Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/24—Cleaning or pickling metallic material with solutions or molten salts with neutral solutions
• • C—CHEMISTRY; METALLURGY
  • C11—ANIMAL OR VEGETABLE OILS, FATS, FATTY SUBSTANCES OR WAXES; FATTY ACIDS THEREFROM; DETERGENTS; CANDLES
  • C11D—DETERGENT COMPOSITIONS; USE OF SINGLE SUBSTANCES AS DETERGENTS; SOAP OR SOAP-MAKING; RESIN SOAPS; RECOVERY OF GLYCEROL
  • C11D7/00—Compositions of detergents based essentially on non-surface-active compounds
  • C11D7/22—Organic compounds
  • C11D7/26—Organic compounds containing oxygen
  • C11D7/265—Carboxylic acids or salts thereof
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
  • C23G1/08—Iron or steel
  • C23G1/088—Iron or steel solutions containing organic acids
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28G—CLEANING OF INTERNAL OR EXTERNAL SURFACES OF HEAT-EXCHANGE OR HEAT-TRANSFER CONDUITS, e.g. WATER TUBES OR BOILERS
  • F28G9/00—Cleaning by flushing or washing, e.g. with chemical solvents

Definitions

• boiler plants must be etched before placing them in operation.
• the welding and bending work done during assembling and installing causes scaling or soiling. Any such foreign substances must be removed to assure satisfactory boiler and turbine performance. This removal is done by the etching treatment. If the narrow boiler tubes are not etched or not cleaned in due time, they become clogged by iron sludge resulting from rust or scale. This may cause tearing or exploding of the tubes. Etching also affords a more rapid starting-up of the plant, because the steam purity required for turbine operation can be reached within a few hours, whereas without etching several months may be required before this steam quality is achieved.
• the etching or cleaning solution usually composed of one or several acids with additions, is supposed to be inexpensive and to be so constituted that it does not subject the etched or cleaned tube inner surfaces to an attack greater than 20 g./m. This amount is the approximate permissible maximum of chemically removed material, regardless of whether the etching or cleaning operation is completed within a few hours or extended over several days.
• the additions to the etchant are preferably temperature resistant to the extent required for utilizing the favorable cleaning effect in the temperature range of 20 C. up to about 200 C.
• etching or cleaning solutions having an acid content of 1% to 10% and auxiliary substances and inhibitors.
• the inhibitor serves to reduce the acid attack upon the pure metal without aifecting the dissolution rate, for example of the iron oxides.
• inhibitor may consist, for example, of organic amines, unsaturated dioles, pure or substituted coke-tar products. It has also been proposed to use hydrazine or hydroxylamine condensation products as inhibitors. The manifold effects of these and other substances acting as inhibitors are known; these subtances, however, cannot be used in the entire pH- and temperature range, since, in some cases, the above-mentioned total corrosion limit of 20 g./m. can be preserved only within limited ranges.
• etching solutions composed of mixtures of hydroxyl-group containing organic acids which, on the one hand, dissolve rust and scale and, on the other hand, maintain the dissolved iron as a complex compound in solution.
• synergistic mixtures of known complex forming substances among others for example, citric acid, hydroxyacetic acid, gluconic acid, heptagluconic acid, malonic acid derivative, tartaric acid, polyphosphates, ethylenediaminetetracetic acid, methyl tartronic acid or other organic acid, which may also be employed in the form of the ammonium salts or organic amines, with or without addition of mineral acids and/ or organic acids for adjusting to the desired pH value, particularly at temperatures at which the dissolution of metal oxides and other depositions takes place.
• the synergistic mixtures result in a bonding ability for metal ions which is much larger than the sum of the individual components, the required quantities, relative to the equivalents, are smaller than the metal-oxide equivalents present.
• the total quantities of complex forming agents to be used may be successfully limited to a total of 2 to 5 g./l. (grams per liter). In previously operated plants, it is sometimes of advantage to make preliminary tests with tubes in which depositions of respectively different compositions are ascertainable, in order to thus determine a particularly favorable synergistic mixture for the particular plant.
• concentrations of up to 5% and more may be added to the etching solution at a pH value most favorable to the particular depositions.
• concentrated synergistic mixtures may be injected into the plant shortly behind the feedwater pump.
• the sludge formation is almost completely prevented. It is known as such that larger or smaller quantities of Fe O or Fe(OH) will precipitate from solutions having a relatively low content of complex forming agents (for example, citric acid and ethylenediaminetetracetic acid), depending upon time, temperature and pH value. With the known methods, however, the precipitated quantities can no longer be dissolved in the etching solution present. That is, the receptivity of the small amount of complexing agents in these solutions becomes exhausted after a certain limited time, whereafter large amounts of F6203 or Fe(OH) will precipitate. The resulting sludge must be drawn from the system from time to time.
• complex forming agents for example, citric acid and ethylenediaminetetracetic acid
• the predominant share of the dissolved iron is converted to the bivalent form. Only 1 mole of this bivalent iron (ferrous) can be complexed by 1 mole of citric acid, whereas 4 moles of the trivalent ion (ferric) are complexed.
• Other complex-forming agents for example, methyltartronic acid, have a considerably more favorable complex-forming ability than citric acid. If oxidizing agents, such as nitrite, are added, the dissolved iron is present in trivalent form so that the complex-forming ability of hydroxylgroup containing organic acids is still further augmented.
• a preferred mode of performing the method of the invention resides in using as etching solution, organic acids or mixtures of different organic acids capable of dissolving rust and scale and of maintaining the dissolved iron dissolved as a complex compound, the etching solution being constituted by hydroxyl-group containing organic acids or their salts and by combinations of several hydroxyl-group containing organic acids or their salts.
• the complex-forming ability and the stability of the formed complexes are further augmented.
• a combination of citric acid and methyl tartronic acid has a better complexforming ability than the same respective amounts of theindividual components of the combination.
• the complex formation can be increased to a further extent.
• the dissolution rate of rust and scale is essentially affected by the pH value of the etching solution. Dissolution still takes place in the alkaline range. Preferably however, the pH value is kept between 3 and 6. This can be done, particularly when using salts of the mentioned acid mixtures, by adding mineral acids, for ex ample, sulfuric acid, at localities within the circulation at which the pH value has risen too high.
• the etching solution may be given an addition of fluorides.
• the dissolution rate of rust and scale can be additionally or exclusively increased simply by employing higher temperatures, for example, 100 C. and more.
• the method of the invention substantially eliminates the disadvantages of the known etching and cleaning methods operating with mineral acids and organic acids, such as the necessity of operating with particularly acidresistant etchant-circulation pumps, switching the plant from normal operation to operation with such special etchant pumps, and using large quantities of acid and inhibitor additions.
• the precipitation of Fe (OH) at elevated temperatures and almost all pH values, usually occurring when using organic acids, is also virtually avoided.
• the initially chosen optimal pH value may change with the progressing dissolution of coatings, it is sometimes advantageous to correct the pH value in a suitable part of the plant with the aid of organic acids and/or mineral acids, for example sulfuric acid. It can be ascertained from the metal content of the solution during etching, whether or not the initial dosage of complexforming agents is suflicient so that an excessive consumption of chemicals can be avoided.
• This method of the invention removes metal oxides from the metal surface by locally selective dissolution and converts the oxides to a stable solution.
• the layers of scale often splitting off at the beginning of the etching or cleaning operation are virtually entirely dissolved due to the obtaining operating conditions of the plant, namely the effect of flow, temperature, pH value and solubility.
• the base metal is virtually not attacked.
• pH-value Removal q./m. h. 2.0 1.74 3.0 0.81 4.0 0.54 5.0 0.50 6.0 0.50 7.0 0.36 8.0 0.36 9.0 0.26 10.0 0.10
• a useful synergistic effect occurs only with mixtures of at least three component products here mentioned, and that such multicomponent mixtures result in excellent dissolution of iron oxide at pH values between 1.0 and 9.0.
• the process is preferably performed at temperatures between 50 C. and C. and a pH value between 4.0 and 8.0.
• This pH range of the etching and cleaning solution permits using pumps made of normal materials for feeding or circulating the etching and cleaning liquid, such as ordinary boiler feed pumps, booster pumps, condensate pumps or other pumps normally available in the plants.
• the pressure may be increased or decreased as may be desired.
• an oxidizing agent for example, alkali and/ or ammonia-nitrite, -nitrate, -hypochlorite, -chlorate,
• the potential of the synergistic mixture may be changed at a predetermined moment of the etching or cleaning operation by then adding suitable oxidizing agents in the pH range of 2 to 9 at corresponding temperatures. Then, with a constant Fe-content, selectively acting reduction or other reaction agents may also be added to the solution.
• This processing mode is likewise applicable with other etching and cleaning methods departing in other respects from the above-described method.
• Synergistic efiects have been ascertained also also in a higher pH range; and this may be utilized to advantage particularly where very thin coatings or special incrustations are involved.
• Media for increasing the pH value are, for example, NH alkali and similar substances.
• the etching solution is preferably rinsed out of the etching system with the fully desalted water, if desired with an addition of hydrazine.
• the etching circulation is preferably short-circuited, for example, through a device for desalting any condensate present in the plant, for the purpose of expediting the ultimate cleaning.
• the passivation may be performed after rinsing the system with fully desalted Water. This may be done, for exam- 6 ple, by operating the plant for about 30 hours with fully desalted water and an addition of hydrazine at temperatures above C., thus producing a magnetite protected layer on the inner surface of the tubes.
• an oxidizing agent which promotes complex-formation and is selected from the group which consists of alkali metal and ammonium nitrates, nitrites, hypochlorites, and chlorates.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Mechanical Engineering (AREA)
• Organic Chemistry (AREA)
• General Chemical & Material Sciences (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Health & Medical Sciences (AREA)
• Emergency Medicine (AREA)
• Life Sciences & Earth Sciences (AREA)
• Oil, Petroleum & Natural Gas (AREA)
• Wood Science & Technology (AREA)
• Combustion & Propulsion (AREA)
• General Engineering & Computer Science (AREA)
• Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)

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Cited By (11)

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Citations (11)

• 0
  • ZA ZA656244D patent/ZA656244B/xx unknown
• 1964
  • 1964-11-27 DE DE1546191A patent/DE1546191C3/de not_active Expired
  • 1964-11-27 DE DE19641546085 patent/DE1546085A1/de active Pending
• 1965
  • 1965-11-26 GB GB50471/65A patent/GB1119680A/en not_active Expired
  • 1965-11-26 US US510105A patent/US3510351A/en not_active Expired - Lifetime
  • 1965-11-27 JP JP40072800A patent/JPS4927031B1/ja active Pending

Patent Citations (11)

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