Chromized coatings

The compositions of the alloys evaluated in Phase I are summarized in Table III. These alloys represent most classes of high-temperature iron-, nickel-, and cobalt-base alloys that could be considered for coal gasification service. Pack aluminized and chromized coatings on AISI 310 and IN-800 were also evaluated in the test program. 

High-temperature stainless steels, most polycrystalline superalloys, and chromized coatings rely on the formation of a surface layer of chromia for oxidation protection. The effects of reactive element additions are often more dramatic in the case of chromia-forming alloys than alumina formers in that, in addition to improving adherence (Figure 5.41), they decrease the amount of transient oxidation, reduce... 

Figure 10.7 Schematic cross-section of a chromized coating formed on a typical boiler steel.

FIGURE 10.17 Chromium distribution in chromized coating on 0.1% C steel. (From LA. Menzies and D. Mortimer, Corros. Sci. 5,1965, 539 P. Galmiche, Rev. Metall. 47,1950,192. With permission.)... 

The structure of the chromized coating has been examined in detail Menzies and Mortimer [150] have studied the chromizing of iron and plain carbon steels (up to 0.7% C). There is very little variation of structure with duration of treatment, but there are considerable differences with carbon content and process temperature. The observed structures can be classified in two groups ... 

Chromium is cathodic to steel however, economic electrodeposited coatings of chromium are too thin and porous to give protection on their own and are used in conjunction with thick (25 pm) deposits of copper or nickel. Chromized coatings, on the other hand, are relatively thick and can confer protection by their barrier action, although corrosion of the steel substrate will be increased at any discontinuities in the coating. 

The breakdown of chromized coatings during oxidation has been attributed in part to the phenomenon of secondary diffusion, which decreases the chromium concentration at the metal surface. The mechanism of oxidation is therefore complex [167], although probably similar to that of iron chromium alloys, a subject that has been reviewed by Wood [168], among others. Protection is therefore dependent on the formation of a coherent barrier scale or, in the case of carbon steels, a carbide diffusion barrier. 

In Japan, Takei et al. (1986) have examined Y-modified aluminide and Y-modified chromized coatings, where Y is first deposited by ion-plating followed by either Al or Cr pack-cementation. The beneficial influence of Y modification of both aluminide and chromized coatings on Ni-base superalloys (TM-49 and TM-321) has been confirmed in hot-salt, burner-rig and in an operational engine test. 

B.J. Smith and A.R. Marder, Characterization of Chromium Diffusion (Chromize) Coatings in a High Temperature Coal Combustion Atmosphere, Surface Modification Technologies IV, T.S. Sudarshan, D.G. Bhat, and M. Jeandin, Ed., TMS, 1991, p 471... 

The borides are extremely hard (9.8—29 GPa (1000—3000 kgf/mm ) Knoop) and, in the case of molybdenum, >39 GPa (4000 kfg/mm ) (see Hardness). However, oxidation resistance is usually poor unless a subsequent coating is formed, such as silicidi ing or chromizing, which imparts oxidation resistance. SiUcides are generally very oxidation resistant, but not as hard as borides. SiUcide coatings formed on molybdenum (51 pm in 3 h) at 675°C have superior oxidation resistance. At these low temperatures, the molybdenum substrate does not embrittle and the coatings are quite flexible. 

In diffusion coating, the substrate surface is enriched in an element that will provide high-temperature corrosion resistance. Typical elements are chromium (chromizing), aluminium (aluminizing), or silicon (siliconizing). The substrate is involved... 

Reduction and dissociation reactions both result in deposition and can yield a 100% concentration of the coating metal M at the surface with a consequent dimensional and weight increase. In chromizing, however, the experimental results suggest that the iron surface is catalytic, and the rates of these reactions also diminish with time. Fnrthermore, reduction reqnires an excess of H2 and a low vapor pressure of HX. All of the above will be facilitated by the highest possible concentration (vapor pressure) of MX2 and removal of reaction products. 

FIGURE 10.16 Variation of coating thickness with time and temperature during chromizing. (From T.P. Hoar and E.A.G. Croom, J. Iron Steel Inst. 169,1951,101. With permission.)... 

Other elements can influence the formation of the coating by altering the rate of diffusion or by modifying the surface reactions. Sulfur and phosphorus have a poisoning effect on the chromizing media most other elements influence the rate of diffusion by virtue of their effect on the stability of the metallic phases present. 

On iron and low-carbon steels, there is a considerable variation in structure with chromizing temperature, and at 950°C, the coating is mainly composed of columnar ferrite crystals. A lamellar structure, mainly ferrite + carbide, becomes apparent as the temperature is raised, with a layer of martensite as the substrate/coating interface on specimens of higher (0.16%) carbon content. At temperatures >1200°C, the coatings consist mainly of columnar grains of ferrite with a layer of martensite (approximately one-third to one-quarter of the total thickness) at the interface. 

With a few exceptions, coatings and linings are not used on the water and steam sides. In an EPRI project, about 50 turbine blade coatings have been evaluated, but none of these are being routinely applied. To reduce steam side oxidation in reheaters and superheaters, chromizing and chromating have been developed but these treatments are also not routinely applied. There is little use of composite materials with the exception of condenser tube sheets, which could be made of explosively clad stainless steel or titanium on carbon steel, and of the surfaces in the primary cycles of nuclear units where carbon or low alloy steels are protected by weld-deposited stainless steels. In pulp mill black liquor recovery boilers, stainless steel clad boiler tubes are often used. 

As production demands have led to increasingly higher operating temperatures and pressures, protection of carbon steel waterwah tubes has involved studding, application of weld overlay or thermal spray, or the use of composite (stainless-clad carbon steel) or chromized (diffusion-coated carbon steel) tubes to provide resistance to sulfidation and oxidation on the fireside. 

Fig. 10. Diffusion coatings on C45 steel produced by chromizing (a) xl50, and aluminizing (b) x250.

Why calorizing, siliconizing and chromizing are considered diffusion coatings In which industry aluminized coatings are used ... 

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