Powder Coating Surface preparation – CLEANING ALUMINUM

Alkaline Cleaners

Alkaline cleaners for aluminum differ from  those used for steel; they usually have a blend of  mild alkaline salts to avoid attacking the aluminum surface. In some cases, a small to moderate amount of free caustic soda may be present in the cleaner to remove difficult soils, or to provide a desired etch.

In the power spray method of application, the parts to be cleaned are suspended in a tunnel while the cleaning solution is pumped from a holding tank and sprayed, under pressure, onto the parts. The cleaning solution is continuously recirculated. Spray pressure ranges from 4 to 40 psi.

In the immersion method of application, the parts to be cleaned are simply immersed in a solution of the cleaner contained in a mild steel or stainless steel tank. Electrocleaning is a specialized version of immersion cleaning in which a direct current is passed through the solution. The parts to be cleaned are usually the anode, while other electrodes hanging in the tank act as the cathode. Electrocleaning is more effective in cleaning than plain immersion due to the scrubbing action of the oxygen bubbles coming off at the surface of the part. Oxygen results from the electrolysis of the water.

The hand-wiping method of application derives additional benefit from the physical act of removing the soil from the surface by means of a cloth or sponge, with the cleaner helping to solubilize the soils.

Alkaline cleaners are usually applied to aluminum by using a minimum of two stages: the cleaning stage and a water rinse. Additional stages, a cleaning and rinsing, may be used if required. The chemical baths are held at a temperature of from 80 to 200°F (27 to 93″C), typically 100 to 140°F (38 to 60°C) for spray and 140 to 180°F (60 to 82°C) for immersion. Parts are exposed to the chemicals for 30 seconds to 5+ minutes; typically 1 to 2 minutes for spray, and 2 to 5 minutes for immersion. A bath concentration of 1/4 to 16 odgal(2 to 120 g/L) is used; typically 1/2 to 1 odgal(4 to 8 g/L) for spray and 6 to 12 odgal(45 to 90 g/L) for immersion.

Comparing the cost of using the various types of chemical cleaners, the most expensive would be the immersion electrocleaner due to the higher concentrations used and the cost of electricity for the electrocleaner.

The least expensive would be the spray cleaner, with hand-wiping being somewhere between. The alkaline type is, by far, the most effective of the cleaner types and usually the least expensive to operate. In order of decreasing performance, the methods of application would generally be rated as: electro cleaning, spray cleaning, immersion  cleaning, and hand-wiping.

Acid Cleaners

Acid cleaners for aluminum are composed of mildly acidic salts or a phosphoric acid base. In either case, any oxide film on the aluminum will be removed by the acidic medium. The acid cleaners are usually not as effective in cleaning common soils as the alkaline cleaners.

In the power spray method of application, the parts to be cleaned are suspended in a tunnel while the cleaning solution is pumped from a holding tank and sprayed, under pressure, onto the parts. The cleaning solution is continuously recirculated.

When the immersion method of application is used, the parts to be cleaned are simply immersed in a solution of the cleaner contained in a mild steel or stainless steel tank. Hand-wiping derives additional benefit from the physical aid of removing the soil from the surface by means of a cloth or sponge, with the cleaner helping to solubilize the soils.

Acid cleaners are usually applied to aluminum using a minimum of two stages, the cleaning stage and a water rinse. Additional stages, a cleaning and rinsing, may be used if required. The acid solutions are held at a temperature of 80 to 200°F (27 to 93°C); typically 100 to 140°F (38 to 60°C) for spray and 140 to 180°F (60 to 82°C) for immersion. Parts are exposed for 30 seconds to 5+ minutes; typically 1 to 2 minutes for spray and 2 to 5 minutes for immersion. Solutions are held at a concentration of 1/4 to 16 odgal(2 to 120 g/ L) for spray and 6 to 12 odgal(45 to 90 g/L) for immersion.

Comparing the cost of using various cleaners, most expensive would be immersion due to the higher concentrations used. The least expensive would be the spray cleaners, with hand-wiping somewhere between. In order of decreasing performance, the methods of application would generally be rated as: spray cleaning, immersion cleaning, hand wiping.

Neutral Cleaners

A neutral cleaner for aluminum may be composed of surfactants only, neutral salts plus surfactants, or surfactants with other organic additives. A solution of a neutral cleaner will usually register between 6 and 8 on a pH scale.

In the power spray application, parts to be cleaned are suspended in a tunnel while the cleaning solution is pumped from a holding tank and sprayed under pressure onto the parts. The cleaning solution is continuously recirculated.

Spray pressure ranges from 4 to 40 psi. The hand-wiping method of application derives additional benefit from the physical act of removing the soil from the surface by means of a cloth or sponge with the cleaner helping to solubilize the soils.

Neutral cleaners usually are applied to aluminum using a minimum of two stages: the cleaning stage and a water rinse. Additional stages, a cleaning and rinsing, may be used if required. Neutral cleaners are held in the temperature range of 80 to 200°F (27 to 93°C); typically 120 to 160°F (49 to 71°C) for spray and 150 to 180°F (66 to 82°C) for immersion. Parts are exposed to the cleaners for 30 seconds to 5+ minutes; typically 1 to 2 minutes for spray and 2 to 5 minutes for immersion. The chemical concentration is between 1/4 to 16 odgal(2 to 120 g/L) typically l to 2 odgal(8 to 15 g/L) for spray and 8 to 14 od gal (60 to 105 g/L) for immersion.

Neutral cleaners are not effective as the primary cleaner. They are more likely used as a precleaner.