When it comes to evaluating the corrosion resistance of materials and surface coatings, two widely used standardized test methods come into play: salt spray testing (SST) and cyclic corrosion testing (CCT) which are both offered through the Qualitest product range. These tests aim to assess how well materials withstand corrosion under accelerated conditions, offering valuable insights for manufacturers seeking to enhance the durability of their products.
Salt Spray Testing (SST): A Closer Look
Salt spray testing, conducted in accordance with the ASTM B117 procedure, involves placing metallic samples with surface coatings in a controlled environment. The samples endure exposure to a dense fog created by atomizing a 5% sodium chloride solution at a temperature of 35⁰C. This test allows for the comparison of the relative corrosion resistance of coatings within an accelerated timeframe, typically ranging from 72 to 1,000 hours.
While SST provides an efficient means of checking corrosion, it's crucial to note that it doesn't perfectly replicate real-world corrosion scenarios. The test doesn't mimic natural environments where corrosion may occur, and it may not be suitable for assessing the corrosion resistance of coatings on non-precoated metals with no galvanized precoating.
Furthermore, the continuous spray during the salt spray test accelerates the wrong corrosion mechanism for galvanized steel. Zinc, known for its corrosion resistance, forms a protective zinc carbonate barrier on its surface. However, the constant spray in SST prevents exposure to the atmosphere needed for this barrier formation. As a result, the salt spray interacts directly with the zinc, leading to rapid corrosion.
Cyclic Corrosion Testing (CCT): Simulating Real-World Conditions
In contrast, cyclic corrosion testing (CCT) offers a more comprehensive approach. Unlike SST, there isn't a universally accepted international CCT standard, but it has gained prominence in the automotive industry. Recognizing that SST doesn't perfectly correlate with real-world corrosion, automotive companies developed their own CCT methods to simulate natural corrosion failures in a controlled laboratory setting.
CCT exposes samples to a series of different environments in a repetitive cycle, providing a more accurate representation of real-world conditions. The test typically includes phases such as salt spray, drying, condensing humidity, and controlled temperature humidity. Each cycle lasts 24 hours, and the overall duration of CCT may range from 40 to 100 cycles, depending on the product and manufacturer.
By incorporating various environmental conditions, CCT methods allow manufacturers and suppliers to predict the service life expectancy of their products more accurately. These methods have evolved according to specific industry requirements and are tailored to the diverse needs of different sectors within the automotive realm.
In conclusion, while salt spray testing remains a valuable tool for assessing corrosion resistance, cyclic corrosion testing stands out as a focussed approach, providing a closer simulation of real-world conditions and offering enhanced predictive capabilities for product durability.