Corrosion of metals and the forming of a protective coating on the inside of pipes carrying thermal waters used by the Reykjavík Municipal District Heating Service

Abstract

The Reykjavík Municipal District Heating Service has now been in operation for nearly 40 years. The oldest system, which has now been replaced, was installed in 1930, and consisted of 6–7 boreholes in the eastern part of the town of Reykjavík. Average temperature of the water was 87°C and it only served a limited section of the town. In this system called ⪡Thvottalaugar⪢ there was never any trouble due to internal corrosion, but the water built up a protective coating on metals, which consisted of metal oxide and silica of varying composition, but also always contained a nearly constant amount of magnesia (MgO). Today the Reykjavik District system has three sources of supply, drawing thermal water from three different thermal areas. The first area to be exploited was the Reykir area, completed with a corresponding distribution system in town, in 1943. Average temperature 86°C. This water contains ca 1 ppm dissolved oxygen which could increase to approx. 2.6 ppm due to atmospheríc influence , and caused a severe internal corrosion on metals in the heating system in the town. Investigation showed that these corrosion damages were solely due to dissolved oxygen in the water and a preventive measure was adopted i.e. addition of sodium sulphite to the water. (10 ppm Na 2SO 3 per 1 ppm O 2). In 1948 the area of Reykjahlid was taken into operation, temperature of the water 87°C. This water contains a certain amount of alkali - sulphide which eliminated most of the oxygen of the Reykir thermal water, as they got mixed. The mixed water (Reykir + Reykjahlid) did occasionally form a protective coating on metals which consisted only of metal oxide and silica, without any magnesia (MgO). The third system to be installed consists of boreholes within the limits of the city of Reykjavík. Average temperature 130°C. This water contains no dissolved oxygen and does not corrode metals but forms, on iron, at the origin, a thin satin-like black coating of magnetite (Fe 3O 4). The mixed thermal water (Reykir + Reykjahlid + city holes) contains only 0.1–0.4 ppm O 2 and does not cause any appreciable corrosion. On iron the water builds with time a white protective coating, consisting mostly of iron oxide and silica, but it also contains a certain amount of magnesia (MgO). This protective coating can form on almost clean metal but also on corrosion-product of different thickness. On copper there is at first formed a thin loose film of copper sulphide which changes with time in such a manner that the sulphur is replaced by silica, leaving a thin white protective coating of curpo silicate and a small amount of magnesia (MgO). Sometimes this white coating forms on almost clean copper at the beginning. It should be noted that there has never been noticed any corrosion on iron due to alkali sulphide in the water. As can be seen from the analysis, the thermal water has a very high content of sodium silicate and it must be believed that this sodium silicate acts as an inhibitor and prevents any appreciable corrosion, even in the absence of a visible protective coating, as is the case for both zinc-coated iron and cast iron. In some countries. mainly in USA, the method of adding sodium silicate to water has been used for the last 50 years, as a protection against corrosion. It has been claimed that thus corrosion in hot water can be reduced by 90% and in cold water by 50%. It is considered that at the beginning a concentration of 8–20 ppm sodium silicate is necessary, which may later be reduced to approx 4 ppm. A protective coating is formed on metals, which for copper and zinc-coated iron only consists of metal oxide and silica, but on iron also contains a certain amount of magnesia (MgO). On closer inspection a number of similarities can be found between the natural protective coating which the thermal water builds and the synthetic coating formed by adding sodium silicate to water. The main difference seems to be that the natural protective coating which the thermal water builds, always contains a certain amount of magnesia. Especially the protective coating which the thermal water in the old system ⪡Thvottalaugar⪢ built, which also consisted mainly of metal oxide and silica, always contained a nearly constant amount of magnesia. Most likely the addition of certain chemicals (magnesia-compounds) to the thermal water will result in such a coating, which would exclude all danger of internal corrosion in the heating systems.