REN Yuxia,GUO Tieming1,＊,XU Xiujie,LI Guangming,TANG Jian,NAN Xueli1,,JIA Jiangang1,
(1.State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals,Lanzhou University of Technology,Lanzhou 730050,China;2.School of Materials Science and Engineering,Lanzhou University of Technology,Lanzhou 730050,China;3.Gansu Province Transportation Planning,Surveydesign Institute Co.,Ltd,Lanzhou 730050,China)
Abstract: The corrosion behavior of Q370qNH steel in the presence and absence of hot-rolled oxide scale in simulated industrial atmospheric environment was studied by dry/wet cycle accelerated corrosion experiments.The experimental results show that the corrosion type of bare steel is uneven overall corrosion and large size pitting corrosion in small areas;that of oxide scale sample is local dissolution corrosion and small size pitting corrosion in large areas,and corrosion rate is much smaller than that of bare steel.The corrosion products of both steels are composed of α-FeOOH,γ-FeOOH,Fe2O3,and Fe3O4,but the formation mechanism is different.The bare steel generates α-FeOOH and γ-FeOOH through “acid regeneration cycle mechanism”;the oxide scale sample generates hydroxides mainly through the gradual dissolution of the oxide film,and then through “the acid regeneration cycle mechanism”.With the extension of corrosion time,the electrochemical stability of the sample with oxide scale increases,but the change of tafel curve of bare steel sample is not obvious.In simulated industrial atmosphere,the existence of hot-rolled oxide scale can facilitate the formation of dense rust layer on the surface of Q370qNH steel,which is more protective than bare steel.
Key words: Q370qNH steel;oxide scale;NaHSO3;corrosion behavior
Bridge weathering steel is made by adding trace alloying elements to improve the composition and structure of the rust layer on the steel surface,and the principle of its non-coating use is “rust to prevent rust”,that is,the formation of dense and stable hydroxyl oxides on the surface during long-term use,especiallyα-FeOOH,thus preventing further corrosion of the steel substrate.The hot rolling temperature range of Q370qNH steel is usually from 800 to 1120 ℃,so a layer of hot-rolled oxide scale is inevitably formed on the steel surface,which plays a protective role that can reduce the corrosion loss in the storage and transportation of steel plates due to its high chemical stability[1-4].The physical phase composition of hotrolled oxide scale is usually Fe2O3,Fe3O4,and FeO,which is different from the corrosion products mainly formed by oxyhydroxide formed by weathering steel in the atmosphere.Whether the hot-rolled oxide scale can prevent the corrosion of steel substrate for a long time and how it affects the formation of hydroxyl oxides in different atmospheric environments has attracted more and more attention of bridge workers with the promotion of weathering bridge steel without coating.
At present,the research on the corrosion behavior of steel surface oxide scale at home and abroad is mostly focused on hot-rolled strip steel,pipeline steel,ship plate steel,high-speed steel,etc.There are also some studies on weathering steel,mostly in Clmedium.The mechanism of the influence of oxide scale on corrosion is mainly explained in terms of the thickness of oxide scale,microstructure integrity,pores and other defects[5-7].For example,Sunet al[8]studied the effect of the content of eutectoid structure in mild steel on the corrosion resistance of oxide scale and found that the presence of eutectoid structure was extremely detrimental to the corrosion of oxide scale in NaHSO3solution,and the oxide scale had almost no protective effect on the substrate when the eutectoid structure exceeded 70%.While the oxide scale without eutectoid and proeutectoid reaction had the best atmospheric corrosion resistance.Han et al[4]studied the influence of the structural characteristics of the oxide scale on the surface of weathering steel on simulated marine atmospheric corrosion behavior,and found that the dense oxide scale significantly retarded the corrosion process at the early stage,but it was difficult to transform into corrosion products during the long-term corrosion process and retained in the rust layer as impurities and defects,thus promoting corrosion.Donget al[6]studied the electrochemical corrosion behavior of hot-rolled strip steel in chlorine solution and found that dense oxide scale can reduce steel corrosion.Although there are some studies on the composition and structure,damage mechanism,and corrosion resistance of hot-rolled oxide scale,there are few reports the difference between the evolution mechanism and protection of rust layer caused by oxide scale and that of bare steel when bridge weathering steels are subjected to long-term wet and dry cyclic corrosion in simulated industrial atmospheric media.
The research team studied[9,10]the corrosion behavior of bridge steel with oxide scale in simulated three atmospheric environments,and compared the influence of environmental media on the corrosion resistance of weathering steel.It was found that the oxide scale protects the substrate differently in different media.In this paper,Q370qNH steel was used as the research object to study the influence of the presence of oxide scale on the corrosion behavior and rust layer formation mechanism of weathering steel by comparing the dry and wet cycle corrosion behavior of steel samples with and without oxide scale in NaHSO3medium,so as to provide some reference values for the practical application of bridge steel.
2.1 Experimental materials
The material used in this experiment is Q370qNH steel produced by a large domestic steel mill.After the controlled rolling and cooling process,it was made into 40 mm×40 mm×8 mm large sample and 10 mm×10 mm×8 mm small sample by wire cutting method.The large samples were used to observe macroscopic corrosion morphology and measure corrosion weight gain,and small samples were used to observe microscopic morphology,phase analysis and electrochemical testing.The specific chemical compositions of Q370qNH steel are shown in Table 1.
Table 1 The chemical composition of the Q370qNH/wt%
Table 2 Linear fitting results by double log
The large sample surfaces of the bare steel were sandpapered to 800 mesh from coarse to fine,alcohol scrubbed,acetone degreased,and then the five surfaces were wrapped up with epoxy resin,leaving a smooth upper surface after grinding,dried and weighed,and then put into a vacuum drying oven for backup.
2.2 Wet/dry cyclic corrosion experiments
The experiment simulated the corrosion behavior of bridge weathering steel under the industrial atmosphere in the northwest region through wet/dry cycle corrosion experiments,and the corrosion solution configured was 0.01 mol/L NaHSO3solution according to TB/T2375-1993 “Periodic immersion corrosion test method for railroad weathering steel”.Corrosion cycle to 24h as a cycle,a total of 20 cycles,and samples were taken and observed at 24,72,144,288,and 480 h,respectively.The specific experimental steps are shown in Fig.1.
Fig.1 Experimental flow chart
The corrosion weight gain is calculated by the following formula:
where,ΔWis the corrosion weight,m0the initial mass(mg),mithe mass after thei-th sampling (mg),andSthe corrosion area of the test sample (cm2).
2.3 Analysis of rust layer phase and corrosion morphology
The physical phase composition of the rusted layers of the corrosion samples with and without oxide scale was analyzed using a Rigaku Ultima IV X-ray diffractometer (XRD) with Cu target,tube voltage of 40 kV,tube current of 40 mA,scanning range of 10°-80°,and scanning speed of 4 °/min.The surface macroscopic morphology,microscopic morphology,and cross-sectional morphology of the corrosion samples were observed with a digital camera and a FEG-450 cold field emission scanning electron microscope (SEM).After the samples were descaled with rust remover (500 mL hydrochloric acid+500 mL distilled water+20 g hexamethylenetetramine),the corrosion morphology and the distribution of pits on the surface of both samples were observed and the size and depth of the surface pits were measured by LSM800 laser scanning confocal microscope (LSCM).
2.4 Electrochemical test
The standard three-electrode (Shanghai Chenhua CHI660e electrochemical workstation) system was used for the electrochemical polarization curve test,with Pt electrode as the auxiliary electrode,standard saturated glycerol electrode (SCE) as the reference electrode and corroded specimen as the working electrode.The electrolyte solution was 0.1 mol/L Na2SO4aqueous solution,the scan rate was 0.5 mV/s,and the scan range was -1.5 -1 V.
3.1 Corrosion kinetics
Fig.2 shows the corrosion kinetic curves (Fig.2(a))and corrosion rate curves (Fig.2(b)) of Q370qNH steel with and without oxide scale in 0.01 mol/L NaHSO3corrosion medium.From Fig.2(a) can be seen,the weight gain of bare steel is much greater than the sample of oxide scale,and the corrosion process is divided into two obvious stages: before 96 h,the corrosion weight gain rises slowly;after 96 h,the corrosion weight gain increases rapidly with a linear trend.From Fig.2(b) corrosion rate curve can be seen the corrosion rate with oxide scale sample is gradually declining before 192 h,and the corrosion rate changes a little and remains stable after 192 h.The corrosion rate of bare steel sample changes greatly,showing a downward trend before 96 h,a rapid increase between 96 and 192 h,and a slow increase trend after 192 h,and finally stabilized.
Fig.2 Corrosion kinetics curves of Q370qNH steel with and without oxide scale in NaHSO3 corrosive medium: (a) Weight gain;(b) Corrosion rate
Fig.3 is the fitting curve of corrosion weight gain of the two samples.From the point of view of corrosion kinetics,the atmospheric corrosion of steel follows the law of power function[11-15]: ΔW=A·tn,where,ΔWis the corrosion weight gain per unit area(mg·cm-2),tthe corrosion time (h),Athe corrosion constant,representing the initial corrosion degree,which is related to the material and environment,andna measure of the growth law of corrosion products,reflecting the characteristics of the kinetic process.Whenn>1,kinetics corresponds to the process of increasing corrosion rate,indicating that the rust layer has no protective effect on the substrate,and the corrosion process is accelerated;whenn<1,in contrast,the corrosion process is a deceleration process.Therefore,the smaller the value ofn,the better the corrosion resistance.Table 2 for the fitting results table,in the whole corrosion stage,sample with oxide scale ofnvalue are less than 1,and the 2nd stage ofnvalue is less than the 1st stage after 48 h.The corrosion rate slowes down,indicating that the oxide scale has a certain protective effect on the substrate.The n value of the bare steel sample is less than 1 before 96 h and greater than 1 after 96 h,and the corrosion tendency increases.
Fig.3 Fitting curves of ΔW as a function of log: (a) With oxide scale;(b) Without oxide scale
3.2 Phase analysis of rust layer
Fig.4 shows the XRD patterns of Q370qNH steel corroded in 0.01 mol/L NaHSO3solution for different times.It can be seen from Fig.4(a) that the main phases of oxide scale are Fe2O3and Fe3O4.From Figs.4(b)and (c),the products of the two samples after different corrosion time includeα-FeOOH,γ-FeOOH,Fe2O3,and Fe3O4,and the proportion of various corrosion products in both samples gradually changes with the extension of corrosion time.Fig.5 is the change curve of the strongest peaks ofα-FeOOH,γ-FeOOH and Fe2O3+Fe3O4in the two samples with corrosion time.The content ofα-FeOOH gradually increases in the samples with oxide scale (Fig.5(a)),andγ-FeOOH appears obvious peak after 288 h,while the peak strength of Fe2O3+Fe3O4gradually decreases,indicating that the oxidation products change from Fe2O3and Fe3O4to oxyhydroxide after corrosion.
Fig.4 XRD patterns of Q370qNH steel in NaHSO3 solution after different corrosion time: (a) Original sample of oxide scale;(b) Corrosion sample with oxide scale;(c) Corrosion sample without oxide scale
Fig.5 The time-dependent peaks of α FeOOH,γ-FeOOH,Fe2O3+Fe3O4 in two steels: (a) With oxide scale;(b) Without oxide scale
The characteristic peak of α-Fe (44.29°) of the bare steel at the initial stage(24 h) is particularly obvious (Fig.4(c)),which is caused by the thinner surface rust layer.With the increase of corrosion time,the peaks of corrosion products gradually increase.Different from the sample with oxide scale,the peak ofα-FeOOH appears in the initial corrosion period of bare steel,but the increase of peak strength is not obvious in the later corrosion.
Throughout the corrosion process,with the extension of corrosion time,the content of α-FeOOH andγ-FeOOH gradually increase in both steels,and in the sample with oxide scale is higher than that in the bare steel.Another difference between the two samples is that the peaks of the corrosion products of the bare steel are significantly wider than that of the sample with oxide scale,indicating that the corrosion products of the bare steel have smaller grain sizes.
3.3 Analysis of rust layer morphology
3.3.1 Surface macroscopic morphology
Fig.6 shows the macroscopic corrosion morphology of Q370qNH steel with and without oxide scale after corrosion in NaHSO3medium for different times.It can be seen from Fig.6(a) that the sample with oxide scale undergoes local corrosion at the initial stage (24 h).As the corrosion time increases,the number of corrosion pits increases,and the rust layer is covered unevenly.The surface appears bubbling phenomenon,which is due to the concentration of internal stress.When the fracture strength of the rust layer can not bear the internal stress,the bubbles will fall off,and the fall off at the performance of dark brown.The surface of the bare steel is covered by a yellow rust layer at the initial stage (24 h) (Fig.6(e)).With the progress of corrosion time,a large number of gullies and bulges appeare,the color becomes reddish brown,and accompany by a small amount of black rust layer.The rust layer is uneven.To the late stage (480 h)(Fig.6(h)),the rust layer gradually becames flat and uniform,and the compactness increased.
Fig.6 Macroscopic corrosion morphology of Q370qNH steel with and without oxide scale: (a -d) With oxide scale;(e -h) Without oxide scale;(a,e) 24 h;(b,f) 192 h;(c,g) 288 h;(d,h) 480 h
3.3.2 Surface microscopic morphology
Fig.7 shows the microstructure of bare steel and microscopic morphology of surface oxide scale of Q370qNH steel.It can be seen from Fig.7(a) that the microstructure of Q370qNH steel is mainly bainite after hot rolling,and the microstructure is fine and uniform.From the micro-topography of the oxide scale(Fig.7(b)) that can be seen the surface of the oxide scale is relatively rough,with some defects such as holes and uneven oxidation products,and the defect distribution is uneven.Magnified observation at the uneven oxidation products (Fig.7(c)) shows that the oxides are relatively dense and have low porosity.
Fig.7 Microstructure and surface oxide scale morphology of Q370qNH steel: (a) Metallographic structure;(b) Microstructure of surface oxide;(c) Partial enlargement
Fig.8 is the surface micromorphology of the two samples after corrosion for different times.It can be seen from Fig.8(a) that at the early stage of corrosion(24 h),the oxide film of the steel with oxide scale is partially dissolved.Through magnifying observation of the dissolved part,it is found that granular (α-FeOOH) and petal-shaped needle-like(γ-FeOOH) corrosion products are formed.After 144 h of corrosion,a large area of the oxide film dissolves,the pores appear,and the granular corrosion productsα-FeOOH[16,17]increase,which was consistent with the XRD results.After corrosion for 480 h,the corrosion area increases,fine microcracks appear on the surface,and product is reconstructed.After local amplification of the reconstructed site,it was found that the products are petal-shaped needle-like structure,which isγ-FeOOH[18],and later it will be transformed into α-FeOOH and Fe3O4[19].After magnification observation(Fig.8(c)),it is found that cracks occur between petalshaped needle-like corrosion products,and the cracks crisscross,but were denser and more uniform than at the initial stage,and the stability of the rust layer is improved.At the initial stage of corrosion(24 h)(Fig.8(d)),the surface of the bare steel has obvious delamination.The outside is flake-like,and the inside is also fine granular and petal-shaped needle-like,which areα-FeOOH andγ-FeOOH,accompanying by pits and cracks of different sizes.Corrosion to 144 h (Fig.8(e)),the outer rust layer falls off,a large number of fine granular products are generated on the surface of the sample,and a few holes and larger cracks existed.After corrosion for 480 h (Fig8.(f)),the granular corrosion products increase,and the distribution of the products is uniform.At the same time,the defects decrease,and the density of rust layer increases.Although the corrosion products ofα-FeOOH andγ-FeOOH are formed in both steels,the rust layer in the steel with oxide scale is more dense and uniform than that of bare steel in the later corrosion period,and have fewer defects and more corrosion products than the bare steel.
Fig.8 Microstructure of surface rust layer of Q370qNH steel with and without oxide scale: (a -c) With oxide scale;(d -f) Without oxide scale;(a,d) 24 h;(b,e) 144 h;(c,f) 480 h
3.3.3 Analysis of cross-sectional morphology and element distribution
Fig.9 shows the cross-sectional microscopic morphology of the oxide scale of Q370qNH steel.The average thickness of the oxide scale measured is 0.0518 mm.It can be seen from the figure that the inner layer connecting the oxide scale and the substrate is relatively complete and dense,and the combination with the steel substrate is relatively tight,while the outer layer has defects such as block breaks and cracks.
Fig.9 Microstructure of oxide scale section of Q370qNH steel
Fig.10 shows the cross-sectional morphology of the rust layer of the two steels after corrosion for different times.In the early stage of corrosion(24 h),the oxide scale is locally damaged,from completing to become broken(Fig.10(a)),indicating that the oxide scale occurs in the local dissolution,and the corrosion products are not obvious at this time.After 144 h of corrosion (Fig.10(b)),corrosion products are divided into three layers: loose outer rust layer on the surface,relatively uniform and dense inner rust layer in the middle,and an incompletely dissolved oxide scale between the inner rust layer and the substrate.A small number of cracks exist in the local area of the rust layer,indicating that the internal stress of the rust layer has exceeded the fracture strength of the rust layer.The corrosion products in the late stage of corrosion (480 h)(Fig.10(c)) are still divided into three layers,but the thickness of the dense inner rust layer is significantly increased,the thickness of loose outer rust layer and undissolved oxide scale between rust layer and substrate decreases,and the corrosion layer is relatively dense,but there are still a few defects.Combined with the macroscopic morphology (Fig.6(h)),it can be seen that the corrosion of the oxide scale at 480h is still local corrosion,but the corrosion area is increased,and the observation site is a severe local corrosion site.
Fig.10 Microstructure of cross-section rust layer of Q370qNH steel with and without oxide scale: (a-c) With oxide scale;(d-f) Without oxide scale;(a,d) 24 h;(b,e) 144 h;(c,f) 480 h
Bare steel after 24 h corrosion (Fig.10(d)),the surface forms a relatively uniform thickness of the rust layer,but it can be seen that the rust layer is loose.After 144 h of corrosion (Fig.10(e)),the thickness of rust layer increases obviously,but there are a lot of holes and cracks in the rust layer.In the late stage of corrosion (480 h),the rust layer is divided into obvious inner and outer layers (Fig.10(f)),and the inner rust layer is relatively dense with few defects,while the outer rust layer is relatively loose,while is due to the corrosion resistance elements Cu,Cr,and Ni in the weathering steel are enriched at the interface between the substrate and the rust layer and at the defects of the rust layer,which promotes the formation of the inner rust layer[20-22].It can be seen that the local corrosion layer thickness of the sample with oxide scale is greater than that of the bare steel,but the rust layer is denser than that of the bare steel.
Fig.11 shows the cross-sectional elemental surface sweep of the rust layer of Q370qNH steel with oxide scale sample (Fig.11(a)) and bare sample(Fig.11(b)) corroded for 480 h.It can be seen from the figure that the penetration of element S in the corrosion medium into the bare steel is more serious than that of the steel with oxide scale,indicating that the rust layer density of the sample with oxide scale is better than that of the bare steel.In the steel with oxide scale,the alloying element Cr is mainly concentrated in the inner rust layer at the interface between the substrate and the rust layer,and the distribution is relatively uniform.Ni exists in the whole rust layer,but is locally enriched in the inner rust layer.In bare steel,Cr is also enriched at the inner rust layer,but the distribution is not uniform,and Ni is evenly distributed in the rust layer.The distribution of Cu in the two kinds of rust layers is relatively uniform.
Fig.11 Q370qNH with oxide scale (a) and without oxide scale (b) corrosion of 480 h rust layer section element surface sweep
3.3.4 Analysis of laser scanning confocal corrosion morphology after rust removal
Fig.12 shows the 3D LSCM morphology of the surface of the two samples after corrosion and descaling for 480 h.Scanning the surface along the arrow direction to obtain the corresponding waviness curve below.Table 3 shows the size parameters of the largest pits obtained by scanning.It can be seen that large area dissolved corrosion occurs in the samples with oxide scale,and there are a large number of small corrosion pits in the corrosion parts (Fig.12(a)).The bare steel sample is non-uniform overall corrosion,and there are large corrosion pits in local area (Fig.12(b)).Although the number of pits is small,the depth and width of the pits are larger than those of the oxide scale sample.Combined with the element surface scan in Fig.11,it can be found that the content ofSelement in the rust layer of oxide scale sample is significantly lower than that of bare steel,which indicates that the rust layer density of oxide scale sample is higher than that of bare steel,so it can significantly prevent the penetration of corrosive medium.Therefore,the concentration of corrosive medium on the surface of substrate is low,which shows a small size of small pore corrosion.However,due to the non-compactness of the rust layer of bare steel,the corrosion medium diffuses into the substrate at the defects of the rust layer,resulting in a larger size of the corrosion pits,which further indicates that the existence of oxide scale can promote the formation of dense rust layer.
Fig.12 3D LSCM morphology and waviness curve of the surface after corrosion for 480 h rust removal: (a) With oxide scale;(b) Without oxide scale
3.4 Electrochemical test
Fig.13 and Table 3 show the polarization curves and electrochemical parameters of the samples with oxide scale and bare steel in 0.1 mol/L Na2SO4solution after corrosion for different time.It can be found that the polarization curves of the two samples have a larger difference of change law.
It can be seen from the polarization curve of the sample with oxide scale in Fig.13(a) that as the corrosion time increases,the Tafel curve moves to the lower right obviously.The self-corrosion potential shifts to the right,the self-corrosion current decreases,and the anodic dissolution current density in the steadystate corrosion zone gradually decreases.It can be seen from Table 4 that although the self-corrosion current density of the sample with oxide scale fluctuates,it gradually decreases with the increase of time,indicating that the generated rust layer is stable and compact,which has a certain protective effect on the substrate[23].The electrochemical test results are in good agreement with the cross-section morphology of the rust layer.At the initial stage of corrosion,the slope of cathodic polarization curve is larger and gradually decreases with the extension of corrosion time.This is because the cathodic reaction process is mainly controlled by oxygen dissolution limit diffusion and oxide reduction (Fe2O3+6H++2e-→2Fe2++3H2O;Fe3O4+8H++2e-→3Fe2++4H2O).In the early stage of corrosion,the oxide is in contact with the large areamedium,and its dissolution reduction rate is faster.As the surface is covered by corrosion products (mainly FeOOH),the cathodic reaction process is controlled by the charge transfer of γ-FeOOH reduction instead of oxygen dissolution limit diffusion,and the reaction rate gradually decreases,thus protecting the substrate and inhibiting corrosion[24-26].
Table 3 The pitting size parameters of the two samples after 480 h corrosion/μm
Fig.13 Polarization curves of two steels in 0.1 mol/L Na2SO4 solution after different corrosion time: (a) With oxide scale;(b) Without oxide scale
Table 4 Electrochemical parameter of two steels in 0.1 mol/L Na2SO4 solution after different corrosion time
Unlike the sample with oxide scale,although the Tafel curve of the bare steel sample shifts to the right,the change is not significant (Fig.13(b)).In the early stage of corrosion (24-72 h),the degree of rightward shift of the self-corrosion potential (move 0.0538V to the right) is greater than that of the oxide scale sample(move 0.0186V to the right),but the selfcorrosion potential decreases between 72 h and 288h,and increases slowly between 288 -480 h.The selfcorrosion current density first increases (24-72 h) and then decreases (72-144 h),then increases (144-288 h)and then decreases (288 -480 h),which indicates that the rust layer is loose and unstable in the process of formation in the middle stage of corrosion.In the late stage (480 h),with the formation of dense inner rust layer,the protection of rust layer to substrate is enhanced,and the self-corrosion current density decreases.For bare steel,the cathodic reaction process is mainly controlled by the oxygen dissolution limit diffusion in the early stage,and in the late stage is mainly controlled by the charge transfer ofγ-FeOOH reduction.The slope change of cathodic curve in the early stage of corrosion is not obvious,because there is no the cathodic reaction generated by the reduction of oxide scale.
3.5 Analysis and discussion
In the NaHSO3solution (simulating the SO2environment,the main corrosive gas in the industrial atmosphere),the rust layers of the two steel samples with and without oxide scale are composed of several corrosion products ofα-FeOOH,γ-FeOOH,Fe2O3,and Fe3O4,but the reaction mechanism is different.
In the initial stage of corrosion,there are two processes in the cathodic reaction of wet cycle for steel with oxide scale: one is that the surface oxide directly reacts with acid corrosion medium to cause the following dissolution reaction:
Second,the following cathodic reactions will occur:
The Q370qNH steel used in this experiment adopts controlled rolling and controlled cooling technology (TMCP).According to XRD analysis,the surface has formed an oxide scale structure mainly composed of Fe2O3+Fe3O4.From the surface morphology,it can be seen that the defects such as pores and oxide breakage often occur in the rolling process.The existence of defects provides a channel for the corrosive medium to immerse into the steel substrate.At the same time,as the corrosion progresses,the oxide scale will gradually transform or dissolve,causing local defects such as cracks in the oxide scale,and the corrosive medium invades the steel substrate,the anode reaction occurs on the steel substrate:
At the same time,the HSO3-in the liquid film on the surface of the oxide scale is gradually oxidized to SO42-:
A large amount of Fe2+generated by the dissolution and reduction of oxide scale and anodic reaction are easy to interact with SO42-in corrosive medium to form an “acid regeneration cycle”mechanism[27,28],which accelerates the dissolution of oxide scale,mainly including the following reactions:
As the corrosion enters the drying stage,part of the γ-FeOOH undergoes the reduction reaction:
Reaction (8) mainly occurs at the defects of oxide scale,and with the extension of corrosion time,uneven corrosion will cause potential difference in some areas,thus forming a “big cathode,small anode” corrosion micro cell between oxide scale and defects,which further promotes the development of corrosion in depth.
In the industrial atmosphere,the FeOOH produced is mainlyα-FeOOH andγ-FeOOH,but HSO3-can increase the content of the stable productα-FeOOH,and at the same time,the dissolution of oxide scale promotes the cathode reaction,thereby accelerating the formation of dense rust layers.With the continuous progress of corrosion,the rust layer becomes thicker and gradually stabilizes,so that the resistance of the rust layer increases,the corrosion rate decreases,and the corrosion is inhibited.The corrosion mechanism is shown in Fig.14(a)[19].
Fig.14 Schematic diagram of corrosion mechanism and rust layer development of steel with and without oxide scale in simulated industrial atmosphere
The corrosion of bare steel in NaHSO3solution is different from that of the sample with oxide scale in that the cathodic reaction reduces the dissolution of oxide scale (reactions (1) and (2)),but other reactions are the same as the above reactions (3) to (9),that is,they follow the mechanism of “acid regeneration cycle”.In the early stage of corrosion,due to the sufficient supply of O2and HSO3-on the surface of the bare steel substrate,and the thin rust layer can not effectively prevent the entry of O2and H2O,so the corrosion rate of the sample is higher.As the corrosion process continues,the rust layer gradually becomes thicker,and it becomes difficult for O2and H2O to enter,which makes the later corrosion rate slower than the previous one.The corrosion mechanism of bare steel is shown in Fig.14(b).
It needs to be explained that the corrosion kinetics curve shows that the corrosion weight gain of bare steel is much greater than that of the sample with oxide scale,and from the corrosion thickness can be seen local corrosion thickness of the oxide scale sample is greater than the bare steel,which is mainly for two reasons: first,from the above corrosion mechanism can be seen,part of the corrosion products of oxide scale sample come from the dissolution transformation of oxides;second,the corrosion of bare steel is uneven general corrosion and pitting corrosion,while the corrosion of oxide scale sample is local corrosion.
a) In the NaHSO3corrosive medium,the corrosion of Q370qNH bare steel is uneven overall corrosion and small area and large size pitting,the initial corrosion rate is small,and the later corrosion trend increases;The corrosion of steel with hot-rolled oxide scale is local dissolved corrosion and large area and small size pitting corrosion,and the corrosion weight gain is much smaller than the bare steel.From the appearance of rust removal,although the number of pits of bare steel is small,the depth and width of pits are larger than that of the oxide scale sample.
b) In the NaHSO3corrosion medium,the corrosion products of the two steels are composed ofα-FeOOH,γ-FeOOH,Fe2O3and Fe3O4,but the formation mechanism is different.Bare steel generatesα-FeOOH andγ-FeOOH through the mechanism of “acid regeneration cycle”.Oxide scale sample mainly generates hydroxyl oxides through the gradual dissolution of oxide film,and then through the mechanism of “acid regeneration cycle”.
c) In the simulated industrial atmospheric environment,with the extension of corrosion time,the self-corrosion potential of the sample with oxide scale obviously shifts to the right,the self-corrosion current decreases,and the anodic dissolution current density in the steady-state corrosion zone gradually decreases.Although the Tafel curve of bare steel moves to the right,the change is not significant.Because the “acid regeneration cycle” reaction accelerates the transformation of oxide scale to rust layer,the existence of hot-rolled oxide scale can promote the formation of dense rust layer on the surface of Q370qNH steel and effectively prevent the penetration of corrosive medium,and the rust layer is more protective than bare steel.