When Heat Becomes the Enemy Sulphidation, Creep, and the Shutdown Crack in Refinery High-Temperature Exchangers

In the first two parts of this series, corrosion was driven by chemistry and by flow. In the refinery hot-exchanger, a third agent takes command — temperature itself. Above a few hundred degrees, steel stops being a passive bystander to its environment and begins to react with it: sulphur attacks it, hydrogen invades it, and under sustained load it slowly deforms and tears. And in a final irony, the most dangerous crack of all appears not while the exchanger is running hot, but in the quiet hours after it has been shut down and opened to the air. This closing part is about the failures that heat makes possible.

This is the third and final part of the series. Part 1 covered chemical and petrochemical exchangers; Part 2 covered condensers. Here we deal with the feed-effluent and process exchangers of the refinery hot train — crude units, hydrotreaters, hydrocrackers, and vacuum service — where metallurgy, temperature, and sulphur meet.

Where Temperature Changes the Rules

A refinery high-temperature exchanger recovers heat from a hot effluent stream and uses it to preheat a colder feed. That duty places one side of the bundle in the hundreds of degrees, often carrying sulphur compounds, hydrogen, and organic acids. At these temperatures the failure mechanisms are no longer simply electrochemical — they are thermal, metallurgical, and time-dependent. A tube may look sound for years and then fail, because the damage has been accumulating invisibly inside the metal itself.

As in every exchanger, the damage concentrates in specific zones — but here the zones are defined by temperature and by phase, not merely by flow. The diagram below shows where each high-temperature mechanism takes hold.

Figure 1 — Zones of damage in a refinery high-temperature exchanger. The hot inlet, the bundle interior, the cold outlet, the welds, and the bends each face a different temperature-driven mechanism.

The Three Signature High-Temperature Failures

Three mechanisms dominate the hot-exchanger investigations my teams have handled. They differ in a way that is worth grasping clearly: one is driven by sulphur, one by sustained stress and time, and one — the most treacherous — by the shutdown itself.

Figure 2 — The three signature high-temperature failures. Sulphidation thins the wall, creep deforms and splits it, and polythionic acid cracks it during shutdown — each with a distinct signature.

Sulphidation. Above roughly 260°C, sulphur compounds in the hydrocarbon stream react directly with the steel to form an iron-sulphide scale. The wall thins uniformly beneath a thick, adherent, dark scale, and the rate is governed by temperature and by the chromium content of the alloy — which is precisely why higher-chromium steels are specified for hotter service. It is worst at the hot inlet, where the metal is hottest.

Creep and stress rupture. A tube held under sustained internal pressure at high temperature does not need corrosion to fail. The metal itself slowly deforms, cavities nucleate along grain boundaries, and over years of service the wall bulges and finally splits in a characteristic longitudinal, thick-lipped rupture. Creep is a race against time and temperature, and it is often the mechanism behind a failure in a tube that chemical analysis shows to be entirely on-specification.

Polythionic acid stress-corrosion cracking. This is the mechanism every refinery metallurgist respects most, because it strikes when the unit is safely shut down. It deserves its own explanation.

The Shutdown Crack: How Polythionic Acid Attacks a Cold Exchanger

During operation, sulphur in the stream builds an iron-sulphide scale on austenitic stainless steel surfaces — harmless in itself while the unit is hot and dry. But when the exchanger is shut down and opened to the atmosphere, moisture and oxygen reach that scale and convert it into polythionic acid directly on the metal surface. If the stainless steel has been sensitized in service — its grain boundaries depleted of chromium — the acid attacks those boundaries and drives intergranular stress-corrosion cracks. The exchanger that ran safely for years can crack in the days after it stops.

Figure 3 — The polythionic acid sequence. The scale forms in service, but the acid — and the cracking — appears only after shutdown exposes it to air and moisture. The defence is procedural.

The crucial insight is that the defence is not metallurgical but procedural. Because the acid forms only when air and moisture reach the sulphide scale, the crack is prevented by keeping them away — neutralizing the surface with a soda-ash wash, or blanketing the equipment under dry nitrogen, before and during the turnaround. This is why shutdown and lay-up procedure, not just material selection, sits at the heart of hot-exchanger integrity. As in Part 2’s condenser, the most damaging period can be the one when the equipment is doing nothing.

The Damage Mechanisms of High-Temperature Service

Beyond the three headline failures, the refinery hot exchanger is exposed to a broader family of temperature-driven damage. The table below is the working map my teams use.

#	Damage Mechanism	Typical Location	Visual Signature	Common Driver
1	Sulphidation	Hot inlet, shell, hot tube rowsHottest tubes under sustained stress	Uniform wall loss, thick adherent sulphide scale	Sulphur compounds attacking steel above ~260°C
2	Polythionic acid SCC	Sensitized SS, cold outlet, shutdown	Intergranular branched cracks	Sulphide scale + moisture + air on shutdown
3	Creep / stress rupture	Welds, C-½ Mo in H₂ service	Bulging, longitudinal split, creep voids	Sustained stress at high temperature over time
4	High-Temperature Hydrogen Attack	Bends, elbows, high-velocity zones	Internal fissuring, decarburization	Atomic hydrogen + temperature + partial pressure
5	Flow-accelerated corrosion	Hot crude/vacuum service, high-velocity	Smooth scalloped wall thinning	Turbulence stripping the protective oxide film
6	Naphthenic acid corrosion	Hot crude/vacuum service, high-velocity	Sharp-edged grooves, clean pits	Naphthenic acids at 220–400°C, high TAN feed
7	Carburization / metal dusting	Very high temperature, carbon-rich gas	Coke pitting, loss of ductility	Carbon ingress at elevated temperature
8	Temper embrittlement	Cr-Mo welds/base after long service	Brittle intergranular fracture on cooling	Prolonged exposure in 340–565°C range

Several of these mechanisms leave little to see on the surface. High-temperature hydrogen attack and creep do their damage inside the wall, and temper embrittlement changes nothing visible at all until the metal fractures. This is what makes high-temperature failure analysis so dependent on metallography and on knowing the service history — the surface alone will not tell the story.

The TCR Approach: Reading Damage Inside the Metal

The convergence principle established across this series applies with full force here, but high-temperature work demands one addition: the service history is itself a piece of evidence. Metal temperature, years in service, sulphur and hydrogen content of the stream, and the shutdown record often distinguish one mechanism from another that looks identical on the surface.

Visual and dimensional examination locates and quantifies wall loss and distinguishes uniform sulphidation from localized flow-accelerated or naphthenic attack. The SEM resolves the character of any cracking — the intergranular path of polythionic acid SCC, the cavitated grain boundaries of creep, the fissuring of hydrogen attack. EDS interrogates the scale and the crack faces, where the detection of sulphur or the nature of the oxide is frequently decisive. Metallography is the centrepiece of hot-exchanger work: only a polished, etched cross-section reveals creep voids, internal decarburization, sensitization, or the microstructural degradation — spheroidization, carbide precipitation — that tells how hot the metal actually ran and for how long. Hardness traverses and alloy verification then confirm whether the material was correct for the duty. Only when these lines of evidence agree is the conclusion signed.

Damage Is a Symptom — the Refinery Edition

As throughout this series, the tube is the last link in a longer chain. Sulphidation points to an alloy chromium level that did not match the temperature and sulphur of the service. Creep points to a metal temperature that ran higher, or a load that ran longer, than the design assumed. Polythionic acid cracking points to a shutdown procedure that let air reach a sulphide scale on sensitized steel. In each case the failed tube is telling the plant which upstream decision — material grade, operating temperature, or turnaround practice — must change. Replacing the tube without changing that decision simply resets the clock on the next failure.

Closing the Series: The Metal Remembers Everything

Across three parts, one theme has held. In the chemical exchanger, the tube recorded the chemistry it saw. In the condenser, it recorded the flow and the idle hours. In the refinery hot exchanger, it records the temperature it endured, the sulphur it met, and the way it was shut down — and it holds that record not only on its surface but deep within its microstructure. The metallurgist’s task, from the first part of this series to the last, has never changed: to read that record faithfully, to make independent techniques agree before drawing a conclusion, and to trace the failure past the tube to the decision that caused it. Do that, and most heat exchanger failures — chemical, condenser, or refinery — turn out to have been preventable all along.

About this series

• Part 1 — Chemical & Petrochemical Exchangers: under-deposit pitting, chloride SCC, and the convergence principle of investigation.

• Part 2 — Condenser Failures: inlet-end erosion, ammonia grooving, and idle-period microbial corrosion.

• Part 3 — Refinery & High-Temperature Exchangers: sulphidation, creep, and polythionic acid stress-corrosion cracking. (This part.)

About TCR Advanced Engineering

TCR Advanced Engineering is a NABL / ISO 17025-accredited materials testing and asset-integrity firm based in Vadodara, with over 9,000 investigations completed across 18 industries. From high-temperature sulphidation and creep to fitness-for-service under API 579-1 / ASME FFS-1, our metallurgical, NDT and FFS teams help refineries find the root cause — and the upstream decision behind it — not just the leak.

Talk to our failure analysis team: testing@tcradvanced.com +91 8511179948 (24-hour hotline)