Refractories high-temperature corrosion

Ruthenium and osmium have hep crystal stmetures. These metals have properties similar to the refractory metals, ie, they are hard, britde, and have relatively poor oxidation resistance (see Refractories). Platinum and palladium have fee stmetures and properties akin to gold, ie, they are soft, ductile, and have excellent resistance to oxidation and high temperature corrosion. 

Refractories. Calcined alumina is used in the bond matrix to improve the refractoriness, high temperature strength/creep resistance, and abrasion/corrosion resistance of refractories (1,2,4,7). The normal, coarse (2 to 5 )J.m median) crystalline, nominally 100% a-Al202, calcined aluminas ground to 95% —325 mesh mesh are used to extend the particle size distribution of refractory mixes, for alumina enrichment, and for reaction with... 

A few high-temperature corrosive properties of ceramics are mentioned here as examples. The book by Samsonov and Vinitskii has many details on high-tempera-ture reactions with various ambients. Table 6.6 compares the corrosion resistance of several refractories in combustion gases. 

Because of the high temperature corrosion resistance and thermal stability exhibited by magnesium-chromium oxide refractories in copper smelting, converting and fire refining furnaces, little research has been conducted on other refractory families. However, the concern for hexavalent chromium (Cr ) as a potentially hazardous waste in spent refractories from copper production furnaces has prompted the recognition that a more environmentally acceptable material is needed for the copper industry. 

Refractories are used at elevated temperatures for structural purposes, and they are used in many cases to contain a high-temperature corrosive environment. This corrosive environment usually contains liquid (melted) phases that participate in chemical reactions with the refractory at elevated temperatures resulting in refractory consumption or wear. It is usually not immediately obvious, but the oxidation and reduction state of the environment (as redox conditions or oxygen activity ) can participate in and influence the chemical reactions that take place. Along with chemical reactions during corrosion, physical changes occur that may be accelerated by the corrosion process. 

The furnace is constmcted with a steel shell lined with high temperature refractory (see Refractories). Refractory type and thickness are deterrnined by the particular need. Where combustion products include corrosive gases such as sulfur dioxide or hydrogen chloride, furnace shell temperatures are maintained above about 150—180°C to prevent condensation and corrosion on the inside carbon steel surfaces. Where corrosive gases are not present, insulation is sized to maintain a shell temperature below 60°C to protect personnel. 

Chemically Functional. Refractory coatings are used for corrosion-resistant high temperature service in gas turbine and diesel engines, components such as cmcibles, thermocouple protection tubing, valve parts, etc. 

The bath is normally at a temperature in the range 620-710°C, depending on whether the coating material is an aluminium-silicon alloy (for use in high-temperature conditions) or pure aluminium (for corrosion prevention). It is heated by inductors, by resistance heaters or by an external flame. The pot will usually be refractory lined unless cast-iron pots are needed to ensure adequate heat transfer from an external flame. As molten aluminium is extremely aggressive towards ferrous metals, replacement of cast-iron pots is fairly frequent. Refractory-lined pots obviously do not have this drawback, although the bath hardware, in particular the sinker roll and support mechanism, will still be attacked and need replacement at intervals. 

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