Metal Finishing Guide Book


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The basic electrolyte is composed of a certain concentration of sulfuric acid. The amount is determined by two important considerations: 1.) The ability of the solution to conduct electricity efficiently; and 2.) The ability of the electrolyte to dissolve the aluminum oxide coating. The balance that must be determined and maintained for consistency is that between formation and dissolution of the anodic coating. To do that, we must carefully control the parameters that affect that balance. Chemical Parameters. The following diagram demonstrates how the different parameters affect the balance between formation and dissolution. By exercising strict control over these forces, we gain control of the type of coating we wish to achieve. We are then able to generate a coating from the simplest thin decorative film to the heaviest hard coat film. Table 2. Illustration of how the different parameters affect the balance between formation and dissolution. Anodize Formation <---- Favors --- Chemical Parameter --- Favors ----> Anodize Dissolution Lowering <------------------------- Temperature -------------------------------> Raising Lowering <--------------------- Acid Concentration ----------------------------> Raising Raising <-------------------------- Acid Agitation -----------------------------> Lowering Lowering <------------------ Aluminum Concentration ------------------------> Raising Raising <----------------------- Additives/Modiļ¬ers --------------------------> Lowering Heat absorption in pore structure (Promotes anodize formation rate at higher temperatures) Lowering <------------------------ Contaminants -------------------------------> Raising Electrical parameters. The formation of the coating is dependent upon the flow of electrons, which is represented by current (NOT VOLTAGE). The chemical factors influence the flow of current and the chemical dissolution simultaneously as noted above. The electrical parameters primarily determine the formation of the anodic coating, but if the coating is not forming efficiently it is dissolving as a consequence of the chemical parameters. Table 3. Illustration of how electrical parameters determine formation of the anodic coating. Anodize Formation <---- Favors --- Electrical Parameter --- Favors ----> Anodize Dissolution Increasing <-------------------------- Contact ------------------------------> Decreasing Increasing <---------------------- Current Density -------------------------> Decreasing Increasing <------------------- Current Distribution -----------------------> Decreasing Increasing <---------------------Pulse Step Ramp --------------------------> Decreasing (Promotes formation at low voltage) Increasing <-------------------------- Voltage ------------------------------> Decreasing Increasing <----------------------------Time --------------------------------> Decreasing Increasing <-------------------- Metal Conductivity ------------------------> Decreasing There will be other universal electrolytes that will be developed as they become necessary to meet present and future specifications. However, the problemsolving procedural requirements presented herein should become a part of all anodize processing. 230

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