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Corrosion Monitoring of Electric Transmission Line Tower Legs by Electrochemical Methods
E. García, J. M. Malo, J. Uruchurtu.
Instituto de Investigaciones Eléctricas.
email: email: juch@iie.org.mx
Corrosion Monitoring of Electric Transmission Line Tower Legs by Electrochemical Methods
E. García, J. M. Malo, J. Uruchurtu.
Instituto de Investigaciones Eléctricas.
email: email: juch@iie.org.mx
The reliability of the electric power system is essential in the modern world. Failures in their transmission line components amounts to thousands of dollars only in maintenance costs apart from other related expenditures. Many of these failures are corrosion related due to the exposure of the system materials to aggressive atmospheric and/or soil environments. One of such failures is the corrosion of transmission line tower legs below ground level. The condition of these components is essential since the disintegration will provoke collapse of the tower or other towers in the vicinity, with the consequences associated, i.e. suspension of the electric supply.
Preliminary work was carried out into the feasability of establishing electrochemical monitoring of tower legs as part of an integrated maintenance program of transmission power line systems.
As part of this work, soil resistivity, corrosion potential, polarization curves, linear polarization resistance and electrochemical potential noise measurements were performed in several transmission line tower legs. Before measurements, visual inspection above ground level were made in tower legs. Afterwards electrochemical measurements were performed, and the tower legs were excavated in order to assess the corrosion condition and to check out correlations among parameters.
Soil resistivity was performed using the Wenner method, corrosion potentials were obtained using a saturated calomel reference electrode. The E/I curves were obtained sweeping from -200 below to 200 mV above the corrosion potential. The polarization resistance was obtained imposing a 20 mV cathodic pulse and registering the current response, while the electrochemical potential noise was obtained as a series of discrete potential oscillations recorded as voltage against time while the DC trend was removed. Potential noise time series were obtained averaging over five 1000 potential measurement ensembles obtained at a sampling rate of 1 second.
A Solartron electrochemical interphase and a digital voltmeter controlled by a PC computer were used throughout this work to obtain the electrochemical parameters; a calomel reference electrode and a one meter copper rod as a counter electrode placed one meter from the tower leg were used to perform the measurements. The soil was moistured continuosly with running water for conductivity purposes, and the soil resistivity was obtained using a Megger apparatus.
The results obtained in galvanized tower legs are presented in table 1 and can be related to different corrosion conditions: zinc corrosion, steel corrosion and mixed corrosion (galvanic effect) under general and/or localized attack. Active potentials were observed in towers 1, 2 and 6 and more noble potentials were registered in tower legs 3 and 4 while tower 5 presents the most anodic. These indicate that some form of corrosion is taking place, the more noble potentials encountered correspond to the steel surface exposed to the aggressive environment polarizing the galvanic couple Zn/Fe in the anodic direction.. Therefore towers 1, 3 and 4 presents a corrosion potential indicative of active dissolution of steel. Contradicting results were observed in towers 1 and 6 where corrosion potentials obtained were similar, suggesting the same corrosion conditions while tower 5 exhibit an anodic potential possibly related to zinc dissolution.
| TOWER | Ecorr
(mV SCE) | RESISTIVITY
(ohm-cm) | TAFELSLOPE
(A/V) | Rp
(ohm) | Observations
(above ground) |
| 1 | -522 | 420 | 9 | 0.17 | old corroded
fallen tower |
| 2 | -710 | 2100 | 48 | 33.67 | fallen tower |
| 3 | -385 | 910 | 11 | 13.79 | fallen tower |
| 4 | -443 | 4840 | 34 | 58.65 | good condition |
| 5 | -860 | 5530 | 26 | 28.57 | good condition |
| 6 | -567 | 29000 | 375 | 276.60 | good condition |
According to classification, soil resistivity indicates an extremely corrosive environment in towers 1 and 3, corrosive in towers 2, 4 and 5 and less corrosive in tower 6 (1). The Tafel slopes and the polarization resistance are related to the overall corrosion rates. High slopes and large Rp are related to low corrosion rates and viceversa. Figure 1 present the Tafel slopes obtained in towers l, 2 and 3 while figure 2 corresponds to results obtained in towers 4, 5 and 6. Results presented exhibit worst conditions for leg foundations in towers 1 and 3 and the best in tower 6 according to these results (except for the corrosion potential in towers 1 and 6). To try to clarify these, noise measurements were obtained.
Figure 3 to 5 presents examples of the electrochemical potential noise obtained in towers 1, 2 and 3 and figure 6 and 7 towers 5 and 6 (tower 4 is not presented). The nature of the noise fluctuations are: low amplitude associated to general corrosion attack, large amplitude transients intermittently observed associated to localized corrosion and a combination of both where periods of general and localized corrosion take place. Transients observed are in the anodic or cathodic direction (2,3).
Conditions in towers 1,2 and 3 as compared to towers 4, 5 and 6, were the worst according to the soil resistivity (freatic mantle was just below ground level) and electrochemical parameters. The electrochemical potential noise obtained in tower 1 (see figure 3), presents low amplitude oscillations indicative of generalized attack with an active potential associated to steel corrosion. These correlate well with the state of the surface where no zinc coating is present as in figure 8.
Potential noise for tower 2 is presented in figure 4, where low amplitude oscillations combined with a series of high amplitude cathodic transients can be observed. This behaviour is probably associated to the zinc dissolution combined with the sudden appearance of some actve steel sites which polarize the surface transiently until the cathodic protection of zinc or the formation of corrosion products over the active sites depolarizes the galvanized surface. This can be seen in figure 9 where the tower leg foundation presents some areas with steel corrosion products and other with galvanized steel.
The results obtained for tower 3 and 4 are similar, therefore only the electrochemical potential noise for tower 3 is presented in figure 5. Combined low and high amplitude anodic oscillations are present associated to propagated localized attack(4,5). The nature of these transients is in the form of sudden drops and exponential recoveries reported widely in the literature (6,7). The corrosion potential suggests disssolution of steel confirmed by the visual inspection. The tower leg was basically steel exposed with few galvanized areas left (see figure 10).
Figure 5 presents the potential oscillation of tower 3 (similar to tower 4) where anodic transient behaviour associated to localized corrosion is observed. This type of oscillations and the corrosion potential registered suggest that steel corrosion was taking place over the surface. This was confirmed after excavation and presented in figure 10, where steel corrosion products over most of the surface can be observed.
Potential noise observed in figure 6 corresponds to a typical response for zinc under localized attack in aggressive media, where a low frequency oscillation presents some transients and high frequency oscillations are superimposed to the main low frequency high amplitude fluctuation. This behaviour is associated to a localized corrosion of zinc over the surface, observed and reported before, and confirmed after visual inspection (see figure 11)(8,9).
Finally figure 7 presents the results obtained in tower 6, where as before over a low frequency oscillation some superimposed high frequency components can be observed, being these of very low amplitude. Again this behaviour can be associated to a low corrosion of zinc mainly of a general type due to the presence of zinc corrosion products and confirmed by the corrosion potential and the soil resistivity. Also the Tafel slope and Rp suggest this condition, confirmed after excavation where the state of the tower leg appears in good condition, with most of the surface galvanized.
Periods of general and localized corrosion associated to low and high amplitude oscillations respectively, can be observed during noise measurements(4,7). Depending upon direction (anodic or cathodic) these noise traces probably are related to the corrosion processes occuring over the metal surface depending upon the surface condition of the galvanized steel, i.e.: areas of totally, partially or not exposed steel.
As it was said before, after measurements all tower legs were excavated and visually inspected (some are presented in figures 8 to 11). Good correlation was obtained between measurements and the corrosion condition of all tower legs.
Different corrosion potentials indicative of the corrosion state, Tafel slopes and polarization resistance related to the overall corrosion rate and electrochemical noise traces associated to the type of corrosion attack were observed. After visual inspection good correlation between soil resistivity, electrochemical methods and the corrosion condition of the tower legs tested were confirmed. The preliminary results presented show the sensitivity of electrochemical techniques for corrosion monitoring and its usefulness in taking decisions related to maintenance programs and corrosion control. More work is needed under laboratory conditions to corroborate field measurements.
KEY WORDS: corrosion, tower legs, electrochemical techniques.
Name: Jorge Uruchurtu.
Adress.Instituto de Investigaciones Eléctricas.
Edificio 12-1
Reforma 113
62490 Temixco, Morelos.
Mexico.
Fax:(573)189854.