Aluminum alloys exfoliation corrosion susceptibility

G34-99, Standard test method for exfoliation corrosion susceptibility in 2XXX and 7XXX series aluminum alloys EXCO test. Annual Book of ASTM Standards, ASTM International, Philadelphia, Pa., 2000, p. 124, Vol. 3.02. 

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

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)... 

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]. 

Lee, S. and Lifka, B. W., Modification of the EXCO Test Method for Exfoliation Corrosion Susceptibility in 7XXX, 2XXX and Aluminirm-Lithium Alloys, Afew Methods for Corrosion Testing of Aluminum Alloys, ASTM STP 1134, V. S. Agarwala and G. M. Ugiansky, Eds., ASTM International, West Conshohocken, PA, 1992, pp. 1-19. 

ASTM G34 - test method for exfoliation corrosion susceptibility in 2xxx and 7xxx series aluminum alloys (EXCO Test). 

ASTM G66 - test method for visual assessment of exfoliation corrosion susceptibility of 5xxx series aluminum alloys (Asset Test). ASTM G67 - test method for determining the susceptibility to intergranular corrosion of 5xxx series aluminum alloys by mass loss after exposure to nitric acid (Namlt Test). 

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). 

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. 

As a general rule, aluminum alloys, particularly the 2xxx series, are less corrosion resistant than the commercial purity metal. Some aluminum alloys, for example, are susceptible to intergranular corrosion as a result of low-temperature aging reactions and the subsequent precipitation in the grain boundaries. Susceptibility to intergranular attack in these alloys shows up as exfoliation and stress-corrosion cracking (SCO). 

S. Lee and B.W. Lifka, Modification of the EXCO Test Method for Exfoliation Coirasion Susceptibility in Ixxx, 2xxx and Aluminum-Lithium Alloys, New Methods for Corrosion Testing Aluminum Alloys, V.S. Agarwala and GA1. Ugiansky, Ed, ASTM, 1992, p 1-19... 

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