Category: Forensic Engineering

Erosion degradation on the outside of cold reheat tubes are due to mechanical action, frequently by impingement of wet steam and/or high-velocity flow. Impingement can be controlled by placing baffle plate tray protections upstream the heat exchanger module; high velocity flow can be handled by oversizing inlet nozzle diameter. However, the effects depicted in the photo represent a more complex failure mechanism commonly encounter during the commissioning of a power plant.

Molten salts, commonly used as heat transfer fluids in solar power plants, are essentially eutectic mixtures of sodium-potassium nitrates and the nature of these bonds provides strong ionic conditions and good chemical stability of the thermal solution against metal corrosion.

However, they are very hygroscopic and highly reactive with water molecules, which combined with its tend of thermal dissociation to form NOx, can produce high concentrated nitric acid and lead the system to a quite extreme acid attack.

The corrosion phenomenon can occur within a few hours and the effects are shown in the following photo. Make sure to dry the system after the pressure test and nitrogen purge is a must. We really mean it.

Austenitic Stainless Steels present corrosion resistance ability due to high chromium (Cr) content. Chromium naturally forms a thin, adherent and very protective passive oxide layer when exposed into corrosive aqueous environment which is primarily the reason of its high resistance capabilities.

When austenitic stainless is exposed to a corrosive aqueous environment, chromium oxide enriches at the metal interface due to the formation of passive and resistent corrosion layer.