The effect of Nb and heat treatment on the corrosion behavior of ferritic stainless steel in acid environments
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THE EFFECT OF Nb AND HEAT TREATMENT ON THE CORROSION BEHAVIOR OF FERRITIC STAINLESS STEEL IN ACID ENVIRONMENTS H. Demiroren,1 M. Aksoy,1 and M. Erbil 2 The corrosion properties of ferritic stainless steel with admixtures of 0.5, 1.0, and 3.0 wt.% Nb are studied in 0.1 M H2 SO4 and 0.3 M HCl solutions. The results obtained by the impedance and mass-loss methods demonstrate that the procedure of alloying with niobium and diffusion treatment positively affect the corrosion resistance of steel in these solutions.
Ferritic stainless steels are highly corrosion-resistant in many environments [1]. However, when these alloys are rapidly cooled in the temperature range 900–1100°C, chromium carbides and nitrides precipitate on the grain boundaries and cause intergranular corrosion. One way to overcome this corrosion is to lower the C and N contents [2]. Another way is to add stabilizing elements, such as Nb or Ti. These elements form carbides and nitrides preferentially to Cr [5]. High Cr, Mo, and N contents of duplex stainless steels decrease their high pitting resistance in chloride environments [4 – 5]. The effect of metallurgical properties on corrosion cracking was investigated in the earlier studies [6–7]. Other researchers also reported that copper always improves the local corrosion of FSS [8]. The behavior of austenitic stainless steels was investigated by means of the AC impedance technique for different concentrations of Na Cl [9]. In the present work, the microstructure, the corrosion behavior of FSS with niobium, and the effect of heat treatment are investigated. Experimental Procedure The alloys were obtained from a scrap of ferritic stainless steel by adding 0.5, 1.0, and 3.0 wt.% of niobium. The compositions of alloys are given in Table 1. The FSS were produced by casting in an induction furnace and forging. The samples were 10 × 10 × 10 mm in dimensions and classified into three groups prior to corrosion tests. The samples from the first group were subjected to heat treatment. The samples from the second group were rapidly cooled after homogenization at 1100°C for 30 min. The third group was rapidly cooled after homogenization at 1100°C for 180 min. For metallographic investigations, the working surface area was mechanically polished by using Si C paper to a 1200-grit finish, held on a diamond paste, and dried with washing alcohol. All samples were subjected to electrolytic etching by holding in 50 mliter HNO3 and 50 mliter of pure water [10]. To perform polarization tests, the samples were embedded in epoxy resin leaving an exposed area of 10 × 10 mm2 . The samples were cleaned with 800-grit silicon-carbide paper. The corrosion tests were performed in 0.1 M H2 SO4 and 0.3 M HCl acid solutions. The samples were polarized to a single cycle for each measurement up to ± 10 mV of corrosion potential at a scan rate of 1 mV in a CHI-64 analyzer. The corrosion state of 1 University of Firat, Elazig, Turkey. 2 University of Cukurova, Adana, Turkey.
Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 4
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