| In this paper, Fe24+xCo24-xCr15Mo14C15B6Y2(X=0, 2, 4, 6 and 17) bulk amorphous alloy were fabricated using the non consumable type vacuum arc melting furnace water-cooled copper mold suction casting method, and the amorphous coatings Fe41Co7Cr15Mo14C15B6Y2 were prepared by High Velocity Oxygen-Fuel(HVOF)technology, respectively. The structure and the corrosion morphology of the alloys were characterized using X-ray diffraction(XRD), scanning electron microscope(SEM) and atomic force microscopy(AFM), respectively. The thermodynamic parameters of amorphous alloy have been got by differential thermal analysis(DSC). This article focus on the effects of Co content on the corrosion resistance of Fe-based amorphous alloy and the corrosion resistance test was respectively carried out by neutral salt spray test and electrochemical corrosion experiment. To further discuss the changes of the corrosion characteristics with the addition of Co element, the surface work function distribution, corrosion morphology and elements energy spectrum analysis(EDS) of amorphous alloy have been used in combination. In addition, the rule of the glasses forming ability(GFA) and the corrosion resistance changed with the Co element addition were measured by wedge specimen method and the weight loss law. The corrosion influencing factors between coating and bulk amorphous alloys were investiaged by chemical immersion and electrochemical corrosion method which were based on the surface work function theory.It can be obtained from the structural characteristics analysis of the alloys that the bulk amorphous alloys Fe24+xCo24-xCr15Mo14C15B6Y2(X=0, 2, 4, 6 and 17) and the amorphous coating Fe41Co7Cr15Mo14C15B6Y2 were all proved to be completely amorphous. It shows that when x = 4, the alloy has the widest supercooled liquid region ΔTx and the largest glass-forming ability through differential thermal analysis and wedge sample method. The neutral salt spray test results of Fe24+xCo24-xCr15Mo14C15B6Y2(X=0, 2, 4, 6 and 17) bulk amorphous have been obtained: when x = 4, the alloy has the lowest corrosion rate, followed by x = 2, x = 0, x = 6, x = 17, Ti6Al4 V and 1Cr18Ni9 Ti. The corrosion morphology were observed by SEM and atomic force microscopy(AFM), there have been varying degrees of corrosion morphology, where X = 4, the corrosion pit is shallow of the alloy, and the area and depth of corrosion pits are larger when x=17. In addition, the corrosion of amorphous alloy in horizontal transmission is more serious than vertical transmission. The trends of GFA are consistent with corrosion resistance. They are all first increase and then decrease with the addition of cobalt. The Salt spray corrosion products of the alloys are mainly composed of iron and cobalt oxides and their chlorides.We also use the electrochemical etching method for the further analysis of bulk amorphous corrosion. The experimental results for different Co contents Fe24+xCo24-xCr15Mo14C15B6Y2(x=0, 2, 4, 6, 17) amorphous alloy shows the similar surface work function value. The work function value of x = 2 and x = 4 alloys are relatively high, which both showed good corrosion resistance in the electrochemical etching process. Moreover, when x is up to 4, the alloy shows the widest passive region, relatively high corrosion potential and lower corrosion current density in 1 mol / L HCl and 3.5% NaCl, respectively. The corrosion resistance of the amorphous alloys from strong to weak is: x = 4, x = 2, x = 0, x = 6, x = 17, which are consistent with the results of the salt spray corrosion.Through the comparison of bulk amorphous alloy and the corresponding composition coating amorphous alloy, the effect of porosity on the corrosion resistance of the coating amorphous alloys was investigated. The final results of coating and bulk amorphous alloy immersion in 1 mol / L HCl are as follows: the size and the number of the pits played a significant role on the corrosion resistance of both alloy, while the corrosion degree of the bulk amorphous alloy is more serious than that of the coating amorphous. The Kelvin probe analysis showed that corrosion product will increase the value of the work function of the surface of the alloy, porosity of 9.25% resulted in 50 meV work function difference of coating and bulk; both before and after the corrosion, the work function value of the bulk is higher than that of the coating. The electrochemical corrosion results of amorphous coatings and bulk in 1 mol/L H2SO4,1 mol/L and 2 mol/L HCl are as follows: corrosion potential of the bulk is higher than that of the coating, the corrosion current value of the bulk is an order of magnitude smaller than that of the coating, its passive current density is far less than that of the coating, which showing a stronger corrosion resistance. Coating’s porosity caused a greater corrosion tendency, make it easier to gain and loss of electrons in the corrosion process, which accelerates the reaction proceeds and resulting in two different fitting circuits. The porosity difference close to 9.25% of coating and bulk resulted in the coating corrosion potential lower than that of the bulk and the corrosion current density higher than that of the bulk. Electrochemical transfer resistance Rt values of the two alloys are different by an order of magnitude in HCl solution. |
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