Surface Finishing Corrosion Protection Through Ion-Assisted Vapor Deposition with Aluminum
Since 2016, the AHC Group has been offering the technology of ion-assisted vapor deposition with aluminum. Steel and titanium components can be better protected against corrosion with this technology.
Acorn Surface Technology is a company specializing in surface technology based in Kirkby-in-Ashfield near Nottingham, UK. It has been part of the AHC Group since early 2016. In 1998, Acorn began using the technology of ion-assisted deposition with aluminum, commonly known as IVD (Ion Vapor Deposition), and successfully refined this technology and expanded the capabilities of the process.
IVD aluminum coating was originally developed by McDonnell Douglas as a replacement for cadmium treatment of steel and titanium components. IVD aluminum is used primarily for corrosion protection, typically in the temperature range from 0 to 300 °C. The coating can also be used for steel and titanium components. It can also be used to prevent bimetallic or galvanic corrosion. An example for this is the coating of fastening elements made of titanium with IVD aluminum, to which aluminum assemblies are attached.
There are three different coating classes: Coating Class 1 with a coating of at least 25 μm offers the best corrosion resistance (at least 504 h in salt spray test according to ASTM B117). Coating Class 2 is often used for machined parts where tight tolerances are required. In this case, the coating thickness is generally between 13 and 25 μm. Finally, Class 3 coatings are usually applied to fasteners and other detailed components with tight tolerances. The layer thickness is typically 8 to 13 μm and offers the lowest corrosion resistance. The corrosion resistance can be increased in all cases by converting the surface with a chromate treatment.
IVD aluminum coating of metallic substrates involves a number of important process. First, the parts are degreased to remove machining or dewatering oils. The next process step is fine-grained sand blasting. On the one hand, the surface is mechanically cleaned and on the other hand, a profile with a large surface is produced. The latter supports the physical bonding of the coating to the substrate.
The final step takes place in the vacuum coating chamber. The parts are fixed on an electrically conductive rack during the process. The rack can be stationary or moved during the process. Movement improves the distribution of the coating on complex parts. The parts are moved into the coating chamber, from where the air is evacuated. An inert gas is then introduced into the chamber and an electrical voltage is applied. This leads to a plasma glow discharge, which is clearly visible as a violet glow in the chamber. The result is a very clean surface of the substrate.
As soon as this process is completed, the coating process can begin. Aluminum wire is now fed to a series of superheated ceramic crucibles. A high voltage is applied to generate very high temperatures, and the aluminum evaporates into an electrically charged mist. The negatively charged aluminum atoms move through the vacuum and deposit on the parts to be coated, which are electrically grounded.
After coating with IVD aluminum, the parts have a matt grey appearance and must be treated carefully. It is now necessary to close the pores in the outer surface of the coating. This is done by glass bead blasting. The parts can then be used as "as plated" or, more frequently, the pure aluminum surface is converted into an aluminum chromate layer by a chemical conversion coating.
This article was first published by konstruktionspraxis