Sulfuric acid anodized

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. 

BAC5632, Boric acid-sulfuric acid anodizing. Boeing Aircraft Company, 1990. 

Substrate Characterization. Test coupons and panels of 7075-T6 aluminum, an alloy used extensively for aircraft structures, were degreased In a commercial alkaline cleaning solution and rinsed In distilled, deionized water. The samples were then subjected to either a standard Forest Products Laboratories (FPL) treatment ( 0 or to a sulfuric acid anodization (SAA) process (10% H2SO4, v/v 15V 20 min), two methods used for surface preparation of aircraft structural components. The metal surfaces were examined by scanning transmission electron microscopy (STEM) In the SEM mode and by X-ray photoelectron spectroscopy (XPS). 

Figure 2. Structure of a sulfuric acid-anodized (SAA) aluminum surface.

Sulfuric acid anodizing can be performed in accordance with MIL-A-8625. 

Figure 30 Bode magnitude and phase angle plots of a 5 im thick sulfuric acid anodized film subject to sealing in hot water. Measurements were made on an interrupted basis in cold water. Immersion times are (1) 0 s, (2) 30 s, (3) 60 s, (4) 90 s, (5) 120 s, (6) 180 s, (7) 300 s, (From J. L. Dawson, G. E. Thompson, M. B. H. Ahmadun. p. 255. ASTM STP 1188. ASTM, Philadelphia, PA (1993).)...

Figure 34 Bode plots of EIS data determined in aerated 0.5 M NaCl solution for sealed and unsealed sulfuric acid anodized A1 alloy 2024. (From F. Mansfeld, M. W. Kendig. Corrosion 41, 490 (1985).)...

Aluminum spraying is used to coat less corrosion-resistant alloys. In the case of some composites, corrosion is due to the galvanic action between the aluminum matrix and the reinforcing material. Aluminum thermal spraying has been successfully used for the protection of the discontinous silicon carbide/aluminum composites, and continuous graphite/aluminum. Other protection procedures include sulfuric acid anodizing and iron vapor deposition on aluminum.44... 

In fluorosulfonic acid containing sulfuric acid anodic dissolution of platinum is reported to take place in anhydrous FSO3H an oxide layer is formed, which protects the platinum electrode [298]. 

Carbon steels and stainless steels are the most common construction materials for equipment used in the manufacture, storage, and transportation of sulfuric acid. Anodic protection was successfully used to control the corrosion of the various equipments made of these materials in this acid over a wide range of temperatures. While the corrosion rates of carbon steels in sulfuric acid are mainly functions of temperature, velocity, and acid concentration and purity, particularly the iron content, these rates are greatly affected by minor alloying elements in steel, particularly copper. The corrosion rates of steel in 77 to 100% concentrations are in the range of 20 to 40-mils per year (mpy) at 24 °C [14]. 

Bonding of aluminum comprises greater than 90% of the end uses for epoxy film adhesives. At least three major cleaning methods have been used chromic acid etch, phosphoric acid anodize and sulfuric acid anodize. Each process produces a different microstructure and chemical surface to which to bond. A corrosion-resistant epoxy primer is often used to protect the delicately etched surface during assembly operations. The adhesive must then bond to the primer and provide the myriad of intricate test strengths required by each aircraft producer, each aircraft, and each design. The tedious nature of optimizing each adhesive to meet each detailed specification becomes apparent and will not be further pursued in this discussion. 

Corrosion of metal by sulfuric acid Anodic reaction (M = metal) ... 

FIGURE 3. Sulfuric-acid-anodized surface of aluminum. Reprinted with permission from Reference 10. Copyright 1982 Butterworth Science Ltd. 

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)... 

Many acidic solutions can be used for anodizing aluminum, but sulfuric acid solutions are surely the most common. Chromic, oxalic, and phosphoric acids are also used relatively often for specific applications [28]. The "standard" sulfuric acid anodizing bath (Type 11) produces the best oxides for coloring. The solution consists of approximately 15 percent sulfuric acid and the anodizing bath is maintained at 20°C. As the anodizing temperature is increased, the oxide becomes more porous and improves in its ability to absorb color however, it also loses its hardness and its luster, due to the dissolution action of the acid on the oxide surface. 

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