For many years two ferric chloride corrosion test methods have been used for the detection of intermetallic phases and precipitates in duplex stainless steels, namely ASTM G48 Method A and ASTM A923 Method C. Recently, ISO 17781 and the IOGP S-563 specification have stipulated the use of ASTM G48 method A. In this context, these corrosion tests are specified in conjunction with metallographic examination and impact testing to demonstrate the presence of a suitable microstructure. They are not used as a means of determining expected corrosion resistance in a service environment.
End users have tended to specify one test method, often with modification of the specific test procedure and acceptance criteria. The purpose of the work presented in this paper was to conduct a direct comparison of the two ASTM test methods to establish if they can be considered equivalent.
Different heat treatments were carried out to previously solution annealed and quenched 22%Cr duplex and 25%Cr super duplex materials to precipitate target amounts of intermetallic phases and precipitates. The specimens were then subjected to both corrosion test methods. The effect of test durations of 24 and 72 hours was also investigated for the ASTM G48 method. Except for an increased test temperature for the 25%Cr material when using the ASTM A923 test method, tests were carried out in accordance with the relevant ASTM specification without modification.
Results are presented in terms of acceptance criteria given in International Standards and specifications, and in terms of the corrosion rates seen to establish if both test methods can be considered equivalent in their ability to detect unacceptable quantities of intermetallic phases and precipitates.