Conductivity, exfoliation corrosion

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. 

Standard guide for conducting exfoliation corrosion tests in aluminium alloys Standard test method for exfoliation corrosion susceptibility in 2XXX and 7XXX series aluminium alloys (EXCO test)... 

Annex 4, Salt/SO, Spray (Fog) Testing, may be performed using either a 5 % salt solution or synthetic seawater solution. Like the salt spray test, it is conducted at 35°C. The fog may be continuous or intermittent. Depending upon the chosen cycle, the sulfur dioxide is injected into the chamber four times a day for 1 h or within a cycle of VS-h salt fog, A-h SO injection, and 2-h soak [4]. The test was developed by the Navy to simulate exfoliation corrosion on aircraft carriers [6]. 

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. 

Extensive tests are conducted to demonstrate a new alloy temper has the claimed characteristics. Subsequent production must be sufficiently tested to ensure that current lots are comparable to the initial development lots and to lots of material used to establish the full design characteristics needed for production of a highly engineered product, such as an airplane. Corrosion performaiKe attributes most regularly tested are resistance to SCC in the short-transverse direction, resistance to exfoliation corrosion, resistance to intergranular corrosion, and durability of film or coating applied to an aluminum substrate. 

Thus, In order to avoid the corrosion-exfoliation problems associated with cupronickel HP feed heaters, stainless steel was specified for the HP heaters In the SGHW reactor. Subsequently It was decided to use mild steel In No. 4 unit on an experimental basis to assess Its potentialities for future systems. Dynamic tests conducted In mild steel loops with the appropriate temperature and chemistry Indicated mild steel wastage rates of 2 mdd after 1000 h exposure with 30 release of corrosion products. While some pitting does occur It Is by no means as serious as the exfoliation which could be expected If cupro-nlckel tubes had been Installed In the HP units. 

For some materials, the processing procedures that develop the desired high resistance to corrosion affect other characteristics, which can be tested by methods that do produce numoical data. If a correlation can be established between these other data and the corrosion results desired, then measurement of these other characteristics provides a quick lot-release tool provid-itig high statistical confidence. The most notable examples are the tensile strength plus electrical conductivity criteria used for some of the 7xvc alloys in T7 type tempers that provide high resistance to stress corrosion or exfoliation (Tables 2a and 2b). Examples of alloys for which such relationships have been developed are 7075-T73,7075-T76,7050-T74, and 7150-T77. 

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