Corrosion Behaviour Between Al–Zr Alloy Conductor and Cu Transition Terminal via Sc Addition
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The influence of Sc microalloying on corrosion behaviour between Al–0.2Zr alloys conductor and Cu sheet have been studied in 0.1 M Na2SO4 + 0.05 M NaCl weak acid solution. The observations of corroded morphologies of the Al–0.2Zr alloys conductor were obtained by scanning electron microscopy and transmission electron microscopy. After same short-term service, slight local corrosion and pitting pits occur on the surface of Al–0.2Zr alloy, and corrosion products mainly adhere along grain boundaries of Al–0.2Zr–0.1Sc alloy. After same long-term service, there are many large and saggy corrosion pits on the surface of Al–0.2Zr–0.1Sc alloy, while only some staircase corrosion marks appear on the surface of Al–0.2Zr alloy. Al3Sc and Al3(Zr,Sc) may reduce the electrochemical heterogeneity and the electrochemical reaction of precipitated precipitates on subgrain boundaries and inter-subgrains, thus improving corrosion resistance via Sc addition.
KeywordsCorrosion behaviour Al–0.2Zr alloy Al3Sc and Al3(Zr,Sc) Subgrain boundaries
Overhead Al conductors in long-term service are usually subject to compressive stress at the junction of intermediate Cu transition terminals, which is easy to loose [1, 2, 3]. It will not only increase the transmission temperature of electrical resistance, but also be vulnerable to acid rain corrosion, and some serious cases may lead to short-circuit fire. Therefore, the corrosion behaviour between the conductor and the intermediate Cu transition terminal can seriously affect the life of the conductor.
Among the aluminium conductors, Al–Zr alloy conductors belong to the medium-strength, high-conductivity and heat-resistant cable, they are potential candidates for application to electrical power transmission in high-latitude regions . Al–0.2Zr alloy conductors have been in service for a long time at high temperature (120–150 °C), compared with other Al alloy conductors [5, 6]. Heat will be generated in the transportation process, and the joints of conductors are easier to soften and loosen, so that acid rain is easier to enter the cavity between the conductor and the intermediate copper transition terminal. Therefore, it is necessary to study the corrosion behaviour of Al–0.2Zr alloy conductors in service.
In the long-term service process, the different metal characteristics between the Al conductor and the Cu transition terminal have significant performance differences in electrical connection. Each pressing position forms voids on both sides of the conductor, and a large gap is formed at the contact interface between the conductor and the terminal tube cavity. These voids are exposed to the corrosion medium (acid rain, acid tail gas and rainwater mixture), and the corrosion solution will be penetrated in the voids to form a closed surrounding. The internal corrosion behaviour will change gradually, such as the change of oxygen concentration and PH value, and will also induce more serious corrosion phenomena such as crevice corrosion. Khedrand et al.  studied the corrosion behaviour of pure aluminium in the solution rich in Cu2+, and the results showed that the deposition of Cu2+ cation and the electrocoupling accelerated the corrosion rate of Al. Jorcin et al.  showed that the electrochemical dissolution of Al caused an increase in the pH value at the Al/Cu interface, and a blocked region was formed to rapidly change the composition of the electrolyte. There are few studies on the corrosion behaviour between Al alloy conductors and Cu transition terminal. Therefore, we focus on the corrosion behaviour between Al–Zr series alloy conductors and Cu transition terminal via microalloying addition.
2 Materials and Methods
The alloy with a nominal composition of Al–0.2Zr (wt%) and Al–0.2Zr–0.1Sc (wt%), were melted in air in a furnace using 99.7 wt% pure Al, Al–10 wt% Sc, Al–5 wt% Zr. The master alloys were stirred to achieve complete mixing, and then cast into graphite mold (Ф 50 mm × 200 mm). The two alloys were homogenized at 620 °C for 48 h in an air furnace. Subsequently, the ingots were hot-extruded into rods of Ф 9.5 mm at 550 °C. Extruded aluminum rods were cold drawn into wires with diameters of 6.5 mm. The wires were divided into several parts and then subjected to solid solution at 640 °C for 48 h, aging at 350 °C for 36 h (peak aging was selected, which could be referred to our previous study .
3 Results and Discussion
Hydrogen evolution corrosion of H+ and oxygen absorption corrosion of Al exist in weak acidic corrosion solution. According to Nenster equation, the electrode potential of O2 is higher than that of H+ and oxygen absorption corrosion is more likely to occur, so the corrosion products may be Al(OH)3 or Al2O3.
In short-term service (immersion for 5 h), slight local corrosion and pitting pits occur on the surface of Al–0.2Zr alloy, and corrosion products mainly adhere along grain boundaries of Al–0.2Zr–0.1Sc alloy.
In the long-term service (immersion for 1800 h), there are many large and saggy corrosion pits on the surface of Al–0.2Zr alloy, while the corrosion degree of Al–0.2Zr–0.1Sc sample is lower after long-term service than that after short-term service, only some staircase corrosion marks appear on the surface of Al–0.2Zr alloy.
Al3Sc and Al3(Zr,Sc) may reduce the electrochemical heterogeneity and the electrochemical reaction of precipitated precipitates on subgrain boundaries and inter-subgrains, thus corrosion resistance of Al–0.2Zr–0.1Sc is better than that of Al–0.2Zr alloy.