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
  • C23C22/68—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 aqueous solutions with pH between 6 and 8
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J19/02—Apparatus characterised by being constructed of material selected for its chemically-resistant properties
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C273/00—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups
  • C07C273/02—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups of urea, its salts, complexes or addition compounds
  • C07C273/04—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups of urea, its salts, complexes or addition compounds from carbon dioxide and ammonia
• • 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
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/02—Pretreatment of the material to be coated
• • 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
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
  • C23C8/10—Oxidising
  • C23C8/12—Oxidising using elemental oxygen or ozone
  • C23C8/14—Oxidising of ferrous surfaces

Definitions

• a process for enhancing the corrosion resistance of austenitic stainless steels which are exposed to solutions containing ammonium carbamate.
• the process involves exposing the surfaces of such stainless steel equipment, which are exposed to ammonium carbamate solutions during, e.g., the commercial manufacture of urea and melamine, to the corrosive effects of such solutions for a period of time to induce significant corrosion of such surfaces, followed by contacting said surfaces at a temperature of at least about 1001 C for a period of time with an oxidizing agent.
• ammonium carbamate is first formed at a temperature of between about 170 C and 250 C, under a pressure of about 350 atm. This ammonium carbamate is then converted, subsequently or almost simultaneously, and possibly in the same vessel, into urea and water.
• Reaction 2 is an equilibrium reaction and does not proceed .to completion.
• the synthesis product leaving the urea'autoclave therefore, is a solution of urea in water, containing ammonium carbamate.
• This synthesis solution possibly after having been partially expanded, e.g., at reduced pressure, is subsequently heated to cause the ammonium carbamate present therein to decompose. Quantities of ammonia and carbon dioxide are thereby set free, and are separated from the rest of the solution. The balance of the solution is then treated in one or more (further) reduced pressure stages to remove the rest of the unconverted ammonium carbamate from the urea product solution.
• This heating and decomposition of carbamate can be carried out in several known ways, for instance, in a high pressure distilling column or in a so-called stripping column which is supplied with a stripping gas, which gas may be, for instance, one of the reaction components or an inert gas, such as synthesis gas for an ammonia synthesis, in order to expel ammonia and carbon dioxide present in the solution.
• a stripping gas which gas may be, for instance, one of the reaction components or an inert gas, such as synthesis gas for an ammonia synthesis, in order to expel ammonia and carbon dioxide present in the solution.
• ammonia and carbon dioxide stream mixture is treated in an absorption column.
• An aqueous solution of ammonium carbamate can then be obtained which is recycled to a urea synthesis.
• this invention relates to a process for increasing corrosion resistance of stainless steels, containing at least 16 percent chromium and at least 8 percent nickel against the corrosion normally experienced by contact of such materials with solutions containing ammonium carbamate.
• the process according to the invention is characterized in that those surfaces of the austenitic stainless steel material, as will be in contact with said ammonium carbamate solutions, are first exposed for some time to the corroding action of such a solution in the substantial absence of oxygen and are then subsequently contacted at a temperature of at least about 100 C with a medium containing an oxidizing agent, said later contact being maintained for some time.
• this invention provides a procedure for preparing the metal to be used in the contact with the corrosive ammonium carbamate solution, whereby the corrosion-preventive technique of processing such solutions in the presence of a small amount of oxygen becomes more effective over extended periods of time.
• the process according to the invention can be applied to advantage, inter alia, in processes for the preparation of urea, in which the synthesis solution is subjected to the previously mentioned stripping treatment.
• This treatment is usually carried out at a very high pressure, preferably at synthesis pressure, so as to be able to effect the condensation of the expelled ammonia and carbon dioxide at a high as possible a temperature level without the intermediary of pumps or hot-gas compressors and, at the same time, to limit the quantity of water expelled together with these gases. This is done in view of the detrimental influence of water on the conversion in the reactor to which the gases expelled are recirculated after condensation.
• urea synthesis solution was each time passed through a testing tube at a temperature which was considerably higher than the one usual in practice, while, also, no oxygen-containing stripping gas was supplied at all.
• the decomposition of the ammonium carbamate present in the solution and the expulsion of the liberated ammonia and carbon dioxide were effected purely thermically by supply of heat via the tube wall. The conditions, therefore, were purposely chosen in such a way that the occurrence of corrosion was strongly favored.
• the occurrence of corrosion was established by judging the color of the urea synthesis solution flowing out of the testing tubes. For, when the material changes over from the passive into the corroding condition, the quantity of Fe dissolving per cm and per time unit increases in very short time to such an extent that it brings about a distinct discoloration of the solution. In addition to this visual judgement, the Fe contents of the urea synthesis solutions flowing in and out were determined periodically. The data thereby obtained confirmed, in all experiments, the visual judgments. Each time a distinct discoloration was established, a corrosion rate of between 40 and 60 mm/year was found.
• EXAMPLE 1 A tube made in stainless steel of the composition 18 of Cr, 12 of Ni and 2.5 of Mo was contacted, in the condition in which it was received from the supplier, (e.g., with normal nitric and passivation treatment) with a urea synthesis solution, while the external wall had a temperature of 210 C.
• the composition of the inlet solution was 30 by weight of NH, 20 by weight of CO 40 by weight of urea, the balance being water.
• the solution contained 5 ppm of oxygen, which corresponds to 0.5 in the carbon dioxide supplied to the urea synthesis.
• the tube started to corrode virtually immediately, at a rate of approximately 50 mm a year. This tube was then emptied and rinsed with water, whereupon air purge was applied for 12 hours at a temperature of 180 C. Subsequently with the same urea synthesis solution having the same oxygen content and at the same temperature as before the treatment, the corrosion rate was found to amount to only 0.050.l mm a year after at least 250 hours, whereupon the test was concluded.
• EXAMPLE 2 At a pressure of 125 atm a urea synthesis solution containing 5 ppm of oxygen was made to flow through a tube made of the above-mentioned 18-12-2 A Cr- NiMo-steel and being in the condition in which it had been delivered. The external wall of the tube was kept at a temperature of 230 C. Again, corrosion was found practically immediately. The corrosion rate amounted to 60 mm a year. This condition was maintained for 6 hours, following which the tube was emptied and flushed with water, whereupon, for 16 hours, a flow of carbon dioxide containing a small quantity of water vapor and 0.5 percent of oxygen by volume was passed through, the temperature of the external tube wall then amounting to 210 C.
• the tube was subjected to a pressure of 125 atm. with the aid of carbon dioxide, following which urea synthesis solution was again passed through, while the temperature at the external wall of the tube again amounted to 230 C. Now corrosion was found only after 13 hours.
• Example 3 The experiment of Example 2 was repeated, now using air as passivating medium instead of carbon dioxide with water vapor and oxygen. In this test no corrosion was found for a period of over 50 hours.
• Example 4 The experiment of Example 2 was repeated once more, passivating now being effected for 16 hours, alternately with water and air. As from the moment the passing through of the urea synthesis solution was resumed, corrosion to a measurable extent was not found to occur for 53 hours.
• EXAMPLE 5 In this case the tube was made of CrNiMo 25-25-2 Ti/stabilized steel. While the temperature of the external wall was kept at 230 C, a urea synthesis solution was again passed through this tube at a pressure of 125 atm. The inlet solution again contained 5 ppm of oxygen. Corrosion occurred within a few hours. This condition was maintained for 24 hours, following which the tube was emptied and rinsed with water. Next, the temperature of the external wall of the tube was adjusted to 210 C and flushing thereof was applied for 12 hours with carbon dioxide containing 0.5 percent by volume of oxygen and with water which flowed along the wall as a film. Upon completion of this treatment, the temperature of the external wall was again brought to 230 C and the supply of urea synthesis solution resumed. Corrosion was not found for 96 hours, whereupon the test was concluded.
• the practice of this invention involves exposing the surface of austenitic stainless steel, containing at least 16 Cr and 8 Ni to an aqueous corroding solution containing ammonium carbamate at elevated temperatures and under conditions and for a period of time sufficient to induce measurable, unacceptable corrosion of said surface, which if continued, would exhibit corrosion at a rate equivalent to at least about 25 mm/year, i.e., to induce corrosion of an amount equivalent to at least about 0.02 mm., or such that 10 mg.
• the initial corrosion operation can be carried out at varying combinations of temperatures and periods of time, the significance being that this corrosion operation is conducted under such conditions which can be readily selected by those skilled in the art such that there is induced that degree of corrosion, if the conditions continued, which induces an unacceptable degree of corrosion which is here indicated as being at least about 25 mm. per year.
• subsequent passivation operations may be conducted with various combinations of temperature and periods of time, the advantage of the invention being indicated by the observation that after such passivation step the re-exposure of the corroded steel surface to the corroding operation will not exhibit such a measurable unacceptable corrosion rate for at least a period of time about twice the initial time involved in the initial corrosion.
• a process for increasing the resistance of an austenitic stainless steel containing at least 16% of Cr and at least 8% of Ni against corrosion resulting from contact thereof with aqueous solutions containing ammonium carbamate said process consisting essentially in that the surfaces of said steel is pre-treated by first exposing the same to an aqueous corroding solution containing ammonium carbamate at elevated temperatures and under conditions and for a period of time sufficient to induce a measurable corrosion of said surface which, if said conditions continued, would exhibit corrosion at a rate equivalent to at least about 25 mm per year, and thereafter contacting the thus-corroded surface at a temperature of at least about 100 C with an oxidizing agent of sufficient strength, said oxidizing agent selected from the group consisting of molecular oxygen and an oxygen emitting substance, for a required period of time sufficient to induce at least a degree of passivation of said surface such that said measurable corrosion thereof upon reexposure to said corroding solution and conditions will not be observed for at least about twice the initial period of time
• said oxidizing agent comprises water containing a small quantity of an oxygen-emitting substance.
• said oxidizing agent comprises carbon dioxide containing a small quantity of oxygen.

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• General Chemical & Material Sciences (AREA)
• Preventing Corrosion Or Incrustation Of Metals (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
• Chemical Treatment Of Metals (AREA)

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Cited By (4)

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Citations (4)

• 1970
  • 1970-04-02 NL NL7004678.A patent/NL164328C/xx not_active IP Right Cessation
• 1971
  • 1971-03-31 ES ES389762A patent/ES389762A1/es not_active Expired
  • 1971-04-01 US US00130483A patent/US3720548A/en not_active Expired - Lifetime
  • 1971-04-02 CA CA109,447A patent/CA949436A/en not_active Expired
  • 1971-04-02 BE BE765197A patent/BE765197A/xx not_active IP Right Cessation
  • 1971-04-02 FR FR7111732A patent/FR2085804B1/fr not_active Expired
  • 1971-04-02 JP JP46020270A patent/JPS515630B1/ja active Pending
  • 1971-04-02 DE DE19712116201 patent/DE2116201A1/de active Pending
  • 1971-04-19 GB GB26820/71A patent/GB1287710A/en not_active Expired

Patent Citations (4)

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