In steam generation systems there is a little-known phenomenon that can cause severe damage: FAC.
FAC (Flow Accelerated Corrosion) is a corrosion mechanism that progressively attacks the wall of carbon steel piping, acting through the continuous dissolution and removal of the protective magnetite layer, aided by flow velocity and turbulence. It requires neither oxygen nor acidic pH to occur, and rarely shows external signs before failure.
Factors That Influence FAC
- Velocity: velocities above 2.4–3 m/s (8 to 10 ft/s) can be enough to induce FAC;
- Turbulence: the transport of soluble iron away from the oxide surface is strongly increased by turbulence, accelerating metal loss;
- Temperature: FAC temperature curves typically have a bell shape. It occurs between 100 and 250°C. For single-phase flow, maximum loss occurs between 130 and 150°C; for two-phase flow, between 150 and 200°C;
- pH: as pH rises, material loss decreases. This is an important line of defense, and the presence of dissolved CO₂ can compromise this control;
- Dissolved oxygen: in single-phase systems, O₂ inhibits FAC by stabilizing the oxide layer. In two-phase systems, this protection is less effective;
- Steel composition: certain alloys increase resistance to FAC. Chromium and molybdenum alloys show greater resistance.
Ignoring FAC is a common mistake. It's also a common mistake to evaluate each of these parameters in isolation and conclude the risk is low, when their combination says otherwise.
Knowing which points of the system are most exposed is the first step. In Part II, we cover how to identify critical regions and what a monitoring program needs to cover to be effective.