Study on Corrosion and Growth Resistance of Ductile Iron

Automotive exhaust is mainly composed of CO, CH and NOX. It directly discharges into the air not only pollutes the atmospheric environment seriously, but also causes certain harm to human health.Considering environmental protection, attention is often paid to improving thermal conductivity or thermal conductivity in material design to maintain high temperature condition in exhaust manifold and purify the exhaust gas. Water in exhaust gas condenses at the muffler area and generates condensate. Water generally containing NH+ condenses at the muffler area and generates condensate, which generally contains NH+4, CO2-3, SO2, SO2-3 and SO2-3.-4 and Cl-plasma, because these substances pass through the exhaust manifold before they form, have little effect on them.

Metal corrosion refers to the chemical or electrochemical damage of materials in the surrounding environment, while corrosion at room temperature is mostly electrochemical.Automotive exhaust manifolds are in direct contact with the atmosphere and can also undergo electrochemical action to form corrosive primary batteries.In addition, the polluted atmosphere often contains sulphur and sulphur oxides, and the higher operating temperature of the exhaust manifold accelerates the corrosion rate of the vehicle exhaust manifold.In addition, snow melting agents are often used in winter in North China, especially in Northeast China, to keep roads clear of snow on the roads.It mainly includes potassium acetate and chloride salt. Potassium acetate is expensive. Therefore, chloride is generally used in highway and other facilities.The commonly used chloride salt snow melting agents are sodium chloride, calcium chloride, magnesium chloride, potassium chloride, etc. NaCl is more commonly used.

According to previous studies, the surfaces of high silicon-molybdenum ductile iron and high nickel ductile iron used to produce automotive exhaust manifolds have different phases, grain boundaries, crystal defects, inclusions, cyclic thermal stresses, etc.The inhomogeneity of these structures causes different electrode potentials at different parts of metal surface microscopy and forms corrosive micro-batteries. The lower part of the potential corresponds to the anode and the higher part corresponds to the cathode.Alloy elements are often added to the structure to improve the corrosion resistance of cast iron.The elements Mo, Nb, Ti, Si, Ni and Cr have better effects.

Mo carbides form, which can reduce the potential difference between matrix and graphite, and reduce the rate of self-corrosion. Silicon oxides have a certain protective effect to prevent corrosion. At the same time, silicon can increase the austenitic transformation temperature and stabilize the austenitic structure. With the addition of nickel element, infinite solid solution with iron lattice can be achieved, austenitic phase can be strengthened and stable intermetallic bonding with iron can be formed.Solid solution in the matrix decreases the diffusion rate of other elements in cast iron, while both nickel and chromium are important corrosion resistant elements in the alloy, and a small amount of manganese can improve the corrosion resistance of the alloy.According to existing reports, under atmospheric corrosion conditions, the corrosion resistance of grey cast iron, ductile iron and malleable cast iron differs slightly, and the corrosion rate of cast iron is generally lower than that of steel.

This is not yet clear.Once this corrosion occurs, it will not only bring huge economic losses, but also endanger the life and safety of passengers.Therefore, it is of great theoretical and practical significance to study and analyze the corrosion behavior of high silicon-molybdenum ductile iron and high nickel ductile iron used in the production of automotive exhaust manifold for predicting or analyzing the anticorrosion ability of the material and saving material to guarantee safety.

Components made of many materials grow irreversibly and increase in volume when they return to room temperature after working at high temperatures, resulting in dramatic changes in material properties.Vehicle exhaust manifolds work at high temperatures for a long time, and their size changes affect the sealing and structural stability of the exhaust manifolds to the engine or the front pipes connected to them.Therefore, the growth resistance of metal material is an important parameter for the production and application of automobile exhaust manifold material and one of the important indexes to measure its heat resistance.

The electrochemical corrosion behavior of high silicon-molybdenum ductile iron and high nickel ductile iron under the environment of electrochemical comprehensive test system is simply tested and analyzed, and the basic data of their service behavior under corrosive environment are mastered.The growth resistance of the materials is reflected by measuring the dimensions at room temperature after different working hours at various operating temperatures with a thermal expansion meter.

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