Hard anodising (Topic: 14120)

Hard anodising is a term used to describe the production of anodic coatings with film hardness or abrasion resistance as their primary characteristic. These anodic coatings are usually thick by normal anodising standards, and they are produced using special anodising conditions. The thicknesses are usually between 25 and 250 µm, and the anodising time can be up to 240 minutes. Most hard anodising processes are based on sulphuric acid, and the parameters are commonly within the following limits:

electrolyte concentration: 5ñ15%(vol.) sulphuric acid

current density: 2ñ5 A/dm2 (DC) or 2ñ10 A/dm2 (DC and pulse)

voltage: 23ñ120 volts

temperature: -5 to +5ƒC

The reason why anodic oxide coatings produced under hard anodising conditions have high hardness values and very good abrasion resistance, is due to the low anodising temperature the low electrolyte concentration, and the high current density compared to normal sulphuric acid anodising.

To keep the temperature on the low level good cooling and agitation are of great importance. At these temperatures water cooling is not possible, and some type of refrigerant must be used. Air agitation is sometimes not sufficient, but agitation can be provided by pumping the electrolyte through an external heat exchanger which thus combines cooling with agitation.

One of the main developments in hard anodising has been the use of more complex power supplies with biased, pulsed or interrupted current used to reduce burning problems and make highly alloyed materials easier to process. Much of the work has used AC superimposed on DC.

All the alloys comprised by this manual are suitable for hard anodising, specially AlZnMg alloys (e.g. 7108). Alloys with a very high copper or silicon content are less suitable. The colour of the coatings depends on the alloy and the coating thickness. AW 6061 has a tan or grey colour which darkens to jet black at 75 µm.

After hard anodising the profiles may be sealed in boiling distilled water, dichromate solution, dewatering oil or wax (80ƒC, 15ñ30 minutes). Dichromate sealing improves the fatigue properties, but decreases the abrasion resistance to some extent (in common with other aqueous sealing solutions). For this reason, aqueous sealing processes are not normally used where high wear resistance is required.

Hard anodised coatings may be impregnated with oils, silicones or dry lubricants for the improvement of anti-friction properties. Corrosion resistance could be improved by PTFE impregnation. This treatment gives no reduction in abrasion resistance of the coating.

For the best resistance to wear and abrasion it is preferable to use pure aluminium or alloys with the lower values of alloying additions. It is usually necessary however to couple the hard surface with a higher mechanical strength and hardness in the basis metal than is possible with pure aluminium. The preferred alloys are AW 6063 and 6082. (See Strength level of main alloy groups-table and Chemical composition).

The hardness should be greater than 350 HV to be called a hard anodised coating. Hardness values up to 1400 HV are reported to be obtained from a mixed electrolyte [27].

Hard anodised aluminium shows a good heat resistance, and a hard anodic oxide coating of 75 µm withstands short exposures to temperatures of the order of 2000ƒC [27]. The coatings give also very good electrical insulation. This property can be further improved by hot water sealing and waxing.

Hard anodic oxide coatings find application in the engineering industry for components which require a very wear resistant surface such as pistons, cylinders, and hydraulic gear. Another application is in the coating for the production of flame and chemically resistant surfaces.