Sulfur Damage and Its Countermeasures in Telecommunication Equipment and Facilities

2.1 Mechanism of sulfur damage

The corrosion reaction that causes sulfur damage occurs when a water film is formed on the surface of a material by condensation or moisture adsorption. As shown in Fig. 2(a), when H₂S is absent, the corrosion of iron is caused by oxygen in the atmosphere dissolving in the water film, followed by reduction of the dissolved oxygen on the surface of the iron and simultaneous dissolution of the iron. When sea salt is also dissolved in the water film, the corrosion rate of iron will increase due to the increased electrical conductivity of the water film. When there is H₂S present (Fig. 2(b)) in addition to the reactions illustrated in Fig. 2(a), the H₂S dissolves and dissociates in the water film to produce H⁺ ions, which results in an acidic environment in the water film. This acidity accelerates the dissolution of iron and results in a high corrosion rate.

Fig. 2. Corrosion mechanism.

2.2 Actual cases of sulfur damage

2.2.1 Sulfur damage to outdoor facilities

Examples of sulfur damage to outdoor facilities at a hot-spring area are shown in Fig. 3. Significant rusting (reddish brown) was found on the metal bands at the lower part of the guy-line rod and scaffolding bolts on the utility pole. Thinning can be seen in the central part of the guy-line rod, so the rod may break when corrosion progresses. Although this type of corrosion can be caused by either salt or sulfur damage, since this hot-spring area is located in a mountainous area far from the coast, the effect of salt damage is considered small. Moreover, measurements of gas concentration at the locations shown in Figs. 3(a) and (b) detected H₂S at 1.5 ppm (parts per million) and 0.9 ppm, respectively. The above corrosion phenomena are thus considered to be sulfur damage, the main factor of which is H₂S. The regulation value for atmospheric concentration of H₂S stipulated by the Offensive Odor Control Law is 0.06 to 0.2 ppm [1], and the Industrial Safety and Health Act stipulates that the concentration of H₂S should be less than 1 ppm to ensure a safe working environment [2]. Accordingly, the measured H₂S concentrations (1.5 and 0.9 ppm) at these facilities indicate that H₂S exists at high concentrations at those locations.

Fig. 3. Examples of sulfur damage to outdoor facilities.

2.2.2 Sulfur damage to indoor equipment

The corrosion rate of metals in an indoor environment is generally lower than that in an outdoor environment because the intrusion of sea-salt particles and H₂S is hindered. However, if indoor spaces (rooms, etc.) are not sufficiently sealed, corrosion may occur due to the intrusion of sea-salt particles or H₂S. Since H₂S is a gas, it is difficult to completely prevent it from entering indoor spaces.

TASC has conducted a number of field investigations of indoor equipment after receiving requests for technical consultations from service personnel. An example of sulfur damage to indoor equipment is shown in Fig. 4, which illustrates sources of H₂S (Fig. 4(a)), examples of metal corrosion in indoor equipment (Fig. 4(b)), and micrographs of a circuit board inside a piece of terminal equipment (Fig. 4(c)). In this example, the concentrations of H₂S near the hot-spring-water vat and fumaroles, which are thought to be the sources of H₂S, were 0.1 and 0.9 ppm, respectively. It was also confirmed that metal fixtures, such as windows and faucets, were significantly corroded in the indoor environment, and it is considered that the corrosion is caused by the intrusion of H₂S from outdoors.

Fig. 4. Examples of sulfur damage to indoor equipment.

In the other case investigated by TASC, a ppb (parts per billion, where 1 ppb = 1/1000 ppm) level of H₂S concentration was detected in indoor environments. From the results of scanning electron microscopy - energy dispersive X-ray spectrometry (SEM-EDS) analysis, the main component of the corrosion depicted in the figure was sulfide, which indicates that a reaction between the metal and H₂S had occurred. Black corrosion was found on copper wiring and around through holes by microscopic observation of a circuit board collected from a faulty piece of terminal equipment installed in the same indoor space. SEM-EDS analysis of this black corrosion revealed that it contained sulfur in a similar manner to the indoor metal fixtures described above, and X-ray-diffraction analysis of the corrosion identified the main component as copper sulfide. When the copper wiring pattern on a circuit board is corroded by H₂S and becomes copper sulfide, it grows on the circuit board and connects to adjacent wiring, through holes, etc. It is thought that since the copper sulfide is conductive, a short circuit occurred between the wiring and through holes, which should not be connected, causing the equipment failure.

3.1 Basic requirements for countermeasures against sulfur damage

As the installation standard for terminal equipment, JEITA IT-1004 (Standard for Operating Conditions of Industrial Computer/Control System) of the Japan Electronics and Information Technology Industries Association [3] recommends controlling H₂S concentration to 3 ppb or less. There are several methods of suppressing corrosion caused by H₂S, as listed in Table 1. Taking costs and other factors into consideration, the basic requirements of countermeasures against sulfur damage to telecommunication equipment and facilities were determined as suppressing contact with H₂S and reducing H₂S concentration.

Table 1. Methods of suppressing corrosion caused by sulfur and their application to telecommunication equipment and facilities.

3.2 Countermeasures to suppress corrosion due to sulfur outdoors

Since concentration of H₂S is higher outdoors than indoors, namely, at the ppm level, countermeasures are required to provide higher corrosion protection performance. To meet this requirement, it is necessary to ensure that outdoor facilities and equipment are prevented from coming into contact with H₂S. One way to prevent contact is to coat the surface of the base metal with a paint that has stronger adhesion. By increasing the adhesion between the paint coating and base metal, it is possible to suppress (i) direct contact between H₂S and the metal and (ii) the adhesion of moisture (water film) containing H₂S to the metal. Therefore, it is possible to reduce the occurrence of sulfur damage. An example of metal bands coated with a powder coating, which has stronger adhesion and higher corrosion resistance than conventional coatings, attached to a concrete utility pole is shown in Fig. 5.

Fig. 5. Example countermeasure against sulfur damage to outdoor telecommunication facilities (metal bands with powder coating).

3.3 Countermeasures to suppress corrosion due to sulfur indoors

An example of a countermeasure against sulfur damage to indoor telecommunication equipment is shown in Fig. 6. The concentration of H₂S can be reduced by installing the terminal equipment inside a sealable acrylic box and placing H₂S adsorbent inside. In fact, a customer who had been experiencing failures of terminal equipment every two months or so has been free of failures for more than 10 months after starting to use an acrylic box.

Fig. 6. Countermeasure against sulfur damage to indoor telecommunication equipment.