Anodising

Anodising Aluminum

Aluminum is anodised both to increase corrosion resistance and to allow dyeing.

While pure aluminum creates a natural oxidation layer its alloys are more prone to corrosion and are therefore anodised for corrosion resistance. Most aluminum aircraft parts including major components are anodised. Anodised aluminum can be found in many consumer products like mp3 players, flashlights, cookware, cameras, sporting goods, and many other products both for corrosion resistance and the ability to be dyed.

The Aluminium oxide coating is grown from and into the surface of the Aluminium. Because of this it is not prone to peeling or cracking like organic coatings such as paint. In most consumer goods the dye is contained in the outermost portion of the Aluminium oxide layer. While highly wear resistant the anodised surface can still be worn. If wear and scratches are minor then the remaining oxide will continue to provide corrosion protection even if the dye is removed.

There are three major processes for aluminum anodising. Type I is Chromic Acid Anodising, Type II is Sulfuric Acid Anodising and Type III is hardcoat anodising.

Type I and Type II Anodising

Aluminium, when exposed to the atmosphere, forms a passive oxide layer, which provides moderate protection against corrosion. This layer is strongly adherent because it is chemically bound to the metal surface as compared to oxidation (corrosion) in steel, where rust puffs up and flakes off, constantly exposing new metal to corrosion. In its pure form aluminum self-passivates very effectively, but its alloys, especially 6000 series due to the magnesium content, are far more prone to atmospheric corrosion and therefore benefit from the protective quality of anodising.

Before being treated, the aluminum, if wrought, is cleaned in either a hot soak cleaner or in a solvent bath and may be etched in sodium hydroxide (normally with added sodium gluconate), ammonium bifluoride or brightened in a mix of acids. Cast alloys are normally best just cleaned due to the presence of intermetallics unless they are a high purity alloy such as LM0.

In aluminum anodising, this aluminum oxide layer is made thicker by passing a DC current through a sulfuric acid solution, with the aluminum object serving as the anode (the negative electrode). The current releases hydrogen at the cathode (the positive electrode) and oxygen at the surface of the aluminum anode, creating a buildup of aluminum oxide. Anodizing at 12 V DC, a piece of aluminum with an area of 1 square decimeter (about 15.5 square inches) can consume roughly 1 ampere of current. In commercial applications the voltage used is more normally in the region of 15 to 21 V.

Conditions such as acid concentration, solution temperature, and current must be controlled to allow the formation of a consistent oxide layer, which can be many times thicker than would otherwise be formed. This oxide layer increases both the hardness and the corrosion resistance of the aluminum surface. The oxide forms as microscopic hexagonal "pipe" crystals of corundum, each having a central hexagonal pore (which is also the reason that an anodised part can take on color in the dyeing process). The film thickness can range from under 5 micrometres on bright decorative work to over 25 micrometres for architectural applications.The older Type I (chromic acid) method produces thinner, more opaque films that are softer, ductile, and to a degree self-healing. They are harder to dye and may be applied as a pretreatment before painting. The method of film formation is different from using sulfuric acid in that the voltage is ramped up through the process cycle