Materials Performance

NOV 2017

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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60 NOVEMBER 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 11 CHEMICAL TREATMENT degreased in acetone and dehydrated etha- nol by ultrasonic sound, and then air dried. Electrochemical tests were conducted in a conventional three-electrode cell: a satu- rated calomel electrode (SCE) as the refer- ence electrode, a platinum plate as the counter electrode, and the specimen as the working electrode. Potential dynamic scan- ning was performed at a rate of 5 mV/s in a 3.5 wt% sodium chloride (NaCl) solution. The potentials ranged from –1.4 to –0.2 V vs. SCE. Results and Discussion Hydrogen Concentration Figure 1 presents the hydrogen concen- tration for various aging states of 7050 alu- minum alloy under dif ferent hydrogen charging times. The hydrogen concentra- tion increased with increasing charging tim e. Th e aging stat es inf lu enced th e hydrogen concentration under the same chargin g c onditions—th e al loy in th e un deraged state had the largest hydrogen concentration, the alloy in the over-aged state had the smallest hydrogen concentra- tion, and the hydrogen concentration for the alloy in the peak-aged state was in the middle of the other two. Th e p e ak- a ge d st at e re fers to th e mechanical properties (tensile strength) of the material. Song, et al. 14-15 found that with increased aging, the concentration of Mg s e g r e g a t i o n a t t h e g r a i n b o u n d a r y decreased. According to the phase transfor- mation-Mg-H theory, the interatomic affin- ity between Mg and H atoms is greater than that of Al and H atoms; thus, the increased concentration of Mg segregation at the grain boundary led to an increase of hydro- gen content in charged specimens. Effect of Hydrogen Charging on Electrochemical Corrosion Th e p o t entio dy n ami c p o l ari z ation curves of hydrogen-charged 7050 alumi- num alloys in 3.5 wt % NaCl solution are presented in Figure 2, and the fitting results of the polarization cur ves are shown in Table 1. This combined information illus- trates that under the same aging condi- tions, the alloy 's free corrosion potential (E corr vs. SCE) became less electronegative with increased charging time while the cor- TABLE 1. FITTING RESULTS OF POLARIZATION CURVES OF 7050 ALUMINUM ALLOY UNDER VARIOUS AGING STATES Aging States Charging Time (h) Free Corrosion Potential (E corr )/V (vs. SCE) Corrosion CD (I corr )/(A·cm –2 ) Corrosion Rate (mm/A) Underaged 0 –1.065 19.42 × 10 –6 211.58 × 10 –3 6 –1.087 44.71 × 10 –6 487.03 × 10 –3 12 –1.095 53.74 × 10 –6 585.37 × 10 –3 18 –1.129 76.97 × 10 –6 838.51 × 10 –3 24 –1.143 92.92 × 10 –6 1,012.2 × 10 –3 Peak-aged 0 –1.072 3.11 × 10 –6 33.915 × 10 –3 6 –1.092 12.69 × 10 –6 138.27 × 10 –3 12 –1.110 48.93 × 10 –6 533.03 × 10 –3 18 –1.120 51.29 × 10 –6 558.68 × 10 –3 24 –1.138 58.60 × 10 –6 638.31 × 10 –3 Over-aged 0 –0.893 1.34 × 10 –6 14.574 × 10 –3 6 –0.925 9.94 × 10 –6 108.3 × 10 –3 12 –0.955 22.01 × 10 –6 239.78 × 10 –3 18 –1.005 34.87 × 10 –6 379.8 × 10 –3 24 –1.031 51.66 × 10 –6 562.78 × 10 –3 FIGURE 3 Relationship between the alloy's corrosion rate and hydrogen concentration.

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