FAC Corrosion

When a steel surface is in contact with water under specific chemical and thermodynamic conditions, an oxide protective layer of magnetite (Fe3O4) is developed. Controlling the growth mechanism, the morphology and the solubility of such corrosion product it is possible to minimize the risk of failure and extend the life-time of the plant. The presence of O2 in the water chemistry is well known to be the most harmful corrosion catalyst.

For this purpose, oxide scavenger is generally dosed as protective agent which also contributes to build a film of magnetite. However, this common practice can be quite detrimental in the low temperature regions of the plant where the thermodynamic solubility equilibrium cannot be reached, and magnetite is been formed in the temperature range of maximum dissolution. As a result, single-phase FAC mechanism will lead to general corrosion.

Solution: keep O2 concentration high to change the Schikorr reaction kinetic to the formation of ferric oxide hydrate (FeOOH) which is stable at low temperature. The following photo shows the frontal baffle plate of a feedwater heater affected by flow-accelerated corrosion