Metallurgical Question: Chrome on Aluminum?

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Not too long ago, chromium plating on aluminum just wasn't done; brass and ferrous metals, but not aluminum. IIRC, the chrome layers had a propensity to leap off the metal, just like a $99.00 paint job.

Apparently that's changed, judging from the sheer number of chromed alloy wheels I'm seeing, both OE and aftermarket.

1. Why wasn't aluminum chromed?

2. What has changed to make this now possible?

Eddie
 
Well, the purpose of chroming steel is to prevent it from rusting. Since Aluminum doesn't rust it doesn't need to be painted or plated. It probably wasn't done before because it isn't nessecary.

I don't know if you were aware of this, but chromed aluminum isn't just aluminium with a chromium plating. It is usually plated with zincate, copper, nickel, and then chromium. I imagine that if someone tried to just chrome bare aluminum it would just fall off.
 
Yeah, I read an article in Hot Rod a long time ago explaining "triple chrome plating", which meant a base "strike" of copper, followed by nickel, followed by the chrome.

It was probably the same article speaking to non-plating of aluminum.

That would have been, oh, somewhere around 1966. :oops:

I used to have brain cells then, you know. :shock:

Eddie
 
I'm working off the impression that Aluminum is vacuum sputter plateing, not elctrolytically plated. Anyone care to enlighten me further?
 
As it was explained to me in the past, there is no problem chroming Aluminum, it just isn't done very often because it can be polished to a similar finish and due to aluminum being a less dense metal it is prone to denting, which causes the chrome and substrate to disbond from the aluminum base. It sounded good to me at the time.
jp
 
Eduardo, have you browsed the posts at "Finishing.com"? There's often interesting stuff there.
 
chrome can be applied to anything that will conduct electricity, hence the term electroplating.
Chrome does stick to aluminum quit well, but the fact that aluminum is so much softer than steel, as soon as the aluminum gets a rock chip the underlying surface gets a small depression, allowing the elements to get under the chrome, and then it comes off in pieces or flakes

most chrome is a triple plate system, copper, nickel, and then the chrome
and the nickel layer is what gives the chrome its nice shiny color and the chrome layer is the top coat

the copper provides a better surface for adhesion of the chrome

a well done chrome job should last almost indefinitely, poor jobs last.... not very long!
 
We use anodized aluminum saw guides in rotary gang resaws in the sawmill business quite extensively. It is a black anodized finish that is quite hard and serves to protect the aluminum from wear and is applied electrically.
Joe
 
Anodizing is very different from crome plating, though. Anodizing creates a layer of aluminum oxide on the surface of the part, most hard anodizing like the saw guides you mentioned is around .0015" thick, and is very hard. But this process actually creates a layer of oxidation on the surface. Chrome plating deposits a layer of chromium on the surface of the part, but it is much less of a metillurgucal bond as much as it is a physical bond. That is why it is prone to chip off. A cheap chrome job on steel can peel off just as easily, but since steel is harder than aluminum, and therefore does not bend and deform as much, it is not as common. However, a good chrome job will last as long as the part it is on. Almost all military rifle barrels are hard chromed, and they are certainly put through alot of stress and abuse. Than again, I don't think anyone is making barrels of aluminum.
 
When I was workin in fla, there was a plating company just up the street.
I was in there one day and asked what kind of stuff they plated they said mostly plastic. So I asked how in the world do you get plastic to conduct electricity ? dude told they static charge it first to get it copper plated, befor going to the chrome. you learn something every day.
 
It sounds as though plating aluminum just wasn't done because there was really no demand and because the technology to achieve durability wasn't there as well.

Looking at current chrome-plated alloy wheels, the key word is "alloy", not "aluminum". Obviously, they've licked the soft base material problem, or ameliorated it to an acceptable point.

Addo, thanx for the link--WOW!
 
My '84 Mercedes 300D has chrome plated aluminum bumpers. Mostly to save weight I think. They are about a quarter inch thick but unexpectedly light when you pick one up. Got the car about ten years ago, and the bumpers look fine after 20 years of not so special treatment. I don't know how long MB used them before that (the 300D began in the mid '70's) and I imagine they didn't invent the process. Perhaps cost may have something to do with it's use or non-use more than anything else. It can be done very successfully, so that's not the problem.

I'm pretty sure "most chrome" is not triple plated anymore, and hasn't been for a long time. Hence the rust coming through the plating or big flakes coming off. I think it would be rare to see that on an undamaged triple plated part. I've seen old parts where the chrome has actually worn off exposing the copper, but still no rust or flaking.

Modern manufacturing dictates that just barely good enough is all that's necessary or even desired. If a step can be left out or a part made somehow cheaper (or left out all together) and still achieve a "satisfactory" result, you can bet it will be. If it can't be reduced anymore or was so bad that it had to be made better, then you're going to pay more for it. It's happening to everything. Smaller, thinner, less of it, none of it, fake metal (chromed plastic), fake everything. Fake "fake stuff". I think the ultimate goal is to get us to buy the imaginary vision of what we want, like the emperor's new clothes. Manufacturing Nirvana. Get the product reduced to the point where only the image remains and then sell that. "Look, it's the same thing, don't you see?"

Almost all "aluminum" is an alloy of some type or another. There are many types. I'm not sure if pure aluminum is used for anything. Kind of like gold. Pure gold sounds great but it's not real useful, even if only for jewelry. It's just too soft even for that.
 
About the only thing pure aluminum is used for is electrical wires and soda and beer cans. As far as the quality of chrome on your mercedes, well, its a mercedes, and as cars go, they have some of the best quality products on the market, esp. from a longevity standpoint. And as far as fake and cheap, you are 100% right. They even come up with fake works for fake. But it seems the average comsumer is interested more in cheap than in quality, so that is what manufacturers must do to make a decent profit.
 
So far the chromed aluminum wheels on my mothers Park Ave Ultra are holding up great.


-=Whittey=-
 
Slant,

Yes it seems most people only want cheap. And then they complain about shoddy products and companies sending jobs overseas. All these companies are doing is trying to keep their products cheap to be competitive and stay in business.

"Yes, I want $27 an hour. I also want total phone service and wireless with video for $2.99 a month plus a $1.50 rebate. But why are you sending all the phone company jobs to India?"

These people are just doing it to themselves.

But that's a different subject.
 
Another use of pure aluminum is found in aircraft sheetmetal. Sometimes called "Alclad", this is usually 2024 or 7075 sheet with a thin layer of nearly pure aluminum (1100, if memory serves) to resist corrosion and keep looking good.

I think the "chrome" plating on plastic model parts is straight tin.

P.S., if you ever have to repair-weld a chrome-plated part, grind off ALL the chrome where you are going to weld.
 
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