How does an aluminum oxide coating hold up over time without anything else?

[Gary Allan]

I got to do something new at the foundary today. I got to aluminum oxide coat/blast these parts in the sand blasting booth. Now we'll ignore the fact that the one glove has a hole in it ..and if you don't watch it ..you'll coat part of your hand (really it's an "ouch" type thing)...but you can't have it all, I guess.

Anyway, this makes bland aluminum look real nice. Almost like it has a grey ceramic like appearance. I'm jumping to the assumption that all aluminum oxidizes ..and you're just assuring a uniform shell of the stuff so you don't get the pitting and random blemishes that you see on our intakes and such under the hood.

(Yes, I'm already thinking of stuff I can get permission to coat on my own time at the foundary

..what would a set of aluminum wheels that the clear coat is pealing look like with this stuff??)

 

[Matt_S]

It has to do with the molecular length of a metal vs. the metal oxide. If the ratio of the molecular lengths is close enough to 1, the oxide molecules will stay bonded to the base metal, as in AL or cast iron. If it's not, the oxide will flake off, as in steel.

 

[Kestas]

Gary, are you saying that grit blasting with aluminum oxide grit coats aluminum parts with aluminum oxide?... I believe this thinking is flawed. Grit blasting aluminum removes some material from the surface and leaves a pleasing matte texture.

I actually did some work for a ricer, grit blasting aluminum underhood parts for his car. We don't usually contract such work and our lab isn't cheap, but he had deep pockets!

I plan on doing the aluminum wheels of my 91 Grand Marquis. They're peeling and look awful. I'll just be doing the parts that are peeling then recoat with a clear paint. I'm not going for any show condition (though the car is nearly mint), I just want to arrest the peeling and make it look somewhat better.

Matt, aluminum oxide is what we call an 'adherent oxide', because there is no volume change as the aluminum converts to aluminum oxide. Iron increases in volume as it oxidises to certain forms of iron oxide (some iron oxides are more adherent), which results in the flaky appearance. (Metals and metal oxides aren't referred to as having molecular lengths.)

 

[Matt_S]

Kestas, that was the way it was explained to me in my college materials class. I think we're referring to the same thing, though. The fact is, either way you look at it, an aluminum oxide particle is similar in size to aluminum, whereas with low carbon steel, the sizes are quite different. I remember being taught a ratio of molecular size where, if it was close enough to 1, the oxide would adhere to the base metal and form a coating, stopping further oxidation.

 

[chas3]

it hard to get a good finish on aluminum cheaply. alternatives would be irriditing/annodizing or powder coat.

 

[1sttruck]

Aluminum oxide is tough stuff, it's used an abrasive, it forms naturally on bare aluminum, people use to just let it form on telescope mirrors for protection after deposition, but the type of aluminum will tend to determine how good it looks with age. Some of the 2000 series aluminum, often used in aircraft, tend to develop intragranular corrosion, creating lots of little white dots of abrasive material on the surface. I don't know what they actually use but the liners in Swiss Army knives tend to develop it, where they get hard to open even if they're oiled. 6000 series aluminum doesn't do it, and makes for a better cosmetic surface.

 

[Gary Allan]

Well, maybe I am ..I dunno

Aluminum oxide is the blasting medium ..and a casual google search asserted that these particles bond and are imbedded in the blasted body (or so I thought). It leaves a "coating" on stainless steel (I put two stainless steel rods in the booth my last tour in it). Which I assume will quickly erode off of the dissimilar metal

quote:

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norganic coatings can be produced by chemical action, with or without electrical assistance. The treatments change the immediate surface layer of metal into a film of metallic oxide or compound which has better corrosion resistance than the natural oxide film and provides an effective base or key for supplementary protection such as paints. In some instances, these treatments can also be a preparatory step prior to painting.

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As a follow up question ...aren't there cheaper mediums to use to establish a mat finish on aluminum??


I guess not

quote:

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Aluminum oxide is an extremely sharp, long-lasting blasting abrasive that can be recycled many times. It is the most widely used abrasive in blast finishing and surface preparation because of its cost, longevity and hardness. Harder than other commonly used blasting materials, aluminum oxide grains penetrate and cut even the hardest metals and sintered carbide.

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So whatever coating appears on the stainless steel is just mashed Aluminum Oxide. I doubt that I scored the stainless steel (or did I

)

 

[Kestas]

Yes Matt, we are referring to the same thing. It's your vernacular I was commenting on. I phrased it the way I learned it from my schooling.

Gary, the matte finish from grit-blasting, whether it's on aluminum or stainless steel, is from the surface finish left formed by the grit-blasting action, not from particles embedding themselves onto the surface. Though some particles embed themselves on the surface, which is why it's not a good idea to use this treatment on electrical contacts or parts that will be subsequently plated, they are few in number and cannot be attributed to the overall finish texture.

The treatment you're trying to describe is called anodizing, a very popular treatment for aluminum surfaces. It's basically a reverse plating process that forms a relatively thick layer of aluminum oxide on the surface. It is used chiefly for either of two reasons. One is decorative as it leaves a pleasing shiny surface on aluminum. The other or for wear resistance on engineered parts.

 

[XS650]

quote:

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Originally posted by Kestas:


The treatment you're trying to describe is called anodizing, a very popular treatment for aluminum surfaces. It's basically a reverse plating process that forms a relatively thick layer of aluminum oxide on the surface. It is used chiefly for either of two reasons. One is decorative as it leaves a pleasing shiny surface on aluminum. The other or for wear resistance on engineered parts.

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Type II and type III anodizing do have considrable build up. They build up about 50% of the total thickness of the anodized layer.

Type III build up.

They are electrolytic processes done in a chemical bath.

I believe Type I is the decorative process which has essentially no build-up