Damage/corrosion mechanism: flow accelerated corrosion (FAC)

 Flow accelerated corrosion (FAC) is an electrochemical corrosion process which causes wall thinning of low-alloy and carbon steel components in steam and feedwater systems. 

Influencing factors

During FAC, the normally protective oxide surface layer (magnetite Fe₃O₄) dissolves into a flow of water (single-phase) or wet steam (two-phase). Metal loss occurs due to a continuous formation and subsequent dissolution of magnetite. Thinning rates up to 3 mm/yr have been reported.

Figure 1: 8. Kain V, Roychowdhury S, Ahmedabai P, Barua D K, Engg Failure Analysis 18 (2011) 2081

Thinning rates due to FAC are besides fluid velocity influenced by temperature, pH, oxygen content/open circuit potential (ORP) among others. The highest FAC rates are observed at 150-180 °C. Higher pH values reduce metal loss due to FAC. Presence of oxygen/increasing ORP can reduce FAC by converting the magnetite to hematite (Fe₂O₃).

Figure 2: Solubility of Magnetite as a Function of Temperature at Various Ammonia Concentrations adapted from P. Sturla, “Oxidation and Deposition Phenomena in Forced Circulating Boilers and Feedwater Treatment”, Fifth National Feedwater Conf., Prague: 1973. (in French) - Guidelines for Control of FlowAccelerated Corrosion in Fossil and Combined Cycle Plants 3002011569 Final Report, December 2017 EPRI

How to identify FAC

• FAC leads to a general wall thickness decrease.
• The surface may exhibit a continuous scalloped or orange peel appearance oriented in the direction of flow. This surface differs from the irregular, sharp pits caused by cavitation or from the relatively sharp pits caused by droplet impingement.
• There is a very thin oxide layer or no oxide remaining on the surface. This can be confirmed by metallographic investigation.

Read more about FAC in Technical report ‘Guidelines for Control of Flow-Accelerated Corrosion in Fossil and Combined Cycle Plants’, EPRI, December 2017.