Hot dipping aluminising

Although hot-dip aluminising of sheet might appear to resemble hot-dip galvanising very closely, there are several important differences which combine to make the process more difficult to operate. The main difficulties are ... 

Fabricated articles are less commonly hot-dip aluminised now than in the past. Coatings are applied after cleaning the work, e.g. pickling in hydrochloric acid in the case of steel and then preheating. The work is then immersed in a molten salt bath, a flux or reducing atmosphere, prior to... 

Calorised and heat-treated mechanically-clad products have coating structures similar to hot-dip aluminised coatings, but the degree of alloying with iron is variable (Fig. 13.5). With Calorised products the surface layers usually contain 25-50% aluminium. 

Hot-dip aluminised coatings have been tested by the American Society for Testing and Materials over a 23-year period in rural and marine atmospheres. In most cases, coatings of 0-025-0-050mm thickness show no deterioration, but at the more severe marine site some rusting has occurred. 

Spray-aluminised coatings are used for exhaust valves in automobile engines, exhaust and silencer systems (double and triple life), tyre moulds, gas ducting, heat-treatment pots, furnace ladles, carburising boxes and fans handling hot gases. Similar applications utilise Calorised and hot-dip aluminised coatings. Hot-dip aluminised steel wire has been used in steel-cored aluminium conductors for overhead transmission lines. For some... 

Glasbrenner, H. et ah. Development of a Tritium Permeation Barrier on F82H-mod, in Sheets and on MANET Tubes by Hot Dip Aluminising and Subsequent Heat Treatment, Forschungszentrum Karlsruhe GmbH, Karlsruhe, Germany, 1998, p. 30. 

Some of the principles of hot dipping are common to galvanising, tinning, terne coating and aluminising. 

It is essential that the effect of heating of the basis metal during the hot-dipping process be borne in mind at the design stage, especially with aluminising when temperatures of about 700°C are reached and also with... 

A hot-dipped tin coating on steel has a layer of FeSnj at the interface between the tin and the steel. This alloy grows very slowly in comparison with the more complex alloy layers formed in galvanising and in aluminising. It usually comprises 10-20% of the total coating thickness. 

Hot Dipped Coatings Major attempts have been made to improve the quality of aluminised steel strip. Requirements on coating thickness and uniformity have been imposed. It is the speed of sheet movement, length of path in the molten bath, temperature and composition of the bath that control the thickness of the intermetallic layer which lies below the aluminium outer surface. The process of intermetallic alloy formation is diffusion controlled, and it is usual that some dissolution of iron into the molten aluminium does occur at a rate, Ac/At, which is given by... 

Aluminised steel produced by hot dipping is used in the construction of parts of many exhaust systems of road vehicles. Failure of some of these exhausts does take place well within the expected two-year average life. This arises in the rear end of the exhaust where dew point corrosion occurs on the inside of the system. Acid dew of pH 2.7-3.1 is produced in the exhaust gases at temperatures below 48°C and this concentrates as the system eventually heats up towards 100°C. The aluminised coating is attacked at weak positions, e.g. where holes have been punched and the aluminium does not completely coat the steel. Eventually, the aluminium coating is undermined and the steel severely attacked. It is estimated that the use of aluminium coatings can increase the life of unprotected steel by at least 12%. 

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