Pack cementizing

In the case of refractory metals, coatings generaHy are sHicides, appHed by pack cementation or slurry processes. Typical sHicide compositions are... 

Each type of metallic coating process has some sort of hazard, whether it is thermal energy, the reactivity of molten salt or metal baths, particulates in the air from spray processes, poisonous gases from pack cementation and diffusion, or electrical hazards associated with arc spray or ion implantation. 

In pack cementation, the part to be coated is placed in a retort and surrounded with a powdered pack consisting of the coating component and an activator the latter reacts with the coating component to form the carrier vapor, usually a haHde or an inert diluent, to prevent the pack from sintering together and to permit vapor transport of the alloying component through the pack. 

Protection of niobium and its alloys from oxidation in air is accomplished by coating, e.g. with zinc deposited by holding in zinc vapour at 865°C or coating with a layer of chemically stable oxide, nitride or silicide. Silicide coatings applied by pack cementation, fused slurry or by electrolytic methods have been found to be one of the most effective means of preventing oxidation of the metal. 

Calorised Coatings The nickel- and cobalt-base superalloys of gas turbine blades, which operate at high temperatures, have been protected by coatings produced by cementation. Without such protection, the presence of sulphur and vanadium from the fuel and chloride from flying over the sea promotes conditions that remove the protective oxides from these superalloys. Pack cementation with powdered aluminium produces nickel or cobalt aluminides on the surfaces of the blade aerofoils. The need for overlay coatings containing yttrium have been necessary in recent times to deal with more aggressive hot corrosion conditions. 

Present oxidation-protection systems are based on silicon carbide (SiC), which is applied by pack cementation or by chemical-vapor infiltration (CVI) (see Ch. 4).d ] Boron, zirconium, and other... 

Shen, J.-N. et al.. Effect of alumina film prepared by pack cementation aluminizing and thermal oxidation treatment of stainless steel on hydrogen permeation, Yuanzineng Kexue Jishu/Atomic Energy Science and Technology, 39, 73 (2005). 

CVD can be performed in a so-called closed or open system. In closed CVD, both reactants and products are recycled for reuse within an enclosure [11], The closed reactor CVD method is primarily used for metal purification and protection coating for aero-engine blades (e.g. pack cementation for aluminising, chromising, etc.). In most cases, CVD happens in an open state which is described in Chapter 1 and Figure 1.1. 

The aluminizing of Ni-base alloys via the pack-cementation process has been a commercially viable process for many years [32]. The typical coating morphologies for the... 

Cockeram and Rapp have evaluated the kinetics of silicide coatings on Ti [103] and have used a halide-activated pack-cementation method to form boron- and germanium-doped silicide coatings on orthorhombic alloy substrates [104]. The coatings greatly decreased the cyclic oxidation kinetics and microhardness measurements did not indicate diffusion of oxygen into the substrate. 

G. W. Goward, L. W. Cannon Pack Cementation Coatings for Superalloys A review of History, Theory, and Practice , Paper 87-GT-50, Gas Turbine Conf., ASME, 1987. 

Figure 10.1 Schematic diagram of the apparatus used for aluminizing by pack cementation.

The most common method of aluminizing is pack cementation, which has been a commercially viable process for many years. This process, which is shown schematically in Figure 10.1, involves immersing the substrate in a mixture of powders. The mixture contains a powder source of Al, either A1 metal or a suitable master alloy, a halide activator, and a filler, which is usually alumina and taken to be inert. A pack will usually contain 2-5% activator, 25% source and the rest filler. The purpose of the filler is to support the component and to provide a porous diffusion path for the gases generated by reaction between the source and activator. 

Fligh-temperature corrosion resistance in molten salts of Si difiiision layers on stainless steel was studied by Fukumoto et al. [60]. They used the pack-cementation method to alloy Si onto SUS430 and SUS304 stainless steels. The oxidation of the siliconized stainless steel was lower than that of nontreated stainless steel. The authors attributed this to the surface alloying of Si with stainless steel alloys, confirmed by electrochemical studies. 

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