Process vessel corrosion protection is not tank lining with a higher pressure
Process vessel corrosion protection starts from the damage mechanism, not from the coating. Reactors, columns, separators and pressure vessels are attacked by process chemistry, deposits, liquid-vapour interfaces and acid condensates; the remedy is an alloy, a cladding, an internal lining or a local repair, always within the regulatory frame of a pressure boundary.
A storage tank holds a product at near-atmospheric pressure. A process vessel reacts, separates, heats, cools, condenses and is depressurized, and its wall is usually a registered pressure boundary. Lining selection for storage is covered in our guide on choosing an industrial tank lining; inspection, rating and repair under the in-service code are covered in API 510 and pressure vessel repair decisions. This article covers what corrodes inside process equipment, and how it is protected and repaired.
Where process vessels corrode: the usual mechanisms
Corrosion in high-corrosion process equipment concentrates where the chemistry, the phase or the flow changes. API RP 571 catalogues damage mechanisms in refining and gives the owner, the inspector and the coating contractor a common vocabulary.
| Mechanism | Where it appears | What drives it |
|---|---|---|
| Process-side corrosion | Wetted shell and heads, internals | The process fluid itself: acids, chlorides, dissolved gases, temperature |
| Under-deposit corrosion | Bottom heads, low points, trays, behind scale | Concentration and aeration cells under sludge or scale; shelter for bacteria |
| Liquid-vapour interface | Normal liquid level band, splash zone | Wet-dry cycling, concentration of species at the interface, oxygen access |
| Acid condensates | Column overheads, top heads, vapour outlets, cold spots | Vapour cooling below its dew point and condensing acidic water; salt deposition |
| Pitting | Anywhere a passive film or coating breaks down locally | Chlorides, stagnant conditions, deposits, coating holidays |
| Erosion-corrosion | Inlet nozzles, impingement zones, around agitators | Velocity, turbulence and solids removing protective films |
In column overheads, the first water to condense can be strongly acidic, and process salts can deposit and absorb moisture. The attack moves with operating conditions, which is why process control (temperature, wash water, neutralization, inhibition) is the first line of defence and a lining or alloy upgrade the second.
Pressure vessel pitting is a frequent internal inspection finding: a vessel can lose very little average thickness while individual pits progress deeply. The mechanics are explained in pitting corrosion explained; what matters here is that a field of pits is both a coating problem (the pits must be filled before lining) and an integrity question (the remaining wall under each pit).
Insulated vessels also corrode from the outside; see our corrosion under insulation guide.
Corrosion protection options for process and pressure vessels
For corrosion protection of steel production equipment, the options fall into two groups. Metallurgical options are design and fabrication decisions made by the owner's materials engineer and the vessel manufacturer. Organic linings and local repairs can be applied to an existing carbon steel vessel during a shutdown.
| Option | Principle | Usually considered when | What to weigh |
|---|---|---|---|
| Corrosion-resistant alloy construction | The pressure wall itself resists the process | New vessel or replacement; severe or variable chemistry | Alloy selection against the actual fluid, including chlorides and temperature; cost and lead time |
| Clad plate or weld overlay | A thin alloy layer metallurgically bonded to a carbon steel wall | Thick-walled vessels where solid alloy is impractical | Fabrication and repair under the construction code; inspection of the bond |
| Welded alloy sheet lining | Alloy sheets welded to the inside of an existing shell | Upgrading an existing vessel without replacing it | Weld integrity, venting behind the sheet, code acceptance |
| Glass-lined steel | Vitreous enamel fused to steel | Chemical and pharmaceutical reactors | Sensitivity to impact and thermal shock; repairs per the manufacturer's procedure |
| Organic internal lining | Coating or sheet lining isolates the steel from the process | Carbon steel vessels in services within the lining's resistance | Temperature, pressure cycling, steam-out, internals; inspection access |
| Process control | Changes the environment rather than the barrier | Overhead condensates, under-deposit attack | Depends on operating discipline; monitoring required |
These options combine: an alloy top section where condensates form over a lined carbon steel bottom, or a lining that protects a vessel until a planned alloy replacement. Cladding, overlay and sheet lining are metallurgical work by qualified fabricators under the construction code, not coating work.
Internal linings for lined vessels: families and vessel-specific checks
The resin families are the same ones used in tanks; what changes is the duty. The table is qualitative and does not replace the manufacturer's resistance data.
| Family | Where it is usually considered in process vessels | Vessel-specific points to verify |
|---|---|---|
| Epoxy phenolic | Hot water, condensate, hydrocarbons, a range of solvents | Many grades need a heat cure or force cure to reach rated resistance; tight recoat windows |
| Epoxy novolac | Acidic process streams, solvents, higher temperatures than standard epoxy | Rigidity under thermal cycling; strong oxidizers checked on the chart |
| Vinyl ester (glass flake or laminate) | Strong and oxidizing acids, aggressive chemical service | Styrene in confined spaces, short working time, post-cure |
| Rubber linings | Abrasive slurries, many mineral acids, scrubbers and process tanks | Vulcanization method, seam design, temperature and solvent limits by compound |
| PTFE and other fluoropolymers | Services beyond the reach of thermoset linings | Usually shop-applied or loose sheet linings; permeation and vacuum behaviour per supplier |
Beyond chemical resistance, a vessel lining must be checked against:
- Temperature and thermal cycling. Batch reactors and columns cycle; a lining qualified in steady immersion may crack or disbond under repeated heating and cooling.
- Pressure cycling and depressurization. In services containing dissolved or free gas, gas that has permeated the film can blister it when pressure drops quickly. Ask the manufacturer whether the system is qualified for the depressurization rate of the process.
- Steam-out and cleaning. The cleaning regime is often harsher than the process.
- Vacuum. Columns and reactors under vacuum load the lining in tension.
- Internals and nozzles. Trays, supports, baffles, coils and small nozzles multiply edges. Each is lined, made of a resistant material, or designed out. NACE SP0178 (now AMPP) covers design and weld finish for vessels to be lined, and ISO 8501-3 defines preparation grades for welds and edges.
Pressure vessel surface treatment before lining
Preparation is half the lining system, done here in a confined space with internals in the way.
- Decontamination. Process residues and deposits are removed before blasting; steel from acid or chloride service can hold salts in pits that blasting alone does not remove.
- Abrasive blasting. Usually to near-white metal or white metal, as the lining manufacturer specifies for immersion; see our NACE No. 2 surface preparation guide and the definition of near-white metal blast.
- Soluble salts and dust. Soluble salts are measured (ISO 8502-6 and -9) against the specified limit; dust is assessed per ISO 8502-3.
- Profile. Surface profile measured per ISO 8503 or ASTM D4417 and matched to the data sheet.
- Climate. Steel temperature kept above the dew point, commonly by at least 3 °C per ISO 8502-4, with dehumidification inside the vessel when needed.
Pressure vessel pitting: local repair with metal-filled epoxy
Isolated pits are commonly filled with a metal-filled epoxy after blasting and salt testing, then overcoated with the lining. The filler gives the lining a continuous surface instead of a thin film bridging a void.
Two questions come before the filler:
- Is the remaining wall acceptable? Pit depth and distribution are measured and assessed against the required thickness. API 579-1/ASME FFS-1 includes an assessment procedure for pitting corrosion, and the fitness-for-service result decides whether the vessel continues as is, at reduced conditions, or needs a pressure-boundary repair.
- Has the mechanism been addressed? Filling pits without changing the conditions that created them only resets the clock.
The filler and lining are not credited as wall thickness. Where pits go below the required thickness, restoring the pressure boundary is a separate repair under the applicable code.
The regulatory frame around a pressure boundary
| Reference | What it governs |
|---|---|
| CSA B51 and provincial regulations | Boiler, pressure vessel and pressure piping requirements in Canada, administered by each province through its own authority |
| API 510 | In-service inspection, rating, repair and alteration, where adopted by the owner within the provincial regime |
| ASME PCC-2 | Repair methods for pressure equipment and piping: welded, mechanical and non-metallic repairs; see our article on ASME PCC-2 composite repairs |
| API 579-1/ASME FFS-1 | Fitness-for-service assessment of damage such as general metal loss, local metal loss and pitting |
Three consequences for maintenance planning:
- A repair to the pressure-retaining wall follows the applicable code and requires an engineering evaluation and acceptance by the authorized inspector and, where required, the provincial authority.
- An internal lining does not replace required thickness. It protects the steel that is there; it is not counted in the pressure calculation.
- Lining work and repairs are recorded in the vessel file, so the next inspection knows what is under the coating.
Inspecting internal linings during shutdowns
An internal lining is inspected at every vessel entry: a defect found early is a patch; a defect left for a run becomes a pitting repair.
- Visual inspection. Blistering, cracking, disbondment, rust staining and mechanical damage, rated with ASTM D714 or ISO 4628 so that results compare between shutdowns. Attention goes to the liquid-vapour band, nozzles, welds, internals and the bottom head.
- Discontinuity detection. A holiday test on suspect areas and repairs, per NACE SP0188 or ASTM D5162, at the method and voltage the lining manufacturer allows.
- Thickness. Dry film thickness measured per SSPC-PA 2 or ISO 19840 where wear or erosion is suspected.
- Adhesion. Pull-off testing (ASTM D4541) is destructive and reserved for doubtful areas, with the test spots repaired afterwards.
Local defects are repaired by abrading back to sound lining, preparing the exposed steel, and applying a compatible repair system with the same thickness and holiday-test acceptance as the original.
FAQ
Can an internal lining restore a pressure vessel's minimum thickness? No. A lining protects the remaining steel against the process; it is not counted as pressure-retaining wall. Wall loss below the required thickness is assessed for fitness-for-service and, if needed, repaired under the applicable code.
Is filling pits with epoxy a pressure vessel repair? Filling pits to restore a surface for lining is surface restoration, not a pressure-boundary repair. Whether the pitted wall remains acceptable is a separate fitness-for-service decision, made before filling.
What lining suits a column overhead? It depends on the condensate chemistry, temperature and cleaning. Overhead corrosion is first addressed by process control; where a lining is considered, the manufacturer must confirm resistance to the actual condensate.
How often should a vessel lining be inspected? At every internal inspection or vessel entry set by the inspection program. The interval comes from the vessel's inspection plan, not from the lining alone.
In practice
Induscoat applies internal linings to process vessels and carries out local pitting repairs with metal-filled epoxy during shutdowns, as part of its protective coatings service, with surface preparation, film thickness and holiday testing documented for the vessel file. Pressure-boundary decisions stay with the owner, the inspector and the jurisdiction. For vessels in Quebec and Ontario, see our tank and vessel lining application and request a quote with the process fluid, the operating temperature and pressure, and the latest inspection report. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.
Hicham M, P Eng, PMP
Engineer and project manager (PMP) at Induscoat. Over 16 years of experience in industrial coatings, composite repairs and wear protection on mining, energy and petrochemical sites in Canada and internationally.