Study On The Adhesion And Sulfuric Acid Dew Point Corrosion Resistance Of Enamel Coating On BTC1 Steel

Facing the increasingly serious problems of environmental pollution,China has set strict standards for thermal power station in terms of the permitted emission amount of atmospheric pollutants.To meet these standards,most of the thermal power stations in China have built flue gas desulfurization systems(DSS)by the way of adding or synchronizing,and the gas-gas heat exchanger is an important part of the system.At the same time,buring of sulfur-containing fuels will produce SO2 and SO3.These acid gases will combine with water in the flue gas to form sulfuric acid vapor.Then these vapors will condense in the cold section of the heat exchanger to form sulfuric acid droplets or liquid films,which will cause serious corrosion of the equipment.In addition to choosing corrosion-resistant materials,the application of chemical inert coatings on the surface of workpieces is the ordinary practice in industry,among which borosilicate enamel coating attracts much attention.It has the characteristics of high chemical stability,good bonding with steel,convenient preparation and low cost.Little is known,however,about adherence of enamels to low carbon steel,as well as the corrosion mechanism of enamel coating in sulfuric acid dew point corrosion environment.Although domestic researchers have done a lot of research on enameled heat exchangers,the preparation technology and the protective performance of enamel coating needs to be improved.Therefore,it is of great scientific significance and application value to study the bonding between enamel coating and matrix and its corrosion behavior in this special environment.The effect of adherence promotors on the adhesion of enamel coating was studied.In this thesis,a basic enamel based on SiO2-B2O3-Al2O3-CaF2-R2O(R=Na and K)system was designed,and two kinds of adherence promotors(NiO and CoO)were introduced,respectively.The effects of two kinds of oxide on the adhesion properties of enamel coatings were studied.The results show that NiO and CoO can produce Fe-Ni and Fe-Co precipitates at the interface between the coating and the substrate during the firing of enamel coatings,which can significantly improve the adhesion of enamel coatings.The effect of CoO and NiO on the improvement of the adherence of enamel coatings is different.The direct reason is the difference of precipitation morphology of microalloys at the interface.The underlying physical mechanism is that the reaction kinetics of CoO and NiO with FeO is different.For NiO-bearing enamel,a large number of Fe-Ni sub-micron particles were formed at the interface between the coating and the substrate,and gradually grew into microalloy sheets parallel to the surface of the substrate during the firing process.With the increase of firing time,these microalloys connect with the matrix to form anchor structure,which improves the adhesion of the coating.For the enamel coatings with CoO,a small number of Fe-Co microalloys with large size were formed at the initial stage of firing,and then these microalloys precipitated vertically and grew together with the matrix.Finally,a dendritic structure that perpendicular to the surface of the matrix was formed,which greatly improved the interface roughness.The effect of network former on sulfuric acid dew point corrosion resistance of enamel was studied.Corrosion of three bulk enamels with different silica contents in sulfuric acid was studied.Catastrophic corrosion happened for enamel with 54.6 wt.%silica.Its weight loss reached 36.90 mg/cm2 after seven-day corrosion.A porous leached layer with tremendous cracks formed at surface during corrosion,which favored acid invading and further corrosion.On the contrary,the durability of enamels with silica content over 60 wt.%was guaranteed by their highly connected silicate network.Their weight losses were less than 0.2 mg/cm2 after seven-day corrosion.The effect of emulsifier on the adhesion and acid-alkali corrosion resistance of enamel coatings was studied.The effect of element Ti on adherence between a CoO-bearing single-layer enamel coating and steel was investigated.Falling-weights tests were carried out.Results show that addition of Ti02 reduced adherence of the enamel coating by hindering the formation of anchor-like alloy precipitates.The reduction of anchor points lies in the following three reasons:(1)The diffusion path of Co2+ ions to the interface was lengthened because of the blocking effect of rutile and the FeTi03 crystals;(2)Formation of CoTiO3 crystals leads to a reduction of free Co2+ions;(3)Co-Fe precipitates form away from the enamel/substrate interface,as FeTiO3 crystals provide extra surface for the nucleation of Fe-Co alloy precipitates.Corrosion of three enamels with different ZrO2 content in H2SO4 and NaOH solution was studied.Results shows that in a wet-dry cycle sulfuric acid corrosion environment,cracking occurs in the gel layer formed on the enamel surface,so that the gel layer cannot be used as an effective diffusion barrier.The addition of ZrO2 reduced acid resistance of enamels.Especially for enamel with 5 wt.%zirconia,the weight loss reached 4.04 mg/cm2 after 30-day corrosion.On the contrary,substituting ZrO2 for SiO2 greatly improved the alkali resistance of enamel,since the weight loss of enamel with ZrO2 was only 0.3 mg/cm2 after 20-hour corrosion,which decreased to less than one third of that of enamel without ZrO2.