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There are a number of recognised tests when it comes to the intergranular corrosion testing of corrosion resistant alloys CRA. This document describes all three of these tests for four common standards, and describes the differences. Just remember that these tests were devised for specific groups of alloys and should not be applied to all CRAs!
This document also points out which alloys the tests are intended for, and recommends the most appropriate test when none is specified. Intergranular corrosion tests are primarily designed to detect carbide precipitates on grain boundaries. However, although some tests also claim to detect intermetallic phases, inter granular corrosion tests should not be relied on for this, as the response of CRAs to the test solutions vary widely.
These tests were introduced when the carbon content in CRAs typically hit 0. This means that carbide formation during welding is highly unlikely, and modern CRAs should easily pass one of these tests. The latter is a German test, but it is often called up in European projects. The exception to this is ASTM G28 method A, where rates for the onset of intergranular corrosion are described in reference 1.
For many alloys, the onset is at 0. Some suggested criteria for the listed alloys are shown in Table 2, below. Good material should have a corrosion rate substantially less than the threshold value. For alloys not listed in this standard, it will be necessary to carry out some tests on material in various conditions to determine the threshold. Where a bend test is required, the sample must be bent over a mandrel of a diameter related to the specimen diameter, without any cracking, as determined by inspection under a low power microscope and a microsection in any suspicious areas.
The specifications for bend testing are shown in Table 4. All bend tests require a tensile test machine or similar, with suitable grips and sufficient strength to bend the strongest alloy. Samples may be rectangular or cylindrical to suit the test vessel, but where a bend test is required, it is suggested that the sample be a minimum of mm long.
The most convenient size is often a cylinder 10mm diameter by mm long, but other dimensions are possible to suit particular product forms. Some standards require the test sample to be aged prior to testing, to simulate the precipitation that might occur during welding.
It does not say what to do about duplex or super austenitic alloys. ASTM G28 does not require samples to be heat treated. ISO is specifically for duplex and all austenitic stainless steels, and recommends ageing at two different temperatures:. Both of these tests should be followed by water quenching. Alternatively, a welded test-piece in the as-welded condition may be tested.
SEP does not require ageing, although it might be required in a specification calling up this standard. The test methods in Table 1 were designed with specific alloys in mind, but the proliferation of new CRAs has meant that the selection of a suitable test method is difficult. ASTM A was designed for austenitic stainless steels, specifically the series alloys.
The Huey test method C is only for molybdenum-free grades, such as L, as alloys with molybdenum show high rates of attack in this test. Hence, it is normally only used for stainless steels intended for nuclear service, or another application involving nitric acid.
It has been used for the duplex stainless steel UNS S , which has been used for nitric acid cooler tubes. Methods B and E are suggested for all grades of series austenitic alloys, while method F is suggested for cast type alloys. ASTM G28 was designed for nickel-based alloys, and a list of applicable alloys for methods A and B is in the standard. For an alloy that is not listed, look for an alloy with a similar composition to a listed alloy to determine the most suitable test.
ISO is the most rigorous in terms of defining test methods, and says which alloys should use which test methods as follows:. It can be seen that some alloys fall into two categories, and no guidelines are given for selecting the most appropriate. Where a standard is included in a purchase specification, this must be followed, although it is advised that unsuitable tests, such as the Huey test for duplex and super austenitic stainless steels, are queried.
More suitable alternatives are offered in Table 5, below. Where no specific test is specified, Table 5 suggests the most appropriate alongside a second choice, if the first is declined. When a test has been selected, follow the detailed directions in that standard for making up the test solution, the preparation of test samples and the finish to be used. There are other requirements in these standards over details that are not discussed in this document, and these should be followed, such as conducting bend tests.
Intergranular corrosion occurs as a result of precipitation of nitrides , carbides , and other intermetallic phases , such as sigma phase, that occurs along the grain boundaries. If materials with incorrect heat treatment enter service, they are liable to crack or fail by intergranular corrosion IGC much more rapidly than properly treated materials; ASTM A is a screening test to help find batches that are incorrectly processed. Intergranular corrosion shows up as the dark black lines around the grain boundaries. How do you select the right ASTM corrosion testing method for your material? The first and fastest step is the ASTM A Practice A, a rapid same-day screening method in Oxalic acid to determine the susceptibility to intergranular corrosion.
Historical Version s - view previous versions of standard. Work Item s - proposed revisions of this standard. More A This specification covers the standard practices for detecting susceptibility to intergranular attack in austenitic stainless steels. Methods for preparing the test specimens, rapid screening tests, apparatus setup and testing procedures, and calculations and report contents are described for each testing practice. The etch structure types used to classify the specimens are: step structure, dual structure, ditch structure, isolated ferrite, interdendritic ditches, end-grain pitting I, and end-grain pitting II. This abstract is a brief summary of the referenced standard.
Intergranular Corrosion Testing: An Overview of ASTM A262