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
  • C23F13/005—Anodic protection
• • 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
  • C23F13/02—Inhibiting corrosion of metals by anodic or cathodic protection cathodic; Selection of conditions, parameters or procedures for cathodic protection, e.g. of electrical conditions
  • C23F13/04—Controlling or regulating desired parameters

Definitions

• the invention relates to a method of providing electrochemical prevention of corrosion in changing conditions.
• the method may be used in providing both cathodic and anodic corrosion prevention systems.
• Electrochemical corrosion prevention refers to a method where the electrochemical surface potential of a metal surface being protected is changed, i.e. polarized in an advantageous direction by passing electrical current to the surface.
• the electrical current may be derived either from less noble i.e. sacrificial electrodes galvanically connected to the object being protected or alternatively from an external source of direct current supplied through a separate electrode.
• Electrochemical surface potential is measured by a reference electrode which is placed close to the surface to be protected and galvanically insulated from the surface.
• a reference electrode which is placed close to the surface to be protected and galvanically insulated from the surface.
• Many types of reference electrodes are known and their usefulness depends on the chemical and physical properties of the prevailing electrolyte.
• a characteristic of a usable reference electrode is a specific potential that remains relatively constant in operating conditions.
• the direction of polarization which is advantageous with respect to electrochemical protection may be sought either in the potential range of an optimally dense oxide layer or alternatively in the immune range of a metal that is being protected, i.e. in the potential range in which the metal atom is thermodynamically stable and thus does not corrode.
• protection potentials are well known and constant
• optimal protection potential can be determined using known corrosion examination methods, such as, determination of a polarization curve, potentiostatic weight loss tests or resistance measurements of an oxide layer
• the potential of the protected object can be affected only by varying the number and location of the anodes
• potentiostatic protective systems accomplished by means of an external current source, the potential of the object is measured and it is sought to be maintained as close as possible to a preselected target potential by automatically changing the current supplied by the current source
• potentiostatic corrosion prevention method is well-known and va ⁇ ous ways of its application are desc ⁇ bed, for instance, in US patents 4,528,460 and 4, 713,158 In the corrosion prevention method in accordance with US patent 4,713,158, potential is allowed to move within predetermined limits
• the potentiostatic method does not operate in the best possible way in an environment where protection is based not on an absolute potential value but, for example, on realized polarization
• Conditions which are difficult to control or which cannot be controlled by means of conventional potentiostatic corrosion inhibition include almost all batch processes of the chemical industry.
• the changes in conditions may be dependent on time or alternatively the change is triggered by some process quantity, such as, temperature or the start of chemical supply.
• the discontinuities causing problems include, for example, changes in the type of product that is being produced. In them, the change is triggered by a change in the source of raw material.
• the potentiostatic method it is also difficult to control the cathodic protection of reinforcement steels, in which, instead of an absolute potential value, the magnitude of a change caused in potential is generally used as the criterion for protection.
• corrosion examination methods offer a possibility of monitoring the development of corrosion aggressiveness also such that optimal protection potential can be determined at short time intervals.
• the examinations have typically been momentary tasks which relate to the determination of potentiostatic protection and which have been performed in the laboratory on a sample taken from a process.
• Continuous corrosion examination of the monitoring type directly carried out in the process environment is noticeably uncommon and until now there has not even existed any need for it because in a stable environment a limited sampling rate suffices to get an idea of the environment. It is, however, already possible to measure in real time a number of process variables affecting corrosion reactions.
• An object of the invention is to provide a method in which the prevention of corrosion adapts its operation to changing corrosion conditions automatically without delay.
• a more specific object of the invention is to provide a method that can utilize both the measurement data of process variables and corrosion measurement data generated by the method itself, and change the current or voltage supply to the corrosion prevention provided by an external current source in such a way that optimal protection potential for conditions prevailing at any given time is achieved.
• a broader object of the invention is also to be able to utilize data measured from the process and processed on the bases of corrosion directly or indirectly for control of the process such that the corrosion aggressiveness of the process diminishes.
• At least one detector device is placed in contact with the electrolyte present in an object that is being protected such that the detector device is electrically insulated from the object that is being protected;
• the electrochemical properties of said electrolyte or the properties of said electrolyte affecting the rate of corrosion reactions are measured by said detector device at time intervals shorter than the time it takes for the electrolyte to change significantly in terms of corrosion;
• the measurement results of said detector device are passed to a measuring and data processing unit, and a new optimum potential is determined on the basis of said measurement results;
• the inventors have realized the possibility to apply commonly known electrochemical and other methods of measuring quantities that affect the rate of corrosion reactions to the determination of an optimum potential for corrosion prevention accomplished by means of an external current source in such a way that the optimum potential is continuously redetermined during operation, thereby making it possible to automatically adapt to changing corrosion conditions.
• the data from measurements performed previously may be utilized in determining a new optimum potential.
• a corrosion prevention control system redetermines and changes the optimum potential for corrosion prevention when the corrosion conditions change.
• the optimum potentials are preferably determined by electrochemical corrosion examination methods, such as, for example, a polarization curve run, linear polarization resistance or CER (Contact Electric Resistance).
• a depolarization test is preferably used in which the realized polarization of the steels is measured by switching off the protection current for a given period of time and measuring the depolarization from which the voltage drop (IR drop) caused by electric field and the specific resistance of the electrolyte has been eliminated.
• the determination of optimum potential in accordance with this invention is not limited to the above-mentioned methods, but all electrochemical corrosion rate measurements can be made use of for the purpose of the invention.
• the measurement of said process variable may be used for controlling the change of optimum potential.
• the automatic change of the optimum potential in corrosion prevention during operation in accordance with the invention does not necessarily require electrochemical corrosion measurements during operation.
• At least one detector preferably several detectors, is/are fitted in the object to be protected such that the detectors will be in contact with the electrolyte present in the object that is being protected.
• the detectors are electrically insulated from the object that is being protected and they are so designed that they withstand the chemical and physical conditions of the electrolyte.
• Measurement electronics and a data processing unit may be situated in connection with the detector device or, in the case of several detector devices, centrally such that one data processing unit operates several detector devices.
• the determination of optimum potential may take place at given preset intervals that can be selected or the determination can be triggered when a given process variable affecting corrosion conditions in a known fashion changes. In the case where process sequences are repeated unchanged in their corrosion conditions, the optimum potential values determined previously may be utilized. Said optimum potential values may also be determined by a periodic corrosion examination in the process or at the laboratory.
• the method in accordance with the invention is suitable for controlling all corrosion prevention systems accomplished by means of an external current source, most advantageously when the corrosion conditions in the environment of the object to be protected vary considerably.
• Figure 1 graphically shows examples of polarization curves in changing conditions when variable process quantities are pH and Cl ! content.
• Figure 2 is a schematic view of an advantageous embodiment of a measurement system used in the method in accordance with the invention.
• Fig. 1 it is seen how polarization curves change with changing process conditions.
• pH and C1G content are on the level shown in Fig. 1.
• the C1G content drops, at the point of time t : the pH drops, at the point of time t 3 the C1G content rises, and at the point of time t 4 the C1G content drops again.
• the pH remains constant.
• the measurement system in accordance with the invention is generally denoted with the reference numeral 10.
• the measurement system 10 comprises a direct current source 12, a current supply electrode 13, a measuring detector 14, and a measuring and data processing unit 17.
• the object to be protected which in this embodiment is a container containing a process liquid 15, is denoted with the reference numeral 1 1.
• the measuring system 10 additionally comprises a measuring detector 16, which may measure any process variable or process variables, such as, for example, pH, temperature T, concentration C, etc.
• the measuring detector 16 measures the chemical and physical properties of the process liquid 15.
• the detector device 14 is placed in contact with the electrolyte 15 present in the object 1 1 that is being protected in such a way that the detector device 14 is electrically insulated from the process equipment.
• the detector device 14 serves to measure the electrochemical properties of the electrolyte or its properties affecting the rate of corrosion reactions at time intervals shorter than the time it takes for the electrolyte to change significantly in terms of corrosion.
• the detector device 14 also measures the electrochemical potential of the protected object 1 1.
• the measurement results of the detector device 14 are passed to the measuring and data processing unit 17, and a new optimum potential B is determined based on the measurement results.
• the measuring and data processing unit 17 delivers a control signal to the current source 12, and the current or voltage supplied by the current source is changed such that the new optimum potential B is reached. In addition, the measurement results of the measuring detector 16 are passed, if needed, to the measuring and data processing unit 17.
• the invention allows the target potential of electrochemical corrosion prevention to be automatically changed when changing corrosion conditions or the fulfilling of the criteria for protection so require.
• the method in accordance with the invention may be employed in providing both cathodic and anodic corrosion prevention systems.
• the invention also makes it possible that the data measured in the process and processed on the bases of corrosion can be utilized directly or indirectly for control of the process such that the corrosion aggressiveness of the process diminishes.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Testing Resistance To Weather, Investigating Materials By Mechanical Methods (AREA)
• Prevention Of Electric Corrosion (AREA)

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Publications (1)

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Cited By (4)

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Citations (8)

• 1999
  • 1999-05-17 FI FI991116A patent/FI119150B/fi not_active IP Right Cessation
  • 1999-07-21 JP JP11205602A patent/JP2000328273A/ja active Pending
• 2000
  • 2000-05-16 AU AU45719/00A patent/AU760526B2/en not_active Ceased
  • 2000-05-16 CA CA002372920A patent/CA2372920A1/en not_active Abandoned
  • 2000-05-16 WO PCT/FI2000/000440 patent/WO2000070124A1/en active IP Right Grant
  • 2000-05-16 BR BR0010644-5A patent/BR0010644A/pt not_active IP Right Cessation
  • 2000-05-16 EP EP00927286A patent/EP1190113A1/en not_active Withdrawn
• 2001
  • 2001-11-06 NO NO20015430A patent/NO20015430L/no not_active Application Discontinuation

Patent Citations (8)

Non-Patent Citations (3)

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