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Synopsis The corrosion resistance of Type 316 stainless steel, partly embedded in concrete, partly exposed to stagnant seawater and partly exposed to flowing seawater, has been determined over various periods up to about 12; years duration. The exposure tests were carried out in full immersion and tidal conditions, on specially designed concrete blocks. Mild steel specimens were also tested for comparison. Corrosion of exposed stainless steel was localized, not extensive, and affected neither the strength nor ductility of the specimens. Contrary to expectation, crevice corrosion occurred on only one of the 42 test specimens, and only after 12½ years total immersion. It is considered that the alkalinity of the concrete was responsible for minimizing corrosion on both embedded and external areas of stainless steel. Ordinary Portland cement concrete gave more protection to the stainless steel than did sulphate-resisting Portland cement concrete due, it is thought, to the higher cement content of the former and consequent greater reserve of alkalinity. The higher proportion of tricalcium aluminate in the OPC concrete and its known effect in complexing chloride ions and delaying their ingress into concrete was also considered to be of great significance.

Two major exposure site programs have been undertaken to provide comparative information on the corrosion susceptibility of steel in concrete. In the first (current for 14 years), a comparative examination of the performance of zinc-coated and uncoated mild steel in a range of concretes made with both dense and lightweight pulverized fuel ash (pfa) aggregates has been made. In the second (which has been underway for five years and where corrosion has been accelerated by the addition of calcium chloride to the concrete), evaluation of the performance of galvanized steel (in this case on a substrate of high-yield bar) has been supplemented by studies of American Iron and Steel Institute (AISI) Types 405, 430, 302, 315, and 316 stainless steel bar and a comparison with the performance obtained from high-yield deformed bar. Both programs have employed exposure of small reinforced concrete prisms in which the concrete cover to the reinforcement has been carefully set by locating the test specimens on a supporting frame. In the five-year tests, the prism specimens have been augmented with trials on reinforced beams which have been stressed to give cracks in the concrete cover to the steel. This paper discusses the results obtained from the galvanized steel specimens and compares them with those for the untreated steel examined in these studies. The results can be divided into a number of categories. Similar good performance (in terms of cracking of the cover induced by expansive corrosion of the reinforcement) has been exhibited by mild, high-yield, and galvanized steel in dense-aggregate good-quality chloride-free concrete. Where dense aggregate has been substituted by lightweight aggregate, cracking due to corrosion of the unprotected steel occurred at low cover, whereas identical prisms containing galvanized bars remained uncracked. The addition of high levels of calcium chloride (3.0 percent by weight and above with respect to the cement) to the dense-aggregate concrete caused severe corrosion of the high-yield bar and resulted in massive cracking of the cover. To date, this cracking has been less severe in similar specimens reinforced with galvanized steel, but results of weight loss measurement indicate extensive zinc loss in some specimens. However, much less loss of zinc has been measured on bars removed from concrete made without deliberate addition of chloride and from concrete to which up to 1.5 percent calcium chloride (by weight of cement) has been added. In the more permeable mixed without added chloride, where carbonation has reached the test bar, plain steel has corroded, resulting in cracking of the cover, whereas with galvanized bar some zinc loss has occurred but without fracture of the cover. The results suggest that, although some delay in cracking of the cover is achieved by the use of galvanized reinforcement, the greatest benefit would occur where it has been used in low-quality relatively permeable concrete containing low or minimal quantities of chloride.

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