Chromate treatments aluminium

Alkaline chromate treatments for tin, e.g. the Protecta-Tin processes, have been developed by the Tin Research Institute. The solutions resemble the M.B.V. compositions for treating aluminium, but are more alkaline. Thin invisible films which resist staining by heat and sulphur-bearing compounds and give protection against humid atmospheres at pores are deposited. 

Painting zinc and aluminium makes special demands on the pre-treatment of the substrate. Chromate treatment provides a good basis for paint, and it is still in widespread use in spite of the toxicity of chromate. Phosphate treatment or washpriming can also be used. Alternatives for aluminium are light blast cleaning or anodizing. 

Magnesium alloys do not retain a high lustre like the aluminium alloys and cannot be electroplated with the same ease as the zinc alloys. Finishing is usually achieved by painting or lacquering. Chromate treatments are used to protect against corrosion, but have little decorative value. 

Aluminium alloys are widely used in aircraft construction, extrusions for building purposes, skins of caravans (aluminium coil) and beverage cans. In each case they are protected by organic surface coatings on interior and exterior surfaces. Prior to coating, the metal is pretreated to improve corrosion protection, normally with a chromate treatment. The most widely used of the well established chromating treatments are the amorphous chrome phosphate process. 

Filiform corrosion is characterised by the formation of a network of threadlike filaments of corrosion products on the surface of a metal coated with a transparent lacquer or a paint him, as a result of exposure to a humid atmosphere. This phenomenon first attracted attention because of its formation on lacquered steel, and for this reason it is sometimes referred to as underfilm corrosion, but although it is most readily observed under a transparent lacquer it can also occur under an opaque paint film or on a bare metal surface. Filiform corrosion has been observed on steel, zinc, magnesium and aluminium coated with lacquers and paints, and with aluminium foil coated with paper. Surface treatment of the metal by phosphating or chromating lessens the tendency for filiform corrosion to occur, but it is not completely... 

The processes are dealt with fully in Chapters 11, 14 and 15. Because many paint systems include an initial surface pretreatment, e.g. chromated aluminium or phosphated steel, BS4479 1990, Part 3 deals with conversion coatings and should be consulted by designers. Whatever the method of treatment, liquids must be able to drain quickly and freely from the surfaces. Crevices where liquids can become entrapped are best avoided. The surface configuration needs to be such that active solutions can be washed away, leaving the surface to be painted completely free from unreacted pretreatment solution. Failure to achieve the requisite level of freedom from the surplus chemicals causes paint failure, e.g. osmotic blistering. 

The anodic oxidation of magnesium does not normally produce a film that has sufficient corrosion resistance to withstand exposure without further protection by painting, and the solutions used are complex mixtures containing phosphates, fluorides and chromates. In the case of aluminium, a relatively simple treatment produces a hard, compact, strongly adherent film of oxide, which affords considerably increased protection against corrosive attack . 

Several immersion treatments using solutions containing chromates have been developed for aluminium. It is not always clear to what extent the films formed can properly be called chromate films, i.e. films containing a substantial amount of a slightly soluble chromium chromate, but even if the film consists largely of aluminium oxide or hydroxide or other salt with chromate physically absorbed, it will still provide a reservoir of soluble chromate at the metal surface. Treatments fall into two classes alkaline and acid. The latter are of more recent development. 

Acid treatments The principal acid processes were developed in the USA under the name Alodine, and are marketed in the UK as Alocrom and under other names. The original solutions were based on acid solutions containing phosphate, chromate and fluoride ions. Immersion for up to 5 min in the cold or warm solution leads to the deposition of a greenish film containing the phosphates of chromium and aluminium, and possibly some hexavalent chromate. The more recent Alocrom 1 200 process uses an acid solution containing chromate, fluoride and nitrate. Room-temperature immersion for 15 s to 3 min deposits golden-brown coatings which contain chromate as a major constituent. 

The chromate sealing treatment imparts to the anodic film a distinct yellow to brown colour, which is probably due to a basic aluminium chromate or alkali chromate adsorbed on to aluminium hydroxide. The film gives appreciable protection against marine exposure. 

The importance of a clean surface for painting is paramount (see Engineering surfaces of metals). For metallic substrates, either mechanical or chemical surface treatment is almost always needed to remove surface contamination and promote adhesion. Chemical pre-treatment of metals prior to painting is an effective method of surface conversion that can provide both corrosion resistance and improved paint adhesion. For ferrous metals, the most common chemical pre-treatment is a phosphate for aluminium (and many other non-ferrous metals), the most effective pre-treatment is chromate. Metals must first be cleaned with an alkaline cleaner and then rinsed before they can be converted chemically in an immersion bath or by spray. 

A summary of the progress in evaluating adhesive bond performance over a 25-year period has been pubhshed by Minford. More recent papers relating to Famborough work include a comparison of anodized and chromate etch treatments for clad aluminium (the durability of the former was superior, see also FPL etch. Anodizing, Pre-treatment of aluminium) and a comparison of the natural weathering and accelerated ageing of joints. ... 

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