30R Dhonau in Metal Finishing No 11 1993 p 35 No 5 p

31. T P. Driscoll, in Metal Finishing, No. 11, 1989, p 19 Alternative Technologies

Much research has focused on finding an alternative to chromium-base compounds for use in conversion coating formulations. In fact, the scientific and patent literature show that attempts have been made to incorporate most elements in the periodic chart (except for radioactive elements and the rare gases) into films. This article gives the rationale for various research paths, along with specific examples where appropriate. Three principal subjects are discussed: deposition of organic-based coatings from aqueous solution, deposition of multivalent metals from aqueous solution, and corrosion-resistant surface layers formed from the gas phase, with or without the use of high-energy light or particle beams (e.g., laser and ion beams).

Organic-Based Coatings. Given that a large number of water-soluble organic corrosion inhibitors are known to exist (Ref 32, 33), conversion coatings based on organic molecules are logical alternatives to chromium. For example, molecules containing the azole functional group (i.e., those having at least two ring nitrogens in a five-membered ring, such as benzotriazole) are known to be effective corrosion inhibitors for copper (Ref 34) and zinc (Ref 35) and are used commercially in cooling water applications to protect copper piping.

The difficulty in making organic-based conversion coatings of sufficient thickness is that organic species are normally poor oxidizing agents, such as chromic acid. This prevents film thickening because of aluminum oxidation and formation of insoluble oxide and hydroxide species. Typical inhibitor-formed films have thicknesses of only 100 A or less, making their use in severely corrosive environments impractical. In addition, the time required to form such films can be hours or more (Ref 35) unless it is possible to accelerate their deposition through use of surface activators such as fluoride ion. Aqueous solubility can also be a limitation for some molecules.

Even though these films may be thin, they have application in areas such as the treatment of architectural aluminum (since this material is not usually continuously exposed to corrosive environments). In addition, organic-based conversion coatings have the potential of being excellent undercoats for organic (paint) finishes, for adhesion can be expected to be strong between similar types of molecules. Both sets of molecules contain various active functional groups that can interact (e.g., through hydrogen bonding or possibly formation of cross-linked or intertwined structures). It is likely that organic-based treatments will find some application in replacing chromium-base systems, but great difficulties exist in attempting to produce treatments that can pass the rigors of 168 and 336 h exposure to salt spray, as required by MIL-C-5541 and MIL-C-81706 on active aluminum alloys such as 7075-T6 and 2024-T3.

Multivalent Metals. The most logical method for obtaining a chromium replacement is to choose another transition, or even a rare earth metal, that has at least two stable oxidation states, is a good oxidizing agent, and has high corrosion resistance. To this end, patents and patent applications exist for using most of these metals in some type of formulation to yield a conversion coating. Not all metals are good oxidizing agents, but the ability of certain metals to oxidize can be enhanced through complexation with the appropriate species.

The fluoride-based formulations used in chromate conversion coatings cannot be used with most metals because of the insolubility of many metal fluorides. Other surface activators need to be chosen, and peroxide, nitrate, and chloride are common substitutes for fluoride (see examples below). The initial choices would be molybdenum-based and tungsten-based reagents, because these elements lie under chromium in the same group of the periodic table and both metals are well known for their corrosion resistance. These metals have been extensively researched, but formulations based on them have not been able to match, let alone surpass, the performance shown by chromate coatings (Ref 36). Molybdates and tungstates have found some application as accelerator replacements for the ferricyanides. Three multivalent metal-based systems that have recently received considerable attention in the open literature and/or have reached the trial stage in various aluminum industries are described below.

Rare Earth Metals. Treatments based on Ce(III) and other rare earth metals were examined first in Australia (Ref 37,

38, 39) and later in the United States (Ref 40, 41, 42). Coatings in excess of 1000 A in thickness and rich in cerium + oxygen species were formed on aluminum alloy 7075 after a 20-day exposure to a 100 ppm CeCl3 solution at pH 5.8 (Ref 43). X-ray photoelectron spectroscopy (XPS) indicated that the film contained both Ce(IV) and Ce(III) species, which likely existed as CeO2, Ce(OH)4, and Ce(OH)3 (Ref 43). X-ray absorption near edge structure (XANES) studies likewise indicated the presence of a mixed cerium valence film (Ref 44). Coating process time was decreased to 10 min by adding hydrogen peroxide, lowering pH, and increasing the solution temperature (Ref 45). Immersion of the film in NaCl solution converted all of the Ce(III) to Ce(IV) (Ref 43). Measured corrosion rates of treated 7075 indicated that a 50% reduction in corrosion rate from that of an untreated substrate can be obtained (Ref 45). No mention of its effect on pitting corrosion was made, but excellent paint adhesion (comparable to that on chromated surfaces) was observed.

A cathodic mechanism has been proposed to account for cerium deposition: pH is increased at the cathodic sites, because of either oxygen reduction or hydrogen evolution, to form excess hydroxyl ions (Ref 37). The hydroxyl ions can attack the metal surface and react with metal ions in solution. The mixed metal oxide/hydroxide then precipitates onto the surface to form the corrosion-resistant film.

The development of "stainless aluminum" has also been claimed for cerium-treated pure aluminum and aluminum alloy 6061-T6 (less satisfactory behavior was obtained for aluminum alloy 2024-T3) (Ref 46). The treatment involves a 2 h exposure to three separate solutions: boiling 10 mM Ce(NO3)3, boiling 5 mM CeCl3, and anodic polarization in the passive region in deaerated 0.1 M Na2MoO4. Excellent corrosion resistance was found upon immersion of treated samples in 0.5 N NaCl. Scratched surfaces also showed excellent resistance. No mention of salt spray testing of the cerium-based treatments was made, however.

Ce(III) molybdate has shown some promise as a corrosion inhibitor in an epoxy/polyamide primer but still does not match the performance of strontium chromate pigmented primers (Ref 47).

Manganese-based treatments for aluminum and aluminum alloys have recently been patented (Ref 48, 49, 50). One of the treatment steps involves exposure of the aluminum alloy surface to permanganate ion, which contains manganese in the +7 oxidation state. Like chromate, the permanganate ion is an excellent oxidizing agent, suggesting that the mechanism of film formation is similar to that of chromate. Although no information on film thickness or composition is given in the patents, one would expect that the manganese found in the film is in some reduced oxidation state (probably either +4 or, more likely, +2). This is a multistep treatment in which many of the steps require elevated temperatures. The last step, which involves a "seal" with alkali metal silicate, is probably necessary to block the pores created in the film during deposition. Good corrosion resistance, as evidenced from salt spray exposure, has been observed for high-copper-containing aluminum alloys.

Trivalent Cobalt. The final system is based on the use of basic solutions containing complexes of trivalent cobalt, for example, co( nh4)3+ (Ref 51). CoCl2 has shown some promise as an inhibitor for aluminum alloy corrosion (Ref 52). It is likely that Co(II) compounds have been examined in the presence of fluoride, for CoF2 does possess appreciable solubility in water. This new system deposits a corrosion-resistant cobalt-containing film on aluminum alloys. Preliminary examination of this coating with electrochemical impedance spectroscopy (EIS) indicates that the coating has corrosion-resistant properties similar to those of a chromate treatment on aluminum alloy 2024-T3 (Ref 53). Good corrosion and paint adhesion properties are also claimed (Ref 51).

Gas Phase. Although gas-phase/high-vacuum techniques will probably not replace chromating of large parts or continuous chromating of high volumes (e.g., aluminum alloy coil stock for beverage cans) in the near future, these processes should not be ignored, for they offer the possibility of forming entirely new corrosion-resistant films. Experiments have already been undertaken in Japan to produce large surface areas of galvanized steel sheet through gas phase deposition of zinc (Ref 54). A partial selection of these techniques as they apply to the formation of corrosion-resistant films on aluminum and magnesium is briefly reviewed below.

Sputter deposition and ion implantation are nonequilibrium alloying techniques that allow solid solution alloys to be formed, which are unattainable by conventional alloying techniques. Corrosion-resistant aluminum-molybdenum and aluminum-chromium alloys have been formed by cosputter deposition of aluminum with each of the two metals (for details see Ref 55 and 56). Ion implantation of silicon, chromium, zirconium, niobium, and molybdenum into pure aluminum have been found to enhance pitting resistance of the aluminum, whereas implantation of magnesium and zinc had no effect on pitting resistance or lowered pitting resistance, respectively (Ref 57, 58). Beneficial results have been observed for iron and boron implantation into magnesium and magnesium alloys (Ref 59, 60). Ion implantation can easily form layers up to several thousand angstroms thick, depending on the mass of the species being implanted and the ion beam energy. Even thicker layers can be produced by ion beam mixing, which combines ion implantation with a vapor deposition process such as evaporation or sputtering.

Corrosion-resistant oxide mixtures can be formed on aluminum and magnesium by laser irradiation of a surface that contains a thin vapor-deposited metallic film such as chromium or molybdenum (Ref 61, 62, 63, 64). Laser irradiation is performed in an air or oxygen atmosphere to enhance the formation of oxides. The rapid heating and cooling rates provided by a laser can produce an amorphous, metastable surface composition that enhances the corrosion resistance of the substrate. Laser-irradiated chromium and molybdenum films on pure aluminum have been found to be very resistant to pitting by chloride ion (Ref 64).

Metal organic chemical vapor deposition (MOCVD) can also be used to deposit corrosion-resistant oxides onto low-melting metal substrates. MOCVD involves the deposition of thin oxide films by thermal decomposition of organometallic compounds. Metal alkoxides are the most commonly used precursors and have been used to deposit metal oxides such as SiO2, TiO2, Cr2O3, and Al2O3 (Ref 65, 66, 67, 68). The metal alkoxides generally have high vapor pressure and readily decompose at low temperatures (as low as 150 °C, or 300 °F). For example, Al2O3 films have been obtained by decomposition of the aluminum alkoxides: tri-tert-butoxide, tri-sec-butoxide, tri-n-butoxide, tri-isopropoxide, and acetyl-acetonate (Ref 69). Interestingly, the introduction of water vapor to aluminum acetyl-acetonate produces an extremely ligand-free, pure Al2O3 with a smooth surface morphology (Ref 70).

Additional information on alternatives to chromium is available in the article "Chromium Elimination" in this Volume.

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