Study On The Initial Rust Evolution Of Mild Steel And Weathering Steel In Typical Atmospheric Environments

Atmospheric corrosion of steel structures seriously reduces the service life of facilities and brings a substantial loss,thus it is important to improve the corrosion resistance of the steels in their service atmospheric environments.The evaluation of corrosion resistance of steel and the researches on the composition of protective rust layer are often based on long-term outdoor exposure and indoor accelerated corrosion experiments,However,little attention has been paid to the initial corrosion evolution of steel in atmospheric corrosion,and there is no systematic study on the initial dynamic evolution and rust layer evolution of steel in typical atmospheres.Time can greatly affect the composition and structure of the rust layer,and even determine the appearance or disappearance of intermediate or secondary compounds,which may directly affect the stability and the density of rust layer in the later period.However,at the initial stage of atmospheric corrosion,the crystallinity of corrosion products is poor,and there are a lot of amorphous phases.As a mean of bridging these gaps in knowledge,this research selected some widely used low-carbon steel(Q235B)and low-alloy steels(Q420NH,16Mn,MnCu,MnCuP)as test materials.Using cyclic corrosion test(CCT),X-ray diffraction(XRD),scanning electron microscope(SEM),X-ray photoelectron spectroscopy(XPS),Raman spectroscopy,electron probe microanalyzer(EPMA),electrochemical method and other relevant techniques,the initial dynamic evolution,rust evolution and corrosion process of Q235B steel and Q420NH steel in typical coastal,industrial and coastal-industrial atmospheric environments were systematically discussed and the relationship between corrosion resistance and rust evolution was established.Furthermore,in simulated coastal-industrial atmosphere,the influence of SO2 concentration on the evolution of the rust layer formed on Q420NH steel and the influence of alloying element Cu/P on the composition of the rust layer formed on low alloy steel were explored.The research findings provide necessary theoretical basis for the development of weathering steel and the stabilization treatment of rust layer.During the CCT test of 60 cycles,the initial atmospheric corrosion kinetics of Q235B steel and Q420NH steel sectionally conformed to the double logarithmic law due to the oxidation component β-FeOOH/β-FeOOH in corrosion products participating in the cathodic reduction process.In the simulated coastal atmosphere,in the early stage,the inner rust layer of Q235B steel was mainly composed of γ-Fe2O3 and a small amount of β-FeOOH and Fe3O4,and the outer rust layer was composed ofγ-FeOOH.The inner rust layer of Q420NH steel was mainly composed of γ-Fe2O3,ferrihydrite and sporadic distributed α-FeOOH,and the outer rust layer was composed of β-FeOOH and γ-FeOOH.With the increase of CCT cycle,the content of the stable component α-FeOOH in the rust layer increased.α-FeOOH was only detected in the middle of the rust layer of Q235B steel,while the rust layer components near the substrate were still Fe3O4 and γ-Fe2O3.However,in addition to γ-Fe2O3 and ferrihydrite in the inner rust layer of Q420NH steel,α-FeOOH was detected at different depths of the rust layer,which may have resulted in the corrosion rate of Q420NH steel being lower than that of Q235B steel.In the simulated industrial atmosphere,in the early stage,a large amount of Fe3O4 distributed in the inner rust layer of Q235B steel.These provided sites for electron transfer,which was conducive to cathodic reduction reaction and thus promoted corrosion.In addition,the inner rust layer also contained a small amount of α-FeOOH,and the outer rust layer mainly consisted of γ-FeOOH.The inner rust layer of Q420NH steel was mainly α-FeOOH and ferrihydrite,of which rust reduction resistance was large,inhibiting the cathodic rust reduction process.With the increase of CCT cycle,more stable component αFeOOH accumulated in the inner rust layer.Compared with Q235B steel,Q420NH steel was more likely to form an inner rust layer containing a large amount of αFeOOH,which made the rust layer of Q420NH steel much more protective than that of Q235B steel.Under the simulated coastal-industrial atmosphere,in the early stage,the inner rust layer with γ-Fe2O3 and Fe3O4 as the main components and the outer rust layer with γ-FeOOH as the main component were formed on the surface of Q235B steel.The main components of the inner rust layer of Q420NH steel were ferrihydrite,γ-Fe2O3 and α-FeOOH.Later,with the increase of CCT cycle,the inner rust layer of Q235B steel and Q420NH steel gradually evolved into γ-Fe2O3 and α-FeOOH.However,the accumulation of α-FeOOH in the rust layer of Q420NH steel was obviously more than that of Q235B steel,and the existence of α-FeOOH was detected at almost all depths of the rust layer of Q420NH steel.The influence of SO2 deposition on the evolution of weathering steel rust layer in coastal-industrial atmosphere was investigated by adjusting the Na2SO3 concentration in the simulated solutions.After 1 CCT,compared with the corrosion in the simulated coastal atmosphere,the existence of SO32-in the thin liquid film in the simulated coastal-industrial atmosphere destroyed the coordination of Fe3+with Cl-and OH-,thus inhibiting the formation and crystallization of β-FeOOH and γ-FeOOH in the corrosion products.At the same time,the low concentration of SO32-promoted the formation of α-FeOOH in the corrosion products.The higher concentration of SO32promoted the formation of Fe8O8(OH)6(SO4)·nH2O and Na21Fe2+(SO4)10Cl3 in the corrosion products,which were only the intermediate phases in the evolution of atmospheric corrosion and rapidly transformed into α-FeOOH in the subsequent corrosion process.Therefore,from the long-term corrosion evolution point of view,the influence of SO32-on the composition of the rust layer is that it promoted the formation of α-FeOOH and inhibited the formation of γ-FeOOH and β-FeOOH,and this promotion/inhibition effect was more obvious with the increase of SO32concentration.Using 16Mn steel,MnCu steel and MnCuP steel,the effects of alloying elements Cu and P on the composition of the initial rust layer and distribution of phases of weathering steel in a simulated coastal-industrial atmosphere were investigated.The addition of Cu promoted the formation of the stable component α-FeOOH in the inner rust layer of weathering steel.The synergistic effect of Cu and P further promoted the formation of α-FeOOH in the rust layer of MnCuP steel,which was mainly distributed in the dense inner rust layer,and inhibited the formation of Fe3O4/γ-Fe2O3,making the rust layer more protective.The corrosion rate of weathering steel containing Cu was lower than that of 16Mn steel,and the addition of Cu/P obviously inhibited the anodic dissolution and cathodic reduction process of the rusted electrodes of weathering steel.