Home  |  Link  |  Favorites 
China Time: 07:16  11 / 19 / 2017  Sunday
Home   │   About CMI   │   Factory   │   Quality   │   Products   │   Technical Forum   │   Contact
Technical Forum
Technical Forum
 
Technical Forum
 
Introduction to Chrome Plating
2009-06-05

What is “Chrome”

Chrome is slang for Chromium, one of the 91 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 occasionally aluminum, brass, copper, plastic, or stainless steel).
A cause of occasional confusion is the fact that people may tend to describe any shiny finish as "chrome" even when it really has nothing to do with chromium. For example, brightly polished aluminum motorcycle parts, electropolished stainless steel boat rigging, vacuum metallized balloons and helmets, shiny painted wheels, and nickel plated oven racks are sometimes called 'chrome' by the lay person.
Indeed it's not always easy to tell chrome plating from other finishes if the parts are not side by side. When a decorative chrome electroplated finish sits right next to another bright finish, however, the other finish usually won't compare very favorably :-)
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 yardstick against a bright finish, and see how many inches of numbers you can clearly read in the reflection -- you can clearly see the clouds in the sky reflected in chrome plating. And there's a hard to define "glint" to top quality chrome plating that nothing else has.

What's the difference between "Chrome Plating", "Chrome Electroplating", "Chrome Dipping", "Chroming", etc.?

Nothing. Chrome is always applied by electroplating, it is never melted onto parts in the fashion of chocolate on strawberries, or applied in any other way than by electroplating. Note the previous paragraph, though, that everything that is somewhat bright is not necessarily chrome.
Is all chrome plating about the same, then?
Not quite. There are two different general applications for chrome plating: "hard chrome plating" (sometimes called 'engineering chrome plating') and "decorative chrome plating".
Hard Chrome Plating
Most people would not be very familiar with hard chrome plating. Hard chrome plating is chrome plating that has been applied as a fairly heavy coating (usually measured in thousandths of an inch) for wear resistance, lubricity, oil retention, and other 'wear' purposes. Some examples would be hydraulic cylinder rods, rollers, piston rings, mold surfaces, thread guides, gun bores, etc. 'Hard chrome' is not really harder than other chrome plating, it is called hard chromium because it is thick enough that a hardness measurement can be performed on it, whereas decorative chrome plating is only millionths of an inch thick and will break like an eggshell if a hardness test is conducted, so its hardness can't really be measured directly.
Hard chrome plating is almost always applied to items that are made of steel, usually hardened steel. It is metallic in appearance but is not particularly reflective or decorative. Hard chrome plating is not a finish that you would want on a wheel or bumper.
There are variations even within hard chrome plating, with some of the coatings optimized to be especially porous for oil retention, etc., others "thin dense chrome", and so on.
Many shops who do hard chrome plating do no other kind of plating at all, because their business is designed to serve only engineered, wear-type, needs. If a shop says they do 'hard chrome only', they have no service that most consumers would be interested in. Follow the link for a list of some shops that offer hard chrome plating
Decorative Chrome Plating
Decorative chrome plating is sometimes called nickel-chrome plating because it always involves electroplating nickel onto the object before plating the chrome (it sometimes also involves electroplating copper onto the object before the nickel, too). The nickel plating provides the smoothness, much of the corrosion resistance, and most of the reflectivity. The chrome plating is exceptionally thin, measured in millionths of an inch rather than in thousandths.
When you look at a decorative chrome plated surface, such as a chrome plated wheel or truck bumper, most of what you are seeing is actually the effects of the nickel plating. The chrome adds a bluish cast (compared to the somewhat yellowish cast of nickel), protects the nickel against tarnish, minimizes scratching, and symbiotically contributes to corrosion resistance. But the point is, without the brilliant leveled nickel undercoating, you would not have a reflective, decorative surface.
By the way, there is no such thing as "decrotif chrome plating". That is just a misspelling of 'decorative'.
Buzzwords: "Show chrome", "Triple Chrome Plating", "Double Nickel-Chrome"
"Show chrome" probably means chrome that is good enough to be on a winning entry in a car show. Although most OEMs rely on the "self-levelling" property of nickel plating to give sufficient reflectivity to roughly polished steel, chrome-lovers believe that the key to "show chrome" is to copper plate the item first and then buff the copper to a full lustre before starting the nickel plating.
Whether you start with bare steel or buffed copper, at least two layers of plating follow -- a layer of nickel and a layer of chrome. But high quality plating requires a minimum of two layers of nickel.
Salespeople are always looking for advantage, and they will use any good-sounding terms they can get away with! There are no laws that define what triple chrome plating actually means, so salespeople will be prone to call their service "triple chrome plating" if there are a total of 3 layers of any kind of plating, or "quadruple chrome plating" if there are 4. So those terms mean little.
By the way, tri-chrome is not an abbreviation for triple chrome, and hex chrome does not mean six layers. But more on that later.
The most important issue for durable chrome plating for outdoor exposure such as on a vehicle is that it MUST have at least two layers of nickel plating before the chrome: namely semi-bright nickel followed by bright nickel. The reason for this involves galvanic corrosion issues. The bright nickel is anodic to the semi-bright nickel, and sacrificially protects it, spreading the corrosion forces laterally instead of allowing them to penetrate through to the steel. OEMs demand very close control of this factor, and there is a test (the Chrysler developed STEP test) which large shops run daily to insure the right potentials. Careful control of this issue is probably the principal reason that today's chrome plating greatly outlasts the chrome plating of earlier times.
Experts argue whether copper plating provides any additional corrosion resistance at all, but with or without copper plating, chrome on top of a single layer of nickel will not hold up to the severe exposure of a vehicle! Industry professionals call the two layers of nickel "duplex nickel plating", and that would be a much better term to use than "triple chrome" and such.
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.
Restoration Work
When an item needs "rechroming", 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 replating 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. Follow the link if you're looking for a list of some chrome plating shops serving the public.
Peeling chrome?
If your chrome plating is peeling, this is virtually always a manufacturing defect due to insufficient adhesion of the plating to the substrate. Although exposure conditions can certainly harm chrome, and discolor it or make it pit, they won't make it peel! It can be very difficult for a plating shop to get good adhesion on some things (most commonly on alloy wheels because they are not pure aluminum), but if they can't do it they shouldn't sell it. If your parts have peeling chrome, you should complain and not be deterred by nonsense about chemicals in your garage, how frequently you wash the wheels, etc. We'll say it again, we're that sure: peeling chrome is virtually always a manufacturing defect

 

Home   │   About CMI   │   Facilities   │   Quality   │   Products   │   Technical Forum   │   Contact

技术支持:宏图腾设计