Effect of Molybdenum on the Corrosion of Low Alloy Steels in Synthetic Seawater

Abstract

The alloying effect of Mo on the seawater immersion corrosion for low alloy steel was investigated using weight loss tests and electrochemical 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 that the MoO₄²⁻ ions which were oxidized from Mo form the compounds which disturb the approach of aggressive ions.

1. Introduction

High strength low alloy (HSLA) steels, which provides high mechanical properties, weldability and corrosion resistance, are widely used as structural material such as bridge, building, pipe line and so on. Especially, HSLA steels are used in the plants and structures on severe environmental conditions such as the ocean because it necessitates the use of anticorrosive steels to retard corrosion.^1–3) A water ballast tank is a compartment filled with ballast water in ship, boat and marine construction manufactured by HSLA steel with thermo-mechanical control (TMCP) process. The tank is essential to maintain ship's balance under different operation systems.⁴⁾ Seawater can be used for ballast water in the tank. Therefore, inside of the tank is exposed to severe corrosive environment under the influence of the seawater whose temperature reached up to 60℃ by solar heat.⁵⁾

A lifetime of water ballast tank can be extended by corrosion protection method such as protective coatings and cathodic protection.^6–8) However, it costs a lot to operate and maintain the protective system.⁹⁾ For these reasons, it was required to develop steels which have excellent corrosion resistance and maintain good mechanical properties such as weldability and mechanical strength compared to commercial steels.

Low alloy steel was mainly used for structure of ships including the water ballast tank.^10,11) The low alloy steel has carbon content of less than 0.2 mass% containing mainly Cu, Cr, Ni, P and Mn of a few weight percent maximum. The alloying elements are added in order to enhance corrosion resistance and improve lifetime of the steels.^5,9,12,13) There have been many developments of low alloy steel applied to marine structures based on alloy design. Recently, the steel plates used for container ships and upper deck of ballast tank were reported.^14,15)

The formation of compact and adherent rust layer is important to enhance corrosion resistance of low alloy steel.¹²⁾ If the steel surface is protected by the dense oxide layer, oxygen reduction reaction which occurs at the interface of oxide and electrolyte is decreased.¹⁶⁾ In other words, the electronic properties of the oxide affect the rate of oxygen reduction reaction and corrosion rate.¹⁷⁾ Therefore, the study on the rust layer is important for the design of the corrosion-resistant steel.

Alloying elements such as Cr, Cu, Ni and P have an effect on properties of the rust layer, resulting in lowered corrosion rate.^18–21) However, the role of Mo on the rust layer has not been identified. Mo added to stainless steels was incorporated in the Cr oxide layer in the form of MoO₄²⁻, which improved localized corrosion resistance.^22–24) For low alloy steel, Schultze et al. informed that the favorable effects of Mo appeared after long immersion in seawater.²⁵⁾ Whereas it was reported that Mo had no effect on the low alloy steel in seawater environment.⁵⁾ Therefore, a detailed study is required in order to solve the controversy about the role of Mo-added low alloy steel.

The purpose of this paper is to evaluate the alloying effect of Mo on the corrosion resistance of low alloy steel in the synthetic seawater by weight loss test, electrochemical test and surface analyses.

2. Experimental Procedures

2.1 Materials and test condition

Low alloy steels used in the study were produced by a thermo-mechanical control process (TMCP). The specimens were heated and hot-rolled through a two-stage controlled rolling process. The first stage rolling process was conducted in the recrystallized austenite region and the second rolling process was implemented in the nonrecrystallized austenite region range above Ar₃. After the process of controlled-rolling, the hot-rolled plate was immediately water-cooled and then air-cooled.

The chemical compositions of the steels were shown in Table 1. The steel plates of 1.5 cm thickness were cut into 1 cm × 1 cm pieces. The pieces of the steels were subsequently polished mechanically to 600-grit silicon carbide (SiC) paper and washed with ethanol and distilled water. All experiments were carried out at the temperature of seawater in a water ballast tank, 60℃ which was always maintained using water bath, and under an aerated condition. A synthetic seawater solution for the experiments was prepared by a method according to the ASTM D1141 standard.²⁶⁾

Table 1 Chemical composition of the low alloy steels (mass%).

Specimen	C	Si	Mn	P	S	Mo	Fe
Blank steel	0.07	0.3	1	0.012	0.003	-	Balance
0.05 Mo steel	0.07	0.3	1	0.012	0.003	0.05	Balance
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

2.2 Weight loss tests

Weight loss tests were implemented on low alloy steels in accordance with ASTM G31-72.²⁷⁾ The initial mass (m_i) of the steel pieces (1 cm × 1 cm × 1.5 cm) was measured. Each specimen was immersed in the synthetic seawater solution for 15 and 30 days by hanging with a plastic wire. After the immersion period of 15 and 30 days, the specimens cleaned with ethanol and distilled water were pickled in a solution for 10 min. The cleaning solution was a mixture of 3.5 g hexamethylene tetramine and 500 mL HCl to which distilled water was added to make it 1000 mL. The specimens were subsequently degreased in ethanol using an ultrasonic cleaner for 10 min, then cleaned with distilled water and dried by drying machine. Finally, the final mass (m_f) of these specimens was measured. To ensure reproducibility of the test, the experiment was repeated at least two times.

2.3 Electrochemical tests

A three-electrode electrochemical system was employed for electrochemical tests. The system was comprised of a steel specimen with the surface area of 0.8 cm × 0.8 cm, two graphite rods and a saturated calomel electrode (SCE) as the working, counter and reference electrodes, respectively. An open-circuit potential (E_ocp) of the specimen was measured for about 2 hours to reach the stable electrochemical state before measurements.

Electrochemical impedance spectroscopy (EIS) measurements for the interpretation of corrosion behavior were carried out using an EG&G PAR VMP2 potentiostat/galvanostat. The impedance tests were conducted after recording open-circuit potential during 10 min of the exposure of the specimen to the synthetic seawater. The impedance spectra were measured over a frequency range from 100 kHz to 10 mHz with an amplitude (V_a) of 10 mV. The operation was done once in 1 day, 15 days and 30 days and repeated at least two times for the reliability of experimental data.

2.4 Surface analyses

The surface analyses were carried out to trace the mechanism for the alloying effect of Mo-added low alloy steel. The analyses were conducted to observe surface morphology of the specimen and investigate element distribution in rust layer using a scanning electron microscope (SEM, S-3000H, Hitachi). The chemical composition of rust layer was identified by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo) using monochromatic Al Kα energy source. All of specimen surfaces were observed after the immersion for 30 days in the synthetic seawater.

3. Results and Discussion

3.1 Corrosion properties

The effect of Mo concentration in low alloy steel was examined in the synthetic seawater at 60℃. The weight loss tests were conducted for 15, 30 days of immersion period. The corrosion rate was calculated by the following equation:²⁸⁾   

\[Corrosion \ rate (mm/y) = \frac{87{,}600 W}{At \rho}\](1)

where W is the weight loss (g), A is the area of exposure (cm²), t is the immersion time (hour) and ρ is the density (g/cm³).

The average corrosion rates with error bars calculated by weight loss tests are shown in Fig. 1. After 15 days of immersion period, all of Mo-containing steels enhanced corrosion resistance beside blank steel. However, the corrosion rate of 0.2 mass% Mo steel was only lower than that of blank steel after 30 days of immersion period. The corrosion rates of 0.05 and 0.1 mass% Mo steel were similar to that of blank steel. Therefore, it is obvious that corrosion resistance of low alloy steel was improved by the addition of Mo (0.2 mass% or more).

Fig. 1

Average corrosion rates by weight loss tests for 15, 30 days of immersion periods.

3.2 Electrochemical behavior

Figure 2 shows EIS Bode plots of blank steel and three steels with Mo contents for 30 days in the synthetic seawater at 60℃. The Bode plot represented impedance parameter (Z) and phase angle as a function of the frequency.²⁹⁾ In Bode plot, the Z of high frequency implies solution resistance, whereas the Z of low frequency is total resistance including solution and polarization resistance. The solution resistance of all specimens during 1 day has similar values and the little differences are due to the distance between specimen and reference electrode. Also, the increase of solution resistance with immersion time is caused by the change of resistivity of solution. As the conductive ions such as chloride, sulfate in the solution react with specimen, the concentration of conductive ions is decreased. For all of specimens, the polarization resistance generally decreased as the immersion time increased, which means a reduction in corrosion resistance. It shows different results of weight loss tests. Zou et al. investigated great deviations of electrochemical estimate from weight loss result after long-term immersion.³⁰⁾ It was explained that β-FeOOH in inner rust layer with higher reduction reactivity can attribute to the strengthening of cathodic reduction reaction, which increases corrosion reaction due to small polarization for electrochemical test.³¹⁾ Therefore, the Z value did not precisely reflect corrosion resistance of the specimens for EIS test in the synthetic seawater due to the interference of rust.

Fig. 2

Bode plots with immersion time: (a) Blank, (b) 0.05 mass% Mo, (c) 0.1 mass% Mo, and (d) 0.2 mass% Mo.

However, EIS is one of the useful techniques in long immersion tests, because it does not interfere with the experimental system.³²⁾ Especially, the Bode plot provides a plain explanation of behavior dependent on electrochemical frequency.³³⁾ Through interpretation of EIS Bode plot, it provides information of surface/electrolyte solution interface. The spectra of all specimens showed two time constants after 15 days, indicating that the rust layer was formed at the time by a reaction between the steel and electrolyte.³⁴⁾ The splitting in the phase angle-frequency curve indicates the presence of other phase maximum.³⁵⁾ Also, the phase angle maxima for all specimens decreased away from −90° and shoulder on the phase angle shifted to lower frequency with increasing of immersion times. It means non-homogeneity for specimens increased and the rust layer formed by uniform corrosion was thicken.^36,37) The phase spectra of Mo-containing steels were not separated more distinctly than that of Mo-free steel (Fig. 2). This is due to the relative homogeneity of rust on Mo-containing steel. The extent of splitting decreased as Mo contents increased. Especially, it seemed that the phase spectra of 0.2 mass% Mo have one time constant despite the increase in immersion time.

Figure 3 is the equivalent electrical circuit models for fitting of the EIS data. The circuit with one time constant is shown in Fig. 3(a), and the one with two time constants is shown in Fig. 3(b).³⁸⁾ The circuit was used for EIS data in 1 day, 15 days and 30 days of immersion times, respectively. The circuit of one time constant has the following elements: the solution resistance (R_s), the constant phase elements (CPE1), charge transfer resistance (R_ct) and Warburg impedance (W) which means oxygen diffusion controlled system. Whereas, the circuit of two time constants has the following components: the solution resistance (R_s), the constant phase elements (CPE1, CPE2), the rust resistance (R_rust) and charge transfer resistance (R_ct). The Warburg impedance is rarely appeared in 15 days and 30 days due to the change of metal surface state. The CPE substitutes the capacitance to consider the depression angle and the impedance of the CPE can be expressed in the following equation:^39,40)   

\[Z_{CPE} = \left( \frac{1}{Y_0} \right) \left[ \left( j \omega \right)^n \right]^{-1}\](2)

where Y₀ is the magnitude of the CPE and n is the depression parameter (0 ≤ n ≤ 1). The CPE displays ideal capacitance when n = 1. In other words, the parameter means the deviation from an ideal capacitive behavior of the interface.⁴¹⁾ The depression angle (α) can be expressed as:⁴²⁾   

\[\alpha = \frac{2 \left(1 - n \right)}{\pi}\](3)

Fig. 3

Equivalent circuit models for interpretation of the EIS data: (a) 1 day, and (b) 15 days and 30 days.

An increase of depression angle has been ascribed to the heterogeneity of the surface, which means the increase of surface roughness and the formation of porous layers.^43–47) Figure 4 shows the calculated depression angle of rust layers with immersion time. The depression angle of blank steel, 0.05 mass% Mo and 0.1 mass% Mo steel increased slightly for 30 days. The depression angle of 30 days was less than that of 15 days for 0.2 mass% Mo steel, indicating that relative nonporous rust layer was formed on the steel surface. Hashimoto et al. mentioned that the addition of Mo produced the formation of Mo (VI) oxyhydroxide or molybdate at active sites, which renders a beneficial effect of decrease of the dissolution rate. The activity of active sites decreased by the adsorption of Mo compounds leads to the formation of homogeneous layer on steel surface.⁴⁸⁾

Fig. 4

Depression angle with immersion time.

3.3 Surface analysis

The SEM image represents the cross section of rust layer for the Mo-free steel and Mo-containing steels after 30 days immersion tests in the synthetic seawater, as shown in Fig. 5. In rust layer, the epoxy and corrosion product are co-existed: black area is epoxy and grey area is corrosion product. The specimens without Mo and with 0.05 mass% Mo and 0.1 mass% Mo had considerably porous rust layer on the steel surface. Whereas comparatively dense rust layer was observed on the specimen with 0.2 mass% Mo, as shown in Fig. 5(d), which means that the homogeneity of the rust layer was greater than that of other steels. These results support EIS results, indicating that 0.2 mass% Mo steel had the lower depression angle. The 0.2 mass% Mo steel can be expected to improve corrosion resistance due to the formation of nonporous and protective rust layer on the surface.

Fig. 5

SEM images of cross sections of the rust layer after immersion tests: (a) Blank, (b) 0.05 mass% Mo, (c) 0.1 mass% Mo, and (d) 0.2 mass% Mo.

Figure 6 shows the XPS peaks of the specimens after immersion tests for 30 days in synthetic seawater. The analysis was conducted to search the chemical composition of the rust layer. The spectra for Mo-containing steel indicated that Fe, O and Mo ions existed on the steel surface. The Fe peaks were detected in all of the specimens including blank steel but the peak shift was observed depending on Mo contents. The Mo-containing steels show the peak at the lower binding energy. Table 2 indicates possible chemical compounds in rust layer and binding energy by the analysis of XPS peaks. For Mo-containing steels, not only the two peaks for Fe_2p appeared at 710.5 eV (2p_3/2) and 724.1 eV (2p_1/2) which indicates the FeMoO₄, but also other ferrous and ferric oxidation states are detected which are the same as Mo-free steel. It can be inferred from these data that the iron oxides such as FeO, FeOOH, Fe₂O₃ and Fe₃O₄ existed in rust layer for all of the specimens.^39,49–51) Unlike the Fe peaks, the Mo peaks were only detected in the 0.2 mass% Mo steel noticeably. These results suggest that Mo compounds were sufficiently precipitated in the rust layer for 0.2 mass% Mo steel, which improved corrosion resistance of the steel effectively. In Table 2, the two peaks of Mo_3d spectra only appeared at 231.9 eV (3d_5/2) and 235.1 eV (3d_3/2) in 0.2 mass% Mo steel, in which oxidized molybdenum existed in the hexavalent state such as molybdate (MoO₄²⁻) and MoO₃.⁵²⁾ According to the Pourbaix diagram of Mo, the Mo (VI) could form molybdate ion in weakly alkaline solution.⁵³⁾

Fig. 6

XPS spectra of the specimens after 30 days immersion tests: (a) Fe_2p, and (b) Mo_3d.

Table 2 Analysis of the XPS peaks for the surface of the specimens.

Analyses of the XPS spectra	Product	Binding energy (eV)
Spectrum of Fe2p	FeO	710.0, 709.3
	FeOOH	711.5, 724.3
	Fe2O3	711.0, 724.0, 710.8
	Fe3O4 (Fe2+)	708.3
	Fe3O4 (Fe3+)	710.2
	FeMoO4	710.5, 724.1
Spectrum of Mo3d	FeMoO4	231.7, 232, 235
	Na2MoO4	231.9
	MoO3	232.1

The effect of molybdate ion on the corrosion inhibitor has been studied extensively in literature.^{49,52,54–56)} Figure 7 shows a schematic representation of the molybdate ion acting as a barrier of electrochemical reaction for Mo-containing steel. In initial state, oxygen is diffused to steel surface and it dominates the corrosion rate as oxygen diffusion-controlled. During the anodic reaction, the steel is corroded, then the Fe ions and Mo ions are dissolved into solution. The Fe ions generate the iron oxide layer on steel surface and it weakly protects the steel.^54–56) As the Mo ions are dissolved sufficiently, the Mo compounds were adsorbed on oxide layer, which prevented ion penetration such as Cl⁻ and SO₄²⁻ ions and the layer acts as cation selective phases.^49,57) From results of XPS studies, the Mo compounds in the rust layer were identified as FeMoO₄ and Na₂MoO₄.^50,58) Especially, the FeMoO₄ formed by the reaction with molybdate ion and dissolved Fe²⁺ ion is an insoluble compound which acts as a stable passive layer.^56,59)

Fig. 7

Schematic of the molybdate ion acting as a barrier of electrochemical reaction in seawater.

4. Conclusions

This study investigated the alloying effect of molybdenum on the corrosion of low alloy steel in the synthetic seawater at 60℃ using weight loss tests, electrochemical impedance spectroscopy (EIS) and surface analysis. It comes down to the following conclusions:

• (1)   Weight loss tests revealed that only the 0.2 mass% Mo-containing steel presented much higher corrosion resistance than Mo-free steel.
• (2)   EIS interpretation of the specimens suggested that Mo-containing steel has relative homogeneous rust layer, showing that the spectra of the phase angle were not separated more distinctly. Especially, the depression angle of 0.2 mass% Mo-containing steel decreased during immersion periods, which means the steel had nonporous corrosion rust layer on the steel surface.
• (3)   Through SEM and XPS analyses, the rust layer of 0.2 mass% Mo-containing steel was much tighter and denser than that of Mo-free, 0.05 and 0.1 mass% Mo-containing steels. The 0.2 mass% Mo-containing steel composed of mainly of Fe, Fe oxides and Mo compounds such as FeMoO₄.

The positive effect of Mo on the corrosion resistance was presented when the Mo was added about 0.2 mass% as an alloying element. Therefore, it is reasonable to add at least 0.2 mass% Mo in low alloy steel in order to increase the lifetime of water ballast tank. Also, the effect of Mo is expected to become more apparent as time goes on.

Acknowledgements

This research was supported by the Hyundai Steel Company.

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