Deep Chemical Etching
For etching the surface you can use a couple different chemicals since aluminum is very reactive. Most people are not aware of this because all aluminum is corroded with an aluminum oxide layer so thin it is transparent and prevents other reactions. If you try to make a thick corrosion layer, you will find pure aluminum oxide to be a white solid. There is not a black version of aluminum oxide is not a distinct chemical compound with a different formula, but rather a grade of aluminum oxide that has been created by fusing it with other oxides, such as iron oxide (Fe2O3), silicon dioxide (SiO2), and titanium dioxide (TiO2). These impurities are what give the material its dark color. Therefore, not all etching chemicals will leave a dark by-product in the “surface pits” of the aluminum however. These are ones I have tested that are readily available with moderate success. YOU NEED GOOD GLOVES, APRONS, EYE PROTECTION AND VENTILATION FOR ALL OF THESE. SERIOUSLY!
Hydrochloric Acid Etching
This is darkening due to the deep pitting of the surface. It is an optical effect and there is no pigment or compound that is making it a slightly darker matte grey color. You need a Muriatic Acid that does not have an inhibitor in it to protect metals. I used Champion brand Muriatic Acid which comes in two types.
- Champion Muriatic Acid: This is 31.45% HCl by mass, a standard, multi-purpose muriatic acid.
- Uses: It is for water treatment and can be used for pool pH control, brightening concrete, and general cleaning.
- Where to buy: Available online and in-store from retailers like Ace Hardware, Family Farm and Home, and DK Hardware Supply.
- Champion AcidBlue Muriatic Acid: This is 31.45% HCl by mass with a low-fume formula designed for a safer, more comfortable application.
- Features: It contains “Vapor Reduction Technology” (VRT) that decreases harmful fumes by up to 90%. Some AcidBlue products also include “Skin Protec” technology for added safety against skin burns.
- Uses: It performs the same functions as standard muriatic acid, including lowering pool pH and cleaning concrete, without the strong fumes.
- Where to buy: Available online and in-store from retailers such as Walmart, Amazon, Leslie’s Pool Supplies, and EZ Pools.
I like the idea of having less fumes, but I chose the standard muriatic acid because I don’t know what the buffering compound is that they are using to react with the chlorine ions to prevent the gas from forming. I suspect it would inhibit the metal oxidizing properly. These are both strong concentrations. In my laboratory, I use the strongest reagent grade hydrochloric acid which is typically 37% by mass (12.0 M). This is a powerful acid. It is 120,000 times the concentration of your stomach acid. I diluted the acid to one-fourth (8% by mass) and I still got rapid results. This will degas hydrogen gas and will form an HCl aerosol as the reaction takes place rapidly and produces heat. The more you dilute this, the safer the process gets, but the more time it will take to have the desired reaction. You will quickly learn that if you leave it in the solution, the entire plate will dissolve away.
Ferric Chloride Etching
This is another great metal etchant for a large variety of metals (aluminum, steel, copper…). This does a great job of darkening the aluminum.
The reaction equation: 3 FeCl3 +3 Al → 3 Fe+3 AlCl3 + heat
Secondary reaction: Fe + O2 → black FeO and Fe3O4 mixing into new Al2O3 layer
Notice that pure iron metal is a product of the reaction. This means that some iron will deposit in the etched surface and then form an black iron oxide. This not only darkens the metal, but later you can get it to change its oxidation and have a “rusted” effect. This truly confuses people to think the aluminum parts are actually a steel alloy.
Just like the HCl etching, there is a lot aerosols and heat. Diluting it will make it safer.
Best deal I could find was MG Chemicals Liquid Ferric Chloride Etchant Solution, 1 Gallon Bottle
Direct Link to Purchase: https://mgchemicals.com/products/circuit-board-design/copper-etchants/ferric-chloride-etching
Material Safety Data Sheet: https://www.mgchemicals.com/downloads/msds/01%20English%20Can-USA%20SDS/sds-415-l.pdf
Blackening solutions:
Products designed to blacken aluminum parts. They are often fast-acting, parts can be dipped or applied with a brush.
Prepping the Surface
When these companies say the surface must be clean, they mean CLEAN. After washing all the machine oils off the plates with detergent. Rinse thoroughly with tap water, then give a rinse with distilled water. I would even recommend lightly buffing the surface with fine steel wool after they are dry. I am still not impressed with the durability of these Blackening agents. I think they work much better as a secondary step after the HCl etching process.
Applying the blackener
I prefer dipping the entire object into the solution. Dipping provides the most uniform concentration and exposure time on the surface. It also is the most efficient with your chemical usage. I use copper wire hooks to move my parts into and out of the chemical baths and rinse stations. If it is a large piece, you can apply the solution using a bristle brush or using a spray bottle. I only use sponges while wearing thick nitrile gloves to rub an area to make the coat more uniform, not really for the direct application.
JAX Aluminum Blackener ($36/qt)
JAX aluminum blackener causes a chemical reaction that fizzes and creates a powdery black residue, producing an antique black finish on the aluminum surface. It is using a reaction with selenous acid that results in the formation of a black selenium oxide to bind with the aluminium oxide.
Direct Link to Purchase: https://jaxchemical.com/shop/jax-aluminum-blackener/
Material Safety Data Sheet: https://jaxchemical.com/wp-content/uploads/2024/10/JAX-Aluminum-Blackener-SDS.pdf
Birchwood Casey Aluminum Black ($81/qt)
This is a more expensive blackener. It is double the cost of JAX brand, but I suspect the concentration of the chemicals is greater. The compositions are listed on the MSDS, but patent protection avoids providing exact proportions. It also includes an additional etching acid, as well as the formation of a black copper oxide as well as the black selenium oxide to bind with the aluminium oxide.
Direct Link to Purchase:
Material Safety Data Sheet:
Rinsing
Once the desired darkness is achieved, rinse the metal thoroughly with water. I have three basins of water and I dip them in sequence, knowing that the first basin is going to become contaminated the fastest and the last basin will remain relatively clean til the end.
Surface Treatments
Relief the high surfaces: To create the antique look, use a Scotch-Brite pad or steel wool to rub off some of the blackening, revealing the underlying silver metal on the edges and high points. Rinse with water again and dry the piece.
Hard Coating: The chemical blackeners can wear off easily. It is possible to assist its adhesion with a clear coat. Cerakote Clear – Aluminum MC-5100 has been specifically engineered for performance on all common alloys of aluminum for outdoor applications.
https://www.cerakote.com/shop/cerakote-coating/MC-5100/cerakote-clear-aluminum
Getting A Rainbow of Colors
Anodizing:
$600 Setup Minimum! This is a common method for coloring aluminum by manufacturers but it is an expensive process to set up. The process creates a porous aluminum oxide layer that is then dyed black using organic dyes, inorganic dyes, or electrolytic dyeing with metal salts. I don’t want to do it. It requires a dedicated space, requires a method of chemical storage/disposal, has the most expensive setup and is labor intensive. It is my least recommended method for attempting at home.
Direct Link to Purchase:
https://caswellplating.com/standard-anodizing-kit.html
Powder Coating
Spray painting with Rustoleum or “Tractor Paint” doesn’t last as long as you would like with all the impacts and abrasions that take place with armor. Powder coating is far better than liquid spray paints. The coating is generally flexible and thick, which helps it resist damage from impact better than a brittle coating might. If you go this way, choose the Polyester Powder Coating paints since they are more UV resistant than epoxy based paints. The application “guns” are approximately $100 and a bottle of paint is around $10. They are easily available. Which oven you are going to cure them in is a different matter. Don’t ruin your kitchen oven. I bought an old electric oven from a neighbor, placed it in my garage and put a metal box inside of it in order to bake and cure my projects.
Cerakote
This is not a normal paint job. This is a thin film ceramic coating. It is actually the best performing coating for aluminum. Yes, it is better than anodizing. It is a cheaper and easier setup than anodizing. But that is still kinda a false comparison. It is still an expensive and time consuming process. But it should have great results for the investment. C-Series is the easiest to apply, since it doesn’t require mixing before application and will air cure over a period of 5 days. The E-Series forms the hardest coats, but requires catalyst mixing and oven curing.
Direct Link to Purchase:
https://www.cerakote.com/shop/cerakote-kits $180-$200 starter kits