Pitting corrosion tests are essential for evaluating the corrosion resistance of stainless steels and Nickel- and Chromium-based alloys. During these tests, samples are immersed in a solution of ferric chloride (FeCl3) at a constant temperature for a specified period of time. Subsequently, the samples are rinsed, weighed and examined. The main characteristics evaluated are mass loss, average and maximum pit depth, and their density on the sample surfaces.

ASTM G 48 standard

ASTM G 48 establishes procedures for determining the temperature at which the corrosion process is initiated, known as the Critical Pitting Temperature. This value is initially estimated by a formula involving the percentages of Chromium, Nickel and Molybdenum. During the test, if the pit depth reaches or exceeds 0.025 mm, the test is repeated at a lower temperature; if the depth is less, the test is repeated at a higher temperature. This process makes it possible to accurately identify the critical pitting initiation temperature.

Description of the phenomenon

Pitting is a form of localized corrosion that occurs when the passivating layer on the surface of metals is compromised due to chlorine ions. Within the pit, ions become concentrated, the pH is lowered, and oxygen is consumed, turning the area into an anode. The surrounding area acts as a cathode, triggering reduction reactions. The rate of corrosion increases in proportion to the ratio of cathodic to anodic area.

Schematization of the pitting mechanism
The pitting mechanism can be schematized as follows:

• Pit interior: behaves as an anode with high ion concentration, low pH and oxygen consumption.
• Surrounding area: functions as a cathode where reduction reactions take place.

Understanding this phenomenon is crucial to developing more corrosion-resistant materials and improving the longevity of metal components in aggressive environments.