The Environmental Protection Agency (EPA) has set the guidelines for the total elimination of chromate conversion coatings (CCC) in protecting aluminium alloys against corrosion. This has led to increased research in the search for urgent and appropriate replacement for the chromate conversion coatings. It is a very difficult task given the high performance levels of chromate conversion coatings. In fact there is nothing that is easy to coat and capable of performing better than the chromates. The high levels of efficiency that come with chromate conversion coatings make the search for their replacement to be very hard. The conductive polymer coatings are the best placed to replace the chromate conversion coatings because of their good qualities. They have high levels of performance and are relatively easy to apply on the metal surface. The purpose of this study is to find a suitable replacement for chromate conversion coatings as a method of protecting aluminium alloys.
Introduction
Corrosion is the degradation of material because of the reaction with the environment. It often leads to the physical deterioration of the properties of the material. The weakening of the material can be because of a loss of some of the cross-sectional area, cracking of the polymer due to sunlight or shattering of the surface because of hydrogen embrittlement. Chromate conversion coatings have been for a long time the preferred material in protecting surfaces from the adverse effects of corrosion. They are normally used to increase the resistance to corrosion and paintabilty of aluminium alloys. Chromate conversion coatings have high levels of efficiency in performance; however their use in surface finishing is facing regulatory extinction because of the high levels of toxicity in chromates. This is why the Environmental Protection Agency (EPA) and the Navy are concerned with the dangers that come with the usage of chromate conversion coatings. The handling of these toxic materials, their leaching into marine environments and the economic and environmental aspects of their safe disposal are some of the key issues that need to be addressed. The replacement of the hexavalent chromium and heavy metal additives used in coatings has the potential of eliminating the health and environmental hazards. This paper presents the results from the research in investing the use of double strand conducive polymer in the protection of materials used in the aircraft industry and marine industry against corrosion.
Properties of Aluminium Alloys
The corrosion of aluminium alloys does not happen in a manner similar to the other common metals. They do not corrode uniformly but form pits on the surface with a unique chemistry inside the aluminium surface walls. In the marine environment, these pits form at very alarming rates because the chloride ions can easily attack and dissolve. The underlying surface is often exposed to the salt electrolyte through an anion exchange mechanism. In the copper rich alloys used in the aircraft industry the galvanic coupling often sets up a mini-battery with the aluminium atoms as the anode and Cu2Al acting as the cathode. In the aluminium alloys, the aluminium atoms are often oxidised to Al+3 cations and then the surface automatically begins to dissolve.
A localised lowering of the pH then follows at the aluminium dissolution site preventing the reformation of the native oxide and in effect accelerates the dissolution process at the site. This is how the pits are formed and the cycle goes on. The pits are very dangerous phenomena for corrosion because they don’t occur uniformly. The process of their formation is unpredictable and their rate of growth is also relatively faster. They offer a more serious problem of structural failure because of the eventual creation of a hole in the bulk material of the alloy. Chromate conversion coating is based on dipping the materials in acidic chromium trioxide bath. The coatings work through the formation of a strong, tight oxide layer of Al2O3 on the surface of the alloy. The resultant oxygen layer is hexagon shaped and grows from the surface of the alloy with a narrow circular hollow channel at the hexagon’s centre. The chromium atoms that have been reduced to the +3 oxidation state then enter this channel and seal the pores in the process.