Research On The Dynamic Migration And Existent Appearance Of Cr Element In Glass-based Coating Melts

During the heating treatment,the high-temperature corrosion reaction between the metal material and the oxidizing atmosphere caused the severe oxidation and burns of steel.It not only greatly reduced the yield of steel and affected the surface quality of steel,but also caused serious waste of resource and energy.The high temperature protecting coating was an effective protective measure with low price and simple operation to reduce the weight loss.The glass-based coating melts could protect the stainless steel substrate effectively.It depended on the formation of Cr enriched oxide layer on the surface of steel,which had a good synergistic effect with the glass-based coating melts.Therefore,a large amount of research work had been carried out on the protection of the surface of carbon steel by using glass-based coating melts with Cr.It was found that a complete Cr enriched layer formed at the interface between steel substrate and glass-based coating melts during the high temperature reaction,which was of great importance to improve the corrosion resisitance of the steel substrate with the glass-based melts.Therefore,the glass-based coating melts with Cr was developed in this paper.The dynamic migration and existent appearance of Cr element in the glass melts during heating process was studied.Meanwhile,the protective mechanism of glass-based coating melts with Cr element for carbon steel was clarified under high temperature oxidizing atmosphere.The results were shown as follows:(1)The high temperature oxidation behavior of several typical carbon steels and stainless steel were carried out to assist the design of glass-based coating melt with Cr element.The high temperature oxidation mass transfer and diffusion coefficient of J55 carbon steel in the range of 800? to 1100? were investigated emphatically.Meanwhile,the effect of oxygen content on the oxidation behavior of J55 carbon steel in the range of lvol%?15vol%was studied.The high-temperature protective mechanism of glass-based coating with Cr2O3 for the 420 stainless steel(containing 12wt%?14wt%Cr)was also certified.(2)The glass-based coating melts with Cr element consisting of Al2O3-SiO2-K2O-Na2O-Cr2O3 was developed.It was based on the matching properties of coating material and steel substrate,and the design principle of low solubility of Cr element in glass material.The glass transition temperature of glass material was 730?,and the thermal expansion coefficient was 8.0×10-6 K-1 at range of 30? to 600 C.The dissolution range of Cr element in glass material was 0.52wt%?0.83wt%from 900 C to 1200?.(3)Cr element enriched layer formed at the interface between 20MnSiNb steel substrate and glass-based coating melts by the SEM-EDS.The experimental results showed that the glass-based coating melts with Cr had good protective properties for 20MnSiNb steel.The anti-oxidation effect was up to 86%at 1050? for 60 min,and which was bigger than 75%from 1000? to 1100? for 120 min.The oxidation rate constant of 20MnSiNb steel was significantly reduced,which was only 1/7-1/10 of blank sample.And the oxidation reaction apparent activation energy had nearly been doubled to 160.06 kJ/mol.It had also been proved that the glass melts with different Cr source and the glass melts changed by the addition of Na,Ti and Sn element could significantly control the migration process of Cr element in glass melt.(4)The dynamic migration and existent appearance of Cr element in Al2O3-SiO2-K2O-Na2O-Cr2O3 glass-base melts were analyzed.The study indicated that the diffusion coefficients of Cr in glass melts was significantly affected by the temperature.It increased from 1.63×10-10 cm2/s to 2.91×10-9 cm2/s at the range of 900? to 11000?,and which increased by 17.85%.The XPS test indicated that the Cr element in the glass melts mainly existed in the form of Cr3+ and Cr4+ at high temperature.(5)The protective mechanism of glass-based coating melts with Cr element for 20MnSiNb steel was clarified.It was might be by the following reseans:first,the dense molten glass-based coating melts blocked the contact between iron and oxygen ions and lelayed the oxidation reaction of the carbon steel;second,the Cr enriched layer formed by the migration and enrichment reaction of Cr element in the glass-based coating melts played an important role in improving the anti-oxidation property of steel substrate;third,the formation of FeCr2O4,(Fe0.6Cr0.4)2O3 and Cr2(FeO2)6 by the Cr,Fe and O elements in the glass-based coating melts effectively reduced the diffusion of oxygen and iron ions.In addition,it was confirmed the glass-based coating melts with Cr element could obviously prevent the formation of oxide layer by the outward diffusion of iron ion.(6)The dynamic behavior and microscopic phase of glass-based coating melt with Cr element was studied.It was found the Cr element had migration and enrichment reaction with the increasing heating time at 800?,and formed complete enriched layer at 800?for 60 min(the relative content of Cr element was up to 60%).The migration and diffusion reactions of Cr element in glass-based coating melts and the formation of Cr enriched interface layer were analyzed from the following multi factor conditions:the formation structure of Fe,Cr and O elements,the solubility of Cr element in silicate glass melts and the formation of new phase by reaction.