ASTM A262 Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels

Get Certified ASTM A262 Compliance for Stainless Steel Evaluation 

ASTM A262 test services are available for assessing susceptibility to intergranular attack in austenitic stainless steels from accredited laboratories. These services provide detailed analytical reports with etch structure classification (Practice A), corrosion rate results (Practices B, C, and F), pass/fail results (Practice E), and compliance documentation. In highly corrosive applications like chemical processing, nuclear, and high-temperature use, certified testing is performed to ensure that the stainless steel material is able to withstand the corrosive effects. Testing can validate the quality of the materials, determine compliance with industry standards, and prevent premature failure through intergranular corrosion. Select a knowledgeable testing provider that is ISO/IEC 17025 accredited to ensure your stainless steels are meeting the intergranular corrosion resistance specifications.

The scope of ASTM A262 includes:

• Material types: Austenitic stainless steels, including molybdenum-bearing grades 316, 316L, 317, and 317L.
• Purpose: Determination of susceptibility to intergranular attack from chromium carbide precipitation, and assessment of material performance in particular corrosive environments.
• Properties evaluated: (Practice A) Etch structure classification. Structures corroded by weight loss: (Practices B, C, and F) Corrosion rate by weight loss.
• Testing Method: Five separate practices: Practice A (Oxalic Acid Etch Test); Practice B (Ferric Sulfate-Sulfuric Acid Test); Practice C (Nitric Acid Test); Practice E (Copper-Copper Sulfate-Sulfuric Acid Test); and Practice F (Copper-Copper Sulfate-50% Sulfuric Acid Test).
• Result: Etch structure classification (step, dual, ditch, isolated Ferrite, interdendritic ditches, end-grain pitting I and II), corrosion rate, or acceptable/nonacceptable classification.
• Limitations of test: The results of these tests do not necessarily reflect the performance of the material in other corrosive environments. The tests are not used to predict the resistance to other forms of corrosion, like general corrosion, pitting, or stress-corrosion cracking.
• Use conditions: Practice A is a screening test, a rapid test to eliminate unacceptable specimens and prevent costly hot acid immersion tests. The quantitative evaluation of material performance is done with practices B, C, E, and F.
• Applications: Chemical processing equipment, nuclear reactor components, high-temperature service materials, and any austenitic stainless steel application for which intergranular corrosion resistance is important.

What are the Uses of ASTM A262 Testing?

ASTM A262 testing helps labs determine susceptibility to intergranular attack for austenitic stainless steels. The results are used for material qualification, quality assurance, and specification compliance by manufacturers, engineers, and quality control personnel. This standard-

• Rapidly screens acceptable specimens using oxalic acid etch test (Practice A).
• Quantitatively measures corrosion rates by weight loss (Practices B, C, and F).
• Measures resistance to intergranular corrosion of molybdenum-containing grades (Practice F)
• Detects sensitization due to precipitation of Chromium carbide.
• Helps in material selection for chemical processing and nuclear applications.
• Confirms acceptable intergranular corrosion resistance as per industry specifications.
• Helps manage quality in the manufacture of stainless steel and stainless steel fabrication.

What Materials Can Be Tested Under ASTM A262?

ASTM A262 is for austenitic stainless steels that are capable of being prepared for intergranular corrosion testing. The method is used for wrought or cast austenitic stainless steels, including molybdenum-bearing types like 316, 316L, 317, and 317L. This oxalic acid etch test is only to be used for the grades provided in the individual hot acid tests. The analysts must establish the type of material, grade, and service environment to choose the suitable test practice.  

Why is ASTM A262  Important? 

ASTM A262 is a standard for testing for susceptibility to intergranular attack in austenitic stainless steels. The intergranular corrosion results from the precipitation of chromium carbides in the grain boundaries, which leaves the matrix around the grain boundary without sufficient chromium to resist corrosion. The condition is called sensitization and can cause premature failure in a corrosive environment. The oxalic acid etch test can be used to quickly accept particular material lots without the need to conduct time-consuming and expensive hot acid immersion tests. The test results enable the manufacturers and engineers to confirm material quality, identify suitable materials for certain environments, and guarantee materials’ performance when they are applied to demanding tasks. 

ASTM A262 Testing Process and Data Collection

Technicians choose the right test practice according to the grade of material and the conditions of use. In Practice A, the technician prepares the sample surface and then etches it by passing an electric current through the oxalic acid etching solution, after which the technician studies the etched structure under a microscope and classifies it. The technician prepares the specimens for Practices B, C, E, and F and immerses them in the solution for an appropriate duration, measures weight loss for the purpose of calculating corrosion rate for Practices B, C, E, and F, or visually inspects bend specimens for cracks. The lab then puts together a detailed report. The technicians document the test practice, test conditions, sample identification, and results. 

Common Challenges and Troubleshooting

The ASTM A262 testing is subject to error when the technician does not prepare the specimen correctly, use the correct solution concentrations, or maintain the appropriate temperatures. Sigma phase in wrought chromium-nickel-molybdenum steels can result in high corrosion rates only in nitric acid. High corrosion rates can occur in nitric acid and ferric sulfate-sulfuric acid solutions in the presence of sigma phase in titanium- or columbium-stabilized alloys and in cast molybdenum-bearing stainless alloys. The presence or absence of intergranular attack in these tests is not necessarily a measure of the performance of the material in other corrosive environments. Technicians always prepare specimens per the practice and ensure proper instrument calibration. 

• The etch structure classification reported in the lab is for Practice A (step, dual, ditch, isolated ferrite, interdendritic ditches, end-grain pitting I and II). A step structure means acceptable material; a ditch structure means suspect material.
• The corrosion rate is reported for Practices B, C, and F, which provide a quantitative measure of the relative performance of specimens evaluated.
• The laboratory reports the results of the bend test for practice test E as either acceptable or unacceptable, depending on what they see.
• Analysis takes into account the material grade and service environment. For grades stabilized for nitric acid and molybdenum grades, a 240-h nitric acid test is required.
• Analysts compare results to product specifications or customer requirements, and a report will be provided stating whether the tested material does or doesn’t meet the applicable specifications.
• Engineers compare the results of intergranular corrosion testing to relevant specifications to determine if the material is suitable for the application.

Link to ASTM A262