Copper-Accelerated Acetic Acid Salt Spray (CASS) Test: Method & Guide
Corrosion is one of the most pervasive and costly challenges in materials engineering. From vehicle body panels to aerospace structural components, protecting metals and coatings from corrosive degradation is an engineering imperative. The Copper Accelerated Salt Spray (CASS) test is one of the most aggressive and revealing accelerated corrosion tests available — and it is a standard qualification tool for electrodeposited decorative coatings, aluminum alloys, and chromium-plated components.
What Is the CASS Test?
The Copper Accelerated Salt Spray test (standardized in ASTM B368) is an accelerated corrosion testing method that evaluates the corrosion resistance of metallic materials, coatings, and surface treatments under highly aggressive corrosive conditions. It is a modified version of the conventional salt spray (fog) test (ASTM B117), enhanced by the addition of acetic acid and copper sulfate to the test solution to significantly intensify the corrosive attack.
CASS testing is particularly valuable for:
- Aluminum alloys — including anodized aluminum surfaces
- Chromium plating on steel and zinc die castings
- Nickel/chromium and copper/nickel/chromium electrodeposited decorative coatings — for which the test was originally developed
The corrosive acceleration effect is dramatic: CASS testing at 50°C is reported to reproduce approximately eight times the corrosion rate of conventional neutral salt spray testing at the same elapsed time — making 24 hours of CASS exposure broadly equivalent to approximately 8 years of natural outdoor exposure for certain material-coating combinations.
The CASS Test Solution and Conditions
The CASS test solution is prepared as follows:
- A standard ASTM B117 salt solution is prepared: 95 parts water to 5 parts sodium chloride (NaCl) by weight
- Reagent-grade copper chloride dihydrate is added at a rate of 1 gram per 4 liters of salt solution, introducing the accelerating copper ions
- The pH of the solution is adjusted to 3.1 to 3.3 by adding reagent-grade glacial acetic acid — making the solution significantly more acidic than the neutral salt spray
Test temperature: The exposure zone is maintained at 49°C ± 1°C (approximately 120°F) — significantly higher than the 35°C used in conventional NSS testing, further intensifying the corrosive attack.
Exposure duration: CASS test exposures are limited to a maximum of 48 hours due to the aggressive nature of the test solution. Longer exposures may be specified in specific product standards.
The test apparatus is a continuous spray (fog) chamber — the CASS solution is atomized and maintained as a fog throughout the test chamber for the specified duration.
What CASS Testing Evaluates
CASS testing assesses several critical performance parameters:
Resistance to corrosion penetration — how effectively the coating system prevents corrosive species from reaching the substrate.
Corrosion of the base material — quantifying the extent of pitting, blistering, or general corrosion visible after exposure.
Coating integrity after exposure — adhesion loss, blistering, discoloration, and dimensional change of the coating following corrosive attack.
Comparison of coating systems — CASS testing enables direct performance comparison of different coating processes, thicknesses, or chemistries under identical conditions.
Applications and Industry Uses
CASS testing is applied across several key industries:
Automotive — chrome-plated and anodized trim components, exterior hardware, and decorative elements must pass CASS testing to meet OEM corrosion resistance specifications. Automotive CASS requirements are commonly specified in manufacturer standards such as GM, Ford, and BMW engineering specifications.
Aerospace — anodized aluminum structural and cosmetic components are evaluated for their corrosion resistance under CASS conditions, supporting material qualification for aircraft interiors and exteriors.
Electronics — connector plating, device housings, and decorative hardware undergo CASS testing as part of reliability qualification programs.
Marine — hardware, fasteners, and decorative fittings exposed to marine atmospheres are validated through accelerated corrosion testing including CASS.
How CASS Differs from NSS (Neutral Salt Spray)
| Parameter | NSS (ASTM B117) | CASS (ASTM B368) |
| Test solution | 5% NaCl, pH 6.5–7.2 | 5% NaCl + CuCl₂ + acetic acid, pH 3.1–3.3 |
| Temperature | 35°C | 49°C |
| Corrosion rate | Baseline | ~8× more aggressive |
| Max exposure | 1000+ hours common | 48 hours maximum |
| Best suited for | General metals, painted surfaces | Decorative electroplated coatings, anodized aluminum |
Why Infinita Lab for CASS Testing?
Infinita Lab’s accredited testing network provides professional CASS testing services with precise solution preparation, chamber control, and post-exposure evaluation in accordance with ASTM B368, ISO 9227, and applicable product specifications. Expert analysts document corrosion patterns, rate coverage, and coating degradation in comprehensive test reports suitable for regulatory submission and customer qualification.
Frequently Asked Questions (FAQs)
What materials are commonly tested using CASS? CASS testing is typically used for aluminum alloys, chromium plating, and nickel/copper/chromium electroplated materials. It is also suitable for evaluating corrosion resistance in automotive, marine, and aerospace applications.
How does CASS testing differ from traditional salt spray tests? CASS testing is more aggressive, employing a mixture of saltwater, acetic acid, and copper sulfate at a higher temperature (50°C) than traditional testing, which uses only saltwater at 35°C.
What is the CASS test solution composed of? The solution is 5% sodium chloride in water, with the addition of copper chloride dihydrate (1 g per 4 L) and acetic acid to adjust pH to 3.1–3.3.
Why is the exposure time limited to 48 hours in CASS testing? The highly aggressive nature of the CASS solution means that most coatings reach meaningful evaluation endpoints within 48 hours. Longer exposures may destroy the sample without providing additional useful discrimination between coating systems.
How does CASS testing compare to natural environmental exposure? CASS testing at 50°C is approximately 8× more aggressive than conventional NSS testing per elapsed time, and 24 hours of CASS exposure is broadly equivalent to approximately 8 years of natural outdoor exposure for certain coating systems.
