What is the purpose of salt spray testing?
1. Understanding Salt Spray Testing
Salt spray testing is a laboratory simulation technique that exposes materials, usually metals and coated metals, to a highly corrosive saline mist or fog in a controlled chamber. The goal is to replicate and accelerate corrosion processes that would occur naturally over a longer time period.
The test environment is created by atomizing a salt solution (typically 5% sodium chloride in water) within a sealed chamber and maintaining specific temperature and humidity levels. The duration of the exposure can range from a few hours to several weeks, depending on the test objectives.
2. Purpose of Salt Spray Testing
a. Evaluate Corrosion Resistance
The primary purpose of salt spray testing is to evaluate the corrosion resistance of materials and coatings. This includes:
Base materials like steel, aluminum, and zinc.
Protective coatings such as paint, powder coatings, electroplating (zinc, chromium, nickel), anodizing, or conversion coatings.
Finished components or assemblies subjected to environmental exposure.
Manufacturers use salt spray testing to determine whether a material or coating system will perform adequately in its intended service environment.
b. Quality Control
Salt spray testing is a valuable quality control tool. By running periodic tests on production batches, manufacturers can verify that their coating processes are consistent and meet required corrosion resistance standards. This helps to ensure that defects such as inadequate coating thickness, poor adhesion, or surface contamination are identified before the product reaches the customer.
c. Product Development and Comparison
During the R&D phase, salt spray tests allow engineers to:
Compare different coatings and finishes.
Test new materials or treatments.
Optimize corrosion protection systems.
This enables selection of the most cost-effective and durable materials for specific applications.
d. Compliance with Industry Standards
Various industries have their own salt spray testing requirements to ensure product reliability in real-world applications. Salt spray testing helps manufacturers comply with:
Automotive (e.g., ASTM B117, ISO 9227, SAE J2334)
Aerospace (e.g., MIL-STD-810)
Construction (e.g., ISO 12944)
Marine and offshore equipment
Consumer electronics and appliances
Passing these tests often serves as a prerequisite for certification, customer approval, or market entry.
e. Simulating Harsh Environments
Though accelerated, salt spray testing simulates harsh real-world environments where metal surfaces are exposed to saltwater spray, humidity, and heat-conditions typically found in coastal, industrial, or winter road settings. This makes it particularly useful for:
Automotive parts (e.g., bolts, chassis, suspension components)
Marine equipment (e.g., propellers, boat fittings)
Outdoor products (e.g., lighting, fencing, HVAC systems)
Construction materials (e.g., metal panels, screws, fasteners)
3. How Salt Spray Testing Works
a. The Test Chamber
A salt spray test is conducted in a sealed chamber that maintains a controlled environment. Key features include:
A reservoir for salt solution (typically 5% NaCl).
Atomizers that generate a fine mist.
A heating system to maintain 35°C (95°F) or higher.
Racks for holding test specimens at a 15–30° angle.
Collection funnels to monitor fog deposition rate.
b. The Salt Solution
The standard salt spray solution is 5% sodium chloride (NaCl) dissolved in distilled water, adjusted to a pH of 6.5–7.2. For more specialized tests, additives like acetic acid or copper chloride may be introduced to simulate different environments (e.g., acid rain or polluted industrial atmospheres).
c. Test Duration
Salt spray test durations vary widely based on material type and standards. For instance:
24 to 96 hours for zinc-plated parts.
250 to 1,000+ hours for powder-coated or painted components.
Some tests may extend to 2,000 hours or more for high-performance coatings.
4. Types of Salt Spray Testing
Several variations of salt spray testing exist, each with specific purposes:
a. Neutral Salt Spray (NSS) – ASTM B117
The most common test method. It uses a neutral pH salt solution and simulates general corrosion conditions.
b. Acetic Acid Salt Spray (AASS) – ASTM G85 Annex A1
Acidified salt solution (lower pH) simulates more aggressive environments, especially for decorative coatings like nickel-chrome plating.
c. Copper-Accelerated Acetic Acid Salt Spray (CASS) – ASTM G85 Annex A3
Adds copper chloride to further accelerate corrosion. Used for highly corrosive applications, especially in automotive and decorative finishes.
d. Cyclic Corrosion Testing (CCT)
More realistic simulation that cycles through salt spray, dry, humidity, and sometimes UV exposure. Better correlates with field conditions than continuous salt fog.
5. Salt Spray Testing Standards
Several international standards define salt spray test methods:
ASTM B117: Most widely used standard for NSS testing.
ISO 9227: Defines NSS, AASS, and CASS test procedures.
JIS Z 2371: Japanese Industrial Standard for salt spray.
MIL-STD-810: U.S. military standard for environmental testing.
Each standard specifies conditions, procedures, and pass/fail criteria depending on the material or industry.
6. Interpreting Test Results
After testing, components are visually inspected for signs of corrosion:
White rust: Zinc corrosion products.
Red rust: Iron-based corrosion, considered a failure for steel.
Blistering or peeling: Indicates coating failure or poor adhesion.
Creepage: Corrosion creeping from a scribed area or edge.
Results are often reported as hours to first corrosion, percentage of corroded area, or number and type of corrosion defects.
7. Benefits of Salt Spray Testing
Fast, repeatable results to compare materials.
Inexpensive compared to long-term field testing.
Simple setup and relatively easy operation.
Widely accepted across multiple industries.
Helps in product qualification and supplier audits.
8. Limitations of Salt Spray Testing
While useful, salt spray testing has limitations:
Does not replicate all real-world conditions (e.g., UV light, temperature changes, physical stress).
Accelerated conditions may not correlate directly with actual service life.
Only tests coating surface behavior, not internal corrosion resistance.
May favor certain coatings (e.g., sacrificial zinc) over barrier coatings like paint.
Results can vary due to test setup, sample geometry, or even salt purity.
Thus, salt spray testing is often used in conjunction with other tests like cyclic corrosion, UV aging, or field trials.
9. Industries That Rely on Salt Spray Testing
Automotive: Testing fasteners, brake parts, underbody coatings.
Aerospace: Evaluating aluminum alloys, conversion coatings.
Electronics: Corrosion protection for casings and connectors.
Construction: Coated steel panels, structural components.
Consumer Products: Appliances, bathroom fixtures, outdoor tools.
10. Conclusion
Salt spray testing remains one of the most important and widely used methods for assessing the corrosion resistance of metals and coatings. Its primary purpose is to provide a controlled, accelerated environment to evaluate how well a material or finish will hold up under corrosive conditions.
Despite its limitations, salt spray testing is invaluable for quality assurance, product development, and regulatory compliance. When interpreted correctly and used in the right context, it helps engineers and manufacturers deliver more durable, reliable, and safe products.
At PowerWinx, we offer professional salt spray testing services to ensure the corrosion resistance and durability of your parts and coatings. Our testing process complies with international standards like ASTM B117 and ISO 9227, helping guarantee product quality and longevity. Whether for quality control or material validation, PowerWinx delivers reliable results tailored to your project needs.
