Research On Microstructure And Properties Of Seawater Corrosion Resistant Low-alloy Cast Iron

With the increasing demand for water pipelines and other seawater corrosion resistant materials in coastal area, higher requirements to the properties of seawater corrosion resistant materials have been put forward. The strength, toughness and seawater corrosion resistance of cast iron can be improved by spheroidizing the internal graphite. The microstructure and further the corrosion resistance and mechanical properties of ductile iron can also be improved by microalloying Al, Cr, Cu and Ni.In this study, four kinds of ductile irons are prepared:with1%Ni and no Cr, Cu or Al, with2%Ni and no Cr, Cu or Al, with Cr, Cu, Al and no Ni, and with Cr, Cu, Al and1%Ni. The microstructures, mechanical properties and seawater corrosion resistances of these four kinds of alloys are compared with those of industrial gray iron and ductile iron. The optical microscope is used to observe the microstructures including the shape, size as well as distribution of the graphite and the composition of the matrix. The mechanical properties are detected by tensile and hardness tests. Besides, SEM and EPMA are used to observe the surface and cross-section of the samples after corrosion, and also the element enrichment. The compositions of the samples after corrosion are analyzed by XRD. In addition, the polarization curve is introduced to study the effect of microalloying on the microstructures, properties and corrosion resistance performance of low-alloy cast iron. The main conclusions are as follows:(1) The low-alloy ductile iron with the alloying element component of0.6%Cr、0.5%Cu、0.8%Al and1%Ni processes the optimal microstructure, properties and corrosion resistance performance.(2) With the increasing of Ni from1%to2%in the low-alloy ductile iron, the matrix structure is improved, and further the electrode potential, hardness, tensile strength and corrosion resistance performance are all improved.(3) The addition of Cu and Cr can increase the amount of pearlite and relatively decrease the amount of ferrite. Besides, the pearlite structure is stabilized and the hardness and tensile strength are increased. The Cu addition increases the electrode potential of the substrate, reduces the corrosion current and improves the corrosion resistance, while Cr with strong passivation results in firm and dense oxide film on the metal surface. The film is closely located on the rust layer, which increases the layer density and prevents the contact between etching solution and the metal substrate. This greatly reduces the corrosion rate and increases corrosion resistance of cast iron. Because Al is easy to form Al2O3protective film on the surface, it improves the corrosion resistance of the substrate a lot.In conclusion, a moderate amount of Cu (0.5%), Al (0.8%), Cr (0.6%) and1%Ni addition can enhance the sea water corrosion resistance, and it can take place2%Ni cast iron. Therefore, the consumption of Ni and then the production cost of cast iron with good seawater corrosion resistance can be reduced.