Co-based alloys

Stellite 21 laser coating (Nd:YAG, 1.3 wt.% Fe) exhibited potentiodynamic polarization curves nearly identical to wrought and low diluted Inconel 625 and Alloy 59 laser coatings including slight increase in current density between +450-+500 mV. Erp was equivalent to Alloy 59 and substantially higher than those for the best Inconel 625 alloys. Icorr values settled between remelted and wrought Inconel 625. In addition, Stellite 21 laser coating was immune to crevice corrosion under the gasket and pitting corrosion. Instead, SEM studies of exposed surface revealed preferentially dissolved areas in micro-level, where Cr- and Mo-rich interdendritic regions, in consequence of microsegregation, dissolved considerably less than dendrite cores as shown in Figure 68. Consequently, dendrite cores became susceptible to pitting corrosion due to lack of Mo and Cr. In given conditions, Mo and Cr contents were, however, high enough to resist pitting. According to EDS point analyses, CID/CD for Mo was 3.8 and for Cr 1.4. Overlapped areas, where the scale of the microstructure is clearly coarser, were revealed by dissolution as shown in Figure 69a.

Figure 68. SEM image of the polarization tested surface of Stellite 21 laser coating. Light regions are rich in Mo and Cr due to microsegregation. Dark regions are preferentially dissolved. Average composition of the light region is Co-34.9Cr-15.0Mo-2.3Ni-1.4Si-0.9Fe, whereas the dark region is Co-25.4Cr-4.0Mo-3.4Ni-1.1Si-1.2Fe in wt.%. CID/CD for Mo was 3.8 and for Cr 1.4. The widths of the segregated regions are less than 1.0 µm. Small size and uneven surface quality may have caused some error in quantitative EDS analyses (EDS point analysis; 20 kV, tilt 0º). According to interaction volume simulation carried out by Edax Electron Flight Simulator, X-rays generated in the ball shape volume, which was 2.2 µm in diameter. Nominal composition of Stellite 21 was used in simulation.

Compared with more heavily diluted PTA coating (24.0 wt.% Fe), laser coating showed substantially better corrosion performance as shown in Figures 54, 56 and 57 and in Table 15.

Anodic current densities were distinctively higher for PTA coating and slight increase in current density took place at potential range as low as +230-+400 mV indicating Fe’s detrimental effect on passive film. Moreover, Erp was significantly higher for laser than that for PTA coating. Similar to PTA overlay welded Inconel 625, Stellite 21 PTA coating possessed positive hysteresis loop. Instead of severe crevice corrosion under the gasket observed in PTA overlay welded Inconel 625, Stellite 21 PTA coating exhibited slight pitting corrosion in the central regions of the exposed areas while crevice corrosion under the gasket was absent (Figures 69b and 70). This susceptibility to pitting can be attributed to the higher dilution and microsegregation. If polarization curves for PTA overlay welded Inconel 625

(25.1 wt.% Fe) and Stellite 21 (24.0 wt.% Fe) are compared (Figure 71), it can be noted that Stellite 21 possessed higher anodic current densities and increase in it took place at lower potentials. E was also substantially lower for PTA overlay welded Stellite 21. rp

a) b)

Figure 69. Optical macrographs of cyclic polarization tested surface of a) laser clad and b) PTA overlay welded Stellite 21. Inter-track advance in laser cladding was 2.5 mm. Three black dots shown on the surface of laser coating are pen marks. Corrosion pits are clearly seen on the surface of PTA overlay welded coating.

Figure 70. SEM images of the polarization tested surface of PTA overlay welded Stellite 21.

Dark regions are rich in Mo and Cr. Average composition of the dark region is Co-32.2Cr-17.8Mo-1.4Ni-1.4Si-16.8Fe. Average composition of the light region is Co-20.3Cr-4.4Mo-2.7Ni-1.3Si-23.3Fe. Widths of these dark regions vary approximately from 2.0 to 3.0 µm.

Corrosion pits are clearly seen in lower magnification micrograph.

-1.000 -0.500 0.000 0.500 1.000

1.E-10 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 log I [A/cm²])

E vs. Ag/AgCl (V)

625 PTA St 21 PTA

Figure 71. Representative potentiodynamic polarization curves for PTA overlay welded Inconel 625 (3) and Stellite 21 (3) coatings measured in 3.5 wt.% NaCl solution at RT.

Low diluted Stellite 6 coating (1.6 wt.% Fe) produced by Nd:YAG laser showed corrosion performance comparable with Stellite 21 in given circumstances, that is, anodic current densities and Erp were nearly identical and increase in current density occurred at +450-+550 mV. SEM examination of the exposed surface revealed that crevice corrosion under the gasket and pitting corrosion in central regions was absent. Instead, analogous with Stellite 21 laser coating, preferentially dissolved areas were detected as displayed in Figure 72. That is, Cr- and W-rich interdendritic regions consisting of fcc-Co and carbides were less attacked than Cr- and W-depleted dendrite cores.

Compared with Stellite 6 produced by HIPping (1.0 wt.% Fe), laser coating exhibited identical polarization curves except for Erp, which was lower for HIPped alloy (Figure 55, Table 15). Similar to laser coating, depleted matrix was more severely dissolved than Cr-rich carbides (Figure 73). Crevice and pitting corrosion was absent. Apart from the interconnected network of carbides in laser coating, discrete spherical carbides were found from the HIPped alloy and matrix contained more W than carbides.

Figure 72. SEM images of the polarization tested surface of Stellite 6 laser coating. Light interdendritic regions are mixture of Cr-rich M C7 3 carbides and small amounts of fcc Co.

Dark Co (fcc) matrix is preferentially dissolved. Average composition of the interdendritic region is Co-40.0Cr-7.5W-1.6Ni-0.6Si-0.5Mo-1.2Fe, whereas for the matrix it is Co-23.9Cr-3.8W-2.4Ni-0.9Si-0.1Mo-2.0Fe in wt.% (EDS point analyses, 15 kV). C /C_{ID D} for Cr was 1.7 and for W 2.0. Overlapped regions were coarser than central regions of bead.

Figure 73. SEM image of the polarization tested surface of HIPped Sellite 6. Light regions are Cr-rich carbides. Dark matrix is preferentially dissolved. Average composition of the carbides is Co-77.3Cr-3.3W-0.4Ni-0.1Si-0.5Mo-0.3Fe (EDS point analyses, 15 kV). Average composition of the dark matrix is Co-20.8Cr-6.7W-2.2Ni-1.0Si-0.4Mo-1.2Fe. C_carbide/C_matrix for Cr was 3.7 and for W 0.5. As opposed to Stellite 6 laser coating W content is higher in matrix than in carbides. This is due to W’s tendency to segregate into interdendritic liquid during solidification, which takes place in laser cladding. On the contrary Cr content in carbides is much higher than in laser coating.