Intergranular corrosion susceptibility in supermartensitic stainless steel weldments J.M. Aquino * , C.A. Della Rovere, S.E. Kuri São Carlos Federal University (UFSCar), Materials Engineering Department, Rodovia Washington Luís, km 235, CEP 13565-905, São Carlos, SP, Brazil article info Article history: Received 31 March 2009 Accepted 8 June 2009 Available online 14 June 2009 Keywords: C. Intergranular corrosion C. Pitting corrosion Stainless steel Welding abstract The intergranular corrosion susceptibility in supermartensitic stainless steel (SMSS) weldments was investigated by the double loop – electrochemical potentiokinetic reactivation (DL-EPR) technique through the degree of sensitization (DOS). The results showed that the DOS decreased from the base metal (BM) to the weld metal (WM). The heat affected zone (HAZ) presented lower levels of DOS, despite of its complex precipitation mechanism along the HAZ length. Chromium carbide precipitate redissolu- tion is likely to occur due to the attained temperature at certain regions of the HAZ during the electron beam welding (EBW). Scanning electron microscopy (SEM) images showed preferential oxidation sites in the BM microstructure. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Intergranular corrosion is a selective process that occurs in sen- sitized regions of stainless steels as a result of inadequate heat treatments, welding, or high-temperature service [1]. In the classi- cal sensitization process there is the formation of chromium rich precipitates at grain boundaries, with the subsequent impoverish- ment of that element in the adjacent matrix. Consequently, passive films formed over those depleted regions are not stable, thus, it leads to a more susceptible region to a corrosion attack. In that as- pect, intergranular corrosion could resemble a galvanic cell in which the grains are the cathodic area, and the corresponding grain boundaries are the anodic one; which results in a high cathodic area in relation to an anodic area [2]. Martensite induced sensitiza- tion is another type of sensitized region susceptible to an intra- granular corrosion attack [1,3]. This process occurs due to the chromium carbide precipitation in the martensitic lath boundaries [3] which is more intensive in tempered than in quenched mar- tensite [4]. Other metallurgical phases may contribute to influence the corrosion resistance. According to the literature [4] austenite promotes carbon and nitrogen dissolution, having a consequent reduction in the chromium and molybdenum precipitates. Thus, pit potential demonstrated to be dependent on the austenite con- tent; which gave noble potentials [5]. Otherwise, the d-ferrite phase presented in low carbon 13%Cr steels deteriorates the corro- sion resistance [6] due to the higher carbide precipitation around that phase, that is occasioned by its low carbon solubility [7]. Other corrosion and mechanical problems are associated with that phase [8,9]. In welded joints, chromium and molybdenum carbonitride pre- cipitations in the heat affected zone (HAZ), particularly in the supermartensitic stainless steel (SMSS), are responsible for its sus- ceptibility to a corrosion attack. Those precipitations mainly occur at the prior austenite grain boundaries and also at the martensite/ d-ferrite phase boundaries during a multipass welding, with an additional intensification promoted by the post weld heat treat- ments (PWHT) [10]. Carbide precipitates occur due to the combi- nation of carbon matrix saturation and tempering effect of the subsequent welding passes. That process is not expected to occur in the cap layer or in the single pass welded joints [10]. Intergran- ular stress corrosion cracking (IGSCC), which was not expected in the HAZ of the SMSS, can occur due to small amounts of Cr-carbide precipitates at the prior austenite grain boundaries [11]. According to the literature [12,13], the HAZ is the most susceptible region to a crack initiation, due to its sensitization in weldments made by con- ventional welding processes. Additionally, the welding technique exerts influence on the cor- rosion behavior of the weldment. The high power density pro- cesses are interesting due to their high heat input; which is confined in a small region of the workpiece, and also to their high cooling rates. These characteristics promote the dissolution of Cr- carbide precipitates as well as their suppression. That precipitate suppression is caused by very short time periods in the tempera- ture precipitation zone [14]. Corrosion resistance and mechanical properties, when comparing the high power density to the conven- tional processes [14,15], can lead to better welded joints. Pitting corrosion is also likely to occur at chromium depleted regions [5] as well as near the MnS inclusions [16–21]. However, a direct relationship was not established. According to the litera- ture [22], the HAZ of a high power density process weldment is not the most susceptible region to the pitting corrosion due to 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.06.009 * Corresponding author. Tel.: +55 16 33518506; fax: +55 16 33615404. E-mail address: dsek@power.ufscar.br (J.M. Aquino). Corrosion Science 51 (2009) 2316–2323 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci