How to Select Compatible Tube and Tubesheet Materials for Heat Exchangers
Tube and tubesheet material compatibility is a critical part of shell-and-tube heat-exchanger reliability, but it should not be treated as a simple exercise in matching two alloy names.
The tubes, tubesheet faces, tube holes, tube-to-tubesheet joints, weld metal, cladding, weld overlay, channels, shells, baffles, and fasteners can experience different combinations of fluids, temperature, stress, flow, deposits, and fabrication effects.
A tube alloy may perform well in the bulk process fluid but fail at a crevice inside the tubesheet. A tubesheet facing may resist the channel-side fluid while the backing material remains vulnerable at an exposed edge or machined tube hole. Two alloys that are individually corrosion resistant may still create an unfavorable galvanic couple when electrically connected in a conductive electrolyte.
A reliable material combination must therefore be evaluated as a complete corrosion, mechanical, thermal, fabrication, inspection, and maintenance system—not as two independent datasheet selections.
![Tube and tubesheet material compatibility for shell-and-tube heat exchangers](https://emilypipe.com/wp-content/uploads/2026/07/Hastelloy-C276-tube16.png")
The first question should not be:
“Can Alloy A tubes be installed in an Alloy B tubesheet?”
It should be:
“Which surfaces contact each fluid, which damage mechanisms are credible at every interface, how will the joint be manufactured, and how will the completed exchanger be inspected and maintained?”
Compatibility Is a System Property
Material compatibility includes several different questions.
| Compatibility Area | Main Question | Possible Failure if Overlooked |
|---|---|---|
| Chemical compatibility | Can each exposed material resist its actual fluid? | General corrosion, pitting, crevice corrosion or cracking |
| Galvanic compatibility | Will electrically connected dissimilar materials form an unfavorable couple? | Accelerated attack of the anodic material |
| Mechanical compatibility | Can the materials support expansion, welding and operating loads? | Pullout, loosening, deformation or cracking |
| Thermal compatibility | Can the exchanger accommodate differential thermal movement? | Fatigue, joint loading or tubesheet distortion |
| Metallurgical compatibility | Can the materials be welded or overlaid without harmful dilution or phases? | Cracking, porosity or reduced corrosion resistance |
| Fabrication compatibility | Can the tube be expanded, welded and inspected reliably? | Leaks, tube thinning or inconsistent joints |
| Code compatibility | Are the materials and construction accepted by the governing design rules? | Unapproved pressure construction |
| Inspection compatibility | Can the specified NDT detect relevant discontinuities? | Undetected flaws or misleading acceptance |
| Maintenance compatibility | Can the tube bundle be cleaned, examined and repaired? | Unmanageable fouling or shortened service interval |
| Supply compatibility | Can the same materials and conditions be supplied consistently? | Uncontrolled substitutions and requalification |
Compatibility does not mean that the two materials must be identical.
It means the completed system must remain within acceptable limits under its defined service and lifecycle conditions.
Start with the Applicable Design Framework
Material selection should be connected to the governing exchanger and pressure-equipment requirements.
ISO 16812:2019 specifies requirements and recommendations for mechanical design, material selection, fabrication, inspection, testing, and shipment preparation of shell-and-tube heat exchangers for petroleum, petrochemical, and natural-gas service.
API Standard 660 provides a related framework for shell-and-tube heat exchangers in those industries.
The TEMA heat-exchanger standards provide widely used mechanical-design and construction practices, including detailed treatment of tube-to-tubesheet joints.
Where the exchanger is a pressure vessel, the applicable pressure code may include ASME BPVC Section VIII or another national construction code.
These documents serve different purposes.
| Document Type | Typical Role | What It Does Not Replace |
|---|---|---|
| Pressure-vessel code | Pressure design, permitted materials, fabrication, inspection and certification | Process corrosion assessment |
| Heat-exchanger standard | Exchanger configuration, construction and purchaser–manufacturer requirements | Complete application-specific material qualification |
| Material product standard | Chemistry, mechanical properties, dimensions and delivery requirements | Equipment design or service-life guarantee |
| Corrosion test standard | Defined comparison or exposure procedure | Complete simulation of operating service |
| Purchase specification | Project-specific materials, joints, testing and documentation | Engineering validation when the specification is incomplete |
| Manufacturer procedure | Controlled welding, expansion and inspection process | Purchaser design responsibility |
| MTR | Batch-level material test results | Finished exchanger conformity |
The project should identify the governing documents and their revisions before material procurement begins.
Map Every Material and Exposure Zone
A tubesheet is not exposed to only one environment.
The following surfaces may experience different conditions.
| Zone | Possible Exposure | Important Questions |
|---|---|---|
| Tube bore | Tube-side process fluid | What are the chemistry, velocity, deposits and cleaning methods? |
| Tube outside surface | Shell-side process fluid | Are there baffle deposits, vibration or boiling conditions? |
| Channel-side tubesheet face | Tube-side inlet or outlet fluid | Is this face solid alloy, clad or overlaid? |
| Shell-side tubesheet face | Shell-side fluid | Is the base material exposed on this side? |
| Tube hole | Joint crevice, leakage or mixed fluids | Is the corrosion-resistant layer continuous through the hole? |
| Tube end | Inlet turbulence, erosion and weld heat | Is inlet protection or extra thickness needed? |
| Expansion zone | Residual stress and crevice conditions | Is expansion controlled and qualified? |
| Seal or strength weld | Weld metal, HAZ and residual stress | Is the weld compatible with both tube and facing material? |
| Cladding termination | Base-metal exposure and dilution | Is the transition protected and inspectable? |
| Pass partition groove | Stagnation and gasket crevice | Can fluid concentrate or leak into the interface? |
| Backing material | Possible leakage or exposed edges | What happens if the corrosion barrier is locally breached? |
| Channel and bonnet | Tube-side fluid and cleaning | Does a different channel alloy create additional galvanic couples? |
| Fasteners and plugs | Crevices and maintenance exposure | Are bolting and plug materials compatible? |
The materials diagram should show not only nominal alloy names but also the actual exposed surfaces.
Define Both Fluid Environments
The tube-side and shell-side fluids should be evaluated independently.
| Fluid Data | Tube Side | Shell Side |
|---|---|---|
| Main chemical components | Required | Required |
| Concentration range | Required | Required |
| Temperature range | Required | Required |
| Design temperature | Required | Required |
| Pressure and vacuum | Required | Required |
| pH range | Required | Required |
| Chloride and halides | Required | Required |
| Dissolved oxygen | Required | Required |
| Oxidizing species | Required | Required |
| Reducing species | Required | Required |
| Sulfur compounds or H₂S | Required | Required |
| Solids and slurry content | Required | Required |
| Water content | Required | Required |
| Organic phase | Required | Required |
| Gas phase and non-condensables | Required | Required |
| Velocity | Required | Required |
| Boiling or condensing | Required | Required |
| Fouling and deposits | Required | Required |
| Cleaning chemistry | Required | Required |
| Shutdown conditions | Required | Required |
| Startup transients | Required | Required |
| Upset or contamination cases | Required | Required |
The tubesheet joint may experience a third environment created by:
- Leakage;
- evaporation;
- deposit concentration;
- oxygen gradients;
- fluid mixing;
- stagnant crevice chemistry.
That local environment may be more aggressive than either bulk fluid.
Do Not Rely on the Main Chemical Name
A service described only as “seawater,” “hydrochloric acid,” “cooling water,” or “process condensate” is incomplete.
| Missing Variable | Why It Can Matter |
|---|---|
| Temperature | Changes corrosion kinetics and passive-film stability |
| Concentration | Can change corrosion mechanism and material ranking |
| Dissolved oxygen | Influences cathodic reaction and passivity |
| Chloride | Promotes localized corrosion in susceptible alloys |
| Fluoride | Can significantly affect titanium suitability |
| Ferric or cupric ions | Can increase oxidizing power |
| H₂S and sulfur species | May introduce cracking or sulfide-related risks |
| Solids | Can create erosion, under-deposit attack or plugging |
| Flow velocity | Can improve mass transfer or cause erosion and vibration |
| Stagnation | Encourages deposits and crevice-like conditions |
| pH excursions | May occur during cleaning, startup or contamination |
| Cleaning agent | May be more aggressive than the process fluid |
| Carryover | Can introduce unexpected salts or treatment chemicals |
| Steam-out conditions | May expose dry hot deposits or concentrated residues |
| Shutdown moisture | Can create a different oxygen and chloride condition |
For mixed or changing streams, the material review can also refer to the internal guide on selecting materials for mixed chemical media in process equipment.
Identify the Governing Damage Mechanisms
Failure-Mechanism Matrix
| Mechanism | Conditions Required | Likely Locations | Evidence Needed |
|---|---|---|---|
| General corrosion | Corrosive fluid and susceptible material | Tube surfaces, tubesheet faces and channels | Application-specific corrosion data |
| Pitting | Passive alloy, aggressive ions and sufficient potential | Tube surface, weld tint and tubesheet face | Localized-corrosion testing and surface review |
| Crevice corrosion | Shielded gap with restricted mass transfer | Expanded joints, gaskets, deposits and tube holes | Representative crevice testing |
| Galvanic corrosion | Dissimilar materials, electrical contact and electrolyte | Tube–tubesheet interface, plugs and cladding edges | Environmental potential and area-ratio review |
| SCC | Susceptible alloy, tensile stress and specific environment | Expanded zones, weld HAZ and cold-worked tube | Stress, metallurgy and environmental assessment |
| Corrosion fatigue | Cyclic stress plus corrosive environment | Tube joints, baffle contacts and thermal transition zones | Cyclic testing or design assessment |
| Fretting corrosion | Repeated small movement and contact | Baffle supports and tube-to-support locations | Vibration and wear assessment |
| Erosion-corrosion | Fluid velocity, particles or phase change | Tube inlet, bends and impingement zones | Velocity and particle review |
| Cavitation | Local pressure collapse | Pumps, inlets or restrictions | Hydraulic assessment |
| Hydrogen uptake | Cathodic or reducing conditions in susceptible material | Titanium or other reactive alloys | Electrochemical and environmental review |
| Intergranular attack | Susceptible microstructure and environment | Weld HAZ or improperly heat-treated alloy | Heat-treatment and corrosion testing |
| Dealloying | Specific alloy and environment | Copper or other susceptible alloy systems | Alloy-specific assessment |
| Thermal fatigue | Repeated temperature gradients | Fixed tubesheets, welds and thick sections | Thermal-cycle stress analysis |
| Mechanical fatigue | Vibration or repeated pressure loading | Tube spans, joints and supports | Vibration and fatigue analysis |
| Creep | Sufficient temperature, stress and time | High-temperature equipment only | Code allowable stresses and creep data |
One test cannot address all of these mechanisms.
Understand Galvanic Corrosion Correctly
AMPP galvanic-corrosion guidance explains that galvanic corrosion occurs when dissimilar materials are electrically coupled in a corrosive electrolyte.
Four conditions should be reviewed.
| Requirement | Review Question |
|---|---|
| Dissimilar electrochemical behavior | Do the materials develop different corrosion potentials in the actual fluid? |
| Electrical contact | Are the materials connected through welding, expansion or metallic contact? |
| Conductive electrolyte | Does the fluid bridge the exposed surfaces? |
| Unfavorable polarization and area ratio | Can the cathode support a high current concentrated on a small anode? |
If one of these conditions is absent, a macroscopic galvanic cell may not operate.
The Galvanic Series Is Environment-Specific
A galvanic series measured in seawater should not be treated as a universal ranking for:
- Acids;
- alkaline solutions;
- deaerated water;
- high-temperature brines;
- hydrocarbon systems;
- mixed solvents.
Passive alloys can also change potential when:
- Their passive film breaks down;
- oxygen changes;
- temperature changes;
- deposits form;
- local chemistry becomes acidic;
- the surface is welded or damaged.
The project should use data relevant to the actual electrolyte whenever galvanic risk is important.
Area Ratio Can Control Severity
A small anodic area connected to a large cathodic area is generally unfavorable because galvanic current can be concentrated on the smaller anode.
| Area Relationship | General Risk Direction |
|---|---|
| Large anode and small cathode | Often less severe anodic current density |
| Similar exposed areas | Intermediate, environment-dependent |
| Small anode and large cathode | Potentially severe localized anodic attack |
This is especially important where:
- A large bundle of passive titanium or nickel-alloy tubes is connected to a small exposed area of steel;
- a small defect exposes the tubesheet backing material through a large noble cladding system;
- a small carbon-steel plug or fastener contacts a large corrosion-resistant surface.
The area assessment must use the wetted exposed area, not merely the nominal component size.
Galvanic Corrosion Is Not the Only Dissimilar-Metal Risk
Two materials can have acceptable galvanic behavior but still be incompatible because of:
- Thermal-expansion mismatch;
- welding difficulty;
- excessive hardness difference during expansion;
- filler-metal dilution;
- different heat-treatment requirements;
- differential stiffness;
- crevice formation;
- maintenance or repair limitations.
Likewise, using the same alloy does not prevent:
- Pitting;
- crevice corrosion;
- SCC;
- vibration;
- poor welding;
- leakage.
“Same alloy” and “compatible system” are not synonyms.
Assess Thermal Expansion as Part of the Exchanger Design
Different coefficients of thermal expansion can create relative movement between the tubes, tubesheets, shell, and channels.
The significance depends on:
- Metal temperatures;
- temperature difference between shell and tubes;
- exchanger length;
- number of cycles;
- restraint;
- tubesheet stiffness;
- tube flexibility;
- joint strength;
- exchanger configuration.
Exchanger Configuration and Differential Expansion
| Configuration | General Thermal-Movement Feature | Material-Compatibility Implication |
|---|---|---|
| Fixed tubesheet | Tubes and shell are restrained at both ends | Differential expansion may create significant axial loads |
| Fixed tubesheet with shell expansion joint | Shell movement can be accommodated by the joint | Expansion-joint fatigue and corrosion require evaluation |
| U-tube | Tubes can flex through the U-bend | Cleaning, tube replacement and bend condition also matter |
| Floating head | One tubesheet or head can move relative to the shell | More components, seals and maintenance complexity |
| Kettle or special configuration | Thermal behavior depends on detailed arrangement | Project-specific analysis is required |
The solution is not always to choose materials with matching coefficients.
It may instead involve:
- A different exchanger type;
- an expansion joint;
- modified tube length;
- reduced restraint;
- adjusted startup procedure;
- a different joint design.
Consider Temperature Gradients, Not Only Bulk Temperatures
The relevant temperatures may include:
- Tube-side fluid temperature;
- shell-side fluid temperature;
- tube-wall temperature;
- tubesheet-face temperature;
- average tubesheet temperature;
- startup metal temperature;
- cleaning temperature;
- steam-out temperature;
- local boiling or condensing temperature.
A thick tubesheet may develop internal gradients that differ from the fluid temperatures.
The design should also consider:
- Rapid startup;
- emergency quench;
- loss of flow;
- blocked tube conditions;
- one-sided operation;
- bypass;
- shutdown with trapped hot fluid.
Tube-to-Tubesheet Joint Selection
The joint is both a mechanical connection and a potential corrosion interface.
| Joint Type | Main Function | Potential Advantages | Important Risks |
|---|---|---|---|
| Mechanical roller expansion | Mechanical grip and sealing by deformation | Established process and no fusion welding | Crevice, overexpansion, underexpansion and residual stress |
| Hydraulic expansion | Controlled pressure-based expansion | Potentially uniform expansion in qualified conditions | Requires validated pressure and geometry |
| Explosive expansion | High-energy expansion for selected applications | Can create strong contact in special designs | Specialized qualification and material response |
| Seal weld | Primarily limits leakage path | Can close the face-side crevice | Should not be assumed to carry structural load |
| Strength weld | Designed to transmit specified mechanical load | Can provide structural joint capacity | Requires weld design, procedure and inspection |
| Expanded plus seal welded | Combines mechanical contact and face sealing | Common where both functions are needed | Sequence, expansion zone and weld interaction matter |
| Strength welded plus expanded | Combines welded strength with controlled contact | May reduce crevice or improve joint performance | Must follow qualified project procedure |
The exact definitions and requirements should follow the governing code, exchanger standard, purchaser specification, and qualified manufacturer procedure.
Seal Weld and Strength Weld Are Not Interchangeable
A seal weld is normally intended primarily to prevent leakage at the joint.
A strength weld is designed to transmit defined mechanical loading.
The purchase order should identify:
- Joint category;
- required load responsibility;
- weld geometry;
- filler metal;
- number of passes;
- tube projection or recess;
- inspection;
- expansion requirement;
- qualification mock-up.
The expression “tube ends welded” is not sufficient.
Expansion Must Be Qualified
Insufficient expansion may produce:
- Low contact pressure;
- leakage paths;
- tube movement;
- crevice conditions.
Excessive expansion may produce:
- Tube-wall thinning;
- work hardening;
- cracking;
- tubesheet-hole deformation;
- residual-stress increase;
- damage to a weld or cladding layer.
The qualified process may need to define:
| Expansion Variable | Required Control |
|---|---|
| Tube OD and wall | Actual production dimensions |
| Tube material and condition | Same or representative metallurgy |
| Tubesheet material | Solid, clad or overlaid condition |
| Hole diameter | Tolerance and surface condition |
| Hole finish | Roughness and groove details |
| Expansion length | Start and stop location |
| Expansion pressure or torque | Qualified process range |
| Wall reduction or contact criterion | Project-defined acceptance |
| Sequence | Relationship with welding |
| Equipment calibration | Controlled and recorded |
| Operator qualification | Project or manufacturer requirement |
| Mock-up inspection | Macro, pullout, leak or other specified tests |
A percentage wall-reduction value should not be copied into every alloy combination without qualification.
Joint Crevices Need Special Attention
A rolled or expanded joint may contain a narrow annular region.
Local chemistry can change because of:
- Restricted oxygen transport;
- evaporation;
- hydrolysis;
- chloride concentration;
- deposit accumulation;
- fluid mixing;
- incomplete sealing.
A tube alloy that resists open-surface exposure may not provide the same resistance inside a hot, acidic crevice.
The review should include:
- Tube alloy;
- tubesheet facing;
- exposed backing metal;
- weld metal;
- filler;
- expansion length;
- operating temperature;
- shutdown exposure;
- cleaning.
Material Architecture Options
A tubesheet does not always need to be manufactured from one solid alloy.
| Architecture | Description | Potential Advantages | Important Limitations |
|---|---|---|---|
| Solid carbon or low-alloy steel | Entire section uses structural steel | Cost and code familiarity | Limited corrosion resistance |
| Solid stainless steel | Entire section uses stainless steel | Uniform composition and fabrication route | Cost and localized-corrosion limits |
| Solid nickel alloy | Entire section uses selected nickel alloy | Continuous alloy through holes and thickness | High cost and availability |
| Solid titanium | Entire section uses titanium | Continuous corrosion-resistant material | Cost, stiffness, fabrication and code considerations |
| Roll-bonded or explosion-bonded clad plate | Corrosion-resistant layer bonded to structural backing | Reduces use of expensive alloy | Bond, edges, holes and weld transitions require control |
| Weld overlay | Corrosion-resistant weld metal deposited on backing | Flexible alloy selection and repairability | Dilution, cracking, thickness and surface finish |
| Machined insert or facing | Local corrosion-resistant component | Targeted protection | Sealing, attachment and crevice risks |
| Coating or lining | Non-structural barrier layer | Potential chemical protection | Damage, permeability, adhesion and repair limitations |
The selection should consider:
- Pressure-code acceptance;
- effective structural thickness;
- corrosion-barrier thickness;
- machining allowance;
- tube-hole configuration;
- welding;
- NDT;
- repair;
- lifecycle inspection.
Clad Tubesheets Require Continuity Review
A corrosion-resistant facing protects only the surfaces it continuously covers.
Potential weak locations include:
- Tube holes;
- pass-partition grooves;
- gasket grooves;
- bolt holes;
- nozzle transitions;
- clad edges;
- weld preparations;
- repaired regions.
The engineering drawing should show:
- Clad thickness;
- backing material;
- bond method;
- bond inspection;
- tube-hole treatment;
- overlay extension;
- corrosion-barrier welds;
- allowable machining;
- repair procedure.
“Titanium-clad tubesheet” or “Alloy 625-clad tubesheet” is not a complete specification.
Tube-Hole Details Can Determine Compatibility
For a clad or overlaid tubesheet, the tube hole may expose:
- Only the facing alloy;
- both facing and backing material;
- an overlay deposited inside the hole;
- a sleeve or insert;
- a transition weld.
The joint design must establish whether the process fluid can reach the backing material through:
- The annular gap;
- a weld defect;
- an unsealed hole;
- damage during expansion;
- corrosion of the facing layer.
A corrosion-resistant face with vulnerable tube holes may not provide the intended protection.
Material-Family Screening
The following table is an initial engineering screen, not a universal recommendation.
| Material Family | Potential Strengths | Important Limitations | Possible Heat-Exchanger Context |
|---|---|---|---|
| Carbon and low-alloy steel | Strength, cost, code familiarity | General corrosion and galvanic vulnerability | Backing plate, shell or controlled noncorrosive service |
| 304L stainless steel | Fabrication and general corrosion resistance | Limited chloride localized-corrosion margin | Mild process or utility services |
| 316L stainless steel | Improved chloride resistance relative to 304L in many environments | Still susceptible to pitting, crevice corrosion and SCC | Water, chemical and sanitary services where validated |
| 6Mo austenitic stainless | Higher localized-corrosion resistance | Cost, welding and availability | More aggressive chloride services |
| Duplex stainless steel | Strength and selected chloride/SCC resistance | Phase balance, welding and temperature limitations | Project-approved seawater or process applications |
| Super duplex stainless steel | Higher localized-corrosion margin | Fabrication, code and thermal limitations | Severe chloride service where qualified |
| Nickel-copper alloys | Useful resistance in selected seawater and chemical environments | Oxidizing-media and galvanic limitations | Condensers and marine systems under defined conditions |
| Alloy 600/690 family | High-temperature and selected water/chemical resistance | Not universal for chloride or mixed-acid service | Project-specific high-temperature or water service |
| Alloy 625 | Strength and broad resistance in selected chloride and chemical environments | Cost, machining and no universal chemical immunity | Tubes, local components and severe services where justified |
| Alloy 825 | Resistance in selected acid and chloride environments | Application-specific limits | Chemical processing under defined conditions |
| C22/C276 family | Broad resistance in selected severe mixed chemical environments | Cost, fabrication and service-specific limits | Aggressive chemical exchangers |
| Titanium Grade 2 | Strong resistance in many oxidizing chloride and seawater environments | Fluoride, reducing acids, hot crevices and hydrogen-related limits | Seawater condensers and selected chemical service |
| Titanium Grade 7 | Enhanced resistance in selected acidic or crevice conditions | Cost and exact environment still require review | More severe titanium applications |
| Titanium Grade 12 | Improved strength and selected crevice-corrosion performance | Not universally superior to Grade 2 or 7 | Project-specific chemical and heat-exchanger service |
| Zirconium or tantalum | Exceptional resistance in selected media | High cost, limited supply and specialized fabrication | Highly specialized chemical service |
The correct combination should be derived from the complete environment and construction route.
Titanium Tube and Tubesheet Considerations
ASTM B338 titanium heat-exchanger tubes covers seamless and welded titanium tubes for condensers and heat exchangers.
ASTM B265 titanium plate covers titanium strip, sheet, and plate.
These are separate product standards.
A titanium tube supplied under B338 does not automatically define:
- Tubesheet plate requirements;
- joint design;
- weld filler;
- expansion procedure;
- corrosion compatibility;
- exchanger code compliance.
Titanium Screening Questions
| Question | Why It Matters |
|---|---|
| Is the environment oxidizing or reducing? | Titanium passivity depends on environmental conditions |
| Are fluorides present? | Fluoride can destabilize the passive film |
| Can hot acidic crevices form? | Crevice conditions may be more aggressive than bulk fluid |
| Is cathodic protection present? | Excessive cathodic polarization may promote hydrogen uptake |
| Is the backing material exposed? | A large titanium cathode may accelerate attack of a small anodic area |
| What is the joint method? | Welding and expansion routes differ from steel systems |
| Is the tubesheet solid or clad? | Tube-hole protection and repair strategy differ |
| What cleaning chemicals are used? | Cleaning may create the limiting condition |
| Is erosion at the tube inlet credible? | Titanium corrosion resistance does not prevent every mechanical damage mode |
Titanium should not be described as a universal solution for all chloride-containing media.
Nickel-Alloy Tube and Tubesheet Standards
ASTM maintains separate product standards for nickel-alloy tube, pipe, plate, sheet, strip, bar, and forging products.
Examples include:
| Product Form | Example Standard Route | Typical Use in Procurement |
|---|---|---|
| Nickel-alloy condenser and heat-exchanger tube | ASTM B163 | Selected seamless nickel and nickel-alloy exchanger tubes |
| Alloy 625 seamless pipe and tube | ASTM B444 | UNS N06625 pipe and tube |
| Alloy 625 plate, sheet and strip | ASTM B443 | Tubesheet facing or solid plate where applicable |
| Ni-Cr-Mo seamless pipe and tube | ASTM B622 | Selected C-family alloy tubes |
| Ni-Cr-Mo plate, sheet and strip | ASTM B575 | Selected C-family tubesheet or facing material |
| Ni-Cr-Mo rod and bar | ASTM B574 | Plugs, bars and machined parts |
| Nickel-alloy forgings | Applicable B564 or project standard | Forged components where permitted |
The current alloy applicability and edition should be confirmed in the ASTM nickel-alloy product-standards listing.
A quotation should not state only:
- Inconel tube;
- Hastelloy plate;
- nickel-alloy tubesheet.
It should identify:
- UNS designation;
- product standard;
- revision;
- seamless or welded construction;
- heat-treatment condition;
- dimensions;
- inspection requirements.
Weld Metal Is a Separate Material
A tube-to-tubesheet weld introduces another metallurgy into the system.
The project should identify:
- Autogenous or filler-metal weld;
- filler classification;
- dilution from tube and facing;
- weld-metal corrosion resistance;
- HAZ behavior;
- heat input;
- interpass temperature;
- purge requirements;
- post-weld cleaning;
- PT or other NDT;
- repair method.
A filler with high nominal alloy content may still produce an unsuitable deposited weld if dilution or welding conditions are uncontrolled.
Dissimilar-Metal Welding Needs a Joint-Specific Review
Potential issues include:
- Solidification cracking;
- brittle intermetallic formation;
- excessive dilution;
- porosity;
- lack of fusion;
- thermal-stress concentration;
- reduced corrosion resistance;
- incompatible heat treatment.
Some material combinations should not be fusion welded directly.
An expanded joint, clad transition, sleeve, insert, or another engineered transition may be required.
The raw-material supplier should not approve a dissimilar weld only from the base-alloy names.
Surface Condition Can Change Corrosion Performance
Relevant surface conditions include:
- Mill finish;
- pickled;
- mechanically polished;
- electropolished;
- heat tinted;
- ground;
- blasted;
- welded;
- contaminated by iron;
- covered by deposits.
Surface treatment cannot correct:
- An unsuitable alloy;
- an open crevice;
- poor weld penetration;
- exposed backing material;
- excessive residual stress.
The corrosion test specimen should represent the intended production surface where the surface can influence performance.
Fouling and Deposits Change the Local Environment
Deposits can:
- Restrict oxygen;
- concentrate chlorides;
- trap acids;
- create differential-aeration cells;
- increase wall temperature;
- promote under-deposit corrosion;
- change heat-transfer performance.
Fouling may occur because of:
- Scale;
- biological growth;
- solids;
- polymerization;
- coke;
- corrosion products;
- process contamination.
The material-selection team should review:
- Fouling tendency;
- cleaning interval;
- cleaning chemistry;
- deposit composition;
- allowable mechanical cleaning;
- access to tubes.
A material that performs in clean laboratory solution may behave differently beneath process deposits.
Flow, Erosion, and Inlet Attack
Tube-inlet damage may result from:
- High velocity;
- turbulence;
- suspended solids;
- droplets;
- flashing;
- two-phase flow;
- poor inlet distribution.
Potential controls include:
- Different tube material;
- greater wall thickness;
- ferrules or sleeves;
- impingement protection;
- inlet-zone design changes;
- reduced velocity;
- improved distribution.
Changing the entire tubesheet alloy may not solve a tube-inlet erosion problem.
Vibration and Fretting Are Not Material-Corrosion Problems Alone
Tube vibration can be influenced by:
- Cross-flow velocity;
- baffle spacing;
- unsupported span;
- tube natural frequency;
- two-phase flow;
- fluidelastic instability;
- vortex shedding;
- acoustic excitation.
Damage may occur at:
- Baffles;
- support plates;
- tubesheet entrances;
- U-bends.
The result may include:
- Fretting;
- wall thinning;
- fatigue cracks;
- leakage.
A premium corrosion-resistant tube will not prevent failure caused by an unsuitable vibration design.
Corrosion Allowance Has Limits
Corrosion allowance can support design against predictable, relatively uniform metal loss.
It is not a reliable primary control for:
- Pitting;
- crevice corrosion;
- SCC;
- fatigue cracking;
- galvanic attack at a small anodic area;
- localized tube-hole attack.
High-alloy and titanium materials may be specified with limited or no conventional corrosion allowance in some designs, but that does not mean localized corrosion can be ignored.
Verification Strategy
A robust verification plan should address the specific risks.
| Verification Level | Typical Method | Main Purpose | Limitation |
|---|---|---|---|
| Material identity | MTR, PMI and marking | Confirm alloy and heat | Does not prove service compatibility |
| Chemistry | Heat or product analysis | Confirm elemental limits | Does not prove microstructure or corrosion |
| Mechanical properties | Tensile, hardness and other specified tests | Confirm product-level properties | Does not prove joint performance |
| Tube dimensions | OD, wall, length, straightness and ovality | Confirm manufacturing requirements | Does not detect every internal flaw |
| Tube NDT | Eddy current, ultrasonic or other methods | Detect specified discontinuities | Method and references must be defined |
| Tubesheet NDT | UT, PT or clad-bond inspection | Assess plate, weld or bond integrity | Coverage depends on method |
| Joint mock-up | Production-representative tube and tubesheet | Qualify welding and expansion | Must match production variables |
| Macro examination | Sectioned joint | Review fusion, penetration and expansion | Destructive and sample-based |
| Pullout or shear test | Joint mechanical test | Compare joint strength | Does not alone prove leak tightness |
| Leak testing | Hydrostatic, pneumatic, helium or other method | Evaluate pressure boundary or joint leakage | Sensitivity and test condition vary |
| Immersion corrosion | ASTM G31-type controlled exposure | Screen general corrosion under defined conditions | Does not cover all localized or flow effects |
| Localized corrosion | ASTM G48-type comparison | Rank selected alloys in oxidizing chlorides | Not a direct service-life test |
| Cyclic testing | Thermal or pressure cycling | Assess repeated loading | Requires representative boundary conditions |
| Baseline tube examination | Eddy current or UT after fabrication | Establish future inspection reference | Requires repeatable data and records |
Understand the Limits of Corrosion Tests
ASTM G31 laboratory immersion guidance addresses variables involved in laboratory immersion corrosion tests.
A useful test report should identify:
- Exact alloy and heat;
- product form;
- heat treatment;
- base metal or welded condition;
- surface finish;
- solution composition;
- impurities;
- temperature;
- pressure;
- atmosphere;
- agitation or flow;
- specimen area;
- exposure time;
- cleaning method;
- mass loss;
- localized attack;
- photographs.
Average corrosion rate alone should not be used to approve:
- Point-corrosion resistance;
- crevice resistance;
- SCC resistance;
- galvanic compatibility;
- fatigue;
- service life.
ASTM G48 can compare pitting and crevice-corrosion resistance in defined ferric-chloride conditions.
It should not be described as an exact simulation of every:
- Seawater service;
- process acid;
- cooling-water system;
- condensate condition;
- cleaning process.
Test Dissimilar Couples as Couples When Necessary
Testing isolated samples may miss galvanic effects.
Where galvanic corrosion is credible, testing may need to represent:
- Both materials;
- actual electrical connection;
- realistic exposed area ratio;
- surface condition;
- fluid;
- temperature;
- oxygen;
- flow;
- crevice geometry.
A 1:1 laboratory coupon area ratio may not represent a tube bundle connected to a small exposed tubesheet area.
Tube-to-Tubesheet Mock-Up Matrix
| Mock-Up Variable | Must Represent Production? |
|---|---|
| Tube alloy and heat-treatment condition | Yes |
| Tube OD and wall | Yes |
| Tubesheet or facing alloy | Yes |
| Solid, clad or overlay construction | Yes |
| Tube-hole diameter and finish | Yes |
| Hole grooves where used | Yes |
| Tube projection or recess | Yes |
| Welding process | Yes |
| Filler metal | Yes |
| Purge and shielding | Yes |
| Expansion method | Yes |
| Expansion sequence | Yes |
| Operator and equipment class | As required |
| Cleaning and surface treatment | Where performance-relevant |
| Inspection method | Yes |
| Acceptance criteria | Yes |
A mock-up using carbon-steel plate may not represent expansion behavior in a hard nickel-alloy overlay or titanium-clad tubesheet.
Inspection of Heat-Exchanger Tubes
ASTM practices include methods for electromagnetic examination of suitable tubular materials.
ASTM E426 eddy-current examination applies to seamless and welded tubular products made from titanium, austenitic stainless steel, and similar alloys.
Inspection requirements should define:
- Standard;
- tube material;
- diameter and wall range;
- encircling coil or probe method;
- reference standard;
- artificial discontinuity;
- inspection speed;
- frequency;
- signal threshold;
- coverage;
- data recording;
- disposition of indications.
“100% eddy current tested” is incomplete without these details.
Product Documentation Matrix
| Document | What It Can Demonstrate | What It Cannot Prove Alone |
|---|---|---|
| Original mill MTR | Heat chemistry, mechanics and specified tests | Service compatibility |
| Certificate of Conformance | Supplier declaration against the order | Actual results unless listed |
| EN 10204 3.1 | Defined batch-specific inspection document | Exchanger design compliance |
| PMI report | Alloy identity at tested locations | Heat treatment or full chemistry |
| Tube dimensional report | OD, wall, length and other stated dimensions | Corrosion resistance |
| Eddy-current report | Examination under a defined procedure | Absence of every discontinuity |
| UT report | Volumetric examination under defined conditions | Surface condition or leak tightness |
| Plate or clad-bond report | Defined plate or bond integrity results | Tube-hole continuity after machining |
| Welding procedure | Approved welding parameters | Production execution unless records exist |
| Welder qualification | Personnel qualification for defined range | Acceptance of each production joint |
| Joint mock-up report | Representative procedure performance | Entire exchanger service life |
| Corrosion report | Results under stated test conditions | Universal material suitability |
| Hydrostatic-test record | Pressure integrity during the test | Future corrosion or fatigue life |
| Leak-test report | Leakage below stated method sensitivity | Long-term joint reliability |
| Final data dossier | Controlled set of project documents | Automatic suitability if requirements were incomplete |
MTR Does Not Establish Compatibility
An MTR may confirm:
- Alloy;
- UNS designation;
- heat number;
- chemical composition;
- tensile properties;
- heat-treatment condition;
- product standard;
- specified tests.
It does not normally establish:
- Galvanic behavior;
- crevice-corrosion resistance;
- joint strength;
- weld-metal compatibility;
- thermal-fatigue life;
- tube vibration resistance;
- completed exchanger reliability.
The engineering team must connect the MTR to the design and service requirements.
Material-Pairing Screening Matrix
This matrix illustrates the questions that should be asked. It does not approve the combinations.
| Tube Material | Tubesheet Architecture | Primary Questions |
|---|---|---|
| Stainless tube | Solid stainless tubesheet | Is the grade sufficient for both fluids and the joint crevice? |
| Stainless tube | Carbon-steel tubesheet | Is steel exposed, and is galvanic or general corrosion acceptable? |
| Stainless tube | Stainless-clad steel tubesheet | Is cladding continuous through holes and weld zones? |
| Nickel-alloy tube | Solid same-family alloy | Are cost, code and expansion properties acceptable? |
| Nickel-alloy tube | Nickel-alloy-clad steel | How are tube holes and overlay transitions protected? |
| Nickel-alloy tube | Stainless tubesheet | Is the stainless alloy the small anode in the actual electrolyte? |
| Titanium tube | Solid titanium tubesheet | Are fabrication, stiffness, cost and code requirements satisfied? |
| Titanium tube | Titanium-clad steel tubesheet | Is steel fully isolated from the wetted joint and tube holes? |
| Titanium tube | Stainless tubesheet | What is the galvanic area ratio and crevice condition? |
| Titanium tube | Nickel-alloy tubesheet | Can the joint be manufactured and validated without harmful metallurgy? |
| Dissimilar tube bundle | Multi-alloy tubesheet system | Can cross-coupling, repairs and maintenance be controlled? |
Each combination requires application-specific review.
Supplier Evaluation Matrix
| Review Area | Evidence to Request | Warning Signs |
|---|---|---|
| Exact alloy identity | UNS and product standard | Only trade names are stated |
| Product form | Tube, pipe, plate, forging or clad plate | One standard is used for every form |
| Original manufacturer | Original mill identity and MTR | Recreated certificate without origin |
| Heat treatment | Actual delivery condition | “Standard condition” without clarification |
| Dimensions | Actual capability and inspection | Catalog tolerance copied without size review |
| Tube manufacture | Seamless or welded route | Route changes without notification |
| NDT | Method, reference standard and results | “100% tested” without procedure |
| Clad or overlay | Bonding route, thickness and inspection | No tube-hole protection detail |
| Welding support | Material and filler data | Supplier claims to approve exchanger weld design |
| Traceability | Heat-to-piece controls | Cut pieces lose heat identity |
| Corrosion testing | Exact medium and specimen condition | Generic “corrosion resistant” certificate |
| QMS | Valid certificate and scope | Certificate covers only sales office |
| Laboratory | Method-specific competence | Accreditation unrelated to test |
| Change control | Mill, process and standard notification | Unilateral substitution allowed |
| Production capacity | Repeatable sizes and quantities | Prototype source cannot support production |
| Nonconformance | Quarantine and corrective action | Replacement without investigation |
Practical Material-Selection Workflow
| Step | Engineering Action | Main Output |
|---|---|---|
| 1 | Identify exchanger type and governing code | Design-basis document |
| 2 | Map tube-side and shell-side environments | Fluid-condition table |
| 3 | Map all wetted materials and interfaces | Materials exposure diagram |
| 4 | Identify credible damage mechanisms | Corrosion and failure matrix |
| 5 | Screen tube-material families | Tube shortlist |
| 6 | Screen tubesheet architectures | Solid, clad or overlay shortlist |
| 7 | Review galvanic behavior and area ratio | Dissimilar-metal assessment |
| 8 | Evaluate differential thermal movement | Thermal-stress design decision |
| 9 | Select tube-to-tubesheet joint | Joint specification |
| 10 | Review welding and expansion feasibility | Fabrication assessment |
| 11 | Select product standards and conditions | Material procurement specification |
| 12 | Define corrosion and mechanical tests | Qualification plan |
| 13 | Produce and test representative mock-ups | Joint-qualification report |
| 14 | Define production inspection and NDT | Inspection and test plan |
| 15 | Review supplier documents and traceability | Approved material dossier |
| 16 | Establish baseline in-service inspection | Lifecycle inspection record |
| 17 | Control future substitutions and repairs | Change-control procedure |
Common Mistakes in Tube and Tubesheet Selection
| Mistake | Why It Is Risky | Better Practice |
|---|---|---|
| Selecting tubes first and tubesheet later | The joint may become impossible or vulnerable | Select the material system together |
| Using a generic galvanic series | The ranking may not represent the service electrolyte | Use environment-specific data |
| Ignoring area ratio | A small anode may corrode rapidly | Calculate exposed wetted areas |
| Assuming two passive alloys are automatically compatible | One passive film may break down locally | Evaluate polarization and crevices |
| Assuming the same alloy prevents corrosion | Same-alloy crevice or SCC can still occur | Review every credible mechanism |
| Matching CTE without reviewing exchanger configuration | Mechanical flexibility may control stress | Perform system-level thermal analysis |
| Using the most expensive alloy | Cost does not guarantee mechanism control | Select against defined risks |
| Selecting Alloy 625 only because chlorides are present | Chloride concentration alone is insufficient | Define complete chemistry and temperature |
| Selecting titanium for every seawater application | Fluoride, crevices and cathodic conditions may matter | Review titanium limitations |
| Using a clad face without reviewing tube holes | Backing material may remain exposed | Detail the corrosion barrier through holes |
| Treating seal weld as a strength weld | Joint capacity may be overestimated | Specify joint responsibility |
| Expanding every tube to the same generic reduction | Alloy hardness and geometry differ | Qualify the production process |
| Assuming welding eliminates crevices | Defects or incomplete sealing may remain | Inspect and leak-test representative joints |
| Testing polished base-metal coupons only | Production welds and crevices are not represented | Include welded and creviced specimens |
| Using ASTM G31 as a complete service simulation | It does not cover all local or cyclic mechanisms | Use a mechanism-specific test program |
| Treating G48 temperature as service limit | It is a defined comparative laboratory result | Interpret only within the method |
| Accepting an MTR as corrosion evidence | MTR data are not application-specific | Request relevant corrosion evidence |
| Specifying only “Inconel” or “Hastelloy” | Alloy and product route remain ambiguous | State UNS and standard |
| Using the tube standard for the tubesheet | Product forms require different standards | Specify tube and plate separately |
| Ignoring tube vibration | Corrosion-resistant tubes can still fret through | Complete vibration review |
| Ignoring cleaning chemistry | Cleaning may govern material limits | Include all cleaning operations |
| Ignoring shutdown conditions | Wet deposits can become more corrosive | Evaluate idle and startup exposure |
| Allowing mill or heat-treatment changes | Properties and fabrication may change | Require formal notification |
| Expecting the raw-material supplier to approve the design | Responsibilities become unclear | Keep final approval with the exchanger designer |
RFQ Checklist for Tube and Tubesheet Materials
| Category | Information to Provide |
|---|---|
| Project | Project name, location, industry and design life |
| Exchanger | TEMA type, orientation and exchanger service |
| Design rules | Pressure code, exchanger standard and revisions |
| Tube-side fluid | Full composition, phase, impurities and concentration |
| Shell-side fluid | Full composition, phase, impurities and concentration |
| Temperature | Normal, minimum, maximum, design and transient values |
| Pressure | Normal, design, test and vacuum conditions |
| Cycles | Startup, shutdown, pressure and thermal-cycle counts |
| Chemistry | pH, chlorides, fluorides, oxygen, H₂S and oxidizers |
| Solids | Particle concentration, size and hardness |
| Flow | Velocity, two-phase condition and distribution |
| Fouling | Deposit type, fouling factor and cleaning interval |
| Cleaning | Chemicals, concentration, temperature and method |
| Tube alloy | Grade and UNS |
| Tube standard | ASTM, ASME, EN or project specification |
| Tube route | Seamless or welded |
| Tube size | OD, wall, minimum or average wall, and length |
| Tube tolerances | OD, wall, ovality and straightness |
| Tube condition | Heat treatment and surface |
| Tubesheet base | Material, grade and product standard |
| Corrosion barrier | Solid, clad, overlay, insert or lining |
| Facing alloy | Grade, UNS and minimum thickness |
| Tube holes | Diameter, tolerance, finish and grooves |
| Joint type | Expanded, seal welded, strength welded or combination |
| Expansion | Method, length, pressure or torque and acceptance |
| Welding | Process, filler, purge, joint geometry and inspection |
| Mock-up | Required production-representative variables |
| Galvanic review | Exposed areas, electrolyte and isolation strategy |
| Thermal review | CTE, metal temperatures and exchanger flexibility |
| Corrosion testing | Method, medium, temperature, surface and criteria |
| Tube NDT | ET, UT, hydrostatic or other method |
| Tubesheet NDT | UT, PT, bond testing or other requirements |
| Leak testing | Hydrostatic, pneumatic, helium or specified method |
| PMI | Coverage and reporting |
| MTR | Original mill document and actual results |
| Inspection document | EN 10204 type where applicable |
| Traceability | Heat-to-piece and bundle identification |
| Third-party inspection | Hold and witness points |
| Baseline inspection | Initial ET or UT record |
| Packaging | End protection, segregation and contamination control |
| Change control | Mill, alloy, route and process notification |
| Documentation | Final manufacturing and material dossier |
| Record retention | Required retention period |
Frequently Asked Questions
Must tubes and tubesheets use the same alloy?
No. Dissimilar materials are widely used, but chemical resistance, galvanic behavior, thermal movement, fabrication, joint design and code requirements must all be evaluated.
Does using the same alloy eliminate galvanic corrosion?
It generally removes the macroscopic dissimilar-metal couple between those two components, but it does not prevent pitting, crevice corrosion, SCC, welding defects or vibration damage.
Can titanium tubes be used with a stainless-steel tubesheet?
They may be used only after an application-specific review. The exposed stainless-steel area, titanium cathode area, electrolyte, crevice condition, temperature and joint design can create a serious galvanic risk. It should not be approved from a generic compatibility chart.
Is a titanium-clad carbon-steel tubesheet equivalent to a solid titanium tubesheet?
No. The clad construction introduces backing metal, bond interfaces, clad edges, machined tube holes, weld transitions and repair considerations that are absent from solid titanium.
Is Alloy 625 always suitable for chloride-containing heat exchangers?
No. Performance depends on chloride concentration, temperature, pH, oxidizers, reducing species, deposits, crevices, stress and fabrication condition.
Is C276 always better than Alloy 625?
No universal ranking applies. They have different compositions, strengths, fabrication characteristics and corrosion profiles. Selection must follow the actual chemical and mechanical requirements.
Does thermal-expansion mismatch require identical materials?
No. Exchanger configuration, tube flexibility, shell expansion joints, temperature gradients and allowable stress can accommodate different coefficients in a properly designed system.
What is the difference between a seal weld and a strength weld?
A seal weld primarily controls leakage. A strength weld is designed to transmit specified mechanical load. The project specification should define which function applies.
Should tubes be expanded before or after welding?
There is no universal sequence for every joint. The sequence should follow the qualified procedure, governing standard and design requirements.
Does a low ASTM G31 corrosion rate prove that the joint is safe?
No. Average immersion corrosion rate does not establish galvanic compatibility, crevice resistance, SCC resistance, fatigue or leak tightness.
Does ASTM G48 provide the maximum safe service temperature?
No. G48 provides results under defined ferric-chloride laboratory methods. Its critical temperatures should not be treated automatically as service limits.
Does an MTR prove tube and tubesheet compatibility?
No. It confirms the batch-level material information reported. Compatibility requires a separate engineering and application assessment.
Who should approve the final material combination?
Final approval should involve the exchanger designer, corrosion or materials engineer, pressure-equipment engineer, fabricator, welding engineer, owner or user, and inspection authority where applicable.
Conclusion
Tube and tubesheet material compatibility is not determined by comparing two alloy datasheets.
A defensible decision must connect:
- Tube-side chemistry;
- shell-side chemistry;
- leakage and mixed-fluid conditions;
- normal and transient temperatures;
- pressure and vacuum;
- general and localized corrosion;
- galvanic potential;
- cathode-to-anode area ratio;
- differential thermal expansion;
- exchanger configuration;
- tube vibration;
- product form;
- tubesheet architecture;
- tube-hole design;
- expansion;
- welding;
- cladding and overlay;
- surface condition;
- qualification testing;
- NDT;
- traceability;
- maintenance;
- supplier change control.
ISO 16812, API 660, TEMA, pressure-vessel codes, ASTM material specifications, corrosion tests, MTRs, joint qualifications, and final exchanger inspection all provide different forms of evidence.
None should be used as a substitute for all the others.
The objective is not to select the most corrosion-resistant tube and then attach it to the least expensive available tubesheet.
The objective is to create a complete material and joint system in which every exposed surface, transition, weld, crevice and structural component can be manufactured, inspected, maintained, and shown to remain suitable for the defined heat-exchanger service.
