INTERGRANULAR CORROSION TESTING

 

INTERGRANULAR CORROSION TESTING: AN OVERVIEW OF ASTM A262

·         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. 

·         To understand intergranular corrosion testing or intergranular attack (commonly abbreviated as IGC or IGA), it is important to understand what causes the process to occur.

·         Metals like stainless steels and aluminium 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

·         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. 

·         ASTM A262 testing is a popular method of choice due to the variety of practices available and the relatively short turnaround for results.

·         All five methods within this specification involve exposing specimens to a chemical mixture designed to encourage corrosive behaviour.

·         At the conclusion off the test, specimens are either visually examined or measured for weight loss and compared to an established corrosion rate for that specific material type.

The five intergranular corrosion testing methods within ASTM A262 are:

·         The Oxalic Acid Test

·         The Strauss Test

·         The Huey Test

·         The Streicher Test

·         The Copper Sulphate Test

Oxalic Acid Test (ASTM A262 Practice A) 

·         This simple etching technique is used as a quick screening method to ensure that a material is free of intergranular corrosion susceptibility.

·         The Oxalic Acid test is only used to ensure that no corrosion exists; samples are labelled either "Acceptable" or "Suspect".

·         A different ASTM A262 test method must be used to quantify the level of corrosion or to disqualify a material for use. 

·         The Oxalic Acid test is also only applicable to corrosion associated with chromium carbide precipitates and is only useful for specific material grades.

·         Please refer to the specification for more details.

Streicher Test (ASTM A262 Practice B)  

·         Generally used for stainless steel and nickel alloys, the Streicher test involves boiling the specimen in a Ferric Sulphate - Sulphuric Acid solution for 24-120 hours and evaluating the results.

·         The level of corrosion is determined by mass loss. 

Huey Test (ASTM A262 Practice C) 

·         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. 

Strauss Test (ASTM A262 Practice E) 

·         This test is used exclusively to determine corrosion in areas high in chromium carbide formations, and is not as effective for intergranular attack caused by other means.

·         Strauss test specimens are boiled in a Copper Sulfate - Sulfuric Acid mixture for 15 hours.

·         A bend test followed by a visual or micro-examination are used to determine "Pass" or "Fail" of test specimens.  

Copper Sulfate Test (ASTM A262 Practice F) 

·         This method is a 120 hour boiling test for "as received" specimens of stainless steel. The Copper Sulphate Test is especially effective for determining susceptibility to intergranular attack for low carbon steels.

·         Element performs all methods of ASTM A262 at our ISO 17025 and Nadcap accredited laboratories across the US and Europe.

·         Our knowledgeable scientists have the experience and expertise necessary to determine the most appropriate intergranular attack testing methods for your material while providing actionable results you can rely on.