Darkening after polish

**bcgreen** · 02-11-2015, 06:16 PM

I was wondering what causes alumin. to darken after using a polish?
I did a set of alumin. wheels and it caused some worry on my part, but I was able to get the polish off and get the desired chrome like finish.

**Niblick** · 02-11-2015, 06:32 PM

looks like the polish has reacted with it. aluminium can be funny stuff. did the polish say 'suitable for use on aluminium' on the container? if not, id suggest looking of one that does

**bcgreen** · 02-11-2015, 07:01 PM

Originally Posted by Niblick

looks like the polish has reacted with it. aluminium can be funny stuff. did the polish say 'suitable for use on aluminium' on the container? if not, id suggest looking of one that does

This is from Mike P. and another chap, Todd Helme, when he did the bed of a truck. The alumin. in the whole bed of the truck was dark.

**FUNX650** · 02-11-2015, 07:59 PM

Originally Posted by bcgreen

I was wondering what causes alumin. to darken after using a polish?

I was able to get the polish off and get the desired chrome like finish.

The "Darkening" is probably a mixture of the remnants of the passivation coating...Aluminum oxide...and those of the polish/compound.

Bob

**Setec Astronomy** · 02-11-2015, 08:14 PM

Originally Posted by bcgreen

I don't think that's really "darkening" as much as changing the reflectivity of the surface. Or bringing out the "color" as they would say in the metal buffing industry.

**davey g-force** · 02-11-2015, 08:25 PM

Hmm... I always thought it was the "real" color of the aluminium underneath, once the al. oxide has been removed from the surface.

Interesting to hear other views / facts.

**aim4squirrels** · 02-11-2015, 09:15 PM

Aluminum oxide is white, which is why the unpolished section appears chalky. Essentially, that is how aluminum "rusts". Unlike iron rust though, aluminum oxide builds a layer that protects underlying aluminum from further oxidation over time. Iron oxide is very porous and allows more oxygen to reach bare iron, which is why the process continues until all the iron eventually becomes iron oxide.

The polished side is oxidation free so you don't see the effects of looking through the top layer of oxidation down to the reflective aluminum. At least that's my theory.

Looks like the shiny side of the foil to me.

**davey g-force** · 02-11-2015, 10:48 PM

^^ Yep, that's what I was thinking...

**Niblick** · 02-12-2015, 05:10 AM

I did think that initially, but it looks just a bit too dark tbh. Might just be the way it's lit of course

**Vegas Transplant** · 02-12-2015, 07:01 AM

Originally Posted by Setec Astronomy

...Or bringing out the "color" as they would say in the metal buffing industry.

Perfectly polished aluminum colors well. It is one characteristic of the process that sets my rig apart from the other hundred or so parked on the line. Chemically brightened aluminum doesn't color and is usually streaked.

Also, IMO, colored metal tends to retain the bling for a longer period of time than chemically brightened metal.

Originally Posted by bcgreen

... but I was able to get the polish off and get the desired chrome like finish.

Parked side-by-side at the shipper, my Alcoas more closely resemble a chromed look than aluminum when viewing the chemically treated wheels parked beside me.

Even after a 600 mile trip throught salt and what not.
A simple rinse to remove the grit, a light wash followed by patting them dry, the end product still looks like chrome.