Study On Fe(Ⅱ) Catalyzing Phase Transformation Of Steel Corrosion Product

Iron and steel, as one of the basic pillars of the national economy, plays an important role in the modern industrial and agricultural production, national defense construction, communication and transportation and people’s daily lives. Steel can be divided two classes: carbon steel and weathering steel according to its chemical composition. Due to external environmental factors, such as time and humidity and so on, iron and steel produces corrosion problems in the process of service. The corrosion products of iron and steel are different in various environments. Fe（Ⅱ） ions play a promoting role in the process of γ-FeOOH--the initial product of iron and steel corrosion in rural environment transforming into a more stable phase, such as α-FeOOH, Fe₃O₄. Both Fe（Ⅱ） ions in the electrolyte film on the surface of steel and Cl- in the environment can promote the transformation from β-FeOOH, the main product in marine environment to the more stable phase, such as α-FeOOH, Fe₃O₄. The formation of α-FeOOH can reduce the corrosion rate of iron and steel. In the industrial environment, the sulfide, nitrogen oxides and other substances in the atmosphere can oxidize Fe（Ⅱ） ions to generate δ-Fe OOH rapidly. With the further corrosion, γ-FeOOH, α-FeOOH, α-Fe2O3 and other composition appeared in the corrosion product gradually. Fe（Ⅱ） ions play an promoting role in process of the transformation of δ-FeOOH and the generationg of γ-FeOOH、α-FeOOH and other composition. In this paper, the effect and mechanism of catalytic conversion of the Fe（Ⅱ） ions in the electrolyte film on the surface of steel on γ-FeOOH, the initial productof steel corrosion, and the effect of Fe（Ⅱ） ions on the composition of rust in different environment were studied.People can realize the rule of steel corrosion and corrosion mechanism by studying the mechanism of Fe（Ⅱ） ions on the formation of rust layer in the process of corrosion. And it also can provide a theoretical basis for steel anti-corrosion or slowing down steel corrosion. The main results of this paper include:（1）The effect of Fe（Ⅱ） ions on the lepidocrocite-to-goethite transformation and magnetite formation in atmospheric corrosion have been investigated. Fe（Ⅱ） was added into the initial corrosion products of a γ-FeOOH suspension at pH 7.0 and 30 °C to simulate the further reaction. Results indicate that Fe（Ⅱ） ions accelerated the dissolution of lepidocrocite. Both dissolution–reprecipitation and the oriented attachment mechanism play important roles in the lepidocrocite-to-goethite transformation. With an increase in the quantity of Fe（Ⅱ） ions, magnetite was formed by the reaction of Fe（Ⅱ） ions and lepidocrocite. Magnetite formation is classified as a topotactic transformation.（2） Rearching the effect of marine environment on the corrosion products of iron and steel. The products of steel corrosion include γ-FeOOH, α-FeOOH, β-FeOOH in marine environment. γ-FeOOH was aging in 3.5% of the NaCl solution for 5 d, and α-FeOOH appeared in the product. Cl- accelerated the conversion from γ-FeOOH to α-FeOOH. In the presence of Fe（Ⅱ） ions in the reaction system, γ-FeOOH was aging in 1.5%, 2.5%, 3.5% of the NaCl solutions, the productsincluding γ-FeOOH, α-FeOOH andβ-FeOOH. The reason for the phase transition may be that electron transfer between Fe（Ⅱ） ions and γ-FeOOH and produce a new Fe（III） ion. Part of the new-born Fe（III） ions hydrolyzed to β-FeOOH in rich Cl- environment.In marine environment, β-FeOOH was the initial product. Fe（Ⅱ） ions accelerated the phase transition ofβ-FeOOH. There were α-FeOOH and Fe₃O₄ appeared in the product with R=0.06. When the concentration of Fe（Ⅱ） ion was increased, the content ofFe₃O₄ in the product was also increased, but the main product was still α-FeOOH. The reason was that electron transfer between Fe（Ⅱ） ion and β-FeOOH produced new Fe（III） ion. New-borned Fe（III） ions hydrolyzed to produce α-FeOOH and a small amount of Fe₃O₄.（3） Rearching the effect of Fe（Ⅱ） ions on the stability of δ-FeOOH. The Fe（Ⅱ） ions effecting on δ-FeOOH was simulatedin natural environment. The results show that, during the experimental period, part of δ-FeOOH transformed to γ-FeOOH spontaneous due to poor crystallization. Fe（Ⅱ） ions could accelerate the dissolution of δ-FeOOH, and the products wereα-FeOOH, γ-FeOOH and δ-FeOOH. When R=0.4, the productswere α-FeOOH and γ-FeOOH. And it was found that increasing temperature was beneficial to accelerate the phase transformation of δ-FeOOH. The research showed that there was no significant difference in the aging process of pH in the range of 6-9.δ-FeOOH transformed to γ-FeOOH then α-FeOOH. The reason was that at the beginning of aging,Fe（III） ion in the solution was low, and γ-FeOOHwas more stable with smaller particles, so γ-FeOOHwas first formed bylow molecular weight substances directly precipitating.Then larger particles of α-FeOOH aggregated.