Flow Accelerated Corrosion (FAC), also known as flow-assisted corrosion, is a degradation mechanism in which a normally protective oxide layer on a metal surface dissolves in a fast flowing fluid. The underlying metal corrodes to re-create the oxide, and thus the metal loss continues until eventually the remaining wall thickness is insufficient to contain the internal pressure and rupture occurs.
Many pipe failures caused by FAC have been reported in power plant piping systems. Under certain water chemistry, fluid velocity, and operating conditions, FAC can cause internal wall thinning of condensate and feed water piping, heater drip and drain lines, and other carbon steel piping. In some cases, this thinning has led to catastrophic failures and fatalities.
Figure shows the Mihama accident damage in 2004 and the rupture location.
Mihama accident 2004; pipe (left), rupture location (right)
FAC typically occurs in the temperature range of approximately 80°C to 300°C and is commonly found in economizer tubing, low pressure evaporator tubing, drum internals and feed water systems. In addition, the propensity for forming FAC damage is greater in areas of high turbulence, mostly in elbows and its damage may extend to the straight pipe section up for approximately a length of 1 pipe diameter upstream and 2 pipe diameters downstream.