The Pros and Cons of Aluminum
The light metals are a great category of materials to make things from. Engineers love the light metals. Aluminum has seen the most wide spread usage, but titanium and magnesium are also gaining ground in many applications. The excellent strength to weight ratio, ease of manufacturing and machinability as well as the many different forms and alloys available makes aluminum an attractive material.
The issue with aluminum is it’s reactivity on the surface. Aluminum is a very reactive metal, even in its alloyed state with commercial grades the surface of aluminum will react with the oxygen in the air and start producing a variety of aluminum oxide phases. This can be seen in a variety of films and residue on the surface of parts. After handling aluminum typically a black smut is formed, and can transfer to anyone handling the parts. Corrosion in an aqueous environment produced a white powder, similar to red rust on steel. Even if the part appears clean it will still have a clear aluminum oxide film on its surface. This reactivity is what has lead the coating industry to develop a multitude of surface protection coatings for aluminum parts.
The most popular treatment for aluminum is anodizing. Anodizing produces a well adhered oxide on the surface. This is done through the use of current applied to the part in an acidic bath, either sulphuric or chromic based acids. This film prevents any further oxide from forming, and thus protects the parts. The drawback is the film is porous. This was turned to an advantage by filling the pores with different coloured dyes to produce a nice colourful look to the coating. The dye is then sealed to prevent it from leaching out. This coating is great for most parts, and in a variety of mild use conditions. The coating is not very thick, on the order of about 5-10 microns and is not very hard, which means it can scratch if handled roughly. For most light duty applications this coating is a good choice for aluminum parts.
If the part requires more scratch resistance, or has a more sever wear application, then a hard anodizing can be used. By alerting the bath and current applied to the part, a thicker film, on the order of 25 microns, and a harder phase of aluminum oxide is produced. This film uses the aluminum in the part to produce the oxide, and there for a 25 micron thick film will only add about 12 micron in thickness to the surface of the part. This needs to be taken into account if your part has some tight tolerances – you may not get back the part at the size you were hoping to receive.
Other Aluminum treatments are more suited as a pre-treatment for other subsequent processing. Chemical Conversion coatings put a light film on the surface of the part, usually in preparation for painting. This coating has the advantage of not requiring current and so there are no touch marks on the part from electrical contact in anodizing.
Similar light films are being developed for aluminum now which also imparts colours, such as a black film. This is akin to Black Oxide with steel, only for aluminum. Again the advantage is not requiring the use of electrical current, so that parts can be dipped on a rack, in a barrel or in a basket, reducing handling and processing time.
Finally for the most demanding of applications there is electroless nickel. Plating aluminum parts with electroless nickel provides the best corrosion protection and wear resistance of all the aluminum coatings. It is not as hard as hard anodizing, however with its much lower coefficient of friction it out performs hard anodizing in most wear applications. Electroless nickel can also be applied with a co-deposit of Teflon® to further enhance the lubricity. The plating can be applied with very tight tolerances over a wide range of thicknesses and creates a complete corrosion barrier over the part, with no porosity.
Whichever coating you chose for your application, aluminum should never be used in its raw state. Any one of these coatings mentioned here, or others also available, will provide some protection for the part, and eliminate the issues with working with such a reactive surface.
Michael Zuraw