A few notes & tips on anodising from our experience to get reliable and consistent results
Aluminium naturally oxidises in an oxygen-bearing environment, which is why you rarely see truly-shiny aluminium; the natural oxide though is thin, soft, and disorganised. Anodising is an electrolytic conversion of the surface of the metal into an oxide, but one in which the oxide forms a hexagonal matrix very much like the cells of a honeycomb. That structure is thicker, more robust and electrically insulating - but the cells have to be sealed after anodising otherwise the porous surface is rapidly contaminated. Un-dyed (type II) anodising is clear so either you just get the colour of the oxide matrix which is a light grey; or those cells can be loaded with a dye before sealing, which can give a variety of colours through to black (there is no useable white dye for anodising though). The honeycomb does not bend well around corners, so aim for a minimum of 0.5mm radius on any edges.
You can find references to Type II and Type III anodising. Type II is the most common, and is normally the one to use if you wish to dye the item. Type III (sometimes referred-to as Mil-Spec or “hard-cote”) is a special process carried out with very high current densities and chilled electrolyte; which results in a far smaller pore size which makes it far harder-wearing but the cells are so small they cannot take much dye.
The item to be anodised needs to be 'pure’ aluminium - can be various alloys but normally no inserts of other metals. Castings can be a pain to anodise due to imperfect mixes leading to blotchy results. Any welding needs to be with wire of exactly the same composition as the base alloy, otherwise it shows to a greater or lesser degree. If you want a matt surface, either etch it for longer, or blast with glass beads or similar media (never use sand or grit, as it remains embedded and leaves un-anodised spots) or media which has had ferrous contamination.
As it is an electrical process, getting a totally conductive surface and making good connections are key. The items needs to be grease free - either straight from the machine, or degreased. We use brake cleaner and then dip in a heated degreaser. The test is to check if the item will pass the water-break test - spray the item with water and if it beads up anywhere the surface is contaminated - you need to see a continuous water film with no breaks.
After degreasing, you need to be sure there is no insulating oxide etc. on the surface. If necessary, the item is then dipped in an alkaline strip/etch to get down to bare metal (especially necessary if the item has been anodised before); longer etching can also be used to give a more matter finish. As almost all aluminium products are alloys, you need to watch for smut on the surface - the strip/etch will remove sufficient aluminium but other constituents such as copper can be left on the surface. Various alloys will have different levels of smut or soot left on the surface, so the next step which may be needed is a quick dip in a “de-smut”.
Between each tank, its essential to avoid carrying-over process chemicals, so a good technique is to hold the item over the tank it has just come out of, spray and/or squirt from a wash-bottle with water (especially threaded/blind holes etc.), dip in a tank or bucket of clean water, spray again and then move to the next stage. You don’t normally dry the item between stages.
Talking of water - anodising is a chemical process and all solution make-up and rinsing needs to be with distilled or reverse osmosis / DI water.
The actual anodising can be done to a number of different “recipes”. The one we use is one part battery acid ADDED TO three parts water (ALWAYS ADD ACID as otherwise the exothermic reaction will spit acid at you). We then run the tank at 68-70 F (too cold and you will get “winter grey” where the matrix is too small to accept dye properly; too hot and you will get chalky surface from poorly-formed matrix and acid dissolution).
Racking - you need to suspend the item for all tanks (never let it sit on the bottom) and in the ano tank you need electrical contact as well. The contact needs to be 'agressive’ as during the anodising, the aluminium will be trying to grow an insulating layer even beween the workpiece and your contacts. The only three metals you can usually have in the acid electrolyte are aluminium, lead and titantium. You can use aluminium wire but it is renowned for losing contact so the best is Ti - either wire/rod bent into a spring or purpose made clips from the likes of ServiSure. You will also benefit from aeration/circulation to prevent hydrogen bubbles building up and forming minute insulated under-anodised spots.
Cathode - Lead is often used for the cathode plate (code 4 roofing flashing or similar) and carry the lead up & out of the electrolyte and make connections away from the reach of acid vapour. Aim for at least as much exposed cathode (i.e. not including that against the side of the tank) as the overall area of the the workpiece; and if you can, have the cathode wrap-around the sides/bottom of the tank to get more even results.
Power supply - you can just use a fixed voltage and “let her rip” but for consistent results you are better using a constant-current power supply. You calculate the total wetted surface area of the part (i.e. if its a cylinder, you need to allow for the inside as well as the outside), multiply that by the chosen current density (we use 6 A / sq foot), and set that on your constant current supply.
Duration - if you’re using a constant current supply, you can then predict the time needed to achieve your target oxide thickness; light colours need about 0.5 thou; dark colours/black need a full 1 thou. With the method we use, at 6 ASF it takes 120 minutes to grow a full 1 thou or 1 hour to grow 0.5 thou.
Dyeing - you can use clothes dye or food colouring; but for predictable results you are best using proper anodising dyes - these are designed to fit into the anodising matrix and give predictable results. They can be used cold, but are best used warm (up to maybe 120F) but not too hot, otherwise the matrix is sealed prematurely. Dying can take from a couple of minutes to 15-30 minutes for deepest colouration.
After dying and usual rinse, the item is then hung in the boiling sealant. You can seal in pure boiling DI water (or even just in steam) - that is best for anything which is associated with the food chain or which will be in skin contact (as some folk are nickel-intolerant) - otherwise sealants such as boiling nickel acetate solution provide maximum surface protection.
After removing from the boiling sealant, a good final stage is a quick wipe-down with WD40 to prevent bloom.
PPE at your own discretion - we use boots, battery-apron, gauntlets & face shields; and work in a well-ventilated environment with eye-wash to hand just in case.
You can buy kits - we started with a kit from Caswell and then grew to our current setup (used to anodise housings etc. for our own manufacture of motorsport TV / filming equipment, and for others).
Have fun & stay safe !