Corrosion performance of Al–Al2O3 cold sprayed coatings on mild carbon steel pipe under thermal insulation☆

Xiaomin Bai,Jianqun Tang,Jianming Gong*,Xiaoliang Lü

School of Mechanical and Power Engineering,Nanjing Tech University,Nanjing 211816,China

1.Introduction

In the petrochemical industry,thermal insulation is commonly utilized on piping and vessels to(i)conserve energy by reducing heat loss;(ii)control surface temperatures of the structures;and(iii)prevent vapor condensation at surfaces having a temperature below the dew point of the surrounding atmosphere[1,2].However,with the wide use of thermal insulation,corrosion under insulation(CUI)appeared on the external surfaces of piping and vessels.As early as in 1956,the failures caused by external stress corrosion cracking(ESCC)of stainless steel under thermal insulation were reported in the literature[3].CUI acted in an insidious pattern and was difficult to detect,so it might result in sudden and hazardous leaks(a safety concern)or shutdowns with high losses of production(economical concern)[4].

CUI occurred when water and oxygen were present on the steel surface,and the corrosion rate was dependent on the type of insulation,chemical content of water,the availability of oxygen,and temperature[5,6].Insulation material contributes to CUI in the following three ways:(i)providing an annular space which can collect water and other corrosive media(Fig.1);(ii)leaching out contaminants that accelerate the corrosion process,and(iii)wicking and/or absorbing water and holding it against the substrate.

Good prevention strategies should provide long term and reliable prevention of CUI and move towards inspection-free and maintenancefree piping systems with significant maintenance cost reductions.Coated piping or vessel before installing insulation is one of the CUI prevention approaches.Thermals pray aluminum(TSA)coatings and organic coatings are conventional coatings to protectpiping and vessels against CUI[6].For instance,the corrosion rate of carbon steel without TSA under mineral wool insulation was 1.0 mm·a?1,while when carbon steel coated with TSA,corrosion rate decreased to 0.003 mm·a?1[7].Halliday proposed cold spray aluminum(CSA)coatings,which had more excellent anticorrosive properties over a wide temperature range than conventional coatings[8].D.Ifezue suggested replacing the inorganic,zinc-rich,ethyl silicate primer coating by CSA coating in high-temp/intermittent systems due to its high-temperature-resistance and adequate corrosion resistance under insulation[9].Using a ceramic–metal mixture,Al–Al2O3,as cold spray material could enhance the coating quality by reducing the porosity and increasing the bond strength of the coating to the substrate[10,11].

However,there is no literature to discuss the ability of Al–Al2O3CS coatings to mitigate CUI.Meanwhile,the current knowledge on CUI mainly comes from field and case studies[12,13].Studies in laboratory are limited.The aim of this work was to evaluate the efficiency of an Al–Al2O3CS coating to mitigate CUI of mild carbon steel pipe using a laboratory CUI apparatus.

2.Material and Coating Preparation

2.1.Material and coating preparation

The substrate material was mild carbon steel of chemical composition(wt.%)0.2C–0.21Si–0.41Mn–0.015P–0.07S–0.06Cr–0.16Cu–0.05Ni,balance Fe.Prior to spray,the substrate was ground with 1200 grit SiC paper and cleaned with acetone,and then sandblasted(using 24 mesh alumina grit)to remove surface contamination and improve surface roughness in order to obtain high deposition efficiency during spraying.Spherical commercial purity Al and platelet α-Al2O3powders were used as the feedstock material for coatings.The weight fraction of α-Al2O3in feedstock were 0%(pure Al coating),30%(denoted as coating30)and 70%(denoted as coating70).The coatings were deposited on the substrate using a CS system provided by Institute of Metal Reasearch Chinese Academy of Science(M6000).The spraying standoff distance was maintained at 25 mm away from the nozzle exit.Compressed air was used as acceleration gas and carrier gas for powder feeding.During the entire process,the acceleration gas was pre-heated to 350°C with a stagnation pressure of 2.1 MPa.The substrate was fixed by a holder,and the spray gun traversed across the substrate surface at 2 mm·s?1.The powder was fed at 10 g·min?1.The whole spraying process was controlled by a pre-compiled computer program to ensure good reproducibility of coating specimens.

Mild carbon steel tubes with dimension of Φ22.8 mm × 3 mm ×200 mm were sprayed and then cut into small rings of Φ22.8 mm ×3 mm×10 mm used for CUI tests.The insulation material used in CUI tests was mineral wool with conductivity and density of 0.044 W·m?1·K?1and 128 kg·m?3.

Coated specimens were characterized with a scanning electron microscope(SEM,JSM-6360LV)equipped with energy dispersive spectrometer(EDS)analysis software.Metallo graphically prepared samples were identically etched to clearly distinguish the particle/particle and the coating/substrate interfaces.

Fig.1.Schematic of pipe-coating-insulation structure.

Microhardness measurements were performed on mounted samples using a conventional Vickers hardness tester with a 200 g load and a dwell time of 15 s.In each coating,the test points were chosen in different regions through the thicknessdirection.Each value presented was the average of five measurements.

2.2.Laboratory CUI test equipment

The corrosion behavior of coatings under insulation was examined by laboratory testing.The laboratory apparatus was based on ASTM G189,Standard Guide for Laboratory Simulation of Corrosion under Insulation.A representative schematic of the laboratory test set up is shown as Fig.2.Brie fly,the test set up consists of:(i)six testing ring samples for weight loss and surface pro filometry measurements,(ii)ploytetra- fluoroethylene(PTFE)rings for separating testing ring samples,(iii)insulation material wrapped around the outside of the rings,and(iv)internal heater and thermocouple to produce hot pipe surfaces.To evaluate the protection efficiency of Al–Al2O3cold spray under insulation,factors and their levels considered in this CUI test are listed in Table 1.The isothermal temperature condition was 80°C.Thermal cycling consisted of 80 °C for 20 h followed by 120 °C for 4 h.In these two conditions,the insulation was kept wet by adding test solution constantly.The wet/dry cycle consisted of 80°C during wet period for 20 h and 120°C during dry period for 4 h,when temperature shifted to 120°C,test solution addition was stopped to achieve a dry surface.The specimens were weighed on an analytical balance with an accuracy of+0.1 mg.

Table 1Factors and levels considered in CUI tests

3.Results and Discussion

3.1.Coating characterization before exposing to corrosive environment

Fig.3 shows the microstructure of Al and α-Al2O3powders used in spraying.The size of the spherical Al particles(Fig.3a)was in the diameter range of 1–30 μm.The size of the α-Al2O3particles(Fig.3b)was about 5 μm.

Fig.2.Schematic of CUI testing apparatus.

Fig.4 shows the cross-sectional morphologies of the coatings with different α-Al2O3contents in the starting powder.Fig.4a shows that there are some cracks and pores between splats in the pure Al coating.On the contrary,in coating30 and coating50,defects of cracks and pores are much less in amount compared to the pure Al coating,which could be attributed to dramatic tamping effect of α-Al2O3on the pre-deposited Al splats.To be more specific,the hard ceramic α-Al2O3particles were fragmenting into small pieces and deposited in the interface between Al splats as shown in Fig.4b and c.Besides,the morphologies of coatings exhibit of lamellar microstructures with the long axis of impacted splats oriented along the substrate surface.The thicknesses of the three coatings were 389 μm,550 μm and 520 μm estimated by cross-sectional morphologies.

Deposition efficiency(DE)was determined as the ratio of mass of the coating deposited on a substrate to the mass of powder fed to the nozzle.mafterandmbeforeare the substrate mass before and after spraying,respectively.mpowderis total mass of powder fed to the nozzle.

Fig.3.SEM images of Al and α-Al2O3 used in experiments:(a)Al powders.SEM images of Al and α-Al2O3 used in experiments:(b)α-Al2O3 powders.

DE for three coatings was 7.78%,17.31%and 11.31%,respectively.The highest DE is obtained at an Al2O3mass fraction of 30%.Ceramic particles Al2O3could increase DE by acting as an “anchor”during spray process.However,the more Al2O3is included in the feedstock,the higher is probability that an Al2O3particle hit another Al2O3particles.So,when Al2O3mass fraction increased to 50%in feedstock,interaction of Al2O3particles dominated the spray process rather than deposition,which results in a decrease in DE.Similar trends were reported in references[14,15].

Vickers hardness for various sites is shown in Table 2.Microhardness for the cold sprayed coating is mainly dependent on porosity and work hardening produced by plastic strain of deposited particles[16,17].The coatings withα-Al2O3had lower porosity than pure Al coatings as mentioned before,so the microhardness of coating30 and coating50 was higher than pure Al coating.It is interesting to find that mcirohardness at carbon steel substrate side of coating/substrate interface was even higher than carbon steel.This is because when Al/Al2O3particles strike carbon steel surface with a high velocity,500–1000 m·s?1[1],the high kinetic energy of particles results in significant plastic deformation of the substrate surface,producing substantial work hardening.So the microhardness near coating-substrate interface is even higher than carbon steel substrate.

Fig.4.Etched cross-section image(SEM)of(a)Pure Alcoating.Etched cross-section image(SEM)of(b)coating30.Etched cross-section image(SEM)of(c)coating50.

Table 2Values of the micro-hardness for CSA coated specimens in variable region

3.2.Corrosion behavior of coatings under isothermal,thermal cycling and wet/dry cycling

Fig.5 shows the cross-sectional morphologies of three coatings after testing for 21 days using 0.01 wt.%NaCl as test solution under isothermal 80°C.It can be noted that all these three coatings could protect mild carbon steel from CUI,and no corrosion occurred at the coating substrate interface.The degradation of coatings during these CUI tests was mainly in the form of uniform thickness reduction.And these micron-sized cracks and pores formed in “horizontal”pattern were more evident than the “vertical”ones(Fig.5b and c).None of these pores or cracks was continuous from the outside surface of the coatings to the mild steel substrate,which indicated the coatings were still impermeable to the solution[17].

Fig.6 shows the cross-sectional morphologies of coating30 after CUI test for 7 days,14 days and 21 days using 0.01 wt.%NaClsolution as test solution under thermal cycling.When the CUI test continued for 7 days,the top surface of the coating became rough due the corrosion of Al and exfoliation of α-Al2O3,and the coating's structure retained density relative to a non-corroded one(Fig.6a).When the CUI test continued for 14 days,the coating continued to thin and many pores developed(Fig.6b).Fig.6c shows the coating microstructure after 21 days,a large piece of coating fell off and many horizontal cracks developed,but the remaining coating covered the substrate closely,which was able to isolate the substrate from the corrosive solution.In conclusion,during the exposure period,coatings became thinner,looser and partially exfoliated.The similar phenomenon was found in pure Al coating and coating50.

Fig.6.The cross-sectional morphologies of coating30 after CUI tests in 0.01 wt.%NaCl solution under thermal cycling:(a)7 days.The cross-sectional morphologies of coating30 after CUI tests in 0.01 wt.%NaCl solution under thermal cycling:(b)14 days.The cross-sectional morphologies of coating30 after CUI tests in 0.01 wt.%NaCl solution under thermal cycling:(c)21 days.

Fig.7 shows the cross-sectional morphologies of coating30 under wet/dry cycling for 7,14 and 21 days using 0.01 wt.%NaCl as testing solution.Thickness reduction of coating was not as much as in thermal cycling testing.Due to the coating surface shifting to dry when stop the solution at 120°C,the coatings degraded slower with time.The corrosion was also in the forms of pores,cracks and exfoliation as in other conditions.The interface of the coatings and substrate also remained intact.

Fig.7.The cross-sectional morphologies of coating30 after CUI tests in 0.01 wt.%NaCl solution under wet/dry cycling:(a)7 days.The cross-sectional morphologies of coating30 after CUI tests in 0.01 wt.%NaCl solution under wet/dry cycling:(b)14 days.The cross-sectional morphologies of coating30 after CUI tests in 0.01 wt.%NaCl solution under wet/dry cycling:(c)21 days.

3.3.The in fl uence of chloride ion concentration on corrosion behavior of coatings

Fig.8 shows the cross-sectional morphologies of coating30 when test solutions were 0.01 wt.%,0.1 wt.%and 1 wt.%NaClunder isothermal condition for 21 days.With the increase in chloride ion concentration,the thickness of coating30 decreased significantly and the microstructure became loose.When the test solution was 0.01 wt.%NaCl,the coatings became thinner due to the corrosion of Al and exfoliation of α-Al2O3(Fig.8a).More pores appeared when the test solution was 0.1 wt.%NaCl,micron-cracksremained in a “horizontal”pattern as mentioned before(Fig.8b).When the test solution was 1 wt.%NaCl,a large piece of coating fell off,many long horizontal cracks developed,but the mild carbon pipe surface was still under protection of the remaining coating,and the interface of the coating and substrate remained dense as before the CUI tests.

Fig.8.The cross-sectional morphologies of coating30 after CUI tests when the test solution were(a)0.01 wt.%NaCl solution.The cross-sectional morphologies of coating30 after CUI tests when the test solution were(b)0.1 wt.%NaCl solution.The cross-sectional morphologies of coating30 after CUI tests when the test solution were(c)1 wt.%NaCl solution.

3.4.Corrosion rate of coatings based on weight loss during CUI test

The corrosion rate based on mass loss of different coatings was calculated according to ASTM G1-03:

Where,ΔW=mass loss(g);A=exposed area in cm2,T=time of exposure in days.

Fig.9a shows the corrosion rate when the test solutions were 0.01 wt.%,0.1 wt.%and 1 wt.%NaClunder isothermal condition.The corrosion rates were 0.28,0.35 and 0.48 g·m?2·d?1to three test solutions,respectively.It can be seen with the increase in NaCl concentration,the corrosion rate increased,which was consistent with the result in Section 3.3.The corrosion rates in three different thermal conditions of coating30 were 0.28,0.54 and 0.39 g·m?2·d?1shown in Fig.9b.As mentioned above,the coating degraded most seriously under the thermal cycling condition because during the whole exposure period,the coating surface was kept wetted.

Fig.9.Corrosion rate of coatings under:(a)different NaCl concentration.Corrosion rate of coatings under:(b)isothermal,thermal cycling and wet/dry cycling.

Fig.10.SEM images of coatings after CUI tests under isothermal condition in 0.1 wt.%NaCl:(a)pure Al coating.SEM images of coatings after CUI tests under isothermal condition in 0.1 wt.%NaCl:(b)coating30;SEM images of coatings after CUI tests under isothermal condition in 0.1 wt.%NaCl:(c)coating50.

3.5.Surface micrographs of coatings after CUI test

Typical surface micrographs of the three coatings after 21 day CUI tests under the isothermal condition are shown in Fig.10,with the test solution of 0.01 wt.%NaCl.The surface revealed corrosion of all coatings in CUI tests.However,EDS analysis of the coatings revealed the specimen surface constituted of aluminum,oxygen,chloride,and a little iron.Silicon was coming from the insulation material(Fig.11),which illustrates that the coatings isolated the substrates from the corrosive electrolyte during CUI tests.The addition of α-Al2O3adds little passive effect on anti-corrosion ability of the coatings when compared to pure Al coating.

4.Conclusions

From the experiments of the current study,Al–Al2O3composite coatings with different metal/ceramic compositions were successfully coated on carbon steel pipe surface using the CS technique.The coatings exhibited higher hardness and denser structure when additional α-Al2O3was added to the spraying powder.Hardness was highest at the interface of the coating and substrate because in this region the deposited particles were repeatedly impinged upon by the subsequently deposited particles.

From the CUI experiments,the CS Al–Al2O3coatings had good performance in protecting the carbon steel pipe from CUI.During exposure to the corrosive environment,coatings degraded in forms of general thinning,pores and cracks,but the remaining coatings could protect substrate from the corrosive medium as a result of the lamellar microstructures.Additionally,there was no evidence that the addition of α-Al2O3had a detrimental effect on corrosion resistance.An increase in chloride ion concentration accelerated the degradation of the coatings.While under a condition of thermal cycling,the coatings degraded more seriously than isothermal or wet/dry conditions.

Fig.11.EDS analysis of coatings after CUI tests:(a)pure Al coating;EDS analysis of coatings after CUI tests:(b)coating30.EDS analysis of coatings after CUI tests:(c)coating50.

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