Copper salts accelerate the acetic acid salt spray test

Copper Salt Accelerated Acetic Acid Salt Spray Test (CASS Test) – Accredited ISO/IEC 17025 Corrosion Resistance Assessment for the Croatian Market

The Copper Salt Accelerated Acetic Acid Salt Spray Test, commonly known as the CASS test, is a highly accelerated corrosion test method used to evaluate the corrosion resistance of decorative and protective coatings on metallic substrates, particularly electroplated coatings (such as copper‑nickel‑chromium, copper‑nickel, and zinc‑based coatings), as well as anodized aluminum, stainless steels, and other corrosion‑resistant surface treatments. The CASS test is more aggressive than the conventional neutral salt spray test (NSS) due to the addition of copper chloride (CuCl₂) and acetic acid (CH₃COOH), which lower the pH and introduce copper ions that accelerate the electrochemical corrosion process. This method is essential for predicting the long‑term corrosion performance of components used in automotive, aerospace, marine, architectural, and consumer goods applications, where exposure to aggressive environments (such as coastal atmospheres, road de‑icing salts, and industrial pollutants) is common. In the Croatian market, where the Hrvatski zavod za norme (HZN), the Ministarstvo gospodarstva i održivog razvoja, the Državni inspektorat, and the Carinska uprava enforce strict quality, durability, and environmental standards aligned with EU directives and HRN EN (Croatian standards based on European norms), the accurate evaluation of corrosion resistance using the CASS test is essential for product certification, supplier qualification, quality control in manufacturing, and import‑export processes. Our laboratory offers a comprehensive CASS testing service, applying standardized methods such as ASTM B368, ISO 9227, and HRN EN ISO 9227 to simulate severe corrosive conditions and to evaluate the performance of coatings and surface treatments. All tests are performed under our ISO/IEC 17025 (CNAS) accreditation, and the resulting reports are fully accepted by Croatian authorities and notified bodies, making them indispensable for regulatory compliance, product validation, and market access in Croatia and the European Union.

Test Samples and Coated Components We Regularly Examine

Our laboratory receives a wide variety of coated metal components and materials for CASS testing. Typical samples include:

• Electroplated components – copper‑nickel‑chromium (Cu‑Ni‑Cr) plated parts, nickel‑chromium (Ni‑Cr) plated parts, and zinc‑based (Zn‑Ni, Zn‑Fe) plated components for automotive, plumbing, and hardware applications.
• Anodized aluminum and aluminum alloys – with various anodizing thicknesses and sealing treatments for architectural and decorative applications.
• Stainless steel and corrosion‑resistant alloys – for marine and chemical industry applications.
• Decorative and protective coatings – paints, lacquers, and powder coatings on metal substrates.
• Fasteners and hardware – screws, bolts, nuts, hinges, and locks with protective coatings.
• Automotive and motorcycle components – trim parts, bumpers, wheels, and exhaust components.
• Prototype and new coating formulations – submitted by manufacturers for validation of corrosion resistance before series production.
• Field‑retrieved coated components – for failure analysis and remaining life assessment.

Test Principle and Standard Methods – Copper Salt Accelerated Acetic Acid Salt Spray

The CASS test is performed in a salt spray chamber where the test specimens are exposed to a continuous fog of a solution containing 5 % sodium chloride (NaCl), 0.26 % copper chloride (CuCl₂·2H₂O), and acetic acid (to adjust the pH to 3.1–3.3). The test is conducted at a chamber temperature of 50 °C ± 1 °C, which is higher than the standard NSS test (35 °C). The copper ions and the acidic environment significantly accelerate the corrosion process, making the CASS test particularly suitable for evaluating decorative coatings that must withstand severe corrosive conditions.

• Copper salt accelerated acetic acid salt spray test (ASTM B368 / ISO 9227 / HRN EN ISO 9227 / NTC 5600 – for accelerated corrosion testing) – we place the test specimens in a salt spray chamber and expose them to a continuous atomized fog of the CASS solution at 50 °C. The test duration is specified by the product standard or the customer (e.g., 24, 48, 96, 144, 240, 480, or 720 hours). After the exposure, the specimens are removed, rinsed with deionized water, and dried. They are then inspected for corrosion products (rust, pits, blistering), the extent of corrosion, and the loss of coating integrity. We report the test duration, the pH of the collected solution, the chamber temperature, the visual appearance (rating according to ASTM D610, ASTM D714, or other specified standards), and the pass/fail status.
• CASS test for electroplated coatings (NTC 5601 – for Cu‑Ni‑Cr and Ni‑Cr coatings) – we perform the CASS test on electroplated components to evaluate the porosity, the adhesion, and the overall corrosion resistance of the plating system. The performance is rated according to the time to the first corrosion (the appearance of red rust or nickel corrosion). We report the time to first corrosion, the corrosion rating, and the classification.
• CASS test for anodized aluminum (ASTM B368 – variant for anodized aluminum, NTC 5602) – we expose anodized aluminum specimens to the CASS test for a specified duration (e.g., 48, 96, or 240 hours). The test evaluates the resistance of the anodic coating to pitting and the sealing quality. We report the pitting density, the maximum pit depth, and the rating.
• CASS test for stainless steels (NTC 5603 – for evaluating pitting resistance) – we use the CASS test to evaluate the pitting resistance of stainless steels and to detect any susceptibility to chloride‑induced corrosion. The test is performed for a specified duration, and the specimens are inspected for pitting. We report the pit count, the pit depth, and the pass/fail status.
• CASS test for powder coatings and paints (NTC 5604 – for organic coatings) – we use the CASS test to evaluate the corrosion protection provided by powder coatings and paints on metal substrates. The test is performed for a specified duration, and the specimens are inspected for blistering, delamination, and corrosion creepage from a scribe line (if a scribe is applied). We report the blister rating, the creepage distance, and the condition of the coating.

Test Conditions and Quality Control – Ensuring Accurate and Reproducible Results

The accuracy and reproducibility of the CASS test depend on the strict control of the test conditions, including the solution pH, the temperature, the salt concentration, and the solution collection rate. Our procedures follow the requirements of ASTM B368 and ISO 9227, and we perform regular quality control checks on the test equipment.

• Solution preparation and pH control (NTC 5610 – for the test solution quality) – we prepare the CASS solution by dissolving 5 % NaCl and 0.26 % CuCl₂·2H₂O in deionized water, and then we adjust the pH to the range of 3.1–3.3 by adding glacial acetic acid. The pH is measured using a calibrated pH meter before and during the test. We report the pH of the solution and the collection rate.
• Temperature control and chamber calibration (NTC 5611 – for the test environment) – we maintain the chamber temperature at 50 °C ± 1 °C using a calibrated thermostat. The temperature is monitored continuously, and the chamber is calibrated regularly using a reference thermometer. We report the chamber temperature and the calibration status.
• Fog collection rate (NTC 5612 – for the solution deposition rate) – we measure the amount of solution collected from a specified area (typically 80 cm²) over a 16‑hour period. The collection rate must be between 1.0 mL/h and 2.0 mL/h per 80 cm², and the pH of the collected solution must be in the range of 3.1–3.3. We report the collection rate and the pH of the collected solution.
• Air pressure and atomization (NTC 5613 – for the fog quality) – we control the air pressure to the atomizer to produce a fine, uniform fog. The air is filtered and humidified before entering the atomizer. We report the air pressure and the fog quality.
• Specimen placement and orientation (NTC 5614 – for the reproducibility of the test) – we place the specimens in the chamber at an angle of 15° to 30° from the vertical, to allow the fog to condense on the surface and to ensure uniform exposure. The specimens are spaced to avoid contact and to prevent cross‑contamination. We report the specimen placement and orientation.

Evaluation and Rating of Corrosion Damage – Visual and Microscopic Inspection

The corrosion damage is evaluated by visual inspection, and by microscopic examination when required. The evaluation is performed according to the rating standards (e.g., ASTM D610 for rust, ASTM D714 for blistering, and ISO 10289 for the rating of the protection and the appearance).

• Visual inspection and rating (ASTM D610 / NTC 5620 – for the rust rating) – we inspect the surface of the specimen for the presence of rust (red rust for steel, white rust for zinc, or corrosion products for other metals). The rust rating (from 10 = no rust to 0 = completely rusted) is assigned according to ASTM D610. We report the rust rating and the condition of the specimen.
• Blistering rating (ASTM D714 / NTC 5621 – for the coating blistering) – we inspect the surface for the presence of blisters (if the coating is organic or plated). The blistering is rated according to the size (density) and the frequency. We report the blistering rating and the blister density.
• Microscopic examination (NTC 5622 – for the pitting and the crack detection) – we use an optical microscope or a scanning electron microscope (SEM) to examine the specimens for pitting, cracks, and any other damage. The pit depth is measured, and the pit density is counted. We report the pit depth, the pit density, and the SEM images.
• Measurement of the corrosion creepage (NTC 5623 – for the scribe creepage) – for coated specimens with a scribe line, we measure the distance from the scribe line to the nearest corrosion blister or delamination. The creepage distance (in mm) is reported. We report the creepage distance and the corrosion rating.
• Cross‑section analysis (NTC 5624 – for the evaluation of the coating integrity) – we prepare a cross‑section of the specimen and examine the coating, the substrate, and the interface for any attack. The thickness of the coating and the depth of the corrosion are measured. We report the coating thickness, the corrosion depth, and the cross‑section images.

Factors Affecting the CASS Test Results – Material, Coating, and Test Variables

The results of the CASS test can be influenced by the test variables, the material properties, and the coating characteristics. Our tests evaluate the effect of these factors, and we provide guidance for the interpretation of the results.

• Effect of coating thickness (NTC 5630 – for the thickness‑performance relationship) – we perform the CASS test on specimens with different coating thicknesses (e.g., 5 μm, 10 μm, 20 μm) to evaluate the relationship between the coating thickness and the corrosion resistance. We report the corrosion rating and the thickness‑performance curve.
• Effect of the substrate material (NTC 5631 – for the substrate compatibility) – we test the same coating on different substrates (e.g., steel, aluminum, brass) to evaluate the effect of the substrate on the corrosion performance. We report the corrosion rating for each substrate.
• Effect of the pre‑treatment and the surface finish (NTC 5632 – for the surface preparation) – we test specimens with different surface finishes (e.g., polished, bead‑blasted, and chemically etched) to evaluate the effect of the surface preparation on the adhesion and the corrosion resistance. We report the corrosion rating and the adhesion rating.
• Effect of the test duration (NTC 5633 – for the long‑term corrosion prediction) – we perform the CASS test for different durations (e.g., 24, 48, 96, 240 hours) to evaluate the progression of the corrosion and to predict the long‑term performance. We report the corrosion rating as a function of the test duration.
• Correlation with the service life (NTC 5634 – for the real‑world performance) – we use the CASS test results to estimate the service life of the coating in a specific environment (e.g., coastal, industrial, or automotive). The correlation is based on field data and the established relationships between the accelerated test and the real‑world exposure. We report the predicted service life and the confidence interval.

Complementary Tests – Coating Thickness, Adhesion, and Microstructure for Corrosion Correlation

To provide a comprehensive assessment of the corrosion performance and to understand the mechanisms of failure, we complement the CASS test with coating thickness measurement, adhesion testing, and microstructural examination.

• Coating thickness measurement (ASTM B499 / NTC 5640 – for the non‑destructive measurement) – we measure the thickness of the coating (in μm) using a magnetic induction gauge (for steel substrates) or an eddy‑current gauge (for aluminum and non‑ferrous substrates). The thickness is measured at multiple points on the specimen. We report the average thickness, the minimum thickness, and the thickness variation.
• Adhesion test (ASTM D3359 / NTC 5641 – for the coating adhesion) – we perform a cross‑cut adhesion test on the coating (before and after the corrosion test) to evaluate the adhesion of the coating to the substrate. A loss of adhesion indicates a corrosion‑induced failure. We report the adhesion rating and the change in the adhesion.
• Microstructural examination (ASTM E3 / NTC 5642 – for the coating and the interface) – we examine the microstructure of the coating (the grain size, the phase distribution, and the porosity) and the coating‑substrate interface using an optical microscope and a SEM. The microstructure is correlated with the corrosion resistance. We report the microstructural observations and the correlation.
• Hardness testing (ASTM E18 / NTC 5643 – for the coating hardness) – we measure the hardness of the coating (e.g., the microhardness of the electroplated layer) to verify the quality of the coating and to correlate it with the corrosion resistance. We report the hardness and the correlation.
• Chemical composition analysis (EDS – NTC 5644 – for the coating composition) – we use energy‑dispersive spectroscopy (EDS) to analyze the composition of the coating (the alloying elements and the impurities). The composition is correlated with the corrosion resistance. We report the composition and the correlation.

Test Report and Recognition in the Croatian Automotive, Marine, and Manufacturing Sector

All procedures described are within the scope of our ISO/IEC 17025 accreditation, with equipment calibrated periodically (salt spray chambers, pH meters, thermometers, and thickness gauges) and traceability to international standards (NIST, PTB). Our test reports are issued in English (with a Croatian summary available on request) and include:

• Full identification of the specimen (material, coating type, thickness, and manufacturer).
• Detailed description of the test methods applied (ASTM/ISO/HRN EN/NTC standards, test duration, pH, and temperature).
• Numerical results: rust rating (10 to 0), blistering rating, pit depth (μm), creepage distance (mm), coating thickness (μm), adhesion rating, and hardness (HV).
• Graphical data: corrosion progression over time, and photographs of the specimens before and after the test.
• Comparative tables against the values specified by the client or against the limits of the relevant standards (ASTM B368, ISO 9227, HRN EN ISO 9227, and the requirements of the HZN, Ministarstvo gospodarstva, and Državni inspektorat).
• Photographs and micrographs (SEM) of the specimen surface, the corrosion pits, and the coating‑substrate interface.
• Recommendations for material selection, coating optimization, and surface preparation to improve the corrosion resistance and the service life.
• Expanded uncertainty (k=2) for all key measurements, calculated according to the ISO/IEC 98‑3 Guide.

These reports are fully accepted by the Hrvatski zavod za norme (HZN) for the verification of product conformity, by the Ministarstvo gospodarstva i održivog razvoja for industrial and environmental compliance, by the Državni inspektorat for market surveillance, and by the Carinska uprava (Croatian Customs) for tariff classification and quality verification in the import of coated and plated components. Additionally, we offer consulting services for the selection of corrosion‑resistant coatings, the design of durable surface treatments, and the implementation of quality control programs for corrosion performance, contributing to the safety, reliability, and longevity of products in the diverse and growing Croatian market, from the automotive and marine industries to the construction and consumer goods sectors.