Intergranular corrosion acids ASTM

ASTM Standard G 67, Standard Test Method for Determining the Susceptibility to Intergranular Corrosion of 5XXX Series Aluminum Alloys by Mass Loss After Exposure to Nitric Acid, Annual Book of ASTM, 1999, 03.02. 

ASTM A-262 [7.21] has standardized five different methods for testing intergranular corrosion of austenitic stainless steels, after Streicher (two), Huey, Warren and Strauss. The Warren test, using exposure to nitric acid-fluoric acid, is particularly suited for molybdenium-alloyed steels. 

Since intergranular corrosion is limited to corrosion at or immediately adjacent to grain boundaries and is detectable at the microstructural level, simple corrosion rate calculations using weight loss measurements provide no information. Standard ASTM tests, such as the oxalic acid etch test... 

A rather unique mass loss test method, ASTM G 67 (Test Method for Determining the Susceptibility of Intergranular Corrosion of 5xxx Series Aluminum Alloys by Mass Loss After Exposure to Nitric Acid [NAMLT Test]) has been developed to provide a quantitative measure of the intergranular susceptibility of 5xxx series Al-Mg and Al-Mg-Mn allo3rs. The technique is useful to determine whether material has become sensitized by exposure to elevated temperature. Development of the test is reported in Ref 23. 

The most common intergranular corrosion tests are outlined in ASTM A 262 and G 28. Most of these test solutions are strong oxidizing acids or acids with added oxidizers that maintain the specimen at a stable electrochemical potential where accelerated intergranular corrosion will occur in a susceptible material. The test duration is typically 24-120 h, depending on the alloy. Recommended exposure periods for various nickel-base alloys are listed in ASTM G 28 but not in ASTM A 262. Though only austenitic stairrless steels are listed in ASTM A 262, some of the test methods are used for nickel-base alloys. It is important to note that ASTM A 262, Practice A, the cBialic add etch test, is recommended for stainless steels. It is not normally used for nickel-base alloys. 

Intergranular Corrosion Resistance AL 29-4C contains a deliberate titanium addition to stabilize the carbon and nitrogen. AL 29-4C is resistant to intergranular corrosion as determined by the copper-copper sulfate-sulfuric acid tests detailed in ASTM Specification A 763, Practices Y and Z. 

ASTM G66 - test method for visual assessment of exfoliation corrosion susceptibility of 5xxx series aluminum alloys (Asset Test). ASTM G67 - test method for determining the susceptibility to intergranular corrosion of 5xxx series aluminum alloys by mass loss after exposure to nitric acid (Namlt Test). 

It was soon found that among the variables affecting corrosion rates were certain heat treatments that made not only the ferritic stainless steels subject to intergranular attack, but also the austenitic, 18Cr-8Ni alloys. From this simulated service test in boiling 65 % nitric acid, there evolved Practice C of ASTM A 262. Its large-scale use by one... 

Generally, evaluation in the 50 % sulfuric acid ferric sulfate test is by weight-loss. However, on alloys C and C-276 the relatively low chromium content of about 15 % (i.e., 3 % less than in Type 304 steel) results in a somewhat high rate of general corrosion that tends to mask low rates of intergranular attack. Therefore, to establish evidence of intergranular attack, microscopic examination is recommended in ASTM G 28 to supplement the corrosion rate (weight-loss) data. The newer C-type alloys (Table 1) have 20 to 23 % chromium, and therefore do not pose this problem. 

Information on "acceptable and nonacceptable corrosion test results has been given for the two EPR tests in the section above on these tests. Acceptable etch structures (step, and in some cases, dual) are also identified for the oxalic acid etch test. The absence of fissures in bends after certain copper sulfate tests is considered an indication of acceptable results. However, in the case of weight-loss tests (nitric acid, ferric sulfate, and certain copper sulfate tests) the standard ASTM test methods merely show how corrosion rates are calculated without identification of rates which are evidence of the onset of susceptibihty to intergranular attack. 

Remove the Nitric Acid Test from ASTM A 262 and establish it as a separate ASTM Test At present, the boiling 65 % nitric acid test (Practice C) is specified for materials to be used in nitric acid service. Only this test is sensitive to sigma-phase in molybdenum-bearing austenitic stainless steels. Also, problems such as end-grain corrosion associated with hexavalent chromium derived from corrosion products are unique to this solution. While this test also detects susceptibility to intergranular attack associated with chromium carbide precipitates, there are other tests that perform this function in less time and with greater simplicity. 

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