| Due to the economy advantages of the weathering steel and its good quality to resist the atmosphere corrosion,weathering steel has been wildly used in the construction of bridge,architecture,railways and power transmission tower and so on.As we all know,industrial oceanic atmosphere plays an important role in the most developed coastal cities.Therefore,it has a great significance to shed light on the corrosion resistance of the weathering resistance steel under the industrial oceanic atmosphere.This paper is based on the four kinds of weathering steel which have been designed,smelt,and rolled by ourselves with the content of Ni of 0.20%,0.42%,1.50%and 3.55%respectively.Cyclic wet-dry immersion test has been applied to study the corrosive behavior of the Ni containing weathering resistance steel under manufactural industrial ocean atmosphere(the mixed solution with half of the O.lmol/L NaCl and half of the 0.01mol/LNaHSO3 by volume ratio).Subsequently,the microstructure,corrosion kinetics,macroscopic and microscopic morphologies of the rust layer,morphologies and the elements distribution in the cross section of the rust layer,phase composition and its distribution in the rust layer as well as polarization curve and impedance spectroscopy have been deeply analyzed to further insight the corrosion mechanism of this steel and the influence Ni has on the resistance properties of the weathering resistance steel under the industrial oceanic atmosphere.The conclusions are summarized as follows:(1)The major microstructure of the tested sample steel with 0.20%Ni and 0.42%Ni respectively are polygonal ferrite and granular bainite;for samples with 1.50%Ni and 3.55%Ni respectively,there mainly exist quasi-polygonal ferrite,acicular ferrite and granular bainite in the microstructure of the sample;Ni contributes to the transformation from ferrite to bainite.(2)Ni can reduce the corrosion depth and slow down the corrosion rate of the sample steels.The fitting curve through dynamics empirical formula shows that the corrosion rate of the sample steel with 3.55%Ni is at a high speed in the earlier stage,however,in the later period the speed decays the fastest.The result indicates that with the increasing content of Ni,the impedance of the rust layer and the self-corrosion potential will rise,but for the samples with the increasing content of Ni from 1.50%to 3.55%,this principle can only work in the earlier stage while play little effect in the later piorid.(3)During the corrosive time from 144 h to 336 h,the rust layer keeps growing towards the inside direction,and the thickness of the inner rust layer is much greater than the outer one which is made up of ?-FeOOH with compact and stable quality.The inner rust layer is mainly made up of unstable compositions such as ?-FeOOH,?-FeOOH,?-FeOOH and Fe3O4.(4)The corrosive process is as follows:the SO2 in the atmosphere will lead to the formation of hard ?-FeOOH in the surface of the sample steel within a short time,then the outer rust layer forms.The PH value rises in the inner layer which will result in the complicated interconversion between the corrosion products from ?-FeOOH to ?-FeOOH via?-FeOOH.Meanwhile,the distortion energy generated by the difference in volume density will cause cracks in the outer rust layer,which allows acid medium to get across the inner layer to accelerate the formation of ?-FeOOH.Subsequently,the defects of the rust layer will be fixed,leading to the decay of the PH value in the rust layer.These two corrosive process will be conducted alternately,as a result,the defects in the rust layer becomes rare as well as a more compact structure will be obtained.Ni is capable to accelerate the transformation speed of the ?-FeOOH in the inner rust layer and increase the content of ?-FeOOH in t outer layer. |
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