Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
  • F28F19/00—Preventing the formation of deposits or corrosion, e.g. by using filters or scrapers
• • 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
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
  • C23C22/60—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using alkaline aqueous solutions with pH greater than 8
• • 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

Definitions

• ABSTRACT OF THE DISCLOSURE The water and steam containing portions of a steam generator are cleaned and passivated by cleaning with an acid cleaning solution followed by neutralization and then filling the portions with a passivating solution containing 25-l000 p.p.m. hydrogen peroxide or in excess of 300 p.p.m. hydrozine at a pH in excess of 7.0.
• the passivating solution is drained from the portions and the metal surfaces exposed to the atmosphere.
• This invention relates to the prevention of corrosion and more particularly to the passivation of metal surfaces to reduce the possibility of corrosion.
• the present invention is principally concerned with the passivation of oxide-free metal surfaces prior to exposure to the atmosphere to prevent oxidation upon such exposure.
• the invention will be described with particular reference to the cleaning and passivation of the water-side heat transfer surfaces in steam generating units.
• Chemical cleaning is normally accomplished with an acid solution. Freshly cleaned surfaces are free of protective films and thus quite active and more susceptible to oxidation than scaled surfaces. This active metal surface rusts rapidly in the presence of moisture and oxygen thereby causing corrosion problems particularly if the unit is to be out of service for a period of time. If the unit is to be kept in wet storage, i.e., filled with water, an inhibitor is normally placed in the water so as to inhibit corrosion. On the other hand, if the unit must be drained so as to expose the metal surfaces to the atmosphere, it is quite important that the surfaces be passivated prior to draining. It is this latter passivation to which this invention is directed.
• An object of the present invention is, therefore, to provide a novel technique for passivating metal surfaces to prevent oxidation and corrosion.
• A- specific object of the present invention is to provide a technique for chemically cleaning a steam generating unit and passivating the cleaned metal surfaces to reduce corrosion upon exposure to the atmosphere.
• Another object of the invention is to provide a passivating technique utilizing materials which contain no dissolved solids.
• Chemical cleaning procedures generally involve four steps which are: (l) pre-boiling with alkaline materials, (2) contacting of the surfaces with an acidic solvent or a solvent containing a chelate, (3) neutralizing and postboiling with a strong alkaline solution, and (4) passivating the freshly cleaned surfaces if an extended lay-up period is anticipated.
• the alkaline pre-boil is not a necessary procedure and is usually only carried out in preoperational cleaning to remove organics.
• a suitable material for this pre-boil would be a solution of Na CO and Na PO
• the conventional material utilized for the acid cleaning step is a solution of inhibited hydrochloricacid, usually a 5 percent by weight solution.
• the acid be inhibited so that once the mill scale and deposits have been removed, the acid attack on the base metal will be minimized.
• Many inhibitors are commercially available and they vary depending upon the acid with which they are to be used. Polyamines are used in many of these products. Dow Chemical Co., for example, has an inhibitor for hydrochloric acid designated as A-120. This solution is admitted to the boiler and the temperature of the solvent and metal is maintained at about ISO- F.
• suitable acid cleaning materials include organic solvents such as citric acid, formic acid, hydroxy-acetic acid and ethylenediarnine tetraacetic acid.
• Hydrochloric acid is a much more rapid cleaning agent than the organic solvents and the organics are, therefore, used for applications where the chloride ion is objectionable such as with austenitic stainless steels.
• the acid solution is drained from the boiler under a positive pressure of nitrogen.
• the boiler is then rinsed with demineralized water to remove any residual acid and filled with a neutralizing solution consisting of 0.5 percent by weight soda ash.
• the passivating technique of the present invention is carried out.
• This technique is directed to the use of passivating materials which will effectively prepare a boiler for exposure to the atmosphere and which will be volatile and leave behind no solids.
• the invention is particularly directed to the use of hydrazine and hydrogen peroxide as passivating materials.
• Hydrazine has been employed in the past in low concentrations as an oxygen scavenger to remove dissolved oxygen from boiler water during wet storage.
• the present invention is directed to the use of this material in greater concentrations prior to the draining of the cleaning and rinse solutions from the boiler to passivate the metal surface.
• hydrazine should be present in solution at concentrations in excess of 300 p.p.m., although slightly lower concentrations will provide some degree of passivation.
• the upper limit to the concentration of hydrazine in the passivating solution will be determined by economic factors since at a certain level the cost of adding more hydrazine to the solution could not be justified by the small increase in passivating capacity.
• a solution of perhaps 1 percent hydrazine would be a practical upper limit.
• a desirable pH for the hydrazine passivating solution is in the range of 9.5 or 10. The pH is adjusted to the desirable value with ammonia or some other amine which will itself add no solids to the system such as morpholine or cyclohexylamine. Test specimens passivated with hydrazine have, upon exposure, retained their luster for periods of at least two months under various adverse atmospheric conditions.
• the hydrogen peroxide passivating solution of the present invention has a hydrogen peroxide concentration of preferably less than 1000 ppm. and perhaps down to 50 or 25 p.p.m. although such low concentrations would not be very effective.
• the pH of the hydrogen peroxide solution is also adjusted with ammonia or other amines to a value somewhere around 10. Although this pH adjustment is not necessary, the fastest and best passivation can be obtained if the pH is elevated to a valve in excess of 7.0.
• the upper concentration limit of 1000 ppm. was selected since an increase in the concentration would not greatly increase the effectiveness and higher concentrations might be slightly hazardous in the presence of oxidizable materials, such as organics.
• the products of the hydrogen peroxide passivation of pickled steel are the unobjectionable materials of water and oxygen.
• the byproducts of the use of hydrazine as a passivator are also unobjectionable, consisting of nitrogen, hydrogen, ammonia and hydrogen peroxide which, in turn, produces water and oxygen. It can be seen that these materials will leave behind no harmful deposits as would the passivators employing dissolved solids.
• the amount of time that it is necessary to subject the metal surfaces to the passivating solution will, of course, vary depending upon many factors. Some of these factors include the concentration of the passivating material in the solution, the pH of the solution, the temperature and particular metal being treated. The amount of time required is also dependent upon the degree of passivation desired. The longer the treatment period, the more complete will be the passivation up to a certain point beyond which no further action will take place. A period of perhaps 24 hours would yield at least some significant passivation. One factor involved in determining the period necessary will be the amount of time the metal is to be exposed to the atmosphere; the longer the expected exposure, the longer should be the passivation treatment.
• a method of cleaning and passivating the metal surfaces of the water and steam containing portions of a steam generator comprising the steps of filling said portions with an acid cleaning solution, draining said acid cleaning solution from said portions, filling said portions with a neutralizing solution, draining said neutralizing solution, from said portions, filling said portions with a passivating solution having a pH in excess of 7.0 and comprising at least in part a compound selected from the group consisting of hydrogen peroxide and hydrazine wherein the concentration of said hydrogen peroxide is from 25 to 1000 ppm. and the concentration of said hydrazine is in excess of 300 ppm. and then draining said passivating solution from said portions and exposing said metal surfaces to the atmosphere.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Materials Engineering (AREA)
• General Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• General Engineering & Computer Science (AREA)
• Preventing Corrosion Or Incrustation Of Metals (AREA)
• Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)

Priority Applications (5)

Applications Claiming Priority (1)

Publications (1)

Family

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Family Applications (1)

Country Status (4)

Cited By (7)

Citations (5)

• 1965
  • 1965-03-12 US US439457A patent/US3415692A/en not_active Expired - Lifetime
• 1966
  • 1966-03-04 NL NL6602847A patent/NL6602847A/xx unknown
  • 1966-03-05 ES ES0323833A patent/ES323833A1/es not_active Expired
  • 1966-03-07 BE BE677475D patent/BE677475A/xx unknown

Patent Citations (5)

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