Abstract
Abstract The beneficial effect of ethylene diamine tetraacetic acid (EDTA) in the formation of a better protective magnetite film at high temperatures (473 to 573 K) on carbon steel is well known. But when the Monel† 400 (UNS N04400) alloy is present, even in a galvanically decoupled condition, using the usually adopted simple alkaline pH treatment for magnetite formation seems to be more advantageous than using the EDTA treatment. This reversal in the role of EDTA has been attributed to the preferential dissolution of nickel from Monel by the sequestering agent, giving rise to species such as (NiEDTA)2− and Ni(OH)2 in the aqueous medium, which seem to be responsible for forming a magnetite film of reduced passivity. The above conclusion is based on monitoring the oxide formation on carbon steel and UNS N04400 under static autoclave conditions by weight loss kinetics, surface analysis of elements in the oxide coating, bulk analysis of elements released from the base metal to the medium, scanning electron ...
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