Basics of Colour Anodising

Anodising is the process of anodic oxidising where a oxidic protective layer on the aluminium is produced. This process does not actually apply a protective layer on the material but creates a thin oxide layer by converting the top metal layer. The aluminium is anodised by immersing it in an acid (the electrolyte) and applying Direct Current (DC) electricity, with the work-piece positively charged. The electricity splits some of the water in the acid, into positively charged hydrogen (H+) and negatively charged oxygen (O-), where it reacts with the aluminium to form aluminium hydroxide and related compounds. It is important here to remember the dual nature of the acid;- whilst it helps to conduct the electricity; it also attacks the anodized film at the same time. The acid helps with forming the anodic structure and at the same time works to dissolve it.

This is not necessarily a bad thing. Rather, the simultaneous formation and dissolving of the anodized film creates a coating that has a porosity in the form of microscopic “drill holes” extending from the surface almost to the raw aluminium.

In colour anodising, the pores in the thus created coating absorbs the dye, and then the coating is “sealed”, locking the dye in. The strength of the colour will depend on how thick the coating produced during the anodising process is, and thus how much dye it can absorb. So light pastel colours can often be obtained with a minimal anodising thickness. Howver to get a fully saturated dark black (jet-black) colour may require a greater anodised oxide thickness. All of our dyes used for anodising aluminium are industrial strength specialised organic dyes, with good colour-fastness and are the same as those being used by commercial anodising companies.

Whilst we have heard of DIY users using simple dyes such as food, leather and textile dyes; these are not suitable as those have limited to no resistance to sunlight. I certainly don’t understand why one would go to the expense and trouble of making a beautiful crafted aluminium article only to then colour it with an inferior dye. 

Personal Safety is Important

Personal safety equipment such as goggles, gloves as well as splash gown must be used all the time during all operations in the anodising process. When handling acids, it is preferable to use a full-face face-mask but at least wear safety goggles to protect the eye. Gloves need to be used for hand protection. Also ensure that there is good ventilation to safeguard your breathing system. Adequate personal protection is strongly advisable!

Colour Anodising Sequences

The anodising process is not completed in a single anodising tank but includes a number of pre-treatment steps before the anodising occurs and has a number of post-treatments after it.

The most common sequence, would be the seven steps:-

  • Clean
  • etch
  • de-smut
  • Anodise
  • Neutralise
  • Dye
  • Seal

each with a rinse after the process as in the diagram above. Depending on the grade of aluminium used, not all steps may be necessary.

If a matt finish is required, the alkaline etch process does produce a matt finish on the anodized parts. The longer the treatment time, the deeper the etch and coarser the surface obtained. Similarly , a chemical or electro-polishing process can be used to brighten up the parts prior to anodising.

Detailed process flow for anodising

corrosive

!!! Do not allow any extra water, caustic soda, sodium bicarbonate or similar near the acid bath. !!!! 

1. Mix up ~20% Sulphuric Acid solution with pure distilled water.

Remember, never add water to acid, always add acid to water. USE PERSONAL SAFETY EQUIPMENT !!! 

Make up enough to fill whatever container you are going to use to about 2/3 full. Leave to cool (mixing acid is exothermic and thus generates lots of heat). This mixture can be used many hundreds of times for anodizing runs. It will eventually pick up impurities and become less effective. The anodising needs to take place in a cool anodizing bath (±20 °C) hot baths cause pitting and surface imperfections

2. Cover your working area with something disposable. Putting the anodizing bath on a big sheet of heavy duty plastic is a good idea – keeps any splashed acid off the work surface. Make sure that you have a bucket of Sodium Bicarbonate solution handy for dipping things in. Suggest getting a carton of sodium bicarbonate. If you do spill any amount of acid, its essential to have some good, easy to dispense, alkali handy to neutralise it.

3. Prepare your aluminium piece. Finish is everything – anodizing does not hide a poor finish in fact it will enhance it. Remember that the colour will accentuate any blemishes. Clean the item then, clean the item then……… clean it again !

4. Degrease parts in a non-etching cleaner solution at ~55 °C for 4-5 min. ….Rinse.

5. If required etch the part in a 50 – 100 g/L sodium hydroxide (caustic) solution at ~ 50°C for one minute (or longer to achieve the required finish. If the aluminium was already anodised, it is possible to remove the anodized layer by leaving it in the caustic bath for longer (beware; if it is a critical part, it will reduce in size if you do this). Rinse.

6. De-smut the object if etched; this dissolves the gray-black alloying ingredients present in the alunimium (such as silicon, copper, zinc, and magnesium) from the surface of the part, typically done at room temp for 1 minute in a 10 % nitric acid solution. Without nitric acid, just try to clean up the part as best you can with hot soapy water and then rinse. Nitric acid is an acid where personal protection is an absolute must. Rinse

Grease from finger prints will leave a mark on the finished item finger prints

7. Suspend the aluminium part in the acid anodising tank, so it is completely immersed using some kind of aluminium wire or aluminium strut. The only metals allowed in the bath are aluminium and lead. Make sure you get a good electrical connection. Anodise the part(s) using Voltage of ~12V at 1 – 1.5 A/dm2 for 30 – 45 minutes duration; at room temp (20 – 22°C). Upon completion of the anodising process, Rinse twice. The parts should stay in the final rinse around 1 minutes or longer. 

8. Dip the parts at room temperature into dye solution. Timing can range from seconds to 30 min (depending on shade desired); using elevated temperatures ( max 60 oC ) can also benefit the development of colour intensity. 

HERE Experimentation is the key! The dye mix can be used many times over. Rinse

9. Seal the parts with either a water boil (for 1 hour) or make use of a hot chemical sealant (at least 20 min boil). Rinse

QUICK REFERENCE VALUES

  • Sulphuric acid (free)  165 g/L – 225 g/L
  • Current density  1.2 – 1.8 A/ dm2
  • Voltage  12 V – 20 V
  • Current  Direct Current (D.C.)
  • Temperature  18° – 22°C
  • Anodising Time  25 – 40 minutes
  • Coating thickness  8 – 12 um

COMMENTS ON PROCESSES

Comments on processes

Cleaning

It can not be stressed enough that the parts need to be clean prior to all process steps. The cleanliness of the parts can be checked by what is called water break test . After cleaning, do not touch the parts with bare hands as sweat, dirt and oil sticks on the parts, requiring it to be cleaned again.

Anodising

The anodising process involves placing aluminium parts into an electrolyte solution, a weak sulfuric acid solution, passing a low direct current (DC) through it. The cathode can be made of lead, titanium or graphite (preferably not aluminium which needs to be cleaned regularly) to ensure conductivity. The aluminium parts that are to be anodised are connected to the positive (Anode) side. By passing a direct current onto the anode, the aluminium surface is being oxidized to form aluminium oxide. The oxide layer looks like a honeycomb structure having many microscopic pores formed from the aluminium surface. These pores have tube-like structures and therefore accepting dyes to be adsorbed into them.

Colouring & Mixing

All of our specialised industrial grade aluminium dyes are water soluble and available in concentrated liquid form. There are many different ways to apply the colours onto a fresh anodized surface. The most used method is by immersion of the part into the dye solution for a period of time. There are also other dyeing techniques which are prevalent for the artistic application which include spraying, brushing, silk screen and etc. Mixing of different dyes (Pre-mix) to form a full spectrum of colours is possible, e.g., blue plus yellow forms a green colour. Multiple over-dying (dye into 2-3 separate colour tank) can also be used to achieve the required result. If the pre-mix or over-dying method is used we normally advise to dye at room temperature.

Sealing

The advantage of using a hot sealing method is minimisation of the colour bleeding during the sealing process. The alternative newer cold sealing process works at room temperature (around 25°C) but we suggest to only use that option for non dyed aluminium

EQUIPMENT

There is a lot of information about this on the internet but some comments

Tanks

Polypropylene tanks in rectangle or circular size can be used for the anodising line. Buckets are also suitable containers.

Aeration

An aquarium air pump will help to ensure a good circulation of the electrolyte inside the anodising bath (make sure these are oil free pumps)

Bath heating & Insulation

A ceramic type of heater is a good source for heating up a hot sealant solution. Do not leave the heater in the sealing tank unattended (as the solution boils away the tank will ignite). It may be advisable to use insulation material to minimize the heat loss of the hot sealant solution.

Power supply

A current density of 1-1.8 A / dm2 can be used to work out the capacity of the rectifier required for the anodising line. A small laboratory rectifier can be an ideal power source. Depending on the surface area being anodized, a 12 V rectifier delivering a current range of 10 – 30 A should be enough for a DIY small scale anodising line. For hobby applications a car battery charger can be used

Racking

The racking system can either be made of titanium or aluminium. If aluminium is being used please ensure that this is stripped regularly as otherwise current may not flow though the parts