SOIL CORROSION ON STEEL

Press, Volume XCI, Issue 27614, 22 March 1955, Page 8

SOIL CORROSION ON STEEL

PROGRESS MADE IN RESEARCH The corrosive effect of soil on mild steel has been investigated for many years in New Zealand in order to ground pipe lings. Within recent yean these investigations have included (studies of soil corrosion of concrete [and cement, asbestos, says a state* iment by the Department of Scientific [ and Industrial Research. I Small mild-steel plates 4 were buried around Auckland, Wellington and Christchurch in 1934 by the New Zealand Institution of Engineers, to gain information on the corrosiveness of soils and ultimately to lead to an increase of the service life of underground pipe lines. Thq, Department of Scientific and Industrial Research did not take an active part in the investigations until 1946 when the Soil Bureau agreed to examine the corrosion on these experimental plates. Its research workers concluded that the corrosion of 75 per cent of these test plates might have been predicted from knowledge of field conditions and the chemical and physical properties of the soils, in the other 25 per cent, the corrosion would not have been predicted from knowledge of soil corrosion or from the experiences of research workers overseas. There are many factors involved, and while corrosion can be confidently predicted in extreme cases it is much more difficult when a number of these factors have intermediate values. These tests gave interesting information on the soils around towns and cities where the bulk of pipelines are situated. The present ana more comprehensive tests, started in 1951 by the soil bureau, were designed to cover all major soil types so as to be of value in predicting corrosion in a wide variety of places for instance, new settlements. Some of the results of the first examination of these plates are now on hand, and the most obvious conclusion is the considerable range of corrosiveness of the different soils: ■ from negligible where no coating would, be needed, to very severe (maximum pit depth of 0.07 inches in 2J years) where the most durable coating would be justified. In general, clay, clay loam and silt loams, which hold water well, show the greatest corrosion, while sands and sandy loams show the least. Soils high in salts are also very corrosive. Prevention of Corrosion Overseas in recent yean a new method known as “ cathodic protection ” has been used to prevent corrosion of steel underground. Corrosion or rusting is caused by a flow of electricity from the metal to the surrounding soil. To combat it, sufficient current is passed through in the reverse direction, that is, from soil to steel pipeline to counterbalance the natural corrosion current This countercurrent keeps corrosion at bay almost indefinitely. The current can be obtained from a magnesium anode, buried in the ground, and connected to the pipe, in the same way that zinc provides the current in a dry cell, or from power mains. Catholic protection can be applied to bare steel but it is almost always used in conjunction with coatings. The coating protects the bulk of the pipe and cathodic protection prevents attack where there are flaws in the coating. Used in this way the current required is quite small. Little use of this method has been made in New Zealand but its use is bound to extend. Cathodic protection is equally applicable to steel tanks (in contact with the ground) and to ships. The alternative materials to steel for underground pipes are concrete and cement asbestos. These may also corrode but from rather different causes. In New Zealand the main , cause of corrosion of concrete and cement asbestos is carbon dioxide in the ground wateA This is present in wet soils where there is decomposition of organic matter by bacteria and in areas of thermal activity. Field trials by the soil bureau with these materials have been laid down at five sites. Preliminary results from these tests will be available in about three years. • In the meantime, existing pipelines are being examined. Dense concrete and cement asbestos of good quality that had been in wet ground for four years showed attack to a depth of 1/lOth inch. Porous concrete drain tiles of poor quality, however, had failed in a few months from the aggressive action of the carbon dioxide in the ground water, and for the same reasons concrete fence posts had been seriously attacked. The economic importance of this research is obvious. Outstanding results are sought, but even if a ■ few years are added to the life of underground pipes the research will be well worthwhile, because pipes are both extensive and troublesome to replace.