Exfoliation corrosion of aluminum

Sinyavski, V. S., Kalinin, V. D., and Dorokhina, V. E., "Study of Correlations Between Structural, Electrochemical and Physical Characteristics and Resistance to Exfoliation Corrosion of Aluminum Alloys," Proceedings of the Second International Conference on Aluminum Alloys-Hieir Physical and Mechanical Properties, International Academic Publishers, A Peigamon-CNPIEC Joint Venture, 1990, pp. 692-697. 

Sprowls, D. O., Walsh, J. D., and Shumaker, M. B., "Simplified Exfoliation Testing of Aluminum Alloys," Localized Corrosion— Cause of Metal Failure, ASTM STP 516, ASTM International, West Conshohocken, PA, 1972, pp. 38-65. 

Aluminum finds wide applications in what would be classified as outdoor atmospheres. Corrosion of aluminum is self-limiting in all outdoor classifications whether it be seacoast, urban, industrial, or rural, and the rate of corrosion becomes very slow within 5 years except for alloys and tempers that are subject to exfoliation. 

Figure 9.49 Leaf-like exfoliation corrosion of corrugated A92024 aluminum alloy. (Courtesy of Kingston Technical Software)...

Intergranular and Exfoliation Corrosion, Corrosion of Aluminum and Aluminum Alloys, J.R. Davis, Ed., ASM International, 1999, p 63-74... 

Ion vapor deposition of aluminum is attractive because it avoids the environmental and toxicological problems associated with cadmium. It does not cause hydrogen embrittlement of steel or solid metal embrittlement of steel or titanium, and it should be more galvanically compatible with aluminum alloy structure and avoid the exfoliation corrosion of sensitive alumimun alloy structme. However, views on the ability of alumimun to protect steel fasteners appear to vary. A view is that the presently available pure aluminum coatings are not able to provide adequate sacrificial protection to steel in a chloride ion environment, and for this reason a recommendation is made for the development of alruninrun coatings containing small amounts of zinc or other elements for improved protection. 

Czyrca E., Hack H.P., Corrosion of aluminum alloys in exfoliation resistant tempers exposed to marine environments for two years. Naval Ship Research and Development Center, report AD/ A-002 234, November 1974. 

Flo 1 1 Exfoliation corrosion of an aluminum alloy stabilizer bracket, (a) Heavy surface corrosion on the stabilizer bracket, (b) Cross section through the bracket showing corroded surface grains and corrosion of grain boundaries of elongated grains... 

D.O. Sprowls, J.D. Walsh, and M.B. Shumaker, Simplified Exfoliation Testing of Aluminum Alloys, Localized Corrosion—Cause of Metal Failure, STP 516, ASTM, 1972, p 38-65... 

Exfoliation corrosion is especially prevalent in aluminum alloys. The grain structure of the metal determines whether exfoliation corrosion will occur. In this form of corrosion, degradation propagates below the surface of the metal. Corrosion products in layers below the metal surface cause flaking of the metal. 

G112-92, Standard guide for conducting exfoliation corrosion tests in aluminum alloys. Annual Book of ASTM Standards, ASTM International, Philadelphia, Pa., 2000, p. 489, Vol. 3.02. 

G66-99, Standard test method for visual assessment of exfoliation corrosion susceptibility of 5XXX series aluminum alloys... 

Exfoliation is a related type of anodic path corrosion in which attack of rolled or extruded aluminum alloy results in surface blisters followed by separation of elongated slivers or laminae of metal. It occurs in various types of aluminum alloys in addition to the copper-bearing series. Proper heat treatment may alleviate such attack. 

Exfoliation is commonly experienced on exposure of susceptible aluminum alloys to marine atmospheres. Simulation in the laboratory is accomplished by controlled intermittent spray with 5% NaCl containing added acetic acid to pH 3 at 35-50°C (95-120°F) [28]. In practice, severe exfoliation corrosion has been experienced in water irrigation piping constructed of type 6061 alloy [29]. The cause in this instance was ascribed to pipe fabrication methods and presence of excess amounts of impurity elements (e.g., Fe, Cu, and Mn). 

G 66 Method for Visual Assessment of Exfoliation Corrosion Susceptibility of 5XXX Series Aluminum Alloys (Asset Test)... 

Historically, exfoliation corrosion problems have been associated principally with high-strength aluminum alloys used in aircraft construction and by the U.S. Department of Defense. This chapter, therefore, will deal mainly with test procedures developed for alloys of aluminum. 

Severe exfoliation has been reported with graphite/tJu-minum metal matrix composites in marine environments [5]. Localized corrosion along wire-wire and wire-foil diffusion bonds resulted in disbondment of the precursor wires and aluminum foils. See also the discussion by Hihara on corrosion of metal-matrix composites in Section VI of this manual. 

Laboratory corrosion tests for exfoliation corrosion susceptibility are a necessary tool for research and quality control engineers however, the validity of such accelerated tests depends on their relationship to realistic service conditions and their sensitivity to various degrees of susceptibility. The tests must be discriminating and yet not so severe as to be unrealistic. For the majority of engineered structures, exposure to outdoor atmospheres provides a baseline that is representative of many service conditions, except for structures that are subjected to unusual chemical environments. Experience has shown that seacoast conditions are more corrosive to aluminum alloys than inland urban and industrial conditions (see Fig. 2), and seacoast atmospheric exposure tests have been particularly useful for the validation of accelerated exfoliation tests [9]. 

Accelerated tests do not precisely predict long-term corrosion behavior however, answers are needed quickly in the development of new materials. For this resison, accelerated tests are used to screen candidate alloys before conducting atmospheric exposures or other field tests. They also are used for production control of exfoliation-resistant heat treatments for the AA2XXX, AA5XXX, and AA7XXX aluminum alloys. ASTM has standardized several laboratory tests for susceptibility to exfoliation corrosion in recent years. 

Historically, various investigators in Europe have used hydrochloric acid-px>tassium dichromate solutions for testing the exfoliation susceptibility of all types of aluminum alloys. An example is the Russian Stanchtid GOST 9.904-82, Aluminium Alloys Methods of Accelerated Exfohation Corrosion Testing [22]. It should be noted, however, that hexavalent chromium (Cr ) solutions are considered undesirable for health and environmental considerations and have been prohibited in some countries. 

Thompson, J. J., "Exfoliation Corrosion Testing of Aluminum-Lithium Alloys," New Methods for Corrosion Testing of Aluminum Alloys, ASTM STP 1134, V. S. Agatwafa and G. M. Ugiansky, Eds., ASTM International, West Conshohocken, PA, 1992, pp. 70-81. 

Technical Report Exfoliation Corrosion Testimony of Aluminum Alloys 5086 and 5456, The Alurrtinum Association. 

Summerson, T. J., Tnterim Report, Aluminum Association Task Group on Exfoliation and Stress-Corrosion Cracking of Aluminum Alloys for Boat Stock, Proceedings of the Tri-Service Corrosion Military Equipment Conference, Technical Report AFML-TR-75-42, Vol. II, Air Force Materials Laboratory, 1975, pp. 193-221. 

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