Corrosion Failure Analysis | Advanced Power Plant Chemistry

Wall Loss

Minimal

outside failure zone

Fracture Type

Thick, Brittle

little deformation

Location

Furnace Wall

near known deposit history

Mechanism	Wall Loss Pattern	Fracture/Surface Character	Typical Location
Oxygen pitting	Narrow, deep, isolated cavities	Often tubercle-capped	Idle/stagnant equipment
Caustic gouging	Irregular, elongated, gouged	Rough, grooved texture	High heat flux, deposit zones
Acid phosphate corrosion	Sharply defined wastage	Deposit remnants, iron phosphate	Deposit zones, low Na:PO₄ history
Hydrogen damage	Minimal external loss	Thick-edged, brittle rupture	Prior acid corrosion site
FAC	Gradual, scalloped thinning	"Orange peel" texture	Elbows, tees, flow disruption

Click any mechanism to see whether it fits this failure. The finding: minimal wall loss, a thick-edged brittle rupture, on a furnace wall tube with a documented deposit history.

Select a mechanism to test it against the findings.

The Failure Report

A furnace wall tube has failed unexpectedly during normal operation. Inspection findings: the fracture is thick-edged with little visible deformation — a brittle-style rupture. Wall thickness measurements taken circumferentially around the failure location, away from the immediate rupture, show only minimal loss — nothing close to the thinning that would typically be expected to explain a wall failure through simple material loss. Maintenance records show this tube location had a documented history of localized deposit accumulation identified during a prior inspection, and the plant's phosphate treatment program had experienced a multi-day ratio drift toward low Na:PO₄ roughly eight months before the failure, similar in character to the Module 2.7 scenario, though it was caught and corrected at the time.

Working Through the Table

Using the comparison table in the Diagram tab, work through each mechanism against the two most distinctive findings: minimal wall loss, and a thick-edged brittle fracture.

Ruled out — oxygen pitting (Module 4.2): Pitting does produce isolated, narrow, deep cavities with comparatively little surrounding wall loss, which is superficially consistent with "minimal wall loss." But pitting does not typically produce a thick-edged brittle rupture fracture appearance — a pit that eventually perforates does so as a small hole through continued dissolution, not as a brittle crack. Also, pitting is most associated with idle/stagnant conditions, not an operating furnace wall location. Doesn't fit well.

Ruled out — caustic gouging (Module 4.3) and FAC (Module 4.6): Both mechanisms are defined by progressive, visible metal loss — irregular gouging or gradual scalloped thinning, respectively. Both would be expected to show meaningful wall loss at or near the failure, which directly contradicts the "minimal wall loss" finding. Neither produces a brittle fracture character. Doesn't fit.

Partially fits, but incomplete — acid phosphate corrosion (Module 4.4) alone: The documented Na:PO₄ ratio drift and deposit history are strong contextual matches for acid phosphate corrosion conditions having existed at this location. But acid phosphate corrosion itself is a surface dissolution mechanism, and dissolution alone doesn't explain a brittle, thick-edged fracture with minimal measured wall loss. This finding is a critical clue pointing toward what happened as a result of that acid corrosion, rather than being the complete answer on its own.

Best fit — hydrogen damage (Module 4.5), originating from acid phosphate corrosion: This is the pattern that fits every finding together. The documented low-ratio period created exactly the acid corrosion conditions described in Module 4.4, at a location with known deposit accumulation — precisely the setup Module 4.5 identified as the typical starting point for hydrogen damage. The resulting atomic hydrogen diffused into the steel, and the eventual failure presents exactly as Module 4.5 described: a thick-edged, brittle rupture with comparatively little external wall loss, because the damage was internal (grain boundary weakening) rather than external material removal.

Why the Eight-Month Gap Matters

The ratio drift was corrected eight months before the failure — which might seem to argue against it being the cause, since the "problem" was fixed. But this is consistent with hydrogen damage's nature as described in Module 4.5: the internal grain-boundary weakening from methane accumulation doesn't reverse itself once the acid corrosion conditions that triggered it are corrected. The damage, once done, remains in the steel's microstructure, weakening it progressively until a stress event — thermal cycling, a pressure transient, or simply accumulated fatigue — triggers the eventual brittle fracture, potentially long after the root chemistry condition was resolved.

Field note: This scenario illustrates why chemistry excursion history (Module 1.5) needs to be retained and reviewable well beyond the immediate response window. An excursion "closed out" and corrected months or years ago can still be directly relevant root-cause information for a failure investigation much later — hydrogen damage in particular has exactly this kind of delayed-consequence profile.

The Diagnostic Skill This Module Is Building

Match physical findings (wall loss pattern, fracture character, location) against the mechanism table first — this narrows the field quickly.
When physical findings alone don't fully explain the failure, look for a root-cause mechanism that explains the findings as a downstream consequence, not just a direct match.
Pull operational and chemistry history for the specific location, not just current conditions — as this scenario shows, root causes can precede failures by a long interval.

Root-Cause Chain

A sequence in which one corrosion mechanism (such as acid phosphate corrosion) creates the conditions that trigger a distinct downstream mechanism (such as hydrogen damage), rather than a single isolated cause.

Delayed-Consequence Failure

A failure whose root cause occurred and was potentially even corrected well before the failure itself manifests, characteristic of mechanisms like hydrogen damage where internal damage persists after the triggering condition ends.

Failure Analysis

The systematic process of matching physical evidence (appearance, location, wall loss pattern) and historical context against known corrosion mechanisms to determine root cause.

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