Materials Performance

DEC 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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Page 44 of 76

42 DECEMBER 2018 W W W.MATERIALSPERFORMANCE.COM alloy changes with changing the tempera- ture and aging states, but the extent of change is very small. However, the corro- sion current increases with increasing tem- perature under the same aging treatment, while the corrosion current under the tem- per of PA is the highest, that of DPA is mod- erate, and that of RRA is the lowest at the same temperature. The corrosion rate is consistent with the change tendency of corrosion current. Therefore, it can be con- FIGURE 2 Typical EIS of AA7003 under the temper of DPA in NaCl solution at different temperatures. FIGURE 3 Equivalent circuit diagram of AA7003 in NaCl solution. cluded from the polarization curves that th e c or ro si on re si st a n c e of th e al l o y decreases with increasing temperature and increases with the aging treatments PA, DPA, and RRA in turn. Electrochemical Impedance Spectroscopy Figure 2 shows the typical EIS of AA7003 in NaCl solution at different temperatures under the temper of DPA. It shows that the capacitance arc radius decreases with increasing temperature, indicating that the electroch emical corrosion of th e alloy becomes greater with the temperature increasing. The EIS is composed of a high- frequency capacitive reactance and a low- frequency inductive reactance. When the temperature is 30 and 50 °C, the reactance almost does not exist. Yu, et al. 15 considered that the presence of reactance indicates the initiation of corrosion in its initial stage. Note the pitting corrosion of alloys at 30 and 50 °C is not serious in the initial stage of corrosion. Figure 3 shows the equivalent circuit diagram, where R s is the solution resis- tance, and R t, and CPE d1 (constant phase element of the original) are the charge transfer resistance and electric double- l ayer cap a c itors . Th e s e w ere u s ed to describe the electrochemical impedance spectroscopy in the capacitive arc. R L and L describe the inductive arc. The curve fitting values are given in Table 2. R S decreases with increasing temperature, indicating t h a t t h e c o n c e n t ra t i o n o f c o r r o s i o n medium increases with increasing temper- ature , an d th e re si st an c e of s o lution decreases with increasing electrochemical corrosion rate in the solution. R P decreases with increasing t emp erature, w hich i s because the temperature increase pro- motes the movement of anions in the solu- tion and accelerates the erosion of the matrix, hence R P decreasing. Morphologies of Corrosion Attack Figure 4 shows the corrosion surfaces of AA7003 under PA in 3.5 wt% NaCl solution at different temperatures for 24 h. It can be seen that the corrosion type of the alloy at different temperatures is the same, and the surface corrosion is formed due to the high temperature of the solution. When the tem- perature was 30 °C, the surface of the speci- men was covered with corrosion products, but the corrosion cracks were almost invisi- ble on the surface. When the temperature was raised to 50 °C, the corrosion products were also covered with the matrix surface, and there were few corrosion cracks. When the temperature was raised to 70 °C, there were many corrosion cracks and corrosion products. These cracks are all caused by active pits. First, the Cl – in corrosion solu- MATERIALS SELECTION & DESIGN

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