Chemical Conversion Coating (Alodine)
Chemical Conversion Coatings on aluminum increases corrosion resistance, and improves paint adhesion.
There are two classes of coatings:
Class 1A is a thick film coating typically found on fuel tanks, tubing and component structures where paint finish is not required in the interior but is required on the exterior. This is class is commonly recognized as Alodine 1200.
Class 3 is a thin film coating typically used in electrical bond applications, commonly recognized as Alodine 600.
The only difference between the two classes is the coating thickness, which is determined by immersion time, therefore the same chemicals are used.
Chemical Conversion Coating can be applied by two methods: immersion or brush application.
For immersion processing, aluminum parts are wired or placed into pinch racks and then lifted by hoist and lowered into the tanks. Parts are alkaline cleaned, etched and then deoxidized; this ensures that the parts are clean, free of foreign contaminants and ready to for coating. Parts are then immersed into the tank and allowed to dwell the requisite time for the specified coating thickness.
For brush application aluminum parts are cleaned and deoxidized then rinsed with cold water. Brush conversion coat solution is applied to the wet part with an acid resistant brush or swab. The surface is kept wet with the solution for the requisite dwell time for the specified coating thickness.
Processed parts are inspected using a dualscope to measure coating thickness. Inspectors also visually check for corrosion, pitting or pockets in the material that could be cause for rejection. If the parts are deemed unacceptable by the inspector, parts are either written up for internal rework or returned to the customer for evaluation.
After processing and inspection, parts are moved to the next operation, typically paint, mask for electrical bond, or final inspection.