Analysis of corrosion behaviour of aluminium alloy AA6012 samples processed by ECAP and cryogenic treatment [Analisi del comportamento a corrosione di campioni di alluminio AA6012 sottoposti a ECAP e trattamento criogenico]

Severe Plastic Deformations (SPD) have been used in order to obtain ultrafine-grained alloys improved in strength and wear resistance with an adequate ductility. Equal Channel Angular Pressing (ECAP) is one of these techniques that consists in pressing through a die, with two intersecting channels, equal in cross section, a sample that is forced to deform by shearing at the intersection of these channels. The sample retains the same cross-sectional area after pressing, so that it is possible to repeat the pressing several times. Modifications of the alloys microstructure produced by ECAP significantly influence their corrosion behaviour. In this work, the corrosion behaviour of the aluminium alloy AA6012 samples solutioned at 550°C/6h and then pressed through ECAP technique was investigated; some of these samples were submitted to a cryogenic treatment before being processed by ECAP. With more details, the samples examined in this work are as received extruded billets (TQ), solutioned at 550 °C/6h (TT), processed only one pass through ECAP (TT-ECAP 1) and submitted to cryo treatment before ECAP (TT-cryo ECAP 1). The samples TT-ECAP 1 were pressed into the ECAP die at room temperature until one pass. The cryogenic treated billets (TT-cryo ECAP 1) were dipped into liquid nitrogen for at least 1 minute immediately before being introduced into the ECAP die. During shear deformation, the cryotreated billets never reached room temperature. The evolution of microstructure was studied by means of optical microscope and TEM observations. The analysis of corrosion behaviour has been carried out at room temperature by means of electrochemical characterization in two different aqueous solutions at the same chloride concentrations (0.1M NaCl): a deareated solution at pH= 2 (solution 1) and an aerated solution at pH= 6.5 (solution 2). For this purpose, polarization resistance, potentiodynamic polarization and electrochemical impedance spectroscopy tests were performed. A three-electrode cell was used with a saturated calomel electrode (SCE, +0.241 V vs NHE) as a reference and two short-circuited platinum sheets as counter, all connected to a Gamry Reference 600 potentiostat. Polarization resistance measurements were carried out from Ecorr- 5 mV to Ecorr + 5 mV, Tafiel plot from Ecorr - 150 mV to Ecorr +150 mV and potentiodynamic polarizations, anodic and cathodic separated tests, from the corrosion potential (scan rate 0.166 mV/s). Electrochemical Impedance Spectroscopy (EIS) measurements were performed with an AC signal amplitude of 5 mV rms, 5 points/decade and a frequency range of 100 kHz- 10 mHz. Fig. 1 shows grain refinement of the AA6012 samples processed by ECAP (TT-ECAP 1 and TT-cryo ECAP 1) with respect to TT samples. Fig. 2 illustrates representative Tafel curves for each sample examined in deareated solution 1 (pH=2). All the samples showed active corrosion. TT and TT-cryo ECAP 1 samples present lower corrosion rate (icorr) than TQ and TT-ECAP 1 (symbols in Fig. 2). Polarization resistance results (Fig. 3) confirm the better corrosion behaviour of TT sample and the improvement in corrosion resistance of TT-cryo ECAP 1 samples with respect to TT-ECAP 1 ones in solution 1 (pH=2). Corrosion behavior in solution 2 (pH= 6.5) is mainly governed by oxygen diffusion and only limited differences are visible in Fig. 4 between the analyzed samples. Therefore, EIS measurements were carried out in order to obtain more information about the system under examination, which typically presented the impedance response shown in Fig. 5. Through the analysis of this response by the equivalent circuit of Fig. 6, the charge transfer resistance has been determined for all samples (Fig. 7). These results confirm the better corrosion behaviour of TT samples, the worsening of ECAPed samples and the improvement of the corrosion resistance given by cryogenic treatment before ECAP. In conclusion, solutioned samples (TT) show a better corrosion behaviour than as received (TQ) and ECAPed ones. Although severe plastic deformations worsen the corrosion behaviour of solutioned samples, cryogenic treatment before ECAP avoids the loss of the corrosion resistance of AA6012 alloy due to ECAP.