The selected tube must withstand the actual chemical environment, design temperature, internal or external pressure, flow conditions, thermal cycling, fabrication process, cleaning regime, and tube-to-tubesheet joining method. It must also be available in the required product form, dimensions, condition, inspection level, and project schedule.

A tube can meet its ASTM material specification and still be unsuitable for the exchanger if the selected alloy does not match the service environment or if the design ignores localized corrosion, galvanic interaction, vibration, fouling, cold work, welding, or shutdown conditions.

A reliable alloy tube selection process begins with a complete design basis. Buyers and engineers should evaluate both shell-side and tube-side fluids, normal and abnormal operating conditions, corrosion mechanisms, code design requirements, fabrication compatibility, inspection scope, traceability, availability, and lifecycle consequences before approving an alloy grade.

Engineer evaluating alloy tube selection for heat exchangers

The correct question is not:

“Which alloy is best for heat exchangers?”

It is:

“Which material, product form, condition, wall basis, inspection plan, and joining procedure are suitable for this specific exchanger and its full operating life?”

This guide presents a structured method for answering that question.

Alloy Selection Is a System Decision

The tube does not operate independently.

Its performance is affected by the tubesheet, shell, baffles, tube-to-tubesheet joint, process chemistry, cleaning method, supports, welding consumables, and operating procedures.

Selection Area	Variables to Evaluate	Possible Consequence if Overlooked
Process chemistry	Main fluid, contaminants, concentration, pH and redox condition	General or localized corrosion
Temperature	Normal, design, start-up, shutdown and cleaning temperatures	Accelerated corrosion, strength loss or thermal fatigue
Pressure	Internal pressure, external pressure, vacuum and transient pressure	Rupture, collapse or deformation
Flow	Velocity, turbulence, two-phase flow and suspended solids	Erosion-corrosion, vibration or impingement
Heat transfer	Thermal conductivity, wall thickness and fouling	Larger area requirement or reduced duty
Tube geometry	OD, wall, length, straightness and U-bend geometry	Assembly, expansion or vibration problems
Tubesheet system	Material, cladding, hole geometry and joint type	Galvanic corrosion, crevice attack or leakage
Fabrication	Bending, welding, expansion, heat treatment and cleaning	Cold-work damage or altered corrosion resistance
Inspection	NDT, pressure testing, dimensions and corrosion testing	Undetected discontinuities or incomplete acceptance
Documentation	MTC/MTR, heat number, procedures and reports	Traceability or compliance failure
Supply chain	Availability, MOQ, mill route and lead time	Schedule delay or unauthorized substitution
Lifecycle	Maintenance, inspection, downtime and replacement	Misleading lowest-price decision

Material selection should therefore be treated as a multidisciplinary engineering decision rather than a purchasing comparison between alloy names.

Start With a Complete Design Basis

The TEMA Heat Exchanger Specification Sheet requests design and operating information for both sides of the exchanger, as well as tube dimensions, material, wall basis, construction, joints, and other mechanical details.

This reflects a fundamental principle:

A material cannot be selected accurately from an application name alone.

“Seawater cooler,” “acid exchanger,” “feedwater heater,” or “chemical condenser” does not provide enough information.

Minimum process data to obtain

Design Input	Required Information	Why It Matters
Tube-side fluid	Full composition and phase	Defines internal corrosion and erosion exposure
Shell-side fluid	Full composition and phase	Defines external tube exposure
Main chemical concentration	Normal, minimum and maximum	Corrosion behavior may change sharply with concentration
Trace contaminants	Chlorides, sulfides, oxygen, ammonia, fluorides and metals	Small impurities can control localized corrosion
pH	Normal and abnormal range	Affects passive-film stability
Redox condition	Oxidizing, reducing or variable	Strongly influences nickel and titanium alloy behavior
Dissolved oxygen	Normal and shutdown levels	May improve or destabilize passive behavior depending on material
Operating temperature	Inlet, outlet and expected metal temperature	Corrosion and strength depend on temperature
Design temperature	Maximum and minimum design values	Required for code and material assessment
Operating pressure	Tube and shell sides	Defines normal mechanical loading
Design pressure	Internal and external cases	Required for pressure design
Vacuum condition	Full or partial vacuum	May control tube collapse resistance
Flow velocity	Normal, minimum and maximum	Influences fouling, erosion and film stability
Solids	Size, hardness and concentration	May cause erosion or blockage
Gas fraction	Single-phase or two-phase flow	Changes vibration and impingement risk
Cleaning chemistry	Acid, alkali, oxidizer, solvent or biocide	Cleaning may be more aggressive than operation
Cleaning method	Hydrojet, mechanical, chemical or steam	Affects erosion, surface and access requirements
Start-up and shutdown	Duration and chemistry	Stagnant or transitional conditions may control corrosion
Upset conditions	Contamination, loss of flow or temperature excursion	May expose material outside normal limits
Design life	Intended service period	Supports lifecycle comparison
Inspection interval	Planned access and monitoring	Influences acceptable risk and repair strategy
Applicable code	ASME, EN, PED, TEMA or project rules	Controls material acceptance and design calculations

Evaluate Both Sides of the Tube

The tube has two exposed surfaces.

The inside may contain the process stream, while the outside may contact cooling water, steam, condensing vapour, boiling fluid, flue gas, or another process medium.

The more aggressive side is not always obvious.

Tube Surface	Possible Exposure	Selection Concern
Inside surface	Acidic process fluid	General corrosion, pitting or erosion
Outside surface	Cooling water	Deposits, MIC, crevice corrosion or galvanic interaction
Tube inlet	High-velocity entry flow	Impingement or erosion-corrosion
Tube outlet	Flashing or two-phase flow	Cavitation or unstable flow
Tube-to-tubesheet joint	Crevice and residual stress	Localized corrosion or SCC
U-bend	Cold work and deposit accumulation	Residual-stress or crevice-related attack
Support contact	Baffle or support interface	Fretting, crevice corrosion or vibration wear
Vapour-liquid interface	Changing chemistry and wetting	Local concentration or condensation corrosion

A tube material that is compatible with the process fluid may still be unsuitable for the cooling-water side or joint region.

Normal Operation Is Only One Exposure Condition

Heat exchanger failures can originate during conditions that receive less attention than normal operation.

Conditions to include in the material review

Fabrication
Pickling and cleaning
Hydrostatic testing
Storage
Commissioning
Start-up
Normal continuous operation
Intermittent operation
Shutdown
Stagnant wet lay-up
Dry lay-up
Chemical cleaning
Steam-out
Sterilization
Loss of cooling
Loss of flow
Process contamination
Emergency depressurization

Condition	Possible Material Risk
Hydrotest water left stagnant	Chloride concentration, MIC or deposit formation
Shutdown with deposits present	Under-deposit and crevice corrosion
Acid cleaning	Attack of an alloy selected only for operating media
Caustic cleaning	Stress-corrosion or general corrosion in unsuitable materials
Steam-out	Temperature above normal operating range
Loss of flow	Increased wall temperature or deposit concentration
Air ingress	Change from reducing to oxidizing chemistry
Process contamination	Unexpected chlorides, sulfides or fluorides
Repeated start-stop cycles	Thermal fatigue and changing surface chemistry

The material selection document should state which conditions have been assessed and which are outside the approved operating envelope.

Identify the Credible Corrosion Mechanisms

The first corrosion question should not be:

“What is the general corrosion rate?”

It should be:

“Which corrosion or environmentally assisted cracking mechanisms are credible in every part of the tube system?”

Corrosion-mechanism screening table

Mechanism	Typical Enabling Conditions	Why It Matters
General corrosion	Chemically aggressive bulk fluid	Progressive wall loss
Pitting corrosion	Passive alloy, halides and suitable electrochemical potential	Small penetrations with limited overall mass loss
Crevice corrosion	Shielded areas, deposits, gaskets or tube joints	Local chemistry becomes more aggressive
Chloride SCC	Susceptible alloy, chlorides, tensile stress and sufficient temperature	Cracking may occur with limited general corrosion
Sulfide stress cracking	H₂S environment and susceptible material condition	Requires service-specific material rules
Intergranular corrosion	Susceptible microstructure or heat-treatment condition	Preferential grain-boundary attack
Galvanic corrosion	Dissimilar metals electrically connected in an electrolyte	Accelerated attack of the less noble component
Erosion-corrosion	High velocity, turbulence, solids or unstable protective film	Localized rapid wall loss
Cavitation	Pressure fluctuation and bubble collapse	Surface damage and accelerated corrosion
Fretting corrosion	Vibration at supports or contact points	Local wear and oxide removal
Under-deposit corrosion	Deposits, concentration cells and restricted transport	Hidden localized attack
MIC	Microorganisms and suitable water chemistry	Local corrosion influenced by biological activity
Hydrogen damage	Hydrogen generation or cathodic charging	Loss of ductility or cracking in susceptible materials
Corrosion fatigue	Cyclic stress in a corrosive environment	Lower fatigue resistance than dry service
High-temperature oxidation	Hot oxidizing gases or steam	Scale growth and metal loss
Sulfidation	Sulfur-bearing high-temperature gas	Rapid attack of unsuitable alloys
Carburization	Carbon-rich high-temperature environments	Carbon ingress and microstructural damage

No alloy should be approved until the relevant mechanisms have been identified and screened.

Pitting and Crevice Corrosion Require More Than a PREN Comparison

PREN calculations may help compare some stainless steels, but they do not replace environment-specific corrosion evaluation.

They do not fully account for:

Surface condition
Heat treatment
Welds
Crevice geometry
Oxidation-reduction potential
Temperature
pH
Deposits
Fluid concentration
Fabrication contamination
Actual alloy microstructure

ASTM G48-25 provides accelerated ferric-chloride tests for comparing the initiation resistance of stainless steels and related alloys to pitting and crevice corrosion.

However, ASTM states important limitations:

The tests rank relative performance under their specified conditions.
They do not establish resistance in non-chloride environments.
They do not determine localized-corrosion propagation rates.
Surface preparation can materially influence results.
Exceptions exist when correlating laboratory ranking with actual service.

A G48 result should therefore be treated as controlled test evidence, not as a direct service-life guarantee.

Stress-Corrosion Cracking Is Environment-Specific

The term “SCC-resistant alloy” is incomplete without identifying the cracking mechanism and environment.

Examples of different cracking concerns

Cracking Concern	Important Variables
Chloride SCC	Alloy family, temperature, chloride activity, oxygen, stress and cold work
Sulfide stress cracking	H₂S partial pressure, pH, material strength, hardness and condition
Caustic cracking	Caustic concentration, temperature, stress and alloy
Polythionic-acid SCC	Sensitized material, sulfur scale, oxygen and moisture
Ammonia SCC	Material family, ammonia and oxidizing species
Hydrogen-assisted cracking	Hydrogen charging, stress and material susceptibility

For oil and gas production environments containing H₂S, NACE MR0175/ISO 15156 provides requirements and recommendations for selecting and qualifying metallic materials.

The following statement is therefore too broad:

“A higher-nickel alloy will solve SCC.”

The correct approach is:

Identify the cracking mechanism.
Define the environment.
Confirm the material grade and condition.
Review hardness, cold work, welding and residual stress.
Apply the relevant service standard.
Use qualification testing where required.

Galvanic Compatibility Must Include the Complete Exchanger

A corrosion-resistant tube may be electrically connected to:

Carbon-steel tubesheet
Stainless-steel tubesheet
Titanium-clad tubesheet
Nickel-alloy weld overlay
Copper-alloy components
Sacrificial anodes
Cathodic-protection systems

Galvanic behaviour depends on more than the two alloy names.

Important variables include:

Electrolyte conductivity
Relative electrochemical potentials
Cathode-to-anode area ratio
Coating condition
Crevice geometry
Flow
Temperature
Deposits
Electrical continuity
Cathodic-protection potential

Material Combination Issue	Possible Concern
Noble tube with exposed carbon-steel tubesheet	Local attack of a small steel area
Titanium with aggressive cathodic protection	Hydrogen charging under unsuitable conditions
Nickel alloy with dissimilar weld overlay	Local galvanic or dilution-related behaviour
Mixed tube materials in one bundle	Different potentials and maintenance complexity
Coated tubesheet with coating defects	Small exposed anodic areas
Tube-end inserts of another alloy	Local electrochemical and flow disturbance

The Nickel Institute guide to galvanic corrosion provides case-based background, but project-specific evaluation remains necessary.

Temperature Changes Both Corrosion and Mechanical Behaviour

An alloy that performs well at room temperature may not retain the same corrosion resistance, strength, ductility, or phase stability at the exchanger design temperature.

Temperature-related questions

Topic	Question
General corrosion	Does the rate increase significantly with temperature?
Localized corrosion	Is there a temperature above which pitting or crevice risk rises sharply?
SCC	Does the credible cracking mechanism require a temperature threshold?
Strength	What allowable stress applies at design temperature?
Creep	Is time-dependent deformation relevant?
Oxidation	Can a protective scale remain stable?
Thermal fatigue	How many heat-up and cool-down cycles are expected?
Phase stability	Can harmful precipitates form during service or fabrication?
Thermal expansion	How will tubes, shell and tubesheet expand relative to each other?
Cleaning	Is cleaning temperature higher than operating temperature?

A generic statement such as “nickel alloys are high-temperature materials” is not enough. Each alloy has its own allowable-temperature, corrosion, phase-stability, and fabrication limits.

Pressure and Wall Thickness Require Code-Based Design

The tube wall should not be selected only from a supplier catalogue or a comparison of alloy strength.

Pressure design may need to consider:

Internal design pressure
External pressure
Vacuum
Design temperature
Allowable stress
Tube OD
Manufacturing tolerance
Corrosion or erosion allowance
Tube support spacing
Ovality
U-bend thinning
Tube expansion or welding
Applicable code equations
Inspection and quality factors

The ASME Boiler and Pressure Vessel Code includes Section VIII rules for pressure vessels and heat exchanger construction. TEMA also provides mechanical-design rules and specification requirements for shell-and-tube exchangers. :contentReference[oaicite:9]{index=9}

Why stronger does not automatically mean thinner

A higher-strength alloy may provide a higher allowable stress under some conditions, but final wall thickness can still be controlled by:

Minimum commercially available wall
External-pressure collapse
Handling damage
Erosion allowance
Corrosion allowance
Tube-expansion requirements
Welding procedure
Vibration resistance
Minimum bend wall
Cleaning loads
Standard dimensional tolerances

The correct statement is:

Higher strength may influence the code-calculated wall, but it does not automatically justify a thinner tube.

Minimum Wall and Average Wall Must Not Be Confused

Some heat-exchanger tube specifications permit ordering by either:

Outside diameter and average wall; or
Outside diameter and minimum wall.

ASTM B163-22 explicitly includes both bases within its scope.

Wall Basis	Meaning	Procurement Risk
Nominal wall	Stated target dimension	Does not define local minimum by itself
Average wall	Average around a section or defined measurement basis	Local regions may be below the stated average
Minimum wall	Local wall must not fall below the specified minimum, subject to standard rules	Usually requires different manufacturing allowance
Design minimum	Minimum required by mechanical design	Must be reconciled with product tolerance
Final minimum wall	Minimum after bending, forming or other fabrication	May be lower than the original tube wall

The purchase order should clearly state the required wall basis.

Writing only “19.05 × 1.24 mm” may be insufficient if the project does not clarify whether 1.24 mm is nominal, average, minimum, or design minimum.

Flow Velocity Cannot Be Evaluated From Hardness Alone

The original draft associated higher flow or abrasive media with a need for harder alloys.

That is incomplete.

Erosion-corrosion depends on:

Fluid velocity
Flow direction
Turbulence
Tube-inlet geometry
Solids concentration
Particle size and hardness
Two-phase flow
Cavitation
Surface-film formation
Alloy-environment interaction
Temperature
Local obstruction
Tube protrusion
Deposit removal

Flow Condition	Possible Risk
High inlet velocity	Tube-end impingement
Partially blocked tube	Local acceleration
Suspended solids	Particle erosion
Two-phase flow	Droplet or bubble impingement
Poor inlet distribution	Uneven tube loading
Tight entrance geometry	Turbulence and local attack
Low velocity	Deposition, fouling and MIC
Varying flow	Repeated film removal and rebuilding

A hard material can still experience erosion-corrosion if its protective film is unstable in the flowing medium.

Likewise, a softer material may perform acceptably where its protective film remains stable and the exchanger geometry controls impingement.

Flow-Induced Vibration Can Control Tube Reliability

Tube selection is sometimes treated as a corrosion-only decision.

However, shell-side flow can produce:

Vortex shedding
Turbulent buffeting
Fluid-elastic instability
Acoustic resonance
Fretting at baffles
Tube-to-tube impact

Material properties relevant to vibration include:

Elastic modulus
Density
Fatigue behaviour
Damping
Wear resistance
Surface hardness
Corrosion-fatigue resistance

But changing the alloy alone may not solve the problem.

Vibration control may require changes to:

Tube OD
Wall thickness
Unsupported span
Baffle spacing
Baffle cut
Support design
Flow distribution
Shell-side velocity
Tube layout
Anti-vibration bars

The thermal and mechanical designer should therefore confirm that the selected tube material and geometry remain compatible with the vibration assessment.

Heat Transfer Is Controlled by the Full Thermal Resistance

The heat exchanger's function is to transfer heat, but alloy thermal conductivity should not be evaluated in isolation.

The overall thermal resistance can include:

Tube-side fluid film
Tube-side fouling
Tube wall
Shell-side fouling
Shell-side fluid film
Contact or deposit resistance

A simplified conceptual relationship is:

Total thermal resistance = Tube-side film resistance + Tube-side fouling resistance + Tube-wall resistance + Shell-side fouling resistance + Shell-side film resistance

This means that a material with lower thermal conductivity may still be acceptable when:

A thinner code-compliant wall is available
Corrosion allowance can be reduced
Fouling is lower
Higher velocity is permissible
Equipment remains clean longer
Tube area can be adjusted

Conversely, high thermal conductivity cannot compensate for rapid corrosion, thick deposits, leakage, or frequent shutdown.

Thermal selection questions

Question	Why It Matters
What is the required duty?	Defines total exchanger performance
What fouling factors are used?	Fouling may dominate wall resistance
Can the selected alloy use the proposed wall?	Changes metal resistance
Does the surface remain clean?	Corrosion products can reduce heat transfer
Is chemical cleaning compatible?	Determines whether performance can be restored
Does the alloy affect tube count or area?	Influences bundle size and cost
Is enhanced tubing planned?	Product standard and inspection may change

Use Material Families for Screening, Not Final Approval

The following matrix is a preliminary screening tool.

It is not a universal material-selection chart.

Material Family	Possible Strengths	Important Limitations to Review
Austenitic stainless steels	Availability, fabrication and broad general use	Chloride pitting, crevice corrosion and SCC
Duplex stainless steels	Higher strength and improved chloride resistance relative to common austenitic grades	Phase balance, welding, temperature and hydrogen-related limits
Super austenitic stainless steels	Improved localized-corrosion resistance	Availability, fabrication and environment-specific limits
Super duplex stainless steels	High strength and strong chloride resistance in suitable conditions	Welding control, temperature range and phase stability
Copper-nickel alloys	Established cooling-water applications and thermal performance	Polluted sulfide-bearing water, ammonia, velocity and galvanic coupling
Commercially pure titanium	Strong passive behaviour in many oxidizing chloride waters	Crevice severity, reducing conditions, hydrogen charging and fabrication cleanliness
Corrosion-enhanced titanium grades	Improved performance in selected severe crevice environments	Higher cost, availability and exact grade qualification
Nickel-copper alloys	Useful in selected seawater and reducing environments	Oxidizing environments and alloy-specific velocity limits
Nickel-chromium-iron alloys	High-temperature and selected corrosion applications	Limited resistance to some localized-corrosion environments
Nickel-iron-chromium-molybdenum-copper alloys	Useful resistance in selected acid and chloride services	Environment-specific limits and fabrication control
Nickel-chromium-molybdenum alloys	Strong localized-corrosion resistance in many severe services	Cost, availability, forming and alloy-specific limitations
Nickel-molybdenum alloys	Strong performance in selected reducing acids	Oxidizing contaminants can materially change behaviour
Zirconium alloys	Useful in selected severe acid services	Cost, availability, ignition/fabrication controls and code acceptance

The material engineer should narrow the candidates using environment-specific corrosion data and then confirm mechanical and fabrication suitability.

Screening Common Nickel-Alloy Tube Candidates

Nickel alloys should be specified by formal alloy designation and UNS number rather than by a broad trade name.

Alloy Example	UNS	Possible Screening Context	Important Questions
Nickel 200/201	N02200/N02201	Selected caustic and reducing services	Temperature, sulfur compounds, carbon content and code requirements
Alloy 400	N04400	Selected seawater and reducing environments	Aeration, velocity, sulfides, galvanic coupling and strength
Alloy 600	N06600	Selected high-temperature and process applications	Water chemistry, SCC mechanism and localized-corrosion requirements
Alloy 625	N06625	Chloride-bearing and mixed corrosive services with strength requirements	Exact chemistry, temperature, heat treatment, product standard and fabrication
Alloy 800/800H/800HT	N08800/N08810/N08811	Elevated-temperature services	Correct grade, grain requirements, code stress and carburization/oxidation
Alloy 825	N08825	Selected acid and chloride-containing services	Acid concentration, temperature, reducing/oxidizing balance and weld condition
Alloy 20	N08020	Selected sulfuric-acid applications	Acid concentration, contaminants, temperature and chloride exposure
Alloy C-276	N10276	Severe mixed corrosive services	Whether service data support it over other Ni-Cr-Mo candidates
Alloy C-22	N06022	Selected oxidizing and chloride-bearing mixed environments	Specific chemistry, weld condition and comparative corrosion data
Alloy B-family candidates	Alloy-specific UNS	Selected reducing-acid environments	Oxidizing contaminants, fabrication and exact product availability

This table identifies possible candidates only.

It does not replace corrosion data, code review, or project qualification.

Screening Titanium Tube Candidates

ASTM B338-17(2021) covers numerous grades of seamless and welded titanium tubing for condenser, evaporator, and heat-exchanger service.

Titanium Candidate	General Screening Context	Important Questions
Grade 1	Lower-strength, more formable commercially pure titanium	Pressure design, wall availability and fabrication
Grade 2	Common industrial commercially pure titanium candidate	Crevice temperature, galvanic effects, hydrogen risk and cleaning chemistry
Grade 3	Higher-strength commercially pure titanium	Forming, expansion and product availability
Grade 7	Palladium-bearing corrosion-enhanced grade	Whether the more severe environment justifies the grade
Grade 11	Lower-strength palladium-bearing grade	Formability, availability and exact specification
Grade 12	Nickel-molybdenum-bearing titanium grade	Specific crevice, temperature, strength and weld requirements
Grade 16	Lower-palladium corrosion-enhanced grade	Qualification data and availability
Other B338 grades	Project-specific	Code approval, corrosion evidence and fabrication procedure

Titanium may be a strong candidate for many seawater, brine, chlorine-related, and oxidizing environments, but the following must still be reviewed:

Crevice geometry
Temperature
pH
Fluorides
Reducing acids
Hydrogen generation
Cathodic protection
Galvanic coupling
Iron contamination
Welding shielding
Hydrotest water
Shutdown conditions

The International Titanium Association discussion of titanium in seawater service provides useful application background but should not be treated as a substitute for project-specific engineering.

Nickel Alloys and Titanium Solve Different Problems

The comparison should not be framed as “which family is better.”

Selection Topic	Nickel-Alloy Candidates	Titanium Candidates
Chloride water	Alloy-specific resistance; some Ni-Cr-Mo grades may be candidates	Frequently considered in oxidizing seawater and brine
Reducing acid	Some nickel-molybdenum and Ni-Cr-Mo grades may be candidates	Performance may be limited without sufficient passivation
Oxidizing acid	Certain Ni-Cr-Mo grades may perform well	Several titanium grades may perform well under suitable conditions
Mixed acid chemistry	Requires alloy-specific corrosion data	Strongly dependent on redox condition and contaminants
High-temperature gas	Several nickel alloys are established candidates	Usually requires separate high-temperature evaluation
Weight	Higher density	Lower density
Thermal expansion	Alloy-specific	Different from common steels and nickel alloys
Welding	Requires alloy-specific filler, cleanliness and heat-input control	Requires strong protection from atmospheric contamination
Tube expansion	Depends on strength, hardness and condition	Depends on grade, condition and wall
Galvanic behaviour	Must be evaluated with tubesheet and electrolyte	Noble passive behaviour can affect coupled metals
Hydrogen exposure	Alloy-specific	Hydrogen absorption can become relevant under charging conditions
Availability	Varies widely by grade and dimension	Varies by grade, tube form and mill route

Neither material family can be approved solely from this comparison.

Fabrication Compatibility Must Be Evaluated Before Purchase

The selected alloy must be compatible with the intended fabrication route.

Fabrication operations that may affect tube performance

Operation	Possible Effect
Cold drawing	Strength, hardness, residual stress and dimensions
Straightening	Local residual stress and ovality
U bending	Wall thinning, ovality and cold work
Tube expansion	Work hardening and residual contact stress
Welding	Heat-affected zone, dilution, residual stress and contamination
Heat treatment	Strength, grain structure, phase precipitation and surface oxide
Pickling	Surface condition and dimensional loss
Grinding	Local wall reduction and surface contamination
Cleaning	Residue or chemical attack
Marking	Surface damage or contamination

Questions to resolve

Is the tube seamless or welded?
What condition is required at delivery?
Will the tube be bent?
Will it be mechanically or hydraulically expanded?
Will it be seal welded or strength welded?
Is post-bend heat treatment required?
Is post-weld heat treatment permitted?
Are local repairs permitted?
What surface cleaning is required?
Does the material require dedicated tooling?
Are iron contamination controls needed?
Must the production procedure be qualified?

An alloy that is corrosion-resistant in laboratory coupons may still fail to meet project expectations if fabrication creates an unsuitable final condition.

Seamless and Welded Tubes Should Be Evaluated Neutrally

Seamless does not automatically mean defect-free or dimensionally superior.

Welded does not automatically mean unsuitable.

Topic	Seamless Tube	Welded Tube
Longitudinal weld	None	Present
Possible dimensional concern	Eccentricity and drawing variation	Weld geometry and strip-forming variation
Inspection focus	Base-metal discontinuities and dimensions	Base metal plus weld seam
Forming	Depends on condition and wall	Depends on condition, weld and weld orientation
Availability	May be limited for some dimensions	May offer efficient thin-wall availability
Acceptance	Product standard and project requirements	Product standard, weld quality and project requirements

ASTM B338-17(2021) differentiates seamless, welded, and welded/cold-worked titanium tubes and their manufacturing or testing routes.

The purchaser should select the product form according to:

Code acceptance
Service criticality
Dimensions
Wall thickness
Weld quality
Inspection capability
Forming requirements
Availability
Previous qualification

Verify That the Material Standard Covers the Ordered Product

Writing an alloy and a familiar ASTM number does not guarantee that the combination is correct.

Standards and their roles

Standard or Document	What It Addresses	What It Does Not Prove
TEMA Standards	Mechanical design and exchanger-industry practices	Specific alloy corrosion resistance in every environment
ASME BPVC Section VIII	Pressure-vessel construction rules	Complete corrosion suitability
ASTM B163	Listed seamless nickel and nickel-alloy condenser/heat-exchanger tubes	Welded tubes or alloys outside its scope
ASTM B338	Listed seamless and welded titanium heat-exchanger tubes	Suitability for a specific fluid
ASTM B829	General requirements for listed seamless nickel-alloy pipe and tube standards	Replacement for the particular product specification
ASTM G48	Comparative pitting and crevice-corrosion testing in oxidizing chloride solution	Actual service life or non-chloride performance
ASTM G28	Intergranular-corrosion susceptibility of specified nickel-rich alloys	Universal corrosion resistance
NACE MR0175/ISO 15156	Metallic materials for H₂S-containing oil and gas production environments	All chemical-processing SCC environments
EN 10204	Inspection-document types	Independent confirmation that the material is suitable for service
Project data sheet	Project-specific requirements	Compliance unless linked to inspection and acceptance evidence

Confirm the following for every standard

Correct title
Correct material scope
Correct product form
Correct grade or UNS
Correct standard edition
Correct dimensional range
Correct supplied condition
Referenced general-requirements standard
Supplementary requirements
Conflict hierarchy between project and material standards

Review the MTC or MTR as a Technical Record

An MTC or MTR provides batch-specific information, but it must be checked against the physical product and purchase requirements.

MTC/MTR review table

Document Field	Verification Required
Manufacturer	Is this the actual tube producer?
Manufacturing site	Is it the approved location?
Customer and order	Does it relate to the correct contract?
Product description	Tube, pipe, seamless, welded or another form
Alloy	Correct formal grade and UNS designation
Material standard	Correct designation and edition
Heat number	Matches tube or bundle markings
Lot number	Matches inspection and manufacturing records
Dimensions	Correct OD and wall basis
Quantity	Reconciles with packing and inspection records
Chemical composition	Within standard and supplementary limits
Tensile strength	Meets required limits
Yield strength	Meets required limits and forming expectations
Elongation	Meets standard and process needs
Hardness	Correct method, location and limit where required
Heat treatment	Correct condition and cycle designation
Grain size	Present where required
NDT	Method, extent and result
Pressure test	Method and result where required
Corrosion test	Correct method and acceptance where specified
Certificate type	Matches contractual requirement
Authorized validation	Signature, digital verification or certification control

Warning signs

Generic certificate without heat-specific values
Trade name without UNS number
Incorrect product standard
Dimensions that do not match the order
Heat number absent from physical marking
Test values repeated identically across unrelated heats
Missing material condition
NDT listed without method or coverage
Certificate issued by a trader without linkage to the producing mill
“Equivalent material” supplied without written approval

The document should support traceability and compliance, not merely accompany the shipment.

Corrosion Tests Must Be Specified With a Purpose

A request for “corrosion testing” is incomplete.

The buyer should identify:

Target corrosion mechanism
Test standard
Test method or practice
Specimen location
Surface preparation
Heat-treatment condition
Test temperature
Exposure time
Acceptance criterion
Reporting format
Whether test material represents the finished tube

Examples

Concern	Possible Test Approach	Important Limitation
Chloride pitting ranking	ASTM G48 method selected for the material	Accelerated comparative test
Chloride crevice ranking	ASTM G48 crevice method	Crevice former and specimen geometry matter
Nickel-alloy IGC susceptibility	ASTM G28 method applicable to the alloy	Service relevance must be established separately
SCC	Environment-specific bent-beam, C-ring or slow-strain-rate testing	Must reproduce relevant chemistry and stress
H₂S cracking	Applicable NACE/AMPP or ISO method	Material condition and environment limits are essential
General corrosion rate	Immersion or autoclave test	Laboratory chemistry may not represent plant variation
Weld corrosion	Welded test coupon	Must represent filler, heat input and final condition

Testing should answer a defined technical question.

Adding unrelated tests can increase cost without reducing the relevant project risk.

Define the Tube Inspection Scope

Material selection and tube quality are related but separate.

A correctly selected alloy can still be rejected because of manufacturing defects or dimensional nonconformance.

Inspection Method	Typical Purpose	Important Limitation
Visual inspection	Surface condition and workmanship	Limited subsurface capability
Dimensional inspection	OD, wall, length, ovality and straightness	Sampling and measurement uncertainty matter
Eddy-current testing	Surface and near-surface discontinuities in conductive tubing	Calibration and geometry affect sensitivity
Ultrasonic testing	Wall and internal discontinuity assessment	Coupling and reference standards are required
Hydrostatic testing	Pressure integrity	Does not detect every non-leaking flaw
Pneumatic testing	Leakage or pressure integrity	Safety and sensitivity must be defined
PMI	Alloy-family or grade verification	Does not replace full chemical analysis
Hardness testing	Condition and consistency indicator	Does not prove complete mechanical compliance
Flattening or flaring	Fabrication quality and ductility under specified test	Must be required by the product standard or order
Surface roughness	Finish verification	Does not directly prove corrosion life
Metallography	Grain, weld and phase assessment	Normally destructive and sample-based

The RFQ should state the method, extent, acceptance criteria, timing, and required reports.

“100% NDT” alone is not a complete requirement.

Evaluate Supplier Capability Beyond a Quotation

A supplier should demonstrate the ability to deliver the exact material, form, dimensions, condition, and documentation required.

Supplier capability checklist

Evaluation Area	Evidence to Request
Alloy experience	Similar grade and application history
Tube manufacturing	Seamless or welded production route
Size capability	Comparable OD, wall and length
Heat treatment	Furnace range, atmosphere and records
Cold drawing	Process route and dimensional capability
Welding	Qualified process and weld inspection
U bending	Radius, wall thinning and ovality capability
NDT	Equipment, personnel and calibration
Dimensional inspection	Calibrated gauges and procedures
Corrosion testing	Qualified laboratory and specimen control
Traceability	Heat and lot control through all operations
Quality system	Valid certification and actual site coverage
Subcontracting	Approved and controlled external processes
Change control	Notification and approval procedure
Documentation	Sample MTC, ITP and final dossier
Packaging	Surface and end protection
Delivery control	Raw material, manufacturing and inspection milestones

Questions marketing material cannot answer

Who produced the original tube?
Which mill heat is offered?
Is the tube form covered by the cited standard?
Is heat treatment performed in-house?
Which operations are subcontracted?
Can the supplier preserve heat traceability after cutting?
Can the required NDT method cover the entire length?
Has the supplier manufactured the exact alloy and wall before?
What changes require purchaser approval?
What happens if test results fail?

Avoid Uncontrolled “Equivalent Material” Substitutions

Two materials may appear similar while differing in:

Chemistry limits
UNS designation
Product form
Heat treatment
Mechanical properties
Grain requirements
NDT
Dimensional tolerances
Code listing
Corrosion data
Weld procedure
Availability

An equivalent-material proposal should include:

Required Comparison	Supplier Evidence
Formal designation	Original and proposed grades
UNS numbers	Exact identification
Standards	Product standard and edition
Chemistry	Side-by-side limits and actual heat analysis
Mechanical properties	Required and actual values
Allowable stress	Applicable design-code data
Corrosion performance	Relevant environment-specific evidence
Product form	Seamless, welded and condition
Fabrication	Welding, bending and expansion compatibility
Inspection	Equivalent or improved examination
Previous use	Relevant, verifiable service or qualification
Approval	Written purchaser and designer authorization

No substitution should be made solely because a material is described as “similar.”

Include Tube-to-Tubesheet Compatibility

The selected tube must work with the intended joint.

Possible joint arrangements include:

Mechanical expansion only
Hydraulic expansion
Seal welding
Strength welding
Expanded and welded
Welded and expanded
Grooved tubesheet holes
Clad or overlay tubesheets

Joint-related material questions

Question	Why It Matters
Can the tube be expanded in its supplied condition?	Strength and ductility affect deformation
Is hardness controlled?	May affect expansion and cracking risk
Is welding required?	Filler, shielding and heat input must be compatible
Is the tubesheet solid or clad?	Dilution and galvanic effects may change
Is post-weld cleaning required?	Surface restoration may be necessary
Is PWHT permitted?	It can change tube and weld properties
Are grooves used?	Local deformation and crevice geometry change
Is a mock-up required?	Verifies the specific material combination
Is the joint exposed to corrosive fluid?	Crevice and residual stress become more important

Material compatibility with the bulk fluid does not prove compatibility with the tube-to-tubesheet joining process.

Compare Lifecycle Cost Rather Than Tube Price Alone

The lowest tube price is not always the lowest exchanger cost.

However, the most expensive alloy is not automatically the best investment either.

Lifecycle cost categories

Cost Category	Possible Components
Procurement	Tube price, MOQ, testing and documentation
Fabrication	Bending, welding, expansion, cleaning and rework
Equipment design	Tube count, area, shell size and supports
Installation	Handling, joining and commissioning
Operation	Pumping energy, fouling and thermal efficiency
Inspection	NDT, leak testing and access
Maintenance	Cleaning, plugging and repairs
Downtime	Lost production and restart costs
Replacement	Retubing, bundle replacement and disposal
Risk	Safety, environmental and contractual exposure
Supply continuity	Future availability of matching material

A useful comparison framework

Candidate	Initial Cost	Fabrication Risk	Corrosion Risk	Maintenance Burden	Supply Risk	Lifecycle Assessment
Candidate A	Lower	Low	Moderate	Moderate	Low	Review expected replacement frequency
Candidate B	Medium	Moderate	Low	Low	Moderate	Review qualification and lead time
Candidate C	Higher	Higher	Very low in defined service	Low	Higher	Confirm that added capability is necessary

The ratings must be based on project evidence rather than generic assumptions.

Build an Alloy Selection Decision Matrix

A structured matrix makes assumptions visible.

Selection Criterion	Weight	Candidate A	Candidate B	Candidate C
Tube-side corrosion	20
Shell-side corrosion	15
Localized-corrosion risk	10
SCC or hydrogen risk	10
Design-temperature strength	10
Pressure and wall feasibility	5
Fabrication compatibility	10
Inspection feasibility	5
Availability and lead time	5
Lifecycle cost	10
Total	100

The score should be accompanied by technical notes and evidence.

A numerical total should not override a mandatory safety, code, or corrosion requirement.

A Practical Material-Selection Workflow

Step 1: Collect complete process data

Obtain both-side chemistry, temperature, pressure, flow, contaminants, cleaning and upset conditions.

Step 2: Define the mechanical design basis

Confirm code, TEMA class, exchanger configuration, tube OD, wall basis, length, external-pressure case and vibration requirements.

Step 3: Identify credible damage mechanisms

Screen general corrosion, localized corrosion, SCC, hydrogen damage, erosion, vibration and high-temperature mechanisms.

Step 4: Develop a shortlist

Compare material families before selecting specific grades.

Step 5: Confirm code and product-standard coverage

Verify that each proposed alloy and tube form is permitted by the applicable material and design standards.

Step 6: Review fabrication compatibility

Evaluate drawing, bending, expansion, welding, heat treatment, cleaning and marking.

Step 7: Obtain environment-relevant evidence

Use service history, authoritative corrosion data, laboratory testing or project qualification.

Step 8: Review supplier capability

Confirm the actual manufacturing mill, process route, NDT, traceability and change control.

Step 9: Compare lifecycle consequences

Consider initial cost, fabrication, maintenance, downtime and replacement.

Step 10: Freeze the specification

Issue a controlled data sheet, drawing, purchase specification, ITP and document requirements.

Step 11: Approve changes formally

Require written authorization before material, mill, condition, manufacturing route or inspection changes.

Step 12: Verify delivered material

Cross-check documentation, markings, dimensions, surface, NDT and physical identity.

What Buyers Should Include in the RFQ

RFQ Category	Required Information
Exchanger	Type, orientation and TEMA designation where applicable
Code	ASME, EN, PED or project basis
Tube-side fluid	Full composition and phase
Shell-side fluid	Full composition and phase
Operating conditions	Temperature, pressure and flow
Design conditions	Maximum and minimum temperature and pressure
Upset conditions	Contamination, loss of flow and excursions
Cleaning	Chemistry, temperature and method
Material	Grade and UNS number
Product standard	ASTM/ASME/EN designation and edition
Tube form	Seamless, welded or welded/cold worked
Condition	Annealed, solution annealed, cold worked or other
Dimensions	OD, wall, length and quantity
Wall basis	Minimum or average wall
Tolerances	OD, wall, ovality, straightness and cut length
Tube geometry	Straight or U-bend
Tube joint	Expanded, welded or combined
Surface	OD and ID requirements
Cleanliness	Oil, chloride, particle and contamination limits
Heat treatment	Required condition and records
Mechanical tests	Tensile, yield, elongation and hardness
NDT	Method, extent, timing and acceptance
Pressure testing	Hydrostatic, pneumatic or another method
Corrosion testing	Standard, method and acceptance
Documentation	MTC/MTR, EN 10204 type and reports
Traceability	Heat, lot, tube or bundle-level requirement
Inspection	Purchaser and third-party witness points
Subcontracting	Approval and notification requirements
Packaging	End protection, moisture and surface protection
Delivery	Milestones, destination and shipment terms

Common Alloy-Selection Mistakes

Mistake	Why It Is Risky	Better Approach
Selecting by application name	Service details are incomplete	Use complete process data
Selecting by trade name	Grade remains ambiguous	Add UNS and product standard
Reviewing only tube-side fluid	Outside surface may control failure	Evaluate both sides
Using only normal conditions	Cleaning or shutdown may be more severe	Include full lifecycle conditions
Comparing only general corrosion rate	Localized attack may control life	Screen all credible mechanisms
Assuming more nickel is always better	Alloying effects are environment-specific	Use grade-level corrosion evidence
Assuming titanium is immune in seawater	Crevices, hydrogen and galvanic conditions still matter	Review exact service and grade
Choosing wall from pressure only	External pressure, tolerance and vibration may control	Apply complete mechanical design
Choosing harder material for erosion	Film stability and geometry also matter	Review flow and erosion mechanism
Treating G48 as service-life proof	It is an accelerated comparative test	Use it within its stated scope
Treating MTC as service qualification	MTC proves batch data, not application suitability	Separate product and service approval
Accepting “equivalent” without review	Chemistry, standards or code status may differ	Require formal substitution assessment
Ignoring fabrication	Bending, welding or expansion can change condition	Qualify the final manufacturing route
Selecting on price alone	Maintenance and downtime are excluded	Compare lifecycle consequences
Ignoring future availability	Replacement material may be difficult to source	Review supply continuity

Incoming Material Review Checklist

Documentation

[ ] Correct purchase order
[ ] Correct alloy and UNS number
[ ] Correct product standard and edition
[ ] Correct tube form
[ ] Correct supplied condition
[ ] Correct MTC/MTR type
[ ] Heat and lot numbers match
[ ] Chemistry reviewed
[ ] Mechanical properties reviewed
[ ] Heat treatment reviewed
[ ] NDT reports reviewed
[ ] Pressure-test report reviewed
[ ] Corrosion-test report reviewed where required
[ ] Third-party release reviewed

Physical inspection

[ ] Quantity confirmed
[ ] Marking matches documents
[ ] OD measured
[ ] Wall basis confirmed
[ ] Wall thickness measured
[ ] Ovality checked
[ ] Straightness checked
[ ] Tube ends inspected
[ ] OD surface inspected
[ ] ID surface inspected
[ ] Transport damage checked
[ ] Cleanliness checked
[ ] Heat separation preserved
[ ] Packaging condition documented

Technical release

[ ] Material is code-permitted
[ ] Material is corrosion-approved
[ ] Fabrication route is approved
[ ] Tube-to-tubesheet procedure is approved
[ ] No unapproved substitution occurred
[ ] Deviations are closed
[ ] Material is formally released for production

Frequently Asked Questions

What is the first step in heat exchanger tube material selection?

Collect complete design and process data for both sides of the tube, including normal, design, cleaning, shutdown and upset conditions. Selecting a material before these data are available creates a high risk of under-specification or unnecessary over-specification.

Is a nickel alloy always better than stainless steel?

No. A nickel alloy may provide additional capability in certain environments, but stainless steels, duplex grades or other materials may be fully suitable under less severe conditions. Selection should be based on the credible corrosion mechanisms, temperature, pressure, fabrication and lifecycle requirements.

Is titanium always the best choice for seawater?

No material should be approved from the word “seawater” alone. Titanium is widely used in seawater and desalination equipment, but grade, temperature, crevice conditions, galvanic coupling, hydrogen charging, water chemistry and cleaning method still require review.

Does a higher nickel content always provide better SCC resistance?

No. SCC depends on the alloy, environment, stress, temperature, cold work and cracking mechanism. Chloride SCC and sulfide stress cracking are different problems and may be governed by different material-selection rules.

Can ASTM G48 predict the service life of a heat exchanger tube?

No. ASTM G48 is an accelerated laboratory method for comparing pitting and crevice-corrosion initiation resistance under defined ferric-chloride conditions. It does not directly predict corrosion propagation or actual equipment life.

Why is the wall basis important?

Average wall, minimum wall, nominal wall and design minimum are not interchangeable. The wall basis affects manufacturing tolerance, pressure design, heat transfer and the remaining wall after bending or expansion.

Can a higher-strength alloy use a thinner tube wall?

Possibly, but not automatically. Final wall thickness must satisfy the applicable design code and may also be controlled by external pressure, vibration, corrosion allowance, manufacturing tolerances, bending, handling or joining requirements.

Is thermal conductivity the deciding heat-transfer property?

No. Overall performance also depends on wall thickness, fouling, film coefficients, flow distribution, surface condition and exchanger area. Thermal conductivity should be considered within the complete thermal design.

Is an MTC enough to approve the material?

No. An MTC confirms reported batch-specific material information. The buyer must also verify physical traceability, correct product form, material condition, standard coverage, project requirements and application suitability.

What is the difference between a material standard and a corrosion test standard?

A material standard defines product requirements such as chemistry, dimensions, mechanical properties and tests. A corrosion test standard defines a specific test method. Neither alone proves suitability for every service environment.

Should the cheapest compliant alloy be selected?

Not automatically. “Compliant” must include corrosion, mechanical design, fabrication and inspection requirements. Lifecycle cost should consider maintenance, cleaning, plugging, downtime and future replacement.

What should buyers do when a supplier proposes an equivalent alloy?

Request a formal comparison of grade, UNS number, chemistry, mechanical properties, product standard, code status, corrosion evidence, fabrication, inspection and availability. Do not approve a substitution based only on a supplier statement.

Can one alloy be used for all exchangers in a plant?

Usually not without a detailed review. Different exchangers may experience different fluids, temperatures, pressures, contaminants, cleaning cycles and damage mechanisms. Standardization can be useful, but it should not override service suitability.

How should sour-service tubes be selected?

Where NACE MR0175/ISO 15156 applies, selection should follow its environmental limits, material requirements and qualification rules. The phrase “sour service alloy” is not enough without defining H₂S conditions and material state.

Which supplier documents are most important?

The core package commonly includes the MTC/MTR, heat and lot traceability, dimensional report, NDT reports, pressure-test report, heat-treatment record, corrosion-test report where required, inspection release and packing list.

Conclusion

Alloy tube selection for shell-and-tube heat exchangers cannot be reduced to a comparison between stainless steel, nickel alloy and titanium.

A technically defensible decision should consider:

Tube-side and shell-side chemistry
Normal, design, cleaning and upset conditions
General and localized corrosion
SCC and hydrogen-related risks
Galvanic compatibility
Temperature-dependent properties
Internal and external pressure
Wall basis and dimensional tolerances
Flow velocity and erosion-corrosion
Flow-induced vibration
Fouling and total thermal resistance
Tube bending, expansion and welding
Heat treatment and surface condition
Material-standard scope
Corrosion-test limitations
MTC/MTR traceability
Supplier manufacturing capability
Availability and change control
Lifecycle cost

The most reliable process begins with complete design data, develops a shortlist of technically credible materials, verifies applicable standards and manufacturing routes, and documents the final decision through a controlled purchase specification and inspection plan.

Buyers requesting nickel-alloy or titanium heat-exchanger tubes should provide the operating media, concentrations, temperatures, pressures, tube dimensions, wall basis, product form, joining method, testing requirements and documentation expectations with the RFQ.

That information allows the material supplier, exchanger fabricator and project engineer to identify conflicts before production rather than after tubes have been manufactured, delivered or installed.