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
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F13/00—Inhibiting corrosion of metals by anodic or cathodic protection
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
  • F28F21/00—Constructions of heat-exchange apparatus characterised by the selection of particular materials
  • F28F21/08—Constructions of heat-exchange apparatus characterised by the selection of particular materials of metal
  • F28F21/081—Heat exchange elements made from metals or metal alloys
  • F28F21/082—Heat exchange elements made from metals or metal alloys from steel or ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • 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
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F13/00—Inhibiting corrosion of metals by anodic or cathodic protection
  • C23F13/005—Anodic protection
• • 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
  • F28F19/004—Preventing the formation of deposits or corrosion, e.g. by using filters or scrapers by using protective electric currents, voltages, cathodes, anodes, electric short-circuits

Definitions

• This invention relates to a method for the protection against corrosion of steel parts made of austenitic or semi-austenitic steel during the production of sulfuric acid.
• sulfuric acid is produced by the catalytic conversion of the SO 2 con- tent of gases to obtain SO 3 and - in the case of dry gases - by the subsequent absorption of the SO 3 formed in concentrated sulfuric acid or - in the case of humid gases - by the subsequent condensation of the sulfuric acid formed.
• the usual technical components such as drier, absorber, heat exchanger etc. get in contact with concentrated sulfuric acid starting at about 93 wt-% and an ele- vated temperature.
• This sulfuric acid is extremely aggressive and exerts a fast and strong corrosion on the structural parts to be used. Therefore, the structural parts which get in contact with sulfuric acid must be made of corrosion-resistant materials.
• the materials to be protected are coated with a metal oxide layer which prevents the corrosion at- tack.
• the use of austenitic steels during the production of sulfuric acid is known from EP 0 130 967.
• the steel grades indicated in this protective right are intended in particular for use in heat exchangers.
• the materials used here do not satisfy the requirements which must now be fulfilled by corrosion-resistant materials. For the technical plants now in use smaller corrosion rates are required in particular.
• this object is solved in the above-mentioned method in that at a sulfuric acid concentration of 93 wt-% up to 100 wt-% and a temperature of 140°C up to the boiling point of the sulfuric acid, the steel part is made of austenitic or semi-austenitic steel which has a Cr content of 15 wt-% to 36 wt-% and an Ni content of 9 wt-% to 60 wt-%, and in which the ratio of the chemical elements (Cr + Si)/(Ni + Mo) lies in the range from 0.9 to 1.9 or in which the ratio of the chemical elements Cr/(Ni + Mo) lies in the range from 0.8 to 1.5, and in which the steel part has an anodic corrosion protection, wherein an anode, a cathode and a reference electrode are connected with a potentio- stat which supplies an adjustable direct electric current, and wherein the cathode and the reference electrode are in contact with the sulfuric acid and the an-
• the ratio is particularly favorable when molybdenum is present in a not too large amount of 0 wt-% to 2.5 wt-%.
• austenitic or semi- austenitic steel parts with a molybdenum content of 2 wt-% to 2.5 wt-% can be used.
• What is particularly critical for corrosion are those ranges in which the concentration of sulfuric acid is about 97 wt-% to 99 wt-% or the temperature of sulfuric acid is about 160°C to 230°C.
• heat exchangers such as e.g. plate-type heat exchangers or shell-and-tube heat exchangers, as well as the entire pipe system. Embodiments of the process will be explained by way of example with reference to the drawing, in which:
• Fig. 1 shows the current density/potential curve of an austenitic material
• Fig. 2 is a schematic representation of the anodic protection in a heat exchanger.
• Fig. 1 shows the current density/potential curve of a typical austenitic material containing 16.5 to 18.5 wt-% chromium, 11 to 14 wt-% nickel and 2 to 2.5 wt-% molybdenum.
• sulfuric acid was used as medium with 98 wt-% at a temperature of 200°C.
• cathode there was used a steel cathode made of 1.4404.
• the potential is plotted in millivolt (mV) against a Hg/HgSO reference electrode, and on the ordinate the current density is plotted in milliampere per square centimeter (mA/cm 2 ).
• mV millivolt
• mA/cm 2 milliampere per square centimeter
• reference electrodes such as e.g. a calomel electrode or a cadmium bar.
• the first part of the diagram in the range from 0 to 600 mV shows a peak which is referred to as active potential.
• active potential In the range from 600 mV to 1800 mV then follows the saddle of the curve, the so-called passive potential.
• passive potential The subsequent rise from 1800 mV is referred to as transpassive potential.
• transpassive potential To achieve a corrosion protection as effective as possible in the anodic corrosion protection, the current density must lie within the range of the passive potential.
• the values represented here are exemplary, as they are material- and temperature- dependent.
• Fig. 2 shows the arrangement of the anodic corrosion protection in a shell-and- tube heat exchanger (1) for sulfuric acid.
• cooling medium is introduced into a first chamber (3) of a shell-and-tube heat exchanger (1 ). From there, the cooling medium is distributed and flows through individual tubes (4) into a second chamber (5), from which the cooling medium is discharged again.
• only two tubes (4) are represented here.
• connection (6) hot sulfuric acid (2) is introduced.
• the sulfuric acid flows around the tubes (4) filled with cooling medium and is discharged again via the connection (7).
• the sulfuric acid is cooled.
• a plurality of metal cathodes (8) are mounted between the tubes (4) in the shell- and-tube heat exchanger.
• the representation shows a cathode (8) by way of example.
• the number of cathodes (8) used depends on the size of the heat exchanger and also on the temperature and the concentration of the sulfuric acid.
• the cathode (8) is made of the material 1.4404 and is in permanent contact with the sulfuric acid.
• the cathode (8) is connected with the negative pole of a poten- tiostat (9) by an electric line.
• the potentiostat (9) is a d.c. voltage source whose positive pole (10) is connected with the parts of the shell-and-tube heat exchanger (1 ) to be protected via an electric line.
• a second reference electrode (11 ) is inserted in the shell-and-tube heat ex- changer via a seal and is connected with the potentiostat (9) via an electric line.
• This reference electrode (11) likewise is permanently surrounded by the sulfuric acid and provides the measurement basis for the potentiostat (9).
• the potential required for the corrosion protection is determined and adjusted at the potentiostat (9).
• the corrosion behavior of the materials in accordance with the invention is shown at different temperatures and a sulfuric acid concentration of 98 wt-%.
• the flow rate of the sulfuric acid was 1 m/s.
• the corrosion behavior was determined by immersion tests. In all cases, the test period was 7 days.
• the removal rates were determined by measuring the corrosion flow and by conversion to mm/a.
• the test medium was renewed after each test cycle.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• General Engineering & Computer Science (AREA)
• Thermal Sciences (AREA)
• Physics & Mathematics (AREA)
• Preventing Corrosion Or Incrustation Of Metals (AREA)
• Prevention Of Electric Corrosion (AREA)
• Physical Or Chemical Processes And Apparatus (AREA)
• Spark Plugs (AREA)

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Citations (6)

• 2001
  • 2001-06-08 DE DE10128032A patent/DE10128032A1/de not_active Withdrawn
• 2002
  • 2002-05-28 US US10/479,304 patent/US20040238375A1/en not_active Abandoned
  • 2002-05-28 EA EA200400008A patent/EA006778B1/ru not_active IP Right Cessation
  • 2002-05-28 MX MXPA03011234A patent/MXPA03011234A/es active IP Right Grant
  • 2002-05-28 JP JP2003503852A patent/JP2004529274A/ja active Pending
  • 2002-05-28 KR KR10-2003-7016039A patent/KR20040023612A/ko not_active Application Discontinuation
  • 2002-05-28 ES ES02743117T patent/ES2272733T3/es not_active Expired - Lifetime
  • 2002-05-28 AT AT02743117T patent/ATE340274T1/de not_active IP Right Cessation
  • 2002-05-28 DE DE60214859T patent/DE60214859T2/de not_active Expired - Lifetime
  • 2002-05-28 WO PCT/EP2002/005842 patent/WO2002101106A1/en active IP Right Grant
  • 2002-05-28 EP EP02743117A patent/EP1409756B1/en not_active Expired - Lifetime
  • 2002-06-07 PE PE2002000475A patent/PE20030023A1/es not_active Application Discontinuation

Patent Citations (6)

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