Comparative study on the corrosion behavior of low alloy steels in sulfuric acid solution

The corrosion behavior of a weathering steel SPA-C and two acid resistance steels(S-TEN1 and ND)in sulfuric acid solution was investigated by using potentiodynamic polarization and weight loss measurement techniques.Both potentiodynamic polarization and weight loss measurement tests indicated that corrosion resistance of these three alloy steels was as follows:ND≈S-TEN1>SPA-C.Surface analysis of the corroded samples conducted after the immersion test indicated that two layers were formed on the substrates,with the outer layer comprising FeSO4·H2O and the inner layer mainly comprising iron oxides.The addition of Cu,Cr,and Sb refined the corrosion products and improved the adhesion ability between the rust layer and the substrate.

acid resistance steel; potentiodynamic polarization test; weight loss measurement; corrosion

1 Introduction

At present,the majority of global energy is consumed by the burning of fossil fuels in the form of coal,oil,and natural gas[1].Exhaust loss accounts for the largest share of about 70%-80% of the total pulverized coal furnace heat loss.However,the exhaust gas temperature is generally 20-30 ℃ higher than the design value due to dew point corrosion and viscosity ash deposition problems[2-3].The dew point corrosion is caused by condensed sulfuric acid formed by combining the oxidation of sulfur in the fuel and H2O vapor at low temperature.The condensed acid reacts with viscosity ash and corrodes the material[4].The sulfuric acid dew point corrosion usually occurs within the low temperature parts of coal or heavy oil fired boilers,such as economizers,air preheaters,chimneys,and ducts,which brings about lost productivity and immense economic loss[4].To solve the problem,it is essential to develop low cost steels with good corrosion resi-stance in high concentrated sulphuric acid.

The most economical way to mitigate acid corrosion of steel is to alloy the steel with a small amount of elements to develop an adherent,protective layer on the steel surface.Weathering steels are designed to possess improved atmospheric corrosion resistance by the addition of Cu,P,Ni,and Cr on the basis of plain carbon steels[5].Simul-taneously,many researchers[6-9]have reported the advantageous impact of Cu,Cr,and Sb on inhibiting the anodic and cathodic reactions of steels in corrosive environments.Hongetal.[6]demonstrated the corrosion rate in sulfuric acid solution is inhibited by the addition of Cu because of a high hydrogen overpotential and prevention of the active dissolution.Parketal.[8]suggested that Cr con-taining steel decreases corrosion damage of 10% H2SO4solution by the formation of Cr oxides in the rust layer.Leetal.[9]suggested that the addition of Sb shows excellent corrosion resistance because of the formation of a protective Sb2O5oxide film on the sur-face of Sb containing steels.Furthermore,the addition of Sb also stimulates the development of a Cu2Sb film on the surface to inhibit the cathodic reaction.

This study aims to determine the corrosion perfor-mance of low alloy steels that are frequently used as weathering steel and acid resistance steel in highly concentrated sulfuric acid.In the present work,SPA-C,S-TEN1 and ND was selected as research specimens.The corrosion resistance of the steel was characterized by potentiodynamic polarization tests,weight loss measurements,surface analysis techniques of X-ray diffraction(XRD),and scanning electron microscopy(SEM).

2 Experiments

2.1 Specimen preparation and test condition

Table 1 shows the chemical compositions of the low alloy steels used for the experiments.SPA-C is a weathering steel while S-TEN1 and ND are acid resistance steels.The specimens were heated to 1 200 ℃ for 1 h,hot-rolled to reduce the steel thickness to 3.0 mm,kept at a coiling temperature of 680 ℃ for 1 h,and lastly cooled in a furnace.In the case of the cold working process,cold rolling was performed at room temperature to obtain a steel thickness of 1.0 mm after the hot working process,and then continuously annealed at 780 ℃ for 80 s.Fig.1 presents the microstructures of SPA-C,S-TEN1,and ND imaged by SEM.All low alloy steels present a typical microstructure of ferrite and pearlite.The pearlite appears as an uninterrupted network and is distributed along the grain boundary.

Table 1 Chemical composition of low alloy steels %

The corrosion samples were cut into pieces of 25 mm×20 mm×1 mm,the surface ground by SiC sandpaper up to 1200 grit size,and then cleaned in an ultrasonic cleaner with ethanol for 10 min.All experiments were conducted in 50 vol.% H2SO4that was prepared by diluting analytical grade 98% H2SO4with deionized water.To ensure reproducibility,at least three measurements were performed for each specimen.

2.2 Potentiodynamic polarization test

Potentiodynamic polarization tests were performed by using an EG&G Model 273A potentiostat in a 50 vol.% H2SO4solution at room temperature.A three-electrode electrochemical system was used with a saturated calomel electrode(SCE)and two pure graphite rods,which were used as the reference and counter electrodes,respectively.A stable open-circuit potential(OCP)was established 1 h prior to testing.The potential of the electrode was swept from an initial potential of -250 mV with respect to the OCP,to a final potential of +100 mV(vs.SCE)at a rate of 0.166 mV/s.

2.3 Weight loss measurements

Weight loss measurements were performed according to the Chinese national standard(JB/T 7901—1999).After calculating the area and measuring the initial weight,the specimens were hung on plastic wire and placed in a 50 vol.% H2SO4solution at 70 ℃ for 72 h.Throughout the corrosion process,the specimens were taken out of the aggressive solution after each 24 h corrosion interval.After the immersion test,the specimens were cleaned with distilled water,immersed in a solution of 500 mL 37%HCl+3.5 g hexamethylene tetramine + balanced distilled water for 5 min,degreased in an ultrasonic cleaner with ethanol for 5 min,dried in hot air,and weighed after being stored in a desiccator for 24 h.The corrosion rate was determined by the weight loss and was calculated from the difference between the initial and final weights.

2.4 Surface analysis

To investigate the relationship between the alloying element components and the surface mor-phology of the corrosion products,the corroded surface was imaged by scanning electron microscopy(SEM)and the composition was investigated by X-ray diffraction(XRD)after 48 h of immersion.Energy dispersive spectrometer(EDS)was used to investigate the cross-section of the corrosion products.

3 Results and discussion

3.1 Potentiodynamic polarization test

Fig.2 shows the potentiodynamic polarization curves for the low alloy steels in the 50 vol.% H2SO4solution at room temperature.All specimens showed a typical polarization curve of activation,sulfation,and passivation,with increasing potential[10].During the activation stage,the current density increased with increasing potential because of the fresh metal surface etched by the aggressive sulfuric acid.This stage involved the reduction of H+to hydrogen and the iron oxidized with the formation of ferrous sulfate(FeSO4),as shown in reaction(1):

H2SO4+Fe→FeSO4+H2

(1)

ThisresultedinthenucleationandgrowthofaporousFeSO4film.Thepassivationstagefollowedthroughoxidationofthesulfatefilmandsubsequentpassivationathigheroverpotential[10].Thecurrentdensitydecreasedsharplyandwasmaintainedatalowpassivecurrentdensity(ip),whichtraditionallywasbelievedtobecontrolledbythepresenceofanadherentandprotectivepassivefilm.

Fig.2 Potentiodynamic polarization curves for the three steels in a 50 vol.% H2SO4solution at room temperature

The corrosion current density was determined by Tafel extrapolation method.The polarization resistance(Rp)value was inferred from linear polarization data[11]:

(2)

whereβaandβcare the anodic and cathodic Tafel slopes,respectively;icorris the corrosion current density of each specimen.

The corrosion rate was inferred from the corrosion current density(icorr),based on Faraday’s law[11]:

(3)

where Misthemolarmassofthemetal,g/mol;icorristhecorrosioncurrentdensity,μA/cm2;zisthenumberofelectronstransferredpermetalatom;FisFaraday’sconstant;andρisthedensityofthemetal,g/cm3.AllpotentiodynamicpolarizationvariablesandresultsaresummarizedinTable2.ThecorrosionpotentialsofS-TEN1andNDarelowerthanthoseofSPA-C.However,thecorrosioncurrentdensities(icorr)andthepassivecurrentdensities(ip)ofNDandS-TEN1werelowerthanthoseofSPA-C.ThelowcurrentdensitieswereconsideredtoberelatedtotheinteractionbetweenCu,Cr,Sb,andFecompoundsthatformedacontinuousrustlayer.Thecorrosionratesofthethreesteelswererankedinthefollowingorder:SPA-C>ND≈S-TEN1.

Table 2 Electrochemical parameters of potentiodynamic polarization tests in a 50 vol.% H2SO4solution at room temperature

SpecimenEcorr/mVicorr/(mA·cm-2)ip/(mA·cm-2)βa/mVβc/mVRp/(Ω·cm-2)Corrosionrate/(mm·a-1)SPA-C-360.90.690.06170.955.726.58.1S-TEN1-413.10.240.0351.739.140.32.8ND-H-402.20.270.0270.447.445.63.2

3.2 Weight loss measurements

Weight loss measurements were performed in a 50 vol.% H2SO4solution at 70 ℃ for 72 h and compared with the potentiodynamic polarization tests.For the weight loss method,the average corrosion rate of the steel was determined by the following expression[11]:

(4)

where8.76×106isthemetricandtimeconversionfactor;Wistheweightloss,kg;Aistheareaofexposure,m2;Tistheimmersiontime,h;andDisthedensity,kg/m3.Table3showstheresultsoftheweightlossmeasurements.Theaveragecorrosionratesofthesteelsinthesulfuricacidsolutionfor48hwererankedinthefollowingorder:SPA-C(4.75mm/a)>S-TEN1 (3.90mm/a)≈ND(3.88mm/a),whichwereinagreementwiththeresultsofpotentiodynamicpolarizationtests.

Table 3 Results of weight loss measurements in a 50 vol.% H2SO4solution at 70 ℃

SpecimenImmersiontime/hCorrosionrate/(mm·a-1)SPA-C246.29484.75723.80S-TEN1483.90ND245.86483.88722.90

Fig.3 illustrates the relationship between the corrosion rate and time.The corrosion rate was quite high in the initial 24 h.In the initial stage of corrosion,the fresh surface was severely attacked by the formation of a large amount of hydrogen,resulting in a high corrosion rate.Then,the corrosion products deposited on the surface of substrate,resulting in the formation of a passive film.The passive film blocked the diffusion of the corrosion medium and might also prevent efficient charge transport[12].Therefore,the corrosion rate declined as the corrosion time increased.

Fig.3 Relationship between the corrosion rate and time in a 50 vol.% H2SO4solution at 70 ℃

3.3 Surface analysis

After a holding period of 48 h in a 50 vol.% H2SO4solution at 70 ℃,the substrates of these three steels were all covered with white-colored rust layers observed by the naked eye.For improved detail,an SEM was used to determine the com-positions of the rust layers.As shown in Figs.4(a) and (b),a porous rust layer with cracks,com-prising plate-shaped particles with different sizes,was imaged on SPA-C.The corroded surfaces of S-TEN1(Figs.4(c) and (d)) and ND(Figs.4(e) and (f)) were much more compact and continuous.However,the rust layer of ND comprised slender,needle-shaped corrosion products that were different from the plate-shaped of SPA-C and S-TEN1.According to XRD analysis,it could be concluded the main components of the rust layers of these three steels were FeSO4·H2O(PDF 45-1365),as shown in Fig.5.Diffraction peaks of the iron matrix in S-TEN1 and ND were also detected and indicated that thicknesses of FeSO4·H2O layers on the surfaces of S-TEN1 and ND were thinner than those of SPA-C.

Fig.5 XRD analysis of corrosion products of the three steels after 48 h of immersion

After removal of the white-colored rust layer by ultrasonic cleaning in ethanol,a dark-colored layer is exposed.Fig.6 shows the SEM images and energy spectrum diagrams of the subsurface layers.In Fig.6(a),a discontinuous FeSO4rust layer is still seen to adhere to the partial surface of the substrate composed of some iron compounds.The FeSO4rust layer is brittle,indicated by cracks,and therefore less protective.The cracks easily allow the solution to penetrate and attack the substrate,and the diffusion of corrosion products from the metal surface toward the bulk solution.Therefore,a loosened,porous film forms on the surface of SPA-C.As shown in Fig.6(b),a compact FeSO4rust layer,comprising plate-shaped particles,remains on the substrate surface enriched with Fe and Cu compounds.Both Fe and Cu dissolve in an acid solution containing dissolved oxygen However,the dissolved Cu redeposits as a precipitate of re-crystallized Cu and Cu oxides[13].The accumulated Cu compound on the surface improves the corrosion resistance of S-TEN1 by reducing the active dissolution of the substrate.Similarly,an aggregate of needle-shaped particles constitute a dense rust layer on the surface of ND.The rust layer comprises iron sulfides and a small amount of chromium compound.Cr is a strong oxide film-forming element and the resulting Cr oxides are less soluble than Fe oxide[14].The addition of Cr promotes the formation of an adherent,protective layer on the steel surface,which can effectively prevent further attack against the metal by a concentrated sulfuric acid solution[8].As a result,the different chara-cteristics of the rust layers are affected by different substrate compositions.

Fig.6 SEM images and EDS analysis results of the surface morphology after removal of the loosened,white-colored corrosion layer

Fig.7 shows the SEM images after removal of the corrosion products from the weight loss measure-ments.When compared with S-TEN1 and ND in Figs.7(b) and (c),the surface of SPA-C was more heavily corroded,as shown in Fig.7(a).The grain boundaries of S-TEN1 and ND were slightly corroded and yet some corrosion products adhered on the surface of ND.The morphology of the sub-strate depended on the protection of the corrosion layer.As shown in Fig.4(a) and Fig.6(a),the loosened,cracked rust layer adhered on the surface of SPA-C could not prevent the attack of the cor-rosive medium effectively.This caused the pit to advance with increasing immersion time.In con-trast,the compact rust layers covering the S-TEN1 and ND substrates provided better inhibition against the attack of the corrosive medium.

3.4 Cross-sectional analysis

EDS mapping was performed to determine the distribution and concentration of the alloying elements in the rust layers after 48 h of immersion.Fig.8 shows the cross-sectional EDS mapping results of the alloying elements.As shown in Fig.8(a),an outer layer,with high S and O concentrations,formed and covered the substrate.This indicated sulfate and sulfide compounds formed in the outer rust layer.As shown in Fig.8(b),enrichment of O and Sb was found in the rust layer on S-TEN1,which indicated Sb oxides,such as Sb2O5and Sb2O3,formed.The following mechanism was proposed by Pitmanetal.[9,15]for the formation of Sb oxides:

2Sb + 3H2O=Sb2O3+ 6H++ 6e

(5)

Sb2O3+2H2O=Sb2O5+ 4H++ 4e

(6)

AsreportedbyLeet al.[9],thedecreaseincorrosionratewasbecauseoftheformationofanSb2O5oxidefilmonthesurfaceoftheSb-con-tainingsteelsinachloride-containingsulfuricacidsolution.Similarly,twolayers,anouterlayercomprisingFeSO4andacompactinnerlayercomprisingFeoxides,werealsofoundonthesurfaceofNDasshowninFig.8(c).However,theexactcompositionsofthecompoundsneedfurtheranalysis.

Thedifferenceincorrosionratewascausedbythedifferenceincompositionandquantityofcorrosionproductsformedonthesteelsurface.TheprecipitationofSb,Cr,andtheiroxideswereaddedtoironoxidesintherustlayerandpromotedtheformationofauniform,compactrustlayer,whicheffectivelypreventedfurtherattackagainstthemetalbyaconcentratedsulfuricacid.ThecorrosionrateinS-TEN1wasstronglyinhibitedbytheformationofSb-incorporatedionoxyhydroxidesintheinnerrustlayer.Furthermore,theformationofCrandSb-incorporatedionoxyhydroxideswithintherustlayerofNDreducedthelevelofcorrosiondamagesignificantly.

4 Conclusions

(1) Potentiodynamic polarization tests and weight loss measurements indicated that the cor-rosion resistance in a 50 vol.% sulfuric acid solution of the three steel was as follows:ND≈S-TEN1>SPA-C.Therefore,S-TEN1 and ND were more suitable for serving in sulfuric acid dew point corrosion environments.

(2) Surface analysis of the corroded surfaces conducted after 48 h of immersion indicated that two layers were formed on the substrate.The outer layer comprised FeSO4·H2O while the inner layer comprised mostly iron oxides.

(3) The addition of Cr and Sb refined the corrosion products,improved the adhesion ability between the rust layer and substrate,and effectively prevented further attack against the metal by a concentrated sulfuric acid.

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LI Yongcan,LIANG Gaofei and YAN Yuanyuan

Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China

LI Yongcan; E-mail: liyongcan@baosteel.com

10.3969/j.issn.1674-3458.2016.04.001