Salt spray testing is a corrosion testing method that uses a high-salt environment to measure the corrosion resistance of products, paints, and coatings over an extended period of time. Sometimes called salt forging, salt spray testing is performed in an enclosed salt spray test chamber that can be adjusted to create various corrosive environments.
During testing, a sample is exposed to a specified salt concentration for a predetermined amount of time, depending on the specification, product type, or industry standard. All your questions about salt spray and salt forging tests are answered in today’s guide. Whether you want to know definitions, applications, meanings, equipment, or standards, you will find everything here. Read on to learn more.
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1. What is a spray test?
The salt spray test, also known as the salt spray test or the salt wrought test, is a standardized procedure used to evaluate the corrosion resistance of various materials, especially metals and their coatings. The test exposes the samples to a highly corrosive environment similar to the saline coastal atmosphere.
For example, one of my clients is a manufacturer of floor drains. In one conversation he told me that the surface of floor drains corrode easily because they are in contact with water for a long time. Therefore, understanding the corrosion resistance of the product is closely related to the service life of the product. The best test to test the corrosion resistance of a product is the salt spray test.
By subjecting materials to controlled salt spray, salt spray testing simulates the harsh conditions that can accelerate corrosion in real-world scenarios.
Salt spray testing is used to evaluate the durability of metal surface finishes and coatings. This test can be used to evaluate the effect of different exposure conditions such as humidity and temperature on the corrosion resistance of the coating. It is also used to compare the performance of different coatings and finishes.
1) NSS or neutral salt spray
The Neutral Salt Spray (NSS) test is used to evaluate the effects of the environment on metal surfaces without affecting any protective or decorative properties. Corrosion tends to be slow and uniform. Suitable for aluminum alloy, magnesium alloy, copper alloy, zinc alloy, titanium alloy, and other ferrous and non-ferrous metals. The test cycle is performed at a constant room temperature of 35 degrees Celsius.
2) AASS or Acetate Spray
After adding acetic acid, which enhances corrosion, the Acetate Salt Spray (AASS) test measures the effect of the environment on metal surfaces. Corrosion tends to be more intense and uneven. It is mainly used on ferrous metals such as nickel, steel, and iron. The most suitable temperature is 50 degrees Celsius.
3) CASS or copper-accelerated acetate mist
The Copper Accelerated Acetate Salt Spray (CASS) test is used to evaluate the corrosion resistance of zinc, cadmium, and polymer coatings. You can also use it to evaluate environmental effects on metal surfaces to which glacial acetic acid and other dihydrates have been added.
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2. Why Need a Salt Spray Test?
The main purpose of the salt spray test is to evaluate the resistance of materials to corrosion caused by salt deposition, which is a common concern in marine and coastal environments.
By subjecting test specimens to a continuous accelerated corrosion process, the performance and durability of protective coatings, surface treatments, and material combinations can be evaluated. This test is important for several reasons:
- Quality Control
You rely on salt spray testing to assess the quality and effectiveness of protective coatings, surface treatments, and corrosion-resistant materials. It helps you ensure your products meet industry standards and customer expectations.
- Material Selection
This test helps engineers, architects, and designers select materials suitable for corrosive application environments. By comparing the properties of different materials and coatings, they can make an informed decision and choose the option that best suits their specific requirements.
- Research and Development
Salt spray testing is an invaluable tool for researchers and scientists studying corrosion, developing new materials, and enhancing existing protective coatings. It provides a controlled and accelerated environment to test the efficacy of innovative solutions.
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3. Salt spray, fog, and corrosion test conditions
The salt spray test uses a standardized solution of 5% NaCl (sodium chloride) to create a highly corrosive environment. To simulate specific climates or operating conditions for projects with additional requirements, we can often adjust environmental conditions such as temperature and humidity.
Samples are usually inspected periodically to monitor corrosion levels and record any milestones such as the formation of white and red rust or coating degradation. Our laboratory can provide a detailed test report that includes pictures and descriptions of each test phase from start to finish.
4. Salt spray test procedure
Here’s an outline of the process:
The material or part to be tested is thoroughly cleaned to remove any contamination that may interfere with the results. This step ensures that the only variable tested is the corrosion resistance of the material.
Salt spray chambers are specially designed for testing cleaned samples in a controlled environment. The chamber is sealed to prevent any external contaminants from affecting the test.
3) Salt solution
A saline solution, usually a 5% sodium chloride (NaCl) solution, is prepared and nebulized to a fine mist in the room. The fog consists of tiny salt particles suspended in the air, replicating the corrosiveness of a salty atmosphere.
4) Test time
Depending on the testing standard, samples are retained in the salt spray chamber for a predetermined period of time. Common durations range from 24 hours to several weeks, with intermittent drying cycles to simulate the cyclic nature of real-world corrosion processes.
5) Observation and evaluation
After the test duration has elapsed, the samples are removed from the chamber, cleaned, and inspected for signs of corrosion. Document the extent and nature of corrosion, such as pitting, rusting, or color changes, and compare to predefined or industry standards.
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5. Application of salt forging tests
Testing for salt spray is popular because it is relatively inexpensive, fast, well-standardized, and reasonably repeatable. Although there may be a weak correlation between the duration of the salt spray test and the expected life of the coating in some coatings (e.g. hot-dip galvanized steel), the test is used worldwide for its low cost and quick results Popularity.
Today, most salt spray chambers are not used to predict the corrosion resistance of coatings, but to maintain coating processes on a comparative basis, such as pretreatment and painting, electroplating, galvanizing, etc.
For example, pre-treated + painted parts must pass a 96-hour neutral salt spray test before being accepted for production. Failure to meet this requirement means that the pretreatment chemistry or paint quality is unstable and must be addressed immediately so that upcoming batches will be of the required quality.
The longer the accelerated corrosion test, the longer the process is out of control and the greater the loss of off-spec batches. The main application of the salt spray test is corrosion resistance. The time it takes for oxides to appear on the sample under test is typically compared with what is expected to determine whether the test passes or fails.
Therefore, salt spray testing is most often used in quality audits, for example, it can be used to check the effectiveness of production processes, such as the surface coating of metal parts.
Salt spray testing has little application in predicting how a material or surface coating will resist corrosion in the real world, because it does not create, replicate, or accelerate real-world corrosion conditions. Cyclic corrosion testing is more suitable for this.
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6. Salt spray test vs. salt forging test: What’s the difference?
In fact, from a testing point of view, the salt spray test and the salt forge test are the exact same test. These are often referred to by different names because the amount of salt applied varies according to the specific environmental conditions that need to be replicated. The test typically applies salt and/or sea fog to coated or painted ships, automobiles, and military equipment to test corrosion over time through an accelerated process.
- Test content: The salt spray test simulates the marine environment or the climate of a salt-humid area, and is used to evaluate the salt spray corrosion resistance of products, materials, and their protective layers. There are two types of tests: salt spray test and alternating salt spray test. Under special conditions, it is often used to assess quality and verify failures.
- Type: neutral salt spray test (NSS), acetic acid salt spray test (ASS), copper acetate accelerated salt spray test (CASS)
- The technical indicators of the salt spray test include salt solution concentration, relative humidity, temperature, salt spray time, storage time, test cycle, fog collection volume, and PH value.
- Salt spray test reference standards: GB/T 2423.17, IEC60068-2-11, ISO4628-3, ASTM B117, JIS-Z2371, JIS-G3141, GJB 150.11A-2009, MIL-STD-810F, MIL-STD- 883E, etc.
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7. Testing equipment for salt spray and salt forging tests
Saline (5% NaCl) solution is atomized through nozzles using pressurized air in a closed test cabinet/chamber. Within the chamber, dense salt water mist (also known as mist or spray) creates corrosive conditions that subject test samples to severe corrosion. Chamber volumes vary by supplier.
If a specific salt spray test standard has a minimum volume requirement, it will be clearly stated and should be followed. Historically, it has been generally accepted that larger test chambers provide a more uniform test environment.
Salt spray test solutions vary depending on the material being tested. The most common test for steel-based materials is the Neutral Salt Spray Test (often abbreviated as NSS), which reflects the neutral pH of this type of test solution, which is 6.5 to 7.2. To maintain a neutral pH, add hydrochloric acid or sodium hydroxide to lower or raise the pH to the desired range.
It is usually expressed as test hours in NSS without corrosion products (e.g. 720 hours in NSS according to ISO 9227). Some companies and standards also commonly specify synthetic seawater solutions.
Other test solutions added other chemicals including acetic acid (often abbreviated as ASS) and acetic acid with copper chloride (often abbreviated as CASS) each of which was chosen to evaluate decorative coatings such as electroplated copper nickel chrome, electroplated copper Nickel, or anodized aluminum. These acidified test solutions typically have a pH of 3.1 to 3.3
There is a risk of cross-contamination when alternating ASS and CASS test cabinets during NSS testing, according to some sources. It was stated that it was very difficult to thoroughly clean the cabinets after the CASS test. ASTM does not address this, but ISO 9227 does not recommend this and advocates thorough cleaning if it is to be done.
Although most salt spray tests are continuous, i.e.; the tested sample is exposed to a continuously generated salt spray throughout the test, a few samples do not require such exposure. Such tests are often referred to as modified salt spray tests. ASTM G85 is an example of a test standard that contains several modified salt spray tests that are variations of the basic salt spray test.
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8. Salt spray test improvement
Do you know that ASTM G85 is the most popular global testing standard covering modified salt spray testing? These tests are referred to as Appendices A1 through A5 in ASTM G85. Many of these modified tests first emerged in specific sectors of industry in response to a need for corrosion tests capable of replicating the effects of naturally occurring corrosion and accelerating these effects.
This acceleration is produced through the use of chemically altered salt spray solutions, often in combination with other test climates, and in most cases these test climates cycle relatively rapidly over time. While modified salt spray testing is popular in some industries, it has been superseded in many cases by cyclic corrosion testing (CCT). The type of environmental chamber used for ASTM G85 modified salt spray testing is generally similar to the type of environmental chamber used for ASTM B117 testing, but often has some added features such as automatic climate cycle control systems.
9. Salt Spray Test Standard
Salt spray testing is performed according to various international standards which provide guidelines and procedures for performing the tests. These standards ensure consistency and comparability of results across laboratories and industries. Some commonly followed salt spray testing standards include:
It is the most widely recognized standard for salt spray testing. The American Society for Testing and Materials (ASTM) provides guidelines for conducting Neutral Salt Spray (NSS) and Acidizing Salt Spray (ASS) tests. ASTM B117 outlines testing room requirements, sample preparation, test duration, evaluation criteria, and reporting of results.
The International Organization for Standardization (ISO) has developed standards for salt spray corrosion testing. ISO 9227 provides detailed instructions for conducting NSS and ASS tests. Test conditions, such as temperature, humidity, and salt concentration, as well as sample preparation, test duration, and evaluation methods, are specified.
3) JIS Z 2371
This standard is published by Japanese Industrial Standards (JIS) and is widely used in Japan and other Asian countries. JIS Z 2371 specifies the salt spray test method, including laboratory requirements, test conditions, sample preparation, test duration, and evaluation criteria.
4）DIN EN ISO 9227
ISO 9227 is the European equivalent. It provides guidelines for conducting salt spray tests in European countries. Test parameters, sample preparation, test chamber specifications, test duration, and evaluation methods are outlined in DIN EN ISO 9227.
The Bolt of America (DoD) follows this standard to test the environmental acquisition of military equipment. It includes a section dedicated to salt spray testing (Method 509.6). MIL-STD-810 outlines test requirements, sample preparation, test duration, evaluation criteria, and reporting of results.
It makes sense that these standards may have specific requirements and variations in terms of test duration, test conditions, and evaluation criteria. Therefore, it is critical to refer to the specific standards relevant to your industry and region to ensure accurate and consistent test results.
In addition, it is worth mentioning that some industries or companies may have their own internal specifications or standards for salt spray testing, which may be based on or derived from the above-mentioned international standards. These internal standards should be followed when conducting salt spray tests within a particular organization or industry.
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11. Limitations and Precautions
While salt spray testing is a widely accepted method of evaluating corrosion resistance, it does have limitations. It should be noted that test conditions may not exactly replicate real-world conditions, and results may not directly correlate to the long-term performance of materials in real environments.
Additionally, the test may not take into account other factors such as temperature changes, UV exposure, or mechanical stress that could affect corrosion. Therefore, it is crucial to use the salt spray test as one of several tools in the comprehensive evaluation process.
12. What is ISO 17025 and how does it relate to salt spray testing?
ISO 17025 is an international standard for the assessment and calibration of testing laboratories. It specifies the requirements that a laboratory must meet in order to be accredited. Another international standard, ISO 9227, specifies a method for evaluating the repeatability and reproducibility of salt spray tests using corrosion reference samples.
There are also important relevant standards to know, such as JIS Z 2371 and ASTM G85.
A laboratory’s accreditation ensures that corrosion tests such as salt spray tests are reliable and repeatable. This makes it possible to compare the corrosion resistance of coatings and finishes using test results performed by different laboratories.
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Salt spray testing is an important tool for evaluating the corrosion resistance of materials. By exposing the material to a simulated salt-containing environment, the test provides insight into its performance and durability.
Product manufacturers, researchers, and quality control professionals can rely on salt spray testing to make informed decisions about material selection, coatings, and design improvements. However, it is important for you to remember that test results should be interpreted in conjunction with other tests and real-world observations to ensure a complete understanding of the material’s corrosion behavior. If you have any further questions, please contact Linkotest for consultation.