Aluminium Design and Construction John Dwight
Aluminium Design and Construction John Dwight
Aluminium Design and Construction John Dwight
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3.7.4 Painting<br />
Alloys having durability rating A or B do not normally need to be<br />
painted, except for the sake of appearance (Section 4.7.2). A paint finish<br />
is more likely to be called for with C <strong>and</strong> D-rated alloys, to provide<br />
protection. A brief outline of possible paint systems is given below.<br />
Whatever system is used, a suitable pretreatment is essential.<br />
Mass-produced components can be painted in the factory using powder<br />
coatings which have largely replaced solvent-based paints for such<br />
application. Before powder coating, the pretreated items must first receive<br />
a chemical conversion coating. They then pass through an electrostatic<br />
field, where they are subjected to a spray of dry powdered paint material,<br />
after which they proceed through a tunnel oven for stoving. The stoving<br />
cycle is typically of the order of 200°C for 10 minutes. A feature of the<br />
process is its lack of waste, since any excess sprayed powder falls to the<br />
floor where it can be collected <strong>and</strong> re-used.<br />
For non-factory application, aluminium can be coated in the normal<br />
way using solvent-based paints. It is important that none of these,<br />
especially the priming coat, should contain copper, mercury, tin, lead<br />
or carbon-based compounds. A preferred primer is one based on zinc<br />
chromate.<br />
3.7.5 Contact with other materials<br />
At joints between aluminium <strong>and</strong> most other common metals, where<br />
moisture may be present, it is essential to take suitable precautions to<br />
prevent bimetallic corrosion (Section 4.7.3). British St<strong>and</strong>ard BS.8118<br />
gives detailed advice on the treatment of such joints. The main essential<br />
is to insulate the two surfaces from each other using, in order of efficacy,<br />
a coat of priming paint, an impervious tape, or a non-conducting gasket.<br />
The other requirement is to apply comparable treatment to bolt holes<br />
<strong>and</strong> under bolt heads, <strong>and</strong> to protect the heads of steel bolts with<br />
aluminium paint, or by metal spraying with aluminium.<br />
There are occasions when aluminium needs to be protected from<br />
non-metallic materials with which it is in contact, in the presence of<br />
moisture. Such materials are concrete (while still ‘green’), soil, some<br />
woods (e.g. oak) <strong>and</strong> some insulation boards. Treatment with bituminous<br />
paint is usually efficacious in such situations.<br />
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