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Tomáš Prošek
Technopark Kralupy of the University of Chemistry and Technology Prague, Czech Republic
Stainless steel materials provide excellent service when selected and applied properly. Sufficient knowledge base is generally available for material selection for immersion conditions. This is not true for applications of stainless steels under atmospheric weathering conditions. Several cases of ceiling collapse and other component failures have shown that austenitic stainless steels are prone to stress corrosion cracking (SCC) under specific atmospheric conditions characterized by the spontaneous formation of concentrated chloride solutions under highly soluble chloride deposits even at room or only slightly elevated temperatures. This was observed in indoor swimming pools, for outdoor climbing anchors, under evaporative conditions in oil and gas production, storage, and processing and in other specific cases.
Corrosion performance of austenitic-ferritic (duplex) stainless steels UNS S32101, S32202, S32304 and S32205 and austenitic stainless steels UNS S30403 and S31603 was studied in the presence of chloride deposits simulating non-rinsing atmospheric conditions. The effect of temperature, relative humidity, concentration and composition of the chloride deposits on the tendency for atmospheric low-temperature chloride-induced SCC, pitting and selective corrosion was assessed using pre-stressed samples with a circular weld. In the presence of calcium chloride, SCC was observed at temperatures as low as 20 and 30 °C on the austenitic stainless steels UNS S30403 (304L) and UNS S31603 (316L), respectively. The only cases of SCC of tested duplex stainless steel grades were found at 70 °C, which otherwise suffered mainly selective dissolution of the ferrite phase with one order lower depth of attack. The initiation of SCC and selective/pitting corrosion was governed by the equilibrium chloride concentration in a solution formed by contact with chloride containing deposits and with air at a given relative humidity. Threshold levels of critical chloride concentrations, critical relative humidity
in the presence of specific deposits and maximum temperatures for safe applications of the studied grades were established. The threshold levels were compared to climatic and chloride deposition and retention data obtained in field conditions.
Key words: Stress corrosion cracking; localized corrosion; atmospheric corrosion; chloride deposits; stainless steel
