How Is Fastener Salt Spray Testing Done? How Many Hours Counts as Pass?
Executive Summary
"How many hours of salt spray does this screw pass?" is the most common question in corrosion acceptance, but many people stop at the hour number. Salt spray testing is not a vague "how many hours"; it is a standardized accelerated corrosion test with defined method, solution concentration, pH, chamber temperature, settlement rate, and specimen placement. Based on GB/T 10125-2012 (identical to ISO 9227:2006), this article explains the methods, parameters, duration, and pass criteria.
Table of Contents
What Salt Spray Testing Actually Measures
Salt spray testing accelerates atmospheric corrosion by exposing a coated fastener to a fine mist of salt solution in a controlled chamber, conducted per GB/T 10125-2012 (identical to ISO 9227:2006, corrosion tests in artificial atmospheres - salt spray). The goal is not to predict exactly how many years the fastener will last outdoors, but to compare the relative corrosion resistance of different coatings or processes under standardized conditions. A coating that lasts 720 hours in neutral salt spray is significantly better than one that lasts 72 hours, but you cannot divide 720 by any factor to get a real-world service life. Salt spray is a comparative and quality-control test, not a lifetime predictor.
Three Methods: NSS, AASS, CASS
| Method | Solution | Temperature | Purpose |
|---|---|---|---|
| NSS (Neutral Salt Spray) | 5% NaCl, pH 6.5-7.2 | 35 C | General corrosion comparison, most common for fasteners |
| AASS (Acetic Acid Salt Spray) | 5% NaCl + acetic acid, pH 3.1-3.3 | 35 C | Accelerated test for decorative chrome, nickel, chromate coatings |
| CASS (Copper-Accelerated Acetic Acid Salt Spray) | 5% NaCl + CuCl2 + acetic acid, pH 3.1-3.3 | 50 C | Highly accelerated test for decorative coatings |
For fastener surface treatment, NSS is by far the most commonly referenced. When a supplier says "500 hours salt spray," they almost always mean NSS at 35 C. AASS and CASS are more aggressive and are used mainly for decorative plating, not for structural fastener corrosion qualification.
Standard Conditions: Solution, Temperature, Settlement
The standard specifies exact parameters: the NaCl concentration is 5% plus or minus 1%, the chamber temperature is 35 C plus or minus 2 C for NSS, and the collection rate is 1.0 to 2.0 mL per hour per 80 cm2 horizontal collection area. The compressed air pressure is controlled to ensure consistent droplet size. The specimens are placed at 15 to 30 degrees from vertical, so the mist runs off the surface rather than pooling. These parameters matter: a chamber that is too hot, or a solution that is too concentrated, will give a falsely short test life.
Test Duration and Pass Judgment
Test duration is chosen based on the expected corrosion environment. Common durations for fasteners are 24 h, 72 h, 96 h, 200 h, 480 h, 720 h, and 1000 h. The pass criterion is typically "no red rust on the base metal" at the specified duration. White rust (zinc corrosion product) is expected on zinc-plated parts and is not a failure; red rust (iron oxide on the base steel) is the failure criterion. Some standards also allow a defined amount of white rust before red rust appears.
For example, a trivalent zinc-plated 8.8 bolt might be specified for 72 hours NSS with no red rust. A zinc-nick plated NEV bolt might be specified for 500 hours. A dacromet-coated 10.9 bolt might be specified for 1000 hours. The specified duration should match the environment: 72 hours for indoor use, 200-500 hours for under-hood or outdoor, 720-1000 hours for coastal or underbody exposure.
How to Place and Evaluate Specimens
Specimens are placed in the chamber at the standard angle, not touching each other or the chamber walls. They are usually removed at the end of the test, rinsed gently with water, and examined for red rust. The number and location of rust spots are recorded. For critical applications, cross-section metallography may be used to examine coating adhesion and under-film corrosion. The test report should record the method (NSS/AASS/CASS), duration, appearance before and after, and any rust observations. To verify the chamber itself is performing correctly, the standard uses CR4-grade cold-rolled carbon steel reference specimens (per ISO 3574, 1 mm thick, 150 mm x 70 mm) in a 24 h check, with defined mass-loss windows that confirm the chamber needs recalibration if exceeded.
How to Benchmark: Do Not Treat Hours as Lifetime
The most common misuse of salt spray data is dividing hours by a factor to predict real-world life. There is no universal conversion: a coating that lasts 500 hours in NSS may last 5 years or 15 years outdoors depending on humidity, temperature cycles, UV exposure, pollution, and whether the part is washed by rain or soaked in road salt. Salt spray testing is useful for comparing two coatings on the same fastener and for batch quality control, but it is not a lifetime estimate. For lifetime prediction, use outdoor exposure data or cyclic corrosion testing, which better mimics real weathering.
Interpreting salt spray results correctly. A 500-hour NSS result does not mean the fastener lasts 500 hours outdoors. It means that under the standardized accelerated test, no red rust appeared for 500 hours. Real-world life depends on humidity, temperature cycles, UV, pollution, and whether the part is washed by rain or soaked in road salt. Use salt spray as a comparative tool between coatings and as a batch QC check, not as a lifetime guarantee. For products sold in coastal or road-salt regions, require additional cyclic corrosion testing or outdoor exposure data rather than relying on NSS alone.
Cyclic corrosion testing as a complement. Neutral salt spray is a continuous salt fog at constant temperature. Real outdoor weathering involves wet-dry cycles, temperature changes, and UV exposure. Cyclic corrosion testing (CCT) alternates salt spray, humidity, and drying phases to better mimic real weathering. CCT is more aggressive and more predictive than NSS for outdoor products. For automotive underbody and outdoor equipment, specify both NSS for baseline comparison and CCT for lifetime prediction. The two tests together give a clearer picture of real corrosion performance than either alone. Ask your lab which CCT cycle they recommend for your environment.
Sample preparation and reporting. A proper salt spray test report includes: the test method (NSS/AASS/CASS), solution concentration and pH, chamber temperature, settlement rate, test duration, specimen orientation, appearance before test, appearance after test, and the pass/fail criteria. Without these details, the hour number is meaningless. When comparing two suppliers' salt spray claims, verify that both used the same method and duration. A "500 hours" claim from one lab may not be comparable to another lab's "500 hours" if the conditions differ. Reputable labs follow GB/T 10125 exactly and provide complete reports.
Interpreting salt spray results correctly. A 500-hour NSS result does not mean the fastener lasts 500 hours outdoors. It means that under the standardized accelerated test, no red rust appeared for 500 hours. Real-world life depends on humidity, temperature cycles, UV, pollution, and whether the part is washed by rain or soaked in road salt. Use salt spray as a comparative tool between coatings and as a batch QC check, not as a lifetime guarantee. For products sold in coastal or road-salt regions, require additional cyclic corrosion testing or outdoor exposure data rather than relying on NSS alone.
Cyclic corrosion testing as a complement. Neutral salt spray is a continuous salt fog at constant temperature. Real outdoor weathering involves wet-dry cycles, temperature changes, and UV exposure. Cyclic corrosion testing (CCT) alternates salt spray, humidity, and drying phases to better mimic real weathering. CCT is more aggressive and more predictive than NSS for outdoor products. For automotive underbody and outdoor equipment, specify both NSS for baseline comparison and CCT for lifetime prediction. The two tests together give a clearer picture of real corrosion performance than either alone. Ask your lab which CCT cycle they recommend for your environment.
Sample preparation and reporting. A proper salt spray test report includes: the test method (NSS/AASS/CASS), solution concentration and pH, chamber temperature, settlement rate, test duration, specimen orientation, appearance before test, appearance after test, and the pass/fail criteria. Without these details, the hour number is meaningless. When comparing two suppliers' salt spray claims, verify that both used the same method and duration. A "500 hours" claim from one lab may not be comparable to another lab's "500 hours" if the conditions differ. Reputable labs follow GB/T 10125 exactly and provide complete reports.
Conclusion
Salt spray testing per GB/T 10125 is a standardized accelerated corrosion comparison. NSS at 35 C is the fastener industry standard. Specify the method, duration, and pass criterion (no red rust) clearly. Treat the hour number as a comparative and QC metric, not as a real-life lifetime. For long-life outdoor applications, supplement salt spray data with cyclic corrosion or outdoor exposure evidence.
