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1 S alt Spray Corrosion Behaviors of Arc Sprayed Al Coating on S 355 Steel by Laser Remelting K ong Dejun *1, 3 , S heng Tianyuan 1 , W ang Wenchang 2 1 School of Mechanical Engineering, Changzhou University, Changzhou 213164, China ; 2 School of Petrochemical Engineerin g, Changzhou University, Changzhou 213164, China; 3 Jiangsu Key Laboratory of Materials Surface Science and Technology, Changzhou University, Changzhou 213164, China Correspond ing author e - mail address : kong - dejun @ 163.com Abstract A layer of arc spray ed Al coating on S355 steel w as remelted with a CO 2 laser , the surface - interface morpholog ies, d istribution s of chemical element s, and phases were analyzed with a SEM scanning electron microscopy , EDS energy dispersive spectrometer ), and XRD X - ray diffracti on ), respectively. T he c orrosion performance of Al coating before and after laser remelting was investigated with a NSS neutral salt spray⤀ test, and the products of salt spray corrosion were also analyzed with a XRD . The result shows that the Al coating surface by arc spraying is rougher, the interface b o nding mode is a mechanical bonding, after salt spray corrosion, the surface corrosion phenomenon is obvious, and the corrosion mode is primarily composed of pitting corrosion. After laser remelting, the c oating surface tends to be smooth, the interface b o nding mode is mechanical bonding and metallurgical bonding, while the surface tends to roughness after salt spray corrosion, and the corrosion mode is co nverted into general corrosion. The corrosion potent ial E Corr of arc spray ed Al coating by laser remelting shift s positively , showing that the corrosion resistance is enhanced. Keywords: A rc spraying; Al coating; L aser remelting ; S alt spray corrosion 1 . Introduction A s a new structural steel, S355 steel i s used in the man ufactur ing of marine platform legs , duo to high strength, ductility, and impact toughness [ 1, 2]. T he S355 steel is serviced in the sea water for long term and easily eroded, and a strong electrochemical corrosion happens, which reduces th e mechanical properties of S355 steel and shortens its service life. B ecause offshore platform is far away from the coast [3], it is difficult to carry out maintenance regularly, especially in the wave splash ed zone, the corrosion rate is about 6 times of that under normal circumstances, therefore, there is a significance issue of process ing Al coating on the S355 steel. Recently, researches on S355 steel are mainly focus on welding microstructure and mechanical properties [4 - 7], the investigation s of laser remelting on salt spray corrosion of arc sprayed Al coating are less reported. T he Al coating by arc spraying is the main method for long - term preservation [8 - 10], which plays a role of isolating and cathodic protection, but in the ocean environment, the corrosion resistance of Al coating decreased very fast. Laser remelting has many advantages such as high efficiency, simple process, and good environmental performance [11 - 13], the salt spray resistance of arc spray ed Al coating by laser remelting can be f urther enhanced. The effective thickness of Al coating can be controlled by laser parameters, achieving the purpose of 2 increasing the b o nding force of the Al coating - substrate. I n this study, a layer of Al coating was prepared on S355 steel substrate by ar c spraying , which was processed with laser remelting , forming a laser remel ted layer. T he corrosion resistance was discussed with salt spray corrosion test, which provide a technical support for surface modification treatment of S355 steel in ocean enginee ring. 2 . Experimental The experimental material wa s the European standard S355 structural steel with the chemical composition s ⠀mass, %⤀ as follows : C 0 .17, Mn 0.94, Si 0.55, Mo 0.30, Cr 0.065, Ni 0.065, Zr 0.15, S 0.035, the remainder wa s Fe. The process of arc spray ed Al coating as follows : polishing → chemical washing blasting drying arc spray ed Al coating cooling cleaning. Technical parameters as the following: Al wire with the diameter of 2 mm ; spraying voltage of 20 - 30 V; spraying current of 100 - 200 A. The process of laser remelting was s hown as follows: Al coating → chemical washing → blasting drying laser remelting, the Al coating on the S355 steel was remelted by a CO 2 laser with laser power of 1000 - 1200 W, broadband spot of 1mm휱 mm , scan speed of 6 - 8 mm/s , lap rate of 50 %, Ar as protective gas. The NSS test was carried out according to the GB 6458 - 86 standard of NSS test in China , test conditions: continuous salt spraying of 3.5 넀0.5 % NaCl solution , pH of 6.5 - 7.2, the temperature of 3 0 o C . The electrochemical experiments were carried out on a PS268A type electrochemical workstation . The morphologies, chemical compositions, and phases of the Al coating after salt spray corrosion were analyzed with a JSM - 6360LA type SEM, its configured EDS, and D/max2500 PC type XRD, respectively . 3 . Analysis and discussion 3 .1 Surface - interface morphologies and EDS analysis of arc spray ed Al coating Fig. 1 ⠀a) shows that the Al coating surface was comparatively rough after arc spraying, there were some particles stacked on the sample surfaces , exi sting a few pores . The flaws generated during arc spraying, caused by Al mistes under high temperature and pressure, the coating was coated firmly and sealed well. Fig. 1 ⠀b⤀ shows the results of EDS analysis, the mass frations ⠀mass, %) were shown as follo ws : C 7.14 , O 11.29 , Al 71.35 , Si 3.82 , K 0.72 , Ca 0.85 , Fe 4.83 ; and atom fractions ⠀at, %⤀: C 14.14 , O 16.77 , Al 62.86 , Si 3.42 , K 0.44 , Ca 0.50 , Fe 2.06 . The part of Al coating had been oxidized by the air, a few Fe atoms were detected, which came from the substrate. T he Fe element was so active that it was easily oxidized into Fe 2 O 3 with the loose structure s , affecting anti - corrosion of the coating. T he Si , K , and Ca were impurity elements, which came from Al wire. Fig. 1 c shows that the Al coating wi th the thickness of 300 was formed on the substrate, the bonding interface appeared sawtooth shape, this was because that of the surface irregularities were filled by Al mist s during arc spraying, and mechanical bonding was formed to improve the bonding strength effectively. F ig. 1 d and ⠀e⤀ show s the EDS analysis results of point B and C, the mass fractions ⠀mass, %⤀ of point B were shown as follows : Fe 100; and atom fractions ⠀at, %⤀: Fe 100, the mass fractions ⠀mass, %) of point C were shown as follows : Al 96.4, Fe 3.86; an d atom fractions ⠀at, %⤀: Al 98.10, Fe 1.90, no Al element diffused into the substrate, and only a small amount of Fe element appeared in the Al coating, which was illustrated that no diffusion behavior happened during arc spraying, and it was further proo fed that b o nding mode was mechanical bonding between the coating and substrate. 3 (a) Surface morphology (b⤀ EDS analysis of point A (c⤀ I n terface morphology (d⤀ EDS analysis of point B (E) EDS analysis of point C Fig.1 Surface morphology , EDS analysis, and interface morphology of arc spray ed Al coating 3 .2 Surface - interface morphology and EDS analysis of arc spray ed Al coating by laser remelting Fig.2 ⠀a⤀ shows that the coating surface roughness decreased after laser remelting, this was because that the Al of the coating surface was melted into liquid by the laser energy, during solidification, the liquid Al was di stributed equally on the sample surface. Fig.2 ⠀b⤀ shows the results of EDS analysis of point D , the mass fractions ⠀mass, %⤀ were shown as follows : C 7.07, O 23.32, Al 58.39, Si 4.16, Fe 7.06; and atom fractions ⠀at, %⤀: C 13.14, O 32.51, Al 48.23, Si 3.3 1, Fe 2.81. The Al content by laser remeling decreased, while the O content by laser remeling increased, compared that by arc spraying. It was illustrated that the Al element was further oxidized into Al 2 O 3 , which reduced coating consumption rate effective ly. T he interface morphology by laser remelting was shown in Fig.2 c , the Al coating was dense and flat, a small number of lateral pores existed, this was because that the Al coating became larger during laser remelting, and the Ar gas got into the coati ng. T he Al coating rapidly cooled after laser melting [14], as the temperature decreased, the coating shrank, no liquid metal added into the pores, and the lateral pores appeared. The interface was composed of Al coating - FeAl metallurgical bonding layer - S3 55 steel substrate, among them, the Fe - Al metallurgical bonding layer with the thickness of about 10 was produced at the Al coating - substrate interface. The combination reaction occurred at the surface to form the Fe - Al metallurgical bonding layer, no cracks appear ed , which was ensured that the close integration of the coating - substrate was formed. T he Fe appeared in the Al coating that was closed to the bonding layer, indicating that a small number of Fe atoms got into the Al coating under the impact of Ar gas, and occurred compound reaction with Al to form Fe - Al compound s . The following reaction occur red during laser remelting: Fe+Al FeAl ⠀1⤀ 3Fe+Al Fe 3 Al ⠀2⤀ T he FeAl and Fe 3 Al were two relative stable phases of Fe - Al alloy at high temperature, the interat omic b o nding A B C Substrate Al coating Bonding interface 4 mode were covalent bond and ionic bond, which improve the bonding performance. Fig.2 d shows the EDS analysis results of point E , the mass fractions ⠀mass, %) were shown as follows : Al 51.34, Fe 48.66 ; and atom fractions ⠀at, %⤀: Al 68.59, F e 31.41, the Al and Fe atoms diffused m utual ly, it was further proofed that the bonding mode was metallurgical bonding from the perspective of element content. (a) Surface morphology (b⤀ EDS analysis of po int D (c⤀ I nterface morphology ⠀d⤀ EDS analysis of point E Fig. 2 Surface morphology , EDS analysis, and interface morphology of arc sprayed Al coating by laser remelting 3 .3 L ine scan analysis of arc spray ed Al coating i nterface by laser remelting Fig.3 ⠀a) shows that the Al element maintained a low content in the substrate, and began to increase at the metallurgical bonding layer closed to the substrate, the gradient increase exhibited at the bonding layer . This diffusion, gap diffusion and vacancy diffusion were coexisted, the obvious metallurgical bond appeared at the middle and the outer coating, the Al content maintained a relatively high concentration. Fig.3 b shows that the Fe element had high conte nt in the substrate, and declined rapidly at the diffusion layer, the gradient of Fe element was significantly larger than that of Al element. I t was illustrated that there was almost no Fe element arrived at the Al layer, no Fe 2 O 3 appeared during salt spr ay corrosion, which was ensured the Al coating had excellent corrosion resistance. D E Substrate Al co ating Bonding interface 5 (a) Al content ⠀b) Fe content Fig. 3 Line scan analysis of arc sprayed Al coating interface by laser remelting 3.4 Surface - interface mor phology and EDS analysis of arc spray ed Al coating after corrosion Fig.4 ⠀a⤀ shows that the Al coating surface became loose and occurred cracking phenomenon after salt spray corrosion. T he cracking was the starting point of coating failure, the Cl - and O 2 - could easily penetrate into the substrate from the cracking, which could destroyed bonding performance between the coating and substrate. T he corrosion phenomenon was most evident at the spraying defects, the deep etch pits and corrosion particles appeare d, which was caused by the phenomenon of occluded cell , and the following reaction occurred in occlusive area [15]. O 2 +2H 2 O+4e - 4OH - ⠀3⤀ Al Al 3+ +3e - ⠀4⤀ Al 3+ +3H 2 O Al⠀OH⤀ 3 +3H + ⠀5⤀ Eq.(3) sh ows that a large amount of H + generated during reaction, which improved the sensitivity of Cl - on the Al element of, the Al 3+ was difficult to form Al 2 O 3 , deepening further the pits, the mechanical bonding between the coating and substrate destroyed after the pits connecting, which could lead to the coating peeling and affected its anti - corrosion seriously. T he results of EDS analysis was shown in Fig.4 b , the mass fractions ⠀mass, %⤀ as follows : C 19.67 , O 59.79 , Mg 0.33 , Al 13.40 , S 1.54 , Cl 1.13 , Ca 0. 53 , Fe 3.62 ; and atom fractions ⠀at, %⤀: C 27.10 , O 61.85 , Mg 0.23 , Al 8.22 , S 0.79 , Cl 0.53 , Ca 0.22 , Fe 1.07 . The Al element on the surface severely decreased, which existed in the oxide form. T he Cl and S elements came from etchant residue, the Fe conte nt did not change, which was illustrated that the corrosion behavior occurred on the coating surface. Fig.4 c shows that a small number of corrosion cracks appeared, and the thickness of Al coating changed, which was illustrated that the arc spray ed Al c oating had a certain of salt spray resistance. (a) Surface morphology ⠀b⤀ EDS analysis of point F (c⤀ I nterface morphology Fig. 4 Surface morphology , EDS analysis, and interface morphol ogy of arc spray ed Al coating after corrosion F Substrate Al coating Substrate Al coating Substrate Al coating 6 3 .5 Surface - interface morphologies and EDS analysis of arc sprayed Al coating by laser remelting after corrosion Fig.5 ⠀a) shows that the laser remelted surface was relatively flat after corrosion, a small numb er of corrosion cracks appeared, and the surface porosity decreased, which came from two sources: ⠀1) the Al coating was oxidized into Al 2 O 3 after laser remelting, the structure was more compact, which was less sensitive to Cl - , and it could avoid Al eleme nt directly dissolved into the salt spray etching solution, which reduced the consumption rate of the Al coating; (2⤀ the following reaction occurred during corrosion. Al 3+ +3OH - Al⠀OH⤀ 3 ⠀6⤀ 2Al⠀OH⤀ 3 →Al 2 O 3 +3H 2 O ⠀7⤀ T he surface Al element adsorbed on the pits, then dehydrated into Al 2 O 3 , which had a larger volume to fil l potholes effectively and made the surface smooth. T he corrosion behavior of the coating changed from pitting corrosion into general corrosion. General corrosion was a corrosion mode which was more uniform slower, in this corrosion mechanism, the corrosio n behavior only happened on the coating surface, deep coating and the substrate would not be influenced, which could extend service life of the coating. Fig. 5 ⠀b) shows the results of EDS analysis, the mass fractions ⠀mass,%⤀ as follows : C 12.97, O 27.71, Al 49.42, Cl 1.94, Fe 7.95; and atom fraction ⠀at, %⤀: C 22.30, O 35.78, Al 37.84, Cl 1.13, Fe 2.94. The Al content by laser remelting decreased, but it was far higher than that by arc spraying after corrosion, which was illustrated that laser remelting co uld reduce the effect of Cl - on Al coating. Fig.5 c shows that the metallurgical bonding mode between the coating and substrate did not destroy after corrosion, the pores generated during laser remelting did not grow up after corrosion, illustrating that salt spraying only affected the surface performance. (a) Surface morphology (b⤀ EDS analysis of point G (c⤀ I nterface morphology Fig. 5 Surface morphology , EDS analysis, and interface morp hology of arc sprayed Al coating by laser remelting after corrosion 3.6 XRD analyses of Al coating by laser remelting before and after corrosion T he XRD analyses of Al coating by laser remelting before and after corrosion are shown in Fig.6. Fig.6 ⠀a) show s that the coating surface was mainly covered by - Al 2 O 3 , which was the most stable phase of alumina [16], and it had the advantages such as stable structure, high body density, high strength, and good corrosion resistance. Fig.6 b shows that the coating surface was also covered by - Al 2 O 3 with a lower peak, the Al⠀OH⤀ 3 appeared at 78.12 o and was amphoteric hydroxides, which could soluble in acid and alkali and had an effect on the coating corrosion resistance. G Substrate Al coating 7 (aB efore corrosion fter corrosion Fig. 6 XRD a nalysis of arc sprayed Al coating by laserremelting before and afterrrosion 3.7 Electrochemical est T he polarization curves of Al coating by laser remelting in NaCl sol ution wereshownin Fig.7.Thecorrosion potential E Corr of theAl coating by arcspraying was - 1 100 m V ,while that by laser remelting was - 1 000 m V ,the corrosionpotential movedforwardafter laser remelting,showing thecorrosionresistancehadenhanced. T he corrosionof Al coating in 3.5 % NaClsolutionwas aprocessof Al dissolutionandreductionof dissolved oxygen. Theollowing eactionoccurredring ectrochemical corrosion[ 17 ]. Al Al 3+ +3e - 8) O 2 +4H 2 O+6e - 6OH - +H 2 9 Al 3+ +3OH - AlOH 3 10) 2Al 2 OH 3 Al 2 O 3 H 2 O 11) The highpassivity normally associatedwith Al camefrom thehighly adherent barrier oxideof Al 2 O 3 that formedonthesurface.Inaqueous environments,this protectiveoxidewasformedwith acertainthickness,and as ong hereas the O lement,hic htinuedtorepassivatetheflawswereormed. Fig. 7 Polarization curvesf arc spray ed Alcoating by laserremelting in 3. 5 % NaClsolution 4 . Conclusions 1) TheAl coating surfaceby arcspraying was rough,theinterf acebinding modeisa mechanical bonding,and theorrosionhavior isitting rosion acking rosionfter salt pray corrosion. 2) TheAl coating surfaceby laser remelting becomes smoother,a small number of longitudinal gaps appear at thei nterface,theinterfacebinding modeis mechanical andmetallurgical bonding,and thecorrosion behavior changes intoeneral corrosion. 3) Thesalt spray corrosionproduct isAlOH 3 withasmaller degree of crystallinity,whichisillustratedthat the c orrosionesistance capability of rc s prayedAl c oating ismproved after aser emelting. 8 ⠀4) The corrosion potential of Al coating by arc spraying and laser remelting is - 1 100 m V and - 1000 m V respectively, which is illustrated that the corrosion resista nce enhanced after laser remelting Acknowledgment Financial support for this research by the Key Research and Development Project of Jiangsu Province ⠀Industry⤀ ⠀BE2016052⤀ is gratefully acknowledged. 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