Effect of Salt-bath Nitriding on Corrosive Wear Performance of Pure Iron

As a soft magnetic material, pure iron has excellent magnetic properties such as low coercivity and high permeability. Consequently, it has been widely used in circuit breakers, electromagnetic relays, and other electronic components, such as iron cores, and armatures. However, the application of pure iron in extreme environments has been limited, owing to its poor corrosion and wear performance. Salt-bath nitriding is widely studied as a chemical heat treatment technology that can effectively improve the properties of metal materials. However, there are few reports on applying this treatment to pure iron, and there is a lack of research on the wear properties of pure iron in different environments. In order to improve the hardness, wear resistance, and corrosion resistance of pure iron, and enhance the service life of electrical switch components of marine equipment, pure iron samples underwent a salt-bath nitriding treatment. Scanning electron microscopy was used to determine the micromorphology of the surface and the nitrided layer section, and determine the wear mark morphology. The wear products were analyzed by energy-dispersive X-ray spectroscopy. X-ray diffraction analysis was used to conduct phase analysis of the surface of the nitrided layer, while X-ray photoelectron spectroscopy was used to conduct specific phase composition analysis before and after the removal of the loose layer on the surface of the nitrided layer. The surface and section hardness distribution was tested with a microhardness tester, and the corrosion resistance was tested and analyzed with a neutral salt spray test box and electrochemical workstation. The wear performance of the samples before and after nitriding was tested on the ball-disk friction and wear tester, and the test environment was an air, deionized water, and 3.5 wt.% NaCl solution environment. The results showed that the maximum coercivity and surface hardness values were 31.5 A / m and 508.8 HV0.2, respectively, with the values increasing with an increase in nitriding temperature and time. The 580 celciusx4.5 h process sample had the best resistance to neutral salt spray corrosion, and the thickness of the sample after this process was about 200 mu m. The sample had epsilon-Fe3N and gamma' -Fe4N phases, with the formation of hard epsilon-Fe3N and gamma '-Fe4N phases being the main reason for the increase of surface hardness. The hardness of the cross section of the sample increases slightly at first and then decreases gradually from the nitrided layer to the matrix. The maximum hardness value was 528.1 HV0.2. The self-corrosion potential of the 580 celciusx4.5 h process sample shifted 0.37 V forward, the self-corrosion current density was significantly reduced, and the charge transfer resistance increased by 7.7 times relative to the respective pure iron values. The improvement in the corrosion performance can be attributed to the improved compactness and chemical stability of the nitrided layer. In an air environment, the friction coefficient of the nitriding sample is lower than that of pure iron, and the wear rate of pure iron and nitriding sample is 2.19x10(-5) mm3 /(N center dot m) and 1.06x10(-5) mm3 /(N center dot m) respectively, with the wear rate of the nitriding sample about 50% lower than that of pure iron. Both deionized water and 3.5 wt.% NaCl solution were conducive to the reduction of the friction coefficient, but resulted in an increased wear rate. The wear rates of the pure iron and nitriding samples in the deionized water environment were 2.75x10(-5) mm3 / (N center dot m) and 1.86x10(-5) mm(3 )/ (N center dot m), respectively, while in the 3.5 wt.% NaCl solution environment they were 3.5x10(-5) mm3 / (N center dot m) and 2.28x10(-5) mm3 / (N.m), respectively. The wear rate in the NaCl solution environment was significantly higher than in the other environments; under the synergistic effect of corrosion and wear, the material removal efficiency was higher. Having systematically studied the effect of salt-bath nitriding on the corrosion and wear properties of pure iron, theoretical guidance and technical support can be provided to improving the service life of electrical switch components in marine equipment.