Effect Of Inclusions On Pitting Corrosion In Al-Ti Complex Deoxidation Steel With Magnesium Treatment

As the basis of marine industry development,long service life and safety of corrosion-resistant ocean engineering structural steel is very important.Under the environment with high salt and humidity,pitting induced by inclusions is the most destructive corrosive form to the marine equipment.In this research,Ti-Mg composite deoxidation process was used to reduce the hazard caused by pitting corrosion,through the way of controlling and optimizing the characteristics of inclusions?such as composition,size,distribution and so on?in steel.Experiments were conducted in high frequency induction furnace with Al₂O₃crucible under the protection of Argon atmosphere,five experimental ocean engineering structural steel samples with different deoxidation processes were smelted?Ti/Ti-Mg/Al-Ca/Al-Mg?.The morphology,composition,deformation and distribution of inclusions in the steel samples were characterized by scanning electron microscopy with energy dispersive spectroscopy?SEM-EDS?;and the research on the pitting induced by inclusions in the steel samples was investigated by polarization and corrosion experiment in 3.5%NaCl solution.Through this research,the theoretical basis for the titanium-based composite deoxidation refining process of corrosion-resistant steel can be expanded,which is conducive to the deepening of the theory of inclusion denaturation,promote the coordinated development of metallurgy and corrosion discipline,having great scientific significance.The main conclusions obtained are as follows:?1?Inclusions in the test steel were mainly composite oxides,the composition was related to deoxidation process and a small amount of residual strong deoxidization elements.The inclusions were mainly Al-Ti-Ca-O complex oxides in Ti deoxidized steel,and that were Al-Ti-Mg-O complex oxides in Ti-Mg deoxidized steel;while calcium aluminate oxides were mainly inclusions in Al-Ca deoxidized steel,and that were aluminum-magnesium spinel in Al-Mg deoxidized steel.Compared with Al deoxidized steel,the number density of inclusions in Ti-Mg was higher?more than 500/mm²?,but the average size was smaller?less than 1?m?.In Ti-Mg deoxidized steel,with the increase of Mg content?0-0.0035%?,the Ti content of inclusions decreased,and the number of inclusions increased first and then decreased?from 358/mm²?569/mm²? 508/mm²?,and the size decreased first and then increased?1.1548?m?0.9268?m?0.9720?m?.?2?The results of thermodynamics and experimental observation showed that,MnS precipitated on the surface of the oxide inclusions in heterogeneous nucleation,under the S content level?16 ppm?of this research.Ti-containing oxide inclusions with high number density and small size were effective nucleation cores for MnS,which was conducive to refine MnS.There was no wrapped MnS on the surface of inclusions in Ti deoxidized steel,sulfide was of local dispersion precipitation,the average thickness was 0.5?m;while the sulfide was thicker and coated on the surface of the inclusions in Al deoxidized steel,the average thickness was 1.5?m.?3?The alloying elements and inclusions in steel could affect the corrosion properties.Ti and Ca are beneficial to improve the rigidity of the rust layer,thus,compared with Al deoxidized steel,Ti deoxidized steel possessed better corrosion resistance(The self-corrosion potential(E_corr)of Ti deoxidized steel was around-0.45V and the corrosion weight loss rate?V?was less than 25 g·dm^-2·a^-1,while it was lower than-0.48V and higher than 30 g·dm^-2·a^-1 in Al deoxidized steel);in terms of Al deoxidized steel,Ca treatment process(-0.4869V/31.9549 g·dm^-2·a^-1)was superior to Mg treatment(-0.5162V/39.5450 g·dm^-2·a^-1).For Ti-Mg deoxidized steel,when the content of Mg was at low level?0.002%?,Ti-Mg compound deoxidized steel possessed greatest corrosion resistance(the E_corr and V were-0.4458V/20.9306 g·dm^-2·a^-1,which was the best value).With Mg content increased,the Ti content in inclusions declined and rose in steel matrix.While Mg content was at low level,its decay resistance was greater.It could be concluded that the influence of inclusions to corrosive pitting was superior to steel matrix composition,inclusions modified could improve the corrosion resistance of steel more effectively.?4?The difference in electrochemical stability between the inclusions and the surrounding matrix contributed to the initiation of pitting.Pitting easily initiated around MnS due to its good conductivity,higher electrode potential(E_S/MnS=-0.05V)and corrosion-induced particles such as S^2-and HS^-produced after dissolution.Fine,diffuse oxides could be as the core of heterogeneous nucleation for sulfide,which was helpful to the dispersion,small and morphological control of sulfide,and it was also conducive to reducing the acceleration of corrosion after its dissolution.CaS had a low electrode potential(E_S/CaS=-0.39V)and could be dissolved as an anode in the dissolution to protect the steel matrix.At the same time,the dissolved product was hydrolyzed to produce OH^-,which could slow down the corrosion of acidic materials to steel substrates.Ca was used to generate CaS-based inclusions,it was also beneficial to improve the corrosion resistance of the material.?5?Pitting could also be induced by oxide inclusions,but it was slightly compared with MnS,because the conductivity of oxides was weaker and there was no corrosion-promoting products after dissolved.The TiOx content in the inclusions had a significant effect on the ability to induce pitting,the stability of Ti oxide was better,and it had reducing property.Therefore,for the same kind of oxide inclusions,the higher TiOx content,the weaker the ability to induce pitting.?6?The effect of inclusions'size on pitting was weaker than that of composition,but it could affect the rate of expansion after pitting,and the larger the inclusions were,the larger the corrosion interface was,and the faster the expansion was.In order to improve the corrosion resistance of the material,on the basis of controlling inclusions'ingredients,the size of the inclusions should be as fine as possible.