Chrome Plating
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What is Chrome Plating?
Chrome is slang for Chromium, one of the 92 naturally occurring chemical elements. Chrome is a metal, but it is not useful as a solid, pure substance. Things are never made of solid chrome. Rather, when you hear that something is chrome, what is really meant is that there is a thin layer of chrome, a plating of chrome, on the object (the bulk of the object usually being steel, but sometimes aluminum, brass, copper, plastic, or stainless steel).
Chrome plating is more reflective (brighter), bluer (less pale, grayish, or yellowish), and more specular (the reflection is deeper, less distorted, more like a mirror) than other finishes. Put one end of a tape measure against a bright finish, and see how many inches of numbers you can clearly read in the reflection -- you can see skywriting clearly reflected in top quality chrome plating. And there's a hard to define "glint" to chrome plating that almost nothing else has.
The Process of Chrome Plating
There are many variations to this process depending on the type of substrate being plated upon. Different etching solutions are used for different substrates. Hydrochloric, hydrofluoric, and sulfuric acids can be used. Ferric chloride is also popular for the etching of Nimonic alloys. Sometimes the component will enter the chrome plating vat electrically live. Sometimes the component will have a conforming anode either made from lead/tin or platinized titanium. A typical hard chrome vat will plate at about 1 mil (25 µm) per hour.
Various linishing and buffing processes are used in preparing components for decorative chrome plating. The overall appearance of decorative chrome plating is only as good as the preparation of the component.The chrome plating chemicals are very toxic. Disposal of chemicals is regulated in most countries.
Decorative Chrome Plating
Decorative chrome is designed to be aesthetically pleasing and durable. Thicknesses range from 0.002 to 0.02 mils (0.05 to 0.5 µm), however they are usually between 0.005 and 0.01 mils (0.13 and 0.25 µm). The chromium plating is usually applied over bright nickel plating. Typical base materials include steel, aluminum, plastic, copper alloys, and zinc alloys.
Restoration Work
Chrome plating is hardly a matter of dipping an article into a tank, it is a long involved process that often starts with tedious polishing and buffing, then cleaning and acid dipping, zincating (if the part is aluminum), and copper plating. For top reflectivity "Show Chrome", this will be followed by buffing of the copper for perfect smoothness, cleaning and acid dipping again, and plating more copper, then two or three different types of nickel plating, all before the chrome plating is done. Rinsing is required between every step.
When an item needs "re-chroming", understand what is really involved: stripping the chrome, stripping the nickel (and the copper if applicable), then polishing out all of the scratches and blemishes (they can't be plated over and any scratches will show after plating), then plating with copper and "mush buffing" to squash copper into any tiny pits, then starting the whole process described above.
Unfortunately, simply re-plating an old piece may cost several times what a replacement would cost. It's the old story of labor cost. The new item requires far less prep work, and an operator or machine can handle dozens of identical parts at the same time whereas a mix of old parts cannot be processed simultaneously, but must be processed one item at a time. If a plater has to spend a whole day on your parts, don't expect it to cost less than what a plumber or mechanic would charge you for a day of their time.
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