The causes of the localized character of corrosion on aluminium

Abstract

The pitting and localized attack on aluminium is explained by the autocatalytic character of the anodic corrosion reaction ; theoretical considerations suggest that if once, a t a point, the anodic current density becomes high, i t will tend t o rise higher, until checked by the evolution of hydrogen, which tends t o produce a film of oxides or hydroxide over the point and slow down the reaction. Pitting may be expected to occur on aluminium in neutral solutions and on iron in alkaline solutions-as is found to be the case. The theoretical reasoning is supported by observation o f pH changes around corroding specimens and also in divided cells, as well as by experiments on the corrosion cracking of aluminium-magnesium in a susceptible state.Types of Localized Attack.-Aluminium is a metal which, despite a strong affinity for non-metals, is surprisingly resistant to corrosion-a property due largely to the formation of protective films. When, however, corrosion does occur, i t is often localized, causing damage disproportionate to the amount of metal destroyed. It may take the form of pits, involving local penetration without much general corrosion. On alloys containing copper or magnesium which have received a heat-treatment that produces readily attackable matter along the grain-boundaries, intercrystalline attack, which causes more mechanical weakening than isolated pits, is found. If such alloys are subjected to tension during corrosion, i t becomes possible for the corrosion-cracking to be concentrated on a limited number of grain-boundaries running roughly at right-angles

Effects of chromate and chromate conversion coatings on corrosion of aluminum alloy 2024-T3

“…Synthetic mixed oxides were made by dissolving Al(III), Cr(III) and Cr(VI) species into solution and then precipitated by increasing the pH. A mixed Cr(III)/Cr(VI) oxide was precipitated from a mixture of Cr(NO 3 ) 3 The release of chromate from a CCC was studied by Raman spectroscopy using the set-up shown in Fig. 1.…”

“…) with a lower-valent form that is insoluble and creates an extremely protective film [Cr 2 O 3 or Cr(OH) 3 ]. Localized corrosion is extinguished by chromate reduction according to Eqs.…”

“…Chromate is retained in the CCC [3,4]. Indirect evidence from, for instance, salt spray tests indicates that it is possible for the chromate in the CCC to promote self-healing of damaged areas by dissolving into a local defect and altering the local environment and dissolution kinetics.…”

The Intersection of Design, Manufacturing, and Surface Engineering

“…57À63 In 1927, Evans suggested that the inhibitory action of chromates on iron could be due to its ability to repair the discontinuities in the oxide film on iron. 64 Later, Edeleanu and Evans 65 proposed that aluminum metal when exposed to chromate reduces hexavalent chromium to trivalent chromium and gets oxidized to aluminum oxide during the process.…”

The negative-difference effect during the localized corrosion of magnesium and of the AZ91HP alloy

“…Hydrogen, in this model, is released by the decomposition reaction of the hydride, which is unstable in contact with water: MgH 2 + 2H 2 O → Mg 2+ + 2OH -+ 2H 2 . The negative-difference effect is a phenomenon that is also known for other metals, as aluminum, [14][15][16] aluminum alloys 17 and ferrous materials. 18 In the case of magnesium, however, the investigation of the negative-difference effect is more complex, since the hydrogen gas results from the partial reaction occurring during localized corrosion inside the pits, as well as from the cathodic reaction on the surface outside the pits.…”