What is the ASTM G48 test? Corrosion
To give it it’s full title, ASTM G48 is the “Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution”.
It is a corrosion test, utilising an aggressive solution of ferric chloride. Although this medium is rarely seen in day to day operations, it partly replicates the conditions seen within a corrosion pit in chloride-containing solutions. Therefore, the test has become an accepted measure of the pitting corrosion resistance of stainless steels.
Within the standard there are several different formats, including the use of a crevice forming device to artificially simulate the presence of an undesirable feature. However, all tests follow the basic approach of Method A. A sample of the metal is immersed in the acidic ferric chloride solution, typically for 24 to 72 hours, before examining either the relative weight loss or the presence of pits. The test is performed at a set temperature, dependent upon the alloy. For instance, 2205 duplex (1.4462, F51) is commonly tested at 25degC, whilst super duplex stainless steels such as SAF2507 (1.4410, F53, S32750), S32760 (1.4501, F55, Zeron 100) and Ferralium 255 (1.4507, F61, S32550) are tested at 50degC. Results will usually show zero corrosion, reported as 0g/m2 of the test piece.This indicates an optimised microstructure, with a balance of austenite and ferrite grain, achieved with appropriate solution annealing and quench processes.