|
|
In its Glossary of Terms, Volume 13, Corrosion, p. 6 defines exfoliation as follows:
Corrosion that proceeds laterally from the sites of initiation along planes parallel to the surface, generally at grain boundaries, forming corrosion products that force metal way from the body of the material, giving it a layered appearance. It also indicates that it is synonymous to 'lamellar corrosion'. The Encyclopedia Britannica indicates that this term suggests a composition or arrangement in the form of a thin, flat layer or scale. Nothing in the Glossary restricts the term to aluminum.
Under Materials Selection, p. 334, Greg Kobrin states that exfoliation affects primarily aluminum alloys, attack proceeding laterally from initiation sites on the surface and generally proceeding intergranularly along planes parallel to surface. Don Sprowls discusses evaluation of exfoliation on pp. 242-3, particularly as regards the ASTM tests (e.g., G34, G64, G66, G85, etc,).
Under Temper Effects, p. 295, it is stated that the structural layers are pushed apart by voluminous corrosion products.
I edited NACE Manual No.1, The Forms of Corrosion Recognition and Prevention@ in 1977-78 with a series of authorities on Fontana's Eight Forms of Corrosion. In that version, no mention was made of exfoliation. Dale McIntyre updated this in 1982 as Volume 1 and 2. In the latter, two case histories of exfoliation of aluminum (under Intergranular Corrosion are given from the aircraft industry.
The term seems to have been largely pre-empted by the corrosionists concerned with aluminum. However, in Vol. 13, exfoliation of 80-20 and 70-30 cupronickels is reported under Closed Feedwater Heaters (pp. 989-990), the susceptibility increasing with the nickel content. Allegedly, this problem (at its worst in peaking/cycling service) is reduced by steam or nitrogen blanketing. It has also been reported in superheaters and reheaters.
In ferrous alloys, exfoliation is characterized by excessive internal growth of oxide, which has a volume some seven times that of the steel. Excessive internal growth of oxide can elevate temperature and the exfoliated material damage turbines. Exfoliation occurs in ferritic materials when multilayer growth occurs. Stresses are induced by temperature cycles and by the difference in thermal expansion between the scale and tube. Exfoliation can also occur in austenitic stainless steels, again because of the difference in thermal expansion between the metal and the oxide.
Apparently, we must use the broader term, Lamellar Corrosion, if we concede the much wider occurrence of exfoliation in the light metals. Personally, I don't care but then the Glossary should be rewritten in that wonderful new 2nd edition of Volume 13!
Paul
