To conduct these tests, w e carefully choose a method w hich is suitable for the steel grade and grain boundary composition to be tested. Intergranular corrosion in stainless steels may result from precipitation of carbides, nitrides or intermetallic phases. Only in the most highly oxidizing solutions can intergranular attack be caused by intermetallic phases. When a test is to be restricted to carbides, in a material containing nitrides or intermetallic phases, a less oxidizing solution is chosen. The test is used for acceptance but not rejection of material. This procedure measures the susceptibility of stainless steels and nickel alloys to inter granular attack associated w ith the precipitation of chromium carbides at grain boundaries.
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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. Why were these tests designed? 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. ASTM G28 lists alloys and specifies the test exposure for each. This test should be avoided for most CRAs, except in the circumstances described below, in section 4.
As 15 hours is a difficult test period in practice, and is only a minimum, conducting the test for 20 hours overnight means that the two tests are equivalent. None of these are identical and cross substitution of results is not possible. ASTM G28 method B is a more aggressive test than method A and is intended for more corrosion resistant alloys, as listed in the document.
SEP method 3 is a special environment only suitable for the Ni-Mo alloys, as described below, in section 4. 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. Test Samples 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. 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.
Selection of a Test Method 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. It can be seen that some alloys fall into two categories, and no guidelines are given for selecting the most appropriate.
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. Conducting the Tests 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.
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. We provide accurate, repeatable results along with world-class customer service. We offer ASTM A testing from Houston for customers around the world; with overnight shipping, we can start immediately. We work with you to select the right method for your steel grade, and then deliver clear results in a reasonable time and budget. How do you select the right ASTM corrosion testing method for your material?
Intergranular Corrosion Testing: An Overview of ASTM A262
More A 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. It is informational only and not an official part of the standard; the full text of the standard itself must be referred to for its use and application. ASTM does not give any warranty express or implied or make any representation that the contents of this abstract are accurate, complete or up to date. Significance and Use 6.