What is CCT testing?

The Cyclic Corrosion Test (CCT) method is designed to be a more realistic salt spray test than traditional steady-state exposure tests, such as traditional salt spray testing. Real-world atmospheric exposures typically include both humid and dry conditions, and cyclic corrosion testing can simulate accelerated laboratory testing under these natural cyclic conditions.

Research has shown that cyclic corrosion testing produces results that more closely resemble those observed outdoors in terms of relative corrosion rates, structure, and morphology, providing better correlation with outdoor environmental conditions and effectively assessing various corrosion mechanisms, including general corrosion, galvanic corrosion, and crevice corrosion. It utilizes a variety of methods, including ASTM G85. ASTM G44. and ASTM D5894.

This modified accelerated corrosion technique is designed to simulate the material degradation and failure types found in outdoor corrosion environments. Simple exposures may involve cycling between salt spray and dry conditions, while complex automotive methods require multi-step cycles, including immersion, humidity, condensation, and salt spray and dry conditions.

In recent years, cyclic corrosion testing has gained popularity in the automotive industry. By automatically cycling through different climates, the tested specimens experience the same dynamic environments as those found in nature. The goal is to accelerate the occurrence of naturally occurring failure types, enabling manufacturers and suppliers to more accurately predict product life expectancy.

Before the development of cyclic corrosion testing, traditional salt spray testing was virtually the only option available to manufacturers. However, it had never been used to predict a product's "real-world" life expectancy because its specified test conditions were unrepresentative of natural environments. Results could only be compared with past experience for quality audits, such as paint tests, which can be used to "monitor" the production process and alert potential manufacturing issues or defects.

The lack of correlation between results obtained from traditional salt spray testing and "real-world" atmospheric corrosion in vehicles has left the automotive industry without a reliable method for predicting product life expectancy. This has been a concern in an industry where corrosion warranty periods are increasing, with new vehicles now covered for several years. With increasing consumer pressure for improved vehicle corrosion resistance and the catastrophic commercial consequences of high-profile corrosion failures experienced by some automakers, the automotive industry has recognized the need for different types of corrosion testing. Furthermore, reproducing these diverse environments in an environmental chamber requires more flexible testing procedures than standard salt spray chambers.

Ⅰ. What is cyclic corrosion testing?

Cyclic corrosion testing is a more realistic salt spray test than traditional steady-state exposure. Because actual outdoor exposure typically involves both dry and wet environments to simulate these natural, cyclical conditions, accelerated laboratory testing is valuable. Studies have shown that the relative corrosion rates, structure, and morphology of samples after cyclic corrosion testing closely resemble those observed in outdoor environments. Therefore, cyclic corrosion testing more closely resembles actual outdoor exposure than traditional salt spray methods. They are effective in evaluating many corrosion mechanisms, such as general corrosion, galvanic corrosion, and crevice corrosion. Cyclic corrosion testing is used to assess how materials resist corrosion under changing outdoor environmental conditions, such as moisture, heat, and salt exposure. This laboratory testing accelerates the different corrosion failures that materials may experience in real-world environments. As a result, manufacturers can realistically and rapidly assess how materials degrade over time, gaining insights into durability and providing information for better quality control.

Cyclic corrosion testing aims to replicate the corrosion patterns found in outdoor environments. CCT testing exposes samples to a series of cyclic conditions. Simple exposure cycles, such as the Prohesion test, expose samples to cycles of salt spray and dry conditions. More complex automotive test methods require not only salt spray and dry cycles, but also immersion, humidity, and condensation cycles. Initially, these test cycles were performed manually, with laboratory operators moving samples from a salt spray chamber to a humidity chamber to a dryer. More recently, microprocessor-controlled test chambers have automated these test steps, reducing test uncertainty.

CCT can evaluate various corrosion mechanisms, such as:

Crevicular corrosion: A form of localized corrosion that occurs when moisture and contaminants are trapped in confined spaces, such as crevices.

Galvanic corrosion: A type of electrochemical corrosion that occurs when two metals are in electrical contact and exposed to an electrolyte, resulting in a difference in electrochemical potential.

General corrosion: A type of uniform corrosion that occurs on a material surface due to chemical or electrochemical reactions with the environment.

CCT can be used on a wide variety of materials. For example, metals such as steel, zinc, and aluminum can be tested with CCT, as can non-metallic materials such as composites and plastics. CCT is also very useful in testing protective coatings applied to metals, such as powder coatings.

Ⅱ. Cyclic Corrosion Testing Process

A typical cyclic corrosion testing process includes the following key steps:

1. Specimen Preparation: Prepare specimens that meet specifications according to the test standard or requirements. This may involve cutting, cleaning, and handling the material.

2. Test Chamber Setup: Adjust test chamber parameters such as temperature, humidity, and salt spray concentration to simulate the specific corrosive environment based on the test requirements.

3. Test Start: Place the specimen in the test chamber and initiate the cyclic corrosion test program. The program automatically cycles through different environmental conditions, such as salt spray, dry, humid, and condensing, according to a pre-set schedule.

4. Monitoring and Recording: Throughout the test, closely monitor and record parameters such as temperature, humidity, and salt spray concentration, as well as the corrosion status of the specimen. This typically involves regular visual inspections, measuring corrosion-related defects, and documenting the results.

5. Results Analysis: After the test, analyze the specimen's appearance, structure, and corrosion level. Compare these results against established standards or performance requirements to assess the material's corrosion resistance.

Ⅲ. Application Scenarios of Cyclic Corrosion Testing

Cyclic corrosion testing is widely used in various industries because it can more realistically simulate outdoor corrosion environments:

● Automotive Industry: Automotive materials, particularly metals and coatings, must withstand a variety of harsh environments. Cyclic corrosion testing effectively evaluates the corrosion resistance of these materials over long-term use.

● Aerospace: Aerospace materials must not only be lightweight but also possess extremely high corrosion resistance and durability. Cyclic corrosion testing provides important data for evaluating the performance of these materials in actual flight environments.

● Marine and Offshore Engineering: The marine environment is particularly corrosive to materials. Cyclic corrosion testing can simulate conditions such as salt spray, humidity, and condensation to evaluate the corrosion resistance of materials in marine environments.

● Construction Industry: Building materials such as concrete and steel are susceptible to corrosion in outdoor environments. Cyclic corrosion testing helps evaluate the durability of these materials in actual use.

Ⅳ. The Importance of Cyclic Corrosion Testing

With technological advancements and industrial development, the demand for material corrosion resistance is increasing. As an advanced testing method, cyclic corrosion testing can more realistically simulate outdoor corrosion environments, providing engineers and scientists with a valuable tool for evaluating material durability and performance. Cyclic corrosion testing can more effectively predict the corrosion behavior of materials in actual use, providing a scientific basis for product design, material selection, and improvement, ensuring reliable product operation in harsh environments.

In summary, cyclic corrosion testing, with its close proximity to real-world corrosion environments, diverse testing methods, and wide range of applications, holds a crucial position in the field of material corrosion resistance evaluation. It not only complements traditional salt spray testing, providing a reliable and effective testing method for numerous industries, including automotive, aerospace, marine engineering, and construction, but also becomes a key support for ensuring product quality and improving durability amidst technological advancements and industrial development. In the future, as material performance requirements continue to rise across various industries, cyclic corrosion testing will continue to be optimized and improved, playing a greater role in driving these fields towards higher quality and greater reliability.