Corrosion testing weathering steels

The reaction mechanisms by which the alloy elements influence the rate of corrosion of weathering steel are not precisely known. While the presence of alloy elements improves the corrosion resistance of a carbon steel subjected to humidity cycles, during a simple immersion test weathering steel and carbon steel yield... 

Legault, Mori and Leckie have used open-circuit potential vs. time measurements and cathodic reduction of rust patinas for the rapid laboratory evaluation of the performance of low-alloy weathering steels. The steel specimens are first exposed for 48 h to the vapour of an 0-(X)l mol dm sodium bisulphite solution maintained at 54°C (humid SOj-containing atmosphere) to stimulate corrosion under atmospheric conditions. They are then subjected to two types of test (tt) open-circuit potential-time tests for periods up to 3 000 s in either distilled water or 0 -1 mol dm Na2S04 and... 

Horton et al. [55] observed that when steels containing Cu and Ni are exposed in industrial and marine atmospheres, the Cu and Ni appear in the mst layers both in the loose outer and adherent inner mst on skyward and ground ward surfaces. Also it was shown by chemical analysis that Ni, Cu, Cr and Mn from weathering steel appear in the mst layer and provides protection. Presence of chlorides in the atmosphere accelerates corrosion of steels leading to the formation of basic Fe ", Fe chlorides and jS FeOOH. Townsend et al. [56] conducted 8-year atmospheric corrosion tests on weathering steel in mral, industrial and marine environments with different heated conditions and indicated that heat treatments have no effect on the corrosion resistance/performance of weathering steels. 

In another study, corrosion rates of Mn-steel and Cu-Mn weathering steel in a simulated coastal environment were measured by wet-dry cyclic test. The rust layer was observed and analysed by SEM and XRD. The experimental results... 

Corrosion rate of weathering steels was found lower with respect to mild steels in accelerated laboratory tests. They corrode most rapidly with respect to field exposure test... 

Present research work has been carried out in two parts field exposme tests and accelerated laboratory tests on unexposed panels. Under field exposme test three representative sites PI very close to sea shore and free firom SO2, P2 away from sea with presence of SO2 and P3 industrial environment with presence SO2 were chosen. Analytical techniques (EDX, XRD and Raman spectroscopy) were used for rust characterisation and SEM was employed for understanding the morphological state. Corrosion rates were estimated after different periods of exposure at sites. It is found that though accelerated laboratory test can predict the general trend of corrosion, actual field test data cannot be created in laboratory. So an attempt was made to simulate protective rust coating in the laboratory that is typical of field exposure protective rust formation on weathering steel. 

Many applications for bar, plate, and structural steel require use of painting or other protective measures therefore, there is not a great need for corrosion testing of the bare metal. An important exception to this, however, is the use of weathering steels, such as those specified in ASTM A 242, ASTM A 588, and ASTM A 852 in atmospheric exposure. The corrosion resistance of these steels is such that they can be used in the unpainted condition for many applications, depending on the environmental conditions. For these applications, there is a need for atmospheric corrosion testing to determine the suitability of bare weathering steels in specific locations. 

For specific applications of bare weathering steel, it is desirable to test a mock-up containing the type of bolted and/or welded joints that will be used in the intended structure, and also the orientation conditions that the bare steel will encoimter in the structure (e.g., horizontal, vertical facing north, 30 to the horizontal facing south, etc.). In addition to givittg irrformation on appearance md general corrosion rates in the specific environment, the mock-up can provide mformation on certain design details that should be avoided to prevent excessive localized corrosion. 

Analysis of the results after 1 and 2 years exposure indicates that the influence of SO2 on the corrosion rate of carbon steel, weathering steel, zinc, bronze, sandstone, limestone, and nickel is significant. Conversely, no influence of NO2 has so far been observed in any of the materials studied. This discrepancy between the laboratory exposures and the field tests must be investigated. Perhaps as a result of catalysts or oxidizers in field exposures, which aid in promoting the oxidation of S(IV) to S(VI), the effect of NO2 was hidden. 

The behavior of weathering low-alloy steels in aqueous corrosion tests and applications is unpredictable. In 1953, early tests on weathering steels containing copper, chromium, phosphorus, and nickel showed superior... 

The most widely used simulated service test for static atmospheric testing is described in ASTM G 50, Practice for Conducting Atmospheric Corrosion Tests on Metals. It is used to test coated sheet steels for a variety of outdoor applications. Test materials, which are in the form of flat test panels mounted in a test rack (Fig. 16), are subjected to the cyclic effects of the weather, geographical influences, and bacteriological factors that cannot be realistically duplicated in the laboratory. Test durations can last from several months up to many years. Some zinc-coated steel specimens have undergone testing for more than 30 years. 

Predicting galvanic corrosion is a serious problem. For contacts between common metals, in particular between ordinary steel and stainless steel, experience shows that laboratory testing always leads to more severe results than what is actually observed under conditions of weathering. 

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