Microstructure And Properties Of Micro-alloyed Aluminum Based Anode Materials

Al-Zn-In series alloys are the most promising sacrificial anode materials with better electrochemical properties; corrosion product falls off easy and the alloys show great potential in anti-corrosion industry. But theirs electrochemical properties and nu-uniform surface dissolution need to be improved and no effective solution so far. RE element is preferred to refine aluminum alloy, manganese element can purify effectively the iron impuritity of aluminum alloy, and aluminum alloy containing silicon element with excellent casting performance. Therefore, it is an effective way to solve the above problems by microalloying with adding appropriate content rare earth element to refinement microstructure, adding suitable content Mn element to reduce the adverse effect of impurity element, and adding suitable content Si element to reduce the casting defects.In this paper, based on the composition design, the microalloying Al-Zn-In-Mg-Ti alloys with appropriate content of Ce, Mn and Si elements achieve the expected goal. The alloys have excellent integratation electrochemical properties by tested of Shanghai research institute of materials. The current efficiency of the alloys adding 0.5Ce and 0.1 Si element are 94% and 93%, respectively, with uniformity dissolution. The performances of the alloys are higher than the current sacrificial anode materials used in engineering. So the both alloys have good industrial applications perspective.The microstructure characteristic of multi-microalloying of Al-Zn-In-Mg-Ti alloy is distinctness increased precipitates. It is a fact that the electrochemical properties of Al-Zn-In-Mg-Ti alloy are higher than that of Al-Zn-In alloy. Thus the precipitates have an important effect on the electrochemical properties of the alloy. So the corrosion mechanism of the multi-microalloying anode material can't simply adopt the old corrosion mechanism of simple alloy.Accordingly, the major precipitates phases of the alloy were determined using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Then the corrosion behavior of the alloy was studied by different techniques, such as polarization curves, electrochemistry impedance spectrum (EIS) and so on, combined with the electrochemical noise (EN), based on this to reveal the dissolution mechanism of the alloy. The results show that all the alloys contain MgZn2 precipitates phase, MgZn2 precipitates phase is anode for the a-Al with the advantages of activation the alloy and will not cause the alloy self-corrosion. This ensures that the alloys have a higher current efficiency, but with un-uniformity dissolution lead in the large dendrites and un-uniformity MgZn2 precipitates of the cast alloy.The current efficiency and the un-uniformity dissolution of the alloy adding 0.5Ce were improved resulted in formation of the anode Al2CeZn2 precipitates which refine the grain of the alloy apparently, also act as the activation centers of the alloy. The grain of the alloy adding 0.5Mn was distinctness refine resulted in formation of the anode Al6Mn precipitates. The effect of grain refinement and activation with Mn element is lower than that of the rare earth Ce element. Therefore, the improvement effect of Mn element on the integration performance of the alloy is inconspicuous than that of Ce element. The alloy adding 0.1% Si shown a good stability integrated electrochemical properties. But the improving properties of the alloy are not because of the role of precipitates, mainly because of Si element can increase the casting performance, reduce casting defects, and make the micro structure uniform of the alloy.The dissolution mechanism of the alloy was investigated by the corrosion morphology, electrochemical impedance spectroscopy and electrochemical noise testing in 3.5% NaCl solution at different immersion time. The results show that the initial dissolution of the alloy is caused by pitting corrosion, and pitting get along with precipitates. The activation mechanism of the precipitates phase in the alloy can not be explained by the dissolved-redeposition, so must combine with the mechanism that the priority dissolution of precipitates phase. The EN and the wavelet analysis of the alloy in the early corrosion also verify that the noise energy of the metastable and steady-state pitting take the highest share in the wavelet analysis of the energy bands. At this time, the inductance loop of the EIS in low frequency also correspond with the characterization of the relevant literature to explain the passivation of metal at pitting stage.The mud pits structure appeared in the grain of the alloy in addition to the pitting corrosion continue to expand with the increase of immersion time. It can be detected the trace quantities In elements in the surface of the alloy which could not be detected in the cast alloy by SEM and EDX analysis. The In quantities of mud pits location were significantly higher than that of the rest position in the alloy. It was indicated that the In elements of the alloy has occurred dissolved-redeposition. The EN and the wavelet analysis of the alloy in the corrosion stage also verify that the noise energy of the "dissolved-redeposition" take the highest share in the wavelet analysis of the energy bands. The inductance loop of the EIS at high-frequency which characteriza the pitting and the second capacitive loop which characteriza the dissolved-redeposition also indicated that the corrosion controlled by the pitting and dissolved-redeposition at this corrosion stage. With further increase of the corrosion time, the pittings and the mud structure around the pittings were connected, and finally the entire alloy surface uniformly dissolved. Dissolved-redeposition took place in the entire surface of the alloy which is the major corrosion form at the later dissolution stage. The EN and the wavelet analysis of the alloy in the uniform corrosion stage also confirmed that the noise energy of the dissolved-redeposition with low frequency and regularity take the highest share in the wavelet analysis of the energy bands. The capacitive loop with gradual increase in the low frequency which characteriza the dissolved-redeposition also indicated that the corrosion controlled by the dissolved-redeposition.Therefore, the corrosion of Al-5Zn-0.03In-1Mg-0.05Ti series anode alloy in NaCl solution includes the following three stages:1. the pitting stage resulted in the precipitates phases in the grain boundary at the initially corrosion stages; 2. Subsequently, the alloy dissolve both with the pitting and the dissolved-redeposition by Zn+,In+ internal grain; 3. The dissolution is in mainly uniform corrosion by Zn+,In+ with dissolved-redeposition at the later corrosion stage.The innovation of the article lies in developed two kinds high-performance anode material of Al-5Zn-0.02In-1Mg-0.05Ti-(0.5Ce, 0.1Si) alloys. By identified, the performance of the alloys is higher than the currently materials used engineering. The anode precipitate phases of the alloys cause the alloy pitting and activate the alloy. The corrosion of the materials cause by pitting and dissolved-redeposition together. The corrosion process includes three stages of pitting, pitting and dissolved-redeposition, uniform corrosion.