Microstructural characteristics of the oxide films formed on Alloy 690 TT in pure and primary water at 325 °C

• Oxidation behavior of Alloy 690 TT is influenced by water chemistry.
• The porous, Cr-depleted inner layer formed in oxygenated water is not protective.
• Increase in pH promotes the dissolution of Cr species.
• Higher solution conductivity leads to a thicker oxide film in primary water.
• Sample surface status affects the thickness of inner oxide layer.

Microstructural characteristics of the oxide films formed on Alloy 690 TT after the exposure in oxygenated pure and primary water at 325 °C were investigated. Change in water chemistry from pure to primary water was found to vary the film structure from duplex to triple layer, and lead to change in the morphology of NiO on the surface. The increased pH and solution conductivity of primary water also accelerate the dissolution of Cr species and enhance the electrochemical corrosion. The Ni-rich and Cr-depleted inner layer formed under both water chemistries is porous and not protective. The related oxidation mechanism is discussed.