Corrosion testing sulphur dioxide

Nickel-based alloys-Determination of resis-ance to intergranular corrosion Corrosion tests in artificial amospheres at very low concentrations of polluting gas(es) Test /. .-sulphur dioxide test for contacts and connectors... 

In most districts, however, sulphur dioxide and dust particles are the main corrosive pollutants. It has been demonstrated that there is a direct relationship between sulphur dioxide in the atmosphere and the corrosion of steel exposed to it (see Fig. 3.2). In a series of tests carried out in the Sheffield area, sulphur dioxide accounted for about 50% of the variations in corrosion rate at the different sites. ... 

Various methods can be used to determine the concentration of sulphur dioxide in the vicinity of steel test specimens. They all provide suitable information on the relationship between sulphur dioxide and corrosion. However, the actual amount of sulphur dioxide in contact with the steel surface is more important than the concentration, as shown in the work reported by Walton et al. ... 

In laboratory tests Vernon showed that the relative humidity and the presence of sulphur dioxide have a profound effect on the rate of corrosion of copper, as of many other metals. When the relative humidity was less than 63%, there was little attack even in the presence of much sulphur dioxide, but when the relative humidity was raised to 75%, corrosion became severe and increased with the concentration of sulphur dioxide present. 

Industrial atmospheres usually accelerate the corrosion of zinc. When heavy mists and dews occur in these areas, they are contaminated with considerable amounts of acid substances such as sulphur dioxide, and the film of moisture covering the metal can be quite acid and can have a pH as low as 3. Under these conditions the zinc is dissolved but, as the corrosion proceeds, the pH rises, and when it has reached a sufficiently high level basic salts are once more formed and provide further protection for the metal. These are usually the basic carbonate but may sometimes be a basic sulphate. As soon as the pH of the moisture film falls again, owing to the solution of acid gases, the protective film dissolves and renewed attack on the metal occurs. Hudson and Stanners conducted tests at various locations in order to determine the effect of atmospheric pollution on the rate of corrosion of steel and zinc. Their figures for zinc are given in Table 4.34 and clearly show the effect which industrial contamination has on the corrosion rate. 

Ambler has attempted to find a relationship between the corrosion of zinc and iron and atmospheric salinity in the UK. This followed previous tests in Nigeria, when it was concluded that the governing factor in the corrosion of steel and zinc was airborne salt and that there was a relationship between corrosion and the distance from the sea. In the UK, however, no such relationship was found to exist, and the governing factor in the corrosion of zinc in the atmosphere is confirmed to be the amount of sulphur dioxide pollution. 

Exposure of the samples to a controlled moist atmosphere containing sulphur dioxide, as recommended in BS 1615 1972, Method H, is an example of a test bridging the gap between sealing tests and accelerated corrosion tests. After exposure for 24 h at 25 2°C, poorly sealed films show a persistent heavy white bloom, while good sealing produces at the most a slight superficial bloom. 

The most widely used accelerated tests are based on salt spray, and are covered by several Government Specifications. BS 1391 1952 (recently withdrawn) gives details of a hand-atomiser salt-spray test which employs synthetic sea-water and also of a sulphur-dioxide corrosion test. A continuous salt-spray test is described in ASTM B 117-61 and BS AU 148 Part 2(1969). Phosphate coatings are occasionally tested by continuous salt spray without a sealing oil film and are expected to withstand one or two hours spray without showing signs of rust the value of such a test in cases where sealing is normally undertaken is extremely doubtful. 

Rubber vulcanised-determination of adhesion to, and corrosion of, metals Corrosion of metals and alloys determination of dezincification resistance of brass Sulphur dioxide test with general condensation of moisture... 

Determination Test, Weight loss test, Eschke test, Salt spray test and Sulphur dioxide test at 50°C. Mechanism of inhibition of corrosion and surface study of mild steel was also studied by Metallurgical Resesarch Microscopy and Scanning electron Microscopy. 

If suitable field sites are not available or lack controlled conditions, then corrosion tests must be conducted in the laboratory. Cabinets are constructed in which the atmosphere is controlled and high humidity and temperature can be used to help accelerate the tests. Marine environments are simulated by salt spray and industrial environments by sulphur dioxide or nitrogen dioxide. Figure 18 shows a salt-spray cabinet and the arrangement of test panels. Periodic changes of temperature within the cabinet can be used to simulate night and day. Addition of other aggressive salts or acid into the sprayed solution is further used to accelerate the test. 

A feature of these mainly benign atmospheres is that the ratio of steel to zinc corrosion is less than found further west. The Soviet workers had test sites for which temperature, relative humidity, days with dew and rain, and particulate deposits were recorded over 4 years, while Table 2.21 gives information on chloride ions and sulphur dioxide (Strekalov and Berukshtis, 1965), together with corrosion rates in both open and sheltered exposures. 

---