A Galvanic Corrosion Approach to Investigating Chromate Effects on Aluminum Alloy 2024-T3

Abstract

The effects of chromate treatment on the corrosion of an aluminum/copper aircraft alloy were examined with a galvanic corrosion apparatus composed of two electrodes and a zero resistance ammeter. Combinations of pure Al, pure Cu, and AA2024-T3 electrodes were immersed in 0.1 M NaCl solution, which was saturated with air, O 2 , or argon. Selection of electrode material or differential aeration resulted in partial segregation of corrosion reactions, with one electrode becoming a net cathode and the other a net anode. For the case of an Al/Cu galvanic cell, chromate significantly reduced the observed cathodic current on Cu but had little effect when added to the Al cell. For an AA2024/AA2024 couple, chromate decreased the observed current when present in either the net anode or net cathode compartments. The results indicate that Cr VI in solution, or Cr VI pretreatment, inhibits O 2 reduction on Cu sites. Inhibition is preceded by a spike in the reduction current, implying that Cr III is formed at approximately a monolayer level on Cu. The results for Cr VI effects on the AA2024/AA2024 galvanic couple are consistent with observed polarization curves. Considering the results in their entirety, Cr VI acts as a strong irreversible cathodic inhibitor on Cu and AA2024, but the results do not exclude the action of Cr VI as an anodic inhibitor on AA2024-T3.Aluminum alloy 2024-T3 (AA2024-T3) has been well documented to be highly susceptible to corrosion, particularly localized pitting corrosion, and intergranular corrosion. AA2024-T3 is an in-homogeneous alloy (nominal composition Al, 3.8-4.9% Cu, 1.2-1.8% Mg, 0.3-0.9% Mn, 0.5% Fe, 0.5% Si, 0.25% Zn, 0.1% Cr, 0.05% Ti) containing several types of secondary-phase intermetallic particles that are enriched in alloying elements vs. the alloy matrix.

A study of the mechanisms of corrosion inhibition of AA2024-T3 by vanadates using the split cell technique

“…Galvanic current was only affected by changing aeration and convection on the Cu-cathode side. In good agreement with Clark et al [33], these findings show that the net galvanic current, and consequently the extent of anodic dissolution, was controlled by oxygen reduction at the Cu-cathode. The response of the cell setup to the injection of Cr 6+ ion was also investigated.…”

“…The response of the cell setup to the injection of Cr 6+ ion was also investigated. In agreement with Clark et al, injection of chromate to the cathode of different split cells was characterized by a sharp current spike associated with the electrochemical reduction of Cr 6+ to Cr 3+ [33,53]. Uncontrolled injection of solution can also affect the galvanic current by altering convection.…”

“…However, as explained by Clark et al [33], the spatial separation of the anode and the cathode decreases the local chemical interactions between surface sites. Nevertheless, taking into account such limitation, interesting and useful information can be extracted from the data presented here.…”

“…It has been shown that the formation of a Cr 3+ monolayer inhibits oxygen and further Cr 6+ reduction [18,[27][28][29][30][31][32]. Debate still exists regarding the control over the anodic dissolution of active phases [33]. Nevertheless, the fast and irreversible formation of the Cr 3+ layer seems to play the major role [20,33,34].…”

“…Debate still exists regarding the control over the anodic dissolution of active phases [33]. Nevertheless, the fast and irreversible formation of the Cr 3+ layer seems to play the major role [20,33,34]. In addition, chromate systems exhibit the critical property of self-healing or active corrosion protection.…”