Module 4.3 — Module 2.3 introduced the bulk-vs-local chemistry gap; this module covers the actual attack chemistry and how gouging is identified on inspection.
Click any step to see what's happening. The reaction becomes self-sustaining as long as the local concentrating condition persists — producing the characteristic irregular gouged appearance.
Module 2.3 established the core problem: bulk boiler water free NaOH can read safely under 1 ppm while local concentration under a deposit or steam blanket climbs thousands of times higher. This module covers the actual chemical attack sequence once that local concentration is established, and how to recognize gouging on inspection — the practical skill that closes the loop on the caustic treatment discussion.
Recall from Module 1.1 that magnetite (Fe₃O₄) is the protective layer that forms on carbon steel under proper alkaline pH control, and that it's the reason alkaline chemistry protects against general corrosion in the first place. Caustic gouging is fundamentally an attack on that same protective layer: concentrated sodium hydroxide reacts directly with magnetite, converting it to soluble sodium ferrite compounds and effectively dissolving away the protection the whole treatment program is built around. Once magnetite is gone at a given spot, bare carbon steel is exposed to continued, essentially unimpeded attack by the still-concentrated local caustic — and the reaction is self-sustaining as long as the concentrating mechanism (the deposit or steam blanket from Module 2.3) remains in place.
Caustic gouging produces a visually distinctive appearance: irregular, often elongated cavities with a somewhat rough or grooved internal texture, typically found at locations with known deposit accumulation history or steam blanketing risk — often near the high-heat-flux zones of a furnace wall, or at locations where flow disruption promotes deposit settling. This distinguishes it from the more uniform thinning of general corrosion and from the narrow, deep, often tubercle-capped cavities characteristic of oxygen pitting (Module 4.2). Metallurgical examination can further confirm gouging by identifying characteristic corrosion product chemistry (sodium ferrite compounds) associated specifically with this mechanism.
Module 2.3 identified deposit control as the real lever for managing caustic gouging risk, and the mechanism explained here makes clear why: without a deposit or steam blanket to concentrate caustic locally, bulk water chemistry alone — even at the alkaline pH levels caustic treatment requires — doesn't reach the concentration needed to attack magnetite meaningfully. The entire gouging risk depends on that local concentrating condition existing somewhere. This is why blowdown discipline (Module 2.5) and good makeup water treatment (Module 1.3) are protective against gouging even though neither directly touches free NaOH concentration — they reduce the deposit formation that gouging fundamentally requires.