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Exfoliation corrosion mechanisms in the 7020 aluminium alloy
Author(s) -
Reboul M. C.,
Bouvaist J.
Publication year - 1979
Publication title -
materials and corrosion
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.487
H-Index - 55
eISSN - 1521-4176
pISSN - 0947-5117
DOI - 10.1002/maco.19790301006
Subject(s) - materials science , metallurgy , corrosion , intergranular corrosion , alloy , anode , grain boundary , zinc , galvanic anode , manganese , cathodic protection , microstructure , electrode , chemistry
The first necessary condition for the occurrence of exfoliation corrosion is the existence of a fibrous texture. Transgranular exfoliation corrosion, which can be observed in some instances, develops through grains in which recrystallization is incomplete and which exhibit a substructure with remnants of a fibrous structure. As with intergranular corrosion, the controlling parameter is the formation of continuous anodic zones. In the case of exfoliation corrosion, these anodic zones are localized in planes parallel to the „fabrication orientation”. In the case of the welded 7020‐T 6 alloy, two types of exfoliation corrosion have been observed, depending upon the nature of the anodic zones: The first type is related to the main elements, magnesium and zinc, which precipitate in the grain boundaries as large anodic MgZn 2 particles during a short reheat (welding) at 200° – 250°C. The second type is related to the secondary elements, chromium and/or manganese.It is hypothesized that the precipitation of particle layers, reported by ADENIS, depletes the neighbouring solid solution, thus making it anodic with respect to the rest of the metal. The exfoliation corrosion develops by anodic attack of the following alloys in the T4 condition:\documentclass{article}\pagestyle{empty}\begin{document}$$ \begin{array}{*{20}c} {Al - 5\%Zn - 1\%Mg - 0.3\%Cr} \hfill & {Al - 5\%Zn - 1\%Mg - 0.5\%Mn} \hfill \\ {1050 + Mn} \hfill & {1050 + Cr} \hfill \\\end{array} $$\end{document}For the phenomenon to occur without galvanic current, in addition to chromium or manganese, it is necessary to have iron which depassivates the cathodic reaction and zinc in solid solution (T 4) which lowers, by about 100 m V, the anodic zones potential. The influence of zinc becomes nul when it is precipitated (T 6).