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Corrosion Behaviour Between Al–Zr Alloy Conductor and Cu Transition Terminal via Sc Addition

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Abstract

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.

Introduction

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 [4]. 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. [7] 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. [8] 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.

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 [9].

The test objects are Al–0.2Zr and Al–0.2Zr–0.1Sc alloy conductors, which were immersed and corroded after welding contacted with Cu sheets, as shown in Fig. 1. The silver color in Fig. 1a is Al alloy and the gold color is the Cu, Al and Cu alloys are welded contacting with each other. The exposed part area of the test surface was 0.5 cm2. The corrosion solution was 0.1 M Na2SO4 + 0.05 M NaCl weak acid melt (pH = 6.0–6.5). The Al alloy conductors and Cu sheets were completely immersed in the weak acid solution, as shown in Fig. 1b. The immersion time for the samples in the solution is 5 h (short-term service) and 1800 h (long-term service). SEM (scanning electron microscopy) and TEM (transmission electron microscopy) were used to investigate the morphologies of Al conductor samples. Specimens for SEM and TEM were cut from the alloys, and then mechanically polished to a final thickness of 60 μm for TEM use. Subsequently, foils for TEM observation were punched into 3 mm discs, followed by thinning at − 25 °C and 15 V employing twin-jet electro-polishing in a mixed solution of 25 vol% nitric acid and 75 vol% methanol. TEM observations were performed by a Titan G2 60-300 microscope at an accelerating voltage of 200 kV. SEM images were obtained on a Quanta-200 microscope.

Fig. 1
figure1

The schematic diagram of Al conductor sample

Results and Discussion

The corrosion morphologies of Al–0.2Zr and Al–0.2Zr–0.1Sc samples after same short-term service are illustrated, as shown in Fig. 2. It can be seen from SEM micrographs that Al–0.2Zr and Al–0.2Zr–0.1Sc alloys have different degrees of corrosion. A large number of white corrosion products are attached to the surface of Al–0.2Zr sample after corrosion. At the same time, slight local corrosion occurs on the surface of the alloy, and pitting pits appear, in which the pits are smaller, as shown in Fig. 2b. The occurrence of pitting pits indicates that the passive film on the surface of Al alloy gradually disappears and pitting occurs inside. However, pitting pits are not observed on the surface of Al–0.2Zr–Sc alloy, but a large number of corrosion products are observed. In addition, corrosion products on the surface of Al–0.2Zr–0.1Sc alloy mainly adhere along grain boundaries, as shown in Fig. 2c. The initial corrosion process is the dissolution of Al at the Al–Cu interface because the alkalization of Al is related to the reduction of oxygen on the surface of Cu. The electrochemical reactions related to Al interface are as follows:

$$ \begin{aligned} & {\text{Cathode:}}\;\;{\text{O}}_{{2}} \; + \;{\text{2H}}_{{2}} {\text{O}}\; + \;{\text{4e}}^{ - } \; \to \;{\text{4OH}}^{ - } \\ & {\text{Anode:}}\;\;{\text{Al}}\; \to \;{\text{Al}}^{{{3} + }} \; + \;{\text{3e}}^{ - } \\ & {\text{2Al}}\; + \;{\text{6H}}_{{2}} {\text{O}}\; \to \;{\text{2Al}}\left( {{\text{OH}}} \right)_{{3}} \; + \;{\text{3H}}_{{2}} \\ & {\text{Al}}\left( {{\text{OH}}} \right)_{{3}} \; + \;{\text{OH}}^{ - } \; \to \;\left[ {{\text{Al}}\left( {{\text{OH}}} \right)_{{4}} } \right]^{ - } . \\ \end{aligned} $$
Fig. 2
figure2

Corrosion morphology of the Al conductor samples after same short- term service: a, b Al–0.2Zr alloy; c, d Al–0.2Zr–0.1Sc alloy

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.

Figure 3 shows the corrosion morphologies of Al–0.2Zr and Al–0.2Zr–0.1Sc samples after same long-term service. The corrosion surface of Al–0.2Zr sample is not only slightly corroded, but also has a large and saggy corrosion pit (Fig. 3a). In addition, in Fig. 3a, b more obvious denudation morphology is found, with honeycomb-like appearance inside the holes. From the SEM morphology of Fig. 3a, b, it can be inferred that this kind of denaturation mainly occurs in the corrosion area, due to some Al3Zr precipitates with uneven distribution in the grains. These precipitates are located correctly and form many microcosmic primary batteries with matrix and grain boundary. Under the action of corrosive medium, the matrix near precipitates are dissolved preferentially. Compared with short-term service, the corrosion degree of Al–0.2Zr–0.1Sc sample is lower after long-term service, and there are no obvious corrosion spalling marks on the surface, only some staircase corrosion marks appear, as shown in Fig. 3c, d. This phenomenon may be attributed to the fine and uniformly distributed Al3(Zr, Sc) and Al3Sc precipitates in Al–0.2Zr–0.1Sc samples. This similar phenomenon has also been reported by other researchers. Deng et al. [2] found that adding Sc and Zr elements to Al–Zn–Mg alloy could form a large number of nano-sized Al3(Zr,Sc) precipitates after aging. These fine dispersed precipitates can well inhibit the formation of PFZ (precipitation free zone). Inhibition of PFZ formation can reduce the difference of chemical composition and the electrochemical heterogeneity among grain boundaries, solute depletion zones and grain interiors, thus reducing the anodic dissolution and electrochemical reaction of precipitated precipitates on grain boundaries. Therefore, the corrosion resistance of the alloy was improved by adding Sc.

Fig. 3
figure3

Corrosion morphologies of the Al conductor samples after same long-term service: a, b Al–0.2Zr alloy; c, d Al–0.2Zr–0.1Sc alloy

Obviously, a large numbers of dislocations and precipitates are observed in Al–0.2Zr–0.1Sc samples, as shown in Fig. 4. The precipitated precipitates in Al–0.2Zr–0.1Sc sample are mainly Al3Sc (7 nm in size) and Al3(Zr,Sc) (80 nm in size), marked by red and yellow arrows in Fig. 4a, respectively. Al3Sc precipitates are pinned at dislocations and Al3(Zr,Sc) precipitates are pinned at subgrain boundaries. The passivation film begins with the lattice defects on the surface. High density of subgrain boundaries and dislocations are beneficial to the nucleation of the passivation film on the sample surface [1]. Dislocations and grain boundaries are bound to be more conducive to the formation of passive film on the surface. In addition, a large number of fine dispersed Al3Sc and Al3(Zr,Sc) precipitates are formed in Al–Zr–Sc alloy. These precipitates are uniformly distributed on subgrain boundaries and inter-subgrains, which reduces the electrochemical heterogeneity and the electrochemical reaction of the matrix near precipitated precipitates, thus improving the overall corrosion resistance of the Al–0.2Zr–0.1Sc alloy.

Fig. 4
figure4

TEM morphologies of Al–0.Zr–0.1Sc samples after long-term service: a Al3Sc and Al3(Zr,Sc) precipitates and b dislocations

Conclusion

The influence of Sc micro alloying on corrosion behaviour between Al–0.2Zr, Al–0.2Zr–0.1Sc alloys conductor and Cu transition terminal have been studied in 0.1 M Na2SO4 + 0.05 M NaCl solution. The conclusions are summarized as follows:

  1. 1.

    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.

  2. 2.

    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.

  3. 3.

    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.

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Correspondence to Jiayi Zhang.

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Zhang, J., Wang, W. Corrosion Behaviour Between Al–Zr Alloy Conductor and Cu Transition Terminal via Sc Addition. Chemistry Africa (2020) doi:10.1007/s42250-020-00118-7

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Keywords

  • Corrosion behaviour
  • Al–0.2Zr alloy
  • Al3Sc and Al3(Zr,Sc)
  • Subgrain boundaries