Aluminum alloys exfoliation

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. 

The material conformed to the specification and the aluminum alloy is known to be subject to exfoliation corrosion. The corrosion was extensive at the catch positions and attributed to the stainless steel shims fitted below the catches. The paint between aluminum and stainless steel shims deteriorated, resulting in galvanic corrosion with the stainless steel acting as the noble metal. 

Charitidou, E., Papapolymerou, G., Haidemenopoulos, G.N., Hasiotis, N., and Bon-tozoglou, V., Characterization of trapped hydrogen in exfoliation corroded aluminum alloy 2024, ScriptaMater., 41, 1327, 1999. 

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... 

A variation on selective leaching is exfoliation, which is corrosion spreading below the surface. It differs from pitting in that the attack has a laminated appearance with whole layers of material peeled away in the form of a flaky or blistered surface. It is well known in certain aluminum alloys and is combated by proper heat treatment and alloying. 

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). 

The second annex. Cyclic Acidified Salt Fog Testing, originally ASTM G 43, was used for exfoliation testing on certain aluminum alloys [4]. It is often referred to as the... 

G 34 Test Method for Exfoliation Corrosion Susceptibility in 2XXX and 7XXX Series Aluminum Alloys... 

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. 

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. 

These several tests, developed by the producers of aluminum alloys and U.S. Government laboratories, have been used mainly for research and alloy development. Certain test procedures are also used for production control of exfoliation-resistant materials. If they are considered for quality assurance purposes, however, limits of acceptable performance should be the subject of an agreement between concerned parties. 

Annex A3 describes a cyclic salt spray test (SWAAT) that uses a different exposure cycle and a 5 % synthetic sea salt solution acidified to pH 3 with acetic acid in a spray chamber at a temperature of 49°C (120°F). This test is applicable to the production control of exfoliation-resistant tempers of the AA2XXX, AA5XXX, and AA7XXX aluminum alloys. Wet-bottom operating conditions are recommended with test durations of 1-2 weeks [13,14],... 

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. 

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. 

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. 

Lifka, B. W. and Sprowls, D. O., Relationship of Accelerated Test Methods for Exfoliation Resistance in 7XXX Series Aluminum Alloys with Exposure to a Seacoast Atmosphere, Corrosion in Natural Environments, ASTM STP 558, ASTM International, West Conshohocken, PA, 1974, pp. 306-333. 

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. 

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