Corrosion and Protection of Synthetic Intermetallic Compounds and Bulk Aluminum Alloy Under Simulated Atmospheric Conditions
Authors: Rodič Peter , Milosev Ingrid , Frankel Gerald S. 
Published: February 2024

Aluminum alloy 7075-D6 is often used for aircraft parts and is thus exposed to atmospheric conditions. AA7075-D6 is susceptible to corrosion due to high Zn, Mg and Cu contents in the form of various intermetallic particles having more or less noble electrochemical potential regarding the matrix (1,2). Usually, corrosion of aluminum alloys is simulated using solutions of various concentrations of chloride ions. However, aircraft are exposed to atmospheric conditions, where Cl- is not the only pollutant. Namely, in industrial and urban areas, major inorganic species such as ammonium (NH4
+), sulfate (SO4
2−) and nitrate (NO3
−) ions are found in the form of NH4NO3 and (NH4)2SO4 (or NH4HSO4). Therefore, these should also be considered because they represent corrosive agents for many aluminum alloys (3). Corrosive conditions may be thus better simulated by the so-called dilute Harrisonʼs solution (DHS) (0.35 wt.% (NH4)2SO4 + 0.05 wt.% NaCl), which in addition to Cl− also contains SO4
2− and nitrate NO3
− reflecting the environmental and atmospheric conditions. The first part of our study was focused on the corrosion of AA7075-T6 in DHS and comparison with NaCl solution. In addition to AA7075-T6, major intermetallic compounds (IMCs) synthesized in bulk form to represent the major intermetallic constituents (Al2Cu, Al2CuMg, Al7Cu2Fe and MgZn2) were also investigated. For IMCs, electrochemical measurements were performed using a microcell, and a standard cell was used to study the bulk alloy.

Bulk AA7075-T6 was less susceptible to corrosion in DHS than in chloride solution stemming from the beneficial presence of sulfate ions and lower chloride concentration. The susceptibility of IMCs to corrosion in DHS increased as Al2Cu < Al7Cu2Fe < Al2CuMg < MgZn2 due to the increased dissolution of Mg in the presence of ammonium ions. The relative nobility of IMCs was determined based on the galvanic current density.

In the second part of our study, we addressed the role of cerium salts in the corrosion protection of AA7075-T6 in DHS. Cerium salts are non-toxic compounds and demonstrate corrosion inhibition by inhibiting redox reactions by forming a protective cerium-containing precipitated conversion layer (4). Ce salts acted as inhibitors for the alloy during 14-day testing, forming a Ce hydroxide layer. The degree of protection depended on the type of anion, being the highest for Ce acetate. For the IMC, however, inhibition by Ce salts did not occur during short measurements, indicating the importance of galvanic interaction with the alloy matrix in the inhibition, which was confirmed by long-term measurements of the alloy. Micro and macro approaches give essential information on the galvanic relation between the IMPs and Al matrix under different conditions in the presence and absence of different cerium salts or other inhibitions (5). This brought about valuable information concerning corrosion inhibition and synergism with other ions in the corrosion media.

Figure.
Schematic presentation of (a) effect of various anions (NH4
+, SO4
2
- and Cl-) in dilute Harrison’s solution, and (b) effect of cerium(III) cations and corresponding anions (Cl-, NO3
- and CH3COO-) on the surface of AA7075-T6. Corrosion inhibition due to the deposition of the cerium layer is presented in (c). The precipitation of Ce species proceeds mainly on the cathodic sites and progressively spreads over the alloy surface; meanwhile, the Mg and Al are slowly dissolved from the anodic sites.

References:

Zhao and G. S. Frankel, Corros. Sci., 49, 3064–3088 (2007)

Birbilis and R. G. Buchheit, J. Electrochem. Soc., 155, C117–C126 (2008)

Rodič and I. Milošev, J. Electrochem. Soc., 163, C85–C93 (2016)

Milošev, B. Kapun, and P. Rodič, J. Electrochem. Soc., 169, 091501 (2022)

Rodič, I. Milošev, G.S: Frankel, J. Electrochem. Soc., 170, 031503 (2023)

Figure 1