Ion Vapor Deposition (IVD) refers to a manual vapor deposition process used for coating various substrates and components with pure aluminum to prevent corrosion. The process is carried out in an Ivadizer®, which is a vacuum vessel that comes in various sizes. Water vapor and oxygen present in the atmosphere tend to contaminate pure aluminum. In order to prevent such contamination, a vacuum is used to apply the aluminum coating to the substrates. In addition, by using a vacuum, the aluminum’s atmospheric boiling point is reduced.
IVD Aluminum Coaters
IVD aluminum coaters have been designed to suit particular applications. A rack-type coater is mainly employed for coating large parts. Figure 1 shows a standard rack-type IVD aluminum coater.
Figure 1. A typical rack-type IVD aluminum coater.
Figure 2. Barrel coater.
A barrel coater refers to a coater developed for handling large volumes of small parts. Generally, barrel coaters are utilized for coating bolts, fasteners, nuts, pins, rivets and other small cylindrical-shaped parts. Figure 2 shows a typical barrel coater.
Properties of IVD
For IVD aluminum coating process, a number of properties are essential such as thickness, composition, coverage, adhesion, appearance, corrosion resistance and substrate integrity.
At first, these properties were defined in the military specification for High Purity Aluminum Coatings, MIL-C-83488. Now, these properties are still being carried out in the detail specification for aluminum coatings, MIL-DTL-83488.
Utilization of IVD
Cadmium is known to have superior wear resistance, paint adhesion, corrosion resistance and lubricity characteristics. The Aerospace industry has been using cadmium platings for a number of years. However, this metal is a carcinogen and was added on the EPA 17 list as a toxic substance that should be reduced or removed from the workplace.
Moreover, with the use of the cyanide bath, cadmium exposure poses environmental, health, and safety concerns.
Aluminum, on the other hand, is environmentally friendly, safe and nontoxic, unlike cadmium. To this end, a three-phase program was conducted by Boeing – St. Louis to validate that IVD aluminum is a better alternative to cadmium. Subsequently, a database handbook was compiled for Air Force Air Logistics Center (ALC) use.
This book includes technical data related to cadmium plating and IVD aluminum coating from elaborate testing conducted at Boeing – St. Louis and other industrial and military test sources. Later, more data was created and process development was focused at "areas of concern" applications, which comprise lubricity, erosion resistance and coverage of internal surfaces.
After the phase trials were completed, an advanced IVD aluminum coater was deployed at the Warner Robins Air Logistics Center (WR-ALC). The coater demonstrated that IVD aluminum was a better substitute to the hazardous cadmium-plating process. Furthermore, IVD aluminum eliminates pollution or waste disposal problems and, therefore, restrictive and costly pollution control requirements.
Based on the successful program at the WR-ALC, the Sacramento Air Logistics Center (SM-ALC) implemented two programs. The two programs were titled, "Ion Vapor Deposition Aluminum Qualification Tests" and "Expanded Ion Vapor Deposition(IVD) Aluminum Program." SM-ALC was assisted by Boeing – St. Louis regarding qualification of IVD aluminum for all alloy steel applications requiring cadmium plating.
The concept behind the program was to check the applicability of the IVD aluminum-coating process to new metal alloys, such as alloys of stainless steel, copper and titanium. During these efforts, IVD aluminum proved to be an ideal replacement to cadmium.
Following this, the IVD aluminum-coating process was approved for usage on titanium-based, copper-based and stainless steel alloys. At SM-ALC, IVD aluminum coating improved worker safety by reducing possible exposure to cadmium products and plating solutions.
Cadmium Versus Aluminum
Aluminum was compared to cadmium in terms of corrosion protection properties. It was found that in acid salt fog tests in accordance to ASTM G85, aluminum coatings are better than cadmium platings.
In outdoor exposure tests, aluminum coatings are equal or better than cadmium platings; and in neutral salt fog tests in accordance to ASTM B117, cadmium platings proved equal to or better than aluminum coatings.
Figure 3. Neutral salt fog resistance.
Additionally, IVD aluminum can be used up to 925°F(496°C), while cadmium is restricted to 450°F (232°C). IVD aluminum can be applied on titanium, but cadmium cannot be used due to solid metal embrittlement problems.
Also, cadmium platings sublimate in a vacuum environment and hence prove unsuitable for space applications. Lastly, while cadmium plating is quite lubricious when compared to an IVD aluminum coating, this issue can be overcome by applying suitable lubricants to the IVD aluminum coating.
The IVD aluminum coating process can easily be carried out and is being used worldwide. It creates a clean, safe and environmentally friendly finishing system.
It is a suitable alternative to cadmium plating processes. IVD aluminum also removes toxic waste stream.
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