Aluminium plating from vapour

Ion Vapour Deposition A variant of the process is ion vapour deposition, in which a high negative potential is applied to the workpiece during chemical vapour deposition. The process has been employed on a commercial scale chiefly for depositing aluminium on to steel and titanium in the aerospace industry as an alternative to cadmium plating, which is liable to cause hydrogen embrittlement, especially of high tensile steel components. The aluminium is evaporated from a wire-fed resistance-heated boat . 

Volatile metal halides, usually chlorides and fluorides, also form the heart of several processes used to produce surface layers, rich in aluminium, chromium, or silicon, or combinations of these. In these processes, the workpiece to be coated is buried in a powder bed and heated to reaction temperature. The bed consists of a mixture of inert alumina filler, a master alloy powder that contains the aluminium, etc., and an activator such as ammonium chloride. Basically, at about 630°C, the activator volatilizes and the aluminium chloride vapour reacts with the master alloy to produce a volatile aluminium chloride, which then reacts with the workpiece surface to deposit aluminium. The deposited aluminium proceeds to diffuse into the surface layers of the workpiece to produce a diffusion coating. The process is driven basically by the difference in aluminium activity between the master alloy and the worlqtiece. These processes are well documented in principle, but their execution to provide reproducible and reliable results still involves considerable experience, or rule of thumb. These processes will be described in detail in Chapter 10. Finally, a chlorination treatment is used to remove tin from tin-plated steel. This uses a normally deleterious reaction to advantage and profit in the recovery of both tin and steel for recycling. Fluorination is used in the manufacture of polymers and fluorocarbon consequently, materials suitable for construction of these plants must be resistant to fluorine attack. 

---