Where, specifically, within a steam system, is the risk of FAC highest?
The answer depends on a range of factors such as system geometry, operating conditions and treatment quality. Analyzing the causes of FAC, already described in Part I, it concentrates in turbulent regions that also meet the other conditions: pH below 9, a range in which magnetite solubility increases significantly, and temperature between 100 and 250°C, with maximum loss between 130 and 150°C:
- Bends and elbows, especially short-radius ones, where the change in direction creates localized erosion on the outer wall;
- Diameter reductions, where flow acceleration increases iron ion transport;
- Equipment outlets, such as economizer, heater and flash tank outlets;
- Regions downstream of control valves, where pressure loss generates high downstream turbulence;
- Condensate return lines, particularly in systems with dissolved CO₂;
- Wet steam piping (two-phase flow), where phase interaction amplifies the attack.
A field example illustrates the problem well: excessive oxygen scavenger dosing combined with poor steam distribution at the bottom of a tray-type deaerator was the cause of FAC. The stripping section operated at 1.7 barg, a pressure that corresponds to saturated liquid at 116°C, within the FAC range. A careful study showed that a baffle plate at the steam inlet was responsible for the increased turbulence, combined with excess scavenger dosing and a pH below 8.5, resulting in FAC.
Structure of a Monitoring Program
Monitoring FAC requires a structured program with:
- Chemical monitoring: pH, dissolved oxygen, conductivity, soluble iron and CO₂, at least daily at critical points and weekly elsewhere;
- Thickness inspection: ultrasonic thickness testing (UT) or TOFD at points mapped as critical, at a frequency set by the system's historical corrosion rate;
- Trend analysis: thickness-over-time charts by point — the trend over time is the real indicator.
A well-built monitoring program doesn't prevent FAC. It ensures you know where it is and how fast it's progressing, before residual thickness reaches the critical limit.
In Part III, we close out the topic with the formal risk map and the defenses that actually reduce the attack rate.