Electrochemical Corrosion Testing of Cold-Sprayed Copper Components

Electrochemical corrosion testing of cold-sprayed copper components is a specialized evaluation used to quantify the corrosion behavior, durability, and service reliability of copper deposits produced by cold gas dynamic spraying. Using techniques such as open circuit potential monitoring, potentiodynamic polarization, linear polarization resistance, and electrochemical impedance spectroscopy, accredited third-party laboratories can measure corrosion potential, corrosion current density, corrosion rate, and localized corrosion susceptibility in accordance with ASTM and ISO standards. This article explains the sample types, testing items, methods, data interpretation, and industrial applications relevant to corrosion assessment of cold-sprayed copper.

Technical Overview

Cold spray, also known as cold gas dynamic spraying or kinetic spraying, is a solid-state deposition process in which copper powder is accelerated to supersonic velocities (typically 500–1000 m/s) by a heated high-pressure nitrogen or helium gas stream. Because the process temperature remains well below the melting point of copper, particles bond to the substrate through severe plastic deformation rather than melting, producing dense deposits with low oxidation, high electrical conductivity, and minimal thermal distortion. However, the resulting microstructure differs fundamentally from wrought copper: it contains inter-particle boundaries, residual porosity, work-hardened grains, and occasionally embedded oxide films, all of which can influence electrochemical corrosion behavior.

Electrochemical corrosion measurement provides a quantitative, fast, and highly sensitive means of evaluating how these microstructural features affect performance. Unlike conventional salt spray or immersion exposure testing, which may require days or weeks to produce measurable degradation, electrochemical methods such as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) can determine corrosion current density and polarization resistance within hours, with sensitivity at the micrometer-per-year level. This makes electrochemical testing particularly valuable for process optimization of cold spray parameters, qualification of coatings for conductive and corrosion-critical applications, and comparison of cold-sprayed copper against wrought and annealed reference materials.

Testing Samples

As-Sprayed Copper Coatings on Metallic Substrates

The most common test specimens are copper coatings deposited by cold spray onto engineering substrates such as aluminum alloys, carbon steel, stainless steel, or existing copper components. These samples are evaluated for intrinsic coating corrosion behavior and, where coating porosity provides electrolyte pathways, for galvanic interaction between the copper layer and the underlying substrate. Typical coating thicknesses submitted for testing range from 0.3 mm to several millimeters, and specimens are usually sectioned into coupons with a defined exposed test area, commonly 1 cm², using epoxy mounting to mask edges and substrate contact.

Free-Standing Cold-Sprayed Copper Deposits

For cold spray additive manufacturing and repair applications, free-standing copper deposits are produced by spraying beyond the required thickness and machining away the substrate, or by depositing onto removable mandrels. These specimens allow intrinsic corrosion properties of the deposit to be measured without substrate or galvanic interference, enabling direct comparison with wrought copper grades such as C11000 electrolytic tough pitch copper.

Post-Processed and Heat-Treated Specimens

Cold-sprayed copper is frequently subjected to post-deposition heat treatment (for example, 300–600 °C annealing) to reduce porosity, relieve residual stress, and heal inter-particle boundaries. Both as-sprayed and heat-treated coupons are commonly submitted in parallel so that laboratories can quantify the improvement in corrosion resistance, typically expressed as a decrease in corrosion current density and an increase in charge transfer resistance, achieved by the thermal treatment.

Surface Preparation Conditions

Two surface conditions are typically evaluated: the as-sprayed surface, which reflects the real service condition with its native roughness and oxide content, and a metallographically prepared surface, wet-ground to 1200–2000 grit or polished to 1 μm, which isolates microstructural effects from surface topography. Both conditions are useful; comparative testing reveals whether surface oxides introduced during spraying act as protective barriers or as sites of localized attack.

Testing Items

Electrochemical corrosion evaluation of cold-sprayed copper components typically encompasses the following measurable parameters:

• Open circuit potential (OCP) and corrosion potential (Ecorr), indicating the thermodynamic tendency of the deposit to corrode relative to a reference electrode.
• Corrosion current density (Icorr), determined by Tafel extrapolation, and the derived uniform corrosion rate in mm/year or μm/year.
• Anodic and cathodic Tafel slopes (βa and βc), describing the reaction kinetics of copper dissolution and oxygen or hydrogen reduction.
• Polarization resistance (Rp) via linear polarization resistance (LPR) measurement, calculated using the Stern–Geary relationship.
• Charge transfer resistance (Rct), solution resistance (Rs), and constant phase element (CPE) parameters extracted from EIS equivalent circuit fitting.
• Pitting potential (Epit), repassivation potential (Erep), and hysteresis behavior from cyclic potentiodynamic polarization, quantifying susceptibility to localized chloride attack.
• Galvanic current and couple potential between cold-sprayed copper and coupled substrates such as aluminum or steel, per ASTM G71.
• Effect of exposure time on impedance response, tracking electrolyte ingress through interconnected porosity.

Testing Methods

Test Cell and Electrode Configuration

Measurements are performed in a standard three-electrode electrochemical cell. The cold-sprayed copper coupon serves as the working electrode with a precisely defined exposed area; a saturated calomel electrode (SCE) or Ag/AgCl reference electrode controls the potential; and a platinum mesh or graphite rod acts as the counter electrode for current passage. Testing is conducted per the general conventions of ASTM G3 and the potentiodynamic practices of ASTM G5 and ASTM G59, with ISO 17475 providing complementary guidance on electrochemical corrosion test methodology.

Test Electrolytes

The choice of electrolyte reflects the intended service environment. Neutral 3.5 wt.% sodium chloride solution is the most widely used medium for general and marine service assessment, while synthetic seawater prepared per ASTM D1141 is used for offshore and shipboard applications. Acidic media such as 0.5 M sulfuric acid or dilute hydrochloric acid may be specified for industrial atmospheric or chemical exposure scenarios. Test temperature is typically controlled at 23 ± 2 °C with naturally aerated solution, and dissolved oxygen content is recorded since cathodic oxygen reduction strongly governs copper corrosion kinetics.

Open Circuit Potential Stabilization

Before polarization, specimens are immersed until the OCP stabilizes, generally requiring 30 to 60 minutes and defined as a drift of less than 1 mV/min. Extended OCP monitoring over 24 hours may be performed to capture the evolution of surface oxide films on the as-sprayed surface.

Potentiodynamic Polarization and Tafel Analysis

Potentiodynamic scans are initiated at 200–250 mV below the stabilized OCP and swept anodically at 0.1667 mV/s in accordance with ASTM G5. Tafel extrapolation of the linear portions of the anodic and cathodic branches yields Ecorr, Icorr, and the Tafel constants. The corrosion rate is then calculated using the relationship CR (mm/year) = 3.27 × 10⁻³ × Icorr (μA/cm²) × EW / ρ, where the equivalent weight EW is 31.7 g/equivalent for copper dissolving as Cu²⁺ and ρ is 8.96 g/cm³. For pitting evaluation, anodic scans are extended above the OCP until the breakdown potential is reached, and cyclic polarization per ASTM G61 determines repassivation behavior and hysteresis loop area.

Linear Polarization Resistance

LPR measurements per ASTM G59 apply a small perturbation, typically ±10 mV or ±20 mV relative to OCP, at a low scan rate. The resulting polarization resistance, combined with the Tafel constants through the Stern–Geary equation (B = βa·βc / [2.303(βa + βc)]), provides a rapid, low-disturbance corrosion rate suitable for time-series monitoring of the same specimen.

Electrochemical Impedance Spectroscopy

EIS is performed at the OCP with a sinusoidal perturbation amplitude of 5–10 mV (RMS) over a frequency range of 100 kHz down to 10 mHz, with 5–10 points per decade. Nyquist and Bode plots are fitted to equivalent circuits such as Rs(Rct·CPE) or models with a second time constant representing porous deposit behavior, following the validation principles of ASTM G106. For cold-sprayed copper, depressed semicircles and additional mid-frequency time constants often indicate electrolyte penetration along inter-particle boundaries, and progressive decreases in Rct over immersion time reveal interconnected porosity.

Galvanic Corrosion Testing

When copper is deposited onto aluminum or steel substrates, galvanic current density and couple potential are measured with a zero-resistance ammeter per ASTM G71, controlling the exposed area ratio between coating and substrate. This identifies whether coating porosity permits detrimental galvanic couples to form at defect sites.

Complementary Confirmation Testing

Electrochemical results are frequently corroborated with gravimetric immersion testing per ASTM G31, neutral salt spray exposure per ASTM B117 and ISO 9227, and post-test surface and cross-sectional examination by optical microscopy, SEM, and EDS to identify attack at particle boundaries, pores, and coating defects.

Applications

Electrochemical corrosion testing of cold-sprayed copper supports qualification and quality control across a broad range of industries and uses:

• Thick conductive copper layers deposited on aluminum heat sinks, busbars, and power electronics substrates, where long-term corrosion directly affects contact resistance and current-carrying capacity.
• Cold spray repair and dimensional restoration of cast copper alloy components, marine hardware, and electrical connectors, verifying that repaired regions match the corrosion performance of the parent material.
• Railway and traction current-carrying components, contact pads, and grounding elements exposed to chloride-rich and polluted atmospheres.
• Cold spray additive manufactured copper parts for aerospace, defense, and marine systems requiring corrosion qualification alongside mechanical and electrical verification.
• EMI shielding and corrosion barrier layers on magnesium and aluminum structures, where the copper layer must not initiate galvanic degradation of the substrate.
• Heat exchanger and thermal management components operating in aqueous coolants, for which polarization and EIS data feed into coolant compatibility assessments.
• Process development studies in which gas pressure, gas temperature, powder type, and heat treatment parameters are optimized using corrosion current density and charge transfer resistance as quantitative response metrics.

FAQ

Why use electrochemical testing instead of salt spray testing for cold-sprayed copper?

Electrochemical methods are far more sensitive and quantitative than salt spray exposure. They provide numerical values of corrosion potential, corrosion current density, and corrosion rate within hours, can detect subtle differences between as-sprayed and heat-treated conditions, and directly reveal localized corrosion mechanisms such as pitting and inter-particle boundary attack that may not be visually apparent after lengthy salt spray exposure. Salt spray testing remains a useful complementary method but is qualitative and poorly correlated to real service life.

What do elevated corrosion current density values indicate for a cold-sprayed deposit?

A high Icorr typically points to interconnected porosity, poor inter-particle bonding, or surface oxides that accelerate anodic dissolution of copper. In practice, a marked improvement in Icorr and Rct after annealing demonstrates that heat treatment has densified the deposit and healed particle boundaries, confirming process improvements.

Which electrolyte should be selected for testing?

Neutral 3.5 wt.% NaCl is the standard default for chloride-containing environments and marine exposure. Synthetic seawater per ASTM D1141 suits offshore applications, while dilute acids are used to simulate industrial chemical atmospheres. The electrolyte should always replicate the chloride concentration and pH of the intended service environment.

How is corrosion rate calculated from the measured current?

Corrosion rate is obtained by converting the Tafel-derived corrosion current density through Faraday's law: CR (mm/year) = 3.27 × 10⁻³ × Icorr (μA/cm²) × 31.7 / 8.96 for copper, assuming divalent dissolution. Accredited third-party laboratories report both Icorr and the converted rate together with the Tafel constants used in the calculation.

Can electrochemical testing detect porosity in cold-sprayed copper?

Yes. EIS is particularly effective: interconnected porosity and weak particle boundaries appear as additional time constants in the impedance spectrum and as a progressive decline in charge transfer resistance during extended immersion. Cyclic polarization can additionally reveal preferential localized attack initiating at pore sites.

What acceptance criteria are typically applied?

Acceptance limits are project-specific and usually set by comparison against wrought copper reference material tested under identical conditions, or by drawing-based requirements on maximum corrosion current density, minimum polarization resistance, and the absence of galvanic attack at coating defects. Where localized corrosion is a concern, the requirement that the pitting potential exceed the maximum anticipated service potential is commonly imposed.