Q. Fang et al 08 ◆A2o3 HF + 5% HCI 05 room temperature coO 60 orrosion time(h) Fig. 4. Corrosion rate of alumina against corrosion time 0016 1.5% HF+5% HCI Zro2 0014 room temperatur ▲Sia 0012 iC 0004 0.002 60 Corrosion time(h) Fig. S. Corrosion rate of ZrO2, sialon and SiC against corrosion time. PSZ ZrO2(95%)is stabilised by the addition of MgO, Y2 O, and Sio. CI Silica is added to suppress grain growth during manufacture and to improve the mechanical properties, but it will confer a reduced corrosion to HF and HCl solution. Mgo surface complex ion was suggested as the surface reaction i& molecule. resistance towards as a basic ingredient will reduce resistance to acids, while SiO z has good acids but is easily attacked by HF; the substitution of HF n solution with a Al2O3 is mixed with 15% SiO2. As mentioned above, the presence of Sioz can be predicted to reduce the corrosion resistancc of Al2O3 in HF HCl solution and indeed this is corroborated by our results
20 40 60 80 Corrosion time (h) 0 100 120 Fig. 4. Corrosion rate of alumina against corrosion time. 0.016 1 1.5% HF + 5% HCI 0.014 -~ room temperature 0.012 -- M Urn I I I 0.01 -- I A 0.006 .- A A IZro2 A Sialon 1 l SiC j 1 I 0.006 -- A 0.004 .~ A A 0.002 A A 04. 0. : 0 0 0 0 0 0 20 40 60 80 Corrosion time (h) Y 100 120 Fig. 5. Corrosion rate of ZrOz, sialon and SC against corrosion time - RB SIC contained 2-10% non-reacted Si, which is insoluble in HF and HCl. - PSZ ZrOz (95%) is stabilised by the addition of MgO, Yz03, and Si02. - Silica is added to suppress grain growth during manufacture and to improve the mechanical properties, but it will confer a reduced corrosion to HF and HCl solution. MgO as a basic ingredient will reduce resistance to acids, while SiOz has good resistance towards acids but is easily attacked by HF; the substitution of HF molecules in solution with a surface complex ion was suggested as the surface reaction.‘* - A1203 is mixed with 15% SiOz. As mentioned above,” the presence of SiO;, can be predicted to reduce the corrosion resistance of A1203 in HF + HCl solution and indeed this is corroborated by our results
Effect of corrosion and erosion on ceramic materials Porosity in ceramics and roughness on surface. Reaction bonded SiC(RB SiC) is produced by the reaction of either liquid or gaseous silicon or Sio with carbon in a silicon carbide/carbon compact. This results in a porous body with a continuous silicon carbide phase, however these pores can be filled with non-reacted Si(2-10%)yielding a dense product that results in excellent mechanical properties and corrosion resistance. This is confirmed by the results which show good corrosion resistance of SiC. In the case of Al2O3 it is possible that the sintering agent, 15% Sio2(which is included during manufacture)is preferentially attacked by the corrosive solution which contains hydrofluoric acid Etching would increase the penetration of solution and result in the formation of a surface layer which is depleted in SiO2 and hence mechanically weaker. This is confirmed by XRD and AAS (see below). It is worth noticing that the corrosion rate of Al2O3 decreases with time at a higher rate than for the other three ceramic materials. As the corrosion time increases the depleted layer thickness increases. Access of the corrosive solution would thus be via diffusion through this layer and thus slow down with time. Another factor for the decrease in corrosion rate with time for Al203 compared to the other ceramics could be due to the nitial surface finish which was rough a rougher surface would increase the exposure area to the corrosive solution at the start of immersion In this case Al,O3 had a start surface finish of Ra=0.7 um, while the other materials had a smoother surface of 0.3-0.01 um Strictly speaking, using the rate of ceramic weight loss as a criterion for corrosion rate is not accurate because there are two different corrosion processes: one is that the outer surfac can dissolve into solution, while the other is due to corrosion of one phase leaving a porous and fragile outer layer which is then easily undermined by erosion. The weight loss in the latter case may not be as high as in the first case but the top layer material strength is lost all the same Figure 6 shows the undermined surface layer on the Al2O3 surface after corrosion for 115h. ZrO and sialon also behave in a similar manner while such a top surface degradation is not obvious in Sic Xrd analysis X-ray diffraction peaks of the tested ceramic specimens are shown in Figs 7-9 Figure 7 shows the X-ray difiraction peaks for alumina(85% Al2O3, 15% SiO2)before nd after immersion in 1.5% HF+5% HCl solution for 35h. Prior to immersion of the mple in the corrosive solution, other peaks, apart from Al2O3, which are mainly due to SiO2, exist. These decrease remarkably, as the immersion time increases indicating preferential attack by the corrodent on the Sioz. This means that silica is particularly Fig. 6. Corroded layer on the Alzo3 surface after corrosion for 1 15 h
Effect of corrosion and erosion on ceramic materials 517 Porosity in ceramics and roughness on surface. Reaction bonded Sic (RB Sic) is produced by the reaction of either liquid or gaseous silicon or SiO with carbon in a silicon carbide/carbon compact. This results in a porous body with a continuous silicon carbide phase, however these pores can be filled with non-reacted Si (2-10%) yielding a dense product that results in excellent mechanical properties and corrosion resistance. This is confirmed by the results which show good corrosion resistance of Sic. In the case of A1203, it is possible that the sintering agent, 15% SiOz (which is included during manufacture) is preferentially attacked by the corrosive solution which contains hydrofluoric acid. Etching would increase the penetration of solution and result in the formation of a surface layer which is depleted in Si02 and hence mechanically weaker. This is confirmed by XRD and AAS (see below). It is worth noticing that the corrosion rate of A1203 decreases with time at a higher rate than for the other three ceramic materials. As the corrosion time increases the depleted layer thickness increases. Access of the corrosive solution would thus be via diffusion through this layer and thus slow down with time. Another factor for the decrease in corrosion rate with time for AlzOs compared to the other ceramics could be due to the initial surface finish which was rough. A rougher surface would increase the exposure area to the corrosive solution at the start of immersion. In this case Al203 had a start surface finish of Ra =0.7 urn, while the other materials had a smoother surface of 0.3401 urn. Strictly speaking, using the rate of ceramic weight loss as a criterion for corrosion rate is not accurate because there are two different corrosion processes: one is that the outer surface can dissolve into solution, while the other is due to corrosion of one phase leaving a porous and fragile outer layer which is then easily undermined by erosion. The weight loss in the latter case may not be as high as in the first case but the top layer material strength is lost all the same. Figure 6 shows the undermined surface layer on the A120s surface after corrosion for 115 h. ZrOz and sialon also behave in a similar manner while such a top surface degradation is not obvious in Sic. XRD analysis X-ray diffraction peaks of the tested ceramic specimens are shown in Figs 7-9. Figure 7 shows the X-ray diffraction peaks for alumina (85% AlzOs, 15% SiO$ before and after immersion in 1.5% HF + 5% HCl solution for 35 h. Prior to immersion of the sample in the corrosive solution, other peaks, apart from A120s, which are mainly due to SiOz, exist. These decrease remarkably, as the immersion time increases indicating preferential attack by the corrodent on the SiOz. This means that silica is particularly Fig. 6