Crevice corrosion tests are essential for testing the corrosion resistance of stainless steels and Nickel- and Chromium-based alloys. During the process, metal specimens are attached to tetrafluoroethylene (Teflon) blocks using elastic bands of the same material. This setup creates ideal stagnation zones for observing the localized corrosion phenomenon.

Samples are immersed in ferric chloride (FeCl3) solution at constant temperature for a specific period. Subsequently, the samples are rinsed, weighed and examined to assess the mass loss and average and maximum depth of crevices in the contact areas between the metal and elastomers.

Identification of critical temperature

ASTM G 48 provides guidelines for determining the temperature at which the crevice corrosion process begins. The initial temperature is calculated by multiplying the percentages of Chromium, Nickel and Molybdenum by specific coefficients. If crevices with a depth greater than or equal to 0.025 mm are observed at the calculated temperature, the test is repeated by lowering the temperature. If, on the contrary, the crevices are more superficial, the test is repeated by raising the temperature. This method makes it possible to accurately identify the critical crevice initiation temperature.

Description of the Crevice phenomenon

The term “crevice” denotes a form of localized corrosion that occurs near cracks, crevices and under shielded surfaces, where stagnant solutions can accumulate. Interstitial areas act as anodes, facilitating iron reduction, while surrounding areas promote cathodic reactions, such as water and oxygen reduction. This corrosive mechanism is typically observed at the interface between two metals, a metal and a nonmetal (e.g., ceramics) or a metal and deposits of various kinds (e.g., silicates).