Corrosion behavior of Al 7075, Al 2024, and Al 6061 in the Red Sea water was studied using weight loss (WL) measurements and potentiodynamic polarization (PDP) technique. The corrosion patterns and corrosion products formed on Al alloys were characterized using optical photography (OP), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The results showed that WL data were consistent with bimodal model rather than the power law function and the corrosion rates exhibit a continuous decrease with exposure time. The increasing order of the Red Sea corrosivity on the studied Al alloys can be given as follows: Al 6061 < Al 2024 < Al 7075. The results of temperature effect revealed that an increase in temperature resulted in an increase in both anodic and cathodic current density and a decrease in corrosion potential. Al 7075 was less influenced by temperature than the other alloys. Pitting corrosion was the predominant corrosion pattern detected on all Al alloy surfaces after prolonged immersion in the Red Sea water. The appearance of S peak in EDS spectra of Al 7075 after corrosion gives an indication of the contribution of bacteria in the corrosion process.
Desirable properties of aluminum such as low density (only 2.7 g/cm3), recyclability, thermal and electrical conductivity, and, in some degree, corrosion resistance, make aluminum the most consumed nonferrous metal in the world [
Corrosion is defined by ISO as “physicochemical interaction (usually of an electrochemical nature) between a metal and its environment which results in changes in the properties of the metal and which often leads to impairment of the function of the metal, the environment, or the technical system of which these form a part” [
Various environmental factors such as dissolved gases, temperature, pH, and micro- and macroorganisms can affect corrosion in seawater [
Using three different aluminum alloys (Al 7075, Al 2024, and Al 6061), the present research studied the Red Sea as a corrosive environment. Factors such as immersion time and temperatures on the corrosion behavior of aluminum alloys in seawater were studied using weight loss (WL) and potentiodynamic polarization (PDP) measurements. Change in aluminum alloy surfaces was detected at different conditions using optical photography (OP), scanning electron microscope (SEM), and electron-dispersive energy spectroscopy (EDS).
Three different types of aluminum alloys were used in this study. The chemical composition of aluminum alloys is given in Table
Percentage composition of the studied Al alloys.
Alloy | Cr | Cu | Fe | Mg | Mn | Si | Ti | Zn | Zr | Al |
---|---|---|---|---|---|---|---|---|---|---|
7075 | 0.18–0.28 | 1.2–2.0 | 0.5 | 2.1–2.9 | 0.3 | 0.4 | 0.2 | 5.1–6.1 | 0.25 | Remainder |
2024 | 0.10 | 3.8–4.9 | 0.5 | 1.2–1.8 | 0.3–0.9 | 0.5 | 0.15 | 0.25 | - | Remainder |
6061 | 0.04–0.35 | 0.15–0.4 | 0.7 | 0.8–1.2 | 0.15 | 0.4–0.8 | 0.15 | 0.25 | - | Remainder |
Polished and preweighed Al specimens were placed in airtight glass containers containing 60 mL of seawater for different immersion periods (1, 2, 3, 5, 7, 10, 22, and 32 weeks) at an ambient temperature (laboratory temperature was 21 ± 1°C) under stagnant conditions. To facilitate the identification of specimens immersed in the seawater at different conditions, numbered stickers were used in each case (Figure
The final arrangement for weight loss experiments.
Potentiodynamic polarization measurements were performed in a three-electrode cell. A cylinder specimen of Al alloys was used as working electrode and was embedded in a Teflon holder using epoxy resin, giving an exposed area of 0.699 to 1.269 cm2. Platinum mesh was used as the counter electrode and silver/silver chloride (Ag/AgCl(s)/KCl saturated (aq)) was used as the reference electrode. Potentiodynamic polarization measurements were done using ACM Gill AC Potentiostat/Galvanostat model 1649 connected to a personal computer. Prior to each experiment, the working electrode was treated as described in the weight loss method and then dipped in the test solution. After reaching a steady-state potential, the potentiodynamic curves were carried out by changing, linearly, the electrode potential from the starting potential (−1200 mV) with respect to the reference electrode towards a less negative direction with the required scan rate (1 mV/s) till the end of the experiment at −200 mV. All the electrochemical measurements were done twice at different conditions. Corrosion current densities were determined by extrapolation of cathodic Tafel line to corrosion potential using the ACM Gill software.
Optical photographs of Al alloy specimens were taken after the chemical experiments, to evaluate gross changes in the metal surface and to perform a cursory evaluation of the forms of corrosion (e.g., general, pitting) at different conditions. Optical photographs were taken using VMS-004 USB microscope.
The scanning electron microscopy (SEM) associated with the energy-dispersive X-ray spectroscopy (EDS) was used to investigate the surface morphology of aluminum alloys and analyze the elements on the surfaces of alloys before and after immersion in seawater for five weeks.
Table
Weight loss and corrosion rates of Al alloys at different immersion periods in the Red Sea water.
Immersion period (weeks) | Weight loss (g m−2) | Corrosion rate, | ||||
---|---|---|---|---|---|---|
Al 7075 | Al 2024 | Al 6061 | Al 7075 | Al 2024 | Al 6061 | |
1st | 0.733 | 0.463 | 0.312 | 38.184 | 24.111 | 16.247 |
2nd | 1.227 | 0.781 | 0.539 | 31.956 | 20.328 | 14.034 |
3rd | 1.685 | 0.820 | 0.648 | 29.313 | 14.269 | 11.268 |
5th | 2.284 | 1.721 | 0.997 | 23.813 | 17.950 | 10.392 |
7th | 2.366 | 1.891 | 1.094 | 17.630 | 14.088 | 8.150 |
10th | 2.396 | 1.994 | 1.147 | 12.490 | 10.394 | 5.981 |
22nd | 3.826 | 2.567 | 1.663 | 9.069 | 6.084 | 3.942 |
32nd | 4.173 | 3.241 | 1.880 | 6.796 | 5.279 | 3.062 |
Dependence of weight loss and corrosion rates of Al alloys in the Red Sea water on immersion time.
Visual images of Al 7075 specimens after immersion in the Red Sea water at different time intervals.
Visual images of Al 2024 specimens after immersion in the Red Sea water at different time intervals.
Visual images of Al 6061 specimens after immersion in the Red Sea water at different time intervals.
Most of the previous data sets in corrosion studies appeared to fit the power law function (see (
Corrosion kinetic parameters for Al alloys in the Red Sea water.
Kinetic parameters | Al 7075 | Al 2024 | Al 6061 |
---|---|---|---|
| 12.505 | 9.029 | 5.136 |
| 0.71 | 0.763 | 0.706 |
| 0.420 | 0.326 | 0.355 |
| 0.997 | 0.920 | 0.992 |
| 0.952 | 0.964 | 0.978 |
By examining the data obtained in Table The values of weight loss after one year ( The value of When When When
The reason for this observation can be stated after considering the proposed mechanism of the bimodal behavior.
Schematic bimodal model for long-term corrosion loss and pit depth in marine environments. The change from mode 1 with predominantly oxic corrosion conditions to mode 2 with predominantly anoxic conditions occurs at
Anodic curves can present useful information on passivation or depassivation of a metal dipped in solutions containing aggressive ions [ The values of Figure
Potentiodynamic polarization parameters of Al alloys in the Red Sea water at different immersion periods.
Time (hours) | Al 7075 | Al 2024 | Al 6061 | |||
---|---|---|---|---|---|---|
| | | | | | |
2 | 732 | 5.78 | 842 | 3.20 | 814 | 1.38 |
24 | 731 | 6.29 | 757 | 4.31 | 773 | 2.28 |
72 | 735 | 2.14 | 777 | 1.56 | 855 | 1.28 |
168 | 806 | 1.64 | 777 | 1.19 | 749 | 0.27 |
Potentiodynamic polarization curves of Al alloys in the Red Sea water at different time interval.
Variation of (a)
Polarization curves of Al 7075, Al 2024, and Al 6061 immersed in the Red Sea water for 2 h were recorded as a function of temperature and are presented in Figure
Corrosion potential
Temperature (°C) | Al 7075 | Al 2024 | Al 6061 | |||
---|---|---|---|---|---|---|
| | | | | | |
10 | 715 | - | 797 | 708 | 781 | 700 |
25 | 732 | - | 842 | 708 | 814 | 695 |
45 | 793 | 691 | 909 | 704 | 978 | 689 |
60 | 956 | 685 | 968 | 677 | 1035 | 628 |
Potentiodynamic polarization curves of Al alloys in the Red Sea water at different temperatures.
The obtained results can be interpreted as follows: It is shown that lowering corrosion potentials was accompanied by raising breakdown potential Al 7075 experienced the highest current density followed by Al 2024 and then Al 6061 at all temperatures with the exception of 60°C; Al 2024 exhibited higher corrosion current than Al 7075. Figure
Dependence of (a)
Potentiodynamic polarization curves of Al alloys in the Red Sea water at 25°C and 60°C temperature.
Figure
SEM images of Al 7075, Al 2024, and Al 6061 “A” before and “B” after being immersed in the Red Sea water for 5 weeks. The arrows indicate the pits.
For EDS results represented in Figures
EDS results of Al 7075 “A” before and “B” after being immersed in the Red Sea water for 5 weeks.
EDS results of Al 2024 “A” before and “B” after being immersed in the Red Sea water for 5 weeks.
EDS results of Al 6061 “A” before and “B” after being immersed in the Red Sea water for 5 weeks.
The corrosion behavior of Al 7075, Al 2024, and Al 6061 was investigated in this research and the results revealed the following: The corrosion rates of Al alloys decreased with time and the weight loss data for 32 weeks were consistent with the bimodal model. Increase in temperature led to an increase in both anodic and cathodic current density and a decrease in corrosion potential. Lowering corrosion potential was associated with an increase in breakdown potential which resulted in expanding the passivity region with temperature All the alloys suffered from pitting corrosion and the most corroded alloy was Al 7075. Al 6061 showed the highest corrosion resistance at all the immersion times.
The authors declare that there are no conflicts of interest regarding the publication of this article.
The authors are immensely grateful to Professor Robert E. Melchers, University of Newcastle, for his comments on WL data. He agreed with us that the data are consistent with bimodal model and explained the reason for the observation of the bimodal model with short-immersion time as we wrote in kinetic study section.