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. 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.

In the most commonly known situations, such as, for example, the protection of carbon steel in soil or in sea water, protection potentials are well known and constant.

In relatively stable corrosion conditions, 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.

By using sacrificial anodes, the potential of the protected object can be affected only by varying the number and location of the anodes. In 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.

The potentiostatic corrosion prevention method is well-known and various ways of its application are described, for instance, in US patents 4, 528, 460 and 4, 713, I58. In the corrosion prevention method in accordance with US patent 4,713,158, potential is allowed to move within predetermined limits.

In processes where conditions clearly change as a function of time, the potentiostatic method cannot be applied without risk. If certain chemical or physical variables affecting corrosion reactions, such as, for example, pH, the proportion of aggressive ions, or temperature change considerably, it may lead to a situation where there exists no optimal protection potential covering the entire process period.

Moreover, 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.

In connection with those metals in particular whose resistance to corrosion is based on formation of a protective oxide layer, protection potential is greatly affected by environmental factors. Moreover, the optimum range of the protection potential of

a metal that has reached the limits of its resistance to corrosion is generally very small, and thus there is hardly any margin of error.

In all advanced potentiostatic corrosion prevention systems it is possible to determine protection potential freely, but it always requires that the operator takes an active part in determining and accomplishing a change. If the changes are rapid, the control of protection takes too much working time and is therefore almost impossible. If the changes occur irregularly and infrequently, the risk of failing to detect them increases. Moreover, the operating personnel of factories and plants do not usually have sufficient skill to perform the necessary measurements and draw the necessary conclusions of the need for change.

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. In batch processes, 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. In continuous processes, the discontinuities causing problems inclue, 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. By 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.

It is true that 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. However, 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.

In other words, regarding the current state of the art, it must be noted that although the far advanced potentiostatic corrosion inhibition system operates very well in stable conditions, for changing conditions there exist no method and apparatus which automatically adapt to changes.

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.

The objective of the invention is achieved by a method which is characterized in that in the method: (a) 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; (b) 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;

(c) 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; and (d) the current or voltage supplied by a current source for corrosion prevention is changed such that said new optimum potential is achieved.

In the method in accordance with the invention, 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. In that case, we may speak of potentiodynamic corrosion inhibition. In the case where process sequences are repeated unchanged in their corrosion conditions, the data from measurements performed previously may be utilized in determining a new optimum potential.

In the method in accordance with the invention, 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). In protection of reinforcement steels, in redetermining optimum potential, 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. In the cases where corrosion conditions have a clear correlation with a given process variable or variables that can be unambiguously measured. such as, for example, pH, temperature

or the concentration of some electrolyte component, the measurement of said process variable may be used for controlling the change of optimum potential. Thus, 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.

For the purpose of determining optimum potentials 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.

The invention will be described in detail with reference to an advantageous embodi- ment of the invention shown in the figures of the accompanying drawings, to which embodiment the invention is, however, not intended to be exclusively confined.

Figure 1 graphically shows examples of polarization curves in changing conditions when variable process quantities are pH and CI'content.

Figure 2 is a schematic view of an advantageous embodiment of a measurement system used in the method in accordance with the invention.

In Fig. 1 it is seen how polarization curves change with changing process conditions.

At the point of time 0, pH and CIG content are on the level shown in Fig. 1. At the point of time t, the CIG content drops, at the point of time t, the pH drops, at the point of time t3 the C1G content rises, and at the point of time t4 the CIG content drops again. Beginning from the point of time t,, the pH remains constant.

It is seen in Fig. 1 that in the time period 0-t, the value of optimum potential B is on the level shown on the left-hand side of Fig. 1. In a corresponding way, the point A of the polarization curve indicates the point having a maximum corrosion rate, i. e. current Ic,, is at the maximum. When the potential exceeds a point C on the polariz- ation curve, metal becomes susceptible to pit corrosion. It is seen in Fig. 1 how the polarization curves change such that the optimum potential B, the point A repre- senting the maximum corrosion rate and the point C representing the onset of pit corrosion change with changing process conditions pH, CIG. It is also seen in Fig.

1 that, for example, the optimum potential usable at the time interval t,-t3 causes pit corrosion at the time interval t3-t4 when the conditions have changed.

In Fig. 2, 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 11. In the embodiment shown in Fig. 2, 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. Thus, the measuring detector 16 measures the chemical and physical properties of the process liquid 15.

In the method in accordance with the invention, the detector device 14 is placed in contact with the electrolyte 15 present in the object 11 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 11. 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.

The embodiment shown in Fig. 2 illustrates cathodic protection. If the + terminal and the-terminal in the embodiment shown in Fig. 2 are exchanged with each other, the protection in question is anodic protection.

Above, only one advantageous embodiment of the invention has been described, and it is obvious to a person skilled in the art that numerous modifications may be made thereto within the inventive idea stated in the accompanying claims.