A Study Of Wear-resisting And Anticorrosive Coating On Carbon Steel

In order to compare the growth process of aluminate PEO coating and silicatePEO coating, PEO coatings treated for different time were prepared, and to study theinfluence of PEO coating on organic coating, PEO/organic and organic coating wereprepared on low carbon steel. In the experiment, the current-time curve were recorded;Energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD, RigakuD/Max-2400X-Ray Diffractometer) and scanning electron microscopy (SEM,JSM-5600LV) were employed to analyze the elemental composition, the elementaldistribution in thickness direction and the phase composition, and to observe thesurface and cross-section morphologies of PEO coating, PEO/organic and organiccoating. The bonding force, shock resistance and tribology properties were tested by aMulti-functional Tester for Material Surface Properties (MFT-4000, China), a shockresistance tester and a reciprocating friction and wear test-rig (UMT-2MT), and thewear tracks on PEO coatings and counterpart balls were observed. The oxide types onthe counterpart tracks and in the inner layer were identified by Raman Spectroscope.The corrosion resistance of PEO/organic coating and organic coating were tested byelectrochemical test, salt spray test and immersing test.The results show that, under the condition of positive voltage450V, negativevoltage100V, the growth rate of silicate PEO coating were much larger; it growsinward and outward and consumes more substrate material; its interface with substrateis uneven. However, the aluminate PEO coating shows a smooth interface andconsumes less substrate. The main elemental compositions of aluminate PEO coatingare O, Fe and Al, and they exist in crystal Fe3O4and FeAl2O4; the elements distributeuniformly in the thickness direction. But for the silicate PEO coating, the maincompositions are Si, O and Fe, and exist in amorphous forms. The distribution ofelements in thickness direction show a two-layered feature. To some extent, thetwo-layered structure weakens the bonding force between silicate PEO coating andsubstrate. The main compound in the inner layer of silicate PEO coating is Fe2O3anda little Fe3O4exists too. But in the inner layer of aluminate PEO coating, the maincompound Fe3O4co-exists with secondary compound Fe2O3. The Fe2O3moleculesexisting without water of crystallization during the PEO process will absorb watermolecules from electrolyte or air and turns into Fe2O3.nH2O which results in expansion of PEO coating,s volume and the PEO coating automatically peels off. It isalso found from the tribology experiment that the friction coefficient and wear rate ofaluminate PEO coating are lower than that of silicate PEO coating and the wear rateof counterpart ball corresponding to aluminate PEO coating is lower than thatcorresponding to silicate PEO coating. This is because the Fe3O4in aluminate PEOcoating transferred to the counterpart ball during tribology process and functioned as alubricant to low the friction coefficient and wear rate of both aluminate PEO coatingand counterpart ball. In silicate electrolyte, with positive voltage of240V, negativevoltage of50V, PEO coating, PEO/organic coating and organic coating weresuccessfully prepared on low carbon steel. The silicate PEO coating under thiscondition mainly consists of Fe3O4、Fe2O3and some Si containing amorphous oxideand shows some corrosion resistance. There is a relatively strong bonding forcebetween this PEO coating and substrate, which is a key character to a promisingpretreatment of organic coating on metal. The rough surface structure with a greatmany micropores and cracks is beneficial to the forming of mechanical locking,which can improve the bonding force and increase the interact area, between PEO andorganic coating. The structure makes it possible that more organic will be absorbed bythe PEO coating and a relatively thick organic coating can be obtained. The results ofelectrochemical corrosion test, salt spray test and immersing test indicates that thePEO/organic coating performs a better corrosion resistance and a longer service life.Therefore, PEO is a promising and environmental pretreatment to organic coating onlow carbon steel.