Anodic coatings on aluminum

Dichromates and chromic acid are used as sealers or after-dips to improve the corrosion resistance of various coatings on metals. Eor example, phosphate coatings on galvani2ed iron or steel as well as sulfuric acid anodic coatings on aluminum can be sealed by hexavalent chromium baths. 

The method described by ASTM B 457, Test Method for Measurement of Impedance of Anodic Coatings on Aluminum, is used to study the seal performance of anodized aluminum 141,1421. This method uses a 1-V rms 1-kHz signal source from an impedance bridge to determine the sealed anodized aluminum impedance. The test area is again defined with a portable cell, and a platinum or stainless steel auxiliary electrode is typically used. The sample is immersed in 3.5 % NaCl (see ASTM B 457). 

ASTM Standard B 457, Test Method for Measurement of Impedance of Anodic Coatings on Aluminum. 

An accelerated corrosion test for some electrodeposits for anodic coatings on aluminum. Corrodkote test... 

Probably the most extensive coatings employed in electrochemical systems are passive films that are formed on metal and semiconductor surfaces (21,22). These anodic films are responsible for the corrosion resistance of reactive metals, such as Fe, Cr, Ni, Ti, Zr, Zn, Cu, Sn, and Al, among others, in aqueous environments as well as for the operation of various electrochemical devices (e.g., electrolytic capacitors). Decorative coatings on aluminum, titanium, and zirconium are also formed anodically, with those for aluminum being very highly developed. The principal limitation in the knowledge of the growth and... 

CAS 89-08-7 EINECS/ELINCS 201-881-6 Uses Electrolytic bath additive for production of hard anodic oxide coatings on aluminum and aluminum-based alloys colored coalings are abrasion resist, and attractive for architectural and other uses Features Colors depend on base metal composition, current dens., voltage, film thickness and electrolyte composition, particularly sulfate and aluminum contents Properties 49-51% act. 

Toxicology Corrosive causes burns Uses Prod, of anodic oxide coatings on aluminum and alloys Manuf./Distrib. Aldrich http //www.sigma-aldrich.com, Fallek... 

Oxide coatings on aluminum are produced at room temperature by anodic oxidation of aluminum (called anodizing) in a suitable electrolyte (e.g., dilute sulfuric acid) at current densities of 100 or more A/ml The resultant coating of AI2O3 may be 0.0025-0.025 mm (0.1-1 mil) thick. The oxide so formed must be hydrated to improve its protective qualities by exposing anodized articles to steam or hot water for several minutes, a process called sealing. Improved... 

This form of corrosion is more prevalent under organic coatings on aluminum than on other metallic surfaces, being a special form of anodic undermining. 

Oxide coatings on aluminum are formed by anodic oxidation of aluminum in a suitable electrolyte. The process is known as anodizing and the finished product is known as anodized aluminum. 

W.C. Cochran and D.O. Sprowls, Anodic Coatings for Aluminum, Paper presented at Conference on Corrosion Control by Coatings, Lehigh University, Nov 1978... 

When using metallic coating over the whole bimetallic assembly, the coating metal should be less noble than either of the component metals - or at least the cathodic one. Anodic films on aluminum-based alloys should be considered a part of the dielectric separation. 

In special cases where the surface hardness must be increased or chemical corrosion resistance is necessary (e.g. plasma etching with chlorine), anodized aluminum surfaces can be useful. Alloying elements, impurities, and heat treatment can influence the nature and quality of the anodized coating - typically, the more pure the aluminum alloy, the better the anodized layer. To build up a thick anodized layer on aluminum, it is necessary for the electrolyte to continuously corrode the oxide, producing a porous oxide layer. ASTM Specification B-580-73 designates seven thicknesses (up to 50 microns) for anodization. 

Anodized coatings on SiC/aluminum composites provide satisflictoiy corrosion protection, but they are not as effective as anodized coatings are for unieinfor-ced aluminum because the structure of the anodized layer is affected by the SiC particulates (the particulates interfere with the formation of a continuous barrier layer). TaUe 3 shows a comparison of pitting times and crevice times for sulfriric acid-an ized samples with a hot water seal with those of hard anodized SiCyahiminum. The corrosion resistance of the hard aiaxlized SiC/aluminum is less than that of the conventional anodized SiC/aluminum because the area fraction of the continuous barrier layer for hard anodized SiC/ahiminum is less than that for conventionally anodized SiC/alimiinum. 

Aluminum coatings on steel will perform in a manner similar to zinc coatings. Aluminum has good resistance to many atmospheres in addition, being anodic to steel, it will galvanically protect exposed areas. Aluminum-coated steel products are quite serviceable under high-temperature conditions, for which gooa oxidation resistance is required. 

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