Characterization of the Corrosion Resistance of Composite Peened Aluminum

The base material is a 2-mm-thick flat rolled sheet made of aluminum alloy EN AW-6082 (BIKAR-METALLE—Bad Berleburg, Germany); its chemical composition is depicted in Table 1.

Ceramic particles of alumina of F600 microgrit (Arteka, Backnang–Waldrems, Germany) with a weight-averaged grain size of 9.3 µm ± 1.0 µm, angular shape and median particle size of 10 µm ± 2 µm are used for blasting and as reinforcing material.

Composite peening combines the micro shot peening process with additional heating of the base material to introduce ceramic particles into the surface. The composite peened surface is macroscopically uniform but forms microscopically a hill-valley profile with uneven thickness of the coating. The remaining Al₂O₃-particles are located in dimples on the aluminum surface, while the precipitations of the alloy are indicated by white particles (Fig. 1). The composite peening setup is described in detail by Seitz et al. (Ref 3). Specimens are manufactured at a homologous temperature T/T_s = 0.9 of aluminum, namely 490°C. Slow cooling leaves no residual stress after the peening (Ref 16). The micro shot peening system is operated with a pressure of 7 bar, a nozzle of 0.76 mm in diameter, a working distance of 10 mm and a feed rate of 8 mm/s. A fourfold coverage with a path distance of 1 mm is used.

To modify near-surface parameters, deep rolling is applied with a pressure of 40 bar, a feed rate of 2000 mm/min and trajectories spaced by 0.04 mm from each other. Deep rolling introduces residual compressive stresses of up to −260 MPa into the near-surface regions (Ref 16). Deep rolling reduces the surface roughness from R_Z = 13.7 µm ± 2.0 µm after composite peening to 3.6 µm ± 1.4 µm, while the original roughness is 2.6 µm ± 0.4 µm (Ref 4). Although the surface is more uniform, it is still crossed by valleys. The coating is uneven in thickness but without cracks.

Four states are compared in various experiments: (i) untreated aluminum, (ii) deep rolled aluminum, (iii) composite peened aluminum and (iv) composite peened and subsequently deep rolled aluminum. All the specimens are heat treated to T6 by soft annealing at 540°C, quenched and aged for 6 h at 180°C. The heat treatment is necessary to create comparability of the unpeened and peened specimens. The composite peening is done prior to and deep rolling is done after the heat treatment.

An immersion test, described by Materialprüfanstalt Darmstadt MPA (Ref 17, 18), is applied to evaluate both the corrosion on this alloy and the surface modifications of the composite peening. The solution for this accelerated corrosion test contains the following chemicals and is summarized in Table 2.

Rectangular specimens (11 mm × 9 mm × 2 mm) are cut, cleaned with ethanol in an ultrasonic bath to remove dust and loose particles, weighed with Mettler Toledo ME-T analytical balance (Columbus, Ohio) and immersed in the test solution at 21°C for 120 minutes. After 5 min, 15 min, 45 min and at the end of exposure after 120 min, they are cleaned in ultrasonic bath with deionized water and weighed again. Morphology and depth of the pits are examined by using a μsurf-type confocal microscope by NanoFocus (Oberhausen, Germany). All the specimens are also analyzed macroscopically and in representative cross sections by scanning electron microscope (SEM). The cross sections are analyzed for the surface morphology, pitting depth and intergranular corrosion.

The preparation of the cross sections is challenging due to differences in hardness between the base material and the reinforcement with alumina. A procedure according to Seitz et al. (Ref 19) is used, which applies low load for grinding and polishing to avoid both increased material removal of the matrix and relief formation.

For electrochemical measurements on the material, a potentiodynamic polarization is conducted in 0.5% sodium chloride solution by polarizing with a platinum electrode and measuring with an Ag/AgCl-reference electrode. Specimens of 9 mm ×7.5 mm are mounted in a 3D-printed custom-made electrode holder of polylactide (PLA) and clamped into a corrosion cell. Specimens equilibrate for one hour prior to measuring their corrosion potential (E_CORR) and initiating the scan of ± 200 mV around E_CORR with 2 mV/s. Calculations of E_CORR and corrosion rates are performed according to ASTM G102 (Ref 20).

All four states of the EN AW-6082 alloy are immersed in corrosive solution for 2 h. Figure 2 shows representative macroscopic images of the surfaces after immersion. Both unpeened Al specimens show similar attacks of pit corrosion as shown in Figure 2(a) and Figure 2(b). Their surfaces are less shiny due to superficial corrosion. The composite peened specimens, as shown in Figure 2(c) and Figure 2(d), appeared uncorroded at a first glance, after being cleaned with water. Ultrasonic cleaning after immersion revealed a large extent of corrosion. The composite peened aluminum in Figure 2(c) is affected by only a few pits, which are about tenfold larger in diameter (up to 2 mm) than those of 0.25 mm in the unpeened specimens in Figure 2(a) and Figure 2(b). The surface of the composite peened and subsequently deep rolled specimen lightens up during the corrosion, as shown in Figure 2(d). Pits in this specimen are frequent but smaller compared to the composite peened specimen but significantly larger than in aluminum specimen.

Corroded surfaces were studied in more detail by backscattered electron microscopy (BSE). Comparatively small pits of less than 300 µm in diameter are observed for the untreated states in Figure 3(a) and deep rolled state in Figure 3(b). Both composite peened specimens show pits larger than in the image sections of Figure 3(c) and (d).

Surfaces outside the pits show almost no corrosive damage. However, a crack-like structure is present on the entire surface of all the specimens, which seems to be indicative of intergranular corrosion.

Corrosion can be quantified by measuring the loss of mass as shown in Fig. 4. There is hardly any loss of mass during the first 15 minutes, while after 45 minutes a minor loss of mass is detectable for all the specimens. The deep rolled state remains basically unchanged. After 120 minutes, the composite peened states lost 1% more mass than their corresponding unpeened specimens. Pits reach by then a maximum depth with up to 70 µm. Generally, the composite peened specimens show 20 µm–30 µm deeper pits than the unpeened specimens. Thus, higher loss in mass coincides with deeper pit corrosion.

BSE analyses uncover strong intergranular corrosion in all specimens. Figure 5 shows a representative cross section of all states and a large image of the composite peened specimen. The depth of this intergranular corrosion differs in the four states. The untreated monolithic aluminum shows localized attacks up to 100 µm in depth and the deep rolled specimen up to 160 µm in depth. Both composite peened states are more extensively affected: the only peened specimen with up to 160 µm (60% deeper than untreated aluminum) and subsequently deep rolled specimens with up to 200 µm (25% deeper than in the corresponding state). The alumina layer is detached due to intergranular corrosion with a grain-like structure beneath the surface. The corrosion occurs at the weakest regions, like cracks, hills and thin areas and forms a network with a depth of up to 200 µm approximately. Pits were only found with surrounding intergranular attack.

Potentiodynamic polarization curves are measured to determine the corrosion resistance of the four stages. Due to the geometry of the specimens, commercially available specimen holders cannot be used, and therefore, a customized specimen holder is additively manufactured. It was difficult to design a holder for long-lasting tightness in all the experiments. Therefore, several analyses are performed to obtain results without short circuit. Figure 6 shows the recorded potentiodynamic polarization curves, which have the following aspects in common. Firstly, the order for E_CORR (untreated with the most negative and composite peened/deep rolled AI with the least negative potential, see Fig. 6) is identical. Secondly, the composite peened specimens are shifted to higher negative values than the nobler Al specimens (E_CORR of  −0.924 V for the composite peened specimen versus  −0.779 V for the substrate). Thirdly, the deep rolled states have less negative E_CORR than the corresponding states. Lastly, i_CORR values are lower for both untreated AI and composite peened AI than for the corresponding deep rolled states. Calculated values for corrosion current density (i_CORR) and corrosion rate (CR) are shown in Table 3. In addition, a significantly steep slope on the anodic branch of the polarization curves is observed with unpeened specimens ascending more than a decade higher in current density before flattening. This is in contrast with the symmetrical branches of the peened specimens, which show no sign for pitting corrosion.