A corrosive process may require titanium, a nickel alloy or a high-molybdenum stainless steel at the wetted surface. That requirement applies to the surface the process touches. It does not automatically apply to the full thickness of the pressure boundary behind it.
Clad construction separates those two jobs. A structural base material carries the design pressure, and a comparatively thin corrosion-resistant alloy protects the process-contact surface. Where the wall is thick and the alloy expensive, that separation can change the cost of a vessel substantially. It also complicates every seam, nozzle and attachment.
Clad construction is not automatically cheaper, and solid alloy is not automatically more reliable. The answer belongs to the service.
IN BRIEF
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Clad construction separates the pressure-retaining function from the corrosion-resistance function. A structural base metal carries the design pressure while a thinner corrosion-resistant alloy (CRA) layer takes the chemical attack.
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The comparison follows required thickness, not price per pound. Material savings grow with wall thickness, wetted area and the CRA-to-base price ratio. No universal crossover diameter, thickness or alloy price exists.
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Integrally clad plate, weld metal overlay cladding and applied linings are three different construction methods. ASME Section VIII, Division 1 addresses all three in Part UCL, and they behave differently in fabrication, inspection and repair.
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Nozzles, seams and internal attachments usually decide the fabrication hours. Every penetration interrupts the corrosion barrier, and every interruption must be restored, examined and eventually repaired.
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Cladding does not make alloy selection less important. The corrosion mechanism still sets the wetted material; clad construction changes only how much of it the vessel contains.
On this page: what clad construction is · clad plate vs. overlay vs. lining · what Part UCL covers · clad plate specifications · where the money goes · forming and welding · nozzles · internals and thermal expansion · inspection · repair · when solid alloy wins · when clad wins · common mistakes · what to define before an inquiry · FAQs
What a clad pressure vessel is
A clad pressure vessel is a vessel whose pressure boundary is built from a structural base material with a corrosion-resistant alloy bonded to the process-contact surface. The base material provides the strength that contains pressure. The clad layer provides chemical resistance.
Wall thickness in a conventional vessel answers one question: how much metal contains the design pressure at the design temperature, given the allowable stress of the material. In a clad vessel the base material answers it, and a second question runs alongside: what survives the process chemistry at the wetted surface.
The CRA layer answers that one, and the thickness it needs has little to do with pressure. It follows expected corrosion and erosion over the intended life, plus margin for future weld repair.
Which corrosion-resistant materials get bonded to what
Clad plate is produced with austenitic and chromium stainless steels, nickel alloys, the reactive and refractory metals — titanium, zirconium, tantalum, niobium — and copper alloys, generally over a carbon or low-alloy steel base. Compatibility is not universal: a combination has to be producible by a bonding method that yields an acceptable bond, weldable with a filler system that tolerates the dissimilar joint, and stable at design temperature and through any heat treatment.
Alloy selection itself still starts from the corrosion mechanism in the actual stream, which is where the selection logic for corrosion-resistant alloys belongs.
Clad plate, weld overlay and applied lining are three different methods
These terms get used interchangeably in inquiries, and the substitution causes real problems. They differ in how the CRA layer is attached, how it is restored at welds, and how it is repaired in service.
| Construction | How the CRA layer is attached | Typical use | Principal fabrication consequence |
|---|---|---|---|
| Integrally clad plate | Metallurgically bonded to the base plate before it reaches the fabricator, by explosion or roll bonding | Shells, heads, large surfaces | Bond must survive forming; clad is stripped back and restored at every seam |
| Weld metal overlay cladding | Deposited as weld metal onto the base material | Nozzles, flange faces, tubesheets, local areas, geometry clad plate cannot follow | Surface chemistry is diluted by base metal; normally more than one layer |
| Applied metallic lining | A separate sheet or strip attached at edges and intermittent points, not bonded through its area | Retrofits, cases where bonded product is unavailable | Attachment welds and seams govern; a crevice can exist behind the liner |
| Non-metallic lining | Rubber, fluoropolymer, glass, brick or refractory applied to the inner surface | Aggressive acid service, abrasive slurry service | Different trade, inspection method and repair cycle; usually thicker |
| Solid alloy | No separate layer; the pressure boundary is the CRA | Thinner walls, smaller vessels, high-purity service | Simplest construction, highest alloy quantity |
Integrally clad plate
Integrally clad plate arrives as a single composite plate. Explosion bonding drives the cladder onto the base plate with a controlled detonation, producing a metallurgical bond across the interface, and is the route generally used for titanium and other reactive metals. Roll bonding joins the two metals under heat and pressure in a rolling mill, and is widely applied to stainless and nickel cladding.
What the fabricator receives matters more than the route: plate of a given base and clad thickness, in the sizes the producer can supply. Plate size is a design input, because it sets how many seams the shell has, and each seam is a place where the cladding is stripped, welded and restored.
Weld metal overlay cladding
Weld overlay deposits the corrosion-resistant layer as weld metal. It follows geometry that clad plate cannot — nozzle bores, flange faces, tubesheet faces, transitions and local repairs — and does not depend on a bonded plate being available.
The complication is dilution. The first layer deposited onto carbon steel mixes with melted base metal, so its chemistry is not the chemistry of the filler. Overlay is therefore generally deposited in more than one layer, and the specification should state the required chemistry at the exposed surface.
Applied lining, and why it is not a synonym for cladding
The ASTM specifications for clad plate describe the CRA layer as integrally and continuously bonded to the base metal. That phrase is the distinction. Integral cladding and weld overlay are bonded through their whole area; an applied lining is attached at its edges and at intermittent points, with no metallurgical bond behind it.
The difference drives behavior. A bonded layer transmits heat and shares strain with the base metal, while an applied liner can develop a gap. In corrosive service that gap is a crevice, and a path a leak can travel before it appears. Non-metallic linings are judged on chemical compatibility, temperature limit, permeation and mechanical damage rather than weldability.
What ASME Section VIII, Division 1 Part UCL covers
Clad and lined vessels are built to the same construction code as any other pressure vessel. ASME Boiler and Pressure Vessel Code Section VIII, Division 1 provides requirements for “the design, fabrication, inspection, testing, and certification of pressure vessels operating at either internal or external pressures exceeding 15 psig.” Part UCL, within that Division, addresses welded vessels constructed with corrosion-resistant integral cladding, weld metal overlay cladding, or applied corrosion-resistant linings.
Three points matter more to a specifier than the paragraph numbers.
Part UCL governs construction, not material selection. The Code sets rules for how a clad or lined vessel is designed, welded, examined and tested. It does not identify which alloy resists a given process. That comes from corrosion data for the actual stream, including trace species.
Whether the clad layer may be credited in the design thickness is a Code question tied to the specific material, and it should be taken from the edition governing the project rather than from a rule of thumb. The conservative posture is to size the pressure boundary on the base material and treat the CRA layer as protection. The material specifications point the same way: in ASTM B898, covering reactive and refractory metal clad plate, tensile properties are determined by a tension test on the base metal only.
Code, project specification and good practice are three layers. The Code carries legal force where a jurisdiction adopts it, and adopted editions differ; the National Board’s NB-370 synopsis compiles jurisdictional requirements. Owner and EPC specifications routinely add requirements above it: bond examination coverage, overlay chemistry limits, surface finish, additional NDE. Our overview of pressure vessel design codes covers how those layers stack.
Material specifications for clad plate
| Specification | Cladding covered |
|---|---|
| ASTM A263 | Stainless chromium steel-clad plate |
| ASTM A264 | Stainless chromium-nickel steel-clad plate |
| ASTM A265 | Nickel and nickel-base alloy-clad steel plate |
| ASTM B898 | Reactive and refractory metal clad plate — titanium, zirconium, tantalum, niobium and their alloys |
| ASTM B432 | Copper and copper alloy clad steel plate |
Titles and scopes per current ASTM listings. Where the vessel is Code-stamped, material must be furnished to a specification the Code permits, so confirm the designation and edition invoked for the order.
Two features of these specifications belong in a purchase specification. Each describes the cladding as integrally and continuously bonded to a carbon steel or low-alloy steel base. And each addresses bond quality directly, with A263, A264 and A265 requiring conformance to specified shear and bond strength values. Bond strength is a purchased property with an acceptance criterion, not an assumption.
Clad versus solid alloy: where the money actually goes
The material argument is straightforward. In a solid-alloy vessel, every increment of required thickness is another increment of expensive alloy across the whole pressure boundary. In a clad vessel, most of that thickness is base material, while the CRA layer stays near what corrosion and repair margin require.
The fabrication argument runs the other way. Clad construction carries work a homogeneous vessel does not: stripping and restoring cladding at seams, dissimilar-metal welding, extra filler materials, more examination, more engineering, and nozzle details that take several times the hours of a plain steel nozzle. Those hours are not proportional to plate cost, so they do not shrink when the vessel is small.
Why thickness, not price per pound, drives it
Required thickness scales with design pressure and diameter, and inversely with allowable stress at design temperature. The alloy premium is therefore paid on volume rather than area: doubling wall thickness in a solid-alloy vessel roughly doubles alloy mass, while in a clad vessel it mostly adds base metal.
Allowable stress at temperature matters as much as price. Some corrosion-resistant alloys hold high allowable stress and produce thin sections, which narrows the gap; others lose allowable stress quickly with temperature and need more metal for the same duty.
What moves the comparison
| Driver | Pushes toward clad | Pushes toward solid alloy |
|---|---|---|
| Required wall thickness | Thick pressure boundary | Thin pressure boundary |
| Vessel size and wetted area | Large diameter, large area | Small vessel |
| CRA-to-base price ratio | High premium | Moderate premium |
| CRA thickness the service needs | Thin layer adequate | Heavy corrosion or erosion allowance wanted |
| Geometry | Simple shells and heads | Many complex penetrations and internals |
| Nozzle and attachment count | Few, large | Many, small |
| Availability and schedule | Bonded plate available in useful sizes | Solid plate readily available, schedule tight |
| Repair philosophy | Planned clad restoration accepted | Preference for homogeneous metal |
| Purity and contamination sensitivity | Tolerant service | Tight cleanliness or product-purity limits |
Anything that raises fabrication hours per pound of alloy saved works against clad construction, and anything that increases the alloy mass a solid design would need works for it. A vessel anywhere near the middle should be priced both ways.
Fabrication: forming, welding and clad restoration
Forming clad plate
Forming clad plate means forming two bonded metals, with two objectives added to the usual ones: the bond has to survive intact, and the CRA surface has to arrive clean.
Four concerns govern. Bond integrity has to survive the imposed strain. The clad surface has to be protected from mechanical damage and iron pickup off tooling. Dimensional control has to account for a composite that does not behave exactly like the base metal alone. And any heat treatment must degrade neither the bond nor the clad chemistry. Bend orientation, forming temperature and allowable strain follow the material system and the plate producer’s guidance rather than a general rule.
Welding: one seam, two functions
A weld in a clad vessel does two jobs that should be specified separately. The structural weld joins the base material and carries pressure. The restoration pass re-establishes a continuous corrosion barrier across the joint on the process side.
They typically use different filler materials in a defined sequence, with the cladding stripped back so the structural weld is made in base metal without pickup from the CRA. The restoration is then deposited into the prepared area. Several items are settled together rather than left to the shop:
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Filler selection for the structural joint, the restoration, and any buffer layer between dissimilar systems
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Dilution control, since a restoration diluted by base metal will not deliver the intended surface chemistry
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Cleanliness, particularly for titanium and zirconium, where contamination degrades corrosion resistance
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Weld sequencing and heat input, so distortion and interface conditions stay controlled
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PWHT requirements and their interaction with the material system, since heat treatment can affect both clad chemistry and the interface
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Physical access for the restoration pass and its examination, which is a decision made on the drawing
That does not make clad welding exotic. It makes it a procedure-driven operation where a carbon steel vessel has one weld and one filler.
Nozzles, manways and penetrations decide the fabrication hours
A vessel is a clad cylinder interrupted by nozzles, manways, instrument connections, agitator penetrations and support attachments, and each interruption breaks the corrosion barrier. Penetrations are frequently the most fabrication-intensive detail in the vessel. Several details are in common use, and choosing among them is a project decision:
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Solid-alloy nozzle necks and forgings, with a dissimilar-metal transition and clad restoration around the opening
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Clad nozzle necks, where a bonded product exists in the required size
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Weld-overlaid nozzles, with the bore, weld preparation and flange face overlaid
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Liners or sleeves, which move the difficulty to the terminations and the crevices there
Flange faces need their own attention. If the gasket seats on the CRA, the facing must be that material and the transition to it must be continuous. Small-bore connections raise a related problem: a barrier that cannot be inspected is a barrier of unknown condition, and small penetrations are where inspection access disappears first.
Internal attachments and thermal expansion
Support rings, baffles, tray supports, clips, distributors and dip pipes attach to a surface that is also the corrosion barrier, so those details are deliberate rather than incidental. The recurring issues are corrosion continuity where an attachment interrupts the barrier, local restoration around the attachment weld, and galvanic pairing where the attachment alloy differs from the clad layer. Differential thermal expansion and future inspection or replacement of the attachment belong in the same conversation.
Thermal expansion deserves mention because clad construction bonds two metals with different coefficients of expansion. Austenitic stainless steels expand more than carbon steel over the same temperature rise; titanium expands less. A sound bond accommodates that difference, and in most services does so without difficulty. It becomes a design consideration where thermal cycling is frequent or severe, at transition details that restrain movement locally, and in PWHT decisions. The mismatch is a reason to define thermal cycles in the specification, not a reason to expect clad construction to fail.
Inspection and quality control on clad construction
Clad construction adds inspection targets a homogeneous vessel does not have. Three are worth naming, because they are most often missing from a purchase specification.
Bond integrity. Bond quality is verified ultrasonically, and the requirement belongs in the specification: what coverage, to what acceptance criterion, at what stage — mill, post-forming, or both. General plate standards such as ASTM A578, for straight-beam examination of rolled steel plates in special applications, address discontinuities within the plate rather than bond acceptance for clad products. Bond examination requirements come from the clad plate specification and the project specification, so leaving them unstated leaves them to interpretation later.
Surface chemistry. Positive material identification on the exposed surface answers the question dilution raises. For weld overlay, the useful requirement specifies chemistry at the surface the process contacts. For clad plate, the equivalent question is what remains after forming, stripping and restoration.
Restoration and thickness. Clad restoration at seams, nozzles and attachments needs visual and, where applicable, liquid penetrant examination, plus thickness verification of the CRA layer. A hydrostatic test proves the pressure boundary. It does not prove the corrosion barrier, and the two should not be conflated.
Sorting requirements into Code-required examination, purchaser-specified examination and manufacturer quality control also keeps a bidder from being penalized for pricing what was actually asked for.
Repair and lifecycle
Repair behavior differs by construction method, and it is a specification decision as much as a maintenance one. Solid alloy is the simplest case, since a repair is a weld in the same material. Weld overlay can be built back locally, which is one reason it is used where attention is expected. Integrally clad construction is repairable by local restoration as well, with a limit: repeated repair thins the CRA layer, and below some remaining thickness welding on it stops being sound practice. Applied and non-metallic linings tend toward campaign-style relining rather than local weld repair.
Two questions belong in the specification. How much CRA thickness may be lost before a component is replaced rather than reworked? And who performs the repair — a shop with procedures qualified for that material system, or plant maintenance? Pressure vessel repair, weld repair and re-rating on clad equipment depends on both answers, which is the argument for specifying clad thickness with future repair in view.
When solid alloy is still the better answer
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Thin pressure walls and small vessels, where the added fabrication costs more than the alloy it saves
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Many complex penetrations and internals, where a vessel that is mostly nozzles spends its budget on clad restoration
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High-purity or contamination-sensitive service, where a dissimilar-metal transition, a restored weld or a crevice behind a liner is a product-quality risk
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Severe thermal cycling, where minimizing dissimilar-material interfaces is a reasonable preference
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Procurement, schedule and repair philosophy, where bonded plate has longer lead time, or the owner expects to weld on the vessel through its life
When clad construction becomes the stronger choice
These conditions stack rather than appear individually:
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A thick pressure boundary, driven by design pressure and diameter, over a large wetted area
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A high price ratio between the CRA and the base material
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Process chemistry that genuinely requires that CRA at the wetted surface
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A CRA layer thin enough to be economical yet adequate for the corrosion mechanism and intended life
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Geometry a bonded plate or a practical overlay route can follow
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Lifecycle economics, including repair expectations, that support the construction
Pressure-leach autoclaves illustrate the pattern: large vessels, thick walls, hot acid at the wetted surface, and an established practice of separating the pressure boundary from the corrosion barrier — by bonded titanium cladding in some designs, by membrane-and-brick systems in others. The same logic appears in acid and chloride service across chemical and petrochemical process equipment.
Common mistakes in clad vessel specification
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Treating “clad” and “lined” as the same requirement. They produce different vessels, inspection plans and repair cycles.
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Assuming the clad layer carries pressure. Design credit is a Code and material question.
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Specifying clad thickness from corrosion allowance alone. Repair welding consumes thickness.
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Omitting bond examination. Coverage, acceptance criteria and stage should be stated, not implied.
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Specifying overlay by filler designation only. Dilution means surface chemistry has to be specified where the process contacts it.
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Detailing nozzles and internals late, after material is already ordered.
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Comparing dollars per pound instead of delivered vessel cost.
What to define before requesting a clad pressure vessel
An inquiry carrying pressure, temperature and dimensions alone cannot be quoted accurately, because the construction decision depends on what those numbers leave out.
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Process — service; fluid composition and concentrations; contaminants and trace oxidizing or halide species; solids; expected corrosion mechanisms and available corrosion data
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Thermal — operating and design temperature; thermal cycles; startup, shutdown and upset conditions
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Mechanical — operating and design pressure; dimensions and orientation; cyclic service; nozzle schedule; internal loads and attachments
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Materials — base material; required CRA and its basis; corrosion allowance; clad, overlay or lining preference if already specified; owner or EPC material requirements
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Fabrication and QA — governing code and jurisdiction; NDE and PMI; bond examination; overlay chemistry and thickness requirements; PWHT; hydrostatic test; documentation
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Maintenance — inspection philosophy and access; expected repair approach; acceptable CRA loss before replacement; anticipated future modifications
Harris Thermal fabricates ASME Section VIII, Division 1 pressure vessels and tanks, shell-and-tube heat exchangers and custom process equipment, with in-house mechanical and thermal design. Documented forming, welding and machining experience covers stainless steels, duplex and super duplex, 6-moly grades, nickel alloys, titanium, zirconium, tantalum and clad materials, with nondestructive examination, positive material identification and hydrostatic testing supporting that work. Where corrosion-resistant construction is under consideration, the fabrication implications are worth reviewing once the process, mechanical and materials requirements are defined, and before the construction method is fixed in the specification.
The corrosion mechanism selects the alloy. The pressure boundary sets the thickness. Construction method is the decision that follows those two, and it is the one most often made first.
FAQs about clad pressure vessels
What is a clad pressure vessel?
A clad pressure vessel is a vessel whose pressure boundary is a structural base material, typically carbon or low-alloy steel, with a corrosion-resistant alloy bonded to the process-contact surface. The base metal contains the pressure; the clad layer resists the process chemistry. ASME Section VIII, Division 1 addresses clad and lined vessels in Part UCL.
Is a clad pressure vessel cheaper than a solid-alloy vessel?
Not automatically. Clad construction reduces the expensive alloy in the pressure boundary, and that saving grows with wall thickness, vessel size and the alloy premium. It also adds fabrication work at every seam, nozzle and attachment. For thin walls and small vessels the added fabrication can exceed the material saving, so a borderline vessel should be priced both ways.
What is the difference between clad plate and weld overlay?
Integrally clad plate arrives at the fabricator with the corrosion-resistant layer already bonded to the base plate by explosion or roll bonding. Weld overlay is deposited as weld metal, so it follows geometry clad plate cannot, such as nozzle bores and flange faces. Overlay chemistry is diluted by the base metal, so composition should be specified at the exposed surface.
What is the difference between cladding and lining?
Cladding is bonded to the base material continuously across its area, as integrally clad plate or weld metal overlay. An applied lining is attached at its edges and at intermittent points, with no metallurgical bond behind it, so a gap and therefore a crevice can exist. Non-metallic linings — rubber, fluoropolymer, glass, brick — are judged on chemical compatibility and temperature limits rather than weldability.
Does ASME Section VIII, Division 1 allow clad pressure vessels?
Yes. Part UCL of ASME Section VIII, Division 1 addresses welded pressure vessels constructed with corrosion-resistant integral cladding, weld metal overlay cladding, or applied corrosion-resistant linings. Whether any part of the clad layer may be credited in the design thickness depends on the Code rules in force and the specific material, and should be confirmed against the governing edition.
Can titanium be used as pressure vessel cladding?
Yes. Titanium clad plate is covered by ASTM B898, which addresses reactive and refractory metal clad plate in titanium, zirconium, tantalum, niobium and their alloys, and is generally produced by explosion bonding. In that specification, tensile properties are determined by a tension test on the base metal only, which reflects the usual treatment of the clad layer as corrosion protection rather than pressure-retaining metal.
