Intergranular Corrosion of Low Cr Ferritic Stainless Steel 429 Evaluated by the Optimized Double Loop Electrochemical Potentiokinetic Reactivation Test

Xiao-Lei Li,Yi-Fan Ni,Yi-Ming Jiang,Li Li,Jin Li

doi:10.1155/2015/716874

Abstract

Intergranular corrosion (IGC) of Nb-Ti stabilized ferritic stainless steel (FSS) 429 was investigated using the double loop electrochemical potentiokinetic reactivation (DL-EPR) test combined with the microstructure observation. The results indicated that the optimized DL-EPR test condition for FSS 429 was the solution of 0.5 M H2SO4+ 0.0001 M KSCN with a scanning rate of 100 mV/min at 30°C. Based on this condition, the specimens aging at 400–700°C for different duration were tested and a time-temperature-sensitization (TTS) curve for FSS 429 was obtained, which reveals the sensitization nose was located around 550°C. The criticalIr/Iavalue was determined to be about 3% above which IGC occurred. After aging treatment, Cr depletion zone was detected using energy dispersive spectroscopy (EDS), most possibly due to Cr segregation around intergranular TiC and NbC.

Highlights

Ferritic stainless steels (FSS) had been widely used in many elevated temperature applications due to its superior resistance to pitting corrosion, good machinability, and high thermal conductivity, as compared with conventional austenitic stainless steels [1,2,3,4,5]. They were inevitably exposed to the temperature range above recrystallization temperature; the thermal cycles would lead to the grain growth and sensitization, causing the intergranular corrosion (IGC) followed by the failure of the appliances [6]
During the welding or heat treatment process, Cr compounds such as Cr-carbides and Cr-nitrides precipitated at the grain boundaries, the Cr-depleted zone adjacent to the intergranular precipitates became the initiation sites, and IGC was fully developed by the electrochemical potential difference between the Cr-depleted zone and matrix
100 μm (d) concerning the optimization of the scan rate and the solution temperature were discussed in detail below

Summary

Introduction

Ferritic stainless steels (FSS) had been widely used in many elevated temperature applications (automotive exhaust systems, steam generators of power plants, and so on) due to its superior resistance to pitting corrosion, good machinability, and high thermal conductivity, as compared with conventional austenitic stainless steels [1,2,3,4,5]. During application, they were inevitably exposed to the temperature range above recrystallization temperature; the thermal cycles would lead to the grain growth and sensitization, causing the intergranular corrosion (IGC) followed by the failure of the appliances [6]. Researches on the IGC of low Cr (∼14 wt%) ferritic stainless steels were extremely lacking, especially for FSS 429

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