Corrosion Behavior Of Weathering Steel With Different Ni Content In Simulated High Humidity And Heat Marine Environment

The marine atmospheric environment is relatively worse, more complex, and the chloride ion concentration is higher, compared with the rural and the industrial atmospheric environment Especially, the marine atmospheric environment of south China sea is not only with a higher chloride ion concentration, but also contains high humidity, high heat, intense radiation and etc.. So the general carbon steel and the low alloy steel used in marine atmospheric environment are not properly functional. The low alloy structural steels used in marine atmospheric environment requires better corrosion resistance as more attentions are gotten by the application of steels in engineering at marine environment in our country. The atmospheric corrosion resistance of weathering steels has been improved with the addition of Ni, Cu, Cr and other alloying elements. Therefore, the establishment of a new type of alloy system of weathering steels has became the study direction of the marine engineering steels. It has a great significance for the marine engineering construction and national defense science and technology of China.Early studies showed that the steel with alloying element, Ni can improve the self corrosion potential, so as to increase its stability. Ni element can be enriched in the rust layer and thin the grain of the rust layer in order to increase density of the rust layer. The presence of Ni also raises the proportion of nanophasic (suerparamagnetic) a-FeOOH in the inner rust layer, so as to block the chloride ions through the protective rust layer. Therefore, it has important practical significance for weathering steels to add Ni to change its marine atmospheric corrosion resistance. However, since the Ni is an expensive metal, it lack of comparison and discussion on the corrosion resistance of weathering steels with different Ni content especially high Ni content in the previous research. Therefore, it is valuable to carry out the study of the effect of Ni on the corrosion resistance of the weathering steels.In this paper, the effect of Ni on the corrosion resistance of low carbon structural steels was studied by a wet/dry cyclic test which can simulate the high humid marine atmosphere. And the influence of the environmental factors on the corrosion resistance of weathering steels with different Ni content were exploded by changing the different test conditions. The iron rust layers formed on weathering steels were analyzed by using SEM, XRD and electrochemical measurements.The results of the periodic immersion wet/dry cyclic corrosion test indicate that the corrosion rate of the test steels shows a gradual decrease with the increase of Ni content. The corrosion resistance of the experimental steels is about twice compare with steel Q450NR1 when the Ni content is over 3%. The density of the rust layer increases with the addition of Ni in the steels according to the observations of SEM and XRD on the rust layer. Although the rust layer of the test steels are all composed by Fe3O4, γ-FeOOH and α-FeOOH phase, the stability and protective rust layer phase α-FeOOH increased and its size became smaller with the increasement of Ni content. The results of electrochemical tests showed that the self corrosion potential of the bare steel is improved with the increasement of Ni content, and rusty steels appear passivation area broaden, the current density decreased and the rust layer resistance increased.By changing the chloride ion concentration in periodic immersion wet/dry cyclic corrosion test, the results indicates that with the chloride ion concentration increases, the corrosion rate of the test steel has increased, and it is harmful for the formation of the dense rust layer.By changing the quantity of irradiation in periodic immersion wet/dry cyclic corrosion test, the resluts indicates that the corrosion rate of two steels are all the largest, and shows a linear growth trend under a half day irradiation conditions. In the absence of irradiation conditions, the corrosion rate of the two test steels in two trials have showed a sharp increase in the early days and flatten in the latter. Under the condition of a full day irradiation, the corrosion rate of the compared steel also shows a linear growth trend, but the steel with 3%Ni increases sharply before flatten out. With the increasement of the irradiation time, the content of α-FeOOH increase gradually.