How to Make Aluminum Corrode / Patina?

A. Hi Doug,
Please e-mail to mooney@finishing.com, for posting here, a good pic of the herons, with other stuff in the pic so color can be judged -- and people may hazard a guess. But we have no idea what they look like currently or what you object to about the current finish. Castings are high in silicon so virtually anything you do chemically (except applying hydrofluoric acid which is a terrible safety hazard) will probably darken them rather than whiten them.
Maybe try oven cleaner ⇦this on eBay or Amazon [affil links] in an inconspicuous spot and see what you learn? But although I'm not experienced with his techniques, I think Al Sterling offers the best answer. Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi John. Bare aluminum can corrode quite readily, but the problem is that the corrosion products will not look like what he is envisioning. Corroded aluminum is not brown with a fine grain, like rust on tool steel. Rather, the corrosion products are white, random pitted, crusty volcanos, and somewhat like salt stains. If you have a sliding shower door more than a couple of years old, look at the corner joints -- the extrusions are anodized, but cut to length after anodizing which exposes bare aluminum (at least if you have cheap ones like I do). I'd suggest a decorative rust-like paint or iron paint if you want rust color. Good luck.

A. Try glass bead blasting.

A. Any very alkaline solution (pH8,5 or more) must etch or corrode aluminum. Try sodium carbonate ⇦this on eBay or Amazon] 10% (or even sodium hydroxide 2-5%) -- mix it with some sawdust or sand,c over your object with this mix, after a few days it must be ok. Good luck!

A. Try a mixture of 5% sodium hydroxide with about 5g/l sodium chloride in it (common salt). Mix this with some sawdust and leave it for a few days on the aluminium surface - it will give a corroded finish. If it is not corroded enough, put the sawdust back for a few more days or until it is the finish you need. Make sure you keep the mixture moist.

A. If you put bleach ⇦ bleach/sodium hypochlorite in bulk on eBay or Amazon [affil links] and salt onto the aluminum, especially a gel bleach so that it sticks (I used toilet bowl cleaner) then cover with plastic you will get a pretty good patina in an hour.

Hi Julie. Thanks for the posting and illustration. Only trouble is, your example isn't aluminum :-)

It looks like a tomato juice can or pineapple juice can made of steel with a tin coating. We can see the brown (iron) rust on the lid seam.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
October 2014

A. ferric chloride ⇦this on eBay or Amazon [affil links] (etching fluid) and apply heat.

A. I hope I'm understanding the intent of the question. From nearly 40 years in the custom metal graphics industry, I can share that making one metal alloy item look like another metal alloy item, or exhibit the "natural oxidation" effect of the intended metal (patina), is certainly possible. It is done by applying an appropriate "topical color-inducer/oxidant" or a "topical pigment." I recently created a cast aluminum historical military plaque for a client, and when asked to match the near century old cast bronze plaques already in place, gave him a color palate from which to choose. He found a color chip that closely resembled the natural patina bronze. It was a rather consistent blue-gray. While not exact, it was very close. If he chooses to send the plaque to our facility, we could, if supplied with photos of his memorial and plaques, come a bit closer.

My point is that when using a core metal, and then asking for a coloration on the metal, one's immediate solution is to simply use a topical pigment. The coloration-artist must have enough experience, but even creating a "ferrous oxidation mark" on an aluminum surface is totally doable.

One should also understand and differentiate between a "mechanical finish", which is usually created on the bare metal using some form of abrasive, and the "topical finish", which refers to any chemistry, liquid or solid, applied to the surface of the subject material. I see the solution to this question as being a selection of appropriate topical chemistries. And frankly, this is a very common practice for the entertainment industry and the prop departments and shops.

Know that "sodium hydroxide", as mentioned by others as a rapid acting oxidant, (the basic chemistry of drain cleaner) is sometimes used as an etchant of aluminum; meaning it will quickly oxidize and essentially remove the aluminum as though one were mechanically engraving with a rotary bit. In its place there will be "aluminum oxide." Of course, purposeful etching for graphics also most usually includes creating a protective stencil that, when applied to the virgin surface of the metal/aluminum, only allows the etchant to "selectively etch" the metal surface. I'd estimate that with 3-5 cans of Rust Oleum spray paint, and some stencil material custom cut to mimic a random rust-mark, and after one properly prepares the metal surface, the replication of ferrous oxide marks could be created.

A. TJ ... it's been a while since my post. I appreciate your response and can understand the challenge of trying to translate a client's 'vision.'

While not permitted by forum policy, which I understand, you might want to search for a manufacturer of faux-finishes (in spray cans) that literally has formulated and provides our industry with just about any 'patina' finish you might want to impart on any metal surface. I've such a product at home to turn galvanized pipe into dark rubbed bronze to match plumbing fixtures. With topical finishes, I've found almost anything is possible. Although, its durability, like any clear coating or pigment, is susceptible in its unique way, to handling wear, UV exposure and general weathering.