Materials Transactions, Vol. 57, No. 12 (2016) pp. 2116 to 2121 ©2016 The Japan Institute of Metals and Materials Effect of Molybdenum on the Corrosion of Low Alloy Steels in Synthetic Seawater Su-Bin Shin1, Sol-Ji Song1, Young-Woong Shin1, Jung-Gu Kim1,*, Byung-Joon Park2 and Yong-Chan Suh2 1Department of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon, South Korea 2Heavy Plate R&D Team of R&D Center, Hyundai-Steel Company, 167–32 Kodae-Ri, Songak-Eup, Dangjin 343–823, South Korea The alloying effect of Mo on the seawater immersion corrosion for low alloy steel was investigated using weight loss tests and electro- chemical impedance spectroscopy (EIS) in seawater. The Mo-containing low alloy steel showed an excellent corrosion resistance by the long immersion test due to the formation of homogeneous rust layer preventing active dissolution. SEM and XPS analyses were conducted to observe cross-sectional images of rust layer and indentify chemical composition of oxide formed on the surface after immersion test. The results revealed 2− that the MoO4 ions which were oxidized from Mo form the compounds which disturb the approach of aggressive ions. [doi:10.2320/matertrans.M2016222] (Received June 17, 2016; Accepted September 15, 2016; Published October 21, 2016) Keywords: corrosion resistance, electrochemical impedance spectroscopy, molybdenum, seawater, X-ray photoelectron spectroscopy 1. Introduction gen reduction reaction which occurs at the interface of oxide and electrolyte is decreased.16) In other words, the electronic High strength low alloy (HSLA) steels, which provides properties of the oxide affect the rate of oxygen reduction high mechanical properties, weldability and corrosion resis- reaction and corrosion rate.17) Therefore, the study on the rust tance, are widely used as structural material such as bridge, layer is important for the design of the corrosion-resistant building, pipe line and so on. Especially, HSLA steels are steel. used in the plants and structures on severe environmental Alloying elements such as Cr, Cu, Ni and P have an effect conditions such as the ocean because it necessitates the use of on properties of the rust layer, resulting in lowered corrosion anticorrosive steels to retard corrosion.1–3) A water ballast rate.18–21) However, the role of Mo on the rust layer has not tank is a compartment lled with ballast water in ship, boat been identied. Mo added to stainless steels was incorporated 2− and marine construction manufactured by HSLA steel with in the Cr oxide layer in the form of MoO4 , which improved thermo-mechanical control (TMCP) process. The tank is es- localized corrosion resistance.22–24) For low alloy steel, sential to maintain ship’s balance under different operation Schultze et al. informed that the favorable effects of Mo ap- systems.4) Seawater can be used for ballast water in the tank. peared after long immersion in seawater.25) Whereas it was Therefore, inside of the tank is exposed to severe corrosive reported that Mo had no effect on the low alloy steel in sea- environment under the inuence of the seawater whose tem- water environment.5) Therefore, a detailed study is required perature reached up to 60°C by solar heat.5) in order to solve the controversy about the role of Mo-added A lifetime of water ballast tank can be extended by corro- low alloy steel. sion protection method such as protective coatings and ca- The purpose of this paper is to evaluate the alloying effect thodic protection.6–8) However, it costs a lot to operate and of Mo on the corrosion resistance of low alloy steel in the maintain the protective system.9) For these reasons, it was synthetic seawater by weight loss test, electrochemical test required to develop steels which have excellent corrosion re- and surface analyses. sistance and maintain good mechanical properties such as weldability and mechanical strength compared to commercial 2. Experimental Procedures steels. Low alloy steel was mainly used for structure of ships in- 2.1 Materials and test condition cluding the water ballast tank.10,11) The low alloy steel has Low alloy steels used in the study were produced by a ther- carbon content of less than 0.2 mass% containing mainly Cu, mo-mechanical control process (TMCP). The specimens Cr, Ni, P and Mn of a few weight percent maximum. The al- were heated and hot-rolled through a two-stage controlled loying elements are added in order to enhance corrosion re- rolling process. The rst stage rolling process was conducted sistance and improve lifetime of the steels.5,9,12,13) There have in the recrystallized austenite region and the second rolling been many developments of low alloy steel applied to marine process was implemented in the nonrecrystallized austenite structures based on alloy design. Recently, the steel plates region range above Ar3. After the process of controlled-roll- used for container ships and upper deck of ballast tank were ing, the hot-rolled plate was immediately water-cooled and reported.14,15) then air-cooled. The formation of compact and adherent rust layer is im- The chemical compositions of the steels were shown in Ta- portant to enhance corrosion resistance of low alloy steel.12) ble 1. The steel plates of 1.5 cm thickness were cut into If the steel surface is protected by the dense oxide layer, oxy- 1 cm × 1 cm pieces. The pieces of the steels were subsequent- ly polished mechanically to 600-grit silicon carbide (SiC) pa- * Corresponding author, E-mail: [email protected] per and washed with ethanol and distilled water. All experi- Effect of Molybdenum on the Corrosion of Low Alloy Steels in Synthetic Seawater 2117 Table 1 Chemical composition of the low alloy steels (mass%). using monochromatic Al Kα energy source. All of specimen Specimen C Si Mn P S Mo Fe surfaces were observed after the immersion for 30 days in the Blank steel 0.07 0.3 1 0.012 0.003 - Balance synthetic seawater. 0.05 Mo steel 0.07 0.3 1 0.012 0.003 0.05 Balance 3. Results and Discussion 0.1 Mo steel 0.07 0.3 1 0.012 0.003 0.1 Balance 0.2 Mo steel 0.07 0.3 1 0.012 0.003 0.2 Balance 3.1 Corrosion properties The effect of Mo concentration in low alloy steel was ex- amined in the synthetic seawater at 60°C. The weight loss ments were carried out at the temperature of seawater in a tests were conducted for 15, 30 days of immersion period. water ballast tank, 60°C which was always maintained using The corrosion rate was calculated by the following equa- water bath, and under an aerated condition. A synthetic sea- tion:28) water solution for the experiments was prepared by a method 87,600W according to the ASTM D1141 standard.26) Corrosion rate(mm/y) = (1) Atρ 2.2 Weight loss tests where W is the weight loss (g), A is the area of exposure Weight loss tests were implemented on low alloy steels in (cm2), t is the immersion time (hour) and ρ is the density (g/ 27) 3 accordance with ASTM G31-72. The initial mass (mi) of cm ). the steel pieces (1 cm × 1 cm × 1.5 cm) was measured. Each The average corrosion rates with error bars calculated by specimen was immersed in the synthetic seawater solution for weight loss tests are shown in Fig. 1. After 15 days of immer- 15 and 30 days by hanging with a plastic wire. After the im- sion period, all of Mo-containing steels enhanced corrosion mersion period of 15 and 30 days, the specimens cleaned with resistance beside blank steel. However, the corrosion rate of ethanol and distilled water were pickled in a solution for 0.2 mass% Mo steel was only lower than that of blank steel 10 min. The cleaning solution was a mixture of 3.5 g hexam- after 30 days of immersion period. The corrosion rates of 0.05 ethylene tetramine and 500 mL HCl to which distilled water and 0.1 mass% Mo steel were similar to that of blank steel. was added to make it 1000 mL. The specimens were subse- Therefore, it is obvious that corrosion resistance of low alloy quently degreased in ethanol using an ultrasonic cleaner for steel was improved by the addition of Mo (0.2 mass% or 10 min, then cleaned with distilled water and dried by drying more). machine. Finally, the nal mass (mf) of these specimens was measured. To ensure reproducibility of the test, the experi- 3.2 Electrochemical behavior ment was repeated at least two times. Figure 2 shows EIS Bode plots of blank steel and three steels with Mo contents for 30 days in the synthetic seawater 2.3 Electrochemical tests at 60°C. The Bode plot represented impedance parameter (Z) A three-electrode electrochemical system was employed and phase angle as a function of the frequency.29) In Bode for electrochemical tests. The system was comprised of a plot, the Z of high frequency implies solution resistance, steel specimen with the surface area of 0.8 cm × 0.8 cm, two whereas the Z of low frequency is total resistance including graphite rods and a saturated calomel electrode (SCE) as the solution and polarization resistance. The solution resistance working, counter and reference electrodes, respectively. An of all specimens during 1 day has similar values and the little open-circuit potential (Eocp) of the specimen was measured differences are due to the distance between specimen and ref- for about 2 hours to reach the stable electrochemical state be- erence electrode.