The salt spray method was first used to test the corrosion resistance of materials around 1914. In 1939, the neutral salt spray test was incorporated into ASTM B117 standard. This traditional salt spray standard requires samples to be continuously exposed to a 5% salt spray at 35°C. Although the ASTM B117 test standard has undergone several revisions and improvements over its 80 years of application, it has long been believed that the test results of the "salt spray method" do not correlate well with the actual corrosion effects of outdoor exposure. Even so, ASTM B117 remains the primary standard for salt spray corrosion testing, widely used for corrosion resistance testing of coatings, military components, and electronic components.
With increasing demands for material corrosion protection, engineers and researchers have continuously strived to develop test procedures that can more accurately simulate outdoor corrosion effects. In the 1960s and 1970s, Harrison and Timmons in the UK developed the Prohesion test, particularly suitable for testing the corrosion resistance of industrial protective coatings. In recent years, SAE and AISI have begun researching and developing cyclic corrosion tests for automobiles. The results are currently satisfactory, and several articles have been published (4, 5, 6, 7, 8, 9, 10). Japanese researchers have also developed many cyclic corrosion testing methods.
In modern industry, the corrosion resistance of materials is one of the important indicators for measuring product quality and service life. Faced with complex and ever-changing environmental conditions, ensuring that products maintain stable performance under harsh environments such as climate, salt spray, and humidity has become a pressing issue for manufacturers. Cyclic corrosion testing machines, as important testing equipment in this challenge, play a crucial role. They not only simulate the corrosion process in nature but also provide a scientific and efficient solution for evaluating the corrosion resistance of products through accelerated testing methods.
What is Cyclic Corrosion Testing?
Cyclic corrosion testing is a more realistic salt spray test than traditional constant-state exposure. Because actual outdoor exposure usually includes both dry and wet environments to simulate these natural, periodic conditions, accelerated laboratory testing is meaningful. Studies have shown that after cyclic corrosion testing, the relative corrosion rate, structure, and morphology of the samples are very similar to outdoor corrosion results. Therefore, cyclic corrosion testing is closer to real outdoor exposure than traditional salt spray methods. They can effectively evaluate many corrosion mechanisms, such as general corrosion, electrochemical corrosion, and crevice corrosion.
The purpose of cyclic corrosion testing is to reproduce the types of corrosion in outdoor corrosive environments. CCT testing exposes samples to a series of cyclic environments with different conditions. Simple exposure cycles, such as the Prohesion test, expose samples to a cycle consisting of salt spray and drying conditions. More complex automotive testing methods require cycles of immersion, humidification, and condensation in addition to salt spray and drying cycles. Initially, these test cycles were performed manually, with laboratory operators moving samples from salt spray chambers to humidification chambers, and then to drying devices. Recently, microprocessor-controlled test chambers can automate these test steps, reducing experimental uncertainty.
Exposure Conditions
Cyclic corrosion testing utilizes one or more of the following conditions:
Room Temperature Conditions:
In CCT testing, room temperature refers to laboratory room temperature conditions. Room temperature conditions can typically alter the properties of the test sample very slowly. For example, a sample after salt spraying may be left at room temperature for two hours. The sample essentially undergoes a slow drying process under specific temperature and humidity conditions. Generally, there are no corrosive vapors or gases in "room temperature conditions." There is almost no gas flow, the temperature is typically 25 ± 5°C, and the relative humidity is 50% or lower. Laboratory conditions should be monitored and recorded for each test.
Chamber Conditions:
Non-room temperature conditions typically refer to exposure conditions within the test chamber. Switching between different non-room temperature conditions can be achieved by manually moving the test sample from one chamber to another, or by cycling through conditions within a fully automated chamber.
Temperature and relative humidity must be monitored for each test. If possible, an automated control system should be used. Temperature deviations should be accurate to ±3°C or less.
Salt Spray Conditions:
Salt spray conditions can be achieved in a B117 type test chamber or manually under laboratory conditions. Nozzles spray a mist of salt solution. Generally, electrolytes containing other chemicals besides NaCl (sodium chloride) can be used to simulate acid rain or other industrial corrosion. Figure 1 illustrates salt spray conditions.
Humid Conditions:
CCT testing procedures typically require high humidity conditions. The relative humidity requirement is 95-100%. This requirement is specified in ASTM D224711. Sometimes, this can also be achieved by spraying a pure water mist using a B117 test chamber. Figure 2 shows the Q-Fog salt spray tester under humid conditions.
Dry Conditions:
Dry conditions can be achieved in an open laboratory or within a test chamber. Sufficient air circulation within the space prevents stratification and allows for sample drying. The definition of "dry" is complex, and there is still debate regarding whether it refers to surface drying or complete drying of the sample. As product corrosion penetrates, the time required for complete sample drying may increase. Figure 3 shows the Q-Fog under dry conditions.
Immersion Corrosion Conditions:
These conditions typically involve a specific concentration of electrolyte, generally 5%, with a pH between 4 and 8. The temperature is also usually specific. The solution may become contaminated during use and should be changed periodically.
Water Immersion Conditions:
Distilled or deionized water must be used. Water quality requirements are detailed in ASTM D119312. Immersion containers should be made of plastic or other inert materials. The pH of the immersion solution should be between 6 and 8, the temperature between 24 ± 3°C, and the conductivity should be less than 50 mohm/cm at 25°C.
Operating Procedures
Using this testing equipment typically involves the following steps:
1. Sample Preparation: Select representative samples based on testing requirements, ensuring their surfaces are clean and free of contamination. For certain specific tests, sample pretreatment, such as scratching or coating, is necessary to more accurately assess corrosion behavior.
2. Parameter Setting: Set the various parameters of the testing machine according to the testing standards or customer requirements, including temperature, humidity, salt spray concentration, and cycle time. The accuracy of these parameters directly affects the reliability of the test results.
3. Sample Placement: Correctly place the prepared samples inside the test chamber, ensuring appropriate spacing between samples and between samples and the chamber walls to avoid mutual interference or affecting the test results.
4. Test Start-up: After confirming all settings are correct, start the testing machine to begin the corrosion cycle process. During this time, the testing machine will automatically control changes in environmental conditions according to a preset program.
5. Observation and Recording: During the test, periodically observe and record the corrosion condition of the sample surface, such as corrosion morphology and corrosion rate. Quantitative analysis can be performed using methods such as photography and weighing if necessary.
6. Conclusion and Analysis: After the predetermined test cycle is reached, shut down the testing machine and remove the samples for evaluation. Analyze the corrosion resistance of the material based on the test data and observations, and propose improvement suggestions.
Exposure Test Precautions
Various test conditions in CCT exposure may pose potential problems to the repeatability and reproducibility of test results. The following are some points to note:
1. Chamber Load
A fully loaded chamber may require a longer time to achieve temperature transition. Ensure air circulation during testing, and maintain a uniform load on the chamber.
2. Transition (Ramp) Time
Whether manually operated or in a fully automated chamber, transition time is a factor affecting test results. The impact of transition time on test results requires further investigation; transition time should be monitored and recorded as much as possible.
3. Salt Spray Deposition and Uniformity
In CCT, the salt spray deposition rate cannot be measured during the test operation; it is necessary to collect salt spray deposition data over 16 hours of continuous spraying.
4. Test Interruption
When testing must be interrupted, the sample should be placed under non-corrosive conditions, and all interruption details and sample handling should be recorded. Through these detailed steps and precautions, cyclic corrosion testing (CCT) can more accurately assess the corrosion resistance of materials in natural environments, providing a scientific basis for material selection and application.
In many industries, including electronics, automotive parts, aerospace, and building materials, the corrosion resistance of products directly affects their safety, reliability, and user satisfaction. Cyclic corrosion testing machines simulate corrosion phenomena under different environmental conditions, such as salt spray corrosion, damp heat cycling, and alternating drying and wetting, enabling the rapid assessment of the corrosion resistance of materials or products. This not only significantly shortens product development cycles and reduces testing costs, but more importantly, it helps companies identify and address potential corrosion problems early, ensuring that products meet stringent quality standards and market demands.
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