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This study examined the influence of phosphorus addition on the corrosion resistance of carbon steel for a flue gas desulfurization (FGD) system using electrochemical methods, such as potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) in 10 wt% sulfuric acid (H 2 SO 4 ) solution at room temperature. The potentiodynamic measurements indicated that the addition of phosphorus increased the cathodic hydrogen evolution reaction. X-ray photoelectron spectroscopy (XPS) analyses of the corroded surfaces after the immersion testing indicated that the oxidized product of phosphorus-containing steel was composed mainly of iron(II) sulfate (FeSO 4 ), iron(III) sulfate (Fe 2 (SO 4 ) 3 ), and iron(III) phosphate (FePO 4 ). However, these products seem to have no protective property. The experimental results confirmed that phosphorus has a deleterious effect on the corrosion resistance of carbon steels in H 2 SO 4 solution as a result of the higher hydrogen activity.However, in alkali media, it was reported that corrosion resistance of steel is decreased by the presence of phosphorus. 9 This study is part of an ongoing research program to evaluate the corrosion resistance and corrosion mechanism of corrosion-resistant steel for FGD systems. The alloying effect and mechanisms of phosphorus addition on the corrosion properties of carbon steels in H 2 SO 4 were examined through electrochemical tests and surface analyses. EXPERIMENTAL PROCEDURES Materials and Test ConditionCarbon steels in this study were made by hot working. A mixture of the constituent elements was heated to 1,250°C, held for 1 h, subjected to hot rolling to reduce the steel thickness to 0.35 cm, and then annealed at 900°C. For ease of storage and transport, the semifinished steel was coiled at a temperature of 650°C for 1 h, and then cooled in a furnace.Carbon steel plates, 0.35 cm in thickness, were cut into 2.25-cm 2 pieces. Table 1 lists the chemical composition of these steels. Only the phosphorus content differed among the steels. The specimens were cast in a mold using hot acrylic powder, ground with 600-grit silicon carbide (SiC) paper, degreased in an ultrasonic cleaner with ethanol (C 2 H 6 O) for 5 min, cleaned with distilled water, and dried in hot air. All electrochemical measurements were carried out in 1,000 mL of a 10 wt% H 2 SO 4 solution, which is considered to be most representative of the corrosive environment in Korean FGD facilities. The solution was aerated with pure air at a flow rate of 10 cm 3 /min. At least three measurements were run for each specimen to ensure reproducibility.

High strength cold-rolled bake hardenable steel sheets for automotive use have been developed. In the last decade, the major strengthening methods of extra low carbon (ELC) steel and interstitial free (IF) steel were the solid-solution strengthening with silicon, manganese and phosphorous and the precipitation strengthening with microalloying elements, such as niobium and titanium. When the steels are strengthened with the high amount of solid-solution elements such as silicon and manganese, it shows the low surface quality due to surface oxidation. In addition, phosphorous added ELC and IF steels become susceptible to the secondary work embrittlement because of the lack of grain boundary strength, which is the essential drawback of the steels.In this paper, we propose the extra low carbon bake hardenable steel which is precipitation strengthened by means of the fine distribution of Copper sulfide (Cu 2 S) precipitates instead of solid-solution strengthening and precipitation strengthening using silicon, manganese, niobium and titanium. These Cu sulfides are oriented to the body-centered cubic a-Fe matrix in (001) KEY WORDS: extra low carbon steel; Cu precipitate; sulfide; precipitation hardening; bake hardenability; cold-rolled sheet. 109© 2009 ISIJ Cu and to treat temper annealing.9-11) Thus, in case of small addition of Cu, it is difficult to form the Cu precipitate (eCu).On the other hand, some researchers studied about the solid-solution strengthening and Cu-S precipitation in Ti added ELC steel. Yamada et al. reported that the addition of 0.22 % Cu increases tensile strength about 10 MPa in Tistabilized IF steel. 12) They described that the increment of tensile strength is mainly resulted from grain refinement and solid-solution strengthening. In addition, Kejian et al. and Ishiguro et al. suggested that a residual level of Cu can make large sulfides and inclusion in Ti added steels. 13,14) However, they did not consider the possibility of precipitation hardening of Cu sulfides.Guilet et al. reported that nano-sized Cu-S precipitates can be formed in batch annealing process for Ti free low carbon steels.15) Recently, Liu et al. studied the crystallography and precipitation kinetics of copper sulfide for low carbon steel in strip casting process. 16,17) In this work, we examine the evidence of Cu-S precipitation hardening due to fine Cu 2 S for extra low carbon steels in continuous annealing process. Also, the performance of the Cu added ELC steel is compared with conventional Nb-Ti BH steel. Experiments MaterialThe materials used in this study are extra low carbon steels with a bake hardenability are listed in Table 1. In order to investigate the effect of Cu on the strengthening for the ELC steel, three ELC steels (0Cu ELC, 01Cu ELC and 02Cu ELC) that contain different content of Cu were selected. In addition, the mechanical properties of the 02Cu ELC steel are compared with conventional Nb-Ti added BH steel (Nb-Ti BH). The Nb-Ti added BH steel means that solute elements of C, N, and S are partially sc...

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