Module 4.4 — The low-ratio counterpart to caustic gouging: what happens when the Na:PO₄ ratio drifts the other direction, and how to tell the two mechanisms apart.
Click any zone to see its mechanism. Acid phosphate corrosion is the mirror image of caustic gouging — same deposit-concentration mechanism, opposite chemistry driving the attack.
Module 2.1 introduced the Na:PO₄ molar ratio as CPT's actual control target, with a congruent zone bounded by two failure modes: too high a ratio risking free caustic and gouging (now covered in Module 4.3), and too low a ratio — excess phosphate relative to sodium — risking acid phosphate corrosion. Module 2.7's troubleshooting scenario walked through detecting a ratio drift toward this low-ratio side. This module explains the mechanism that scenario was working to prevent.
Certain sodium phosphate species, when concentrated locally under a deposit through the same wicking and evaporation mechanism described for caustic gouging in Module 2.3, don't remain neutral or alkaline — they can hydrolyze and generate local acidity rather than local causticity. This is the direct chemical consequence of having too little sodium relative to phosphate: the phosphate compounds present under those low-ratio conditions concentrate into a more acidic local chemistry rather than an alkaline one, attacking magnetite through an acid mechanism rather than the direct caustic dissolution described in Module 4.3.
| Factor | Caustic Gouging | Acid Phosphate Corrosion |
|---|---|---|
| Ratio direction | High Na:PO₄ (excess sodium) | Low Na:PO₄ (excess phosphate) |
| Local chemistry | Highly alkaline | Locally acidic |
| Appearance | Irregular, elongated gouges | More sharply defined wastage, often with visible phosphate-related deposit remnants at the attack site |
| Confirming evidence | Sodium ferrite corrosion products | Iron phosphate/acidic corrosion product chemistry on metallurgical exam |
Recall Module 2.7's scenario: phosphate concentration held steady in target range while pH slid downward over four days, with the correct diagnosis being ratio drift toward the acid phosphate zone rather than hideout or a sampling artifact. This module explains exactly what was at stake in that scenario — left unaddressed, that kind of ratio drift is the precursor condition for acid phosphate corrosion actually occurring at a deposit site somewhere in the boiler. The chemistry data sheet monitoring taught in that module isn't an abstract exercise; it's specifically aimed at catching this mechanism before it produces the damage described here.