Scaling cast iron

The largest use for calcium carbide is in the production of acetylene for oxyacetylene welding and cutting. Companies producing compressed acetylene gas are located neat user plants to minimize freight costs on the gas cylinders. Some acetylene from carbide continues to compete with acetylene from petrochemical sources on a small scale. In Canada and other countries the production of calcium cyanamide from calcium carbide continues. More recentiy calcium carbide has found increased use as a desulfurizing reagent of blast-furnace metal for the production of steel and low sulfur nodular cast iron. 

The scale can also be removed by shot blasting using abrasive grits such as dry sand, less than I mm (p. This method is more suited for components not suited to the dip method and cast iron components, in which the acid may become trapped in the porous surfaces. For sheet-metal components and complicated shapes and crevices, the dip method alone is recommended. 

The parameters of pipe bore, flow, length of pipe, and specific gravity can be used to determine pressure loss. The calculators are versatile in that they can be used for pressure losses of under 1 mbar and for up to 50 bar. Scales for both cast iron and steel pipe are available for greater accuracy. 

Borehole waters are generally very hard and cast iron pipes are still used because of the low internal corrosion rates permitted by the scaling which occurs naturally. Acidic waters cause graphitic attack on cast irons. 

Those waters in which the carbon dioxide content is in excess of that required as bicarbonate ion to balance the bases present are among the most aggressive of the fresh waters. Hard waters usually, though not invariably, deposit a carbonate scale and are generally not appreciably corrosive to cast iron, corrosion rates of less than 0-02 mm/y being frequently encountered. Water-softening processes do not increase the corrosivity of the water provided that the process does not result in the development of an excess of dissolved carbon dioxide. 

When cast iron is exposed to high temperatures under oxidising conditions, oxidation of the metal results, with the formation of a surface scale. In addition, the dimensions of the component become distorted. Although such dimensional changes can occur also in inert atmospheres or in vacuum, the evidence available suggests that this growth is frequently associated with oxidation, and accordingly it is appropriate to consider it as an aspect of the corrosion of the iron. 

Fig. 7.18 Scaling rates for cast iron in pure gases (Fe-3-20Cioiai-l 13Si-0-72Mn-0-125S-...

Scaling results for the flake graphite cast irons a, b, d and e in Fig. 7.21 show that the weight increase related to surface area due to scaling rises as the quantity of graphite increases with increase in carbon content (iron a), and as the graphite becomes more widely dispersed in the matrix (iron d). Apart from carbon content, changes in the base metal composition and... 

It is interesting that the first large-scale application of cathodic protection by Davy was directed at protecting copper rather than steel. It is also a measure of Davy s grasp of the topic that he was able to consider the use of two techniques of cathodic protection, viz. sacrificial anodes and impressed current, and two types of sacrificial anode, viz. zinc and cast iron. 

Frit may be milled dry or wet. The long established dry process is used for cast iron baths and for chemical plant. Vitreous enamel application by a dry electrostatic method is being used on an increasing scale. In these cases, the frit is milled alone, or with inorganic colouring or refractory additives. This is achieved in cylinders using balls of porcelain, steatite or more dense alumina, or with pebbles of flint, to produce a fine powder of predetermined size. 

On an industrial scale, diazotization reactions are chiefly carried out in cast iron kettles that are lined with brick or rubber as a protection against acid. Wooden vats also continue to be used. 

Depending on the technical requirements such as corrosion resistance, pressure and temperature stability, industrial scale azo pigment synthesis is carried out in appropriate equipment. Suitable materials include cast iron, stainless steel, steel lined with rubber, acid-proof brick, enamel, synthetic resins supported by glass fiber, and wood. 

Minimum impact energies to initiate the explosion of various exothermic mixtures, used for the continuous casting of steel, were determined. Components used included sodium nitrate, aluminium-iron scale, silicocalcium, ferrosilicon fluorspar, borax, etc. Hazardous mixtures were defined, and improved safety controls were derived. 

Information about the corrosion of boiler piles comes from analysis of scale samples and also from laboratory experiments. Smith and McEaney (1979) used XRD and SEM to follow the initial stages in the development of scale on gray, cast iron in water at 50 °C. At first, the corrosion product was a mixture of magnetite and green rust. Whether lepidocrocite formed depended on the level of oxygen in the system. 

Fig. 18.7 Changes with time in the levels of the three iron oxide components of scale on grey cast iron (Smith and McEaney,...

On a manufacturing scale, an aq. soln. of hydrofluorio acid is made by gently heating the best quality of powdered fluorspar, free from silioa, to about 130° along with cono. sulphuric acid—containing 10 per cent, of water. The retort is a cast-iron pot with a cast-iron cover. The rim of the cover dips into an annular trough the joint is sealed with cone, sulphuric acid. The retorts are connected with a series of lead boxes as condensers. These contain water or dil. hydrofluorio acid to absorb the gas from the retorts. The condensers are submerged in water to keep them cool, and the acid so obtained is collected in leaden bottles. 

Ammonium carbonate is prepared on a large scale by distilling an intimate mixture of ammonium sulphate with twice its weight of chalk in cylindrical cast-iron vessels, 7 to 10 ft. long and 1J ft. diameter, and fitted with movable leaden covers at each end. Several retorts are heated in one furnace. The vapours from the- retorts are led into stone chambers where the carbonate,is deposited as a crust... 

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