What Is Galvanic Corrosion?
The galvanic corrosion test measures corrosion arising from the potential difference between two dissimilar metals in contact within an electrolyte. It is assessed via galvanic current or mass loss.
Purpose of Galvanic Corrosion
The purpose is to determine which metal will corrode and at what rate in joints combining dissimilar metals, and to support material-pair selection.
Which Products Is Galvanic Corrosion Applied To?
- Dissimilar-metal fasteners
- Aluminium-steel hybrid structures
- Heat exchanger components
- Marine environment hardware
- Electric vehicle lightweight structure joints
In Which Sectors Is Galvanic Corrosion Used?
Standards Related to Galvanic Corrosion
- ASTM G71
- ASTM G82
- ISO 21207
- ASTM B117tuz sisi (salt spray) korozyon testi test
- ISO 9227tuz sisi korozyon testi test
- ASTM G85modified salt fog (cyclic) test test
- ASTM G48pitting and crevice corrosion in stainless steels test
- ISO 16701cyclic corrosion test test
- ISO 6270strength in condensation environment test
- SAE J2334cyclic laboratory corrosion testing test
- VDA 233-102cyclic corrosion testing in automotive test
Galvanic Corrosion Conditions
- Electrolyte
- Solution defined in the specification (e.g. NaCl or artificial seawater)
- Area ratio
- The anode/cathode area ratio determines the corrosion rate; defined in specification
- Temperature
- Constant temperature defined in the standard
- Time
- Exposure time defined in the specification
Equipment for Galvanic Corrosion
- Electrochemical measurement system (potentiostat / zero-resistance ammeter)
- Reference electrode and cell setup
- Precision balance
- Temperature-controlled bath
Sample Requirements for Galvanic Corrosion
- The two metal samples must be prepared with the defined area ratio
- Surfaces must be cleaned per the standard
- Initial masses must be measured
- Connection resistance must be negligible
Duration of Galvanic Corrosion
Electrochemical measurement takes hours; mass-loss measurement takes days to weeks.
Galvanic Corrosion Procedure
- 1Samples are prepared and their surface areas and masses recorded.
- 2The metals are brought into contact within the electrolyte.
- 3Galvanic current is continuously measured, or the defined duration is waited out.
- 4At the end of the exposure, samples are cleaned and mass loss is measured.
- 5Galvanic corrosion rate is calculated.
Galvanic Corrosion Acceptance Criteria
The acceptance criterion is given in the specification as maximum galvanic current density or maximum mass loss. In some applications, use of the metal pair is directly prohibited.
Evaluation of Results
The direction of galvanic current shows which metal is the anode. Since area ratio strongly affects the result, it must always be stated in the report.
Galvanic Corrosion Report
- Standard applied
- Metal pair and area ratio
- Electrolyte composition and temperature
- Galvanic current or mass loss
- Calculated corrosion rate
Accreditation
EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.
Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.
Scope and authorisationsRelated Regulations
- Marine and aviation material specifications
- Lightweight structure joint standards
Corrosion Tests — What Does This Testing Field Cover?
The section below describes the field that the Galvanic Corrosion test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.
Corrosion testing assesses under accelerated conditions how long metals and coatings last in a corrosive environment. The family contains two approaches, and the difference between them is the most important knowledge in the field: constant-condition tests and cyclic tests.
Constant-condition salt spray (ASTM B117, ISO 9227) maintains the same environment in the chamber throughout: the same salt concentration, the same temperature, a continuously wet surface. The strength of this approach is its REPEATABILITY; because the conditions are tightly defined, results from different laboratories come out close to one another. That is exactly the property wanted when monitoring production consistency and comparing suppliers.
But the real atmosphere is not constant. A surface gets wet, dries, warms, cools, takes UV and becomes soiled. The wet-dry cycle changes the corrosion mechanism: during drying the salt concentrates, oxygen access increases and corrosion accelerates. The form of damage on a continuously wet surface differs from that on a cyclically wetted one.
Cyclic tests (SAE J2334, VDA 233-102, ASTM G85, ISO 16701) imitate this variability and give results closer to the damage patterns seen in the field. The automotive sector has largely moved to this approach. The practical consequence is this: the same two coatings may RANK DIFFERENTLY in constant and cyclic tests; which test was applied determines the result.
No test in this family defines a DURATION OR ACCEPTANCE CRITERION. How many hours it is applied for and what level of damage is acceptable come from the product standard or the customer specification. The assessment scales are defined in separate standards: different scales are used for degree of rusting, blistering, creep from a scribe and loss of adhesion.
The damage looked for varies with the coating type. For metallic coatings, white rust (corrosion of the coating itself) and red rust (corrosion reaching the base metal) are reported separately, and specifications usually give a separate duration for each. On painted surfaces, creep from a scribe and blistering come to the fore.
Electrochemical methods are a separate body of work and answer a different question. Potentiodynamic polarisation, electrochemical impedance spectroscopy and pitting potential measurements reveal the MECHANISM and rate of corrosion within hours; chamber tests give the result visually over weeks. The two are complementary rather than alternatives.
Stainless steels have a special sub-body in the field: pitting and crevice corrosion and intergranular corrosion tests indicate the quality of the heat treatment and the condition after welding.
Method groups in this field
Constant-condition salt spray
ASTM B117 and ISO 9227 (NSS); for repeatable comparison and process control.
Acidified and accelerated
The AASS and CASS conditions of ISO 9227; preferred for decorative chromium-nickel coatings.
Cyclic corrosion
SAE J2334, VDA 233-102, ASTM G85 and ISO 16701; approaches field behaviour through wet-dry cycles.
Humidity and condensation
Steady damp heat, condensation and cyclic humidity tests; these reveal coating blistering.
Electrochemical
Polarisation, impedance spectroscopy, pitting and crevice corrosion potential measurements.
Stainless steel specific tests
Pitting and crevice corrosion, intergranular corrosion; indicators of heat treatment and weld quality.
Assessment scales
Scales for degree of rusting, blistering, creep from a scribe and loss of adhesion; the ratings come from these standards.
Commonly confused in this field
- Chamber hours cannot be converted into field life; there is no reliable conversion factor.
- Constant and cyclic tests can produce different rankings; whichever the specification states is the one applied.
- A corrosion report without a stated duration and acceptance criterion cannot be interpreted.
- If the coating thickness is not reported, the result cannot be compared; thickness directly affects durability.
- White rust and red rust say different things; the two must be reported separately.
- The position and angle of the specimen in the chamber affect the result; standards govern this and the report must document it.
Which decisions these results support
- Selection of the coating system and supplier approval
- Process control of the coating line and lot acceptance
- Evidence of durability for automotive and outdoor products
- Assessment of field corrosion damage and warranty decisions
- Verification of heat treatment and weld quality in stainless steel
Related Tests
Frequently Asked Questions About Galvanic Corrosion
Why is the area ratio so important?
A small anode combined with a large cathode increases the current density on the anode and accelerates corrosion. The test result therefore cannot be interpreted without stating the area ratio.