Decorative vs Industrial or Hard Chromium Plating
Chromium plating manifests itself in two commercial forms namely “decorative” and “industrial” or “hard” chromium plating.
The nature, functions and purposes of the two types are distinctly different, and they are not interchangeable.
Decorative chromium plating is somewhat of a misnomer in that the bright, pleasing type of metallic surface coating usually associated with automobile trim, electrical appliances and the like for durable appeal and corrosion-resistance is, in reality, a triple coating of three different metals.
This protective coating generally comprises copper, nickel and chromium electrolytically deposited onto the basis metal in that order in three separate operations.
The surface chromium deposit is bright and mechanically hard in itself, but the combined coating is relatively soft reflecting the soft nature of the copper and nickel layers.
In decorative plating, the thickness of the chromium layer is only 0.00001″— 0.00003″, and serves primarily as a non-tarnishing, durable protective coating for the soft nickel undercoating.
Thus, decorative plating is of little or no value for those engineering applications requiring very high surface hardness, wear-resistance and durability under stress.
Industrial or hard chromium plating on the other hand is primarily an engineering aid, electrolytically applied directly onto the basis metal (iron, steel or bronze) without benefit of any intermediate coating of any form.
The direct electro-deposition of chromium is essentially restricted to iron, steel and bronze either because of inability to satisfactorily deposit, poor plating bond strength or tendency of one of the alloying elements in other basis metals to amalgamate with the chromium, rendering it useless.
The latter is especially true with those basis metals such as brass which contain rather large percentages of zinc.
Hard chromium plating is electroplated or electro-deposited in thicknesses ranging from approximately 0.0001″, or slightly less, to 0.050″, and by virtue of this thickness range the coating exhibits the hardness, brightness, wear and corrosion-resistance and low friction characteristics of pure chromium under conditions of relatively severe stressing.
It must be remembered, however, that electrolytically deposited chromium is hard and consequently brittle.
Therefore, the success of any application is predicated upon an intelligent recommendation as to the thickness of coating practical under the conditions of stress and service to which the plated surface is subjected.
The Value of Hard Chromium Plating
Why is hard chromium plating of general value to the plastics, rubber and synthetics industries?
Hard chromium plating is of general value and economical importance to these industries because it is, in essence, an extremely hard, wear and corrosion-resistant surface coating which may be applied to the functional surfaces of all iron, steel and bronze molds and machine parts to:
- 1—Alleviate sticking and staining in molds resulting from the chemical action of the molding materials,
- 2—Alleviate surface corrosion of molds, dies and machine parts which are in intimate contact with plastics, rubber or synthetic materials,
- 3—Reduce surface wear on molds, dies and moving machine parts through lower coefficient of friction and increased surface hardness, thus increasing the useful life of these parts simultaneous with decreasing maintenance costs,
- 4—Produce and maintain better quality of surface finish on molded, calendered, extruded, planished and embossed articles,
- 5—Aid in prevention of corrosion during storage or inactivity of equipment,
- 6—Facilitate flow of material in molds as well as in injection and extrusion equipment, and
- 7—In many instances, dimensionally salvage mold components and machine parts where toolmaker’s mistakes, distortion during hardening or excessive wear has rendered the part useless. Such salvage, when feasible from a practical standpoint, is usually the economical way out of such difficulties.