Degradation mechanisms of AA 1350 aluminium alloy conductor strands in air and chloride solution
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The primary aim of this doctoral thesis is to investigate the degradation mechanisms contributing to the failure of the aluminium strand material (AA 1350) used in steel reinforced aluminium conductors. These conductors are widely used in rural and marine environments due to their combined electrical and mechanical properties of the aluminium and galvanized steel core strands, respectively. The galvanizing is supposed to serve as a sacrificial anode for the steel core and mechanical barrier between the steel core and the aluminium strands to protect the strands against galvanic corrosion. Degradations due to fatigue and wear caused by wind induced vibration are the common causes of failure. Corrosion (galvanic, pitting and crevice) is also assumed to contribute. It was of interest in this thesis to investigate the significance of these mechanisms of degradation in the failure of the aluminium strands of a conductor. In the literature, investigation of the parameters of fretting degradation of aluminium strands has been based on bending fatigue test of the conductors in a laboratory air. The basic understanding of the parameters and mechanisms of fretting and wear has generally been restricted to dry laboratory environments. Studies investigating the effect of marine environments, especially the role of corrosion, are scarce. Therefore, the purpose of this thesis is to investigate the basic degradation mechanisms of the aluminium strand material used for conductor construction under controlled mechanical and electrochemical parameters. The conductor samples investigated in this research were obtained from the utility companies through SINTEF Energy Research AS. The investigation was started with the failure analysis of samples collected from marine environments to see if this effort would reveal the relevance of mutual action of corrosion and wind induced vibration in the failure. Cracks appeared to initiate at slightly worn and corroded areas of the contact surfaces. However, it was not possible to distinguish between corrosion occurring before or after initiation of the cracks. The failure analysis suggested that fretting corrosion and sliding wear-corrosion were responsible for the degradation of the aluminium strands and indicated the need for further investigations in these areas as mentioned above. The crack initiation sites, which led to breaking of the strands, were corroded and fretted indicating that the two degradations contributed to failure. Analysis of the fracture surface suggested that the major portion of the fatigue life of strands was consumed by the crack initiation phase, which involved both fretting and corrosion. In addition, crevice corrosion and pitting corrosion degradations were observed. This indicated that the protection provided by the galvanic coating was not sufficient to avoid corrosion of the aluminium strands. This indicated also the need for further analysis of degradation of the strands by fretting and wear under applied potential (enforced by cathodic protection) in chloride solution. In the light of the failure analysis results above, fretting and sliding wear degradations were investigated in the laboratory in air and 3.5% NaCl solution for better understanding of the role of the electrochemical and mechanical parameters in the degradations involving mutual action of the two. The parameters for the fretting tests were a frequency of 1 Hz, normal force of 5 - 60 N and displacement amplitudes from 3 - 100 µm on a flat polished aluminium strand surface against alumina ball ("ball on flat") for 4200 s. Fretting of alike cylindrical strands were also performed and compared to that of ball on flat surface to investigate the effect of geometrical factors. The sliding wear test conditions were also the same as that of fretting tests except that the displacement and normal force were 5 mm and 2 N, respectively. The effect of applied potential was investigated in the NaCl solution by use of a potentiostat. The fretted and worn surfaces were investigated using scanning electron and optical microscopes. Material loss volume was estimated for the sliding wear tests using an optical 3D confocal microscope. Investigation of sliding wear in NaCl solution at potentials in the range of -0.74 - -1.34 VSCE showed that the electrochemical environment reduced the degradation by wear by acting as a lubricant. Direct contribution of corrosion to the material loss was not significant. The lubrication effect arose from the presence of the solution and the debris of wear and corrosion products, formed largely by the oxidation of the metallic debris into an interfacial layer. The mechanical processes were thus the dominating factors of degradation in the potential range of -0.74 - -1.34 VSCE. However, the degradation mechanism changed considerably with the applied potential. In the potential range of -0.76 - -0.86 VSCE, the metal surface is thermodynamically expected to be passive. However, this condition could not prevent mechanical wear dominated by delamination (shearing and tearing of the surface) and abrasive wear from fragments of debris. Above the pitting potential (-0.75 VSCE), corrosion contributed to the degradation by anodic dissolution of the metal, mainly at the mechanically damaged areas. At more negative potentials down to -1.34 VSCE, the wear mechanism was dominated by plastic deformation and fatigue wear. At potentials more cathodic (negative) to -1.34 VSCE, the corrosion degradation became independent of mechanical wear. At OCP, sliding wear in NaCl solution caused cathodic polarization of the surface to about -1.32 VSCE. This value was determined by the cyclic depassivation, caused by localized exposure of the bare metal by rubbing, followed by repassivation by oxidation. A fretting map, demonstrating the occurrence of stick, mixed stick slip and gross slip regimes within the range of normal force and displacement amplitude used, was established for both tests in air and NaCl solution as stated above. Physically, these regimes are associated with no degradation, cracking degradation and wear degradation, respectively. Of the contact parameters investigated in this work, the mixed stick slip regime was the dominant one, indicating susceptibility of the aluminium strands to cracking degradation (fretting fatigue). However, the boundary of the transition from the mixed stick slip to gross slip contact condition was shifted to smaller displacement amplitudes in NaCl solution compared to that in air, i.e., the gross slip regime was wider in NaCl solution than in air for the same contact parameters. It is concluded, therefore, that the NaCl solution favours the gross slip rather than the mixed slip condition at the contact interface. This indicates the reduction of the tangential friction force under fretting in NaCl solution, such that fretting fatigue crack initiation takes longer time than in air. A tangential friction force arising from fretting in NaCl solution is about a third of that arising in air. The smaller tangential friction force in air is due to the lubricating effect of the NaCl solution, similar to that discussed for sliding wear above. This indicates that fretting fatigue life is expected to be much longer in marine environments than in air as far as fretting fatigue degradation is concerned.