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Journal of Chinese Society for Corrosion and protection  2016, Vol. 36 Issue (2): 121-129    DOI: 10.11902/1005.4537.2015.051
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Effect of Trace Chloride and Temperature on Electrochemical Corrosion Behavior of 7150-T76 Al Alloy
Qingqing SUN1,2,3,Wenhui ZHOU1,Yuehuang XIE2,Pengxuan DONG2,Kanghua CHEN2,Qiyuan CHEN1()
1. Key Laboratory of Resource Chemistry of Nonferrous Metals, Ministry of Education, Central South University, Changsha 410083, China
2. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
3. School of Chemical Engineering, Purdue University, West Lafayette 47906, USA
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Abstract  

3.5%(mass fraction) NaCl has been intensively used as an electrolyte for evaluating the corrosion behavior Al-alloys for decades due to the simulated sea water being close to the real service environment, particularly for the carrier-based aircrafts. But this medium has several limitations such as the lack of acid substances and the absence of pitting potentials in polarization curves for some alloys. According to Arrhenius equation, the corrosion rate is promoted by temperature. In fact, one of the most crucial factors in the corrosion of Al alloys is temperature which has a strong impact on the stability and properties of passive films. In the present work, the influence of trace Cl- and temperature on the electrochemical corrosion of 7150-T76 Al alloy was investigated by measurements of open circuit potentials (OCP) and cyclic polarization curves as well as observation of corrosion morphology. The results showed that the localized corrosion of 7150 Al alloy was promoted by the increase of Cl- concentration and temperature. In the solutions with relatively low temperature and low chloride concentration, pitting corrosion was the main corrosion form, while for higher temperature and higher chloride concentration, the corrosion of the alloy gradually turned to be intergranular corrosion. The presence of pit transition potential Eptp reflects a step-wise propagation of corrosion on very narrow fronts with a sequence of active tip-passive walls. OCP shifts to the negative direction with the increase of Cl- concentration and temperature, respectively. Moreover, OCP decreased drastically with increasing temperature above 60 ℃, indicating the chan-ge of corrosion mechanism in the temperature range from 60 to 70 ℃. In the medium of 0.1 mol/L Na2SO4+1 mmol/L NaCl, the free corrosion potential decreased, while the free corrosion current density increased firstly and then decreased with the increase of temperature due to less dissolved oxygen at higher temperatures. Differences between potential parameters such as ΔE1(Epit-Ecorr), ΔE2 (Ecorr-Eptp) and ΔE3(Ecorr-Erep) were determined as criteria to assess localized corrosion. The value of ΔE3(Ecorr-Erep) increased linearly with the increase of Cl- concentration. However, ΔE3 showed a turning point at 70 ℃ because uniform corrosion occurred at 80 ℃ as deduced from corrosion morphology. It can be concluded that ΔE3(Ecorr-Erep) only increases linearly with the corrosion propagation at the first stage of localized corrosion.

Key words:  7150 Al alloy      trace Cl-      temperature      cyclic polarization      potential parameter     

Cite this article: 

Qingqing SUN,Wenhui ZHOU,Yuehuang XIE,Pengxuan DONG,Kanghua CHEN,Qiyuan CHEN. Effect of Trace Chloride and Temperature on Electrochemical Corrosion Behavior of 7150-T76 Al Alloy. Journal of Chinese Society for Corrosion and protection, 2016, 36(2): 121-129.

URL: 

https://www.jcscp.org/EN/10.11902/1005.4537.2015.051     OR     https://www.jcscp.org/EN/Y2016/V36/I2/121

Fig.1  OCP vs time curves of 7150AA in electrolyte solutions containing different Cl- concentrations
Fig.2  Cyclic polarization curves of 7150AA in 3.5%NaCl solution
Fig.3  Cyclic polarization curves of 7150AA as a function of Cl- concentration
Fig.4  Corrosion current densities Icorr and Irep (a), limit current density at reverse potential (b), current densities corresponding to Epit and Eptp (c), and linear polarization resistances Rcorr and Rrep (d) of 7150AA as a function of Cl- concentration
Fig.5  Ecorr (a), Epit (b), Eptp (c), and Erep (d) of cyclic polarization curves as a function of Cl- concentration
Fig.6  ΔE1 (a), ΔE2 (b) and ΔE3 (c) as a function of Cl- concentration
Fig.7  Corrosion morphologies of Al alloy specimens after cyclic polarization in electrolyte solutions with 0 mmol/L (a), 20 mmol/L (b), 50 mmol/L (c) and 100 mmol/L (d) Cl-
Fig.8  OCP vs time curves of 7150AA in 0.1 mol/L Na2SO4+1 mmol/L NaCl solution at different temperatures
Fig.9  Cyclic polarization curves of 7150AA in 0.1 mol/L Na2SO4+1 mmol/L NaCl solution at 30 ℃ (a), 40 ℃ (b), 50 ℃ (c), 60 ℃ (d), 70 ℃ (e) and 80 ℃ (f)
Fig.10  Icorr and Irep (a), Irev (b) and Rcorr and Rrep (c) of 7150AA as a function of temperature
Fig.11  Ecorr (a), Erep (b) and ΔE3 (c) of cyclic polarization curves as a function of temperature
Fig.12  Corrosion morphologies of 7150AA after cyclic polarization in 0.1 mol/L Na2SO4+1 mmol/L NaCl solution at 30 ℃ (a, a'), 40 ℃ (b), 50 ℃ (c), 60 ℃ (d), 70 ℃ (e, e') and 80 ℃ (f)
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