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
  • 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
  • C23C22/62—Treatment of iron or alloys based thereon
• • 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
  • C23C22/73—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 characterised by the process
  • C23C22/77—Controlling or regulating of the coating process
• • 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 an improvement in the method of passivation of a ferrous metal surface, e.g., steel, especially following removal of metallic copper from the surface.
• Such plated copper must be very thoroughly removed else deleterious bimetallic couple corrosion will take place when the equipment is placed back in service.
• One solution to this problem has been to introduce an oxidizing agent into the cleaning solution after substantially complete removal of scale and iron oxides.
• the copper is oxidized and stripped from the ferrous metal surface, the more readily if the solution contains materials capable of complexing or chelating copper, e.g., ammonia or the ferric or ferrous chelates of the ammonium salt of ethylenediaminetetraacetic acid.
• Suitable oxidizing agents are ferric citrate, the ferric chelate of ethylenediaminetetraacetic acid and its salts, sodium nitrite, and, oxygen or air bubbled through the cleaning solution. Also usable are soluble ferric salts, nitric acid, readily soluble tungstates and molybdates, and ammonium persulfate and ammonium bromate.
• ferrous metal surface is often brought to a passive condition in which the metal appears clean and bright, and the metal does not oxidize or rust readily for a time.
• ferrous metal surface has failed to become passive, and in fact, has carried a thin film of yellow surface oxidation at the conclusion of the cleaning operation. The cause of the seemingly random occurrence of this condition has not been understood and so it has not been possible to avoid the condition with certainty.
• An even broader object of the invention is to provide an improved method of passivating, any clean and bright ferrous metal surface.
• ferrous metal surface is brought to the desired condition when there obtains an appropriate ratio of oxidized ion species, e.g., ferric ion species, to reduced ion species such as ferrous ion species in the solution used for cleaning and/ or passivating.
• the ratio of ferric ion species to ferrous ion species may be measured in any suitable manner. The most convenient Way is simply to measure the oxidation potential of the solution.
• Both a saturated calomel electrode (S.C.E.) and one of a platinum or a ferrous metal electrode are inserted into the solution, employed for cleaning and/ or passivating, and are interconnected through a high impedance voltmeter, conveniently, a digital voltmeter. It is generally desirable to employ a ferrous metal electrode having the same or a similar composition as the ferrous metal surface being treated since such an electrode will, in general, more accurately reflect the condition of the treated surface. However, if the metal electrode cannot conveniently be of the same composition as the metal being passivated, a platinum electrode is more likely to give the most reliable determinations. Under a given set of conditions, a platinum electrode versus a saturated calomel electrode will exhibit about 10 millivolts less potential difference than the iron-saturated calomel couple.
• the cleaning solution is brought to a temperature less than about 195 F., usually to less than 180 F. and preferably to less than about 160 F., but above about F.
• the optimum temperature is approximately F.
• the pH of the solution is adjusted, if necessary, to an alkaline value, preferably in the range of from about 7.5 to about 10.
• an alkaline value preferably in the range of from about 7.5 to about 10.
• iron solutions in contact with ferrous metal exhibit broader more reproducible rest potentials, or plateaus on polarographic examination, than at lower pH values.
• the ferrous substrate would be adversely affected if the solution pH were allowed to drop below about 7.0.
• ferric citrate or an ammonia, amine or alkanolamine salt of a ferric chelate of a polycarboxylic acid chelating agent.
• Such iron complexes or chelates readily react with copper to oxidize it, leaving ferrous species in solution.
• Oxidation of the copper is effected by the addition of an oxidizing agent which generally reacts most readily with such ferrous species which in turn oxidize metallic copper.
• an oxidizing agent is air which is simply bubbled through the solution, substantially does not change the liquid volume, causes little loss of ammonia or volatile amine and builds up no residue of salts or foreign ion species.
• Suitable polycarboxylic acid chelating agents include the alkylene polyamine polyacetic acids of the formula (HOOCCH N[ (CH NCH COOH] CH COOH Used with polycarboxylic acid Amines:
• Ammonia EDTA Ethanolamine EDTA. Ethyla-mine EDTA.
• Ethylenediamine EDTA Diethylenetriamine EDTA.
• Pentaethylenehexamine EDTA Pentaethylenehexamine
• Trimethylamine EDTA Trimethylamine
• Ethanolamine Ethylenediaminetetraproplonic acid.
• iron complexing or chelating agent is suitable for stripping copper and passivating. At least a 2 weight percent solution is preferred and more preferably a 4 to about 20 percent solution. Typically a 6 to 12 percent solution is used in commercial type operations. To avoid undue attack on the ferrous metal substrate, the iron complexing or chelating agent is essentially in the condition of holding 60 to 97 percent of the theoretical maximum amount of iron.
• the iron complexing or chelating agent may be reacted or spent with iron powder to exhaust the requisite proportion of its chelating power, as a practical matter, the iron complexing or chelating agent is normally used in the requisite amounts in a prior cleaning step, involving the removal of iron oxide-containing scale, that the desired degree of binding or chelating is achieved preparatory to copper stripping and/ or passivating.
• Addition of the oxidizing agent selected is continued after stripping of the copper in order to effect passivation with the solution pH and temperature adjusted as described.
• the solution oxidation potential falls.
• Passivation as determined by visual examination of exposed specimens, or coupons, comcrnences on at least part of the steel substrate when the oxidation potential reaches about 250 millivolts (iron vs. S.C.E.) or 240 millivolts (platinum vs. S.C.E.) and diminishes and disappears after about 175 millivolts (iron vs. S.C.E.) or 165 millivolts (platinum vs. S.C.E.).
• the solution oxidation potential When the solution oxidation potential has reached a predetermined value in the stated range, the addition of oxidizing agent is stopped. If there is any time lag in reaction, as with residual copper deposits, the oxidation potential will climb again within a few minutes and further oxidizing agent is required to reach the predetermined oxidation potential. Once the potential becomes stable, the solution is promptly withdrawn from the vessel or equipment being treated, and the vessel or equipment rinsed with water at ambient room temperature. The cleaned ferrous metal surfaces are clean, bright and passive, and ready to be returned to service.
• the passivation process of the invention need not be carried out following copper stripping where the equipment does not contain, and has not just been cleaned of copper. Where copper is not involved, the vessel or equipment is cleaned so as to provide entirely clean and bright surfaces throughout. The vessel or equipment is then substantially filled with an aqueous solution containing an electrolyte such as one or more of the complexing or chelating agents listed hereinabove. It is highly desirable that such complexing or chelating agent has been spent in some manner to hold at least 60 percent by weight of the theoretical amount of dissolved iron.
• the pH is adjusted to an alkaline value, if necessary, preferably in the range of 7.5 to 10, and maintained at an alkaline pH. The temperature is maintained at less than about 195 F., usually at less than 180 F.
• the oxidation potential of the solution is monitored as described hereinbefore, and the addition of oxidizing agent is stopped when the oxidation state of the system, as reflected by the ratio of ferric ion to ferrous ion species, corresponds to an oxidation potential in the range of about 250 to millivolts, and preferably about 210 to millivolts (Fe vs. S.C.E.).
• the solution is then drained, and the vessel or equipment rinsed with clean water.
• ammoniated ethylenediaminetetraacetic acid (ammoniated EDTA) and water were added to a small simulated boiler having a steel pot connected to a glass header, disposed above the steel pot, by means of two foreshortened replaceable steel boiler tubes and a glass downcomer, all three tubes being connected in parallel.
• the boiler was also fitted with an iron electrode, a platinum electrode, and a saturated calomel electrode.
• Sufficient ammoniated EDTA solution was used to bring the liquid level well up into the glass header.
• the boiler tubes in each case, were cleaned by bringing the boiler to 212 F. for a short time. Then enough copper oxide (CuO) was added to the solution to bring the dissolved copper level, on a theoretical basis, to 0.06 percent by weight of copper. This solution was circulated until all the copper oxide dissolved and plated on the steel substrate. The solution was then spent to the desired level with iron powder and cooled, after which the pH was adjusted to about 9 with a concentrated aqueous solution of ammonia.
• CuO copper oxide
• Air blowing was then carried out to remove plated copper and to bring the steel substrate to a passive condition. Air was admitted to the steel pot through a fritted glass plug. The pH remained alkaline during each test. At the termination of the air blowing step the oxidation potential of the solution was recorded. The solution was 6 drained out to permit disassembly of the boiler and The initial potential of 812 millivolts (mv.) became inspection of the boiler tubes. smaller as air was bubbled through the solution. At The solution concentrations, temperatures, and terminal 248 mv., a copper plated steel coupon suspended in the oxidation potentials and the condition of the treated boiler boiler was stripped of copper. Once the potential had tubes are summarized in the following table.
• Chelating agent ammoniated ethylenediaminetetraacetic acid.
• Fe steel.
• S.C.E. saturated calomel electrode.
• the improvement which comprises: monitoring the state of oxidation exhibited by the ferric iron concentration-ferrous iron concentration ratio during the addition of oxidizing agent and stopping such addition when the oxidation potential of the said aqueous solution is in the range of 250 to 175 millivolts as measured with a ferrous metal electrode with reference to a saturated calomel electrode.
• the oxidizing agent is selected from the group consisting of air, and from aqueous solutions of one of, HNO NaNO K CrO alkali metal and alkaline earth metal molybdates, tungstates, ferric citrate, ferric nitrite or an ammonia, amine or alkanolamine salt of a ferric chelate of a polycarboxylic acid chelating agent.

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

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Citations (3)

• 1965
  • 1965-10-24 US US504983A patent/US3413160A/en not_active Expired - Lifetime
• 1966
  • 1966-10-06 NL NL6614093A patent/NL6614093A/xx unknown
  • 1966-10-19 GB GB46816/66A patent/GB1149200A/en not_active Expired
  • 1966-10-21 BE BE688683D patent/BE688683A/xx unknown
  • 1966-10-22 JP JP41069531A patent/JPS512418B1/ja active Pending

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