Chromizing

There is also a two-step process of chromizing foUowed by aluminizing. Above 900°C the chromizing begins to rediffuse and the protective oxide changes to Al O from Cr202. Aluminum oxide is less volatile than chromium oxide and better for high temperature oxidation resistance above 1000°C. 

If the rules for volatiles and thermodynamics of the haUdes are followed, the reaction can be used for aluminizing, silicidizing, chromizing, and similar processing. 

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. 

Titanium bromide [7789-68-6] TiBr, is claimed as a catalyst for olefin polymerizations (18). Chromous bromide [10049-25-9] CrBr2, is used in chromizing. Chromic bromide [10031-25-1], CrBr, and tungsten bromide [13701 -86-5], are catalysts for polymerizing olefins (19). Manganese... 

Electroplating, Chromizing, and Other Chromium-Surfacing Processes... 

Chromizing. The other principal method of obtaining a chromium-rich surface on steel is by chromizing (29). The material to be treated is embedded in a mixture of ferrochromium powder, a chromium halide, alumina, and sometimes NH Cl. The chromium is diffused in by a furnace... 

New Chromizing and Chromium-Aluminizing Processes, Mach. Prod. Eng. (May 3, 1967). 

Incamat, a. flesh-colored, inchromiereu, v.t. (Metal.) chromize. Indanthren-blau, n. indanthrene blue, -gdb, n. indanthrene yellow. 

Chromizing and Related Diffusion Processes. Chromizing is similar to aluminizing. A thin corrosion and wear resistant coaling is applied to low cost steels such as mild steel, or to a nickel-based alloy. In the related boroni/ing process, a thin boron alloy is produced for extreme hardness, wear, and corrosion resistance. Siliconizing is yet another process used especially lor coaling of the refractory metals Ti. Nb. Ta. Cr. Mo. and W. 

Are there other metal treatments beside alonizing (or aluminizing) that are beneficial Chromizing might be tested. Will any of these treatments minimize both the kinetics of coking and also the rate of surface corrosion ... 

At the basic level CVD reactors fall into two classifications—open and closed reactor systems. In the closed CVD system the precursors are loaded into the CVD chamber together with the specimens to be treated. The system is then closed and the temperature increased to initiate the chemical process. The process continues for a time sufficient to produce the required effect. The temperature is then reduced to ambient so that the reactor may be opened and the specimens removed. This reactor design is frequently used for the purification of metals and chromizing parts. The dominant type of CVD reactor is the open type. Flowing precursors continuously enter this reactor and the gaseous by-products of the chemical process are continuously removed from the reactor (usually with a vacuum pump) and appropriately treated for discharge into the environment. 

USE In chromizing manuf of Cr metal and compds as catalyst for polymerization of olefins and other organic reactions as textile mordant in tanning in corrosion inhibitors as waterproofing agent. 

USE In chromizing in prepn of Cr metal in catalysts for organic reactions as 02 absorbent in analysts. 

USE In chromizing in catalytic cracking of hydrocarbons alkylation catalyst in nuclear reactor fuels. Caution A strong irritant. 

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... 

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... 

The underlined symbols refer to species in the soUd substrate.) Displacement reactions are not significant in the aluminizing of Ni-base alloys but often predominate in processes such as chromizing of iron and steels. ... 

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

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