ISO 3651-1 Determination of the Resistance of Stainless Steels to Intergranular Corrosion - Part 1: Austenitic and Ferritic-Austenitic (duplex) Stainless Steels

This part of the ISO 3651 standard, developed by the International Organization for Standardization (ISO), specifies a method for determining the resistance to intergranular corrosion of austenitic and ferritic-austenitic (duplex) stainless steels in nitric acid medium by measuring mass loss (Huey test). It also specifies the purpose that can be assigned to the test.

The method is applicable only to austenitic and ferritic-austenitic (duplex) stainless steels supplied as rolled or forged products, pipes and castings and intended for use in a strongly oxidizing medium (for example, relatively concentrated nitric acid). In general, the Huey test should not be used for molybdenum containing grades unless the material being tested is used in nitric acid service.

It is important to note that the result of the corrosion test is strictly valid only for the corrosive medium used in the test. It provides a basis for estimating resistance to intergranular corrosion, but cannot be used to check resistance to other forms of corrosion (general corrosion, pitting, stress corrosion, etc.).

The user must adapt the specified corrosion test to the use to be made of the alloy. In no case should this test be considered an absolute criterion of the quality of the alloy. The term "intergranular corrosion testing" refers to the corrosion test performed by preferential attack of grain boundaries.

Austenitic and ferritic-austenitic (duplex) stainless steels are susceptible to such attack when kept at a temperature between about 500°C and 1°C. This heat cycle, which can cause sensitization to intergranular corrosion, may occur as a result of incorrect solution treatment during hot forming (forging, rolling) or during a welding operation.

Intergranular corrosion in nitric acid may be associated with one or more of the following:

• Precipitation of chromium carbides
• Precipitation of intermetallic compounds such as sigma phase in molybdenum containing grades
• Separation of impurity elements into grain boundaries

The interpretation of the result (eg maximum corrosion rate) will be the subject of an agreement between the parties involved.

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