Aluminum alloys are pickled and/or etched as a preliminary step prior to anodization or conversion coating. The chemistry of the etching/pickling process must be carefully engineered to avoid the enrichment of the alloying elements such metallic Cu which can interfere with subsequent treatments. Understanding and optimizing the etching process is therefore a major concern for the use of aluminum alloys, and especially, the Cu-rich high strength aluminum alloys.

The etching reaction is a mixed potential process similar to other forms of aqueous corrosion. However, the dynamic nature of the interface makes it difficult to apply conventional electrochemistry to determine the rates and mechanisms of the different reactions and therefore, to predict how the etching process will vary with electrolyte composition. Among the many complicating factors, we can cite the very high dissolution rate of the alloy coupled with the formation of gas and the release of intermetallic particles. The dissolution of alloying elements such as Cu and Mg – the reactions of interest – occur at rates several orders of magnitude below that of Al dissolution. For example, Cu dissolution makes a negligible contribution to either electron exchange or mass loss. Finally, the rapid reaction rates and the short time scale of the process preclude the appearance of a true steady state, necessary for the interpretation of many electrochemical techniques.

Therefore, to gain insight into the mechanism of the acid etching process, we have used an element-resolved electrochemical technique, atomic emission spectroelectrochemistry (AESEC) (Gharbi et al., 2016; Gharbi et al., 2017; Sultan et al., 2022). This permits the direct measurement of the elemental dissolution rates as a function of time, on an element-by-element basis.

In this work, we illustrate the element-resolved electrochemical approach to surface treatment focusing on the acid etching of three alloys: a commercial high-strength AA7449 alloy (containing Zn, Cu, and Mg), and binary Al-Cu and Al-Mg alloys to represent the extremes of alloying element nobility. In sulfuric acid, the binary alloys also represent the extremes of selective versus congruent dissolution. The Al-Mg binary alloy undergoes a simultaneous, congruent dissolution mechanism with Mg dissolving simultaneously with Al. By contrast, the Al-Cu binary alloy exhibits a selective dissolution mechanism with Al dissolving to leave behind dealloyed metallic Cu on the surface of the material. This is ultimately followed by the release of Cu-rich particles due to anodic undermining.

The electrolytes investigate in this work represent a range of oxidizing strengths from pure sulfuric acid to pure nitric acid and various mixtures of the two. The effect of Fe(III), an important component of many state-of-the-art etching solutions, was also investigated. Experimental elemental Evans diagrams were determined to clarify the electrochemical nature of the dissolution reactions, specifically how the elemental dissolution rates were related to one another and how they were coupled to the cathodic reactions.

An example is shown in the Figure (right), based on the 2022 publication of Sultan et al. On the left, we show a cartoon representation of the dissolution process as determined for a high strength AA7449 alloy. On the right, we show the experimental element-resolved Evans diagram for the same alloy in sulfuric acid. It was found that Cu would not dissolve at the open circuit potential in sulfuric acid, but displayed active dissolution with a well-defined Tafel slope of 44 mV/decade at higher potential. The dissolution rate of Cu in the etching solutions was controlled by the redox potential of the electrolyte as determined by the addition of nitric acid and/or Fe(III). Based on our results, the Fe(III) additive in the presence of nitrate appears to serve a catalytic role, enhancing the rate of electron transfer between Cu and nitrate. The dissolution of Al and Mg were independent of potential suggesting simultaneous dissolution across an oxide film. The rates and mechanisms of Cu particle release will also be discussed.

BBM Sultan, D Thierry, JM Torrescano-Alvarez, K Ogle, “Selective dissolution during acid pickling of aluminum alloys by element-resolved electrochemistry”, Electrochim. Acta, 404(2022)139737.

O Gharbi, N Birbilis, K Ogle, “In-situ monitoring of alloy dissolution and residual film formation during the pretreatment of Al-alloy AA2024-T3”, J. Electrochem. Soc. 163 (2016)C240.

O Gharbi, N Birbilis, K Ogle, “Li reactivity during the surface pretreatment of Al-Li alloy AA2050-T3”, Electrochim. Acta 243(2017)207-219.