Identification of Sensitization of Austenitic Stainless Steel to Intergranular Stress Corrosion Cracking by Atomic Force Microscopy

Oil and gas flow lines of 13 Cr weldable martensitic grade stainless steel(WMSGSS) offers a very cost effective alternative to more expensive corrosion resistant alloys.However,laboratory tests and field experience indicates the 13 Cr steel undergoes inter-granular stress corrosion cracking(IGSCC) in the heat affected zone of the welds.The IGSCC mechanism of lean grades is considered to be chromium depletion near carbides at grain boundaries.For medium and rich grades,the cracking has only been found in laboratory tests.For these grades the mechanism has not been clarified,although the most likely mechanism is considered to be similar to that of the lean grades.Atomic force microscopy(AFM) has been extensively used in the study of microstructures of materials and environmentally induced fracture.The sensitisation of rich grade martensitic stainless steel weld to intergranular corrosion/SCC maybe identified by AFM.Before this technique is used in the study,it would be beneficial to start the technique by using sensitised specimens made of standard 304 stainless steel to verify the suitability.In this paper the sensitisation of a series of 304 stainless steel samples with different degree of sensitisation was studied by means of AFM together with scanning electron microscopy(SEM) and energy dispersive spectroscopy(EDS) techniques and the results agree with each other.