Descaling

Descales, B., D. Lambert and A. Martens (1989), Determination des nombres d octane RON et MON des essences par la technique proche infrarouge . Petrole et Techniques (Revue de I Association Frangaise des Techniciens du Petrole), No. 349. 

Metallic sodium is vital in the manufacture of esters and in the preparation of organic compounds. The metal may be used to improve the structure of certain alloys, to descale metal, and to purify molten metals. 

An important iadustrial use of NaH involves its in situ formation ia molten NaOH or ia fused eutectic salt baths. At concentrations of 1—2% NaH, these compositions are powerful reducing systems for metal salts and oxides (5). They have been used industrially for descaling metals such as high alloy steels, titanium, zirconium, etc. 

Alkaline permanganate pretreatment of steel for the removal of heat scale and smut prior to acid pickling results in faster descaling and reduced metal attack (see Metal surface treatments Metal treatments). Stainless steel alloys can also be cleaned by alkaline permanganate followed by pickling in nonoxidi2ing acids (260). 

In some metal-forming operations such as rod and wire drawing, various surface treatments are appHed to the workpiece. These include descaling, cleaning the apphcation of lubricant carriers, and the use of lubricants (see Lubrication and lubricants). Descaling can be mechanical or chemical (pickling). 

Use acid-free methods (mechanical methods such as blasting) for descaling, where feasible. 

Electrolytic Cleaning—method of degreasing/descaling metal surfaces via electrolysis. The metal is utilized as an electrode. 

Abblasehahn, m. blowoff cock, abblasen, v.t. blow away, blow off, blow blast (castings) distil off. — v.i. (of gases) expand. Abblaseventil, n. blowoff valve, abblassen, v.i. fade, lose color, grow pale, abblattem, v.r. i. scale off, exfoliate desquamate shed the leaves. — v.t. descale strip of leaves. 

The subject is also closely related to fuel-ash corrosion which in most cases is caused by a layer of fused salts such as sulphates and chlorides Attention has been focused on the electrochemistry of this type of corrosion and the relevant thermodynamic data summarised in the form of diagrams . Fluxing and descaling reactions also resemble in some respects reactions occurring during the corrosion of metals in fused salts. A review of some of the more basic concepts underlying corrosion by fused salts (such as acid-base concepts and corrosion diagrams) has appeared. ... 

Rohrig, van Duzer and Fellows exposed samples in an experimental superheater fed with steam at 2-6MN/m from a power plant. Some 42 materials were tested for periods of up to 16(XX)h, attack being estimated after test by weight loss following descaling. It was concluded that at 593°C attack continues at a high rate on carbon steel, whereas the rate for most alloy steels decreases with time (Table 7.10). 

Test temperature rc) Weight loss after descaling following a 24 h test (g/mO ... 

When specifying equipment to fabricators, it should be remembered that equipment may well lie exposed on-site before erection and temporary corrosion protective measures should be considered. Any equipment precommissioning treatments specified by the design, e.g. descaling, must be carried out. 

Pitting (Sections 1.5 and 1.6) Pitting of carbon steel is seldom catastrophically rapid in service and can often be accommodated within the corrosion allowance for the equipment. It often takes place under scale or deposits so that regular descaling of equipment can be beneficial. 

Zinc is seldom used as a power-impressed anode. It may be a convenient way of achieving a high initial current density, particularly where descaling is involved, but it does, of course, require the anode to be locally insulated from the cathode. Used as a power-impressed anode the energy rate per... 

Although phosphoric acid can be used for pickling steel, it is seldom used simply for scale removal since it is so expensive and slow in action. Steel plates are often initially descaled in sulphuric acid and then, after rinsing, immersed in 2% phosphoric acid containing 0-3-0-5% iron at 85°C for 3-5 min. The plates are then allowed to drain and dry without further... 

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