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
  • C23G1/02—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
• • 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
• • 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

Definitions

• the invention relates to a novel method for derusting metallic surfaces.
• Rust forming on metal surfaces is a common problem in industry. As a result of the formation of rust deposits, pit corrosion occurs inside steel pipes. Pit corrosion can damage, and causes leakage in pipes. Therefore, cooling installations frequently have to shut down and as a direct result, also the plant for intensive inspection. This all is very time consuming and very expensive. It is known to use lactic acid for rust removal, but this product has the disadvantage that when applied during longer periods itself causes corrosion.
• citric acid When citric acid is used also the combination citric acid-ammonium citrate-tin sulfate is described.
• the metal parts are thereby immersed in the derusting solution for about 15 minutes and then rinsed with a basic solution. It was found that this solution is less suitable for plant installations, which cannot be immersed therein but through which the solution must be pumped for a considerable longer time than used in WO 2005/033365 . When used in that manner unacceptable damaging of steel pipes and the like may occur.
• citric acid is a solid, making dosing and dissolving in cooling water difficult, particularly leading to difficulties at effective pH control.
• the invention pertains to a method for derusting a metallic surface by bringing the surface in contact with an aqueous solution of a buffer having a pH between 5.1 and 6.5, wherein the buffer comprises lactic acid and the sodium, potassium and/or ammonium salt of lactic acid.
• the present invention is of particular importance for removing rust within steel pipes as used in plant installations such as cooling installations. Pit corrosion in such pipes can cause serious damage and leakage of the pipes. Prevention is thus of utmost importance to prevent shut down of the installations and intensive inspection.
• the lactic acid is buffered with its sodium, potassium, and or ammonium salt.
• a buffer system comprising lactic acid and ammonium lactate is preferred. Such systems do not longer require the presence of zinc sulfate, which is further advantage of the present method.
• the invention can be performed in the pH range 5.1 to 6.5, but at the higher pH values the derusting process is much slower. For that reason it is preferred to perform the process at a pH between 5.1 and 5.7.
• the method can be used at ambient temperature but when more quicker removal of rust is required the temperature of the aqueous acid solution may be raised to 70 °C, and if necessary to even higher temperatures until the boiling point of water.
• the method of the invention is of particular interest for plant cooling systems through which the aqueous acid solution is pumped for usually 4 to 12 hours.
• the circulation time can be shortened, which is favorable for shortening the shut down time.
• the panels were partly submerged into a stirred 5% or 10 % (w/w) sodium lactate solution at different concentration and temperatures.
• the glass beakers, containing the partly submerged panels, were covered with aluminum foil. This simulates a closed system in the daily practice.
• the panels were lifted out of the test solutions, immediately rinsed with demi-water (to remove the acid), rinsed with acetone, dried with an air hose, weighed, and finally photographed. At every time interval the time was stopped, until the panels were partly submerged in the test solutions again. After 48 hours the test was stopped.
• Ammonium lactate solution was prepared from commercially available ammonium lactate to obtain a 10.0% (w/w) ammonium lactate solution. This concentration corresponds to a calculated concentration of lactic acid of 8.4% The solution was than brought to pH 5.7 by addition of 0.7% sodium hydroxide.
• citric acid As comparison (di)ammonium citrate was prepared from citric acid as follows. The weight in amount of citric acid was based on the theoretically starting amount of L(+)-lactic acid in the used ammonium lactate solution. This is 8.4 % (w/w) acid. The citric acid was partly neutralized by adding 6.8% of ammonia, 25% to give pH 5.7. As a result, the calculated concentration of ammonium citrate in this solution was 10% as well.
• Table 1 shows that both sodium lactate and ammonium lactate are effective in derusting steel panels, the latter being more effective than sodium lactate. Ammonium citrate is not effective at this pH.
• the prior art pH conditions pH 3-5
• Table 2 shows that the pH conditions (pH 3-5) according to the prior art lead to substantial more corrosion than the instant invention operating at higher pH.

Landscapes

• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• General Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Preventing Corrosion Or Incrustation Of Metals (AREA)

Priority Applications (2)

Applications Claiming Priority (1)

Publications (1)

Family

ID=38191070

Family Applications (1)

Country Status (2)

Citations (6)

• 2007
  • 2007-01-25 EP EP07101166A patent/EP1950327A1/de not_active Withdrawn
• 2008
  • 2008-01-25 WO PCT/EP2008/050893 patent/WO2008090219A1/en active Application Filing

Patent Citations (6)

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