Abstract
The bimodal microstructures of Al6063 consisting of 15, 30, and 45 vol. % coarse-grained (CG) bands within the ultrafine-grained (UFG) matrix were synthesized via blending of high-energy mechanically milled powders with unmilled powders followed by hot powder extrusion. The corrosion behavior of the bimodal specimens was assessed by means of polarization, steady-state cyclic polarization and impedance tests, whereas their microstructural features and corrosion products were examined using optical microscopy (OM), scanning transmission electron microscopy (STEM), field emission scanning electron microscopy (FE-SEM), electron backscattered diffraction (EBSD), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) techniques. The bimodal Al6063 containing 15 vol. % CG phase exhibits the highest corrosion resistance among the bimodal microstructures and even superior electrochemical behavior compared with the plain UFG and CG materials in the 3.5% NaCl solution. The enhanced corrosion resistance is attributed to the optimum cathode to anode surface area ratio that gives rise to the formation of an effective galvanic couple between CG areas and the UFG matrix. The operational galvanic coupling leads to the domination of a “self-anodic protection system” on bimodal microstructure and consequently forms a uniform thick protective passive layer over it. In contrast, the 45 vol. % CG bimodal specimen shows the least corrosion resistance due to the catastrophic galvanic corrosion in UFG regions. The observed results for UFG Al6063 suggest that metallurgical tailoring of the grain structure in terms of bimodal microstructures leads to simultaneous enhancement in the electrochemical behavior and mechanical properties of passivable alloys that are usually inversely correlated. The mechanism of self-anodic protection for passivable metals with bimodal microstructures is discussed here for the first time.
Highlights
In recent years, ultrafine-grained (UFG) Al alloys have received incredible attention owing to their excellent mechanical and corrosion properties [1]
We investigate the corrosion behavior of Al6063 with bimodal grain-size distributions by means of electrochemical impedance spectroscopy (EIS) and polarization tests
The schematic illustration of a bimodal microstructure is shown in Figure 1f, which shows the white CG regions elongated through extrusion direction within the dark UFG matrix
Summary
Ultrafine-grained (UFG) Al alloys have received incredible attention owing to their excellent mechanical and corrosion properties [1]. The results of previous investigations clarify that UFG Al alloys show superior resistance to stress corrosion cracking (SCC). Pitting, decreased cathodic kinetics, and higher resistance in mass-loss testing compared with their coarse-grained (CG) counterparts [7,10,11]. The improved corrosion resistance of UFG materials is attributed to their high affinity to passivation resulting from a high density of grain boundaries. Increased homogeneous surface energy distribution of UFG materials has been proposed as the main source for the enhanced resistance to pitting [12]. A heightened pit initiation resistance has been reported for UFG Al5052 [13], Al5083 [6], Al6082 [14] subjected to the short-term corrosion testing
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