of 3% NaCl solution. Curves 2 - 4 are corresponded to the alloys with 0.005% - 0.05% of additive Berylliums. They have been shifted tomore positive potential compared with curve 1 toward the base Zn5Al alloy which is indicated to a lower rate of anodic dissolution of these alloys.
Potentiodynamic studies of anodic behavior of the alloy show that increasing concentration of Beryllium up to 0.05 wt% will make a gradual decreasing in the corrosion rate and further increasing concentrations of components of the alloy slightly increase the corrosion rate of the initial alloy, but the total value does not exceed the corrosion rate of the original alloy (Figure 2).
Microstructures of initial alloys, Zn5Al and Zn55Al alloys which alloyed with Beryllium are shown in Figure 3. They show that the additive Beryllium have a modifying influence on the structure of original alloys.
Figure 1. Potentiodynamic anodic polarization curves (rate of potential sweep is 2 mV/s) of (1) Zn5Al alloy, (2) 0.005% (3) 0.01% (4) 0.05%, (5) 0.1%, (6) 0.5%, (7) 1.0% and (8) 2.0% wt% contained of Beryllium in the ambience of the 3% NaCl electrolyte.
Figure 2. The influence of Beryllium content C (wt%) on corrosion rate, K (g/m2∙h), of Zn55Al alloy in the ambience of (1) 0.03%, (2) 0.3% and (3) 3% NaCl electrolytes.
Figure 3. Microstructures (300×) of (a) Zn5Al, (b) with 0.05% Be, (c) Zn55Al and (d) with 0.05% Be.
Increasing content of Beryllium due to decrease grain size of Zinc in Aluminum (α-Al) and Aluminum in Zinc (γ-Zn) solid solutions and change grain shapes to spherecal. In fact, there is not any literature and information about the phase diagram of Al-Zn-Be ternary systems and influence of Beryllium on Aluminum and Zinc, thus discussion is not possible in this paper. Although there is an anecdotal evidence of Al-Zn-Mg-Be quaternary systems . It seems that Beryllium with Aluminum, Zinc and Magnesium in the quaternary alloys does not form a new phases and β-Be is formed free as a solid solution. Hence it should be expected that in Zn55Al-Be and Zn5Al-Be alloys and also in the form of solid solution in Aluminum (α-Al) and Zinc (γ-Zn) does not form a complex ternary phases.
Original alloys which compare with Zn5Al and Zn55Al alloys with additive Beryllium shows that alloys with Beryllium are characterized by smaller grains than Zn5Al and Zn55Al original alloys. It gives grounds to conclude that Beryllium is an effective modifier for these alloys and high corrosion resistant of the alloys, which is alloyed with Beryllium compared with the initial alloys is explained by the effect of modification of the original structure of Zn5Al and Zn55Al alloys.
Generally, potentiodynamic studies performed with Zn55Al alloys on Beryllium showed that a small amount of it (0.005 - 0.05 wt%) improves the corrosion resistance of the alloy in the initial alloy two to three fold. The proposed composition of Zinc-Aluminum alloy contained of Beryllium can be used as protective coatings to protect steel products and structures against the corrosion.
Changing regularities in the electrochemical properties of Be-Zn5Al and Be-Zn55Al alloys in the ambience of NaCl electrolyte with concentrations of 0.03%, 0.3% and 3%, indicate that increasing the concentration of chloride ions in the electrolyte contributes to decrease the corrosion potential. It is determined that increasing concentration of the alloying element in alloys, the pitting and repassivating potentials shift to the positive area and increaseing concentration of chloride ions in the solution shift to the negative area. Corrosion rates of Zn5Al and Zn55Al alloys decreases 2 - 3 times in order to add Beryllium up to 0.05%. It is explained by the effect of modification on the structure of original alloys, spherical shape of grains and small grain sizes.
This work was supported by department of Material Science and Engineering form Islamic Azad University, Majlesi Branch, Isfahan, Iran.