Intergranular Corrosion (IGC)

Intergranular corrosion (IGC) is a type of corrosion that occurs along the grain boundaries of a material, making these boundaries more susceptible to corrosion than the rest of the material. This corrosion can weaken the bonding force between crystal grains, leading to a loss of mechanical strength. It is a type of corrosion that occurs along the grain boundaries of a metal, particularly in alloys such as stainless steel along with some nickel-based and magnesium alloys.

IGC typically happens due to sensitization of the material, where the grain boundaries become susceptible to corrosion due to the depletion of certain elements.

Factors such as improper heat treatment, impurities in the material or exposure to corrosive environments can contribute to intergranular corrosion.

This type of corrosion can severely weaken the material, leading to structural failure if left unchecked. Hence, IGC Testing is crucial to assess a material's resistance to this type of corrosion.

Various methods are used for IGC testing, including the Copper - Copper Sulfate - Sulfuric Acid Method, Nitric Acid Method, Nitric Acid - Hydrofluoric Acid Method, and others. These tests help evaluate the susceptibility of materials to intergranular corrosion under specific conditions. ?

Standard like ASTM A262 provide guidelines for conducting these tests and is a popular intergranular corrosion testing method. There are different types of intergranular corrosion tests detailed in the ASTM A262 standard.

In regular applications, corrosion varies by materials and solutions. For example, in highly oxidizing solutions, intergranular attack can occur due to intermetallic phases, while attack of carbides may occur somewhat less oxidizing solutions. Due to the variance of attack in different materials, various methods have been developed for assessment purpose.

ASTM A262, Practice A?- “Oxalic Acid Test”?Oxalic Acid Etch

Practice A, the oxalic acid etch test is used as a rapid technique to screen samples of certain stainless steel grades to ensure they are free of susceptibility to intergranular attack. The test is generally performed for acceptance of materials, but not sufficient for rejection of materials.

ASTM A262 Practice B – “The Streicher Test”?Ferric Sulfate – Sulfuric Acid

Practice B, also known as the Streicher test, uses weight loss analysis to provide a quantitative measure of the materials performance. This practice includes boiling the sample for 24 to 120 hours in the solution above, and measures the materials performance quantitatively. It is typically used for stainless alloys such as 321 and 347, Cr-Ni-Mo stainless alloys, and nickel alloys to evaluate the intergranular attack associated with the precipitation of chromium carbides at grain boundaries.

ASTM A262, Practice C – “The Huey Test”?Nitric Acid

In Practice C, the Huey Test, samples are boiled for five 48-hour periods in a 65% Nitric Acid solution. The weight loss is calculated after each step, and reveals if the sample has been properly heat-treated. Please specify the maximum allowable corrosion rate and any available data on the sensitizing heat treatment performed. The Huey test works well to analyze chromium-depleted regions and intermetallic precipitates, such as sigma phase, and is used for materials in strongly oxidizing environments such as nitric acid

?ASTM A262 – Practice E – “The Strauss Test”?Copper – Copper Sulfate – 16% Sulfuric Acid

Practice E, the Strauss test, is performed to assess attack associated with chromium-rich carbide formation. The sample is boiled in a Cu-Copper Sulfate mixture and then bent 180° over an equal diameter bend. The test uses a visual inspection of the surface of the bent specimen to determine pass or fail.

ASTM A262 – Practice F?Copper – Copper Sulfate – 50% sulfuric acid

Practice F, is a weight-loss based analysis that provides a quantitative measure of the materials performance, and is commonly used to analyzed as-received stainless steels.

IGC testing helps to assess the quality and integrity of materials, particularly stainless steel, to ensure they meet required standards and specifications for various applications, such as in the chemical industries, refinery, aerospace etc.