Influence of Fe‐Rich Intermetallic Inclusions on Pit Initiation on Aluminum Alloys in Aerated NaCl

Abstract

The role that Al3normalFe intermetallic inclusions play during initiation of pitting corrosion on aluminum alloys was investigated in 0.6 M NaCl. In aerated solutions microscopic observations showed the growth of cavities in the host metal adjacent to inclusions. The rest potential of synthetic Al3normalFe was measured in aerated and deaerated NaCl solutions over a range of bulk pH values between 2 and 12 and was found to act as a cathode. In aerated solutions rotating disk electrode experiments on synthetic Al3normalFe electrodes verified that the cathodic reaction corresponded to the reduction of dissolved oxygen. With scanning pH microelectrodes, measurements were carried out near synthetic Al3normalFe electrodes which were coupled in a galvanic cell with Al-6061 in NaCl solution. In addition, the pH measured over Al-6061 at a distance of 25–30 μm from the surface was observed to fluctuate between pH 4 and 8.5 for the first 2 h of immersion in NaCl solutions. Atomic force microscopy images of Al-6061 immersed in buffered aerated 0.6 M NaCl solution (pH 5.5) did not show any evidence of dissolution of the Al around intermetallics. These measurements support the view that Al3normalFe particles in Al-6061 serve as local cathodes, that a high pH develops around the intermetallic particles and creates cavities in the host metal, and that interactions among a cluster of intermetallic particles causes a large number of slowly dissolving alkaline cavities to evolve into a small number of rapidly dissolving acidic pits. © 1999 The Electrochemical Society. All rights reserved.

New insights into the cathodic dissolution of aluminium using electrochemical methods

“…The agreement is good in the whole frequency range; the small difference appearing in the high frequency range is attributed to current and potentials distribution induced by geometry effect, which were not taking into account in the model. It has to be noted that the presence of Fe-rich intermetallic phases can also contribute to the nonuniformity of the surface [22][23][24][25], but their influence were considered to be negligible in comparison with that of the classical nonuniform current distribution on the disk electrode in the present work.…”

Electrochemical Characterization of Al 7075-T6 Surface Oxide After Alkaline Treatments

“…7), line X represents oxygen-water equilibrium, and line Y represents hydrogen-water equilibrium. [13][14][15]23,24 The other common cathodic reaction that occurs during metal corrosion is reduction of dissolved oxygen (Eq. 1.…”

“…The stable potential region for water is between lines X and Y and is expressed by Eq. [23][24][25][26] Because noble intermetallic particles are known to be better oxygen reduction electrocatalysts than oxide surfaces, 24,26 reaction (IV) would be expected to occur in the vicinity of IMCs À 0:242 À 0:0592pH nðvs:SCEÞ 0:986 À 0:0592pH Decreasing the potential of the electrode to a value less than that given by line Y causes the cathodic evolution of hydrogen gas at 1 atm hydrogen pressure from the electrolyte expressed by Eq.…”

Pitting corrosion of naturally aged AA 7075 aluminum alloys with bimodal grain size

“…Consequently, lots of hydroxyl ions are produced causing localized alkalization of the solution near the cathodic particles. A pH value as high as 9.5 was measured around Al 3 Fe in AA6061 alloy [75]. Besides, various modeling studies have indicated that a localized pH (near cathodic particle) approaching 10 is possible for a neutral solution [76,77].…”