Halides, high temperature corrosion

Another problem in high-temperature corrosion can be the effect of the formation of volatile metallic halides which can, in turn, disrupt the integrity of a protective surface oxide. Figure 7.73 shows that in the Ti-O-Cl system at very low oxygen potentials, volatile TiClj can be formed directly from TiO and Ti, whereas from Fig. 7.74 it is clear that in the system U-O-Cl at 450°C the volatile chloride cannot be formed directly from the oxides. 

High-temperature Corrosion of Metals by Cases 643 product oxide, sulfide, or halide ... 

Tatsuya, K. and Shuichi, F., Pitting Corrosion of Titanium in High-temperature Halide Solutions , Proc. 2nd Ini. Conf. Titanium Sci. Technol., 4, 2383 (1973)... 

The outstanding characteristics of the noble metals are their exceptional resistance to corrosive attack by a wide range of liquid and gaseous substances, and their stability at high temperatures under conditions where base metals would be rapidly oxidised. This resistance to chemical and oxidative attack arises principally from the Inherently high thermodynamic stability of the noble metals, but in aqueous media under oxidising or anodic conditions a very thin film of adsorbed oxygen or oxide may be formed which can contribute to their corrosion resistance. An exception to this rule, however, is the passivation of silver and silver alloys in hydrochloric or hydrobromic acids by the formation of relatively thick halide films. 

At the reaction temperatures involved, molten nitrate systems are not particularly corrosive and conventional materials, such as stainless steels, are probably adequate for the reaction, dissolution, and separation steps. If high-temperature volatilization or transfer to a halide system is necessary, then other materials will be required. 

Bismuth fluoride (Bip3) is found in the form of white or gray dimorphic crystals, practically insoluble in water, but soluble in concentrated hydrofluoric acid with the formation of complexes. It volatilizes slowly with partial decomposition at high temperatures. Bismuth bromide (BiBr3) is found as yellowish crystals, soluble in aqueous alkali halides and dilute hydrochloric acid, but practically insoluble in alcohols. It is readily decomposed by water to give bismuth oxybromide BiOBr. Bismuth iodide (BiU) is a black fine crystalline solid with a metallic sheen, practically insoluble in water, but slowly decomposes in hot water. It dissolves in liquid ammonia, aqueous potassium iodide, hydriodic acid and hydrochloric acid, but not so much (ca. 3.5%) in absolute ethanol. When exposed to air for a prolonged time, it is slowly converted to bismuth iodate (Bi(I03)3). Bismuth fluoride of 98% purity costs US 24.30 ( 6200) per 25 g, bromide of 98% purity US 24.90 ( 6200), and iodide of 99% US 34.50 ( 8600). These halides are all corrosive and moisture-sensitive. 

RE enhancing the selective oxidation of Cr to form stronger Cr203 scales Perez et al. [64] studied the effect of Si, Mo, and Ce implantation in AISI 304 stainless steel to test their corrosion resistance. At high temperatures. Si was found to behave similarly to a RE metal. Mo has beneficial properties in halide environments. 

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