Caustic Embrittlement: Caustic embrittlement is a type of boiler corrosion caused by using highly alkaline water in the boiler. During softening process by lime-soda processes, free Na2CO3 is usually present in small proportion in the softening of water. In high pressure boilers, Na2CO3 decomposes to give sodium hydroxide and carbon-dioxide.
Na2CO3 + H2O = 2NaOH + CO2
And this makes boiler water "Caustic". The NaOH containing water flows into the minute hair cracks, always present in the inner side of boiler, by capillary action. Here water evaporated and the dissolved caustic soda concentration increases progressively. This caustic soda attacks the surrounding area, thereby dissolving iron of boiler as sodium ferrate. This causes embrittlement of boiler parts, causing even failure of the boiler. Caustic cracking can be explained by considering the following concentration cell. The iron surrounded by the dilute NaOH becomes the cathodes sides, which the iron in contact with concentrated NaOH becomes anodic part, which is consequently dissolved or corroded.