Protection Performance of Plasma Sprayed Al2O3-13 wt%TiO2 Coating Sealed with an Organic-inorganic Hybrid Agent

ZHANG Kaicheng ,ZHOU Zehua＊ ,DENG Yong ,WANG Guangyu ,WANG Zehua ,WU Lintao ,YANG Guangheng

(1.College of Mechanics and Materials,Hohai University,Nanjing 211100,China;2.Chengdu Customs Technical Centre,Chengdu 610041,China)

Abstract: An organic-inorganic hybrid sealing agent was fabricated and used in the plasma sprayed Al2O3-13 wt%TiO2 coating,and conventional silicone agent was also used for comparison.Protection performance of the coatings was comprehensively evaluated based on both anti-corrosion and anti-biofouling properties.The results reveal that the sealing treatment is remarkably useful to decrease the porosity of the coating,and the porosity of the coating sealed with the hybrid agent is only 0.035%.Immersion corrosion test and Tafel polarization test reveal that the sealed coating with the hybrid agent exhibits a better corrosion resistance by compared with the coating sealed with silicone agent.The corrosion current density icorr of the hybrid agent sealed coating is only 0.7×10-6 A·cm-2.Moreover,anti-biofouling tests both in the outdoor analogue hydraulic environment and in the natural marine environment prove that the mentioned novel coating presents a better combination of corrosion resistance and anti-biofouling property by compared with the other coatings,and it could be used as a protection of metal components in the marine environment.

Key words: plasma sprayed coating;marine corrosion;Al2O3-13 wt%TiO2;corrosion resistance;sealing treatment

1 Introduction

Plasma sprayed ceramic coating on metal substrate exhibits superior protection performances including wear resistance and anti-corrosion in air.Accordingly,it is widely applied in aviation,metallurgical and mechanical industry.However,sealing treatment on ceramic coating is necessary for its application in some severely aggressive conditions such as in the hydraulic environment.Because inevitable pores in the un-sealed ceramic coating (about 5%-8% porosity) prepared by the conventional method,transmitting channels of corrosion medium to the metal substrate,might deteriorate corrosion resistance of coating.

Plasma sprayed ceramic coating treated with sealing materials is deemed to be efficient in improving corrosion resistance under mentioned aggressive conditions.There are different sorts of conventional sealing agents.For instance,plasma sprayed ceramic coating sealed with organic silicone agent presents a good combination of anticorrosion and wear resistance used in the freshwater environment.Based on our early work,the piston rod with the sealed surface of plasma sprayed AlO-13 wt%TiOcoating,instead of the rod with the chromeplated surface,has been widely applied such as in Three Gorges Project (in China).

Among conventional sealing agents,unfortunately,no evidence indicates that plasma sprayed coating sealed with them has a good anti-biofouling property or could be applied in the marine environment.Our early results also revealed that the plasma sprayed ceramic coatings sealed with different materials such as a sort of silicon resin met a severe biosorption.Lots of researches prove that anti-biofouling paints could improve marine biosorption of components,however,these paints on the components aren’t suitable for wear resistance applications.

This study focuses on the preparation of plasma sprayed AlO-13 wt%TiOcoating with superhydrophobic surface treated with a novel sealing agent.The aim is that the protective coating has an excellent combination of corrosion resistance and anti-biofouling performances.Meanwhile,there is no negative effect on wear resistance of the sealed coating because the sealing agents penetrate the pores of the coating.The study will be helpful for protection of metal components sprayed with the mentioned coating in marine applications.

Thus,AlO-13 wt%TiOcoating was plasma sprayed on steel substrate based on optimization of spray process parameters,and then heat treatment was performed to improve microstructure and properties of the coating including porosity and bonding strength between the coating and the substrate.An organic-inorganic hybrid sealing agent was fabricated and sealed in the coating;a conventional organic silicone agent was also used for comparison.Protection performance of the sealed coatings was comprehensively evaluated resulted from both anticorrosion and anti-biofouling properties.

2 Experimental

2.1 Preparation of coating

Commercial high purity AlO-13 wt%TiO(AT13) powder with the average grain size of 30 μm and NiCrAl powder with the average grain size of 75 μm were used to prepare the ceramic coating and the adhesive coating,respectively.Q235 steel was selected as the substrate.

Plasma spray process was carried out in the air by a conventional method using PRAXAIR 3710 plasma spraying system,based on our early optimizing parameters.More details were discussed in Ref.[22].Then the sprayed samples were heat-treated in air in the electric furnace at 560 ℃ for 6 hours.

2.2 Preparation of hybrid sealing agent

Our early works ascertained that the coating sealed with silicone agent had a poor anti-biofouling property.Therefore,based on lots of researches,an organic-inorganic hybrid sealing agent was introduced.The novel agent was composed of 23 wt% silicon resin,35 wt% inorganic components (including SiOwith the average size of 30 nm and pulvistalci with the average of 5 μm,and their mass ratio being 1:2),6 wt%assistant agents (surface active agent of BYK3700,a dispersing agent of BYK161,foam suppressant of BYK066N and levelling agent of BYK306 with conventional ratio) and ethanol as solvent.

The preparing procedure of the novel agent is as follow:silicon resin was first introduced into ethanol solvent and they were mixed in the magnetic stirrer for 30 minutes with the rotating speed of 3 000 r/min;then the solution was mixed with the other mentioned compositions in the dispersing machine for 30 minutes with the speed of 10 000 r/min;after ageing of 4 hours,the solution was put into the oven at 110 ℃ for 1 hour.

2.3 Sealing treatment

Commercial silicone agent was selected as the compared sealing agent.So,both the hybrid agent and the silicone agent were used in sealing treatment.The samples were immersed in sealant for 60 minutes.Then they were taken out and dried in air for 20 minutes.After drying the samples were placed in the oven to heat the sealing agent for 30 minutes at 130 ℃.Besides,the mentioned treatment was repeated twice to ensure a good sealing effect.

2.4 Apparatuses and methods of analysis

In this paper,the sealed surfaces of the tested samples were first polished till the ceramic coatings were exposed to evaluate the properties of the coating.

The corrosion resistance of the coatings before and after sealing treatment was evaluated by both immersion test and electrochemical measurement.The immersion test was performed in a 10 vol% HCl solution according to Chinese Standard GB 10124-88 and the corrosion resistance was evaluated by mass loss method.The electrochemical characteristics (corrosion current density

i

and corrosion potential

E

) of the coatings in 10 vol% HCl solution were determined by the Tafel slope method via PARSTAT 2273 Advanced Electrochemical Workstation with a three-electrode system.

Connecting pores and/or micro-cracks from coating surface to substrate serve as transmitting channels of corrosion medium,iron reagent test was considered to evaluate the porosity of ‘connecting pores and or micro-cracks’ of the coatings.The procedure is in accordance with the Chinese standard of the porosity test method for thermal spray coating (JB/T 7509-94).

The static water contact angle of the sealing surfaces was measured with the JC2000B drop shape analysis system and the final value is an average of three tested samples.Anti-biofouling property was determined by the tests in an outdoor analogue hydraulic environment for over 3 months and in the Yellow Sea (China) for 396 days,respectively.

HITACHI-S3400N scanning electronic microscopy (SEM) was used to observe the microstructure of the coatings.The common camera was selected to observe biofouling characteristic.

3 Results and discussion

3.1 Morphologies and porosities of coatings before and after sealing treatment

Fig.1 shows the surface and polished crosssection morphologies of the coatings at different conditions.The as-sprayed coating presented a typical lamellar structure and both the ceramic coatingadhesive coating and the adhesive coating-substrate exhibited good bonding effects.However,some closed pores and micro-cracks existed in the coating (Fig.1(a)).After heat-treatment,the flaws decreased to some extent (Fig.1(b)),and the reasons are mainly grain growth and corresponding compressive stress.The surface of the as-sprayed coating was flat on the whole,but a few open-pores,partially un-melted particles and even some micro-cracks were observed (Fig.1(c)).In addition,the surface morphology of the heat-treated coating was the same as that of the former,so it is not presented here.The sealed coating after polishing treatment exhibited an excellent sealing effect with a smooth and continuous surface (Fig.1(d)),which means the mentioned flaws on the surface were sealed efficiently.

Moreover,porosities of the coating before and after sealing treatment were tested.The porosity of the as-sprayed coating was 7.12%,while that of the heattreated coating was 3.01%;after sealing treatment,the porosity of the sample with polished surface decreased obviously to 0.035%.For comparison,the porosity of the coating sealed with silicone agent was also tested and the value was 0.049%.The results mean that the novel sealing agent was close to the conventional silicone agent in sealing effect and the sealing treatment was remarkably useful to decrease the porosity of the coating.

3.2 Corrosion resistance of coatings

Mass loss rates of the coatings with/without sealing treatments at different immersion times in 10 vol% HCl solution were measured (Fig.2).The results indicate that sealing treatment improved obviously corrosion resistance of the coatings,and the coating sealed with the hybrid agent presented a rather slight loss rate during the whole corrosion period,which exhibited a better corrosion resistance than that of the coating sealed with the silicone agent.

Fig.1 Surface and polished cross-section morphologies of coatings at different conditions:(a) Cross-section of as-sprayed coating;(b) Crosssection of the coating after heat-treatment;(c) The surface of as-sprayed coating;(d) The surface of coating after polishing treatment sealed with hybrid agent

Tafel polarization curves of the mentioned coatings tested in 10 vol% HCl solution are shown in Fig.3,and the corresponding electrochemical characteristics are listed in Table 1.Apparently,the sealing treatment resulted in a remarkable change in the electrochemical properties of the coatings:the corrosion current densities of both sealed coatings were substantially lower than those of the unsealed coatings.The

i

of the coatings sealed with the novel hybrid agent and the conventional silicon agent were only only 0.7×10and 4.8×10A·cm,respectively.The lower corrosion current density corresponds to the slower corrosion rate,and the remarkable change in current density is mainly because the sealing agent hinders the transport of ions between the coating and the electrolyte,thus greatly reducing the corrosion current density.Meanwhile,an increase in corrosion potential indicates a gradual decrease in corrosion tendency.Moreover,the electrochemical characteristics of the coating sealed with the hybrid agent were better than those of the coating sealed with the silicone agent.The tested evidence matched the fact of the immersion corrosion.

Table 1 Corrosion current densities () and potentials ()of different coatings

Fig.2 Mass loss rates of coatings at different immersion time in 10 vol% HCl solution (1:As-sprayed coating;2:Coating sealed with silicone agent;3:Coating sealed with hybrid agent)

Fig.3 Tafel polarization curves of coatings tested in 10 vol% HCl solution (1:As-sprayed coating;2:Coating sealed with silicone agent;3:Coating sealed with hybrid agent)

3.3 Anti-biofouling properties of coatings

Static water contact angles of the two sealed coatings were measured and the values were 139.7 degrees for the surface sealed with the hybrid agent,and 119.1 degree for that sealed with silicone agent,which means that the former might have a better superhydrophobic surface.

Anti-biofouling property was first tested in the outdoor analogue hydraulic environment.The results revealed that,after over 3 months,severe corrosion was found on the surface of un-sealed coating,while obvious biofouling and slight rusty were observed on the surface sealed with silicone agent.However,no obvious evidence of both corrosion and biosorption was found out on the surface sealed with the novel agent.Furthermore,the protection performances of the coatings sprayed and sealed on the piston rods were evaluated based on the test in the natural marine environment (Yellow Sea,China,396 days),and the test results supported positively the mentioned results.

The anti-biofouling property coincided with the electrochemical test and immersion corrosion results,the reason for this situation was that the sealing agent can effectively provide protection for the coating,and the novel hybrid agent was more effective than the conventional silicone agent for corrosion protection and anti-biofouling property.

4 Conclusions

AlO-13 wt%TiOcoating was plasma sprayed on the steel substrate.An organic-inorganic hybrid sealing agent was fabricated and used successfully in the sprayed coating;conventional silicone agent was also used for comparison.Protection performance of the sealed coatings was evaluated resulted from both anti-corrosion and anti-biofouling properties.The results revealed that:

a) Sealing treatment is remarkably useful to decrease the porosity of the coating.The porosities of the coatings sealed with the hybrid agent as well as the silicone agent were 0.035% and 0.049%,respectively;while the porosities of the as-sprayed coating and the heat-treated coating were 7.12% and 3.01%,respectively.

b) Both immersion corrosion test and Tafel polarization test supported the fact that the sealed coating with the novel agent exhibited a better corrosion resistance by comparing with the other mentioned coatings.The sealing agent has a protective effect on the electrochemical corrosion of the coating,mainly by hindering the transport of the coating with the electrolyte.The corrosion current density

i

of the hybrid agent sealed coating was only 0.7×10A ·cm.

c) Anti-biofouling tests both in the outdoor analogue hydraulic environment and in the natural marine environment showed that the coating sealed with novel agent presented a good combination of corrosion resistance and anti-biofouling property.But the un-sealed coating occurred severe corrosion and the coating sealed with silicone agent presented obvious biofouling and slightly rusty.The organic-inorganic hybrid sealing agent offer better marine corrosion and anti-biofouling than the conventional silicone agent.