Corrosion costs industries billions yearly. Many businesses use salt spray testing to check how well metal parts can resist corrosion before shipping or final approval. If you work with sheet metal, coatings, or custom hardware, you’ve probably heard of this method. But how does it work—and how reliable is it?
Salt spray testing seems simple. But the way it works, what it can (and can’t) do, and how results are read—are often misunderstood. Want to know how it works and why it matters? Let’s break it down.
What is Salt Spray Testing？
Salt spray testing, also known as salt fog testing, is a method used for corrosion testing. It exposes metal parts to a fine mist of saltwater solution inside a controlled chamber. The goal is to create an accelerated corrosive environment. This test helps evaluate how coatings, platings, and surface treatments withstand harsh conditions.
It is used to simulate what might happen to metal parts when they are placed in coastal, humid, or salty environments. The results help determine if a coating is suitable for real-world use.
Corrosion weakens metal parts. This results in reduced performance, shorter product lifespan, and increased safety risks. Salt spray testing provides a rapid method for evaluating corrosion resistance without requiring years of real-world exposure.
Manufacturers use this test to check if a part meets the required durability standards. It is instrumental in various applications, including automotive, aerospace, construction, and marine. Salt spray testing also helps in comparing coatings, identifying quality issues, and enhancing product design.
How Salt Spray Testing Works?
Salt spray testing employs a straightforward setup, but it adheres to strict guidelines to ensure reliable results. The process simulates long-term exposure to salt in a short amount of time.
Overview of the Testing Chamber
The core of this test is the salt spray chamber. It is a sealed box made from corrosion-resistant material. Inside, metal samples are placed at a fixed angle. A nozzle sprays a fine mist of salt solution over them. The chamber runs nonstop for hours or days.
Every part of the chamber—spray nozzles, racks, drainage—must meet industry standards like ASTM B117. These standards help ensure results are accurate and repeatable across different labs.
The Role of the Salt Solution
The test uses a salt solution made by mixing 5% sodium chloride (NaCl) with purified water. This mix mimics the salt level found in ocean water. The solution is atomized into a fog that coats all surfaces evenly.
Salt is aggressive. When it lands on the metal surface, it starts breaking down coatings and exposing flaws. The time it takes for rust or corrosion to appear is measured and recorded.
Temperature and Humidity Control
The test chamber runs at a steady temperature—usually around 95°F (35°C). This creates a warm, humid environment that accelerates corrosion.
Humidity plays a significant role. Inside the chamber, the humidity level remains close to 100%. The combination of salt mist and high humidity creates an extremely harsh environment. This setup helps reveal weak points in coatings faster than real-world outdoor exposure.
Types of Salt Spray Tests
There are several types of salt spray tests. Each type is designed to suit different materials, coatings, and industry needs. The choice depends on how aggressively the test needs to be conducted.
Neutral Salt Spray (NSS)
NSS is the most common type. It uses a neutral pH salt solution made with 5% sodium chloride and deionized water. The pH stays between 6.5 and 7.2. It is used mainly for testing metallic coatings, such as zinc plating or electroplating.
NSS is simple, consistent, and widely accepted in numerous standards, including ASTM B117 and ISO 9227. It gives a good baseline for comparing coating performance.
Acetic Acid Salt Spray (AASS)
AASS is more aggressive than NSS. It adds acetic acid to the salt solution, which lowers the pH to around 3.1–3.3. This creates a more acidic environment.
This test is primarily used for organic coatings on aluminum and decorative finishes, such as nickel-chromium. It helps evaluate corrosion behavior in acidic or industrial atmospheres.
Copper-Accelerated Acetic Acid Salt Spray (CASS)
CASS is even harsher than AASS. It includes both acetic acid and copper chloride. The copper speeds up the corrosion process. The pH stays in the same acidic range.
CASS is used for high-end decorative coatings and some automotive parts. It reveals coating defects in a short time. It’s often specified in standards like ISO 9227 for decorative electroplating.
Modified Salt Spray Tests (e.g., Cyclic Corrosion Tests)
Some parts face changing weather, not just constant salt. For those, cyclic corrosion tests (CCT) are better. These tests combine salt spray with drying, humidity, and sometimes temperature changes.
CCT simulates real-world outdoor conditions more closely. It’s used in automotive and marine industries to test complete assemblies under more realistic exposure cycles.
Standards and Regulations
Salt spray tests must follow strict standards. These rules make sure results are reliable and accepted across industries. Different regions and sectors may use other standards, but the test setup stays similar.
ASTM B117 Overview
ASTM B117 is the most widely used salt spray standard. It was introduced in 1939 and is still the benchmark in many industries. It covers how to set up and run the neutral salt spray test.
The standard specifies details such as solution mix, spray pressure, chamber temperature, pH range, and test duration. It does not set pass/fail limits. Instead, it provides a standardized method for others to develop their test requirements.
ISO 9227 Specifications
ISO 9227 is the international version of salt spray testing. It supports NSS, AASS, and CASS methods. This standard is more commonly used in Europe and Asia.
ISO 9227 also outlines the procedures for preparing samples, measuring corrosion, and conducting the test consistently. It’s often required by OEMs or international suppliers when exporting products across borders.
Other International Testing Standards
Other countries have their standards, too. Examples include:
- JIS Z 2371 (Japan)
- GB/T 10125 (China)
- DIN 50021 (Germany)
Most of these follow similar setups to those of ASTM or ISO. They may include regional tweaks to match local use cases or industries.
Certification and Compliance Needs
Many companies use salt spray test results to meet contract or regulatory requirements. For example, auto parts may need to resist rust for a certain number of hours under NSS or CASS.
Some industries require third-party labs to run the tests. Certification reports help show that a product meets performance expectations before shipping or installation.
Sample Preparation and Requirements
Before testing begins, the samples must be prepared appropriately. Poor preparation can affect the accuracy of the results and lead to false failures or misleading data.
Surface Cleaning Before Testing
Each sample must be clean and free of oil, dust, and fingerprints. Any residue can block the salt mist and change how the surface reacts.
Cleaning is typically performed using solvents such as isopropyl alcohol or deionized water. In some cases, mild detergents may be used. The goal is to expose the actual coating or metal surface to the salt spray.
Masking and Labeling
If only part of a sample needs to be tested, the rest must be masked. Masking uses tape or sealants to block the mist from unwanted areas.
Every sample should also be clearly labeled. Labels must resist moisture and not affect the test. They are often placed on the back or in a non-critical area. This helps keep track of multiple parts that are tested simultaneously.
Size and Material Considerations
Samples should fit in the chamber without touching each other. Spacing is key to ensure each sample gets equal exposure.
Most standards recommend test pieces with flat, even surfaces. But parts with curves or holes can also be tested if needed.
The material and coating should match the final product. Any change in metal type, thickness, or finish can affect the outcome.
Salt Spray Testing Procedure
The test follows a precise and repeatable process. Every step is carefully controlled to ensure that results are reliable and easy to compare across different batches or laboratories.
Step-by-Step Testing Process
First, the operator prepares the samples and places them inside the chamber. Each part is fixed at a slight angle, typically around 15 to 30 degrees, to allow the salt mist to settle evenly.
Next, the salt solution is mixed and placed in a reservoir. The chamber heats up to the set temperature, and the nozzle starts spraying a fine mist.
The test runs continuously for the set duration. No cleaning or drying is allowed during this time. Ultimately, samples are removed, rinsed with clean water, and inspected.
Common Test Durations
Test times vary depending on the coating, material, and standard being followed.
- NSS tests: 24 to 1,000 hours
- AASS and CASS: 16 to 240 hours
Longer times do not always mean better performance. Each standard often suggests a pass/fail threshold based on visible rust or coating breakdown.
Monitoring During the Test
During the test, operators check the chamber conditions every few hours. They monitor temperature, spray pressure, pH level, and the volume of the salt solution.
The chamber must stay stable. If the temperature or spray pattern changes, the test may need to be restarted. Chambers are also inspected for clogs or leaks to avoid uneven exposure.
Result Evaluation and Interpretation
Once testing ends, each sample is inspected for signs of corrosion. The results help determine if the coating or finish is performing as expected.
Visual Inspection Criteria
Most evaluations are done by eye. Inspectors look for changes like rust, blisters, peeling, or discoloration. Some standards may require the use of a magnifier, but most rely on simple visual checks under normal lighting conditions.
The location, size, and amount of corrosion are all noted. Any defects are compared against the standard or customer-specified limits.
Rust, Blistering, and Corrosion Patterns
Red rust is a common sign of failure in steel parts. White rust may appear on zinc coatings. Blistering shows the coating is lifting away from the base metal.
Uniform rusting across a surface can mean the coating is thin or worn. Rust near edges or scratches may suggest weak points in the protection.
Each pattern gives clues about coating quality and how it may fail over time.
Pass/Fail Judgment Methods
Different standards define pass/fail in various ways. Some use time-based criteria, like “no red rust after 240 hours.” Others look at the size or location of defects.
Judgments must follow the test method and project requirements. What passes in one industry may fail in another. That’s why it’s key to match testing to the part’s real-world use.
Photography and Documentation
Photos are often taken before, during, and after the testing process. These give a clear record of the part’s condition and any defects.
Reports usually include test duration, chamber settings, sample IDs, and any observations. Clear records facilitate problem tracking, design improvement, and meeting customer requirements.
Factors Influencing Test Accuracy
Salt spray tests must be conducted under strict conditions. Even small changes can significantly impact the outcome and yield inaccurate results. These factors must be controlled to ensure reliable and repeatable data.
pH Value of Salt Solution
The pH of the salt solution must stay within a specific range. For neutral salt spray (NSS), the range is 6.5 to 7.2.
If the pH is too low, the test becomes more acidic. This accelerates corrosion and may lead to premature failure. If it’s too high, the test becomes less aggressive and may not reveal weak coatings.
Test Temperature and Humidity
The standard chamber temperature for NSS is around 95°F (35°C). This warm environment helps speed up corrosion reactions.
Humidity also plays a role. Inside the chamber, it should be close to 100%. If temperature or humidity drops, the test loses accuracy. Rust might appear slower or not at all.
Concentration of Salt Solution
The salt concentration is usually 5% by weight. If the solution is too strong, it may attack the part too fast. If it’s too weak, it won’t give realistic results.
The solution must be mixed using deionized water to prevent contamination. It should be stirred well and filtered before use.
Salt Spray Deposition Amount
The mist must land evenly on all samples. Most standards require 1.0 to 2.0 mL of salt spray to collect per 80 cm² per hour.
Too much spray floods the surface, skewing the results. Too little spray delays corrosion. A collector inside the chamber checks the amount regularly.
Spray Method
The nozzle setup affects how the salt fog is distributed. It should create a fine, even mist. If the spray is too coarse or directed, it causes uneven coverage.
Spray pressure and nozzle condition must be checked often. A worn or blocked nozzle can reduce accuracy and make results unreliable.
Conclusion
Salt spray testing is a fast and reliable method for evaluating the effectiveness of a coating or surface treatment in protecting metal from corrosion. It uses a controlled chamber to simulate harsh, salty environments. The test helps identify weak points, compare coatings, and ensure parts meet durability standards.
Need corrosion-resistant parts for your next project? Get in touch with our team today. We’ll help you select the ideal surface finish and conduct the necessary tests to achieve your quality objectives.
Hey, I'm Kevin Lee
For the past 10 years, I’ve been immersed in various forms of sheet metal fabrication, sharing cool insights here from my experiences across diverse workshops.
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Kevin Lee
I have over ten years of professional experience in sheet metal fabrication, specializing in laser cutting, bending, welding, and surface treatment techniques. As the Technical Director at Shengen, I am committed to solving complex manufacturing challenges and driving innovation and quality in each project.
