Measuring Corrosion Of Materials In N.Z. Soils

Press, Volume CI, Issue 29765, 7 March 1962, Page 15

Measuring Corrosion Of Materials In N.Z. Soils

Corrosion of materials buried in the soil gets worse, in general, as one goes northward. said Mr H. R. Penhale of the Soil Bureau, Wellington. Mr Penhale is collecting steel plates and concrete pipes which he buried in various types of soils several years ago to check on the rate of corrosion, and at the same time he is burying earthenware pipes for similar tests.

The steel plates have been buried the longest—about 10 years. Sites in the South Island at which they have been buried include Wakefield, Blenheim, Greyrnouth, Hokitika, Motukarara, Tirnaru, Oamaru, Dunedin, Waipori, Alexandra, and Gore. By far the greatest rate of corrosion among these sites is probably at Dunedin, where the plates are buried on a reclaimed area on the foreshore. The accelerated corrosion here, he believes, is caused by anaerobic bacteria which flourish in the poorly-drained soil. The only other of his sites where comparable conditions are found is in the Hauraki plains of South Auckland. Bacteria apart, the soils where the greatest rate of corrosion of steel occurs are acid clays and the salty soils of the seaside or of very dry areas. The acid clays are associated with warm climates and reach their greatest development in Northland. Next in harmful effect after the acid clays and salty soils are the waterlogged soils, which have a comparatively mild action which nevertheless seems to continue at a fairly constant rate for longer than in the case of other soils, where the rate falls continually after a few years Least corrosive are the dry. sandy soils. At Motukarara, the steel test plates are buried close to the racecourse in a wet, saline soil, but the attack on them is only moderate. An even lower rate of corrosion is observed in very wet pakihi soils at Greynx. uth and Hokitika. There is a fairly high rate of attack by a somewhat acid clay in Pigeon valley, outside Wakefield, Nelson, while in saline soil near the coast a few miles south of Blenheim the corrosion seems slight. Each “planting” of steel plates consists of five separate pits three feet deep, each containing four plates 15in long by 9in wide by 3-16 in thick. The steel is sampled at approximately four-year intervals by excavating one of the pits. The plates are of mild steel, but the rate of corrosion of this does not differ significantly, says Mr Penhale. from that of other

types of low-priced steel or of cast iron. As the object is to give a guide to the precautions needed against corrosion of steel pipes or foundation reinforcing in any particular type of soil, there is no point in testing steels which are too expensive for these uses. (Steels such as stainless steel are known to have a much higher resistance to corrosion, but their price is prohibitive.)

The tests on concrete and similar products are being made at four sites, three in the North Island and one in the South Island. As the tests have been going only four years, results are tentative, but at the site in the South Island, near Blenheim, the indication is that corrosion of these materials is negligible in the soil type chosen. Four types of pipe are buried at each site—lean concrete, rich concrete, ordinary cement asbestos, and autoclaved cement asbestos. The most important property of the soil in causing damage to these materials is usually its carbon dioxide content. The earthenware pipe Mr Penhale is now burying are part of a joint experiment with the New Zealand Pottery and Ceramic Research Association. The South Island sites chosen for the burials are at Nelson, Greymouth, Motukarara, Waipori, and Alexandra.