Abstract
This study employs linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and immersion test for weight loss to investigate the corrosion behavior of several anticorrosive alloys, and uses SEM, EDS, XRD, ESCA, and ICP-MS to observe and analyze their microstructure and corrosion morphology, composition of the inert film, and dissolving amount of elements in the immersion solution so as to compare corrosion mechanism of high-entropy AlxCrFe1.5MnNi0.5MoyNbz with that of conventional stainless alloys and to understand the influence of elemental individuality and whole integrity property of alloys on the corrosion behavior.It is seen that in the LSV study, the corrosion current density of both the conventional and the high-entropy in sulfuric acid is greater than in the sodium chloride by about 100 times, whereas the transpassive potential of the two alloys is nearly the same; indicates that for both alloys the corrosion in sulfuric acid is a homogeneous type, while that in sodium chloride is mainly a local pitting. The corrosion resistance of alloys in sulfuric acid is principally influenced by Cr and Ni; the more amount in the alloy, the more corrosion resistance for the alloy. The polarization curves of the alloys in sodium chloride is much unstable (fluctuates) as compared to that in sulfuric acid in that formation of metastable pits competes the recovery of the passive film. The transpassive potential in sodium chloride is much lower than in sulfuric acid; is because of the high pitting ability in the former solution. The more amount the Cr and Mo in the alloys, the higher the transpassive potential of the alloys. EIS analyses show that the growth of the passive film in the high-entropy is slower than in the conventional; is believed to be due to that formation of Al- and Mn-oxides on the surface of the former alloys impedes the growth of a densified passive film on the alloy surface. From the immersion test, it is seen that the corrosion resistance of alloys is obviously affected by Mo because of the rapid formation of passive film by the presence of Mo.It shows from ICP-MS analysis that total dissolution of elements for the Mo-containing alloys, whatever the high-entropy or the low-entropy (the conventional), is 100 times less than that for the Mo-free ones. The Cr dissolution for the Mo-containing alloys is also less than that for the Mo-free ones. Analyses from ESCA data suggest that formation of Cr2O3 and Cr(OH)3, of oxides or complexes of Mo4+ and Mo6+, and of NiO and Ni(OH)2 for Cr, Mo, and Ni, respectively, are principally responsible to anticorrosion for the alloys.The corrosion resistance of the high-entropy alloys is inferior to that of the conventional. Two factors are responsible. One is the property of the element itself in the alloys, i.e., element individuality; the other is the whole property of alloy, i.e., alloy integrationality or cocktail property. Since the high-entropy alloys contain chemically active Al and Mn, and the formed passive film is not dense, hence they are readily corrosive. On the other hand, the stainless steels compose only one or two solid solution(s), whereas the high-entropy alloys, in addition to solid solution(s), have a large amount of corrosive nano-scaled Ni- and Al-rich phase and ρ phase. Therefore, the high-entropy alloys are less corrosion resistant than the conventional.In summary, it is preliminarily concluded that corrosion behavior of materials, especially alloys, depends on the subjective or the intrinsic property of materials themselves and also on the objective or environment conditions such as temperature, pressure, and/or corrosive medium. The corrosion behavior of a material in a specific corrosion environment is a resultant result of the interaction of the material and the environment. The corrosion relation between a material and an environment is a part-and counterpart relationship or a spear-and-shield one; they are of one against the other. There is always no superior or inferior side for each of both sides. It is concluded that the intensity of corrosion resistance for a material or an alloy is always referred to a specific environment only. Based on the electrochemical theory, it is concluded that a single-phase material is more corrosion-resistant than a multi-phase one because of the existence of the local-cell effect for the latter material; the single-phase solid solution that contains specific composition would have superior corrosion property against some specific environment, as was shown in this study. One can thus conclude that the high-entropy effect enlarges the solubility of elements in alloys, enables to obtain a single homogeneous solid-solution phase, and therefore, favors the anticorrosion property. In addition, for good anticorrosion property against a specific environment, this single solid solution must contain suitable composition. The above requirements have to fulfill so that the alloys are really corrosion-resistant. Finally, as to the corrosion resistance issue for high-entropy alloys, one may conclude that the cocktail or collective property from the high-entropy effect is referred to the appearance of a single homogeneous phase, whereas the individuality of each element in the high-entropy alloys appears still in its own anticorrosion property.