ASTM A262 is a common intergranular corrosion testing method that can quickly screen batches of material to determine corrosion susceptibility. The ASTM A262 testing specification contains five unique intergranular corrosion tests. Choosing the correct method(s) rely on a complete understanding of your material and processes, as well as the concept of corrosion itself.
Metals like stainless steels and aluminum contain elements such as niobium and chromium, often integrated because of their natural corrosion resistance. However, when a material is exposed to high temperatures for long periods of time, a process called sensitization may occur. Sensitization causes the grain boundaries of a material to precipitate, creating carbide deposits and causing the material to be susceptible to intergranular attack.
Choosing an Intergranular Corrosion Test Method as per ASTM A262
In everyday 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, numerous methods (Practices B-F) have been developed to asses
Since high temperature processing (such as heat treatment) is a common practice, many manufacturers wish to perform intergranular corrosion qualification testing on each batch, to ensure that the material has been processed correctly and does not show signs of corrosion susceptibility.
The ASTM A262 tests can determine if the proper heat treatment was performed or if the alloys are in danger of intergranular corrosion occurring in use. These tests are often run as a qualification test to ensure each batch of stainless steel is properly prepared. Each ASTM A262 Practice specification includes a list of the grades of stainless steels and the acceptable etch structures for the specific alloys. For example, many low-carbon and stabilized stainless steels (e.g. 304L, 316L, 317L, and 347) must be subjected to a sensitizing heat treatment prior to testing by the oxalic acid etch test (Practice A).
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
Generally used for stainless steel and nickel alloys, the Streicher test involves boiling the specimen in a Ferric Sulfate – Sulfuric Acid solution for 24-120 hours and evaluating the results. The level of corrosion is determined by mass loss.
ASTM A262, Practice C – “The Huey Test” Nitric Acid
Similar to the Streicher test, the Huey test method uses a nitric acid solution and subjects the specimen to five 48-hour boiling intervals. The samples are weighed at each interval to determine mass loss and degree of susceptibility to intergranular attack. This method is preferable for chromium depletions and corrosion in intermetallic phases.
ASTM A262 – Practice E – “The Strauss Test” Copper – Copper Sulfate – 16% Sulfuric Acid
IGC testing A262 Practice E refers to a standard test method used to evaluate the intergranular corrosion (IGC) resistance of stainless steel and high-alloy materials. This test method, described in Practice E of ASTM International standard A262, involves immersing specimens of the material in a solution of nitric acid and ferric nitrate. The solution is used to simulate the corrosive environment that can cause IGC in some materials. The appearance of the specimens is then evaluated to determine the degree of intergranular corrosion that has occurred.
Intergranular corrosion is a type of corrosion that occurs along the grain boundaries of a material. It can be caused by a variety of factors, including improper heat treatment, the presence of impurities in the material, or exposure to a corrosive environment. The A262 Practice E test is used to evaluate the susceptibility of a material to IGC and is commonly used in the aerospace, chemical processing, and petroleum industries, among others.
ASTM A262 – Practice F Copper – Copper Sulfate – 50% sulfuric acid
This method is a 120 hour boiling test for “as received” specimens of stainless steel. The Copper Sulfate Test is especially effective for determining susceptibility to intergranular attack for low carbon steels.
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