Resistance of Anodized Aluminum

In general, corrosion resistance of anodic coatings is greatest in approximately neutral solutions, but such coatings are usually serviceable and protective if the pH is between 4 and 8.5. More acidic and more alkaline solutions attack anodic coatings. 

Weathering of anodic coatings involves relatively uniform erosion of the 

H2SO4 anodic coatings 23 xm (0.9 mil) thick, sealed in boiling water on test panels 100 X 150 mm (4X6 in.) cut from sheet and extrusions  

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Initial attempts to use aluminum for automotive trim were unsuccessful due to the corrosion behavior of the metal. It is therefore anodized for automotive trim applications to provide a protective oxide surface which acts as a barrier coating for corrosion pro tec tion. > 2 Aluminum and its alloys are susceptible to pitting and crevice corrosion in chloride containing environments. The corrosion resistance of anodized aluminum is therefore highly dependent on the quality of the anodized surface and the absence of scratches and other damage sites. 

The corrosion resistance of anodized aluminum is more or less comparable with the untreated aluminum in an acidic and alkaline environment. The formation of pits is decreased under atmospheric corrosion conditions ( pitting corrosion). 

Lei J., Li L., Li. D. and Zhang S. (2006), Synergism between cerium salt and EDTA in improving the corrosion resistance of anodized aluminum. 210th ECS Meeting Abstracts, Cancun, Mexico. 

FIGURE 6.14 Variation of the reciprocal capacitance and resistance of anodic oxide film on pure aluminum rod specimen with film formation potential in contact with 0.5 M H2SO4 solution. (From Moon and Pyun, 1998. J. Solid State Electrochem. 2, 156-161, with permission.)... 

In these equations, A, , represents the exposed area of the specimen, p,. is the Him resistivity, e is the vacuum permittivity, and Eq/ is the relative permittivity of the metal oxide. Consistently, with such equations, the reciprocal capacitance of anodic aluminum oxide films increases on increasing the formation potential in both alkaline (Figure 6.15) and acidic media (Figure 6.14). This behavior reflects the increase in the film thickness on increasing the formation potential. 

ASTM B 117 is the standard test used for evaluating the corrosion resistance of unpainted aluminum alloy surfaces when treated by anodizing, conversion coating, and for plating processes. Aluminum surfaces, which are anodized and then sealed for corrosion protection, must resist the salt spray environment for at least 336 h without pitting. Conversion... 

More recently the author has published the results of eight-year testing in the seacoast of sulfuric-acid-anodized surface aluminum joints and demonstrated the excellent resistance of anodized surface joints to seacoast and marine conditions.The same excellent performance would also be achieved with aluminum adherends that have been anodized in chromic or phosphoric acids.(23,24,91)... 

Fig. 24. Acidic corrosion resistance of two test coupons of anodized aluminum. On the left, anodized aluminum does not show any acidic corrosion in two hours and on the right, anodized aluminum shows severe acidic corrosion after only 10 minutes immersion in the acid.

The HCl bubble test can be processed at any position of etching chamber before and after etching process. In Fig. 25, one process chamber is studied on its corrosion resistance in 5.0wt% HCl solution [76]. This method has received a wide application for the corrosion resistance study of anodized aluminum. 

A systematic study of anodized aluminum made of A16061-T6 11" thick block was carried out. It is obvious that HQ bubble test can be studied at different thickness positions to compare the differences of corrosion resistance [77]. The detail positions of different tests are shown in Fig. 26. Eleven different test methods are applied to the study. 

Where Z is the total impedance, Rb is the barrier layer resistance of anodization, Rp is the porous layer resistance of porous layer of anodized aluminum, CPE is the constant phase element of the porous layer, Rc is the coating resistance, and Rs is the solution resistance. 

Huang, "Automatic Hydrogen Bubble Detection and Recording System for the Study of Acidic Corrosion Resistance for Anodized Aluminum and Surface Coatings", Lam Research Patent Invention Disclosure P2232, Filed for US Patent on January 4, 2011. 

Piebaked anodes aie produced by molding petroleum coke and coal tar pitch binder into blocks typically 70 cm x 125 cm x 50 cm, and baking to 1000—1200°C. Petroleum coke is used because of its low impurity (ash) content. The more noble impurities, such as iron and siUcon, deposit in the aluminum whereas less noble ones such as calcium and magnesium, accumulate as fluorides in the bath. Coal-based coke could be used, but extensive and expensive prepurification would be required. Steel stubs seated in the anode using cast iron support the anodes (via anode rods) in the electrolyte and conduct electric current into the anodes (Fig. 3). Electrical resistivity of prebaked anodes ranges from 5-6 Hm anode current density ranges from 0.65 to 1.3 A/crn. ... 

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. 

CAA. Chromic acid anodization [74-76]. was developed initially as a treatment to improve the corrosion resistance of aluminum surfaces, but it is also used as a surface treatment for adhesive bonding especially in Europe where it is used extensively in aerospace applieations [29,77],... 

The use of anodic films on aluminum alloys is only applicable to some mildly corrosive environments, including architectural purposes and where abrasion resistance is required. 

Aluminum has a low density it is a strong metal and an excellent electrical conductor. Although it is strongly reducing and therefore easily oxidized, aluminum is resistant to corrosion because its surface is passivated in air by a stable oxide film. The thickness of the oxide layer can be increased by making aluminum the anode of an electrolytic cell the result is called anodized aluminum. Dyes may be added to the dilute sulfuric acid electrolyte used in the anodizing process to produce surface layers with different colors. 

The anodic oxidation of sheet aluminum has been used for a long time to protect aluminum against corrosion by a well-adhering oxide layer. Porous oxide layers are formed if acid electrolytes are used that can redissolve the aluminum oxide (mostly sulfuric or phosphoric acid). A compact oxide layer is formed at the beginning of the electrolysis (Fig. 20.3). Simultaneously, the current decreases, due to the electric resistance of the oxide. Subsequently follows a process in which the oxide is redissolved by the acid, and the current increases until it reaches a steady state. The electrochemical oxidation continues to take place with formation of pores. At the end of a pore, where it has the largest curvature, the electric field has its largest gradient and the process of redisolution is fastest. 

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