Cold sprayed copper parts corrosion resistance testing evaluates how copper coatings and components produced by cold gas dynamic spraying perform when exposed to salt fog, humidity, industrial atmospheres, and corrosive electrolytes. Testing is typically carried out by accredited third-party laboratories in accordance with recognized standards such as ASTM B117, ISO 9227, and GB/T 10125. This article explains the sample types, test items, test methods, acceptance criteria, and typical applications involved in the corrosion evaluation of cold sprayed copper parts.
Technical Overview
Cold spray, also known as cold gas dynamic spraying or kinetic spraying, is a solid-state deposition process in which fine metal powder particles are accelerated to supersonic velocities by a heated high-pressure gas stream, typically nitrogen or helium. Because the particles remain well below their melting point, coatings form through severe plastic deformation and mechanical interlocking upon impact rather than by melting. Copper is one of the most suitable materials for cold spray due to its excellent ductility, and cold sprayed copper deposits are widely used for electrical conductivity, thermal management, dimensional repair, and additive manufacturing.
From a corrosion standpoint, cold sprayed copper parts present several unique considerations:
- Residual porosity, typically below 1% to 5% depending on process parameters, can create pathways for corrosive media to reach the substrate.
- Embedded copper oxides and residual particle boundaries influence the electrochemical homogeneity of the deposit.
- When copper is deposited on aluminum or steel substrates, a galvanic couple may form at exposed edges or through interconnected pores, accelerating localized attack on the less noble substrate.
- Service environments such as marine atmospheres, industrial air containing sulfur dioxide, and deicing salt exposure require verified corrosion performance.
Corrosion resistance testing therefore serves to qualify cold spray process parameters, verify coating integrity, evaluate substrate protection, and support service life prediction of finished parts.
Testing Samples
Cold Sprayed Copper Coatings on Aluminum Substrates
These samples are typically produced for electrical conductivity and heat dissipation applications. Corrosion testing focuses on galvanic compatibility, pore sealing, and the risk of aluminum substrate attack through interconnected porosity. A common sample geometry is a flat panel of 100 mm by 150 mm with a copper deposit thickness of 0.3 mm to 2 mm.
Cold Sprayed Copper Coatings on Steel Substrates
Used for repair, conductivity, and surface protection, these samples require careful evaluation because steel is less noble than copper. Any coating discontinuity, edge exposure, or through-pore can trigger localized galvanic corrosion of the steel beneath the deposit.
Free-Standing Cold Sprayed Copper Parts
Additively manufactured or machined copper components are tested as bulk material. Testing evaluates the inherent corrosion behavior of the deposit, including the influence of porosity and oxide content on mass loss rates and localized corrosion susceptibility.
Bimetallic and Functionally Graded Structures
Copper-aluminum and copper-steel transition structures are tested with particular attention to the dissimilar metal interface, which is often the most corrosion-sensitive region of the part.
General sample preparation requirements include:
- Solvent and deionized water cleaning to remove oils, residues, and handling contamination.
- Masking of cut edges with chemically resistant tape or wax so that only the functional surface is exposed.
- Recording of initial mass, dimensions, coating thickness, porosity, surface roughness, and electrical conductivity.
- A minimum of three replicate specimens per test condition, plus uncoated substrate reference coupons for comparison.
Testing Items
Typical corrosion-related test items for cold sprayed copper parts include the following:
- Neutral salt spray (NSS) resistance testing.
- Acetic acid salt spray (ASS) and copper-accelerated acetic acid salt spray (CASS) resistance testing.
- Damp heat, or constant humidity, resistance testing.
- Cyclic corrosion and cyclic salt mist resistance testing.
- Sulfur dioxide, or industrial atmosphere, resistance testing.
- Full immersion and alternate immersion corrosion testing.
- Electrochemical characterization, including open circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy.
- Galvanic couple current and potential testing against substrate alloys.
- Post-corrosion evaluation of mass loss, appearance, corrosion products, porosity, adhesion retention, and electrical conductivity retention.
Testing Methods
Neutral Salt Spray Test (NSS)
Performed per ASTM B117, ISO 9227, GB/T 10125, or JIS Z 2371. Specimens are exposed to a continuous fog of 5% sodium chloride solution at 35 °C, with the pH of collected solution maintained between 6.5 and 7.2 and a collection rate of 1 to 2 mL per 80 cm² per hour. Common test durations are 24, 48, 96, 240, 480, or 1000 hours, selected according to the application environment and product specification.
CASS Test
The copper-accelerated acetic acid salt spray test per ASTM B368 uses a sodium chloride solution modified with cupric chloride and acetic acid, adjusted to pH 3.1 to 3.3, at a chamber temperature of 49 °C. This severely accelerated environment is well suited to copper and copper alloy surfaces and reveals weaknesses in coating density, adhesion, and edge protection.
Damp Heat (Humidity) Testing
Conducted per IEC 60068-2-78, for example at 40 °C and 93% relative humidity, or per ASTM D2247 with condensing humidity at 38 °C. Typical durations range from 48 to 1000 hours. Evaluation covers tarnishing, patina formation, blistering, and any corrosive penetration at the coating-substrate interface.
Cyclic Corrosion and Cyclic Salt Mist Testing
Standards such as ISO 16701, IEC 60068-2-52, and GB/T 2423.18 alternate salt fog, dry, and high-humidity phases. Cyclic tests generally correlate better with real atmospheric service than continuous salt spray and are increasingly specified for transport and outdoor electrical applications.
Sulfur Dioxide Resistance Testing
Tests per ISO 3231, ISO 6988, or GB/T 9789 expose specimens to humid atmospheres containing sulfur dioxide to simulate polluted industrial air. Cold sprayed copper surfaces are evaluated for tarnish film formation, weight change, and the protective behavior of naturally developing patina layers.
Immersion Corrosion Testing
Full immersion testing per ASTM G31, commonly in 3.5 wt% sodium chloride solution at controlled temperature, is used to determine steady-state corrosion rates through periodic mass loss measurement and to observe localized attack at pores and particle boundaries.
Electrochemical Testing
Electrochemical measurements are performed in a three-electrode cell, typically in 3.5% sodium chloride electrolyte. Procedures follow ASTM G3 conventions, with polarization resistance and potentiodynamic scans per ASTM G59 and ASTM G61 to determine corrosion potential, corrosion current density, and passivation behavior. Electrochemical impedance spectroscopy per ASTM G106 provides insight into coating porosity and degradation kinetics over time.
Galvanic Corrosion Testing
Per ASTM G71, cold sprayed copper is electrically coupled with the substrate alloy in the test electrolyte, and the couple potential and galvanic current are recorded. This test quantifies the driving force for substrate attack in the event of coating damage or through-porosity.
Post-Test Evaluation
- Chemical cleaning and mass-loss determination per ASTM G1 to calculate corrosion rates.
- Visual rating of rusting or corrosion defects per ISO 4628-3 or ASTM D610 when steel substrates are involved.
- Adhesion testing after exposure per ASTM C633 or ASTM D4541 to confirm the deposit remains bonded.
- Metallographic cross-sectioning per ASTM E3 to measure attack depth, interfacial corrosion, and electrolyte penetration through pores.
- Porosity verification using indicator methods where applicable to the substrate material.
- Electrical conductivity measurement, for example by four-point probe per ASTM B193, to verify retention of functional performance.
Applications
Corrosion resistance testing of cold sprayed copper parts supports qualification and quality control across many industries:
- Electric vehicles: copper busbars, battery interconnects, and heat dissipation layers deposited on aluminum housings exposed to humidity and road salt.
- Power electronics: cold sprayed copper heat sinks and conductor tracks requiring stable conductivity in humid and salt-laden air.
- Aerospace: conductive deposits and repairs on aluminum and magnesium structures that must pass atmospheric corrosion qualification.
- Rail transit: corrosion qualification of copper deposits in components subject to salt fog and aggressive wash environments.
- Marine and offshore engineering: conductive layers and protective copper surfaces on steel structures.
- Additive manufacturing: qualification of free-standing cold sprayed copper parts for structural and electrical service.
FAQ
What is a typical pass criterion for salt spray testing of cold sprayed copper parts?
Acceptance criteria are application specific. Common requirements include no red or white corrosion products originating from the substrate, no blistering or delamination of the deposit, cross-sectional attack depth below a specified limit such as 10% of coating thickness, and retention of at least 90% of the original electrical conductivity after the designated exposure, for example 96 or 240 hours of neutral salt spray.
Does porosity affect corrosion test results?
Yes. Interconnected, or through, porosity allows electrolyte to reach the substrate and can initiate galvanic attack at the interface. Optimizing cold spray parameters to minimize porosity, or applying a sealing treatment, significantly improves corrosion test performance.
Do salt spray results directly predict service life?
No. Salt spray testing is primarily a comparative and quality control tool. For service life estimation, cyclic corrosion testing, electrochemical data, and environmental data from the actual installation site should be combined.
Can these tests be performed by accredited third-party laboratories?
Yes. Accredited third-party laboratories routinely conduct salt spray, humidity, cyclic, immersion, and electrochemical corrosion tests under ISO/IEC 17025 quality management systems and issue test reports acceptable for supplier qualification and certification purposes.
What information should accompany submitted specimens?
Submissions should include finished or representative coated panels, at least three replicates per condition, uncoated reference coupons, and documentation of the cold spray parameters, powder specification, coating thickness, and any post-processing such as machining, rolling, or sealing.