvanising is done as well, of course, but on a large scale dipping is the general rule. As a protection, zinc must be pure, and this purity is ensured by using the spray method of application. It particularly commends itself to the treatment of small articles and is an extremely economical process, since the thickness of zinc sufficient to prevent rusting and corrosion need not exceed .002in. The thickness of deposit naturally depends upon the use to which the articles are to be put, for most purposes twice the above thickness being usual for the prevention of atmospheric corrosion. Zinc, as has already been mentioned, is often sprayed in conjunction with aluminium, the effect being an ideal, tenacious deposit.

Gold Spraying

Porosity, again, is the chief deterrent in the spraying of gold, . and, since additional thickness would be prohibitive in most cases, the porous nature can only be checked by using suitable varnishes. However, the same remarks here apply as to silver, for both metals are what may be termed “luxury metals," principally being used for ornamental and artistic effect. Sometimes it is necessary to have appliances covered with gold, as in some laboratory apparatus. On steel, the metals gold, silver, copper, and brass appear to be best deposited electrolytically, very little dissatisfaction arising therefrom.

The factor to be taken into consideration in the spraying of metal is the convenience and facility of adhering to the ground surface. As has been indicated, this is of decided advantage in the case of metals like aluminium, the electro-deposition of which is a difficult process—such a difficulty being removed by spraying. We have chiefly dealt with the spraying of metal upon metal, but it is equally important to understand that certain bodies—wood, plaster, unglazed porcelain, etc are quite easily treated by spraying.