Corrosion rate determination for cold-sprayed copper parts quantifies how quickly copper deposits produced by cold gas dynamic spraying degrade in chloride-bearing, marine, or industrial service environments. By combining standardized immersion, salt spray, and electrochemical polarization techniques, manufacturers and accredited third-party laboratories can verify deposit integrity, compare spray process parameters, and confirm that cold-sprayed copper components meet their design life requirements.
Technical Overview
Cold spray (cold gas dynamic spraying) is a solid-state deposition process in which fine copper powder, typically 5–45 µm in diameter, is accelerated to supersonic velocities of roughly 300–1200 m/s by a heated, high-pressure gas stream such as nitrogen, helium, or air. The particles remain well below the melting point and bond to the substrate through severe plastic deformation, producing dense copper coatings and free-standing parts with high electrical and thermal conductivity, low oxide pickup, and near-net-shape geometry.
Because the deposits solidify mechanically rather than from a melt, they contain interparticle boundaries, a few tenths to several percent of porosity, dispersed oxide stringers derived from the feedstock powder, and a work-hardened microstructure. In electrolytes such as seawater or deicing salt solutions, these features can act as preferential anodic paths, electrolyte reservoirs, or micro-galvanic sites, and interconnected porosity can even expose the underlying aluminum or steel substrate to galvanic attack. Corrosion rate determination is therefore a core qualification step for cold-sprayed copper in electrical, marine, and thermal-management applications.
Corrosion rate determination for these materials is performed by accredited third-party laboratories and in-house facilities under an established framework of international and national standards, including ASTM G1, ASTM G3, ASTM G5, ASTM G31, ASTM G59, ASTM G61, ASTM G102, ASTM G106, ASTM B117, ASTM B368, ASTM D1141, ASTM E2109, ISO 8407, ISO 9227, ISO 10289, ISO 11130, ISO 17475, ISO 11845, GB/T 10125, and GB/T 16545. Results are reported as penetration rates in mm/a, µm/a, or mils per year (mpy), or as mass-loss rates in g/(m²·h), together with electrochemical parameters such as corrosion potential (Ecorr), corrosion current density (icorr), polarization resistance (Rp), and charge-transfer resistance (Rct).
Testing Samples
Cold-Sprayed Copper Coatings on Substrates
Flat coupons, commonly 50 mm × 25 mm with copper layers of 0.3–3 mm deposited onto aluminum, copper, or steel substrates, are the standard format for immersion, salt spray, and electrochemical testing. Coupons may be tested in the as-sprayed condition to reflect real surface topography, or machined and polished to a defined finish when comparison with wrought reference copper is required.
Free-Standing Deposits and Additively Manufactured Parts
Thick deposits machined into tab or disc specimens, and sectioned coupons from additively manufactured cold-spray copper components, are used to evaluate the intrinsic corrosion behavior of the deposit itself without any substrate influence. These specimens are particularly valuable for studying the effect of porosity and particle-boundary density on the corrosion rate.
Finished Components and Product Sections
Busbar segments, connector housings, heat-exchanger elements, repaired castings, and other end-use parts can be tested as received. Where full parts are too large, representative sections are cut, edges deburred, and cut faces masked or re-coated so that only the engineered surfaces are exposed to the corrosive medium.
Specimen Preparation and Documentation
- Degrease in acetone or ethanol, dry, and weigh to ±0.1 mg on an analytical balance per ASTM G1 and ASTM G31.
- Measure and record exposed surface area; mask non-relevant surfaces and substrate edges with chemically resistant lacquer, unless deliberate substrate exposure is part of the test plan.
- Prepare a minimum of three replicate specimens per condition; document feedstock powder type (electrolytic, gas-atomized, or water-atomized), carrier gas, porosity level per ASTM E2109, and any post-spray heat treatment.
- For electrochemical tests, mount specimens in epoxy with an exposed area of 0.5–1 cm², attach an electrical connection, and prepare metallographic surfaces per ASTM E3 where required.
Testing Items
Typical corrosion rate determination programs for cold-sprayed copper parts cover the following evaluation items:
- Gravimetric corrosion rate by immersion weight-loss testing (mm/a, µm/a, mpy, g/(m²·h))
- Electrochemical corrosion rate by potentiodynamic polarization and Tafel extrapolation (Ecorr, icorr, anodic and cathodic Tafel slopes)
- Linear polarization resistance (Rp) for rapid, low-destructive rate monitoring
- Electrochemical impedance spectroscopy (Rct, coating/deterioration behavior over time)
- Neutral salt spray (NSS), acetic acid salt spray (ASS), and copper-accelerated acetic acid salt spray (CASS) resistance with appearance rating
- Pitting and localized corrosion susceptibility by cyclic potentiodynamic polarization
- Galvanic corrosion coupling with aluminum or steel substrates per ASTM G71 and ASTM G82
- Particle-boundary and porosity-related attack by cross-sectional metallography and microscopy
- Identification and quantification of corrosion products such as cuprite (Cu2O), nantokite (CuCl), and atacamite (Cu2(OH)3Cl)
Testing Methods
Gravimetric Immersion Testing
Immersion testing per ASTM G31 and ISO 11130 exposes weighed coupons to a controlled electrolyte, most commonly aerated 3.5 wt.% sodium chloride solution or substitute ocean water prepared per ASTM D1141, at 25 ± 2 °C for durations from 24 hours to 30 days or longer. After exposure, corrosion products are removed chemically in accordance with ASTM G1 and ISO 8407 cleaning procedures, and the specimens are reweighed. The corrosion rate is calculated as: CR (mm/a) = 8.76 × 10⁴ × W / (A × T × D), where W is mass loss in grams, A is area in cm², T is time in hours, and D is density in g/cm³. For pure copper D is 8.96 g/cm³, but the actual density of the cold-sprayed deposit, corrected for porosity, should be used for accurate results.
Salt Spray Exposure Testing
Spray cabinet testing per ASTM B117, ISO 9227, and GB/T 10125 subjects specimens to a 5 wt.% NaCl fog at 35 °C with a collection rate of 1–2 mL/h per 80 cm² and pH of 6.5–7.2 for standard durations of 24, 48, 96, 240, 480, or 1000 hours. Accelerated CASS testing per ASTM B368 uses NaCl with cupric chloride and acetic acid at pH 3.1–3.3 and 49 °C for more aggressive screening. Specimens are evaluated for white or red corrosion products, blistering, undercutting at scribes, and weight loss, with appearance ratings assigned per ASTM D1654 and ISO 10289.
Potentiodynamic Polarization and Tafel Extrapolation
In a three-electrode cell per ASTM G5 and ISO 17475, the cold-sprayed copper specimen serves as the working electrode, a saturated calomel or Ag/AgCl electrode as the reference, and platinum or graphite as the counter electrode. After allowing the open-circuit potential to stabilize for 30–60 minutes, a potentiodynamic scan is run from approximately 250 mV below to 250 mV above the corrosion potential at a scan rate of 0.1667 mV/s. Tafel extrapolation of the anodic and cathodic branches over the range of 50–150 mV from Ecorr yields the corrosion current density, which is converted to a penetration rate using Faraday's law per ASTM G102: CR (µm/a) = 3.27 × icorr × EW / D, where icorr is in µA/cm², EW is the equivalent weight (31.77 g/eq for Cu dissolving as Cu²⁺), and D is 8.96 g/cm³; thus an icorr of 1 µA/cm² corresponds to roughly 11.6 µm/a.
Linear Polarization Resistance
LPR testing per ASTM G59 applies a small sweep of ±10 to ±20 mV around the open-circuit potential at 0.1 mV/s. The slope of the potential–current response gives the polarization resistance Rp, from which the corrosion current is estimated using the Stern–Geary relationship, icorr = B / Rp, where B = βa·βc / (2.303 × (βa + βc)). For copper in neutral chloride solutions B typically falls between 20 and 30 mV. Because the perturbation is small, LPR allows repeated, nearly non-destructive rate measurements on the same specimen over time.
Electrochemical Impedance Spectroscopy
EIS per ASTM G106 practice applies a sinusoidal perturbation of typically 10 mV at the open-circuit potential over a frequency range of 100 kHz down to 10 mHz. Fitting the resulting spectra to an equivalent circuit yields the charge-transfer resistance and solution resistance; a decreasing Rct over successive immersion intervals indicates a rising corrosion rate or degradation of protective surface films, making EIS an excellent tool for time-dependent monitoring of cold-sprayed copper.
Post-Test Characterization and Reporting
Following exposure, specimens are examined visually and microscopically. Corrosion products are identified by X-ray diffraction, surface and cross-sectional morphology is documented by scanning electron microscopy with energy-dispersive spectroscopy, pitting depths are measured on sectioned metallographic mounts, and porosity-related attack along particle boundaries is quantified by image analysis per ASTM E2109. A compliant test report states the standards applied, specimen condition and history, electrolyte chemistry and temperature, exposure duration or electrochemical parameters, individual and mean corrosion rates, appearance ratings, and photographic documentation.
Applications
Corrosion rate determination of cold-sprayed copper parts supports qualification, quality control, and design decisions across many industries:
- Marine and subsea electrical systems, where conductive copper layers sprayed onto aluminum or steel housings, connectors, and busbars must resist seawater and chloride-laden atmospheres.
- Power transmission and distribution, including high-current busbars, switchgear contacts, and grounding components exposed to industrial and coastal environments.
- Aerospace and propulsion, where cold-sprayed copper-alloy liners and heat-exchanger passages are evaluated for coolant-side degradation.
- Rail transportation, for current-carrying components, earthing straps, and pantograph-related hardware exposed to rain, humidity, and winter deicing salts.
- Repair and remanufacturing of worn copper and bronze bearings, impellers, and valve components, verifying restored parts in actual service fluids.
- Electronics and electromagnetic shielding, confirming the durability of conductive copper layers under humidity and salt-mist conditions.
- Research and development, benchmarking powder types, carrier gases, spray parameters, porosity levels, and post-spray heat treatments against wrought reference copper such as C11000.
FAQ
What corrosion rate is typical for cold-sprayed copper in chloride media?
Wrought copper in aerated 3.5% NaCl typically shows rates on the order of a few to a few tens of µm/a, falling as protective cuprite films mature. Dense cold-sprayed copper with porosity below about 1% behaves comparably, while higher porosity or heavy particle-boundary oxidation raises the apparent rate and shifts attack toward localized boundary penetration. Actual values always depend on electrolyte, aeration, temperature, and exposure time, so project-specific testing is essential.
Why can mass-loss and electrochemical corrosion rates differ?
Gravimetry integrates all material removed or retained as adherent products over the whole exposure, while electrochemical methods measure the instantaneous faradaic dissolution current, which excludes non-faradaic product spallation and can change as films form. Agreement within a factor of about two is common; discrepancies usually point to film growth or non-uniform localized attack.
Should specimens be tested as-sprayed or machined?
Both are informative. As-sprayed surfaces reproduce service roughness and open porosity; machined and polished surfaces eliminate roughness variables and enable direct comparison with wrought reference copper. Many programs test both conditions to separate intrinsic deposit behavior from surface topography effects.
How is corrosion through porosity to the substrate detected?
Interconnected porosity is revealed by salt spray or immersion testing of unmasked cross-sections and scribed coatings, followed by cross-sectional metallography to measure undercutting and galvanic attack of the substrate. Galvanic current measurements per ASTM G71 and galvanic series construction per ASTM G82 quantify the coupling effect between copper and the substrate alloy.
How long does corrosion rate determination take?
Potentiodynamic, LPR, and EIS tests are typically completed within one day per specimen set. Salt spray exposures run from 24 to 1000 hours depending on the specification, and gravimetric immersion programs usually require 7 to 30 days to obtain stable, statistically meaningful rates from at least three replicates.
Can laboratory results predict service life?
Laboratory rates provide comparative, qualification-level data. For service-life estimation they are combined with atmospheric corrosivity classification per ISO 9223 and guiding corrosion values per ISO 9224, or with environment-specific data such as ISO 11306 for high-temperature aqueous service, always recognizing that chloride concentration, wet-dry cycling, and flow conditions in the field differ from controlled tests.