Industry Knowledge: Salt Spray Test for Stainless Steel Bearings

The salt spray test is a common accelerated corrosion test used in the stainless steel bearing industry. It simulates high-humidity, salt-containing, and corrosive environments to test the rust and corrosion resistance of bearings-this is a must-test item for exported bearings, especially those used in marine, food, chemical, and shipping applications.
The most widely used standards in the industry are ISO 9227, ASTM B117, and GB/T 10125. The test is usually conducted with neutral salt spray (NSS), using 5% salt water at a constant temperature of 35℃.

II. What Do 24h, 48h, 72h and Longer Test Hours Mean?

1. 24-Hour Salt Spray Test

This is the basic entry-level test, mainly used for quick screening to check if the bearing material and surface treatment meet the minimum rust resistance requirements. It is suitable for bearings used in ordinary indoor, dry environments with short-term rust prevention needs. Common applicable materials include 420 stainless steel and ordinary polished 304 stainless steel (without passivation).

2. 48-Hour Salt Spray Test

This is the most common standard in industrial applications, especially for foreign trade orders. It is suitable for bearings used in humid workshops, food equipment, and environments with mild corrosion. The standard 304 stainless steel (with regular passivation) is usually able to pass this test.

3. 72-Hour Salt Spray Test

This is the dividing line for high-performance corrosion resistance. It is required for bearings used in coastal areas, chemical industry, spraying environments, and high-demand equipment. Materials that can pass this test include 304 stainless steel with deep passivation and basic 316 stainless steel.

4. 96h, 120h, 168h, 240h and Longer

These are high-end corrosion resistance levels, designed for bearings used in harsh environments such as ships, marine equipment, and long-term outdoor use. 168 hours (7 days) and 240 hours (10 days) are typical high-end requirements. Applicable materials include 316L stainless steel, duplex steel, and bearings with special passivation and polishing processes.

III. Corresponding Relationship Between Stainless Steel Material and Salt Spray Test Hours (Industry Common Standard)

- 420 stainless steel: 24-hour (basic rust prevention)

- Regular 304 stainless steel: 48-hour

- 304 stainless steel with deep passivation: 72h–96h

- 316 / 316L stainless steel: 72h–120h

- 316L stainless steel with enhanced treatment: 168h–240h+

IV. Judgment Standards (Most Concerned by Foreign Trade Customers)

Pass: No red rust (rust on the bearing substrate). Minor discoloration and white rust (oxide film) are acceptable. The appearance of red rust means the test fails.

V. Why the Same Material Has Different Test Hours?

There are four key factors affecting the test results, even for the same material:

1. Passivation treatment: This is the key factor determining corrosion resistance. Bearings without passivation can only pass 24-hour tests, while those with deep passivation can reach 72h+.

2. Surface finish: The smoother and brighter the bearing surface, the better its corrosion resistance.

3. Material purity: The content of molybdenum, chromium, and nickel in stainless steel directly affects its corrosion resistance.

4. Processing contamination: Residues such as cutting chips and iron filings on the bearing surface will cause rapid rusting during the test.

VI. Application Scenarios (Professional Words for Customers)

- 24h: Ordinary machinery, indoor dry environments

- 48h: Food machinery, textile equipment, general exported equipment

- 72h: Coastal areas, chemical industry, cleaning equipment

- 168h+/240h+: Ships, offshore wind turbines, seawater environments