Estimated and Stationary Atmospheric Corrosion Rate of Carbon Steel, Galvanized Steel, Copper and Aluminum in Iran
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TRODUCTION
OUTDOOR atmospheric corrosion of metals and alloys has serious implications for structural and industrial equipment as approximately half of the total annual cost of all corrosion types of metal is due to the issue. It has also caused catastrophic environmental issues.[1,2] Corrosion behavior of metals in an atmosphere is mainly affected by both their surface condition and environmental factors.[3,4] Based on the atmospheric corrosivity classification provided by the International Standards Organization (ISO 9223),[5] the various types of atmosphere include marine, industrial, urban and rural environments. The main environmental factors and parameters having important repercussions on
MAHDI SHIRI and DAVAR REZAKHANI are with the Metallurgy Department, Niroo Research Institute (NRI), Tehran 14665-517, Iran. Contact e-mail: [email protected] Manuscript submitted November 30, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
atmospheric corrosion are[1,5] time of wetness, SO2 pollution (P) and airborne salinity (S), considered the concentration of Cl ions in the atmosphere.[5] Based on the TOW definition, electrochemical reactions, which are encouraged by forming a liquid film on the metal surfaces (wet layer), account for atmospheric corrosion. Dew, rainfall, melted snow, temperature and humidity level are contributing factors to both the forming and stability of the wet layer.[6,7] In fact, TOW is calculated by estimating the number of hours per year that the relative humidity of the atmosphere is > 80 pct and the temperature > 0 C.[5] Additionally, metal degradation increases in the presence of contaminants, as pollutants are a fundamental cause of larger TOWs and consequently higher corrosion rates.[8,9] In terms of corrosivity, the main atmospheric pollutants are SO2 and Cl ions.[10–12] Sulfur dioxide plays a crucial role in atmospheric corrosion in urban and industrial areas[3] while the effect of chlorine ions on corrosion is more obvious at coastal areas.[13]
The International Organization for Standardization (ISO) has developed two different approaches to calculating the atmospheric corrosion rate. Based on ISO 9223,[5] the predicted corrosion rate can be calculated using dose-response functions (Eqs. [1] through [4]) linked to meteorological and environmental variables such as RH, T, and SO2 and Cl concentrations. In fact, they are derived from results of the previous field tests for calculation of corrosion loss. While based on ISO 8565,[14] the actual atmospheric corrosion rate of metals can be measured by exposing the metals to the outdoor atmosphere at a fixed time. This corrosion rate is related to the weight loss of the exposed metals in the atmosphere. The most used structural metals in the outdoor atmosphere include carbon steel (St), galvanized steel (GS), copper (Cu) and aluminum (Al).[15] Offering good mechanical behavior, carbon steels and galvanized steels have been widely utilized in the construction industry, but they suffer corrosion especially in moist and marine environments.[16
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