Gong, Z-G. Yang/ Material and Design 32(2011)671-681 loor was probably the ferrous sulfate FeSO4, whose feature color is nical compositions of the two manhole doors(wt.6). In order to confirm the element compositions in the black cor- rosion product, XRF was employed. The results showed that the Manhole door B 87. 229 two primary elements were Fe and S, seen in Table 4. However, GB→Asi6Cu4 the detailed sorts of these substances should be further identified As is shown in Fig. 14a, the black solid corrosion product con- requirements of casting aluminum alloy specifications in GB-ALSi6. sisted of a large amount of compounds. Among them, the ferrous Cua standard of China(27). In it, Si, Cu and Zn elements are used to sulfate hydrate( FeSoa4H2 0)was the predominant one according improve the hardness, castability, corrosion resistance of the mate- to the standard powder diffraction file(PDF)card. It can be easily rere also both qualified. components with matrix metals of iron-based materials in the a n al. Based on the results. matrix metals of the two manhole doors inferred that the ferrous sulfate was the corrosion product of the By using the Keller agent(HF 1.0 mL, HCI 1.5 mL HNO3 2.5 mL lone. and then adhered on the manhole door surfaces meanwhi and H20 95 mL), metallographic structures of the matrix metal of le yellow corrosion product was also analyzed by XRD, seen in the perforated manhole door are presented in Fig. 12. As is shown Fig. 14b. It is clear that the simple substance sulfur was the exclu- in Fig. 12a, the material displayed the typical dendritic microstruc- sive composition, which was originated from sublimation of the um alloy. However, as for the fractured sur- fuels. To sum up the analysis results of XRF and XRD, it testified face, obvious micro cracks had already initiated from its edge, seen that the scaling and the perforation of the manhole doors were also in Fig. 12b. This is a significant evidence of intergranular corrosion partly caused by the sulfur related corrosion. that eventually caused perforation on the manhole door. With respect to the thermal properties of the black corrosion roduct, Fig. 15 displays its TGAresult. It is obvious that there were 3. 2. Corrosion products analysis two turning points in the curve, respectively at temperatures of It can be learned from Figs. 4 and 5 that three different mor- 190C and 280C, and the weight losses of them were 18% and phologies of the corrosion products existed on the surfaces of the 55%. In fact, the three segments of the curve virtually represented manhole doors, i.e. the green liquid(Fig. 4c). the black solid three different steps of the decomposition procedures of the black (Fig. 5b)and the yellow solid(Fig. 5c). Analyses including IC, corrosion product. XRD, XRF and tga would then be successively conducted to inves igate the chemical compositions and the thermal properties of Step one(<190"C): dehydration. Dissolved in deionized water, the green corrosion product re- leased the ions it contained. It is displayed in Fig. 13 that the pre- Table 4 chloride ion, and nitrate radical all did not exceed 0.5 ppm. Thus, -ss black corrosion product(wtx). dominant ion was the sulfate radical, whose concentration was up xRE results of th to 17.74 mg/L i.e. 17.74 ppm. Other ions including fluoride ion, s Cr Mn Fe Ni Cu it can be inferred that the liquid corrosion product on the manhole Wt‰0046011.23.610.14512.03.670.0404Rest a)求器( Fig. 12. Metallographic structures of the perforated manhole door (a)100x and(b) polished state 50 °。20-40608010012010 Fig. 13. lon chromatograph results of the green liquid corrosion product.requirements of casting aluminum alloy specifications in GB-AlSi6- Cu4 standard of China [27]. In it, Si, Cu and Zn elements are used to improve the hardness, castability, corrosion resistance of the mate￾rial. Based on the results, matrix metals of the two manhole doors were also both qualified. By using the Keller agent (HF 1.0 mL, HCl 1.5 mL HNO3 2.5 mL and H2O 95 mL), metallographic structures of the matrix metal of the perforated manhole door are presented in Fig. 12. As is shown in Fig. 12a, the material displayed the typical dendritic microstruc￾ture of casting aluminum alloy. However, as for the fractured sur￾face, obvious micro cracks had already initiated from its edge, seen in Fig. 12b. This is a significant evidence of intergranular corrosion that eventually caused perforation on the manhole door. 3.2.2. Corrosion products analysis It can be learned from Figs. 4 and 5 that three different mor￾phologies of the corrosion products existed on the surfaces of the manhole doors, i.e. the green liquid (Fig. 4c), the black solid (Fig. 5b) and the yellow solid (Fig. 5c). Analyses including IC, XRD, XRF and TGA would then be successively conducted to inves￾tigate the chemical compositions and the thermal properties of them. Dissolved in deionized water, the green corrosion product re￾leased the ions it contained. It is displayed in Fig. 13 that the pre￾dominant ion was the sulfate radical, whose concentration was up to 17.74 mg/L, i.e. 17.74 ppm. Other ions including fluoride ion, chloride ion, and nitrate radical all did not exceed 0.5 ppm. Thus, it can be inferred that the liquid corrosion product on the manhole door was probably the ferrous sulfate FeSO4, whose feature color is exactly green. In order to confirm the element compositions in the black cor￾rosion product, XRF was employed. The results showed that the two primary elements were Fe and S, seen in Table 4. However, the detailed sorts of these substances should be further identified by XRD. As is shown in Fig. 14a, the black solid corrosion product con￾sisted of a large amount of compounds. Among them, the ferrous sulfate hydrate (FeSO44H2O) was the predominant one according to the standard powder diffraction file (PDF) card. It can be easily inferred that the ferrous sulfate was the corrosion product of the components with matrix metals of iron-based materials in the cy￾clone, and then adhered on the manhole door surfaces. Meanwhile, the yellow corrosion product was also analyzed by XRD, seen in Fig. 14b. It is clear that the simple substance sulfur was the exclu￾sive composition, which was originated from sublimation of the fuels. To sum up the analysis results of XRF and XRD, it testified that the scaling and the perforation of the manhole doors were also partly caused by the sulfur related corrosion. With respect to the thermal properties of the black corrosion product, Fig. 15 displays its TGA result. It is obvious that there were two turning points in the curve, respectively at temperatures of 190 C and 280 C, and the weight losses of them were 18% and 55%. In fact, the three segments of the curve virtually represented three different steps of the decomposition procedures of the black corrosion product.  Step one (<190 C): dehydration. Table 3 Chemical compositions of the two manhole doors (wt.%). Element Al Si Cu Zn Fe Mn Manhole door A 87.585 5.541 3.300 2.241 1.224 0.109 Manhole door B 87.229 5.454 3.200 2.256 1.153 0.105 GB-AlSi6Cu4 86.9–91.9 5.0–7.5 3.0–5.0 / / 0.1–0.6 Fig. 12. Metallographic structures of the perforated manhole door (a) 100 and (b) polished state 50. Fig. 13. Ion chromatograph results of the green liquid corrosion product. Table 4 XRF results of the black corrosion product (wt.%). Element Si S Cr Mn Fe Ni Cu O Wt.% 0.0460 11.2 3.61 0.145 12.0 3.67 0.0404 Rest Y. Gong, Z.-G. Yang / Materials and Design 32 (2011) 671–681 677