Scale removal

Scale cleaning Scale inhibitors Scale removal Scales... 

Sulfamic acid has a unique combination of properties that makes it particularly well suited for scale removal and chemical cleaning operations, the main commercial appHcations. Sulfamic acid is also used in sulfation reactions, pH adjustment, preparation of synthetic sweeteners (qv), and a variety of chemical processing appHcations. Salts of sulfamic acid are used in electroplating (qv) and electroforrning operations as well as for manufacturing flame retardants (qv) and weed and hnish killers (see Herbicides). 

Metal Cleaning. Citric acid, partially neutralized to - pH 3.5 with ammonia or triethanolamine, is used to clean metal oxides from the water side of steam boilers and nuclear reactors with a two-step single fill operation (104—122). The resulting surface is clean and passivated. This process has a low corrosion rate and is used for both pre-operational mill scale removal and operational cleaning to restore heat-transfer efficiency. 

The most commonly used acids in preplate processes are hydrochloric acid, HCl, more common for mst and scale removal, and sulfuric acid,... 

Salts are obtained by direct neutralization of the acid with appropriate oxides, hydroxides, or carbonates. Sulfamic acid is a diy, non-volatile, non-hygroscopic, colourless, white, crystalline solid of considerable stability. It melts at 205°, begins to decompose at 210°, and at 260° rapidly gives a mixture of SO2, SO3, N2, H2O, etc. It is a strong acid (dissociation constant 1.01 x 10 at 25° solubility 25gper 100g H2O) and, because of its physical form and stability, is a convenient standard for acidimetry. Over 50000 tonnes are manufactured annually and its principal applications are in formulations for metal cleaners, scale removers, detergents and stabilizers for chlorine in aqueous solution. 

Kesselstein, m. boiler scale compass brick, -ablagerung, /. deposit(ion) of boiler scale, -beseitigung, /. removal of boiler scale, -be-seitigungsmittel, n, (boiler) scale remover, disincrustant. -bilder, -bildner, m. (boiler) scale former, -bildung, /. (boiler) scale formation, incrustation. -gegenmittel, n. boiler compound, disincrustant. -kruste, /. incrustation of boiler scale. -losemittel, -losungamittel, n. (boiler) scale solvent, dis ... 

From the corrosion point of view, it is very important to control the deposition of scale. Removal of deposited scale by mechanical means is the first step. Standard, industrial water-treating techniques can be used to control scale deposition in general. In deep, hot wells or geothermal wells it is best to avoid untreated makeup water (i.e., geothermal brines). 

Porosity Gas absorption Remove surface scale Remove surface moisture Cleaner gas shield... 

Mechanism of Scale Removal from Steel with Acid... 

Fig. 11.3 Mechanism of scale removal with acid, (a) High-temperature scale and (b) low-...

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... 

The furnace scales which form on alloy steels are thin, adherent, complex in composition, and more difficult to remove than scale from non-alloy steels. Several mixed acid pickles have been recommended for stainless steel, the type of pickle depending on the composition and thickness of the scale For lightly-scaled stainless steel, a nitric/hydrofluoric acid mixture is suitable, the ratio of the acids being varied to suit the type of scale. An increase in the ratio of hydrofluoric acid to nitric acid increases the whitening effect, but also increases the metal loss. Strict chemical control of this mixture is necessary, since it tends to pit the steel when the acid is nearing exhaustion. For heavy scale, two separate pickles are often used. The first conditions the scale and the second removes it. For example, a sulphuric/hydrochloric mixture is recommended as a scale conditioner on heavily scaled chromium steels, and a nitric/hydrochloric mixture for scale removal. A ferric sulphate/ hydrofluoric acid mixture has advantages over a nitric/hydrofluoric acid mixture in that the loss of metal is reduced and the pickling time is shorter, but strict chemical control of the bath is necessary. 

The inhibitor should not decompose during the life of the pickle nor decrease the rate of scale removal appreciably. Some highly efficient inhibitors, however, do reduce pickling speed a little. It would be expected that since the hydrogen evolution is reduced the amount of hydrogen absorption and embrittlement would also be reduced. This is not always the case thiocyanate inhibitors, for example, actually increase the absorption of hydrogen. 

The mechanisms of oxide dissolution and scale removal have been widely studied in recent years. This work has been thoroughly reviewed by Frenier and Growcock who concluded, in agreement with others", that oxide removal from the surface of steel occurs predominantly by a process of reductive dissolution, rather than by chemical dissolution, which is slow in mineral acids. 

Scale removal is also assisted by the dissolution of the underlying metal by normal acid corrosion processes, which undermines the scale, and by the physical effect of hydrogen gas evolved in this latter reaction. Some authors attribute major effects to the latter. 

In general there does not appear to be any direct correlation between the rate of the chemical dissolution of oxides and the rate of scale removal, although most work on oxide dissolution has concentrated on magnetite. For example, Gorichev and co-workers have studied the kinetics and mechanisms of dissolution of magnetite in acids and found that it is faster in phosphoric acid than in hydrochloric, whereas scale removal is slower. Also, ferrous ions accelerate the dissolution of magnetite in sulphuric, phosphoric and hydrochloric acid , whereas the scale removal rate is reduced by the addition of ferrous ions. These observations appear to emphasise the importance of reductive dissolution and undermining in scale removal, as opposed to direct chemical dissolution. 

Acids injected down hole for scale removal treatments are extremely corrosive to the production tubing and casing liners. Inhibitors are added to the... 

E. D. Burger and G. R. Chesnut. Screening corrosion inhibitors used in acids for downhole scale removal. Mater Performance, 31(7) 40-44, July 1992. 

A 25-year-old Caucasian man presents with itchy lesions on his scalp, chest, back, elbows, and knees. He says these lesions started about a month ago, and seem to be spreading. Upon examination, the lesions are well-demarcated and are reddish-violet in color—easily distinguished from normal skin. They appeared raised and are covered with loose scales. Removing the scales caused pinpoints of bleeding to show up. 

The process was demonstrated on a semi-industrial scale—removal of Ikgh As. The operation of the extractors was reliable. The phase entrainment in each stage was less than 0.05% at an organic/aqueous phase ratio of 1. 

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