Impedance Characterization of Crevice Corrosion of an Aluminum 2024 Structure

• Published in: Corrosion (Houston, Tex.) Tex.), 2003-09, Vol.59 (9), p.780-789
• Main Authors: Su, P., Devereux, O.F.
• Format: Article
• Language: English
• Subjects: Alloys; Aluminium; Aluminum; Applied sciences; Calomel electrode; Capacitance; Circuits; Coating; Coatings; Computer simulation; Corrosion; Corrosion mechanisms; Crevice corrosion; Epoxy compounds; Exact sciences and technology; Hydrogen evolution; Impedance; Mathematical models; Measurement; Metals; Metals. Metallurgy; Parameters; pH effects; Polyurethane; Polyurethane resins; Primers (coatings); RC circuits; Sodium; Sodium chloride
• ISSN: 0010-9312; 1938-159X
• DOI: 10.5006/1.3277607

ABSTRACTThe impedance behavior of aluminum 2024 (UNS A92024), bare and with a MIL-P-23377 epoxy primer/MIL-C-85285 polyurethane topcoat, in aqueous 0.5 M sodium chloride (NaCl) was investigated in experiments that included a simulated crevice. Impedance spectra of the bare alloy were effectively modeled by the Randles circuit; the coated alloy required the addition of a pore resistance in series with, and a geometric coating capacitance in parallel with, the parallel RC circuit representing the metal/coating interface. A coated structure containing a corroding crevice was modeled by a simplified Randles circuit element, to represent the crevice, in parallel with the foregoing circuit. Experiments with a corroding system showed this set of circuit models to be selfconsistent and to yield reasonable values for the parameters characterizing the model circuits. Using these models and parameter values, it was shown that, for the 2024/0.5 M NaCl system, a corroding crevice can be detected in the context of a Nyquist or Bode representation of an impedance spectrum when the active area within the crevice comprises at least 0.1% of the test area. In additional work, a crevice was simulated by resting a microscope slide on the alloy surface and saturating the intervening space with electrolyte. This permitted a semi-quantitative observation of hydrogen evolution (4% of the total cathodic reaction), measurement of Crevice corrosion is problematic in many engineering structures and may be of particular concern with regard to aging aircraft. Aluminum alloys are prone to crevice corrosion, and modern aircraft, fabricated largely with high-strength aluminum alloys, are replete with crevices within which corrosion may be enhanced not only by the crevice geometry but also by galvanic coupling of aluminum alloy structural components to the steel rivets, which are in widespread use.1 Although airborne structural components are typically protected with adhesive organic coatings (e.g., epoxies, sulfonates, and alkyd resins), their protection is neither total nor permanent, and such coatings are less likely to be intact in the vicinity of fasteners. Anecdotal evidence suggests that the average commercial transport or military aircraft may be flying with hundreds of gallons of brackish water entrained within its structure. The autocatalytic nature of crevice corrosion is, of course, well understood in terms of self-acidification of the crevice, inward migration of chloride, isol