materials Article

Comparison of Medium Manganese Steel and Q345 Steel on Corrosion Behavior in a 3.5 wt % NaCl Solution Guanqiao Su and Xiuhua Gao * The State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China; [email protected] * Correspondence: [email protected]; Tel.: +86-24-8369-0360 Received: 16 July 2017; Accepted: 4 August 2017; Published: 11 August 2017

Abstract: A cyclic wet/dry accelerated corrosion test was used to compare the corrosion behavior of medium-Mn steel and Q345 steel. In terms of scanning electron microscope (SEM), using X-ray diffraction (XRD), electron probe microanalysis (EPMA), X-ray photoelectron spectroscopy (XPS), and analysis of the corrosion process, the results showed that the medium-Mn steel did not exhibit higher corrosion resistance than Q345 steel due to the greater content of Mn-rich compounds in the rust layer. Moreover, the effect of a small amount of anti-corrosion elements in medium-manganese steel can regulate the corrosion rate. The conceptual model of the corrosion process of the medium-Mn steel in a 3.5 wt % NaCl solution is proposed. Keywords: corrosion rate; medium-Mn steel; SEM; EPMA

1. Introduction As the world continues development in the exploration and exploitation of offshore oil and gas resources, a large number of high-quality steel materials will replace the traditional offshore platform steel. Due to its low cost and the high strength and toughness, low-carbon medium-manganese steel will play a significant role in the construction of marine engineering projects [1–3]. The corrosion resistance is usually improved with the increase of anti-corrosion element contents. Nickel was added to the steel to achieve a synergistically-enhancing effect, with small amounts of copper enriching the outer layer of the steel substrate [4,5]. Molybdenum may generate insoluble FeMoO4 and become enriched near the outer layer of the corrosion products to serve as a barrier against corrosive ions during the corrosion process [6,7]. The presence of chromium in steels can decrease the corrosion rate through the formation of a compact barrier in the rust film with the formation of a Cr-rich compound. However, this steel may be adversely affected by the seawater environment owing to the lower content of the above anti-corrosion elements. Therefore, it becomes very important to the assessment of the corrosion behavior of this type medium-manganese steel. In the new type medium-Mn steel, alloying with 5% Mn plays a significant role in enhancing the homogeneity of the mechanical properties, and it also improves the hardenability during the heat treatment process [8]. However, in terms of corrosion, few reports showed the effects of Mn on this steel. In consideration of the importance of Mn as an alloying element in new medium-manganese steel, the corrosion behavior of medium-manganese steel under a simulated seawater environment (a 3.5 wt % NaCl solution) was elaborated, and the Q345 steel was introduced into the thesis as reference materials.

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2. Experimental Procedure 2.1. Preparation of the Tested Steels The chemical compositions of medium-Mn steel and Q345 steel are listed in Table 1. In the manufacturing procedures, the medium-Mn steel was melted using a vacuum induction furnace (Benxi Steel Group Corporation, Benxi, China), and was then casted into ingots. In the following procedure, they were forged into billet slabs with a 140 mm thickness. The slab was heated to 1200 ◦ C for 2 h and then hot rolled into a plate with a thickness of 30 mm under multiple passes from 960 to 940 ◦ C. Subsequently, the plate was directly water-quenched to room temperature using an accelerated cooling system. Next, the plate was intercritically annealed for 60 min at 650 ◦ C. Then the plate was air-cooled at 25 ◦ C. The yield strength, ultimate tensile strength, and impact absorbed energy at −60 ◦ C of the tested medium-Mn steel are 737 MPa, 868 MPa, and 206 J, respectively. The Q345 ingot was also prepared by the vacuum induction furnace in the laboratory. The yield strength of Q345 steel is ~345 MPa. Table 1. Chemical compositions of the specimens (in mass %). Samples

C

Si

Mn

P

S

Al

Mo

Ni

Cr

Fe

Medium-Mn Steel Q345 Steel

0.04 0.17

0.2 0.32

5.5 1.38

0.005 0.019

0.003 0.009

0.03 0.04

0.22 -

0.30 -

0.39 -

Bal. Bal.

2.2. Cyclic Wet/Dry Accelerated Corrosion Test The coupons cut from the two tested steels (60 mm × 40 mm × 4 mm and 20 mm × 15 mm × 4 mm) were subsequently polished with 240, 400, 600, and 800 grit silicon carbide papers, respectively. Those two types of coupons were then cleaned with petroleum ether, alcohol, and acetone using an ultrasonic cleaner and dried with cold air. Then the coupons (60 mm × 40 mm × 4 mm) were weighed using a balance with a precision of 0.1 mg and stored in a desiccator before the cyclic wet/dry experiment. A 3-mm diameter hole was machined to hang the coupons with a non-metallic wire. According to EN ISO 11130 [9], the tested solution with a volume of 90 L contained 3.5 wt % NaCl, which was made from deionized water and analytical reagents. The pH value of the solution was around 6.5. Each wet/dry cycle consisted of a drying period (50 min at 25 ◦ C) plus a wetting period (10 min at 25 ◦ C). NaOH was provided to maintain the simulated seawater corrosive solution’s pH value. The accelerated corrosion test was divided into six parts, 24, 72, 168, 288, 432, and 600 h, respectively. Four replicate coupons were used for each corrosion test. Another important factor to consider was the humidity which, in the airtight container, should be maintained at 45% to create a humid environment. After the corrosion tests, the larger coupons (60 mm × 40 mm × 4 mm) were removed from the corrosion products to determine the mass loss. The coupons were pickled in 20% hydrochloric acid (HCl), inhibited with 20 g/L of hexamethylenetetramine (urotropine) and 500 mL distilled water, rinsed subsequently in deionized water and acetone, and dried [10]. Then, the mass loss of those coupons was used to calculate the corrosion rate. 2.3. Morphology Observation and Composition Analysis The corrosion products of the two tested steels were observed by a FEI QUANTA 600 scanning electron microscope (SEM) (Hillsboro, OR, USA). Corrosion phases were detected by using a D/max-2400 X-ray diffraction (XRD) (Rigaku, Tokyo, Japan) with Cu Kα radiation and a step of 0.04◦ , and identified by matching peak positions automatically with MDI Jade software (Livermore, CA, USA) equipped with PDF-2 (2004). A JEOL-8530F electron probe analyzer (EPMA) (Tokyo, Japan) was set to a voltage of 20 kV and an 11 mm working distance to observe the elemental distribution. The composition of the corrosion films was detected with a Thermo ESCALAB 250 X-ray photoelectron spectroscopy (XPS) system (Waltham, MA, USA).

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3. Results and Discussion 3. Results and Discussion 3.1. Corrosion Rate 3.1. Corrosion Rate In general, corrosion rate reflects the corrosion process of the materials. In this experiment, the In general, corrosion rate reflects the corrosion process of the materials. In this experiment, corrosion rates were calculated according to the weight loss (ΔW, g) based on the following equation the corrosion rates were calculated according to the weight loss (∆W, g) based on the following [11] equation [11] Corrosion (mm/y) W))//((AA × × TT××D)D) Corrosion raterate ∆W (1) (mm/y ) ==((KK×× Δ (1) 87,600, aa constant constant for for the corrosion rate evaluated in mm/year, mm/year, A A is is the the total total corrosive corrosive area area where K == 87,600, 2 , T is the exposure time in hours, and D is the density of the tested steels in g/cm 3. 2 3 in cm , T is the exposure time in hours, and D is the density of the tested steels in g/cm . in The corrosion kinetic curves for the two types of steel after cyclic wet/dry accelerated corrosion corrosion The wet/dry accelerated tests with with aa 3.5 3.5 wt % NaCl solution are presented in Figure 1. For For the the two two kinds kinds of tested tested steels, steels, the the tests overall trend of the corrosion rates versus corrosion time was similar. After 24 h and 72 h of corrosion overall the corrosion 72 h of corrosion time, the the corrosion corrosion rate of the tested tested medium-Mn medium-Mn steel was higher than that of the Q345 Q345 steel and and the the time, differences were were close close to 1 mm/year. Thecorrosion corrosionrates ratesof ofthe the two two tested tested steels steels from from 168 168 to to 600 600 hh differences mm/year. The exhibited slight changes. The The final final corrosion corrosion rate rate values values of of the the tested tested medium-Mn medium-Mn steel steel and and the the Q345 Q345 exhibited − 1 − 1 −1, respectively. steel in simulated seawater are 2.07 mm·year mm·year−1 and and1.99 1.99mm·year mm·year , respectively. steel 5.0

Medium-Mn steel

Corrosion rate (mm/y)

4.5 4.0 3.5 3.0

Q345 steel

2.5 2.0 0

100

200

300

400

500

600

Time (h)

Figure Figure 1. Corrosion Corrosion rates rates of of the the tested tested medium-Mn medium-Mn steel steel and and Q345 Q345 steel steel exposed exposed to to aa 3.5 3.5 wt wt % % NaCl NaCl ◦ C. solution for different lengths of time at 25 °C. solution

3.2. 3.2. X-ray X-ray Diffraction Diffraction Figure Figure 22 presents presents the the XRD XRD patterns patterns of of the the corrosive corrosive products products taken taken from from the the surface surface of of the the two two tested tested steels after corrosion tests of various durations. The The corrosion corrosion phases phases of of the the medium-Mn medium-Mn steel steel and and the the Q345 Q345 steel steel are presented presented in Figure 2a,b, respectively. After After the the 24 24 h corrosion corrosion test, test, the the steel steel substrate of of thethe other corrosive phase andand thatthat in Figure 2a was than substrate peak peakwas waslarger largerthan thanthat that other corrosive phase in Figure 2a lower was lower that Figure 2b. Other peaks mainly indicated thethe lepidocrocite than in that in Figure 2b. Other peaks mainly indicated lepidocrocite(γ-FeOOH), (γ-FeOOH),small smallamounts amounts of of (Fe,Mn) x O y , Fe x O y , and goethite (α-FeOOH). The critical peaks of lepidocrocite and goethite corrosion (Fe,Mn)x Oy , Fex Oy , and goethite (α-FeOOH). The critical peaks of lepidocrocite and goethite corrosion phases phases increased increased after after 168 168 h h of of corrosion corrosion time, time, respectively. respectively. However, However, in in the the final final three three corrosion corrosion durations, these peaks started to shrink. Additionally, the critical peaks of (Fe,Mn) x O y and Fe Oyy did durations, these peaks started to shrink. Additionally, the critical peaks of (Fe,Mn)x Oy and FexxO did not process. not change change detectably detectably over over the the entire entire corrosion corrosion process.

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FexOy ♦ (Fe,Mn)xOy ⊗ α-FeOOH Δ γ-FeOOH (a) ♥ (a) ♥ FexOy ♦ (Fe,Mn)xOy ⊗ α-FeOOH Δ γ-FeOOH Fe ♥Δ FeΔ Δ ♥Δ Δ♥ Δ Δ ♦ ♦ Δ♥ 24h Δ 24h ♥Δ Δ Δ ♦ Δ♥ ♥ Δ Δ ♦ Δ♥ 72h 72h ♥Δ Δ ♦ Δ♥ Δ ⊗ Δ ♥ Δ Δ ♦ Δ♥ ⊗ 168h ♥Δ Δ 168h Δ Δ Δ♥ ♥ ⊗ Δ Δ ♦ Δ♥ ♦ 288h ⊗ Δ Δ ♥ 288h ⊗ Δ Δ Δ Δ ♥ Δ ♦ Δ♥ ♥ ♦ 432h ⊗ ♥Δ 432h ⊗ Δ Δ ♦ Δ ♥Δ Δ ♥ Δ Δ ⊗ ♦ ♥ 600h 600h 10 20 30 40 50 60 70 10 20 30 50 60 70 2 Theta40, degree 2 Theta , degree

(b) (b)

♥ FexOy♦ (Fe,Mn)xOy⊗ α-FeOOH Δ γ-FeOOH ♥ FexOy♦ (Fe,Mn)xOy⊗ α-FeOOH Δ γ-FeOOH

Δ ⊗ Δ ⊗ Δ ⊗ Δ ⊗ Δ ⊗ Δ ⊗ ⊗ Δ ⊗ Δ ⊗ Δ ⊗ Δ ⊗ Δ ⊗ Δ Δ

Intensity Intensity

Intensity Intensity

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24h 24h 72h 72h 168h 168h 288h 288h 432h 432h

600h 600h 10 20 10 20

Δ

♥Δ Fe ♥Δ Fe Δ Δ Δ ♥Δ Δ ♥ Δ Δ Δ ♥Δ Δ ♥ ♥Δ ♥Δ Δ Δ ♥Δ Δ ♥ Δ Δ ♥Δ Δ Δ ♥ Δ

30 40 50 30 50 2 Theta40, degree 2 Theta , degree

♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦

♦ Δ♥ Δ♥♦ Δ♥ Δ♥ Δ Δ Δ Δ Δ♥ Δ♥ Δ♥ Δ♥ 60 60

70 70

Figure 2. XRD patterns the tested coupons after different corrosion times exposed to % coupons after different corrosion times exposed to a 3.5 % wt NaCl Figure 2. 2. XRD XRD patterns patternsofof ofthe thetested tested coupons after different corrosion times exposed to aawt3.5 3.5 wt % ◦at NaCl solution for different lengths of time 25 °C: (a) medium-Mn steel; and (b) Q345 steel. solution for different lengths of time at 25 C: (a) medium-Mn steel; and (b) Q345 steel. NaCl solution for different lengths of time at 25 °C: (a) medium-Mn steel; and (b) Q345 steel.

3.3. 3.3. SEM SEM Morphology Morphology Figures and 44 show show SEM SEM images images of of surface surface morphologies morphologies of of the the tested tested medium-Mn medium-Mn steel steel and and Figures 33 and Q345 steel after cyclic wet/dry accelerated corrosion testing with a 3.5 wt % NaCl solution. After 24 Q345 steel after after cyclic cyclic wet/dry wet/dryaccelerated acceleratedcorrosion corrosiontesting testingwith with a 3.5 NaCl solution. After a 3.5 wtwt %% NaCl solution. After 24 h corrosion (Figures 3a and 4a), the rust presented some flaky structures. The flaky structure was 24 h corrosion (Figures 3a and 4a), the rust presented some flaky structures. The flaky structure was h corrosion (Figures 3a and 4a), the rust presented some flaky structures. The flaky typical of γ-FeOOH [12]. Due the existence γ-FeOOH, became loose porous. The typical of of of γ-FeOOH, thethe rustrust became loose andand porous. The rust of γ-FeOOH γ-FeOOH[12]. [12].Due Duetoto tothe theexistence existence of γ-FeOOH, the rust became loose and porous. The rust of the Q345 steel was denser than that of the tested medium-Mn steel. Therefore, the corrosion of the Q345 steel was denser than that of the tested medium-Mn steel. Therefore, the corrosion rate rust of the Q345 steel was denser than that of the tested medium-Mn steel. Therefore, the corrosion rate of medium-Mn higher at initial time. the second time point, the γ-FeOOH of tested medium-Mn steelsteel waswas higher at the initial time. rate of tested tested medium-Mn steel was higher at the the initial time.AtAt Atthe thesecond secondtime timepoint, point, the the γ-FeOOH γ-FeOOH mainly occupied the surfaces of the two tested steels. The size of γ-FeOOH above the medium-Mn mainly occupied the surfaces of the two tested steels. The size of γ-FeOOH γ-FeOOH above the medium-Mn steel smaller than that of of the the Q345 Q345 steel steel (see Figures 3b and 4b). Thus, this this phenomenon phenomenon may steel was was smaller than that may steel (see (see Figures Figures 3b 3b and and 4b). 4b). Thus, cause the gap in the corrosion rates of the two tested steels. As seen in Figures 3c and 4c, the rust cause the rates of of thethe twotwo tested steels. As seen in Figures 3c and the4c, rust above the gap gapininthe thecorrosion corrosion rates tested steels. As seen in Figures 3c4c, and the rust above the medium-Mn steel was denser than that of the previous one with the increasing corrosion the medium-Mn steel was than that the of previous one with increasing corrosion time. above the medium-Mn steeldenser was denser thanofthat the previous one the with the increasing corrosion time. was also than rust of Q345 steel. The at the It wasIt than the rustthe of the steel. The change the morphology of the rustof the time. Italso wasdenser also denser denser than the rustQ345 of the the Q345 steel. Theatchange change at the the morphology morphology ofabove the rust rust above the medium-Mn leads to the corrosion rate decreasing sharply. Furthermore, the rust of the medium-Mn leads to the corrosion decreasing sharply. Furthermore, the rust ofthe therust Q345 above the medium-Mn leads to the rate corrosion rate decreasing sharply. Furthermore, ofwas the Q345 similar to that time point. Thus, the corrosion of the steel similar to that in the timeprevious point. Thus, corrosion the Q345rate steel Q345 was was similar toprevious that in in the the previous timethe point. Thus, rate the of corrosion rate ofremained the Q345 Q345stable steel remained stable 72 h shown in and 4d,e, from 72 h corrosion to 168 corrosion.to shown in FigureAs 3d,e and Figure 4d,e,3d,e the rust remained stable from from 72 h hhcorrosion corrosion toAs168 168 h corrosion. corrosion. As shown in Figures Figures 3d,e and morphologies 4d,e, the the rust rust morphologies presented as a ‘whisker’ morphology, which was a typical of α-FeOOH [13]. presented as a presented ‘whisker’ morphology, which was a typical of α-FeOOH [13].of The transformation of morphologies as a ‘whisker’ morphology, which was a typical α-FeOOH [13]. The The transformation of FeOOH could decrease the corrosion rate. Finally, the rust morphologies above the FeOOH could decrease the corrosion rate. Finally, the rust morphologies above the two tested steels transformation of FeOOH could decrease the corrosion rate. Finally, the rust morphologies above the two achieved aa smooth and (see Figures 3f 4f), the corrosion of achieved smooth and dense state (see Figures 3f and and the corrosion the two rate tested two tested testeda steels steels achieved smooth and dense dense state state (see4f), Figures 3f and and 4f), and andrate theof corrosion rate of the two tested steels achieved stable values. steels achieved stableachieved values. stable values. the two tested steels

Figure 3. Cont.

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Figure 3. SEM morphologies of the tested medium manganese steel after different immersion Figure 3. SEM morphologies of the tested medium manganese steel after different immersion durations: Figure 3. SEM morphologies of the manganese durations: (a) 24h; (b) 72 h; (c) 168h; (d)tested 288 h; medium (e) 432 h; and (f) 600 h.steel after different immersion (a) 24 h; (b) 72 h; (c) 168h; (d) 288 h; (e) 432 h; and (f) 600 h. durations: (a) 24h; (b) 72 h; (c) 168h; (d) 288 h; (e) 432 h; and (f) 600 h.

Figure 4. Cont.

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Figure 4. SEM morphologies of the tested Q345 steel after different immersion durations: (a) 24h; (b) Figure 4. SEM morphologies of the tested Q345 steel after different immersion durations: (a) 24 h; 72 h; (c) 168h; (d) 288 h; (e) 432 h; and (f) 600 h. (b) 72 h; (c) 168h; (d) 288 h; (e) 432 h; and (f) 600 h. Figure 4. SEM morphologies of the tested Q345 steel after different immersion durations: (a) 24h; (b) 3.4. EPMA 72 h;Results (c) 168h; (d) 288 h; (e) 432 h; and (f) 600 h.

3.4. EPMA Results The EPMA images of the tested medium-Mn steel corresponding to the different corrosion times 3.4.The EPMA Results EPMA images of the tested medium-Mn steel corresponding to the different corrosion times presented in Figure 5 shows the main elemental distribution of the rust formed after cyclic wet/dry presented in Figure 5 shows thetested mainmedium-Mn elemental distribution of the rustthe formed after cyclic wet/dry The EPMA images of the corresponding different corrosion accelerated corrosion testing with a 3.5 wt % NaClsteel solution. After 24to h corrosion (Figure 5a),times Fe and accelerated corrosion testing with a 3.5 wt % NaCl solution. After 24 h corrosion (Figure 5a), Fe and Figure 5 showsand the concentrated main elemental of the rust formed Mnpresented elementsinwere oxidized in distribution the outer rust film. The outerafter rust cyclic layer wet/dry contained Mnaccelerated elements corrosion were oxidized and concentrated in the outer rust film. outer (Figure rust layer testing a 3.5 wt % NaCl solution. After 24 hThe corrosion 5a),contained Fe and large amounts of Fe oxides andwith some Mn-rich compounds. The corrosion products between the outer large Fe oxides andand some Mn-rich compounds. products the outer Mnamounts elementsof were oxidized concentrated in the outer The rust corrosion film. The outer rust between layer contained rust film and the matrix presented a diffuse distribution, it indicated that the rust was forming with rust filmamounts and theofmatrix presented a diffuse distribution, indicated that the rustbetween was forming with large Fe oxides and some Mn-rich compounds.itThe corrosion products the outer the corrosion solution and oxygen and this moment was an accelerated corrosion stage from this time therust corrosion solution andpresented oxygen and this moment was an accelerated corrosion from this time film and the matrix a diffuse distribution, it indicated that the rust stage was forming with point. After 72 h corrosion, a completed oxidation rust layer is presented in Figure 5b, and the Mn the corrosion and oxygen and this moment was accelerated corrosion stage from thisthe timeMn point. After 72 solution h corrosion, a completed oxidation rust an layer is presented in Figure 5b, and was enriched in the rust layer from the matrix to the outer rust layer. At this time point, the rust layer point. After in 72 the h corrosion, a completed oxidation rustouter layer rust is presented in this Figure 5b,point, and the was enriched rust layer from the matrix to the layer. At time theMn rust was completely dominated with the Fe corrosion products and the Mn-rich compounds and the waswas enriched in the rust layer from thethe matrix to the outer rust layer. time point, the rust and layerthe layer completely dominated with Fe corrosion products andAt thethis Mn-rich compounds corrosion rate reached the peak. After h corrosion, the density of the Fe element increased was completely dominated with the 168 Fe corrosion products and the Mn-rich compounds and again the corrosion rate reached the peak. After 168 h corrosion, the density of the Fe element increased again and the Mn migrated to the outer rust film (Figure 5c). After 288 h corrosion (see Figure 5d), the rust corrosion rate reached the peak. After 168 h corrosion, the density of the Fe element increased again and the Mn migrated to the outer rust film (Figure 5c). After 288 h corrosion (see Figure 5d), the rust layer to the matrixtoside and the outer layer was loose. Meanwhile, andclose the Mn migrated the was outerrelatively rust film dense (Figure 5c).the After 288 rust h corrosion (seeloose. Figure 5d), the rustthe layer close to the matrix side was relatively dense and outer rust layer was Meanwhile, the Mn-rich compounds can be found in the outer rust layer. Similar to the initial corrosion, the corrosion layer close to the matrix side was in relatively dense theSimilar outer rust layer was corrosion, loose. Meanwhile, the Mn-rich compounds can be found the outer rustand layer. to the initial the corrosion process in compounds the outer rust was affected by rust the Fe andSimilar Mn. The corrosion rate decelerated due to Mn-rich canfilm be found in the outer layer. to the initial corrosion, the corrosion process in the outer rust film was affected by the Fe and Mn. The corrosion rate decelerated due to the theprocess existence of the dense rust affected layer. After 432Fe h corrosion (Figure 5e), the elemental distribution in the outer rustinner film was by the and Mn. The corrosion rate decelerated due to existence of the dense inner rust layer. After 432 h corrosion (Figure 5e), the elemental distribution in in the the existence rust layer similar to rust that layer. after After 288 h432 corrosion. However, the the Mnelemental element distribution was enriched of was the dense inner h corrosion (Figure 5e), the rust layer was similar to that after 288 h corrosion. However, the Mn element was enriched near in the midline rust layerofwas to that after 288 h corrosion. element was enriched near the similar rust layer. It was speculated that theHowever, existencethe of Mn the anti-corrosion elements the midline of the rust layer. It was speculated that the existence of the anti-corrosion elements could nearcause the midline of the rust layer. that the existence of thein anti-corrosion could this phenomenon due Ittowas thespeculated Mn-rich compound being fixed the midline elements of the rust cause this phenomenon due to due the Mn-rich compound beingbeing fixedfixed in the of the rustrust layer. could cause this the phenomenon to the Mn-rich compound inmidline the midline of the layer. Therefore, rust layer achieved a stable cross-sectional morphology and elemental Therefore, the rust layer achieved a stable cross-sectional morphology and elemental distribution after layer. Therefore, rust layer achieved distribution after 600the h corrosion (Figure 5f). a stable cross-sectional morphology and elemental 600distribution h corrosionafter (Figure 600 h5f). corrosion (Figure 5f).

Figure 5. Cont.

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Figure 5. The main elemental distribution of the corrosion products formed in the tested medium-Mn Figure 5. The The main main elemental elemental distribution distribution of the the corrosion corrosion products products formed formed in in the the tested tested medium-Mn medium-Mn Figure steel 5. after different immersion durations: of (a) 24 h; (b) 72 h; (c) 168 h; (d) 288 h; (e) 432 h; and (f) 600 h. steel after different immersion durations: (a) 24 h; (b) 72 h; (c) 168 h; (d) 288 h; (e) 432 h; and steel after different immersion durations: (a) 24 h; (b) 72 h; (c) 168 h; (d) 288 h; (e) 432 h; and (f) (f) 600 600 h. h.

Percentage/% Percentage/%

Figure 6 shows the EPMA line scanning results of Mn distribution in the rust layer of the of Mn The in the rust absorption Figure 6 shows thethe EPMA distribution layer of the medium-Mn steel and Q345 line steelscanning after 432 hresults of corrosion. atomic characteristic steel of corrosion. corrosion. The atomic characteristic medium-Mn the results Q345 steel after 432 of The absorption intensity from theand EPMA reveals that thehhMn was enriched inatomic the rustcharacteristic layer with a content of intensity the EPMA results reveals that the Mn in layer the rust layer with a content ~8% in from the medium-Mn steel, which was higher thanwas thatenriched of the rust in the Q345 steel. It was of speculated that the instability of Mn was concentration in the might produce certain adverse effects ~8% in the medium-Mn steel, which higher than thatrust of the rust layer in the Q345 steel. It was ~8% steel. on the corrosion in of the testing of the medium-Mn steel. speculated that the thebehavior instability ofMn Mn concentration therust rust might produce certain adverse effects speculated that instability concentration ininthe might produce certain adverse effects on on the corrosion behavior in the testing of the medium-Mn steel. the corrosion behavior in the testing of the medium-Mn steel. Medium-Mn steel (after 432 h corrosion)

8

86

matrix Medium-Mn steel (after 432rust h corrosion)

64 2 4 0 2

rust matrix

0

0

Percentage/% Percentage/%

100

200

300

0

8

8 6 4

6 4

100

200

400

500

Distance/um

300

400

500

600

600

700

700

800

800

Q345 steel(after 432 h corrosion) Distance/um

rust matrix

Q345 steel(after 432 h corrosion)

rust matrix

2 0

2

0

100

200

300

400

500

600

700

800

Distance/μm

0 0

100

200

300

400

500

600

700

800

Figure 6. The typical EPMA line scanning results of Mn distribution in the rust layer of the tested Distance/μm medium-Mn steel and the Q345 steel after 432 h of corrosion.

Figure 6. The typical EPMA line scanning scanning results of Mn distribution in the rust layer of the tested medium-Mn steel and the Q345 steel after 432 h of corrosion. corrosion.

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3.5. X-ray X-ray Photoelectron Photoelectron Spectroscopy The chemical chemical composition composition of of the the rust rust layer layer on medium-Mn medium-Mn steel steel was was measured measured by by an X-ray X-ray photoelectron spectroscopy spectroscopysystem. system.However, However,FeFe and elements were obviously detected and MnMn elements were obviously detected withwith XPS XPS measurement compared with other alloy elements, as shown in 7. Figure 7. Theofpeaks 2pFe 3/2 measurement compared with other alloy elements, as shown in Figure The peaks Fe 2pof and 3/2Fe and 2p1/2concentrated were concentrated 711.4 andeV, 724.6 eV, confirmed which confirmed the existence of the oxidation 2p1/2Fewere at 711.4atand 724.6 which the existence of the oxidation state of state of trivalent Fethe ions inlayer the rust [14]. In the addition, peaks ofand Mn Mn 2p3/22pand Mn 2p 1/2 were trivalent Fe ions in rust [14].layer In addition, peaks the of Mn 2p3/2 observed 1/2 were observed at 654.0 642.3 eV, andrespectively. 654.0 eV, respectively. identified the ion bivalent Mn ion state in[15]. the at 642.3 and It identified It the bivalent Mn oxidation stateoxidation in the rust layer rust layer Hence, the results mentioned further identifycompound the Mn-richcomposed compound Hence, the[15]. results mentioned above further above identify the Mn-rich ofcomposed MnFe2 O4 of MnFe Mn 3O4layer in theduring rust layer during theprocess. corrosion process. and Mn32O44 and in the rust the corrosion 4500 4000

2p3/2

2p1/2

Fe

2p3/2

3800

3500

2p1/2

3600

Counts /s

Counts /s

Mn

4000

3000 2500

3400 3200

2000

3000

1500

2800

700 705 710 715 720 725 730 735 740 745

Binding Energy (eV)

630

635

640

645

650

655

660

665

Binding Engery (eV)

Figure the rust layer onon medium-Mn steel after 432432 h inhain 3.5a Figure 7. 7. XPS XPSspectra spectraofofFe Feand andMn Mnelements elementsinin the rust layer medium-Mn steel after wt solution. 3.5 % wtNaCl % NaCl solution.

3.6. Medium-Mn Steel Steel 3.6. Corrosion Corrosion Process Process of of the the Tested Tested Medium-Mn From From the the observed observed results results and and comparison comparison with with the the Q345 Q345 steel steel on on the the corrosion corrosion process, process, it it can can be inferred that the addition of Mn can increase the corrosion rate with the corrosion process. The be inferred that the addition of Mn can increase the corrosion rate with the corrosion process. conceptual corrosion model of the tested medium-Mn steel is ispresented The conceptual corrosion model of the tested medium-Mn steel presentedininFigure Figure 8. 8. In In our our experiment, the electrochemical reaction occurred in a near-neutral pH environment from a 3.5 wt experiment, the electrochemical reaction occurred in a near-neutral pH environment from a 3.5 wt % % NaCl NaCl electrolyte electrolyte solution. solution. Under Under such such conditions, conditions, Fe(II) Fe(II) complexes complexes that that form form during during the the initial initial anodic anodic dissolution favor of of chloride-saturated chloride-saturated dissolution were were oxidized oxidized to to FeOOH, FeOOH, the the formation formation of of which which was was in in favor environments [16]. In addition, Fe xOy acted as another main corrosion product during the entire environments [16]. In addition, Fex Oy acted as another main corrosion product during the entire process. Thesecompounds compoundscan can formed the transformation of the phase; FeOOH phase; this process. These be be formed due due to thetotransformation of the FeOOH this reduction reduction reaction was because favorable oxygenprocess reduction process could notastake placeofasthe a reaction was favorable thebecause oxygenthe reduction could not take place a result result of the limited supply of oxygen at the rust–metal interface [17,18]. The Fe xOy constituted the limited supply of oxygen at the rust–metal interface [17,18]. The Fex Oy constituted the final dense rust final dense rust near the the inner layer under theenvironment. simulated seawater environment. Equilibrium near the inner layer under simulated seawater Equilibrium equations for the stable equations for the stable range of Fe 3O4 during the corrosion process were listed in Equations (2)–(4) range of Fe3 O4 during the corrosion process were listed in Equations (2)–(4) [19,20]. Furthermore, the [19,20]. Furthermore, might in process: two states during (γ-FeOOH) the corrosion FeOOH might appear inthe twoFeOOH states during theappear corrosion lepidocrocite and process: goethite lepidocrocite (γ-FeOOH) and goethite These mightrust originate from FexOy near the (α-FeOOH). These might originate from (α-FeOOH). the Fex Oy near the outer layer, and thethe transformation of outer rust layer, and the transformation of Fe xOy to FeOOH might take place in the porous or cracked Fex Oy to FeOOH might take place in the porous or cracked locations. locations. 3Fe2+2++ 4H2 O = Fe3 O4 + 8H++ + 2e (2) 3Fe + 4H2O = Fe3O4 + 8H + 2e (2) 3FeO++HH2 O 2O==Fe Fe33O O44 + + 2H 3FeO 2H++++2e 2e

(3) (3)

3O H22O O== 3Fe 3Fe22O33 ++2H 2Fe 2Fe 2H++++2e2e 3O 4 4++H

(4) (4)

Previous Previous investigations investigations [21] [21] have have shown shown that that aa relatively relatively high high Mn Mn content content will will result result in a high corrosion current density in the potentiodynamic polarization test. Our results showed that the high content of Mn element element can can increase increase the the corrosion corrosion rate rate by by promoting promoting the the anodic anodic sensitivity, sensitivity, which was similar to one one reported reported by byEliyan Eliyanetetal.’s al.’s[22] [22]study studyon onthe theeffect effectofofthe thechloride chloride content corrosion similar to content onon corrosion of of high strength steel. In our study, the effect of Mn content would be better to promote the hydrogen high strength steel. In our study, the effect of Mn content would be better to promote the hydrogen evolution during the corrosion process which was normally accompanied by charge transfer [22,23].

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evolution during the corrosion process which was normally accompanied by charge transfer [22,23]. Furthermore, a better understanding was revealedininour ourexperiments experiments on on the the effects of Furthermore, a better understanding was revealed of Mn Mn element element in in the rust layer as well as the interaction during the corrosion process. the rust layer as well as the interaction during the corrosion process.

Figure 8. Conceptual model corrosionprocess processofofthe thetested testedmedium-Mn medium-Mn steel: Figure 8. Conceptual model of of thethe corrosion steel: (a) (a)24 24h;h;(b) (b)168 168 h; h; and (c) 432 h. and (c) 432 h.

Through a comparison between the alloy elements in the tested medium-Mn steel and the Q345 Through a comparison the alloy elements inamount the tested medium-Mn steel and steel, it can be found that thebetween tested materials contained some of anti-corrosion elements (Ni, the Q345 steel, it can be found that the tested materials contained some amount of anti-corrosion Mo, Cr). However, the main difference of the two tested steels was the content of Mn. Mn might elements (Ni, Mo, Cr). However, the main difference of the two testedcorrosion steels was the content produce Mn-rich compounds (oxides and hydroxides) during the whole process. Some of Mn.specific Mn might produce compounds (oxides and hydroxides) duringinthe whole corrosion reactions wereMn-rich introduced into the electrochemical reaction, as shown Equations (5)–(8) process. Some specific reactions were introduced into the electrochemical reaction, as shown in [19,20,24,25].

Equations (5)–(8) [19,20,24,25].

Mn2+ + 2FeOOH = MnFe2O4 + 2H+ 2+

(5) +

+ 2FeOOH = MnFe2 O2O 4 4++2H 2FeMn 3O4 + 3Mn2+ + 4H2O = 3MnFe 8H++ 2e 2+ + 2Fe + 4H O = 3MnFe OO4 4+ +5H8H + 2e 3 O4 + 3Mn 3Mn(OH) 3− + 2Fe3O4 +2H+ = 3MnFe22 2O + 2e

3Mn(OH)3 − + 2Fe3 O4 + H+ = 3MnFe2 O4 + 5H+ 2 O + 2e 3MnFe2O4 + 4H2O = 6FeOOH + Mn3O4 + 2H +2e

(6) (5) (7) (6) (8)

(7)

+

+ Mn3 Oadverse + 2e during the corrosion (8) 2 O4 + 4H 2 O = 6FeOOH 4 + 2H effects The generation of3MnFe the Mn-rich compounds can produce process. In terms of corrosion process, theproduce effects ofadverse Mn-richeffects compounds The generation of the the above Mn-rich compounds can duringwere the mainly corrosion manifested in two aspects: (1) Mn-rich compounds (such as MnFe 2O4 and Mn3O4) can usually be used process. In terms of the above corrosion process, the effects of Mn-rich compounds were as an anode material as they have a spinel structure that was helpful for H+ to intercalate/deintercalate mainly manifested in two aspects: (1) Mn-rich compounds (such as MnFe2 O4 and Mn3 O4 ) can and their formation processes were accompanied by a higher H+ generation (from Equations (5)–(8)) usually be used as an anode material as they have a spinel structure that was helpful for H+ to [21,26,27]; and (2) the free energy of Mn-rich compounds were higher than other corrosion products intercalate/deintercalate and their formation processes were accompanied by a higher H+ generation during the corrosion process (Table 2); thus, it can exhibit higher spontaneous electrochemical (from Equations (5)–(8)) [21,26,27]; and (2) the free energy of Mn-rich compounds were higher than reactivity. other corrosion products during the corrosion process (Table 2); thus, it can exhibit higher spontaneous electrochemical reactivity. Table 2. Thermodynamic data of used species during the corrosion process of the tested medium-Mn steel

Table 2. Thermodynamic data of used species during the corrosion process of the tested medium-Mn steel.

ΔG° (298.15 K) Species ∆G◦ (298.15 K) (J−1·mol−1 ) (J·mol ) H2 H2 00 H+ H+ 00 −228,100 Mn2+ Mn2+ −228,100 MnFe2MnFe O4 2O4 −1,121,790 −1,121,790 Fe 00 Fe FeO FeO −256,354 −256,354 FeOOH −485,300 FeOOH −485,300 Species

Source

Source

[25][25] [25][25] [20][20] [25][25] [25][25] [25][25] [19][19]

Species

Species

OO 22 HH 2O 2O − Mn(OH) Mn(OH) 3 3− Mn O Mn 3 3O 44 2+2+ FeFe FeFe 2O 2O 33 FeFe 3O 3O 44

ΔG° (298.15 K) Source Source ∆G◦ (298.15−1K) (J·mol−1 ) (J·mol ) 0 0 [25] [25] −237,191 −237,191 [20] [20] −744,200 −744,200 [25] [25] − 1,283,200 −1,283,200 [25] [25] −78,900 −78,900 [25] [25] −740,986 −740,986 [25] [25] −1,015,450 −1,015,450 [25] [25]

anti-corrosion alloyingelements elements (Cu, (Cu, Mo, Mo, Ni, steel cancan enhance TheThe anti-corrosion alloying Ni, and and Cr) Cr)ininmedium-Mn medium-Mn steel enhance corrosion resistance and weaken the adverse effects of elemental Mn. A lower content of Cu in the

corrosion resistance and weaken the adverse effects of elemental Mn. A lower content of Cu in the designed steel may improve corrosion resistance by forming CuOx compounds that provide nucleation

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sites during the early stage of the corrosion process. An increase in the nucleation rate can inhibit the corrosion rate [28]. The addition of Mo decreased the corrosion rate by selective cation permeability via the formation of the MoO4 2− ion [6,7]. Ni promoted corrosion resistance in steel in amounts less than 0.5% [28,29]. It improved corrosion resistance by stabilizing the structure of the protective Fex Oy phase [28,30]. Cr nucleates Cr compounds (oxides and hydroxides) that can act as a barrier to prevent the infiltration of the corrosive solutions and oxygen during the corrosion process [31,32]. The synergy effects of the above alloy elements can regulate the corrosion process of the tested medium-Mn steel and lead to its corrosion rate being close to that of the Q345 steel. 4. Conclusions The following conclusions can be drawn from this work: (1)

(2)

Through a comparison of medium-manganese steel and Q345 steel on their corrosion behavior in a 3.5 wt % NaCl solution, the medium-Mn steel exhibited lower corrosion resistance due to the adverse effect of Mn. Mn-rich compounds presented adverse effects during the corrosion process. The two tested steels had similar corrosion behavior. Moreover, the rust morphologies of the Q345 steel were much denser than that of the medium-Mn steel. The corrosion rate was regulated because of the existence of the anti-corrosion elements in the medium manganese steel.

Acknowledgments: The authors gratefully appreciate the financial support by the National High Technology Research and Development Program (863 Program), No. 2015AA03A501. Author Contributions: Xiuhua Gao designed the experiments and performed rolling related works of this work; Guanqiao Su was responsible for the corrosion tests; Xiuhua Gao and Guanqiao Su analyzed the data; Xiuhua Gao contributed reagents/materials/analysis tools; All authors co-wrote the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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Comparison of Medium Manganese Steel and Q345 Steel on Corrosion Behavior in a 3.5 wt % NaCl Solution.

A cyclic wet/dry accelerated corrosion test was used to compare the corrosion behavior of medium-Mn steel and Q345 steel. In terms of scanning electro...
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