Filiform corrosion of aluminum

Slabaugh WH, Dejager W, Hoover SE, Hutchinson LL. Filiform corrosion of aluminum. Journal of Paint Technology, 1972 44 76-83. 

Fifiform Cemsien in Automobiles. The inqxx--tance of filiform corrosion of aluminum is increasing due to the increase use of aluminum for outer body sheet in automotive applications. An all-aluminum body shell made entirely of aluminum alloys is about 35% lighter than an all-steel body and meets the same stiffriess requirements (Ref 25). Sheet panels are first conversion coated (phosphated) and then painted with a three-coat paint system consisting of a cathodic electrocoat, a primer/surfacer coat, and a top coat Present experience indicates that the potential for fill-... 

Fig. 17 Effect of sanding prior lo phosphatlng on the filiform corrosion of aluminum alloys. All test specimens received an acid pickling pretreatment before phosphatlng and were in the hardened T6 condition. Source Ref 25...

Coyle, J., Fortier, M., and McKeon, J., Studying Filiform Corrosion of Powder-Clearcoated Aluminum Wheels, Powder Coating, March 1997, pp. 22-35. 

The lowest potential is measured in the center where corrosion (i.e., anodic dissolution of iron) attacks most aggressively. At the edges the potential increases somewhat in this zone oxygen reduction proceeds. The potential changes around the drop imply the presence of an ultrathin electrolyte film because the potential reaches values of the bare iron surface only at a considerable distance from the edges of the macroscopically observed drop [135]. Filiform corrosion of automotive aluminum alloy AA6016 has been studied with SKP [143]. 

C m I I Filiform corrosion of an aluminum aircraft skin around slael fasteners, (a) I Before paint removal, showing paint cracking and blistering. (b) After paint removal... 

Filiform corrosion is observed under thin organic coatings (typically 0.1 mm) on aluminum and steel exposed to a humid atmosphere. It has been observed in aircraft structures, beverage cans, flanges, gaskets, and weld zones [111—113]. Corrosion-resistant alloys of stainless steel, copper, and titanium are not susceptible to fihform corrosion. Metals with thin organic coatings are in contact with water from the humid atmosphere. 

A further advantage of polyaniline dispersion coatings and complete coating systems including interlayers and topcoats is its applicability to metal substrates other than steel. Aluminum is subject to so-called filiform corrosion, which can be effectively beaten by suitable products, in this case CORRPASSIV 4901 (a different kind of primer and top coat) the performance was tested by an independent institute (see footnote 7). In the meantime, the same primer was also successfully tested in various applications on magnesium. 

Filiform corrosion occurs independent of light, metallurgical factors in the steel, and bacteria. Although threads are visible only under clear lacquers or varnishes, they probably also occur under opaque paint films. They have been observed under various types of paint vehicles and on various metals, including steel, zinc, aluminum, magnesium, and chromium-plated nickel. This type of corrosion takes place on steel only in air of high relative humidity (e.g., 65-95%). At 100% relative humidity, the threads may broaden to form blisters. They may not form at all if the film is relatively impermeable to water, as is stated to be the case for paraffin [17]. The mechanism appears to be a straightforward example of a differential aeration cell. 

FIG. 4—Photograph of filiform corrosion that occurred on a medicinal Uni-dose cup during compatibility tests with various pharmaceuticals. The cup was formed from 0.15 mm aluminum foil coated with a clear lacquer. Corrosion did not perforate the cup but resuited in contamination of the liquid. (Photograph courtesy of F. S. Bovard, Alcoa Technical Center.)... 

Resistance to filiform corrosion depends more on factors such as the environment, the type and thickness of coating, metal surface preparation, and coating apphcation procedures than on the metal itself. However, there is evidence that higher copper content aluminum alloys are more susceptible. Primary environmental factors are the presence of moisture and chloride ions. An acidic electrolyte has an accelerating effect. 

Indoor atmospheres having relatively constant air temperatures with the air being dehumidified pose no problem for aluminum or its alloys. However, storage of aluminum products in unheated buildings or tractor trailers can cause serious corrosion problems such as staining, filiform corrosion, and other surface corrosion. Condensation on the metal during cool nights fol-... 

Filiform Corrosion. Filiform corrosion is a special form of crevice or oxygen cell corrosion occurring beneath organic or metallic coatings on steel, zinc, aluminum, or magnesium. Filiform corrosion normally starts at small, sometimes microscopic, defects in the coating. Lacquers and "quick-dry" paints are most susceptible to the problem. 

A specific t) e of delamination that is also driven by a differential aeration cell is filiform corrosion [5,6,62,63]. In most cases it is related to anodic undermining, especially because it occurs mainly on aluminum alloys. Filiform corrosion is a specific t5 e of delamination that occurs only under atmospheric condition with relative humidity between qjproximately 50 and 90%. Small threadlike delamination tracks are formed that in general follow irregularities on the surface (either rolling or grinding marks). New interest in this special form of corrosion was stimulated by many practical cases along the coastal areas in Europe. It may be expected that new mechanistic information will become available within the next 2 years. 

Williams G., McMuiray H.N. (2003), Anion-exchange inhibition of filiform corrosion on organic coated AA2024-T3 aluminum alloy by hydrotalcite-like pigments , Electrochem. Solid Slate Lett., 6, B9-B11. 

In filiform corrosion damage to aluminum, an electrochemical potential at the front of the head of 0.73 V (SHE) has been report-... 

McMurray H. N. and Williams G. (2004), Inhibition of Filiform Corrosion on Oiganic-Coated Aluminum Alloy by Hydrotalcite-Like Anion-Exchange Pigments , Corrosion, 60,219-28. 

H.N. McMurray, A.J. Coleman, G. Williams et al.. Scanning Kelvin probe studies of filiform corrosion on automotive aluminum alloy AA6016, / Electrochem. Soc. 154(7), C339-C348 (2007). 

Filiform corrosion is often mistaken as havitig biological origins because of its wormlike qipearance. Filiform corrosion is commordy obsoved on aluminum sheet, plate, and foil. The ctmosion products are gelatinous and milky in color. When dry, their filaments may take on an iridescent or clear appearance because of internal light reflection (see Fig. 9c). 

Fuselage Skins. On a Boeiiig 707 aircraft operated by a major commercial airline, filiform corrosion occurred on fuselage skins along rows of fasteners (Ref 27). Paint blistering is produced on airfhune aluminum sheet-metal structures when this form of attack occurs (Ref 27). 

Fifl. 10 Schematic diagrams of the filiform corrosion cell in aluminum. Corrosion products and predominant reactions are laMled. Filiform corrosion is a dillerenlial aeration cell driven by differences in oxygen con-... 

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