neqsim-flow-accelerated-corrosion

NeqSim Flow-Accelerated Corrosion and High-Temperature pH Skill

Screening for flow-accelerated corrosion (FAC) — dissolution of the protective magnetite film under mass-transfer control — and for the in-situ pH that governs it.

When this skill applies

• Localised wall thinning or leaks at bends, elbows or circumferential welds in carbon-steel tube bundles, with intact surfaces nearby.
• Closed heating- or cooling-medium loops (glycol/water or water), boiler-feedwater, condensate, WHRU / economiser / steam-generator tubes.
• Operating temperature roughly 90–250 °C, peaking near 150 °C.
• Questions about pH control of an amine-buffered loop, or whether measured chemistry is adequate.

Two mistakes to avoid before anything else

1. Do not judge a hot system by its laboratory pH.

Laboratory pH is measured on a cooled sample, typically at 20–25 °C. Corrosion happens at 150 °C. Two things move, by different amounts:

• The amine pK$_a$ falls, so the buffered fluid becomes less alkaline.
• Neutrality itself falls: neutral water is pH 7.00 at 25 °C but about pH 5.85 at 150 °C.

So the meaningful quantity is the alkaline margin, $\text{pH}(T) - \text{pH}_{neutral}(T)$, not the raw pH. Use AmineBufferedPH.

2. Do not confuse FAC with erosion-corrosion. They occur at the same locations and need different mitigation:

RootCauseAnalyser now raises both as separate candidates.

3. Do not read a CFD wall-shear map as a FAC map at a separated feature.

FAC is mass-transfer controlled. FlowAcceleratedCorrosion gets there through Berger-Hau, $Sh = 0.0165,Re^{0.86}Sc^{0.33}$, with a Stokes-Einstein diffusivity — for a hot glycol/water loop $Sc$ lands near 280, so the concentration boundary layer is roughly $Sc^{1/3}\approx 6.5$ times thinner than the momentum one.