Comparison of corrosion resistance between nodular cast iron and gray cast iron

The mass fraction of carbon in iron is higher than 2%, while that of silicon is between 1% and 3%. Cast iron is widely used in industry because of its low cost, easy pouring, low notch sensitivity, good wear resistance and shock absorption, and corrosion resistance to some media. With its excellent casting, processing and comprehensive mechanical properties, the new ductile iron has become the first choice material for various complex structures such as valves, pumps, pipes, etc. Sea water is a natural electrolyte with high salt content, and it is one of the most corrosive media in nature. There are many factors influencing the corrosion of cast iron in seawater, including the physical and chemical properties and microstructure of the material itself, such as graphite morphology and matrix structure. External environmental factors such as oxygen content, temperature, velocity, pH and marine life. The corrosion properties of common gray cast iron and nodular cast iron under static simulated sea water condition are shown by experiments. On this basis, the corrosion mechanism is analyzed, which provides theoretical and practical basis for material selection and application of cast iron in sea water environment.

The main composition, matrix structure and graphite morphology of common gray cast iron and a new type of nodular cast iron were selected.

The size, specification and corrosion data of coupon test sample are determined according to GJB / T 7901-1999. The test period is 180 days, and the simulated seawater used in the test is 3% (in mass fraction) sodium chloride solution, which is replaced every 10 days. All corrosion data are the average values of three samples of the same material. The corrosion resistance of cast iron is determined by the corrosion rate of the samples.

Polarization curve and AC impedance were measured by cs350 electrochemical workstation produced by Wuhan Kesite Instrument Co., Ltd. The common three electrode system was used in the experiment, the reference electrode was saturated calomel electrode with capillary glass tube, the auxiliary electrode was platinum sheet, and the working electrode was made of test material. The electrochemical test was carried out in a constant temperature water bath at 25 ℃. The scanning range of linear polarization test is – 0.20 ~ 0.20 V (relative to open circuit potential), the rate is 1.00 MV / s, the sampling frequency is 2.0 Hz, and the analysis software is CView 2.0. The frequency of AC impedance test is 0.1 ~ 100 000 Hz, the amplitude of AC signal is 10 MV, and the analysis software is zview 3.0.

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