Is it just for the pretty colour? What even is Anodizing?
I’m glad you asked. Most of the BTCCNC cases are Anodized, and there is good reason for taking the extra time and effort required to do this.
First let me start at the end and explain the result. I’ll cover the technicals a bit later.
It’s Cosmetic, right?
Yes, the pretty orange colour is nice, however there is a more important reason.
Aluminium alloys are quite soft. While they can be polished to a super shiny finish, they will dull quite quickly and are easily scratched. If left alone for a while, the next time it is handled there is a chance that your hands might get slightly dirty. This is because the alloy will begin to corrode ever so slightly, depending on the environmental conditions. That rubs off when handled, and leaves a black smudge on skin.
Anodising is an electro-chemical process that accelerates the growth of a natural layer on the alloy. This layer is Aluminium Oxide and unlike the base metal, this oxide layer is very hard. If grown thick enough the layer will prevent most corrosion and can be dyed to take on a wide range of colours.
After anodizing, the BTCCNC cases are reasonably scratch resistant in normal use, and will maintain their appearance over time.
Anodizing Technicals
Aluminium is a very reactive metal. Almost instantly after being cut or polished, a thin Aluminium oxide layer will form because the metal reacts with the oxygen in the air. It is impossible to ever completely remove the oxide layer, because it reforms very quickly.
Anodising is an electro-chemical technique that rapidly grows this normal oxide layer to be much thicker. Depending on the parameters used in the process, a honeycomb-like oxide structure will grow from the metal surface. Most anodizing processes aim to grow that layer to be around 25 microns thick (about one thousandth of an inch).
This transparent oxide honeycomb-like structure can be filled with dye and then sealed. The outer layer is mostly closed, trapping the dye and enhancing corrosion resistance.
The process passes an electrical current between the item being anodized (the anode) and another metal (the cathode) while both are submerged in a bath of strong acid. The current calculated depends on the surface area of the part, and desired current density. Different current densities produce different oxide layer characteristics; tradeoffs can be made for speed, corrosion resistance, colorability, and scratch resistance.
The Process
The process is theoretically quite simple – dunk that part in acid and run a current through it, wait for a bit then dunk in dye and BOOM! Done!
Yeah, but the devil really is in the details here.
First, the metal must be prepared. Anodizing isn’t a covering like paint so any surface scratches or machining marks will show through. The first step is to prepare the surface to a nice even and consistent finish. This can be done by sanding and polishing, tumbling with special media that will slowly abrade the surface, electro-polishing or a combination of all of those things. Mostly, it’s all of those things.
Second, cleaning. The part must be perfectly clean. Even one touch from a “clean” finger will leave enough contamination behind that it will show up. Once cleaning starts, gloves are a requirement and preferably no manual handling at all. Cleaning is a combination of degreasing, ultrasonic and chemical processes. In the later stages, the part is dipped in sodium hydroxide and then a nitric / hydroflouric / hydrochloric acid cocktail. One downside of this thorough cleaning is a highly polished surface will be etched and therefore dulled slightly. You can’t have your cake and anodize it too.
All processes from cleaning until the end of anodizing are done using distilled water. Tap water contains too many minerals and chemicals which interfere with the process. The end result would be a splotchy mess.
Next, the Anodizing. The Anodizing bath contains 20% Sulfuric Acid, a lead sheet for the cathode, and the part itself suspended on a titanium wire or jig. The lead sheet and the titanium are connected to a constant current DC power supply and the current is slowly ramped up the pre-calculated value. This acid solution is kept at about 20 degrees C and the part remains there for a pre-calculated time, typically 1-2 hrs. The time calculated is based the required anodizing layer thickness.
Once the time is up, the part is sprayed with distilled water to remove most of the acid, and dipped into a sodium bicarbonate solution to completely neutralize any acid that remains. It is then left for a few minutes in a distilled water rinse bath to clean the bicarb off.
Dye is next. The dye is a special synthetic type made specifically for anodizing, which gives good long term resistance to infrared fading. It is heated to 60 degrees C and the part is submerged for about 15 minutes. At this time any failures in the cleaning, handling and anodizing become apparent. This is particularly frustrating and tends to make one very careful and meticulous in the early cleaning stages.
After a quick rinse, the part is transferred into the sealing bath. This can be boiling distilled water, but more commonly it has nickel acetate added which improves the sealing process. The near-boiling water reacts with the very top surface of the oxide layer and the resulting compound seals the holes at the top of the honeycomb structure. This traps the dye and prevents surface contaminants reaching the aluminium and causing corrosion.
After cooling, the part can be wiped off to remove any remaining dye and the process is complete. Phew! Crack open a beer and relax for a bit!
Why Orange?
Orange is the colour of Bitcoin after the mining process. The energy expended by the miner tends to color the ones and zeros with an orange hue, not dissimilar to the process of heating and quenching steel to achieve a blued finish. This can be tricky to see, but ask any experienced Bitcoiner to lift the lid on their full node and to set a point-source light to just the right angle… you’ll see the orange sparkle.
There may also be some cases that are black and maybe sapphire blue… because that’s my favorite colour. Keep an eye out for those.
Never will you see green of any shade. I would not want to accidentally confuse anyone into thinking a green bitcoin is real Bitcoin.