Salt Spray Testing: Key Points You Must Clearly Understand

Salt spray testing is an environmental test method used to evaluate the corrosion resistance of products or metal materials by creating an artificial salt spray environment using specialized test equipment. It is mainly divided into two categories: natural environmental exposure testing and artificial accelerated salt spray testing.

1. Why Conduct Salt Spray Testing

Salt spray testing is performed to verify whether metal components on a product will corrode when exposed to the atmosphere or other environments.

This test is an accelerated corrosion evaluation method conducted in an artificial atmosphere. A salt solution of a specified concentration is atomized and sprayed into a sealed, temperature-controlled chamber. By observing changes in the tested samples after a certain exposure period, the corrosion resistance of the samples can be assessed.

Compared with natural exposure testing, salt spray testing requires significantly less time—days or even hours instead of years—making it an efficient evaluation method.

However, corrosion is influenced by many factors. Salt spray resistance alone cannot represent resistance to all corrosive media. Therefore, salt spray test results should not be considered a direct guideline for corrosion performance in all service environments, nor should test performance be regarded as a direct prediction of real-life durability.

2. Products That Require Salt Spray Testing

• Metals and metal alloys

• Metallic coatings

• Organic coatings

• Anodized coatings

3. Coatings

1) Types of Coatings

Organic coatings:
Such as paint, varnish, powder coating, rust-preventive oil, etc.

Metallic coatings:
Such as electroplating, electroless plating, ion plating, thermal spraying, hot-dip plating, chemical vapor deposition, etc.

Common electroplated coatings include zinc, copper, nickel, chromium, tin, brass, and lead-tin alloys.

2) Functions of Coatings

• Aesthetic purposes (e.g., bright chrome plating)

• Protection of the base metal against corrosion

4. Classification of Salt Spray Test Methods

• Neutral Salt Spray Test (NSS)

• Acetic Acid Salt Spray Test (AASS)

• Copper-Accelerated Acetic Acid Salt Spray Test (CASS)

Commonly used standards include:

• ISO 9227:2006 (Europe, including the UK)

• ASTM B117-03 (USA)

• DIN 50021:1988 (Germany)

Neutral Salt Spray Test (NSS)

Applicable Standards

• ISO 9227:2006

• ASTM B117-03

• DIN 50021:1988

These standards use essentially the same test conditions.

Test Conditions

• Sodium chloride solution concentration: 50 g/L ± 5 g/L

• pH value: 6.5–7.2

• Chamber temperature: 35 ± 2°C
  (ASTM: 35 +1.1 / −1.7°C)

• Salt spray deposition rate: 1.5 ± 0.5 mL per 80 cm² per hour

Sample Preparation

• Sample type, quantity, shape, and size shall be determined according to product standards; if no standard exists, they shall be agreed upon by relevant parties.

• Samples must be properly cleaned before testing, but protective organic coatings intentionally applied must not be removed.

• Cut surfaces of samples must be protected with paint, wax, or tape.

Sample Placement

• Samples shall be placed with the test surface facing upward so that salt mist can freely settle; direct spraying onto the test surface is not permitted.

• The test surface shall be inclined at 20 degrees from the vertical direction.

• Samples may be placed on different horizontal levels but must not touch each other or the chamber walls. Droplets from one sample must not drip onto another.

• Sample racks shall be made of glass, plastic, or other non-metallic materials. Suspension materials must be non-metallic, such as fiber.

Test Duration

• Determined according to product specifications; common durations include 2, 4, 6, 8, and 24 hours.

• For tests exceeding 24 hours, durations shall be in multiples of 24 hours.

Post-Test Sample Treatment

• After testing, remove the samples and allow them to dry naturally indoors for 0.5–1 hour.

• Rinse gently with clean running water at a temperature not exceeding 40°C to remove residual salt solution.

• Dry immediately using a blower.