Salt Spray and Corrosion Testing
Salt spray and corrosion testing measure how long a coated or plated automotive part resists rust and degradation in a corrosive environment. The most common method is neutral salt spray (ASTM B117), and CASS (ASTM B368) is a more aggressive variant for decorative coatings.
What Corrosion Testing Measures
Automotive parts face road salt, moisture, and humidity for years. Corrosion testing reproduces that exposure in a controlled chamber so a coating or plating system can be qualified before it goes into service. The result is a measure of how long the surface resists corrosion, reported as hours to first failure or a rating of the surface after a set exposure.
The exposure is accelerated. A few hundred hours in a salt spray chamber reproduces corrosion that would take years on the road, which lets a supplier qualify a finish without waiting for real-time results.
Salt Spray, CASS, and Cyclic Methods
Several methods exist because different finishes and applications call for different exposures.
Neutral salt spray (ASTM B117) is the baseline. A specimen sits in a chamber filled with a continuous salt fog at a controlled temperature, and the surface is evaluated for corrosion over time.
CASS, or copper-accelerated acetic acid salt spray (ASTM B368), is more aggressive. It adds copper chloride and acetic acid to the solution to accelerate corrosion of decorative chrome and other plated finishes. CASS is the method many OEMs require for bright trim and plated parts because it correlates better with how those finishes fail.
Cyclic corrosion methods alternate between salt exposure, humidity, and drying to better reproduce real-world wet and dry cycling. ASTM D1654 covers the evaluation of coated specimens after corrosion exposure, scoring creep from a scribe and surface degradation.
| Method | What it is | When it applies |
|---|---|---|
| ASTM B117 (neutral salt spray) | Continuous plain salt fog at a controlled temperature | General corrosion baseline for coated and uncoated parts |
| ASTM B368 (CASS) | Copper-accelerated acetic acid salt spray, harsher and faster | Decorative chrome and plated finishes |
| Cyclic corrosion | Alternates salt, humidity, and drying | Real-world wet and dry cycling |
| ASTM D1654 | Rating of coated specimens after exposure | Scoring creep from a scribe and surface degradation |
The Cost of Running the Wrong Exposure
Here is where corrosion testing gets expensive if the method is wrong. These exposures run long. A CASS program can take days, a neutral salt spray program can run a week or more, and a cyclic program longer still. If a decorative chrome finish is run under neutral salt spray when the OEM specification calls for CASS, the result does not answer the OEM’s question, and there is no shortcut to a correct answer except to start over.
That restart is not a same-day fix. It is another full exposure cycle, plus new specimens, plus the requalification schedule slipping by however long the correct test takes. The applicable method lives in the material specification, and confirming it before the first specimen goes in the chamber is the difference between one exposure and two.
How This Fits a Full Program
Corrosion testing often pairs with the rest of an environmental program. A coating may go through weathering, then corrosion, then post-exposure adhesion to confirm it held through the full sequence. The order is defined by the specification, and the corrosion result is one input into whether the finish qualifies.
Frequently Asked Questions
What is CASS testing?
CASS stands for copper-accelerated acetic acid salt spray (ASTM B368). It is an aggressive corrosion test that adds copper chloride and acetic acid to the salt solution to accelerate the corrosion of decorative chrome and plated finishes.
What is the difference between salt spray and CASS?
Neutral salt spray (ASTM B117) uses a plain salt fog and is the general baseline. CASS (ASTM B368) adds copper chloride and acetic acid for a faster, more aggressive exposure used on decorative and plated finishes.
How long does salt spray testing take?
It depends on the specification. Exposures range from a few hundred hours to over a thousand, with the specimen evaluated at set intervals or at the end of the required exposure. Because the runs are long, a wrong method is costly to redo.
Which corrosion method does my part need?
The OEM material specification defines it. Decorative plated finishes often require CASS, while general corrosion resistance may use neutral salt spray. Confirming the method before testing avoids a full second exposure.
Ghesquiere Plastic Testing, Inc. is an A2LA accredited testing laboratory in Harper Woods, MI, specializing in weathering and flammability testing for automotive materials. Request a quote at quotes@gptesting.com.
