What Is Cladding in a Vessel? A Practical Guide to CRA Weld Overlay
Cladding in a vessel means applying a corrosion-resistant or wear-resistant metallic layer to the vessel surface that contacts the process fluid. In most industrial pressure vessels, the base shell is carbon steel or low-alloy steel for strength and cost, while the cladding layer is stainless steel, nickel alloy, duplex alloy, or another corrosion-resistant alloy for surface protection.
The purpose is simple: let the base vessel carry pressure and mechanical loads, while the clad layer protects against corrosion, erosion, chemical attack, or wear. This is usually far more economical than making the entire vessel from an expensive alloy.
In pressure equipment, vessel cladding may also be called CRA cladding, clad plate, weld overlay, weld inlay, corrosion-resistant overlay, or internal vessel cladding. The exact term matters because cladding, weld overlay, and lining are not always the same thing.
Short Answer
Vessel cladding is a bonded metallic layer applied to a vessel’s internal or process-contact surface to protect the pressure boundary from corrosion, erosion, or wear. It is commonly used in oil and gas, chemical, petrochemical, refinery, power, fertilizer, and offshore equipment where the process fluid is too aggressive for bare carbon steel.
For example, a pressure vessel may use a carbon steel shell for strength and a 3 mm stainless steel or nickel alloy clad layer on the inside for corrosion resistance. This gives the vessel the corrosion behavior of a premium alloy at the wetted surface, without the cost of a full solid alloy shell.
Why Are Vessels Cladded?
Vessels are cladded when the process environment is more aggressive than the base pressure material can handle. The base material may be strong and economical, but the internal surface may need special corrosion, erosion, or high-temperature resistance.
The table below explains the main reasons. It matters because cladding should be selected for a specific service problem, not as a vague premium feature.
| Reason for Cladding | What It Protects Against | Typical Vessel Example |
|---|---|---|
| Corrosion resistance | Acids, chlorides, sour service, CO2, H2S, seawater, caustic media | Separators, reactors, scrubbers, columns, chemical vessels |
| Erosion resistance | Solids in flow, slurry, catalyst particles, high-velocity zones | Inlet areas, cyclonic vessels, slurry tanks, process nozzles |
| High-temperature protection | Oxidation, sulfidation, hot corrosion, thermal cycling | Refinery and power process vessels |
| Wall-loss control | Internal corrosion that reduces pressure boundary thickness | Existing vessels with corrosion allowance concerns |
| Cost reduction | Need for premium alloy only at the wetted surface | Large pressure vessels where solid alloy construction is expensive |
| Life extension | Repair or upgrade of an existing asset | Field-applied internal cladding or weld overlay repair |
The buyer conclusion: vessel cladding is usually an economic and integrity decision. It protects the pressure boundary while avoiding the cost and lead time of a full CRA vessel.
Cladding vs Weld Overlay vs Lining
These terms are often mixed together, but they are not identical. For buyers, the difference affects bonding, inspection, repair strategy, and how the vessel should be specified.
Arveng Training gives a useful engineering explanation of cladding as a corrosion-resistant layer on load-bearing base material: What is cladding and when to use it. A practical weld overlay discussion is also available from WH Labs on weld overlay vs cladding.
| Term | Meaning | Bonding | Typical Use |
|---|---|---|---|
| Cladding | Broad term for applying a metallic corrosion- or wear-resistant layer to a base material | Usually metallurgical or strongly bonded | Pressure vessels, heat exchangers, piping, process equipment |
| Weld overlay | A cladding method where filler metal is welded onto the base surface | Metallurgical fusion bond | Completed vessels, nozzles, seams, repairs, complex geometries |
| Clad plate | Plate made by roll bonding, explosion bonding, or similar mill process before fabrication | Metallurgical bond across plate area | New vessel shells, heads, and large fabricated sections |
| Lining | A separate corrosion-resistant layer attached inside the vessel | May be intermittent, mechanical, or not fully bonded | Rubber lining, strip lining, loose liners, selected low-pressure services |
| Thermal spray cladding/coating | Sprayed metallic layer applied to a prepared surface | Mostly mechanical bond | Field corrosion barriers where low heat input is needed |
The conclusion: weld overlay is one way to make a cladded vessel, but not all cladding is weld overlay. Lining can protect against corrosion, but it usually behaves differently from a fully bonded clad layer.
Where Is Cladding Used in a Vessel?
Cladding is usually applied to the vessel side that sees the process fluid. In most cases this means the internal surface, but it can also include nozzles, manways, weld joints, inlet zones, outlet zones, and areas exposed to local erosion or corrosion.
| Vessel Area | Why It May Be Cladded | Key Specification Point |
|---|---|---|
| Shell internal surface | General corrosion resistance | Minimum finished clad thickness and chemistry |
| Heads / dished ends | Same process fluid exposure as shell | Forming method and post-forming inspection |
| Nozzles and weld ends | Local flow turbulence and weld transition corrosion | Continuity of clad layer through the connection |
| Inlet impingement zone | Erosion, particle impact, local corrosion | Higher wear-resistant overlay or replaceable insert |
| Seam weld areas | Clad continuity after shell welding | Weld overlay restoration over seams |
| Manways and attachments | Crevice corrosion and local fluid exposure | Detail design and inspection access |
The buyer conclusion: a vessel cladding specification should mark exactly which areas are cladded and where clad continuity must be restored after fabrication.
Common Materials for Vessel Cladding
The cladding material should be chosen for the actual process fluid and operating condition. It is not enough to say “stainless cladding” or “nickel alloy cladding” without defining the service and acceptance criteria.
| Cladding Material Family | Main Benefit | Typical Vessel Service |
|---|---|---|
| Austenitic stainless steel | General corrosion resistance | Mild to moderate corrosive process fluids |
| Duplex / super duplex stainless steel | Chloride resistance and strength | Offshore, seawater, oil and gas process vessels |
| Nickel alloys such as Alloy 625 type materials | Strong corrosion resistance in aggressive environments | Sour service, chlorides, acids, high-temperature corrosion |
| High nickel-chromium-molybdenum alloys | Severe corrosion resistance | Highly aggressive chemical and petrochemical service |
| Cobalt or carbide hardfacing alloys | Wear, erosion, galling, and hot hardness | Inlet zones, internals, erosion-prone surfaces |
| Thermal spray CRA systems | Low heat input corrosion barrier | Field upgrades and repair where weld heat is undesirable |
The conclusion: material selection should be tied to corrosion data, process chemistry, operating temperature, solids content, and expected design life.
How Is Vessel Cladding Applied?
There are several ways to clad a vessel. New vessels often use clad plate or weld overlay during fabrication. Existing vessels may use field weld overlay, thermal spray cladding, or localized repair depending on access and service risk.
| Method | Best Fit | Strength | Limitation |
|---|---|---|---|
| Roll-bonded or explosion-bonded clad plate | New vessel shells and heads | Uniform clad layer produced before fabrication | Requires planning before vessel fabrication |
| Weld overlay cladding | Completed vessels, nozzles, seams, repairs, complex geometry | Metallurgical bond and flexible application | Dilution, heat input, PWHT, distortion, and productivity must be controlled |
| Automated strip / wire weld overlay | Large internal surfaces and production cladding | Higher productivity than manual overlay | Needs access, fixtures, procedure qualification |
| Laser cladding | Precision local repair or low heat input surface upgrade | Low heat input, low dilution, controlled layer | Higher equipment cost and access requirements |
| Thermal spray / HVTS-type metallic cladding | Field corrosion barrier where low heat input matters | No fusion HAZ and reduced distortion risk | Bond and service limits differ from fused weld overlay |
| Loose or strip lining | Selected low-pressure or chemical services | Can be economical and replaceable | Not fully bonded; crevice and attachment details matter |
The buyer conclusion: the best method depends on whether the vessel is new or existing, how much surface area must be protected, whether a metallurgical bond is required, and whether heat input creates risk.
Why Dilution Matters in Weld Overlay
Dilution is the mixing of base metal into the deposited overlay. In weld overlay cladding, dilution can change the chemistry of the corrosion-resistant alloy. For example, too much iron pickup in a nickel alloy overlay can reduce corrosion performance.
This is why cladding specifications often define minimum finished thickness, layer count, chemistry after overlay, or maximum dilution. The Fabricator discusses the inspection and dilution challenge in CRA weld overlay: Better cladding, better inspection.
| Variable | Why It Matters | Buyer Should Specify |
|---|---|---|
| Minimum finished thickness | Protective alloy must remain after machining or grinding | Final thickness after all finishing, not only deposited thickness |
| Dilution / chemistry | Base metal mixing can reduce corrosion resistance | Chemistry limits or procedure qualification evidence |
| Layer count | Multiple layers may be needed to reach undiluted chemistry | Minimum number of layers where required |
| Porosity and cracks | Defects create corrosion paths and leak risk | Visual, PT, UT, or other NDT acceptance criteria |
| Bonding quality | Lack of fusion can cause overlay failure | Procedure qualification and inspection plan |
| PWHT / heat input | Welding can affect base material properties and distortion | WPS/PQR, interpass limits, heat treatment requirement |
| Surface finish | May affect corrosion, cleanability, or flow behavior | Grinding, polishing, roughness, or acceptance standard |
The conclusion: for weld overlay vessels, the critical question is not simply “what alloy was deposited?” It is “what alloy chemistry and thickness remain at the service surface after welding and finishing?”
Cladded Vessel vs Solid Alloy Vessel
Cladding is often chosen because a solid alloy vessel would be expensive, heavy, or slow to source. A cladded vessel keeps the bulk strength of carbon steel or low-alloy steel and adds the premium alloy only at the fluid-contact surface.
| Factor | Cladded Vessel | Solid Alloy Vessel |
|---|---|---|
| Material cost | Lower for large and thick pressure vessels | Higher because the entire shell uses premium alloy |
| Pressure strength | Base steel carries pressure and mechanical load | Alloy body must provide both strength and corrosion resistance |
| Corrosion resistance | High at clad surface if thickness and chemistry are controlled | High throughout the full wall |
| Fabrication complexity | Requires cladding procedure, inspection, and clad continuity at welds | Requires alloy fabrication and welding expertise throughout |
| Repair strategy | Localized overlay repair or field cladding may be possible | Repair may require expensive full-alloy welding |
| Best fit | Large vessels where only the wetted surface needs CRA | Small or severe-service parts where full alloy is justified |
The buyer conclusion: a cladded vessel is often the best balance of cost and performance, but only when cladding quality is properly specified and verified.
When Laser Cladding May Be Considered
Laser cladding may be considered for localized vessel repair, precision corrosion-resistant overlays, nozzle areas, erosion zones, or components connected to vessels where low heat input and low dilution are important. It is not always the first choice for large internal vessel surfaces, but it can be valuable for high-value repairs and controlled local upgrades.
HALDEN’s related equipment pages include laser cladding machine, high-speed laser cladding machine, and pipe hardfacing equipment. For cylindrical vessels or pipe-like parts, access, rotation, torch reach, and safety enclosure design are as important as cladding power.
Common Buying Mistakes
- Confusing lining, cladding, and weld overlay. This can lead to the wrong bond type, inspection plan, or corrosion protection strategy for the actual pressure vessel service.
- Specifying only the alloy name. A vessel listed as Alloy 625 overlay may still fail if dilution, final chemistry, thickness, or defects are not controlled.
- Ignoring final thickness after machining. The deposited layer may look adequate, but grinding or machining can leave too little CRA to protect the vessel.
- Not defining clad continuity at weld seams and nozzles. Unprotected weld transitions can become corrosion weak points even if the main shell is cladded.
- Skipping NDT and chemistry verification. Hidden porosity, cracks, lack of fusion, or excessive dilution can create corrosion paths and pressure boundary risk.
- Choosing weld overlay without considering PWHT and distortion. Heat input can affect the base material, schedule, and dimensional control, especially in repair work.
- Over-specifying a solid alloy vessel when cladding is enough. This can increase cost and lead time without improving the actual wetted-surface performance.
Buyer Checklist
- What process fluid and temperature will the vessel see? Corrosion chemistry and temperature decide whether stainless, duplex, nickel alloy, or another CRA is required.
- Which vessel surfaces require cladding? Shell, heads, nozzles, weld seams, inlet zones, and internals may not all need the same protection.
- Is this a new vessel or an existing vessel repair? New fabrication may favor clad plate or automated weld overlay, while repair may require field-applied overlay or thermal spray cladding.
- What minimum finished clad thickness is required? Finished thickness after machining is the protective barrier that matters in service.
- How will dilution or final chemistry be controlled? Dilution can reduce corrosion resistance, especially in nickel alloy and stainless overlays.
- What NDT and inspection methods will be used? Visual inspection, PT, UT, hardness, thickness, and chemistry checks may be needed depending on risk.
- Is PWHT required after weld overlay? Heat treatment can affect schedule, distortion, overlay properties, and base material requirements.
- How will clad continuity be restored at seams and nozzles? These transition zones often become weak points if not detailed properly.
- What is the economic comparison against solid alloy construction? Cladding is usually selected to reduce cost, but the full cost should include fabrication, inspection, repairability, and lead time.
What to Send for a Vessel Cladding Quote
A useful vessel cladding quotation needs service and fabrication details, not only vessel size. The supplier must understand the corrosion problem, geometry, and inspection requirements.
| Information to Send | Why It Matters |
|---|---|
| Vessel drawing and dimensions | Shows access, surface area, nozzles, heads, and cladding zones |
| Base material and thickness | Controls weldability, heat treatment, pressure boundary, and procedure selection |
| Process fluid and operating temperature | Determines CRA material and corrosion mechanism |
| Required cladding material | Defines stainless, duplex, nickel alloy, carbide, or other overlay family |
| Minimum finished thickness | Ensures enough protective alloy remains after finishing |
| New build or repair | Affects whether clad plate, weld overlay, field cladding, or local repair is practical |
| Inspection and code requirements | Defines NDT, procedure qualification, documentation, and acceptance criteria |
| Delivery schedule or outage window | Helps compare productivity and downtime impact of each cladding method |
This information lets the supplier recommend whether the job should use clad plate, weld overlay, laser cladding, thermal spray cladding, or another vessel protection method.
Final Recommendation
Cladding in a vessel is a metallic protective layer applied to the process-contact surface so the vessel can resist corrosion, erosion, or wear while using an economical structural base material. It is common in pressure vessels where carbon steel provides strength and a CRA layer provides corrosion resistance.
For buyers, the critical details are not only the word “cladding.” Specify the cladding method, material, final thickness, dilution or chemistry, bonded condition, NDT, PWHT, and clad continuity at welds and nozzles. A well-specified cladded vessel can reduce cost and extend service life. A vaguely specified one can create hidden corrosion risk inside a pressure boundary.
