Deterioration of Telecommunication Equipment and Facilities in Salt-damage Environments—Case Studies of Corrosion in Guy Wires and Maintenance Holes

3.1 Corrosion of a guy wire due to salt damage

First, we present a case study of guy wire deterioration along the coastline. The middle part of a guy wire of a utility pole had significant corrosion. The guy wire is installed about 30 m from the sea, and the region is designated as a high-salt-damage area on a “salt-damage map” developed and provided by TASC [1, 2]. The configuration of the facility in this corrosion case is shown in Fig. 2.

Fig. 2. Facility configuration.

3.1.1 Investigation method

We investigated the guy wire recovered from the site from two perspectives: (i) residual strength of the severe corrosion area and (ii) progress of corrosion in areas other than the severe corrosion area.

3.1.2 Results of investigation

The cross section of the severe corrosion area of the guy wire is shown in Fig. 3. We measured the residual strength of the wires within the guy wire. As listed in Table 1, the residual strength of the wires had decreased by an average of 77%. The progression of corrosion in areas other than the severe corrosion area is shown in Fig. 4, which shows the number of partially broken outer wires (“×” in the figure) increases from the utility-pole side to the bedrock side. This confirms that corrosion areas are more prevalent on the bedrock side than on the utility-pole side.

Fig. 3. Cross section of the severe corrosion area of the guy wire and the state of corrosion of the outer wires.

Table 1. Residual strength of wires of the severe corrosion area.

Fig. 4. Progression of corrosion in areas other than the severe corrosion area of the guy wire (partially broken outer wires: 11 locations in total).

3.1.3 Estimation of the cause of corrosion and countermeasures

The reason that the guy wire had severe corrosion on the bedrock side but light corrosion on the utility-pole side was estimated as follows and illustrated in Fig. 5(a). On the bedrock side, there are no obstacles blocking the air containing sea-salt particles above the sea from being carried by the sea breeze to the severe corrosion area of the guy wire. In other words, sea-salt particles in the air above the sea are blown in as they are. On the utility-pole side, however, the hill acts as an obstacle blocking the sea breeze and captures sea-salt particles blown in by the sea breeze. It can therefore be concluded that the difference in the corrosion state in those two sides of the guy wire is due to the difference in the amounts of sea-salt particles in the air blowing on those sides. This estimation is supported by the meteorological data (wind direction) shown in Fig. 5(b), that is, the hill on the upwind side of location A is located in a position where it obstructs sea breeze from the most-frequent wind direction (northwest).

Fig. 5. Local aerial photograph and meteorological data (wind direction).

For a countermeasure against salt damage, it is first necessary to identify areas where corrosion progresses rapidly and requires careful inspection by referencing salt-damage maps, etc. Second, when conducting inspections, it is necessary to ensure that the entire facility is thoroughly inspected since the corrosion rate may vary—even for the same facility—owing to differences in environmental conditions such as wind exposure, which makes it possible to reliably detect unsafe equipment. When messenger wires and guy wires are renewed, implementing corrosion countermeasures such as replacement with products with stronger anti-corrosion treatment (e.g., powder coating [3]), as shown in Fig. 6, can be expected to extend the service life of equipment.

Fig. 6. Example of anti-corrosion measure for messenger wires and guy wires (powder-coated messenger wires).

3.2 Metal corrosion in a maintenance hole due to salt damage

Cases of metal corrosion in a maintenance hole in a section located along a river are shown in Fig. 7. As shown in Fig. 8, the maintenance hole is located about 20 m from the river and about 400 m from the sea. It was previously confirmed that the bottom of the maintenance hole is 0 m above sea level or lower and that the water level inside the maintenance hole rises and falls due to the ebb and flow of the tide, so it was suspected that river water flows into the maintenance hole.

Fig. 7. Appearance of metal corrosion on fixtures inside a maintenance hole.

Fig. 8. Location of maintenance hole.

3.2.1 Investigation method

Water retained in the maintenance hole, corrosion fixtures, and river water were collected. Ion concentrations of the maintenance-hole water and river water were first analyzed by ion chromatography. The presence or absence of river-water inflow and the cause of corrosion were then analyzed by conducting X-ray structural analysis of the fixtures from the maintenance hole: mounting hardware with loose rust removed, a metal fixture with significant loose rust (metal-fixture adhering substance 1), and metal fixture with minor loose rust (metal-fixture adhering substance 2).

3.2.2 Results of investigation

Regarding the analysis of ion concentration contained in maintenance-hole water and river water, as shown in the “Sample 1: river water” row in Table 2, high concentrations of chlorine and sodium ions were found in the river water. As shown in the “Sample 2: maintenance-hole water” row of the same table, the maintenance-hole water has higher concentrations of chloride and sodium ions than the maintenance-hole water from inland areas, which suggests inflow of river water into the maintenance hole.

Table 2. Ion concentrations in maintenance-hole water and river water [mg/L].

The results of the X-ray structural analysis of the mounting hardware and metal-fixture adhering substances are shown in Fig. 9. They indicate that metal-fixture adhering substance 1 contains iron-corrosion products, indicated by peaks at the spectral positions of hydroxides (goethite: FeO(OH) and magnetite: Fe₃O₄), which are formed when iron corrodes. As shown in Table 3, the other samples (the mounting hardware and metal-fixture adhering substance 2) were confirmed to contain a hydroxide, FeO(OH), which is an iron-corrosion product. Since iron corrosion is accelerated as the concentration of chloride ions increases and FeO(OH) generates Fe₃O₄, the above results suggest the effect of salt damage.

Fig. 9. Results of X-ray structural analysis of metal-fixture adhering substance 1.

Table 3. Identification results of X-ray structural analysis of mounting hardware and metal-fixture adhering substances.

3.2.3 Estimation of the cause of corrosion and countermeasures

From the results of the above investigation, we estimated that salt-containing river water flowed into the maintenance hole and increased the salinity of the water retained in the maintenance hole, which in turn accelerated metal corrosion. Effective countermeasures against corrosion include (i) installing a stopcock in ducts (Fig. 10) to prevent inflow of seawater into the maintenance hole in question and surrounding maintenance holes in the same installation environment; (ii) replacing metal fixtures inside maintenance holes with ones that have powder coating [3] providing robust corrosion resistance (Fig. 11); and (iii) attaching a galvanic anode to the metal fixture, i.e., by connecting a metal with lower electrical potential than the metal of the target fixture, which sacrificially corrodes instead of the target fixture.

Fig. 10. Examples of stopcock.

Fig. 11. Examples of corrosion countermeasures for metal fixtures.