ASTM G31 Standard Guide for Laboratory Immersion Corrosion Testing of Metals

Get Certified ASTM G31 for Reliable Corrosion Resistance Data 

Certification is the documentation process that proves a metal or alloy has the corrosion resistance needed for a design application before it enters service. Representative specimens are immersed in the accredited laboratory according to ASTM G31 guidelines, and then a report is produced that contains the corrosion rate calculated for the test conditions. This step enables engineers to compare materials and choose alloys that serve in corrosive service applications. 

The scope of ASTM G31 includes:

• Material type: Metals and alloys tested by immersion in a corrosive test solution. 
• Purpose: Weight loss or gain; corrosion cup apparatus. 
• Properties measured: Corrosion rate, mass change, and visual corrosion attack (pitting, intergranular corrosion). Place a prepared specimen in a corrosive solution for a specified time and weigh before and after cleaning. 
• Method: Highly trained technicians immerse the prepared specimen in a corrosive solution for a defined period, then measure its mass change before and after cleaning. 
• Result: Corrosion rate (mpy or mm/year) reported, as mass loss or mass gain (g) and localized corrosion observations. 
• Test Limitations: ASTM G31 does not specify a single inflexible procedure for all metals because corrosion behavior is different for different metals, solutions, and exposures. 
• Environmental conditions: When applicable, a large surface-area-to-mass ratio and a low edge-area-to-total-area ratio, typically less than 20 percent. 
• Applications: Material selection, alloy development, quality control, and comparative corrosion resistance evaluation. 

What are the Uses of ASTM G31?

ASTM G31 testing determines the corrosion resistance of a metal or alloy in a specific solution. Engineers then compare the results to decide whether a metal is suitable for a corrosive service condition and to compare the performance of various materials. This standard- 

• Evaluates corrosion rates of metals using mass gain or mass loss.
• Compares corrosion resistance of the various metals or alloys in the same environment.
• Evaluates localized corrosion attack (e.g., pitting, intergranular corrosion)
• Helps to compare the different materials available to use in a particular corrosive environment.
• Helps with quality control and material development of corrosion-resistant alloys.
• Helps to achieve reproducible and comparable results from laboratory to laboratory.
• Helps make decisions on choosing metals for chemical processing, marine, and industrial uses. 

Which materials are under ASTM G31?

ASTM G31 is used for metals and alloys that are tested in an immersion test solution, such as reactive metals like titanium and zirconium, which have tightly bonded oxide layers. With the right sample geometry, the guide assesses the corrosion behavior of a wide variety of metals in corrosive liquids such as acids, bases, and other solutions. Before running the test program, an analyst needs to decide on the metal(s) to test, the test solution, and the time of exposure.

Why is ASTM  Important? 

ASTM G31 provides guidelines for laboratories to develop controlled immersion corrosion procedures for metals and alloys. Proper preparation of specimens, cleaning, and calculation of the corrosion rate ensures reliable results, which enables analysts to make the correct material selections. The guidance provides support for analysts in developing procedures that provide repeatability and comparability of results from laboratory to laboratory and from batch to batch of material.

ASTM G31 Testing Process and Data Collection

The analyst selects a representative metal specimen and prepares it to the recommended surface finish and geometry. The analyst weighs the specimen before placing it in the test solution for the specified time, temperature, and agitation. The operator then removes the specimen at the end of the exposure period, cleans it as per Practice G1, and weighs it again to calculate the mass change. Using the mass change, the specimen surface area, the metal density, and the time of exposure, the corrosion rate is calculated by the analyst and recorded in the compliance report.

Caption: ASTM G31 Immersion Corrosion Testing of Metal Specimens in Corrosive Solution

Alt text: The image shows a metal specimen undergoing immersion corrosion testing under ASTM G31. 

Common Challenges and Troubleshooting

After cleaning, technicians follow Practice G1 to ensure specimens are cleaned consistently and corrosion products are completely removed, preventing bias in the corrosion rate calculation. Consistency of test conditions between specimens eliminates variations in agitation and temperature during exposure that affect corrosion rate repeatability. Analysts use mass gain to calculate the rate of corrosion of reactive metals like titanium and zirconium that form oxide layers that resist conventional cleaning methods. With a testing program, these challenges are minimized with regular equipment calibration and trained staff.

• Technicians rely on the laboratory to provide the corrosion rate in mpy or mm/year, determined from mass change, surface area, metal density, and exposure duration. 
• Technicians record the mass in grams for each sample before and after exposure to determine mass loss or, for reactive metals, mass gain.
• Technicians record any localized corrosion observed in the laboratory, such as pitting or intergranular attack.
• Technicians note that the final report omits an overall performance value and instead compares the measured corrosion rate against expected benchmarks for similar metals and solutions.

Link to ASTM G31