Rust Formation Behavior and Mechanism of Q235 Carbon Steel in 5％NaCl Salt Spray Under Elastic Tensile Stress
As a structural steel material,carbon steel bears a certain extent of elastic tensile stress in actual service.Elastic tensile stress on steel is supposed to impact the electrochemical process and corrosion behavior,which may further influence the rusting behavior and the phase composition and structure of the formed rust layer.However,stresses on the steel substrate slightly influence the rust layer of carbon steel because no intrinsic change exists in the corrosion mechanism.Here,a remarkable effect of elastic tensile stress on Q235 carbon steel was found on the phase composition and structure of the rust layer formed in 5％NaCl salt spray.The effect on the rust layer was studied using SEM,XRD,and electrochemical impedance spectroscopy.The neutral salt spray test with four-point bending was used to preform the rust layer of Q235 steel under various stress levels.The results show that the elastic tensile stress accelerates the anodic dissolution,thereby promoting the generation of γ-FeOOH,which occurs faster in the electrolyte than the transformation of γ-FeOOH to α-FeOOH and Fe3O4/y-Fe2O3 in the solid-liquid interface.Consequently,the mass fraction of γ-FeOOH in the rust layer increases as the stress lev-el increases,whereas the mass fraction of α-FeOOH and Fe3O4/γ-Fe2O3 decreases accordingly.As the stress increases from 0 to 0.95σs(σs is yield strength),the mass fraction of Fe3O4/γ-Fe2O3 decreases from 53％to～46％,the mass fraction of α-FeOOH decreases from～30％to～23％,and the mass fraction of γ-FeOOH increases from less than 17％to～31％.Meanwhile,the phase composition change decreases the density and increases the thickness of the rust layer.Additionally,the acceleration of the anodic disso-lution induced by the elastic tensile stress promotes the growth of the rust layer,which further increases the thickness of the rust layer.The increase in thickness and decrease in compactness of the rust layer jointly enhance the protective capability of the rust layer.The former increases the resistance to the elec-tromigration of ions through the rust layer,and the latter mitigates the occlusion effect under the rust layer.
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