Nickel alloys and titanium are often selected for equipment that must resist seawater, chlorides, acids, high temperatures, or other demanding conditions.

However, installing a corrosion-resistant alloy next to carbon steel, stainless steel, aluminium, copper alloy, or another metal can change the corrosion behaviour of the complete system.

The problem is not simply that two different metals touch each other.

Galvanic corrosion can develop when metals with different electrochemical behaviour are electrically connected and exposed to a common conductive environment. The more active member of the couple may then corrode faster than it would alone. The severity depends on the actual alloys, their active or passive condition, the electrolyte, cathodic reaction, exposed area ratio, geometry, temperature, flow, deposits, coatings, and electrical continuity.

Preventing galvanic corrosion between nickel alloys titanium and other metals

The correct engineering question is not:

“Are nickel alloys and titanium high enough in the galvanic series to be safe?”

It is:

“Under the actual service conditions, which material will act as the anode, how much cathodic area will it support, and can the electrical or electrolyte path be removed or controlled?”

This guide explains how buyers, designers, and equipment manufacturers can assess those questions before material is ordered.

What Conditions Are Required for Galvanic Corrosion?

Four conditions generally need to exist at the same time.

Required Condition	What It Means	Example
Different electrochemical behaviour	The materials establish different corrosion potentials in the service environment	Passive titanium connected to carbon steel in seawater
Electrical connection	Electrons can move between the metals	Direct contact, metallic bolts, grounding, or conductive piping
Common electrolyte	Ionic current can move through a conductive liquid or wet film	Seawater, process fluid, condensation, or contaminated moisture
Sustainable electrochemical reactions	Anodic metal dissolution and a cathodic reaction can continue	Steel dissolution coupled with oxygen reduction on a noble surface

Removing any one of these conditions can stop the galvanic circuit.

This is why two dissimilar metals may coexist without galvanic attack when:

The joint remains completely dry
The metals are electrically isolated
Only one material is exposed to the electrolyte
The electrolyte has very high resistance and remains uncontaminated
The cathodic reaction is strongly limited
Both materials remain at similar potentials under the actual conditions

The last two conditions cannot normally be confirmed from material names alone.

Why the Galvanic Series Is Only a Screening Tool

A galvanic series arranges materials according to their measured corrosion potentials in a defined environment.

ASTM G82-98(2021)e1 provides guidance for developing and using a galvanic series to predict which member of a coupled pair may experience increased corrosion.

The series is useful for determining the likely direction of galvanic attack.

It does not directly predict the corrosion rate.

A Galvanic Series Is Environment-Specific

A series developed in aerated seawater should not automatically be used for:

Hydrochloric acid
Sulfuric acid
Caustic solution
High-purity water
Sour oil and gas fluids
Condensing atmospheres
Hot brine
Deaerated water
Fluoride-containing media

The corrosion potential of an alloy can change with:

pH
Temperature
Oxygen
Chloride concentration
Sulfides
Flow
Biofilm
Surface oxide
Welding
Heat treatment
Deposits
Active or passive condition

Potential Difference Is Not Corrosion Rate

A large potential difference may indicate a stronger thermodynamic driving force.

The actual galvanic current also depends on:

Cathodic reaction kinetics
Anodic polarization
Cathodic polarization
Electrolyte resistance
Distance through the electrolyte
Surface area
Mass transfer
Deposits and biofilms
Coating condition

Two material combinations with similar measured potential differences can therefore produce different corrosion rates.

Are Nickel Alloys and Titanium Galvanically Compatible?

There is no universal yes-or-no answer.

Titanium normally relies on a stable titanium-oxide film for corrosion resistance.

Many nickel-chromium and nickel-chromium-molybdenum alloys rely substantially on chromium-rich passive films. Nickel-copper alloys, commercially pure nickel, and precipitation-hardened nickel alloys should not automatically be treated as having the same electrochemical behaviour.

When titanium and a corrosion-resistant nickel alloy both remain passive in an oxidizing aqueous environment, their potentials may be relatively close compared with either material coupled to carbon steel or aluminium.

That can reduce galvanic driving force.

It does not prove that every nickel alloy–titanium couple is safe.

Questions That Still Need to Be Answered

Which nickel alloy and titanium grade are involved?
Are both materials passive in the actual fluid?
Is either material exposed to reducing acid or fluoride?
Is cathodic protection applied?
Is one material a small insert connected to a large surface?
Are crevices or deposits present?
Will welding, grinding, or heat treatment alter the surface state?
Is the joint exposed continuously or only during condensation?
Can one material become active during shutdown or cleaning?

The combination should be assessed as two specific materials in one defined environment—not as “nickel alloy plus titanium.”

Which Mixed-Metal Combinations Commonly Require Attention?

The following table is a screening guide, not a fixed compatibility rating.

Material Combination	Typical Concern in a Conductive Oxidizing Water	Variables That Can Change the Result
Titanium and carbon steel	Carbon steel may become the anodic member	Area ratio, coating, oxygen, flow, and electrical isolation
Nickel-chromium-molybdenum alloy and carbon steel	Carbon steel may experience accelerated attack	Alloy surface state, steel coating, electrolyte, and exposed area
Titanium and aluminium	Aluminium may become strongly anodic	Aluminium alloy, coating, seawater chemistry, and joint geometry
Nickel alloy and aluminium	Aluminium is often the material requiring protection	Cathodic efficiency, area ratio, and coating defects
Titanium and copper alloy	Copper alloy behaviour may be altered	Exact copper alloy, biofilm, velocity, oxygen, and area ratio
Nickel alloy and stainless steel	Stainless steel may be active or passive	Grade, chloride, temperature, weld condition, and crevice geometry
Titanium and passive stainless steel	Potentials may be relatively close in some environments	Loss of stainless-steel passivity can change the couple
Passive Ni-Cr-Mo alloy and titanium	May have limited galvanic driving force in some oxidizing waters	Reducing conditions, active states, cathodic protection, and area ratio

The table should not be used to approve a design without service-specific review.

Why Cathode-to-Anode Area Ratio Matters

The relative exposed areas of the two metals can be more important than the total amount of metal used.

The most unfavorable arrangement is often:

Small anodic area connected to a large cathodic area

The cathodic reaction occurs over the large noble surface, while the resulting anodic current is concentrated on the small active surface.

This can produce a high local current density and rapid penetration of the anodic component.

Example

Consider a small exposed carbon-steel bolt connected to a large titanium plate in seawater.

The steel bolt may supply anodic current to the much larger titanium cathode.

The opposite geometry—a small titanium fastener in a large steel structure—may create a lower galvanic current density on the steel, although crevice corrosion, coating damage, mechanical requirements, and local geometry still require review.

Area Ratio Questions for Buyers

Question	Why It Matters
What is the exposed cathodic area?	Determines the available cathodic reaction area
What is the exposed anodic area?	Determines where anodic current is concentrated
Are coatings included in the assumed area?	A coating defect can change the effective ratio
Can the anode become smaller as it corrodes?	Local penetration may accelerate
Will a deposit create a small exposed zone?	Deposit edges can concentrate attack
Are both sides of a sheet or tube exposed?	Hidden surface area may be overlooked
Is the joint intermittently wet?	The effective area can change during wetting

Nominal component dimensions do not always equal electrochemically exposed area.

Galvanic Corrosion and Crevice Corrosion Are Different

A mixed-metal joint can suffer more than one form of corrosion.

Galvanic Corrosion

Galvanic corrosion is driven by the electrical coupling of materials with different electrochemical behaviour in a shared electrolyte.

Crevice Corrosion

Crevice corrosion is driven by local chemical changes inside a restricted gap, such as oxygen depletion, acidification, or chloride concentration.

Differential Aeration

Different oxygen concentrations can create electrochemical differences even within one alloy.

Observation	Possible Mechanism
Active metal corrodes near a noble metal	Galvanic corrosion may be involved
Attack occurs beneath a gasket of the same alloy	Crevice corrosion may dominate
Attack occurs under deposits away from a mixed-metal joint	Under-deposit or differential-aeration corrosion
Corrosion occurs only where a coating is damaged near a noble component	Galvanic concentration at a coating defect
Both corrosion and tight joint geometry are present	Galvanic and crevice effects may interact

Replacing one alloy without correcting the crevice may therefore fail to solve the problem.

Environmental Conditions Can Reverse or Intensify the Risk

Electrolyte Conductivity

Higher ionic conductivity generally allows galvanic current to travel more easily.

However, the practical risk depends on the complete chemistry, not conductivity alone.

Low-conductivity water can become more conductive through:

Dissolved salts
Process leakage
Dust
Cleaning agents
Carbon dioxide absorption
Corrosion products
Condensation contamination

Oxygen and Cathodic Reactions

Oxygen can help maintain passive films on titanium and many nickel-chromium alloys.

It can also support oxygen reduction on a large noble surface, increasing the cathodic current that must be supplied by the anode.

Oxygen may therefore improve the resistance of the passive material while increasing galvanic demand on a connected active metal.

Temperature

Temperature can change:

Reaction kinetics
Oxygen solubility
Electrolyte resistance
Passive-film stability
Corrosion-product solubility
Crevice chemistry
Hydrogen-generation risk

The direction and magnitude of the effect should be established for the actual fluid.

Flow

Flow may:

Increase oxygen transport to a cathodic surface
Remove corrosion products
Change mass-transfer control
Prevent deposits
Create erosion-corrosion
Cause cavitation
Produce local turbulence

ASTM G71-81(2024) is intended for galvanic testing where flow is not high enough to cause erosion-corrosion or cavitation. High-velocity systems may require additional testing or analysis.

A Practical Prevention Hierarchy

The most reliable strategy is usually to remove the cause rather than add more protection after the design is complete.

Priority	Prevention Method	Main Limitation
1	Avoid an unnecessary dissimilar-metal couple	May conflict with strength, cost, or fabrication needs
2	Select materials with compatible behaviour in the actual electrolyte	Requires service-specific data
3	Break electrical continuity	Isolation can be bridged or damaged
4	Avoid a small anode connected to a large cathode	Geometry may be constrained
5	Keep the joint dry or improve drainage	Not possible in permanently immersed systems
6	Apply a properly engineered coating system	Coating defects and maintenance must be considered
7	Use corrosion allowance or replaceable sacrificial components	Does not eliminate the corrosion mechanism
8	Apply engineered cathodic protection where appropriate	Requires specialist design and monitoring
9	Monitor the joint during service	Detects problems but does not prevent all damage

Several measures may be required together.

Electrical Isolation Must Break Every Conductive Path

Electrical isolation can be highly effective because it interrupts electron flow between the dissimilar metals.

Possible isolation components include:

Insulating flange gaskets
Insulating bolt sleeves
Insulating washers
Dielectric unions
Nonconductive spacers
Composite transition pieces
Approved lined connectors

However, installing one insulating gasket does not guarantee isolation.

Common Isolation Failure Paths

Metal bolts bypass the gasket
A washer is missing
Bolt sleeves are damaged
Conductive sealant bridges the joint
Seawater or conductive debris accumulates across the gap
Instrument tubing reconnects the two structures
Electrical grounding bridges the isolation
Pipe supports create an alternate metallic path
The isolator cracks under load
Maintenance personnel replace insulated hardware with standard hardware

Isolation Verification

The installation plan should define:

Isolation component specification
Pressure and temperature rating
Chemical compatibility
Assembly torque
Visual inspection
Electrical resistance or continuity test
Acceptance criterion
Inspection frequency
Replacement procedure

Electrical isolation is a maintainable system, not only a purchased gasket.

Coatings Must Be Designed for Damage

Coatings can reduce galvanic corrosion by separating the metal from the electrolyte or reducing the effective cathodic area.

The coating strategy must assume that defects may eventually occur.

Why Coating Only the Anodic Metal Can Be Dangerous

If a large anodic component is fully coated, the initial galvanic current may be low.

If the coating develops a small defect, the exposed area can become a very small anode connected to a large uncoated cathode.

This can concentrate attack at the defect.

Coating Options

Strategy	Possible Benefit	Important Risk
Coat both metals	Reduces electrolyte contact on both members	Requires compatible coating systems and inspection
Coat the cathodic metal	Reduces available cathodic area	Defects and edge exposure still require control
Coat only the anodic metal	Protects while coating remains intact	Small defects may experience concentrated attack
Use a lining at the joint	Separates electrolyte from the interface	Damage during assembly can expose the couple
Combine coating with isolation	Adds two barriers	Requires verification of both systems

The selected coating should be evaluated for:

Adhesion
Permeability
Immersion resistance
Temperature
Chemical resistance
Mechanical damage
Edge coverage
Repairability
Inspection access

Cathodic Protection Is Not a Universal Local Fix

Cathodic protection can be effective for buried or submerged metallic structures when designed as a complete electrochemical system.

Possible systems include:

Sacrificial-anode cathodic protection
Impressed-current cathodic protection

Current NACE SP0169-2024 provides practices for external corrosion control of underground or submerged metallic piping systems.

Its scope should not be extended automatically to every:

Heat exchanger
Bolted flange
Chemical reactor
Instrument connection
Medical component
Atmospheric fastener
Internal process surface

Titanium Under Cathodic Polarization

Titanium is normally protected by a stable oxide film.

Under certain cathodic conditions, hydrogen can be generated and absorbed if the oxide film, potential, temperature, pH, and exposure conditions allow it.

The International Titanium Association seawater guidance discusses galvanic coupling, cathodic conditions, and hydrogen-damage considerations for titanium systems.

A CP design involving titanium should therefore define:

Applied potential
Current density
Temperature
pH
Surface condition
Expected hydrogen-generation reaction
Exposure time
Monitoring method

“More negative potential” does not always mean “more protection” for every material in the system.

Fasteners and Transition Joints Need Specific Review

Fasteners are frequently overlooked because their exposed area is small.

Important questions include:

Is the fastener the anode or cathode?
Is the surrounding structure much larger?
Is the fastener located inside a crevice?
Can moisture remain under the head or washer?
Is a coating damaged during tightening?
Are insulating sleeves and washers installed?
Can the joint be inspected?
Is the fastener replaceable?
Does the fastener need greater mechanical strength than the base structure?

A noble fastener in a large active structure may create a favorable area ratio for the active structure, but the local joint can still suffer from:

Crevice corrosion
Coating damage
Stress concentration
Fretting
Loss of preload
Local electrolyte retention

Area ratio is important, but it is not the only fastener-design criterion.

How Should Galvanic Compatibility Be Tested?

A generic seawater chart may be sufficient for preliminary screening.

Critical systems may require representative testing.

ASTM G71-81(2024) provides guidance for conducting and evaluating galvanic corrosion tests in liquid electrolytes under low-flow conditions.

Representative Test Inputs

Test Variable	What Should Match the Project
Materials	Exact grades, conditions, welds, and coatings
Surface	Production finish, cleaning, and oxide condition
Electrolyte	Full chemistry, contaminants, pH, and conductivity
Temperature	Normal and credible maximum
Oxygen	Aerated, deaerated, or controlled
Area ratio	Representative exposed cathode and anode
Geometry	Joint, gap, distance, and crevice conditions
Electrical connection	Actual connection resistance
Flow	Representative unless outside the test method’s scope
Duration	Sufficient to observe stabilization and damage
Evaluation	Galvanic current, potential, mass loss, and morphology

Possible Test Outputs

Open-circuit potentials
Coupled potential
Galvanic current
Current density
Mass loss
Penetration rate
Local attack depth
Surface morphology
Coating damage
Hydrogen content where relevant

A short-term galvanic-current result should not automatically be converted into long-term equipment life without considering changes in films, deposits, corrosion products, biofilms, and exposed area.

What Material Documents Can and Cannot Prove

A Material Test Certificate can confirm information such as:

Alloy and UNS designation
Chemical composition
Mechanical properties
Heat treatment
Product standard
Heat number
Specified product tests

It does not normally prove:

Galvanic compatibility with another metal
Corrosion rate in the process fluid
Actual coupled potential
Safe cathode-to-anode area ratio
Coating performance
Insulation effectiveness
Cathodic-protection compatibility
Long-term hydrogen behaviour

NDT reports can verify specified discontinuities or product integrity.

They do not establish galvanic-corrosion resistance.

The corrosion design requires separate engineering evidence.

What Buyers Should Include in the RFQ or Technical Enquiry

RFQ Category	Information to Provide
Nickel or titanium material	Exact grade and UNS designation
Other connected metals	Grade, condition, coating, and surface
Product form	Tube, pipe, bar, plate, fitting, fastener, or tubesheet
Joint type	Welded, bolted, expanded, clamped, or threaded
Electrical contact	Direct, isolated, grounded, or externally connected
Electrolyte	Complete chemistry and conductivity
pH	Normal and upset range
Temperature	Operating, design, cleaning, and shutdown
Oxygen	Aerated, deaerated, or variable
Flow	Velocity, solids, turbulence, and stagnation
Exposure	Continuous immersion, splash, condensation, or intermittent wetting
Area ratio	Exposed area of each material
Crevices	Gaskets, deposits, fasteners, and shielded areas
Coatings	Material, thickness, location, and repair plan
Isolation	Gasket, sleeve, washer, and testing requirement
Cathodic protection	Type, potential range, and monitoring
Design life	Required operating period
Inspection	Access and monitoring method
Testing	ASTM G71 or project-specific qualification
Documentation	MTC, coating records, drawings, and test reports

Without these inputs, a material supplier can confirm the ordered alloy but cannot reliably approve the complete galvanic couple.

Common Galvanic-Corrosion Design Mistakes

Mistake	Why It Is Risky	Better Approach
Selecting materials from one seawater galvanic chart	The actual environment may be different	Use environment-specific data
Assuming all nickel alloys behave similarly	Alloy families and passive states differ	Specify exact grade and condition
Assuming titanium is always harmless to connected metals	Titanium may act as a large noble cathode	Evaluate the connected active metal
Ignoring cathode-to-anode area ratio	Small anodic areas may penetrate rapidly	Calculate exposed areas
Coating only the anodic member	Small defects can concentrate attack	Design a damage-tolerant coating system
Installing only an insulating gasket	Bolts or other connections may bridge it	Isolate and test the complete joint
Treating crevice corrosion as galvanic corrosion	Removing one metal may not remove the crevice	Identify the actual mechanism
Using MTC data as corrosion approval	MTC does not measure the galvanic couple	Obtain corrosion-engineering evidence
Applying CP without reviewing titanium	Excessive cathodic polarization can create other risks	Use specialist CP design
Ignoring shutdown and cleaning conditions	Material states and electrolyte can change	Evaluate the full operating cycle
Relying on potential difference alone	Potential does not equal corrosion current	Review polarization and test data
Using supplier experience without project inputs	Experience may relate to another environment	Submit complete service data

Frequently Asked Questions

Can nickel alloys and titanium be connected directly?

They can be used together in some systems, particularly when both remain passive and their potentials are relatively close in the actual environment. The exact grades, surface states, electrolyte, area ratio, crevices, and cathodic-protection conditions must still be reviewed.

Will titanium cause carbon steel to corrode?

In a conductive environment such as seawater, titanium commonly behaves as the more noble member and carbon steel may become anodic. The risk is especially important when a small exposed steel area is connected to a large titanium surface.

Is Inconel 625 galvanically compatible with titanium Grade 2?

No universal approval can be made from the two trade names. Both may remain passive in some oxidizing aqueous environments, but the system still requires review of temperature, fluid chemistry, surface condition, area ratio, welding, crevices, and cathodic polarization.

Can galvanic corrosion occur without direct physical contact?

Yes. The metals only need an electrical connection. The connection may be through piping, bolts, grounding cables, supports, instruments, or another metallic path.

Can galvanic corrosion occur in air?

It can occur when atmospheric moisture, condensation, salt deposits, or another wet film creates an electrolyte. Completely dry metal contact does not form an operating galvanic cell.

Does a larger voltage difference always mean faster corrosion?

No. Potential difference indicates driving force. The actual rate also depends on polarization behaviour, cathodic kinetics, electrolyte resistance, area ratio, mass transfer, and surface films.

Which metal should be coated?

The answer depends on the complete design. Coating only the anodic member can be risky if a defect creates a small exposed anode connected to a large cathode. Coating both members, reducing cathodic area, or combining coating with electrical isolation may provide a more damage-tolerant solution.

Are insulating gaskets enough?

Not necessarily. Bolts, washers, conductive sealant, grounding, instrument lines, supports, or contaminated moisture can bypass the gasket. The completed joint should be tested for electrical isolation.

Can corrosion allowance solve galvanic corrosion?

Corrosion allowance may extend life but does not eliminate concentrated local attack. It is less effective where a small anode can pit or penetrate rapidly.

Does ISO 9001 prove galvanic compatibility?

No. ISO 9001 addresses quality-management processes. Galvanic compatibility requires material, environment, geometry, electrochemical, and design evidence.

Does an MTC prove corrosion resistance?

An MTC verifies reported material data for the supplied heat or lot. It does not prove the performance of a dissimilar-metal couple in a specific electrolyte.

Should every project perform ASTM G71 testing?

No. Testing should be based on risk, available service data, novelty of the material combination, project criticality, and whether the actual environment falls within the method’s scope.

Can cathodic protection be used with titanium?

It may be possible, but the applied potential and operating conditions must be reviewed. Certain cathodic conditions can increase hydrogen-generation and absorption concerns in titanium.

Who should approve the final material combination?

The equipment designer, responsible materials or corrosion engineer, and applicable project authority should approve the complete combination. The material supplier can provide alloy data, certificates, product standards, dimensions, and manufacturing information.

Conclusion

Galvanic corrosion prevention between nickel alloys, titanium, and other metals cannot be reduced to one galvanic-series table.

A defensible assessment should consider:

Exact alloy grades
Active or passive surface condition
Electrical continuity
Electrolyte chemistry
Temperature
Oxygen
Flow and deposits
Cathode-to-anode area ratio
Crevice geometry
Coating condition
Electrical isolation
Cathodic protection
Shutdown and cleaning conditions
Representative testing
Inspection and maintenance

The first priority should be to avoid an unnecessary galvanic couple.

Where dissimilar metals must be used, the design should reduce electrochemical driving force, avoid a small anode connected to a large cathode, interrupt electrical continuity where practical, control electrolyte exposure, and use coatings or cathodic protection only through a verified engineering procedure.

For nickel alloy or titanium tube and bar enquiries, buyers should provide the exact grades, dimensions, connected materials, fluid chemistry, temperature, joint design, coating, isolation, cathodic-protection conditions, inspection requirements, and documentation expectations.

Emily PIPE can review whether the requested nickel alloy or titanium tube or bar grade, product standard, material condition, dimensions, surface, testing, and certification requirements are manufacturable. Final approval of the galvanic couple should remain with the responsible equipment designer or corrosion engineer.

How Can Buyers Prevent Galvanic Corrosion Between Nickel Alloys, Titanium, and Other Metals?