PSI - Issue 13

Vit Krivy et al. / Procedia Structural Integrity 13 (2018) 825–830 Vit Krivy, Monika Kubzova, Katerina Kreislova, Martin Krejsa / Structural Integrity Procedia 00 (2018) 000 – 000

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outdoor environment, see Morcillo et al. (2014) and Kukrus et al. (1985). At the surface of the steel a protective layer of corrosion products, so called patina , is formed under suitable exposure conditions. The course of corrosion processes on the surface of the structural elements depends on the particular environmental conditions, see Albrecht and Hall (2003) and Kreislova et al. (2009). A sufficient patina layer is usually formed after 5 to 8 years of direct exposure, but in case of partially protected surfaces, the development of fully protective patina is slower. Corrosion rates after one year of exposure r corr are the basic characteristics of corrosion behaviour of weathering steels in a specific environment. Corrosion rates can by directly measured using standard specimens according to EN ISO 9223 (2012) or suitable prediction models may be used to predict the corrosion behaviour taking into account the environmental characteristics of the locality. Environmental parameters entering into prediction models are naturally random variables, so it is appropriate to use probability methods when applying prediction models. From a practical point of view, it is possible to divide the prediction models into two groups: (a) prediction models for estimating corrosion rate (corrosion loss) after the first year of exposure; (b) prediction models for estimating corrosion loss after long-term exposure of the structure. Long-term exposure of weathering steel leads, with extended exposure duration, to gradual deceleration of corrosion loss of material on the surface of the structure. The basic input variable for prediction of long-term corrosion loss is the corrosion rate in the first year of exposure of the metal r corr . The value of long-term corrosion loss can be determined in compliance with EN ISO 9224 (2012) according to the formula: = ∙ (1) where D is corrosion depth [g/m 2 , μm ]; t is exposure time [years]; r corr is corrosion rate in the first year of exposure [g/(m 2 ·y), μm/ y] and b is coefficient of time dependence specific for combination iron-environment (usually less than 1). The input value of corrosion rate ( i.e. corrosion loss) after one year of exposure can be determined in two ways: using atmospheric corrosion tests or using analytical prediction equations, so called the dose-response functions . The most accurate way is experimental measurement using steel samples of 150x100x1 mm size that are exposed for one year in the monitored environment, see EN ISO 9226 (2012). However, this straightforward method is time consuming. The most commonly applied method, used for example in corrosion maps, is therefore an estimate using analytical prediction relationships derived from the statistical processing of a large set of corrosion tests. The input parameters of the analytic prediction relations are environmental parameters. However, the inaccuracy of the results is up to 50 %, see Kreislova et al. (2017). For the estimation of corrosion loss, the average annual values of climatic parameters are used in most cases. Analytical relationships for prediction of corrosion loss after one year of exposure were derived on the basis of the results achieved in the implementation of research programs aimed at long-term atmospheric testing, see EN ISO 9223 (2012), Kreislova et al. (2017) and Multi-Assess Report (2017). Long-term measurements made it possible to take into account more environmental parameters in the calculation of corrosion loss, thus achieving a more accurate result which takes into account changes in annual averages of climatic parameters. Analytical relationships for estimating corrosion loss are derived for individual types of metals. Analytical relationships derived from corrosion behaviour of carbon steel can also be applied to weathering steel. For weathering steel, a similar corrosion rate can be expected after one year of exposure as with carbon steel except for very corrosive aggressive environments. 2.1. Prediction of corrosion losses according EN ISO 9223 The standard EN ISO 9223 (2012) provides analytical prediction relationships for the estimation of corrosion loss for individual types of metals. The dose-response functions were derived from large sets of experimentally determined data. The equation for carbon steels has four parameters and takes into account the basic parameters of the atmosphere influencing the course of atmospheric corrosion. For determination of corrosion loss after one year of exposure of structural carbon steel (or weathering steel) the following relationship applies: 2. Estimate of corrosion losses with prediction models