Producing Good Anodic Coatings
Steps to Better Anodic Coatings
A quality and efficient anodizing line starts with quality metals, pre-cleaning, racking, and ends with unracking, clean-up, and inspection.
Pretreatment. Removing extrusion die lines, casting parting lines, adhesives on sheet panels and excessive machining oil is necessary prior to racking of aluminum parts.
Racking. Racking of parts is extremely important in anodizing process and is the first step necessary to producing good anodic coatings. The cleaned parts must make good electrical contact with the rack to carry current to the part during the process. Iridescent appearance, powdery coatings, poor dyed-color match and burning may occur because of insufficient rack-parts contact area. On the other hand, continuing build-up of oxide will break the current flow to the part. The thicker the coating, the more resistance is exerted. Inappropriate racking may also cause parts to loosen when transferring parts from tank to tank. A good rack design should hold parts securely, conduct electricity, good drainage to avoid air pockets and carry as many parts as possible.
Smaller parts with holes in them can be racked using split finger racks. Parts with a small outside diameter can be racked using wire racks. Larger and longer parts can be racked using either box racks or disc racks depends on volume. Pin racks are more flexible and can be bent to fit different parts in different shapes. Large, heavy sections, on the other hand, will require strong work bars of sufficient current-carrying capacity, and possibly bolted contacts.
Processing. Cleaning to remove organic compound is the first required tank process operation. Thorough rinsing after each chemical step is required. Conductivity meters can be used to control water purity and conservation. Tanks should be free of galvanic currents (insulating pads Aden dielectric pipe connections) and be equipped with insulated work-bar pads on the tank ends.
Deoxidizing is usually the step after cleaning and rinsing. The purpose is to remove non-uniform oxide films present on the surface of tech aluminum products. Deoxidizers are often the mixtures of chromic, sulfuric, nitric and phosphoric acids.
Etching is a treatment step to remove natural shine and provide a soft and matte appearance. Etching is carried out for periods of 3-5 minutes at 90-120F in typically 5pct sodium hydroxide solutions. Excess temperature may lead to caustic burning.
Etched parts are sometimes desmutted in acid deoxidizer. It gets rid of the aluminum alloy metallic constituents that are not dissolved by etching and stick to the surface.
Normally, anodizing is carried out in a nominal 15 pct sulfuric acid solution at 70-80F, depending on whether the coating is to be clear or dyed. Temperature should be held within a few degrees to maintain consistency. Agitation is necessary to prevent localized overheating and provide uniform temperature throughout the tank. Cathode location is also important, especially with large sections.
Dyeing operation, when applicable, is carried out next in tanks of various water-soluble, organic-dye solutions. Usually dye concentrate, pH and temperature are recommended by the dyestuff manufacturer.
The very important final step is sealing, which prevents the oxide pores from staining and becoming less corrosion resist. Sealing times can be 3-5 minutes for nickel acetate and up to 20-30 minutes for water. Double seals may also be employed.
Final inspection is a step to check the appearance, coating thickness, and performance of seal tests. Coating weight and salt-spray resistance tests are sometimes required.