Ditch Corrosion Generated in Flowing Boiler Feed Water

摘要

In order to elucidate the cause of extraordinary corrosion which often occurs on the inside wall surface of heat conduction tubes of boilers, corrosion tests were conducted using a jet-in-slit apparatus in various boiler feed water flowing under high pressure at elevated temperature. As a result, ditch corrosion was detected with a rate in excess of 1 mm/y under some combined conditions of material, environment and fluid flow. In this paper, the experimental results obtained to date are summarized with the aim of proposing a mechanism of ditch corrosion occurrence. That is, the influence of fluid flow conditions as well as environmental conditions such as dissolved oxygen content (DO), pH and the temperature of the feed water on the occurrence of ditch corrosion were investigated. The observation of corrosion damaged surfaces and the depth distribution of the damage revealed that the ditch corrosion occurs only under specific conditions where the corrosion products formed on the metal surface are not a single species but consist of two different types of oxides. The process by which these oxides are formed appear to be influenced by how the ferrous ions are solubilized and how the hydroxide ions are located on the metal surface. In order to verify this interpretation, corrosion tests were carried out on carbon steel to successfully reproduce the ditch corrosion at ambient temperature, thus validating the interpretation.