The selection of anodizing equipment should be made with proper consideration of the chemical processes to be used in the installation.
When proprietary solutions are utilized, be sure to get recommendations on materials of construction from the supplier. Unless otherwise indicated, recommendations listed below are for Type II or Type III anodizing of aluminum with sulfuric acid based solutions.
Alkaline processes, such as cleaners and caustic etching solutions, can be held in mild steel tanks. Stainless steel is preferred for nitric acid desmutting baths and some proprietary deoxidizers. Fluoride based materials require plastic tanks.
Many materials have been satisfactorily used for fabrication of anodizing tanks. These range from lead or rubber lined steel and wooden tanks to fiberglass lined concrete. Metal tanks can also serve as cathodes, but care should be taken to prevent shorts and to maintain a suitable anode-to-cathode ratio. Inert materials for the construction of the tank or lining are preferred.
Polypropylene, PVC and fiberglass are all acceptable. These can be fabricated with an encapsulated structural framework for strength. Stainless steel can also be utilized. Concrete is only used for large tanks, up to 30 feet in length, with the advantage of common walls between process steps.
The tank size is determined by utilizing the dimensions of the largest part to be accommodated along with allowances for the racks, cathodes, heating/cooling coils, etc. Temperature control is very important in anodizing, so insulation of tanks is strongly recommended.
Direct current power supplies or rectifiers are essential to the anodizing system. Close control of the current output is needed for quality anodizing. For some alloys, ramping techniques and AC or pulse power may also be required. Computer-controlled rectifiers are available to incorporate current ramping, step changes or even complex waveforms.
The maximum current output for the power supply can be determined by multiplying the surface square footage of the largest load by the current density in amperes per square foot (ASF) for the process. This plus the required process voltage (V) will enable the sizing of the rectifier. The following are suggested current density and voltage ratings for several anodizing processes:
Type I (Chromic Acid) 5 ASF and 40 V
Type II (Sulfuric Acid) 12 ASF and 24 V
Type III (Hardcoat) 24 to 36 ASF and 100 V
Type IV (Phosphoric Acid) 10 ASF and 24 V
Heating and Cooling Systems
The sulfuric acid anodizing tank generates heat and requires cooling. However, several other process steps will need a heating system. Cleaners, etches, dyes and seals may require
temperatures above ambient. In addition, chromic acid anodizing tanks need to be heated at start up. In cold climates, tanks that are normally chilled may also require heating at start up. The heat for each process can be provided by electric immersion heaters, gas or oil fired burners mounted in the tanks, or by steam or hot water in coils or plate-type heat exchangers. Immersion
electric heaters are the simplest and are usually preferred for smaller tanks. The direct fired burners are cheaper to maintain, but take up more tank space. Coils may serve double duty by heating with team or hot water and cooling with cold water as required.
Cooling is essential for sulfuric acid anodizing and may also be needed for phosphoric acid anodizing. Smaller anodize tanks can be kept cold by pumping chilled water or a
glycol-water solution (for Type III anodizing) through coils in the tank. Larger systems are better controlled by an external heat exchanger with the anodizing bath pumped through one side and
the coolant solution through the other. The chiller for the cooling water or other coolant is the heart of the system and also the most expensive component.
Vigorous, uniform agitation is needed to disperse the heat generated in anodizing. Air
sparging is in common use. Porous air tiles or plastic (polypropylene or PVC) tubing can be
utilized. A typical set up would have a 1” diameter tube with 1/8” holes every 3 to 6 inches along its length. Agitation can also be provided by solution pumping, such as when solution is pumped out to a heat exchanger and returned to the tank on a continuous basis. In this case, eductors should be used to take advantage of the pumping action. The air supplied for agitation should be clean and dry. A blower is preferred for this purpose. Compressors should not be used to supply
Racks, called jigs in some areas, have two primary functions: to carry the workload
through the anodizing process from stage to stage and to provide a secure electrical connection leading to the power supply during the anodizing step. The most common materials for rack construction are aluminum and titanium. Aluminum anodizing racks should be of the same or similar alloy materials as the parts being anodized, but 6061 and 6063 are acceptable choices for many uses. In high volume applications, titanium should always be considered.