Caustic Gouging
Sodium hydroxide (NaOH) is used extensively in boiler water treatment to maintain the optimum hydroxyl ion concentration range to form a protective magnetite film on steel surfaces and to help form nonadherent sludges when hardness enters the boiler water. However, excessive sodium hydroxide can destroy the protective film and corrode the base metal . NaOH can concentrate during departure from nucleate boiling (DNB), film boiling or steam blanketing conditions. Concentration also occurs when normal boiler water evaporates beneath deposits leaving behind the caustic at the metal surface. The effect of tube metal gouging beneath deposits is shown in the photomacrograph.

Caustic gouging can also occur due to evaporation along a waterline without significant accumulation of deposit .
In these cases, solubilized sodium ferroite is removed from the base tube metal, but then hydrolyzes and precipitates elsewhere in the boiler as magnetite when the concentrated water is diluted by normal boiler water.