The Protective Ability Of Pure Iron Oxides Film To Low Carbon Steel And Weathering Steel

The surface of structural steel is readily corroded by exposure to the air, and the corrosion products film formed on the steel surface consists of iron oxides, such as ferric oxyhydroxides and hydroxides. Corrosion behavior of steel is strongly affected by the existence of corrosion products formed on the steel surface. In order to understanding the protective effect of the rust layer and the contribution of the pure oxide in the rust layer to the protection of the rust, the pureγ-FeOOH andα-FeOOH film were prepared in situ on Q235 and Cor-TenA by chemical methods. The corrosion behaviors of Q235 with pureγ-FeOOH andα-FeOOH film, and Cor-TenA with pureγ-FeOOH andα-FeOOH film have been studied in aqueous solution containing Cl- and in aqueous solution without Cl- at normal temperature. The influence of pureγ-FeOOH andα-FeOOH film on the corrosion behavior of Q235 and Cor-TenA had been studied in details under different corrosive environments. These results are very useful for people to understand the protective effect of purey-FeOOH andα-FeOOH film and the reciprocal effect of pure oxides, and afford the theoretical base to understand the protective performance of the corrosion products on the surface of steel and predict the life of the steel.The electrochemical corrosion behavior of Q235 with pureγ-FeOOH film and pureα-FeOOH film has been studied in 0.25M Na2SO4 solution respectively. The pureγ-FeOOH film developed in situ on Q235 accelarated the corrosion rate of the steel because ofγ-FeOOH taking part in the cathodic reaction, so pureγ-FeOOH film dose not provide protection. Pureα-FeOOH film developed in situ on Q235 is the porous structure. The porous film has strong reactive action. The corrosion process was controlled by the diffusion process of the corrosive species penetrating the porous rust layers to the substrate where corrosion reactions were promoted. Pureα-FeOOH film taked part in the cathodic reaction too. It is apparent that the pure rust layers on Q235 provide little or no protection to the substrate in aqueous solution without Cl-.The electrochemical corrosion behaviors of Q235 with pureγ-FeOOH film and pureα-FeOOH film have been studied in 0.25mol/L Na2SO4+10-4 mol/L NaCl,0.25mol/L Na2SO4+10-3 mol/L NaCl and 0.25mol/L Na2SO4+10-2 mol/L NaCl solution respectively. Pureγ-FeOOH film developed in situ on Q235 promoted the cathodic reaction, and the cathodic current density decreased strongly with Cl- concentration increasing, that is the cathodic reactin was depressed with Cl- concentration increasing. The different Cl-concentration resulted in the difference of the composition in rust film after the cathodic reaction performed. Pureγ-FeOOH film growed in situ on Q235 provides no protection to the substrates in lower Cl- concentration solution. When Cl- concentration is 10-2 mol/L, Na ofγ-FeOOH film decreased greatly, pureγ-FeOOH film provides protective quality to the substrates. Wheras the electrochemical corrosion process of pureα-FeOOH developed in situ on Q235 is different from that of pureγ-FeOOH film developed in situ on Q235. Although the cathodic current density decreased strongly with Cl- concentration increasing, and Na ofγ-FeOOH film decreased greatly when Cl- concentration is 10-2 mol/L, pureα-FeOOH film provides no protective ability to the substrates in the solution containing Cl-.The alloying additions in Cor-TenA result in pureγ-FeOOH film on Cor-TenA that is denser than that on Q235, and pureα-FeOOH film on Cor-TenA is not enhanced by the alloying elements. The electrochemical corrosion behavior of Cor-TenA with pureγ-FeOOH film and pureα-FeOOH film has been studied in 0.25mol/LNa2SO4 solution respectively. The cathodic reaction of pureγ-FeOOH film growed in situ on Q235 was depressed because of the alloying elements affected. Only partγ-FeOOH changed intoα-FeOOH and Fe3O4. Pureγ-FeOOH film on Cor-TenA provides protection to the substrate in aqueous solution without Cl-. Pureα-FeOOH film on Cor-TenA is more porous than that on Q235 because of the alloying elements affecting. As a result of the poor protection byα-FeOOH film, serious corrosion of the substrate occurred.The electrochemical corrosion behaviors of Cor-TenA with pureγ-FeOOH film and pureα-FeOOH film have been studied in 0.25mol/L Na2SO4+10-4 mol/L NaCl,0.25 mol/L Na2SO4+10-3 mol/L NaCl, and 0.25 mol/L Na2SO4+10-2 mol/L NaCl solution respectively. The semiconductive type of pureγ-FeOOH film in solution containing Cl- changed from p-typeγ-FeOOH film growed in situ on Q235 to n-type that growed in situ on Cor-TenA, because of the alloying elements affected. The electrochemistry ofγ-FeOOH andα-FeOOH was depressed, and the composition and structure of rust film were maintained, pureγ-FeOOH film and pureα-FeOOH film provide no protective ability to the substrates in the solution containing Cl-.That the protection of rust layer contributing toα-FeOOH is not correct. All the results indicated thatα-FeOOH is not the key cause that the rust layers formed in nature provide the protective ability. At atmospheric corrosion, thatγ-FeOOH transformed toα-FeOOH in the rust film is not the basic cause that the films provide the protective ability to the substrate. The protective ability of corrosion product is related not only in the structure, composition and coexisting of oxides in film, but also in the alloying elements in substrate under the film.