|Content Type||Conference Paper|
|Title||Corrosion Testing and Ranking of Anodized Aluminum Produced in Oxalic Acid|
|Authors||Leon Manian, Fluor Daniel Inc.|
|Source||CORROSION 2001, March 11 - 16, 2001 , Houston, Tx|
|Copyright||2001. NACE International|
A corrosion testing program was undertaken to evaluate the corrosion resistance of anodized/oxygen stabilized films on aluminum substrate. In oxalic acid the oxide layer formation is the self-color type with superior corrosion resistance properties. Two test procedures are discussed that were used to complete the evaluation. These included the Kape test, and the potential decay test in natural immersion. Electron microscopy was used to identify the attack pattern. In accordance with the test results a corrosion resistance ranking designation was assigned to various surfaces based on electrical and physical process parameters. Finally, a cross-reference was established to correlate the rankings to various atmospheric environments.
Aluminum and certain other metals exhibit accelerated oxide growth under anodic conditions. When this oxide film is allowed to form under controlled parameters a tough, corrosion resistant barrier layer is formed that is capable of protecting the aluminum substrate from further attack under various atmospheric conditions. Both anodic protection and anodizing are two effective methods of protecting certain metals by promoting an electrochemical reaction between the metal and an electrolyte. In anodic protection once the initial oxide layer is formed the protection current drops to an extremely low value but is always available for repairing any oxide film discontinuities. However, in anodizing under diffusion controlled conditions an amorphous oxide layer is formed with a limiting thickness. The oxide film provides the metal with much improved corrosion resistance. Coupled with good mechanical properties, anodized aluminum has become a desirable material in various industries including electronics, architecture, automotive, and as an insulation for aluminum wire, etc.
CHEMICAL AND PHYSICAL NATURE
Based on the aggressiveness of the electrolyte two types of films can be formed, 1) a compact film when the electrolyte is non-aggressive and 2) a porous film when the electrolyte is an aggressive one such as oxalic acid. The porous type film consists of two distinct parts, first an initial thin barrier layer and a much thicker porous layer, which forms on top of the barrier layer. Porous anodic films are usually sealed in order to preserve their appearance and maintain corrosion resistance. The process of sealing involves thermal conversion of amorphous film material which results in the blocking of pores by a swelling process and plugging of the pore mouths. Commonly used sealants are water and heavy metal salts such as cobalt and nickel acetates. The thickness of the porous anodized film is much greater than that of naturally formed coatings, which renders the metal effectively "noble". The naturally formed film is predominantly crystalline whereas the anodic films are mainly amorphous at room temperature.
PARAMETERS FOR INDUSTRIAL PROCESSES
Industrial processes employ direct current normally in the range 12 to 15 A/~ at 10 to 20 V with the electrolyte bath at 15 to 25 °C for 20 to 40 min. The oxide films are then sealed in either hot distilled water or in dichromate solutions. In oxalic acid electrolytes I the anodized oxide is a self-colored film ranging from bronze to gold to silver depending on the aluminum alloy. It requires only a sealing process to lock the pore entrance. This is in contrast to oxide films produced in sulfuric acids, which are transparent. These oxide films may be dyed in a suitable solution prior to being sealed or they may be sealed as anodized without the dying process.
SPECIMEN PREPARATION AND PROCESS PARAMETERS
The anode specimens were made of 2xl.5 cm pieces of super puri
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