Determine Corrosion in Areas High in Chromium Carbide Formations

Intergranular corrosion (IGC) is a special form of localized corrosion, where the corrosive attack takes place in a quite narrow path preferentially along the grain boundaries in the metal structure. The most common effect of this form of corrosion is a rapid mechanical disintegration (loss of ductility) of the material. Usually, it can be prevented by using the right material and production process. A well-known example relevant to the construction industry is the so-called sensitization of stainless steel. When certain grades of this material are kept at a temperature within the range of 500°C to 800°C for a considerable time, e.g., during a welding process, chromium-rich carbides are formed, resulting in chromium depletion at the grain boundaries. Consequently, the grain boundaries possess a lower degree of corrosion resistance than the residual material, leading to localized corrosive attack.

MATERIALS SUSCEPTIBLE TO INTERGRANULAR CORROSION

The grains in all crystalline materials are separated by grain boundaries which are susceptible to IGC. Some of the common material susceptible to IGC are:

• Steels
• Brasses
• Bronzes
• Aluminum alloys

PREVENTIVE MEASURE OF INTERGRANULAR CORROSION

The prevention methods include:

• Avoiding sensitizing heat treatments;
• Avoiding steels sensitized;
• In promoting the use of steel stabilized with titanium or niobium;
• In restoring the steels previously sensitized through a heat treatment again solubilizes the chromium carbides and limits the metal stay for awareness temperatures;
• Using particular corrosion inhibitors.

TEST SPECIFICATION FOR INTERGRANULAR CORROSION

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

• Practice A- Oxalic Acid Etch Test for Classification of Etch Structures of Austenitic Stainless Steels. The oxalic acid etch test is used for acceptance of material but not for rejection of material. This may be used in connection with other evaluation tests to provide a rapid method for identifying those specimens that are certain to be free of susceptibility to rapid intergranular attack in these other tests. Such specimens have low corrosion rates in the various hot acid tests, requiring from 4 to 240 hrs of exposure. These specimens are identified by means of their etch structures, which are classified according to the certain criteria.

• Practice E – Copper–Copper Sulfate–Sulfuric Acid Test for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels.

This procedure is conducted to determine the susceptibility of austenitic stainless steel to inter granular attack associated with the precipitation of chromium-rich carbides. Once the specimen has been subjected to the solution boil, it is bent through 180° and over a diameter equal to the thickness of the specimen being bent. This test is based on a visual examination of the bent specimen.