Corrosion behavior in seawater of arc thermal sprayed Inconel 625 coatings with sealing treatment

Accepted Manuscript Corrosion behavior in seawater of arc thermal sprayed Inconel 625 coatings with sealing treatment

Il-Cho Park, Seong-Jong Kim PII: DOI: Reference:

S0257-8972(17)30245-1 doi: 10.1016/j.surfcoat.2017.03.009 SCT 22178

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

25 December 2016 2 March 2017 3 March 2017

Please cite this article as: Il-Cho Park, Seong-Jong Kim , Corrosion behavior in seawater of arc thermal sprayed Inconel 625 coatings with sealing treatment. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi: 10.1016/j.surfcoat.2017.03.009

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ACCEPTED MANUSCRIPT Corrosion Behavior in Seawater of Arc Thermal Sprayed Inconel 625 Coatings with Sealing Treatment Il-Cho Park, Seong-Jong Kim* Division of Marine Engineering, Mokpo National Maritime University, Mokpo-si, Jeollanam-do, Korea *

Corresponding author

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Prof. Dr. Seong-Jong Kim

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Mokpo National Maritime University

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91, Haeyangdaehak-ro, Mokpo-si, 58628, Korea E-mail: [email protected]

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Tel.: +82-61-240-7226

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Fax: +82-61 240-7201

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Abstract

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In this study, zinc powder (Zinc), inorganic ceramic (Ceramic), and organic-inorganic composite ceramic (Hybrid) sealing processes were applied to improve the corrosion

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resistance of arc thermal sprayed Inconel 625 coatings. The Inconel 625 coating in seawater solution showed poor corrosion resistance due to its porosity and Cr depletion resulting from Cr oxide formation during the arc thermal spraying process. Zinc sealing formed a stable cathodic protection, but the local corrosion damage occurred largely in the anodic polarization experiment caused by an excessive dissolution reaction and a weak bonding force between the zinc particles. Hybrid sealing gave a better sealing effect, but surface corrosion damage appeared due to interconnected micro-crack defects on the surface. Ceramic sealing gave the best corrosion protection due to a high sealing efficiency and a lower surface damage tendency resulting from effectively blocked defects of the Inconel 625 coating. Keywords: Inconel 625; ceramic sealing; corrosion; arc thermal spray; seawater -1-

ACCEPTED MANUSCRIPT 1. Introduction Corrosion resistance of the steel can be significantly enhanced by modifying the surface with thermally sprayed anti-corrosion coatings [1,2]. The advantage of this method is the substantial cost reduction compared to using bulk alloys [1]. The thermal spraying technology can be divided into several processes according to the thermal energy source and the kinetic

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energy of the sprayed materials, such as flame spraying, plasma spraying, arc thermal

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spraying, and high velocity oxy-fuel (HVOF) spraying [3]. Among these methods, long-term anticorrosion of the large steel structures in the marine environment has been effectively

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achieved by arc thermal spraying technique as a cathodic protection method using the

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sacrificial anode [3]. Their materials as Al, Zn or Al-Zn, and Al-Mg alloys, etc. have been mainly used [4-6]. However, existing anti-corrosive materials are limited for the protection of

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the steel in the severe corrosive environment where physical and mechanical forces are externally applied. To improve the corrosion resistance and durability of the steel substrate,

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therefore, the attention about thermal spray coatings using high grade metallic alloys such as

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Inconel, Hastelloy, Stellite or stainless steel type is growing recently [7,8]. In particular,

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HVOF coating using Inconel 625 powder is available in a wide range of aggressive conditions which contribute to corrosion, erosion and overall wear of the surface [1,7,9-13]. The reason is that the HVOF spraying process has been developed to produce extremely high spray particle velocities of 300-800 m/s to be coated for the substrate and the kinetic energy of their particles is therefore great [7,14]. Hence, the porosity of HVOF coatings is reduced to less than 1%, and their oxide content is also very low [10,12,15]. However, deposited Inconel 625 coating for protection of the steel in the aqueous environment can be damaged by electrochemical corrosion because the coating is not galvanically sacrificial against the steel substrate. It is explained that interconnected porosity and oxide formation at the intersplats are the principal microstructural features affecting the corrosion rate [7,8]. To solve these problems, many researchers carried out various post-treatment onto the Inconel 625 coating 2

ACCEPTED MANUSCRIPT by HVOF process. Neville et al. suggested that corrosion resistance of the coating might be improved by post heat-treatment due to reducing the composition gradient through promoting some inter-diffusion [16]. On the other hand, many researchers have investigated on the application of laser surface melting. As a result, laser surface melting of Inconel 625 coating has been shown to be able to significantly improve corrosion resistance. This is attributed to

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microstructural homogenization such as the elimination of interconnected porosity and the structure consisting of splats, and reduction of composition gradient in the coating [11,12,17].

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In spite of the above efforts, application of Inconel 625 coating onto the large steel structures

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in the marine environment is limited due to low productivity and difficulty of site constructability arising from the nature of HVOF spraying process. In this study, therefore,

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the Inconel 625 coating was conducted by arc thermal spraying technique. This technique has

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good characteristics such as site constructability, low running costs, high spray rates and efficiency to coat large areas at the site [18]. However, arc thermal spraying has a lower

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particle velocity of 100-300 m/s and lower temperature process compared to HVOF spraying

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[18]. The combination of relatively low temperature and particle velocities gives arc sprayed coatings poorer bond strengths and higher porosity levels of 3-5% [18,19]. Moreover, the use

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of compressed air for droplet atomization gives rise to high oxide content in the coating when compared with HVOF spayed coatings [18]. Consequently, it is believed that corrosion resistance of Inconel 625 coating by arc thermal spraying technique will be poorer than that of HVOF coating resulting from high porosity and oxide content of the coating. Therefore, we additionally carried out sealing treatment to improve the corrosion performance onto the arc thermal sprayed Inconel 625 coating using three sealants, respectively. And then corrosion resistance of Inconel 625 coating was evaluated by microstructure analysis and various electrochemical experiments.

2. Experimental method 3

ACCEPTED MANUSCRIPT The popular structural steel, A 36 by ASTM standard specification, was used as the substrate specimen and its chemical compositions (wt. %) are as follows: 0.16% C, 0.19% Si, 0.36% Mn, 0.013% P, 0.011% S, and Fe balance. For surface preparation, a rough surface was formed by grit blasting according to SSPC-SP10 standard to remove the foreign materials and improve the adhesion between the substrate and the thermal spray coating. The grit blasting

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was carried out using compressed air to form a surface roughness (Rmax) of about 80 ㎛. Arc thermal spraying method was used for this experiment, with a robot control system to

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maintain constant spray conditions of the spray gun. (Model: ZPG-400B arc spray equipment,

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Shanghai Xinye Spray Machinery Co., Ltd) The spraying work was performed with twin wires of the Inconel 625. And then Hybrid, Zinc, and Ceramic sealing were applied to thermal

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sprayed Inconel 625 coating by brushing. Hybrid sealing is an organic-inorganic composite

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ceramic product developed by Bukwang Korea Inc. Zinc sealing (product: ZINGA, Zinga Korea) is mainly composed of zinc powder with a purity of 99.995%, aromatic hydrocarbons,

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and binder. Ceramic sealing (Product: 7053 HV-BP124N, Hankook Firetech co., Ltd) is

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divided into the main agent and a harder. The main agent is made by adding distilled water to the cooled silicasol and epoxysilane and then it is allowed to react for 4-12 h at the room

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temperature. The harder is manufactured by adding an amine and an alkoxysilane to isopropyl alcohol solvent. Then it induces a reflux reaction by increasing the temperature to 60℃. Unreacted amines are removed by column chromatography. The structural formula of the main agent and the harder is shown in Fig. 1. Details of the arc thermal spraying parameters and the sealing conditions are given in Table I and Table II, respectively. Every electrochemical experiment was performed in the natural seawater with a working electrode of the exposed area of 1 cm2, Ag/AgCl (KCl saturated) as a reference electrode, and the platinum mesh as a counter electrode of the size of 2.5 cm x 2.5 cm. (Model: US/PCI4/750 Potentiostat/Galvanostat/ZRA, Garmy Instruments, Inc.) The distance between the working electrode and the counter electrode was kept at 8 cm using the flat corrosion cell. The main 4

ACCEPTED MANUSCRIPT chemical compositions (wt. %) of the natural seawater are as follows: 30.62% Na+, 3.68% Mg2+, 1.18% Ca2+, 1.10% K+, 55.07% Cl-, 7.72% SO42-, and 0.63% others. The corrosion potential measurement was conducted for 10 h, and the initial stabilization time for polarization experiment was set to 3,600 seconds. The anodic polarization experiment was performed in the open circuit potential (OCP) at the scan rate of 2 mV/sec up to +3.0 V.

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Besides, after polarization to ±0.25 V at the OCP, the corrosion current density and corrosion potential were determined by a Tafel’s extrapolation method. After the anodic polarization

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experiments, the surface damage morphologies were observed by scan electron microscope

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(Model: S-2150, Hitachi) and 3D microscope (Model: Nextop-3D, Nextec). The porosity measurement of the Inconel 625 coating was evaluated through image analysis (Image

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analysis software: i-Solution DT, IMT i-Solution Inc.) and its value was averaged at five

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points on the cross section of the coating. An energy dispersive spectrometer (EDS), Model EX-250 by HORIBA, was used to characterize the structure of the coating. And the

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micro-Vickers hardness (Equipment Model: HMV-2E-125, Shimadzu) of the thermal spray

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coating layer was measured more 10 times with a 500 g load and a dwell time of 10 sec and

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their average values were determined. 3. Results and discussion

Fig. 2 shows the cross section and EDS analysis results of the Inconel 625 coating. In Fig. 2 (a), the deposited thickness of the Inconel 625 coating was around 220 ㎛. The range of coating porosity including crack, pore, and the interfacial defect was measured at 6-8%. Many pores and lamella structures resulting from the nature of thermal spray coating were clearly observed. High temperatures and velocities of the fully melted particle usually form the significant droplet deformation on impact at a surface, producing thin layers or lamella, often called “splats” that adhere to the substrate surface. Solidified droplets build up rapidly, as a continuous stream of fully melted particles impact to form continuous rapidly solidified 5

ACCEPTED MANUSCRIPT layers. However, the sprayed particles from the nozzle actually reach the substrate in fully melted, semi-melted, and unmelted conditions because the use of wires as a feedstock material produces different size particles by the nonuniform heating and unpredictable drag forces. If the particles are not fully melted or do not have sufficiently high velocity until the collision with the substrate, the particles cannot fully fill the void in the coating and pores are

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formed. Thus, the formation of pores is usually closely associated with large unmelted particles. For this reason, a relatively broad range of micro-Vickers surface hardness was

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observed about 350-450 HV due to the rough surface morphology resulting from the

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irregularly deposited lamella structures and pore defects. Their average value was calculated as about 420 HV. Fig. 2 (b) presents the EDS spectrum of the area indicated in Fig. 2 (a).

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High peaks of Ni, Cr, and Mo as main components of an Inconel 625 wire were observed.

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According to the studies by other researchers, Inconel 625 coating showed the Cr2O3 phase that was not detected in a feedstock materials such as powder, rod, etc. [10, 11]. The same

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result was confirmed by the EDS analysis (Fig. 3) in this study.

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Fig. 3 (a) exhibits the SEM image of a typical coating layer and Fig. 3 (b)-(f) displays the elemental concentration maps of Ni, Mo, Cr, O, and Nb for this area, respectively. In Fig. 3

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(a), the coating layer largely consists of two areas: unmelted particles (U) and lamella structures (L). The concentrations of Cr, Nb, and O were very high in many dark inclusions and grain boundaries of lamella structures, whereas those of Ni and Mo were relatively low. This means that Cr and Nb were oxidized and separated from the substrate during the high temperature coating spray process [10]. Thus, oxides are formed mainly around the grain boundaries and pore defects. At grain boundaries [7], Cr as an alloy element is preferentially oxidized in the fully melted area where mass transportation is the fastest during heating the wire particles. Therefore, Cr2O3 is formed on the particle surface, and the other Cr-depleted droplets are sequentially solidified again. Furthermore, in the case of pores, air molecules are

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ACCEPTED MANUSCRIPT isolated, and pores are formed between unmelted or semi-melted particles that were deposited first and splats of high temperature which are deposited later, thus facilitating oxidation. Fig. 4 shows the surface morphology of Inconel 625 coating after applying various sealing treatments by SEM. In Fig. 4 (a)-(b), Inconel 625 coating shows very rough surface morphology including open pores. This was caused by the formation of surface roughness of

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substrate to enhance the mechanical bonding strength before thermal spray coating work as shown in Fig. 2 (a) and the nature of spray coating that melted particles are consecutively

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deposited at a high velocity. In Fig. 4 (c)-(d), Zinc sealing formed a very smooth surface, and

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the pores were locally observed. In an image at high magnifications, the particle size was smaller than 10 ㎛, and zinc particles with irregular shapes were observed. On the other hand,

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the surface roughness of Hybrid sealing in Fig. 4 (e)-(f) and Ceramic sealing in Fig. 4 (g)-(h)

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was greatly alleviated compared to the as-sprayed Inconel 625 coating due to sealing treatment. However, micro-crack defects were observed on their sealed surface. In particular,

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Hybrid sealing formed much more micro-cracks than that of Ceramic sealing, and the

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micro-cracks were connected each other. Fig. 5 represents the results of potential measurement for 36,000 sec in seawater after

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applying various sealing treatments to the Inconel 625 coating. In the case of as-sprayed Inconel 625 coating, a noble potential of -0.05 V/Ag|AgCl|KCl saturated in the beginning of immersion sharply dropped to about -0.5 V/Ag|AgCl|KCl saturated after about 5,000 sec. The reason for this seems to be the formation of a mixed potential with the substrate which shows a relatively active potential because seawater easily was penetrated into the substrate through the defects of the coating layer such as pore and crack observed in Fig. 2 (a) and Fig. 4 (a)-(b). In general, the effect of porosity on corrosion resistance is very small because the high-quality Inconel 625 coating by HVOF technique forms less than 1% porosity [11]. In this study, the Inconel 625 coating layer, however, was produced by arc thermal spray technique and has a relatively large porosity of 6-8%, and this led to the above results. After this period, 7

ACCEPTED MANUSCRIPT the potential slightly decreased until the end of the experiment when it was measured at -0.57 V/Ag|AgCl|KCl saturated. Zinc sealing maintained a potential of about -0.9 V/Ag|AgCl|KCl saturated until about 5,000 sec of immersion. After this period, the potential dropped to approximately -1.0 V/Ag|AgCl|KCl saturated at around 15,000 sec and presented a stable potential until the experiment end. It is believed that formed ZnO oxide film in the air was

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destroyed by Cl- ion in seawater [20]. At the same time, the effective reaction area of active zinc particles increased as seawater infiltrated through the pores and the gap among zinc

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particles observed in Fig. 4 (c)-(d). Nevertheless, anticorrosive effects by the sacrificial anode

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can be expected because, after 15,000 sec of immersion, Zinc sealing stably maintains a potential that is more active about -0.45 V/Ag|AgCl|KCl saturated than the as-received

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coating and more active than -0.85 V (vs. SCE) which is the protective potential of the steel

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substrate [21-23]. During this period, zinc oxide (Zn1+xO) were formed at the solution and

coating interface and progressed towards the substrate with increasing the immersion

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duration [21]. On the other hand, Hybrid and Ceramic sealing formed noble potentials

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throughout the experiment period, indicating improved corrosion resistance. In particular, Ceramic sealing continuously maintained a stable potential of around -0.15 V/Ag|AgCl|KCl

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saturated during the immersion period. Even though micro-crack defects were observed on the surface in Fig. 4 (g)-(h), their effect is very small because the number is small and a stable corrosion potential was maintained during the immersion time. On the other hand, Hybrid sealing exhibited a gradually decreasing tendency of the potential as well as unstable potential variations with immersion time. This seems to be that seawater infiltrated into the coating through many micro-cracks on the surface as observed in Fig. 4 (e)-(f). Fig. 6 depicts the anodic polarization curves of Inconel 625 coating with sealing treatment. The current density rapidly increased in general with no passive state section along with the increasing potential. Throughout the experiment, the quantity of current density with potential change was in the order of Ceramic < Hybrid < Inconel 625 coating < Zinc. Consequently, 8

ACCEPTED MANUSCRIPT corrosion resistance was improved when Ceramic and Hybrid sealing were applied to the Inconel 625 coating. These different trends of anodic polarization curves can be partially inferred by observing the surface morphologies before and after the experiment. The surface observations after anodic polarization experiments of the Inconel 625 coating with sealing treatment are given in Fig. 7 and Fig. 8. The surface morphology of Inconel 625

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coating underwent a little change after the experiment as shown by comparing Fig. 4 (a) and Fig. 7 (a). However, local corrosion damage was observed on the surface after the anodic

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polarization experiment in the high-magnification image of Fig. 7 (b). The reason for the

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occurrence of local corrosions on the surface of Inconel 625 coating seems to be the local formation of Cr depletion [1,11]. In Fig. 3, the formation of Cr2O3 oxides was concentrated

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around the unmelted particles or in the intersplats where lamella structures are formed. In

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other words, the existence of Cr2O3 oxides indicates the occurrence of Cr depletion in some parts of the as-sprayed Inconel 625 coating because Cr is consumed by the oxidation reaction

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while the coating is deposited. As a result, microstructure areas with various Cr contents exist

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on the surface as well as inside the coating. Thus, galvanic cells were formed, and galvanic corrosion occurred first in the Cr depletion areas. Furthermore, local corrosion damages were

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mostly observed as a micro-crack shape. This seems to be the effect of residual stress which occurs while the melted particles are transformed, solidified, and quenched during the deposition [9,11]. The residual stress of the thermal spray coating layer is attributed to tensile stress due to the volumetric shrinkage during the rapid solidification of the deposited melted droplets [24-26]. In other words, the residual stress is believed to have been released by numerous cracks caused by the active dissolution reaction on the coating surface that contains residual stress during the anodic polarization experiment. Hybrid sealing was partially detached throughout the surface, and Inconel 625 coating was exposed, leading to corrosion damage as shown in Fig. 7 (c)-(d). This was caused by formed micro-cracks on the surface before the experiment. So, in Fig. 7 (c), small surface damages occurred along the 9

ACCEPTED MANUSCRIPT micro-cracks. When the micro-cracks were interconnected as shown in Fig. 4 (f), the Hybrid sealing was easily detached as a flat plate shape, generating greater damages as shown in Fig. 7 (d). Therefore, to achieve effective corrosion protection of the Hybrid sealing, the measures to control the generation of micro-cracks must be previously taken. However, corrosion damages of Ceramic sealing tended to occur at the protrusion (bright area) as shown in Fig. 4

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(h) despite the existence of micro-cracks on the surface like Hybrid sealing. This result seems to be attributed to the nature of the inorganic ceramic sealing material used in this study.

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Liquid Ceramic sealing infiltrates into the pores by capillary action, and promotes the

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diffusion of sealant and the homogenization of microstructure through post heat treatment [27]. Therefore, deep infiltration is more effective if the impregnation time is longer, the

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sealant viscosity is lower, and the wetting angle is smaller [27]. Consequently, the sealing

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effect of the protrusion was lower because the sealant on the protrusion easily flows down onto the rough surface arising from the low viscosity and smaller wetting angle of Ceramic

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sealing as shown in Fig. 4 (a), and infiltrates into the Inconel 625 coating. Zinc sealing

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generated a large surface damage of a crater shape throughout the surface because the Zinc sealing was partially detached as shown in Fig. 8 (a). In Fig. 8 (b), the surface shape is clearly

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divided by the corrosion damaged area and the white corrosion area. The hemispherical surface damages were observed in the corrosion damaged area because the zinc particles were separated from the coating in Fig. 8 (c). On the other hand, white corrosion products were formed evenly on the flat surface surrounding the damaged areas. They were observed as white cotton-shaped products in Fig. 8 (d) and acts as a barrier by being piled up on the open pores. This phenomenon known as “white corrosion” is generated by the oxidation of zinc [3, 23]. Generally, zinc particles are corroded first in the early stage and present the sacrificial anode effect for the cathodic protection of the steel substrate. After that, the pores are covered by zinc corrosion products resulting from a white corrosion phenomenon, and the cathodic protection is terminated. Instead of the cathodic protection, barrier-type protection is 10

ACCEPTED MANUSCRIPT achieved by compact zinc corrosion products [21]. However, in this study, the local corrosion damage occurred on the Zinc sealing layer and little white corrosion products were observed in their surface damage area. This is probably due to the effect of the excessive dissolution reaction of the Zinc sealing layer caused by the anodic polarization. Hence, the weaker part of the bonding force between the particles is considered to be preferentially damaged before the

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compact white corrosion products are formed on the Zinc sealing layer. Fig. 9 shows 3D analysis of the surface after anodic polarization experiments of the Inconel

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625 coating with sealing treatment. Inconel 625 coating and Ceramic sealing which had less

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corrosion damage presented almost no changes in the surface roughness before and after the experiments, indicating the same tendency as the surface observations in Fig. 4 and Fig. 7. On

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the other hand, the surface roughness difference before and after the anodic polarization

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experiment was calculated as 41 ㎛ for Zinc sealing and 54 ㎛ for Hybrid sealing, but the surface roughness of Hybrid sealing (149 ㎛) was significantly larger than that of Zinc

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sealing (106 ㎛) after the anodic polarization experiment. This is because the surface

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conditions before the anodic polarization experiment were very different each other. As

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shown in Fig. 4, Zinc sealing was uniformly applied at a thickness of about 80 ㎛, whereas Hybrid and Ceramic sealing were thinly coated about 20 ㎛ and had the surface shape of Inconel 625 coating. Hence, the surface roughness of Zinc sealing (52 ㎛) was measured to be significantly smaller than that of the hybrid sealing (108 ㎛) before the anodic polarization experiment. Consequently, Zinc sealing showed larger surface damage than Hybrid sealing. Moreover, this is the same tendency as the results of the polarization curves and SEM analysis related to the anodic polarization experiments. Fig. 10 presents the polarization curves for the Tafel analysis of the Inconel 625 coating with sealing treatment. The results of Tafel analysis are outlined in Table III. Due to the nature of the active zinc, Zinc sealing showed the smallest corrosion potential of -0.91 V and 11

ACCEPTED MANUSCRIPT the largest corrosion current density of 2.33 x 10-4 A/cm2. On the other hand, Inconel 625 coating presented a corrosion potential and a corrosion current density of -0.27 V and 1.18 x 10-5 A/cm2, respectively, indicating better corrosion resistance than that of Zinc sealing. The corrosion resistance of Inconel 625 coating is most greatly affected by the porosity of coating and the galvanic corrosion resulting from the local distribution of Cr depletion areas

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[1,7,9-12]. Therefore, the corrosion resistance of Hybrid and Ceramic sealing was greatly improved by the role as a barrier of the sealing that fundamentally blocked the defects of

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Inconel 625 coating. This can be quantified by calculating the sealing efficiency

of

(1)

is the corrosion current density after sealing treatment, and

is the

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Where in

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Hybrid and Ceramic sealing using the following equation (1) [28-30]:

corrosion current density before sealing treatment. The effective application of sealing

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treatment can be confirmed because the sealing efficiencies of Hybrid and Ceramic sealing by

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the above equation were considerably high values of approximately 94.9% and 99.1%, respectively. In other words, this is assumed that most defects such as open pores and cracks

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in the Inconel 625 coating were effectively blocked by Hybrid and Ceramic sealing. Consequently, the most effective corrosion protection performance can be expected when Ceramic sealing is applied to the Inconel 625 coating considering the sealing treatment conditions in Fig. 4 (e)-(h).

4. Conclusion This study aimed to investigate the corrosion behaviors of the arc thermally sprayed Inconel 625 coating with sealing treatment through various electrochemical experiments in natural seawater solution. 12

ACCEPTED MANUSCRIPT In the case of the arc thermally sprayed Inconel 625 coating, the porosity and micro-Vickers hardness were analyzed at 6-8% and around 420 HV, respectively. Furthermore, Cr depletion areas were observed due to the formation of Cr oxides around the unmelted particles and in the intersplat boundaries. Therefore, Cr depletion is believed to be a major cause that degraded corrosion resistance by directly affecting the coating in the

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seawater environment. All sealed Inconel 625 coatings performed well preventing the corrosion of the substrate by

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acting as a barrier or a sacrificial anode against the corrosive natural seawater solution.

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Zinc sealing can expect the effect of corrosion protection by the sacrificial anode characteristics because it maintained a stable corrosion potential which was about -0.45

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V/Ag|AgCl|KCl saturated lower than that of Inconel 625 coating. However, Zinc sealing

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generated the greatest surface damage due to local corrosion damage tendency. This is presumably because the excessive dissolution reaction is induced by anodic polarization on

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the Zinc sealed coating and the local area of the weakly cohesive coating layer is detached

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before the formation of compact white corrosion products acting as a barrier. Corrosion resistance of Hybrid and Ceramic sealing was significantly improved by

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effective blocking the defects of the Inconel 625 coating. However, numerous micro-cracks of Hybrid sealing on the surface were contributed to degrading corrosion resistance. As a result of various electrochemical experiments in this study, it is considered that Ceramic sealing has the best corrosion resistance due to the high sealing efficiency of 99.1% and smaller surface corrosion damage tendency than those of others. However, Zinc sealing is necessary to evaluate the corrosion resistance through the immersion experiment for a long term similar to the actual corrosion environment in further work because its coating has been confirmed to form compact white corrosion products with anticorrosive function. On the other hand, Hybrid sealing is expected to produce a high

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ACCEPTED MANUSCRIPT quality sealed coating if a study is conducted to prevent the formation of crack defects on the surface.

Acknowledgments This research was a part of the project titled 'Construction of eco-friendly Al ship with

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painting, and maintenance/repairment free', funded by the Ministry of Oceans and Fisheries,

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Korea.

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J. Tuominen, P. Vuoristo, T. Mäntylä, S. Ahmaniemi, J. Vihinen, P.H. Andersson, Corrosion behavior of HVOF-sprayed and Nd-YAG laser-remelted high-chromium, nickel-chromium coatings, J. Thermal Spray Tech. 11 (2002) 233-243.

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[17]

I. Gedzevicius, A.V. Valiulis, Analysis of wire arc spraying process variables on coatings properties, J. Mater. Process. Technol. 175 (2006) 206-211.

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[18]

V. Higuera, F. Belzunce, A. Carriles, S. Poveda, Influence of the thermal-spray procedure on the properties of a nickel-chromium coating, J. Mater. Sci. 37 (2002) 649-654.

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[19]

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G.A. El-Mahdy, A. Nishikata, T. Tsuru, Electrochemical corrosion monitoring of galvanized steel under cyclic wet-dry conditions, Corros. Sci. 42 (2000) 183-194.

[20]

H. Marchebois, C. Savall, J. Bernard, S. Touzain, Electrochmical behavior of zinc-rich powder coatings in artificial sea water, Electrochim. Acta 49 (2004) 2945-2954.

[21]

C.M. Abreu, M. Izquierdo, M. Keddam, X.R. Novoa, H. Takenouti, Electrochemical behaviour of zinc-rich epoxy paints in 3% NaCl solution, Electrochim. Acta 41 (1996) 2405-2415.

[22]

A. Meroufel, S. Touzain, EIS characterisation of new zinc-rich powder coatings, Prog. Org. Coat. 59 (2007) 197-205.

[23]

K.H. Baik, J.H. Kim, B.G. Seong, Spray-formed tooling process by arc spraying, Trans. Mater. Process. 14 (2005) 595-600.

[24]

T. Rayment, S. Hoile, P.S. Grant, Phase transformation and control of residual stresses in thick spray formed steel shells, Metall. Mater. Trans. B 35B (2004) 1113-1122.

[25]

D.A. Jones, S.R. Duncan, T. Rayment, P.S. Grant, Control of temperature profile for a spray deposition process, IEEE Trans. Control. Syst. Technol. 11 (2003) 656-667.

[26]

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ACCEPTED MANUSCRIPT L. Pawlowski, The Science and Engineering of Thermal Spray Coatings, second ed., John Wiley & Sons Ltd, West Sussex, 2008.

[27]

Z. Liu, D. Yan, Y. Dong, Y. Yang, Z. Chu, Z. Zhang, The effect of modified epoxy sealing on the electrochemical corrosion behaviour of reactive plasma-sprayed TiN coatings, Corros. Sci. 75 (2013) 220-227.

[28]

Y.J. Yu, J.G. Kim, S.H. Cho, Plasma-polymerized toluene films for corrosion inhibition in microelectronic devices, Surf. Coat. Technol. 162 (2003) 161-166.

[29]

Y.H. Yoo, D.P. Le, J.G. Kim, Corrosion behavior of TiN, TiAlN, TiAlSiN thin films deposited on tool steel in the 3.5 wt.% NaCl solution, Thin Solid Films 516 (2008) 3544-3548.

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[30]

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ACCEPTED MANUSCRIPT Fig. 1 (a) the structure of the main agent, (b) the structure of the harder Fig. 2 SEM morphology and EDS analysis of the Inconel 625 coating. (a) cross sectional morphology, (b) EDS spectrum of area in (a) Fig. 3 SEM micrograph of the Inconel 625 coating and corrosponding EDS mapping of Ni,

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Mo, Cr, O, and Nb. Fig. 4 SEM morphologies of the Inconel 625 coating with sealing treatment. (a)-(b)

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as-sprayed, (c)-(d) Zinc sealing, (e)-(f) Hybrid sealing, (g)-(h) Ceramic sealing

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Fig. 5 Corrosion potential measurements of the Inconel 625 coating with sealing treatment. Fig. 6 Anodic polarization curves of the Inconel 625 coating with sealing treatment.

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Fig. 7 SEM morphologies of the inconel 625 coating with sealing treatment after anodic

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polarization experiments. (a)-(b) as-sprayed, (c)-(d) Hybrid sealing, (e)-(f) Ceramic sealing

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Fig. 8 SEM morphologies of the inconel 625 coating with Zinc sealing treatment after anodic polarization experiments. (a) low magnification showing the local corrosion damage, (b)

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boundary area between corrosrion damaged area and white corrosion area, (c) corrosion damaged area, (d) white corrosion area Fig. 9 3D analysis of the Inconel 625 coating with sealing treatment and after anodic polarization experiments.

Fig. 10 Tafel analysis of the Inconel 625 coating with sealing treatment.

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ACCEPTED MANUSCRIPT Table I Arc thermal spraying parameters

Variable parameter

Value

1

Compressed air (bar)

6.5-7.0

2

Output voltage (V)

30

3

Output current (A)

150

4

Spray distance (mm)

5

Spray angle (°)

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No

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200

18

90

ACCEPTED MANUSCRIPT Table II Sealing conditions of Inconel 625 coating Work

Zinc

Ceramic

Hybrid

1

1st sealing

About 40 ㎛

About 10 ㎛

About 10 ㎛

2

Drying

Atmospheric temperature/24 h

60℃/1 h

60℃/1 h

3

2nd sealing

About 40 ㎛

About 10 ㎛

About 10 ㎛

4

Drying

Atmospheric temperature/24 h

60℃/1 h

60℃/1 h

About 300 ㎛

About 240 ㎛

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Thickness (Inconel 625 coating + sealing)

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Order

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About 240 ㎛

ACCEPTED MANUSCRIPT Table III Tafel analysis results of the Inconel 625 coating with sealing treatment. Zinc

Hybrid

Ceramic

Corrosion potential (V)

-0.27

-0.91

-0.21

-0.16

Corrosion current density (A/cm2)

1.18 x 10-5

2.33 x 10-4

6.01 x 10-7

1.07 x 10-7

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As-sprayed

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ACCEPTED MANUSCRIPT O O O

O O

O

OMe

Si

OMe Si

OMe

OMe

Si O

O

HO

+

OMe

O

Si OMe

O

OMe

O

O

O Si O Si HO O Si O Si O O OO OMe OH Si O O Si O Si O O Si O Si O O OMe O O Si Si O Si O Si OH O Si HO O O O O O Si O O Si O Si O Si OH O O HO O Si Si O Si OH O OH

Si OR1 OR1

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(R: alkyl, aminoalkyl / R1: Methyl, Ethyl) (b)

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O

O

O

R

O

O

O

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OR1

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+

O

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(a)

R NH2

O

OH Si O Si OMe O Si O Si O O O Si OO O Si O O OMe HO Si O O Si O Si O O O O Si Si O Si O Si OH O HO Si O O O O O Si O O Si O Si O Si OH O O HO O Si Si O Si HO O O OH O Si O Si OMe OH HO OMe OH OH H O Si O Si O O O Si O Si O Si HO Si O O O Si O OO Si O O OH Si O O OO Si O O HO Si O Si Si O O O O O HO Si O O Si O Si O O Si O O Si Si O Si O O O Si OH OMe O Si Si O Si HO Si O OH O O O O Si O O O HO Si Si O O OMe O O O Si O Si O Si O O Si OH Si O O O Si Si O Si O O O O O O Si Si O OMe O Si O Si Si O O Si HO OH O HO Si O Si OMe OMe OMe O OMe Si Si OMe OMe O O O

OR1

HO

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OH HO OH Si O Si O Si O Si O O OO O OH Si O Si HO Si O OO O Si O O O Si Si O Si O Si OH O HO Si O O O O O Si O O Si O Si O Si OH O O HO O Si Si O Si OH HO OH

O

OMe

RI

O

OH

OR1 N H Si OR1 NH R

O

ACCEPTED MANUSCRIPT (a)

Element Ni Cr Mo O Nb C Al Ti

(b)

Crack

wt. % 47.29 14.64 7.94 11.80 2.89 14.92 0.32 0.19

Pores

Pore

Interfacial defect

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Substrate

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Fig. 2

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at. % 25.20 8.81 2.59 23.07 0.97 38.86 0.38 0.12

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U U

(b)

Ni

(c)

Mo

(e)

O

(f)

Nb

L

L U

Cr

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(d)

5um

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ACCEPTED MANUSCRIPT (a)

(b)

Inconel 625

500um

(c)

50um

PT

(d) Pores

SC

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Zinc

500um

(f)

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(e)

Hybrid

500um

100um

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(g)

Ceramic

10um

(h)

500um

24

100um

ACCEPTED MANUSCRIPT 0.4

As-sprayed Hybrid

Ceramic Zinc

0.0 -0.2 -0.4 -0.6

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Potential, E/V vs. Ag/AgCl

0.2

-0.8

RI

-1.0 -1.2 10000

20000

Time, sec

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30000

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0

25

40000

ACCEPTED MANUSCRIPT

3.5

As-sprayed Ceramic Hybrid Zinc

2.5 2.0 1.5 1.0

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0.5 0.0 -0.5

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Potential, E/V vs. Ag/AgCl

3.0

-1.5 -9 10

-8

10

-7

10

-6

10

-5

10

-4

10

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-1.0

-3

10

2

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Current density, A/cm

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Fig. 6

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-2

10

-1

10

0

10

ACCEPTED MANUSCRIPT (a)

(b)

Inconel 625

500um

50um

(d)

PT

(c)

500um

(e)

100um

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(f)

Ceramic

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500um

Fig. 7

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Hybrid

27

100um

ACCEPTED MANUSCRIPT (a)

(b)

500um

50um

(d)

SC

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(c)

10um

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Fig. 8

28

10um

ACCEPTED MANUSCRIPT Before

After 129um

135um

52um

106um

PT

Inconel 625

NU

108um

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Zinc

Fig. 9

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Ceramic

117um

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149um

29

116um

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0.0

-0.3

-0.6

-0.9

-9

10

-8

10

-7

-6

10

10

-5

-4

10

10

2

SC

-1.2 -10 10

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As-sprayed Ceramic Hybrid Zinc

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Potential, E/V vs. Ag/AgCl

0.3

Current density, A/cm

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30

-3

10

-2

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ACCEPTED MANUSCRIPT Highlights 

Inconel 625 coating deteriorated corrosion resistance due to the effects of its p orosity and Cr depletion.

 Sealing with zinc powder, inorganic ceramic, and organic-inorganic composite ceramic were performed.  Corrosion behavior was investigated through various electrochemical experiment s.

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Inorganic ceramic was found to have the best corrosion protection performance .

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

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