Slurry erosion-corrosion resistance and microbial corrosion electrochemical characteristics of HVOF sprayed WC-10Co-4Cr coating for offshore hydraulic machinery

Accepted Manuscript Slurry erosion-corrosion resistance and microbial corrosion electrochemical characteristics of HVOF sprayed WC-10Co-4Cr coating for offshore hydraulic machinery

Sheng Hong, Yuping Wu, Wenwen Gao, Jianfeng Zhang, Yugui Zheng, Yuan Zheng PII: DOI: Reference:

S0263-4368(18)30026-X doi:10.1016/j.ijrmhm.2018.02.019 RMHM 4679

To appear in:

International Journal of Refractory Metals and Hard Materials

Received date: Revised date: Accepted date:

10 January 2018 5 February 2018 18 February 2018

Please cite this article as: Sheng Hong, Yuping Wu, Wenwen Gao, Jianfeng Zhang, Yugui Zheng, Yuan Zheng , Slurry erosion-corrosion resistance and microbial corrosion electrochemical characteristics of HVOF sprayed WC-10Co-4Cr coating for offshore hydraulic machinery. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Rmhm(2017), doi:10.1016/ j.ijrmhm.2018.02.019

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ACCEPTED MANUSCRIPT Slurry erosion-corrosion resistance and microbial corrosion electrochemical characteristics of HVOF sprayed WC-10Co-4Cr coating for offshore hydraulic machinery

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Sheng Honga,b,c*, Yuping Wua,c, Wenwen Gaoa, Jianfeng Zhanga, Yugui Zhengc, Yuan

a

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Zhengd

College of Mechanics and Materials, Hohai University, 8 Focheng West Road,

Material Corrosion and Protection Key Laboratory of Sichuan Province, 180

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b

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Nanjing 211100, PR China

Xueyuan Street, Zigong 643000, PR China Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal

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c

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National Engineering Research Center of Water Resources Efficient Utilization and

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d

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Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR

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Engineering Safety, Hohai University, 1 Xikang Road, Nanjing 210098, PR China



Corresponding author: Tel:+86-25-83787233; fax: +86-25-83787233

E-mail address: [email protected] (S. Hong), [email protected] (Y.P. Wu). 1

ACCEPTED MANUSCRIPT Abstract

The slurry erosion-corrosion and microbial corrosion electrochemical behavior of high-velocity oxygen-fuel (HVOF) sprayed WC-10Co-4Cr cermet coatings and the stainless

steel

1Cr18Ni9Ti

were

both

investigated.

The

slurry

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reference

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erosion-corrosion test was performed in a rotating disk rig facility with circulating

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system using distilled water and 3.5 wt.% NaCl slurries. The microbial influenced corrosion behavior in the presence of SRB was evaluated by electrochemical

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measurement. The results showed that WC-10Co-4Cr coating exhibited higher slurry

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erosion-corrosion resistance compared to the stainless steel 1Cr18Ni9Ti in both distilled water and 3.5 wt.% NaCl slurries. The evolution of the slurry erosion

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mechanism of the coatings with the augment of NaCl concentration was cracks and

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microchipping in distilled water slurry, and a combination of detachment of binder phase, microchipping and fragments in 3.5 wt.% NaCl slurry. Potentiodynamic

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polarization and electrochemical impendence spectroscopy (EIS) results showed that

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the WC-10Co-4Cr coating had a comparable microbial influenced corrosion resistance in seawater with SRB as compared to the stainless steel 1Cr18Ni9Ti.

Keywords: High-velocity oxygen-fuel; Slurry erosion; Microbial influenced corrosion; Sulfate-reducing bacteria; WC-10Co-4Cr.

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ACCEPTED MANUSCRIPT 1. Introduction Many flow-handling components of hydraulic power plants situated in the coastal region are exposed to an environment that contains sand particles and microorganisms. When the flow-handling part of offshore hydraulic machinery works, it always

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follows the high-velocity impact of liquid-solid streams result in slurry erosion failure.

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However, microbial corrosion displays a strong tendency to accelerate material

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deterioration of infrastructure when the machine stops [1,2]. Both slurry erosion and microbial corrosion cause reductions in the efficient operation of hydraulic machinery

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and substantial economic losses in offshore hydraulic power plants. This urgent

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problem is generally solved through hydrodynamics optimization, mechanical construction design and material exploitation [3-5]. In particular, WC-Co hardmetals

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are preferred material choice for reductions of sliding wear, abrasion, tribo-corrosion

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and slurry erosion damage due to the combination of high hardness and adequate fracture toughness [6,7]. Besides hardness and toughness, WC grain size and binder

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mean free path also affect the erosion resistance of WC-Co hardmetals [8-10].

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Due to the prevalence of surface treatment techniques in industrial applications, great attentions of the researchers worldwide have been paid to the use of thermal sprayed coatings to improve the slurry erosion and microbial corrosion resistance of steel components for prolonging the service life of offshore hydraulic machinery [11-13]. Among all thermal sprayed coatings, the WC-Co system cermet coatings are often considered to be the protection of the metallic components in high wear, erosion and corrosion-resistant applications. Various spraying techniques, such as detonation 3

ACCEPTED MANUSCRIPT spraying [14,15], plasma spraying [16-18], and high-velocity oxygen-fuel (HVOF) spraying [19-23] have been employed to fabricate the WC-Co system cermet material. Compared with other thermal spraying methods, HVOF spraying is capable of producing a dense WC-Co system cermet coating with low porosity, limited oxide

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content, less decarburization, high hardness and superior bond strength [24]. It is

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evident from literature that slurry erosion resistance of HVOF sprayed WC-Co system

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cermet coating was related not only to impingement velocity and impact angle, solid concentration, size and shape of erodent, but also to carbide grain size, binder matrix

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property, porosity and hardness of the coatings [19,25,26]. The literature survey

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described above demonstrated that most investigations on the slurry erosion behavior of the coatings were used the jet impingement method. However, the application of

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the rotating disc arrangements method in studying the slurry erosion-corrosion

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behavior of HVOF sprayed WC-Co system cermet coatings is still limited, with which a rotation condition of actual hydraulic pump impellers can be simulated. Furthermore,

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it has also been observed that, there are not much studies available in the literature

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related to the microbial influenced corrosion behavior of HVOF sprayed WC-Co based coatings except studies on their corrosion properties under other corrosive environments, such as artificial seawater electrolyte, 3.5 wt.% NaCl solution and acidic solution [27-29]. In particular, sulfate-reducing bacteria (SRB)-influenced corrosion as a predominant form of microbial influenced corrosion involved the iron-derived electron transfer pathway, the consumption of cathodic hydrogen, and the chemical attack by sulfate, sulfite, thiosulfate, hydrogen sulfide and other acidic 4

ACCEPTED MANUSCRIPT organisms [30,31]. In a previous work, the present authors obtained the optimal spray parameter of HVOF sprayed conventional WC-10Co-4Cr coatings by investigating the correlation between the spray parameters and the porosity. The coating obtained by the optimum

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spraying condition exhibited better corrosion resistance and seemed to be an

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alternative to hard chromium coating in 3.5 wt.% NaCl solution [32]. This work is an

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extension of the reported research. In the current study, the slurry erosion-corrosion performance and the microbial influenced corrosion behavior in the presence of SRB

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of the WC-10Co-4Cr coating and the reference stainless steel 1Cr18Ni9Ti were both

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investigated using rotating disc arrangements method and electrochemical methods (e.g., potentiodynamic polarization and electrochemical impendence spectroscopy

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(EIS)), respectively.

2. Experimental procedure

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2.1 Sample preparation

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Feedstock material used in this study was a commercial agglomerated and sintered WC-10 wt.% Co-4 wt.% Cr powder (Large Solar Thermal Spraying Material Co. Ltd, Chengdu, China). The powders were deposited onto 1Cr18Ni9Ti stainless steel substrates using a commercial HVOF spray system (Praxair Tafa-JP8000, USA). Prior to the HVOF spraying, the substrate samples were pre-cleaned in acetone, dried in hot air, and then grit blasted with 30 mesh Al2O3 to enhance the bond strength of as-sprayed coating to substrate by increasing the surface roughness. The HVOF 5

ACCEPTED MANUSCRIPT spraying of the powders was carried out at a kerosene flow rate of 0.38 L·min-1, an oxygen flow rate of 897 L·min-1, a spray distance of 300 mm, an argon carrier gas flow rate of 10.86 L·min-1, a powder feed rate of 5 rpm, and a spray gun speed of 280 mm·s-1. The thickness of the as-sprayed coatings was about 400 μm, which was

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controlled by the number of passes of the spray gun. Then the coating specimens were

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wire cut, ground and polished for slurry erosion and microbial corrosion testing.

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Stainless steel 1Cr18Ni9Ti was also tested in the same condition for comparison based on its wide applications in offshore hydraulic machinery. Details of the

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microstructures, thermostability and formation mechanisms of the HVOF sprayed

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WC-10Co-4Cr coating has been reported in the literature [33].

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2.2 Slurry erosion test

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The slurry erosion experiments were carried out using a rotating disk rig facility with circulating system. Details of the slurry erosion test apparatus and its specimen's

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dimension were described in our previous research [34]. Prior to the test, specimens

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with an average surface roughness Ra=0.02 μm were cleaned with acetone in an ultrasonic bath, dried in hot air, and weighed by an electronic balance. Then, specimens were fixed on the disk that controlled by a motor. In the testing process, the slurry solution consisted of water and commercial silica sand with the grain size of 70~150 mesh. The slurry solution containing 3.5 wt.% NaCl and silica sand was also used for comparison. The sand concentration and the rotation speed of specimen were 10 kg·m-3 and 18 m·s-1, respectively. The slurry erosion test time was set to a constant 6

ACCEPTED MANUSCRIPT value of 10 h. After each slurry erosion test, specimens were degreased, rinsed, dried and weighed periodically by an analytical balance with an accuracy of 0.1 mg to determine mass loss. The surface morphologies of specimens after slurry erosion were observed by a scanning electron microscope (SEM, Hitachi S-3400N, Japan) with an

SC

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to make sure a good repeatability of the experiment result.

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energy dispersive spectroscopy (EDS, EX250). Each test was repeated at least thrice

2.3 Microbial corrosion electrochemical measurement

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The SRB used in this study were obtained from muddy sediment in Jiaozhou Bay of

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Qingdao. Fresh marine sediment was added into a sterile modified Postgate’s C (PGC) medium to enrich anaerobic bacteria. The bacteria were subsequently purified in

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sterile agar plates by picking up several single colonies with a sterile inoculation loop.

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The procedure was repeated until the colonies of single SRB strain were formed on the solid PGC medium. The modified Postgate’s C culture solution contained 0.5 g

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KH2PO4, 1.0 g NH4Cl, 0.06 g CaCl2·6H2O, 0.06 g MgSO4·7H2O, 6 mL 70 % sodium

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lactate, 1.0 g yeast extract, and 0.3 g sodium citrate in 1 L seawater from Qingdao offshore area. The pH value was adjusted to 7.0 using the appropriate amount of sodium hydroxide before the medium was autoclaved at 121 °C for 20 min. All the electrochemical tests were performed at 30 °C using the Solartron SI1287 electrochemical interface and SI1260 impedance/gain phase analyzer control systems. Before tests, working electrode, counter electrode, rubber stoppers and salt bridge were degreased with ethanol using an ultrasonic bath followed by exposure to a UV 7

ACCEPTED MANUSCRIPT lamp for 20 min. The electrodes used in the present study were as follows: a working electrode; a ruthenium-titanium counter electrode; and a silver/silver chloride (Ag/AgCl, 3M KCl) reference electrode (CH Instruments, Inc.). The EIS data were measured at open circuit potential (OCP) by applying a sinusoidal potential excitation

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of 10 mV amplitude in a frequency range of 100 kHz~10 mHz. The experimental data

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were interpreted on the basis of equivalent circuits using the software ZsimpWin3.21

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to obtain the fitting parameters. The potentiodynamic polarization curves were measured with a fixed sweep rate of 0.5 mV·s-1. The corrosion current density (icorr)

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and corrosion potential (Ecorr) were obtained as the intersection point of linear fits to

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the anodic and cathodic polarization curves, according to the Tafel extrapolation

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3. Results and discussion

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technique.

3.1 Slurry erosion-corrosion performance of nanostructured WC-CoCr coatings

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Fig. 1 shows the effects of chloride ions concentration on mass loss rate of HVOF

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sprayed WC-10Co-4Cr coating and stainless steel 1Cr18Ni9Ti. As can be seen, the erosion rate increases as the slurry erosion-corrosion condition becomes more severe, which is in accord with the results observed in previous investigation of HVOF sprayed Fe-based amorphous metallic coatings [34]. This is because that both uniform corrosion and pitting corrosion are more pronounced under the combined action of slurry erosion and corrosion [35]. After eroded for 10 h, the mass loss rates of the WC-10Co-4Cr coating in distilled water slurry and 3.5 wt.% NaCl slurry are 8

ACCEPTED MANUSCRIPT 0.19±0.02 and 0.29±0.04 mg·cm-2·h-1, respectively, whereas the mass loss rates of the stainless steel 1Cr18Ni9Ti in distilled water slurry and 3.5 wt.% NaCl slurry are 1.43±0.09 and 2.37±0.11 mg·cm-2·h-1, respectively. This indicates that the WC-10Co-4Cr coating exhibits a superior slurry erosion-corrosion resistance

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compared to the stainless steel 1Cr18Ni9Ti and the difference between the coating

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and the stainless steel is enhanced with the augment of NaCl concentration. Firstly,

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this could be explained that the hardness of the WC-10Co-4Cr coating (1423 Hv0.1) is about 7 times higher than that of the stainless steel 1Cr18Ni9Ti (203 Hv0.1) since the

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hardness has a great influence on the slurry erosion resistance [36]. Secondly, the low

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dissolution rate of W oxides in the passive film of the WC-10Co-4Cr coating would suppress passive dissolution and inhibit slurry erosion-corrosion damage, although the

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stability of passive film and the ability of repassivation for the WC-10Co-4Cr coating

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and the stainless steel 1Cr18Ni9Ti are both reduced in severe erosive conditions [35,37]. Moreover, high elastic modulus and fracture toughness of the WC-10Co-4Cr

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coating are vital to enhance the wear and slurry erosion resistance, which has been

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indicated by other researchers [12,38]. SEM and EDS were taken to gain further information about the slurry erosion-corrosion mechanisms in different slurry solutions from eroded surfaces of the WC-10Co-4Cr coating and the stainless steel 1Cr18Ni9Ti. Figs. 2 and 3 show the SEM images of the WC-10Co-4Cr coating and the stainless steel 1Cr18Ni9Ti after slurry erosion-corrosion tests for 10 h in distilled water and 3.5 wt.% NaCl slurries, respectively. It can be seen clearly that the WC-10Co-4Cr coating (Figs. 2(a) and 3(a)) 9

ACCEPTED MANUSCRIPT has relatively smoother eroded surfaces as compared to the stainless steel 1Cr18Ni9Ti (Figs. 2(c) and 3(c)) in both slurries. This is consistent with the mass loss rate results. As shown in Fig. 2(b), cracks and microchipping are detected on the surface of the coating after eroded in distilled water slurry for 10 h, confirming that the mass loss

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begins around the peripheries of micropores, propagates into the binder phase, and

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follows by a fatigue process. This phenomenon is in good agreement with the results

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reported by other researchers [14,39]. Fig. 2(d) shows formation of ploughing and signs of lips, microcutting and plastic deformation in the stainless steel after eroded

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for 10 h, which are responsible for the material removal. In case of 3.5 wt.% NaCl

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slurry testing, detachment of binder phase resulted from the micro-galvanic cells between the WC phase and the CoCr binder is observed in Fig. 3(b), which lead to

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formation of more number of microchipping and fragments on the coating surface.

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Under same condition, corrosion pits are seen for the stainless steel, which is verified by the EDS analysis. The chemical composition of point A in Fig. 3(d) is Fe 52.38 -

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Cr 14.80 - Ni 6.51 - C 17.43 - O 7.88 - C 17.43 (at.%), indicating that some iron

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oxides are formed as a result of simultaneous action of localized corrosion and repetitive impingements.

3.2 Microbial corrosion electrochemical behavior in the presence of SRB EIS is a suitable technique to characterize the electrochemical processes in microbial influenced corrosion, including the reactions at the metal/solution interface and the formation of natural oxidation films, SRB biofilms and corrosive products [40]. Fig. 4 10

ACCEPTED MANUSCRIPT shows the Time-dependent Nyquist plots and the corresponding Bode plots of HVOF sprayed WC-10Co-4Cr coating and stainless steel 1Cr18Ni9Ti exposed to the seawater with SRB. As shown in Fig. 4(a), the diameter of Nyquist plots increases over time for the WC-10Co-4Cr coating. However, an increase in the diameter from 3

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h to 8 d is observed for the stainless steel 1Cr18Ni9Ti, and the diameter decreases

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after 8 d of exposure to the seawater with SRB (Fig. 4(c)). The diameters of Nyquist

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plots for the stainless steel are always larger than those for the coating, indicating that the stainless steel 1Cr18Ni9Ti has a slower corrosion rate compared to the

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WC-10Co-4Cr coating. Different from the cases of the coatings where two maxima

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are observed at the Bode phase angle plots during the immersion period (Fig. 4(b)), only one time constant is observed for the stainless steel 1Cr18Ni9Ti from 3 h to 2 d,

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and a broad time constant joined by two time constants appears after 5 d (Fig. 4(d)).

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The conjunction of the two time constants may be attributed to the similar frequency responses of the oxidation of metal surface and the formation of electric double layer

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[31]. For a better explanation to microbial influenced corrosion behavior, equivalent

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circuits are used to simulate the experimental impedance diagrams by ZSimpWin commercial software (USA). The chi square (χ2) value reaches the level of 10-3, which ensures the quality of fitting. The equivalent circuits describing the electrochemical reactions for the WC-10Co-4Cr coating and the stainless steel 1Cr18Ni9Ti in seawater with SRB are shown in Fig. 5. According to the characteristic of Nyquist plots of the coating after immersion for 3 h to 15 d and the stainless steel after immersion for 5 d, the equivalent circuits 11

ACCEPTED MANUSCRIPT with two time constants are used to fit the EIS data. The constant phase element (CPE or Q) is interpreted by dispersion effects from surface roughness and heterogeneous biofilm of the sample [41], which can be described as ZQ=Y0-1(jω)-n (0
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Fig. 5(b) consisting of an additional element Warburg impedance (Zw) is used to fit

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the Nyquist plots of the coating after immersion for 2 d, where the parameter reflects

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the diffusion coefficient, the characteristic of stagnant layer, and the mass-transfer reaction based on the accumulation of corrosive products in pores [42]. According to

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Ahn et al. [43] and Liu et al. [44], the Warburg impedance is usually employed to

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represent semi-infinite diffusion process, which suggests a diffusion controlled mechanism in electrochemical systems. Fig. 5(c) shows an equivalent circuit based on

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a single layer which can be satisfactorily used for fitting the Nyquist plots of the

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stainless steel after immersion for 3 h to 2 d. In the case of the WC-10Co-4Cr coating, Rs, Rp, Rct, Qc, and Qdl represent the resistance of electrolyte solution, the resistance of

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pores, the resistance of charge transfer, the capacitance of the coating, and the

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capacitance of the double layer, respectively. Besides, Rf and Qf correspond to the resistance and the capacitance of surface film for the stainless steel 1Cr18Ni9Ti, respectively.

In this study, the fitting values of Rct shown in Fig. 6 are important parameter for the evaluation of corrosion rate, since the values of Rct are more than thousand times higher than those of Rs, Rp and/or Rf. For the plots of the WC-10Co-4Cr coating recorded at the initial stage, the Rct value decreases from 234 k·cm2 at 3 h to 92 12

ACCEPTED MANUSCRIPT k·cm2 at 5 d. This is associated with the penetration of the corrosion medium to the interior of the coating and the dissolution of the active zones inside the coating, although pores and microcracks are limited. The increase in Rct value from 92 k·cm2 at 5 d to 196 k·cm2 at 8 d is attributed to the plugging of the electron transfer

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pathway that influenced by the accumulation of corrosive products and the formation

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of SRB biofilms on the coating surface [45]. After the increase, Rct value decreases

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slightly during the late period, indicating that the corrosion of oxide films and the surface inhomogeneity are both increasing due to continuous microbial influenced

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corrosion. The characteristic in Rct shift for the stainless steel 1Cr18Ni9Ti is opposite

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to that of the coating at the initial stage, and the variation of the Rct value is much larger. This could be explained that the natural oxidizing Cr2O3 formed on the

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stainless steel sample is a dense passive film, which will hamper the transportation of

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corrosive ions and inhibits the corrosion process [46]. As the immersion time prolongs, the Rct value decreases from 1063 k·cm2 at 5 d to 665 k·cm2 at 15 d. The

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reason may be that the natural oxidizing film breaks and the chemical or

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electrochemical reactions are influenced by SRB metabolic activity. When the sulfide deposits on the surface, the cathodic polarization process within the crevice resulted from the partial destruction of oxidizing film will be facilitated, which increases the electron transfer rate [47]. Furthermore, it can be seen that the Rct values of the WC-10Co-4Cr coating are lower than those of the stainless steel 1Cr18Ni9Ti at the corresponding time points, suggesting that the corrosion resistance of the coating is lower than the stainless steel in seawater with SRB. 13

ACCEPTED MANUSCRIPT Figs. 7(a) and (b) show the potentiodynamic polarization curves of HVOF sprayed WC-10Co-4Cr coating and stainless steel 1Cr18Ni9Ti exposed to the seawater with SRB for different time, respectively. It can be obviously seen that polarization curve behaviors of the coating and the stainless steel are both influenced

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by the adhesion of SRB biofilms. Table 1 exhibits the values of the electrochemical

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corrosion parameters extracted from the curves, such as corrosion potential (Ecorr) and

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corrosion current density (icorr). As shown in Table 1, the effect of SRB on the corrosion is quite different for the coating and the stainless steel at the initial stage.

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The presence of bacteria triggers increases in icorr values of the coating at the first 5 d,

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while the stainless steel exhibits the opposite tendency. This suggests that the combined effects of corrosive ions and SRB metabolites can inhibit the formation of

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the passive film and accelerate the corrosion process of the coating. However, the

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presence of natural oxidizing film on the stainless steel surface effectively prevents microbial adhesion and inhibits biofilm growth for a while. The icorr value of the

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coating first decreases and then increases from 5 d to 15 d. This can be attributed to

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the presence of corrosive products and SRB biofilms on the coating surface, and the corrosion of oxide films accelerated by the corrosive metabolites, respectively. For the stainless steel, the increase of the icorr value from 5 d to 15 d can be explained by the partial breakage of oxidizing film and dissolution of metal under the effect of SRB metabolic activity [47]. The results of the potentiodynamic polarization curves are consistent with the EIS tests as shown in Figs. 4 and 6. In addition, the icorr values have the same order of magnitudes for the coating and the stainless steel, indicating 14

ACCEPTED MANUSCRIPT that the coating with compact and uniform structure can provide effective protection for the steel substrate in seawater with SRB. As discussed above, HVOF sprayed WC-10Co-4Cr coating exhibits a higher slurry erosion-corrosion resistance and a comparable microbial influenced corrosion resistance as compared to the stainless

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steel 1Cr18Ni9Ti, which provides a potentially competitive approach for offshore

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hydraulic machinery.

4. Conclusions

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In this study, the WC-10Co-4Cr cermet coating was deposited on the stainless steel

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1Cr18Ni9Ti by HVOF spraying system. WC-10Co-4Cr coating exhibited a superior slurry erosion-corrosion resistance compared to the stainless steel 1Cr18Ni9Ti in both

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distilled water and 3.5 wt.% NaCl slurries, and the difference between the coating and

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the stainless steel was enhanced with the augment of NaCl concentration. Cracks and microchipping contributed to the slurry erosion mechanism of the coating in distilled

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water slurry, while the main failure mechanism of the coating was detachment of

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binder phase, microchipping and fragments in 3.5 wt.% NaCl slurry. Compared with stainless steel 1Cr18Ni9Ti, the WC-10Co-4Cr coating had a comparable microbial influenced corrosion resistance in seawater with SRB indicated by potentiodynamic polarization and EIS results. This study provides a potentially competitive cermet coating for use in engineering applications of offshore hydraulic machinery.

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ACCEPTED MANUSCRIPT Acknowledgments The research was supported by the National Natural Science Foundation of China (Grant Nos. 51609067, 51579087 and 51339005), the Natural Science Foundation of Jiangsu Province of China (Grant No. BK20150806), the China Postdoctoral Science

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Foundation (Grant No. 2016M590404), the Opening Project of Material Corrosion

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and Protection Key Laboratory of Sichuan Province (Grant No. 2016CL08), and the

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Opening Project of CAS Key Laboratory of Nuclear Materials and Safety Assessment (Grant No. 2016NMSAKF03). Professor Jizhou Duan of Institute of Oceanology, Academy

of

Sciences

was

also

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Chinese

acknowledged

for

providing

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microbiologically induced corrosion equipment.

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ACCEPTED MANUSCRIPT Figure captions:

Fig. 1 Mass loss rates of HVOF sprayed WC-10Co-4Cr coating and stainless steel 1Cr18Ni9Ti at 18 m·s-1 in distilled water and 3.5 wt.% NaCl slurries (sand content:

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10 kg·m-3, sand size: 75~150 mesh)

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Fig. 2 SEM images of HVOF sprayed WC-10Co-4Cr coating (a,b) and stainless steel

content: 10 kg·m-3, sand size: 75~150 mesh)

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1Cr18Ni9Ti (c,d) after eroded for 10 h at 18 m·s-1 in distilled water slurry (sand

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Fig. 3 SEM images of HVOF sprayed WC-10Co-4Cr coating (a,b) and stainless steel

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1Cr18Ni9Ti (c,d) after eroded for 10 h at 18 m·s-1 in 3.5 wt.% NaCl slurry (sand content: 10 kg·m-3, sand size: 75~150 mesh)

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Fig. 4 Nyquist (a,c) plots and Bode plots (b,d) of HVOF sprayed WC-10Co-4Cr

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coating (a,b) and stainless steel 1Cr18Ni9Ti (c,d) in seawater with SRB for different time

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Fig. 5 Equivalent circuits used to fit EIS data of HVOF sprayed WC-10Co-4Cr

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coating (a,b) and stainless steel 1Cr18Ni9Ti (c,d) in seawater with SRB Fig. 6 Time dependence of Rct of HVOF sprayed WC-10Co-4Cr coating and stainless steel 1Cr18Ni9Ti in seawater with SRB Fig. 7 Potentiodynamic polarization curves of HVOF sprayed WC-10Co-4Cr coating (a) and stainless steel 1Cr18Ni9Ti (b) in seawater with SRB for different time

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ACCEPTED MANUSCRIPT Tables:

Table 1 Corrosion current densities (icorr) and potentials (Ecorr) values obtained from the potentiodynamic polarization curves of WC-10Co-4Cr coating and stainless steel

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1Cr18Ni9Ti in seawater with SRB for different time. icorr (μA·cm-2)

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Ecorr (mV) Time

3h

-313

-339

2d

-552

-680

5d

-606

-704

8d

-703

15 d

-610

WC-10Co-4Cr

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1Cr18Ni9Ti

1.158

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WC-10Co-4Cr

1Cr18Ni9Ti 2.195 1.862

2.171

0.629

-612

1.342

0.713

-716

1.616

1.059

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1.359

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ACCEPTED MANUSCRIPT Highlights  WC-10Co-4Cr coatings were prepared by HVOF spraying.  The coating exhibited higher erosion-corrosion resistance than the stainless steel.  Slurry erosion mechanisms with the augment of NaCl concentration were studied.

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 The coating had a comparable microbial corrosion resistance in seawater with

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SRB.

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7