PLATING OF PARTS

Evening Post, Volume CIX, Issue 81, 5 April 1930, Page 26

PLATING OF PARTS

NICKEL AND CHROMIUM

Just as there is a fashion in wearing apparel, so there is a fashion in metals. The metal chromium was isolated nearly 200 years ago, but it is only within the past few years that it has been applied to the plating of radiators, etc. The fashion of nickel plating is far from being dead yet, although it has been in vogue for 25 years or so. A cause that has helped to displace nickel plating somewhat is that articles have been placed upon the market with a deposit of nickel so thin .as to give no possibility of reasonable satisfaction, in wear. An article becomes almost immediately covered with a thin film of nickel on insertion into the plating bath, and this is sometimes sent out as nickel-plated work. The thickness of nickel on the plated parts of a low-priced car to-day varies between 0.0001 inch and 0.0002 inch, or say l-10th the thickness of tissue paper. The paint and enamel are 10 to 20 times as thick as the nickel plating. Chromium plating is from 0.0001 inch to 0.0002 inch in thickness. Nickel plating can be of real protective value as is evidenced by the fact that a 15----year guarantee is given with nickelplated work supplied to ono railway company. The "bumpers" on the American cars have to stand daily impact, but. the best grades do not rust under severe treatment.

The" rust resistance of steel plated with nickel is dependent upon the thickness -and continuity of tho plating, tho protective value increasing more than proportionately to the thickness of the deposit. It is therefore surprising, when ono considers that the difference in the costs of thin and thick electroplate is very small, that some manufacturers do not put on the "extra bit" that makes all the difference.

MANUFACTURING DIFFICULTIES,

The manufacture of chroniium-plated articles has its own difficulties. Great heat is ■ evolved in the process, which has to be kept down for satisfactory deposits. As is well known, chromium is exceedingly hard —it is harder even than glass—so that it is/extremely difficult to buff a dull finish to a high polish. To obviate this difficulty the chromium is often flashed on to a previous deposit of nickel plating. This process is

costly, but certainly secures a maximum degree of' rust resistance. Chromium plating, unlike nickel plating, can stand a great deal of polishing with coarse polishing materials. Like many other good things, chrom-

ium platiug has its limitations;' it is brittle, and is therefore liablo to crack or flake off if the plated article is strained in any way. Thick deposits are liable to crack and chip off. Chromium itself resists tarnish but it does not necessarily protect the underlying metal, such as steel, against corrosion if tlie steel is iv any way exposed. Motorists would do well to bear this in mind when forcing a nut. An American eoinjisny has a novel way of inspecting the quality of their platinsr. The intensity of the light reflected from the surface of the article is measured by means of a photo-elec-tric cell and radio-amplification. A high surface polish, without scratches, reflects light strongly; a lessor degree of polish diffuses and weakens it. _ 1 Chromium plating lias been applies in other directions than as a protee-j tion against atmospheric influence, lor example, it has been tried iv bearings to resist wear. Tho danger of_ usyig it in moving parts of machinery is that if it flakes off, the extreme hardness oi tho particles renders them liablo to cause considerable damage. The cost of chromium is not low, but, with the increased demand, tho cost ot production should decrease and result in its wider application. Many false claims are made for chromium plating which aro to be deplored, as the value of the process ia sufficiently high to make it unnecessary to bolster it up by exaggerating its excellence.