An inherent characteristic of some alloys of austenitic stainless steel is their tendency to crack at stress points when exposed to certain corrosive environments. The mechanisms of ESCC are complex and not completely understood but are apparently related to certain metallurgical properties. Chloride ions concentrated at a stress point will catalyze crack formation. It has been reported that other halide ions do not promote ESCC to the same degree as does chloride using the test technology of Test Method C692 (drip test).
Chlorides are common to many environments, so great care shall be taken to protect austenitic stainless steel from chloride contamination.
Most thermal insulations will not, of themselves, cause stress corrosion cracking as shown by qualification tests. When exposed to elevated-temperature (boiling point range), environments containing chlorides, moisture, and oxygen, however, some insulation systems act as collecting media, transmigrating and concentrating chlorides on heated stainless steel surfaces. If moisture is not present, the chloride salts cannot migrate, and stress corrosion cracking because of chloride-contaminated insulation cannot take place.
Insulation materials are available that are specially formulated to inhibit stress corrosion cracking in the presence of chlorides through modifications in basic composition or incorporation of certain chemical additives.
The ability of the 28-day test to measure the corrosion potential of insulation materials is documented by Karnes, whose data appear to have been used for construction of the acceptability curve used in Specification C795 and other specifications.
The metal for all of the coupons used in this test method (C692) shall be qualified (see Section 14) to ascertain that under conditions of the test, chloride ions will cause the metal to crack, and deionized water alone will not cause cracks.
1.1 This test method covers two procedures for the laboratory evaluation of thermal insulation materials to determine whether they contribute to external stress corrosion cracking (ESCC) of austenitic stainless steel due to soluble chlorides within the insulation. This laboratory procedure is not intended to cover all of the possible field conditions that contribute to ESCC.
1.2 While the 1977 edition of this test method (Dana test) is applicable only to wicking-type insulations, the procedures in this edition are intended to be applicable to all insulating materials, including cements, some of which disintegrate when tested in accordance with the 1977 edition. Wicking insulations are materials that wet through and through when partially (50 to 75 %) immersed in water for a short period of time (10 min or less).
1.3 These procedures are intended primarily as a preproduction test for qualification of the basic chemical composition of a particular manufacturer''s product and are not intended to be routine tests for ongoing quality assurance or production lot compliance. Test Methods C871, on the other hand, is used for confirmation of acceptable chemical properties of subsequent lots of insulation previously found acceptable by this test method.
1.4 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitat......
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