Factors Affecting Corrosion Resistance of Stainless Steel Gas Pipes

Service environment factors inside stainless steel gas pipes

For stainless steel pipes carrying gas, the most damaging corrosion scenarios typically start when a conductive liquid phase forms on the pipe wall. Without an electrolyte (usually water), most internal corrosion mechanisms slow dramatically.

Water presence and gas dew point

Free water is the enabling condition for most internal corrosion. Even if gas leaves a plant “dry,” temperature drops along the route can force water to condense if the water dew point is not adequately controlled. Industry guidance emphasizes dehydration to reduce gas dew point and remove the conditions that promote corrosion.

• Upsets that introduce wet gas (or allow condensation) concentrate risk at low points, dead legs, and downstream of cooling.
• Small volumes of water can be enough if they sit stagnant and accumulate salts, iron fines, or bacteria.

Acid gases, oxygen, and salts that “activate” localized attack

Once water is present, dissolved species drive the severity and failure mode:

• Chlorides (from produced water carryover, hydrotest water, coastal air ingress, or cleaning fluids) are the most common trigger for pitting/crevice corrosion and chloride stress corrosion cracking.
• CO₂ lowers pH in condensed water (carbonic acid) and can raise general corrosion risk in mixed-metal systems; oxygen ingress can further accelerate corrosion in wet regions.
• H₂S changes cracking susceptibility and material qualification requirements in sour environments; material use is commonly governed by MR0175/ISO 15156.

Practical takeaway: control the process so that internal surfaces see dry gas and minimal salt deposition; when that cannot be guaranteed (start-ups, pigging, hydrotests, or off-spec gas), material selection and fabrication quality become decisive.

Alloy chemistry and grade selection: why “stainless” is not one material

Stainless steels resist corrosion because a thin chromium-oxide passive film forms on the surface. In chloride-bearing wetting, the difference between “adequate” and “high” resistance is often dominated by chromium (Cr), molybdenum (Mo), and nitrogen (N) content, which are commonly compared using the Pitting Resistance Equivalent Number (PREN).

Using PREN to compare pitting/crevice resistance

PREN ≈ %Cr + (3.3 × %Mo) + (16 × %N). Higher PREN generally indicates improved resistance to chloride-driven pitting and crevice corrosion (a key issue when wet gas or salty condensate is possible).

Practical takeaway: if wetting with chlorides is credible (condensate, hydrotest residue, coastal exposure, produced water carryover), grade selection should be based on localized corrosion and SCC margin, not just “stainless vs carbon steel.”

Temperature, chlorides, and stress: the SCC “tripwire” for gas piping

Chloride stress corrosion cracking (Cl-SCC) requires three conditions at the same time: tensile stress (residual weld stress can be enough), chlorides on a wetted surface, and elevated temperature. In practice, temperature is the factor that often turns a manageable pitting risk into a cracking risk.

A practical threshold: 60 °C (150 °F) guidance

When stainless steels are fully immersed, it is rare to see chloride SCC below about 60 °C (150 °F). Above that range, susceptibility rises sharply, and even relatively low chloride levels can become problematic—especially with wet/dry cycling that concentrates salts at the surface.

Controls that work in real piping systems

• Keep metal temperatures below the SCC-sensitive regime where feasible (insulation design, routing, and avoiding hot spots).
• Reduce chloride exposure during hydrotest/commissioning and ensure thorough drain-and-dry (residual films can initiate pits that later evolve into cracks).
• If temperature and wet chlorides cannot be reliably avoided, specify duplex/super duplex or higher-alloy materials (and qualify them to the applicable sour/service standards where relevant).

Welding, heat tint, and surface condition: how fabrication can erase corrosion resistance

For stainless steel pipes for gas, many “mystery” corrosion problems trace back to fabrication: heat tint, embedded iron, poor purge on the ID, rough finishing, and incomplete cleaning/passivation. These issues create weak points where the passive layer is damaged or cannot reform uniformly.

Heat tint and oxide scale after welding

Heat tint is more than discoloration: it indicates an oxidized surface and often a chromium-depleted layer at the surface. If left in place, it can markedly reduce localized corrosion resistance right where residual stresses are highest (the heat-affected zone and weld toe).

Pickling and passivation (and why both matter)

Pickling removes weld scale/heat tint and the damaged surface layer; passivation promotes a robust passive film. Standards such as ASTM A380 (cleaning/descaling/passivation practices) and ASTM A967 (chemical passivation treatments) are commonly used to define acceptable processes and verification.

• Use proper ID purge to prevent heavy internal oxidation on pipe weld roots (especially critical for gas piping where internal access is limited after assembly).
• Remove iron contamination from grinding tools or contact with carbon steel (iron pickup can “rust” on the surface and initiate under-deposit attack).
• Specify acceptance criteria for weld finish (smooth transitions, minimal crevices) because geometry drives crevice chemistry and deposit retention.

Design and installation details that drive corrosion performance

Even with the right grade and good welding, design details determine whether corrosive liquids and deposits collect, whether oxygen can ingress, and whether galvanic couples accelerate attack.

Avoid crevices, dead legs, and liquid traps

• Slope lines where practical and provide drain points at low spots to prevent stagnant condensate.
• Minimize dead legs and capped branches; stagnant water is a common driver for microbiologically influenced corrosion (MIC).
• Use gasket/connection designs that do not create persistent crevices where chloride-rich brines concentrate.

Galvanic interactions and mixed metals

If stainless steel is electrically connected to less noble metals (e.g., carbon steel) and an electrolyte is present, galvanic corrosion can accelerate attack on the less noble component and concentrate deposits at the junction—creating localized corrosion risk for stainless as well. Isolation strategies (dielectric unions, careful grounding design, and avoiding “wet” junctions) reduce this risk.

Operations, hydrotesting, and MIC: the “hidden” factors that decide long-term resistance

Many stainless gas piping corrosion failures are triggered not during steady-state operation, but during commissioning, hydrotesting, shutdowns, or process upsets that introduce water and leave residues behind.

Hydrotest water quality and drying discipline

Hydrotest and flush water can introduce chlorides and microbes. Practical industry guidance commonly recommends low-chloride water (often ~50 ppm chloride as a conservative benchmark) and emphasizes cleaning, draining, and drying so that stagnant water does not remain inside the pipe.

MIC risk when water is left stagnant

Microbiologically influenced corrosion (MIC) can occur in stagnant waters—even at relatively modest chloride levels—and has been documented in stainless systems where lines were left undrained after hydrotesting. The immediate control is operational: do not leave stagnant water films, and avoid long stagnant holds without biocide/control measures where permitted by your process and regulations.

1. Define a commissioning sequence that ends with full drain, dry gas blowdown (or equivalent), and verification of dryness.
2. Control oxygen ingress during downtime (blanketing, tight isolation, and leak management) because oxygen in wet regions accelerates attack.
3. Inspect the most vulnerable locations first: low points, dead legs, downstream of coolers, and weld-heavy spools.

Practical decision table: factor, failure mode, and what to do about it

Final takeaway: stainless steel gas pipes perform best when you treat corrosion resistance as a system property—process dryness, chloride management, alloy selection (PREN/SCC margin), fabrication quality, and liquid-management design must all align.

References used for data points and thresholds

• SSINA: Chloride Stress Corrosion Cracking (rare below ~60 °C when fully immersed).
• Unified Alloys: PREN formula and example PREN ranges (PREN equation and typical ranges for common grades).
• PHMSA report: Pipeline Corrosion (dehydration and dew point control to remove conditions that promote corrosion).
• GRI: Internal Corrosion Direct Assessment of Gas Pipelines (dew point definition and water condensation mechanism).
• TWI: Restoring corrosion properties after welding (remove heat tint oxide and chromium-depleted layer).
• Nickel Institute technical note: Pickling and passivation (ASTM A380/A967 references and purpose).
• Nickel Institute: MIC case examples in stainless after hydrotesting (stagnant water as root cause).
• NACE MR0175 / ISO 15156-1 (sour service context and H₂S-related precautions framework).