Caustic corrosion in a boiler waterside tube: Root cause and mechanism

We report a failure occurred in a boiler tube of low alloy steel, SA-210 Grade A-1. A severe thickness reduction was detected in a boiler tube during periodic investigations. Two identical parallel grooves along the horizontal tube were observed. The inner surface of the tube consisted of three distinct parts: (1) dark gray scale under the grooves, (2) hemi-elliptical grooves, and (3) brown porous scale above the grooves. Microscopic investigations and microhardness tests revealed that the microstructure and properties of the base metal is intact along the tube. Moreover, some needle shape crystals which represented Na2FeO2 and NaFeO2 compounds were observed in SEM images taken from the groove scale. An excessive amount of Na and Cu was detected by EDS in the groove. The XRD results confirmed the presence NaFeO2 which is the main product of caustic corrosion. Therefore, it was concluded that the evaporation at the waterline of the partially filled tube leaded to locally caustic concentration which dissolves the protective magnetite layer and subsequently attacked the bare metal.