Rust is the oxide that is formed by open-air oxidation of iron. The chemical composition of rust is mainly hydrated iron(III) oxide (Fe2O3.nH2O), and under wet conditions may include iron(III) oxide-hydroxide (FeO(OH)). Rusting is the common term for corrosion of iron and its alloys, such as steel. Although oxidation of other metals is equivalent, these oxides are not commonly called rust.
I would argue that oxidation of any metal is called "corrosion", "Rust" is a term which relates specificaly to the corrosion of Ferrous based metals, and cannot therefore by used to describe the white powder on the surface of Aluminium and its Alloys. Although having said that, occasionally the term "rust" is used by personel ignorant of its correct meaning. This should not be the case on legal documents such as a warranty.
The most common form of Aluminium alloy corosion on vehicles is Galvanic corrosion caused by the electrolytic attack of one metal - the "lesser" metal. This can easily be cured as can be seen from the following article.....
The most frequent types of corrosion are Galvanic Corrosion, Pitting and Crevice Corrosion.
Galvanic Corrosion:
Galvanic corrosion takes place when two different metals have contact with each other in the prescence of an electrolyte. The less noble metal (anode) of the combination corrodes while the noble (cathode) will be protected. A small surface of the cathode and a large surface of the anode lead to a low corrosion. In reverse case the Aluminium will be attacked quickly. In most combinations with other metals, Aluminium is the less noble. Aluminium is therefore of greater risk of galvanic corrosion than that of the other constructive materials.
These dangers of galvanic corrosion only exist in metallic contact with more noble metals (or other conductors of electrons, for example graphite) and then when an electrolyte with good conductivity exists between the metals. The occurrence of galvanic corrosion will be encouraged because of a disadvantageous construction of the extrusion.
Galvanic corrosion does not happen in dry surroundings, however contrary to this, in surroundings with chloride the risk of galvanic corrosion always exists, for example near the seaside. In these surroundings it could be that copper, carbon steel and stainless steel suffer galvanic corrosion.
It should be noted that problems can occur in the combination of galvanised steel and Aluminium. This is due to the fact that the Aluminium will be protected by the coating of zinc but when the zinc is used up the bare surface of steel can attack the Aluminium. For this reason the Aluminium should be combined with warm zinced material in an aggressive surrounding because warm galvanising, gives a harder coating of zinc, than hot-dipped
However, galvanic corrosion can be prevented if certain steps are taken. One being by electric insulation between the metals, where the insulation has to interrupt the metallic contact completely.
In large constructions where electric insulation is difficult it is possible to use an electrolyte insulator between the two metals for example by painting to interrupt the connection. Often it is beneficial to conceal the surface of the cathodes (that of the nobler metal), however another possibility is the installation of an intermediate layer
Another method of protection is Cathodic protection. Cathodic protection can be achieved in two ways. Often anodes, consisting of less noble metal can be in contact with the surface of Aluminium that has to be protected. Within this process the less noble metal was sacrificed (it is corroding ) and is therefore called a sacrificial anode. The existence of a contact of liquid between the coating which has to be protected and the anode is another condition for prevention of corrosion. Often Z- or Mg-anodes are used for Aluminium. Cathodic protection can be reached by an exterior constant-potential supply and by connecting an Aluminium object to the negative pole.
