Evaluating corrosion tests is crucial for determining the lifespan of materials, but it is not always a straightforward task. In our laboratory, we often encounter various factors that can distort the accuracy of test results, thus affecting the final assessment of the quality of surface finishes on the tested samples.
Assessing Corrosion Damage to the Substrate
One common challenge is identifying the actual corrosion damage to the metal substrate after a corrosion test. The first step is assessing the degree of delamination of the coating. To evaluate degree of delamination, we use TESA 4657 adhesive tape, which mechanically strips off parts of the coating around the scribe that no longer adhere to the substrate after the corrosion test. However, the tape may not always reveal the entire corroded area on the sample. For this reason, we apply a non-corrosive paint stripper, which allows us to remove larger areas of the coating and better assess the extent of corrosion damage to the substrate.
Corrosion Product Drips: How to Deal with Them?
Occasionally, corrosion product drips occur during testing, forming on inadequately coated edges and subsequently dripping onto the test surface. These products can then obscure the true condition of the sample’s surface, making accurate evaluation difficult. Fortunately, there is a simple solution: gently remove these drips using a soft brush and water. This procedure allows us to clean the sample without damaging the coating or the test surface. Once thoroughly dried, we can evaluate the sample's condition without distortion.
The Dual Nature of Blisters in Coatings
Blisters that form in coatings during corrosion testing can be of two types. It is very important to evaluate them as soon as the corrosion test is completed. Blisters may either be filled with water or with corrosion products. Blisters filled with water tend to deflate and disappear over time as they dry out. On the other hand, blisters filled with corrosion products cannot dry out and maintain a larger volume. This difference is crucial for properly assessing the condition of the sample after the corrosion test.
Water-filled blisters
Blisters filled with corrosion products
Anodic vs. Cathodic Coatings: How do They Affect Corrosion?
When evaluating corrosion tests, it is also important to distinguish between anodic and cathodic coatings, as the material composition of the coatings has a significant impact on corrosion behaviour. Anodic coatings (e.g., galvanized steel) corrode uniformly and gradually lose thickness. The metals used for coating are less noble in these coatings, resulting in the creation of a so-called sacrificial anode, hence the term for these coatings. In contrast, cathodic coatings (e.g., nickel-plated steel) protect the substrate until the coating is damaged. If the coating is damaged (e.g., by cracking or imperfect plating), the base metal begins to corrode preferentially. The resulting corrosion products can build up pressure under the coating, causing it to detach or flake off. The cathodic coating itself, however, resists corrosion.
At SQI Laboratory, corrosion testing is part of our daily work. Do you have any questions or need help with specific issues? Feel free to contact us at the details below, and we will be happy to assist you.
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