Corrosion is the process of degrading materials (e.g., metal) chemically and can occur naturally or in controlled acidic/basic environments. Natural atmospheric corrosion involves the oxidation of atoms on the surface of the material. This oxidation occurs due to a combination of slightly acidic water contacting the surface and oxygen in the air reacting with the surface. In some controlled environments, several types of acids are used to simulate situations that could occur naturally or to look for inconsistencies in a material. Two common types of corrosion that are examined are intergranular corrosion (IGC) and pitting/crevice corrosion.
IGC focuses on grain boundaries or the areas next to grain boundaries in the microstructures of metals and alloys. These areas are attacked preferentially because they can locally lack the necessary elements needed to help defend against corrosion. The lack of certain corrosion resistant elements is from improper melt chemistry, chemical segregation, or phase precipitation on the grain boundaries. One method used to test for IGC includes boiling an acid solution and exposing a test piece to it for a pre-determined length of time, usually 15-24 hours. The piece is then bent and examined for cracks that appear with IGC is present.
Ferric Chloride Solution is commonly used to determine pitting resistance in ferrous alloys. Exposure to ferric chloride solution correlates to exposure in low pH chloride-bearing environments and in natural seawater at ambient temperature. Pitting corrosion is a localized corrosion attack that creates cavities in material surfaces. Pitting forms when the protective oxide layer is broken down locally or non-uniformities in the material or environment are present, creating a scenario for accelerated corrosion. Sample mass is measured before or after exposure and this difference is used to calculate a corrosion rate. Increasing the temperature of the ferric chloride solution can accelerate the corrosion rate that the material experiences. When pitting occurs, pit depth can be measured to analyze pitting severity throughout the material.
Common application specifications include:
Entry Originally Posted August 2021