Materials Performance

SEP 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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31 MATERIALS PERFORMANCE: VOL. 57, NO. 9 SEPTEMBER 2018 The extremely high resistance of the WC-Ni brazing layer to wear and corrosion is effective for many applications, such as coal slurry transportation, seawater cool- ing systems, flue gas desulfurization (FGD), and especially power plant tubes, where the combined erosion and corrosion phe- nom enon widely o ccurs. For example, rotating impellors used in coal slurry trans- portation, seawater cooling systems, and FGD often suffer attack from solid particles and corrosion, as shown in Figure 1. In many engineering systems, this is a significant degradation m echani sm . It involves mechanical wear damage caused by the erosion generated by solid particles and surface material loss from exposure to a corrosive medium. Erosion and corrosion may create a synergistic effect that pro- duces a wear rate that is greater than the sum of each process alone. 10-11 In aggressive environments, the discrete understanding of corrosion resistance or mechanical ero- sion resistance is not sufficient to select an effective method of material protection from the combined attack of erosion and corrosion. Coating techniques could provide a solution for protecting the surface; but it is very difficult to uniformly coat an impellor due to its complex geometry. The vacuum brazing technique, however, can apply a uniform coating on an impeller using a rotation system in the vacuum furnace. Until recently, limited information on the combined erosion and corrosion per- formance of WC coating materials was available. M. Reyes and A. Neville 12 investi- gated the degradation mechanism of WC- based cerm ets for drilling tool s. Th ey reported that erosion models have limita- tions for liquid-solid impingement condi- tions because of corrosion and complex particle trajectories. E.J. Wentzel and C. Allen 10-11 noted the effect of different bind- ers on the resistance of WC coatings to combined erosion and corrosion. They also observed that binder removal is the main degrad ation m echani sm during slurr y erosion. In addition, A. Neville, et al., 13 investi- gated the behavior of WC-based metal matrix composites in liquid-solid slurries with different erodent sizes and loading. FIGURE 1 An impeller damaged from erosion and corrosion attack in a FGD system. FIGURE 2 A schematic diagram representing the cross-section of a specimen with the WC sheet and the Ni-based top layer. They reported that the erodent size is a very important parameter that affects the total mass loss of materials. However, little research work has been done on the com- bined erosion and corrosion performance of WC-Ni brazed coating materials. For this article, an improved manufacturing pro- cess for rotary brazing was studied with simulation work to find suitable process conditions. Experimental Procedures Materials The coating powder mixture was made using WC powders with the grade size of 1 μm and a Ni–Cr–Si brazing alloy powder with the grade size of 5 μm. The chemical composition is 8 wt% Cr, 4.5 wt% Si, 3 wt% B, 3 wt% Fe, and the balance Ni. Organic binders were added with the materials by ball milling, and then the mixture was rolled into a f lexible sheet. Polymethyl- methacr ylate was used for the organic binder and was agglomerated into carbide particles and removed by sintering. The organic binder provided good formability to the carbide mixture. The mixture was rolled into a WC sheet using a doctor blade to adjust thickness. Application The 70-mm diameter pilot impellor test specimens for FGD were manufactured from Type 304 stainless steel (SS) (UNS S30400). A WC sheet was placed directly on the surface of the impellor. A f lexible Ni- base alloy sheet was used as the top layer, as shown in Figure 2. Both sheets were placed on the impeller before vacuum brazing. Vacuum Furnace The brazing was performed in a vac- uum furnace, shown in Figures 3(a) and (b), with a box chamber that included a rotary system. The rotary axis is located at the center of the box chamber, which is sealed with a non-contact, magnetic f luid seal that is clean and works in a high vacuum. During the brazing process, the speci- men with sheets was attached to the axis rotary system in the chamber. The chamber was then evacuated to 0.133 Pa and heated at a rate of 15 °C/min. During this process, the binders in the combined flexible sheets were evaporated at just under 400 °C. The Ni-base alloy sh eet star t ed to m elt at 980 °C. Small pores form ed by binder

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