METHOD AND SYSTEM FOR MEASURING THE CORROSION PROPERTIES OF SPECIMENS

Abstract: The invention concerns a specimen testing procedure for studying the corrosion properties of the specimen. As taught by the invention, in the procedure are employed at least two specimens at different potentials, one of them being used as auxiliary specimen (1) which is controlled to carry its potential of repose, and the potentials of the other specimens, or principal specimens (2), being controlled relative to the auxiliary specimen in such a manner that the true corrosion current flows between the principal specimen and the auxiliary specimen. The invention furthermore includes an equivalent potentiostat comprising an auxiliary specimen (1) immersed in the electrolyte and connected to a current source (6); and at least one principal specimen (2), connected to another current source (3) to control the principal specimen to be at the desired potential relative to the auxiliary specimen.

Description: METHOD AND SYSTEM FOR MEASURING THE CORROSION PROPERTIES OF SPECIMENS The present invention concerns a specimen testing procedure as defined in the preamble to Claim 1, and a potentiostat as defined in the preamble to Claim 5. In existing potentiostats, the corrosion prop¬ erties of specimens are studied in the way that the test specimen in an electrolyte is arranged to carry a given potential, whereby then the corrosion currents, or the dissolution of metal from the specimen, can be studied. However, a comparatively high excitation cur— rent is required to establish the given sample poten¬ tial , and this current may generate hydrogen or oxygen on the counter—electrode surface. Also, in an electro¬ lyte with poor conductivity, such as suitable water, the resistance of the electrolyte is comparatively high, whereby the voltage drop is also high, resulting in major errors: at their maximum, the errors may amount to several hundred mV. The measurement of the true corrosion currents will consequently be inaccurate and the water chemistry of the measuring arrangement changes so that it becomes difficult to obtain reliable and comparable results. The object of the invention is elimination of the above-mentioned drawbacks. Specifically, the aim of the invention is to present a novel testing procedure, and an equivalent apparatus, with the aid of which cor— rosion measurements can be carried out free of inter— ference and accurately, and possibly in extensive ser— ieε, employing several different potential levels si¬ multaneously. Regarding the features which are characterist¬ ic of the invention, reference is made to the claims section. In the specimen testing procedure of the in¬ vention the specimen placed in an electrolyte is ar— ranged to carry a given potential , for studying the corrosion properties of the specimen. As taught by the invention, in the procedure are always employed two specimens at different potentials in the way that one of the specimens is used as auxiliary specimen, and this specimen is controlled to carry its own potential of repose so that no current passes therefrom. The rest of the specimens, the principal specimens, are con¬ trolled to carry suitable potentials with reference to the auxiliary specimen in such manner that the true corrosion currents pass between the principal and auxiliary specimens, substantially free from the exci¬ tation current required to establish the potential. Hereby the sole currents which are significant within the limits of measuring accuracy are corrosion currents proper. In an advantageous embodiment of the invention there is a plurality of principal specimens or princi¬ pal specimen groups, stepped to carry different poten¬ tials so that the corrosion currents of all these spec— imens are simultaneously measured. The currents flowing between specimens at different potential levels are then negligible in comparison with the currents between each principal specimen and the auxiliary specimen, whereby adequate accuracy of measurement is maintained. Advantageously, when several principal speci¬ mens or principal specimen groups are used, these po¬ tentials are controlled with the aid of separate, potential level-specific circuits. In another embodi¬ ment of the invention these potential levels of the different principal specimens are controlled with one and the same circuit, in this case the potential levels being interconnected with suitable resistances. For such resistances advantageously steplessly variable resistances are used, in which case the potential lev— els can be steplessly adjusted relative to each other. The potentiostat of the invention comprises an auxiliary specimen immersed in the electrolyte and con— nected to a current source and at least one principal specimen connected to a second current source, to con¬ trol the principal specimen to carry the desired poten¬ tial level relative to the auxiliary specimen. Advan- tageously, the auxiliary specimen is controlled with the aid of said first current source substantially to its potential of repose, whereby virtually the sole currents flowing in the potentiostat are true corrosion currents between the principal specimen and the auxil— iary specimen. In an advantageous embodiment of the invention the potentiostat comprises a plurality of different potential levels to which the principal specimens or groups of principal specimens have been allocated. In that case, in an embodiment of the invention, to each potential level corresponds a specific, separate cur— rent source by the aid of which the potential level can be controlled. In another embodiment of the invention for the principal specimens is provided only one cur— rent source, the various potential levels, or the prin¬ cipal specimens or principal specimen groups carrying them, being interconnected through resistances. Said resistances are in this case advantageously adjustable so that the potential levels can be steplessly adjusted relative to each other in such way as is desired, for adjusting the corrosion currents specimen-specifically to be appropriate. The advantage afforded by the present inven¬ tion over the state of art is that no high currents need be driven through the electrolyte in order to achieve a given potential : changing the potential of the specimen is accomplished through an external sys¬ tem. It is equally possible with the aid of the proce¬ dure to eliminate largely the harmful I*R drop effect because the current flowing in the liquid is minimal. Likewise, owing to the external control the specimen that is being controlled can be forced to very high potentials with reference to the water, even to poten¬ tials which are impossible in the standard procedure. It is also an advantage that thanks to the external potential control no high current delivery capacity is required in the auxiliary potentiostat. It is a further advantage that the current measured between the principal specimen and the auxil¬ iary specimen represents the true corrosion current, not the excitation current required for achieving a given potential, as is the case in existing procedures and apparatus. Furthermore, the water chemistry on the specimen surface is appreciably better than that in any apparatus representing the state of art. This is par— ticularly notable at high potentials because establish— ment of the potential requires no high currents, which would for instance decompose water on the surface of the so—called countei—electrode specimen. An additional advantage is the fact that, when desired, in the system of the invention a truly remarkable number of specimens can be connected to be simultaneously measured. The invention is described in detail in the following, with reference to the attached drawing, whereins— Fig. 1 presents schematically a potentiostat representing the state of art, Fig. 2 presents a potentiostat according to the invention, Fig. 3 presents another potentiostat according to the invention, and Fig. 4 presents schematically an experimental set—up according to the invention. Fig. 1 presents schematically a potentiostat representing the state of art, in this potentiostat being immersed in an electrolytes a specimen 7, a ref— erence electrode 8, and a counter—electrode 9. When in a case like this the specimen is forced to carry a given potential, especially in an electrolyte with poor conductivity a comparatively great interference term iε produced, which arises from the comparatively high cur¬ rent that is required in order to achieve said poten¬ tial. As a consequence, it is difficult to measure the true corrosion currents by means of the experimental set—up of prior art. In Fig. 2 is shown the principle diagram of the specimen testing procedure of the invention. The apparatus of the invention is characterized in that it always comprises at least two separate current systems, one of them, 6, connected to an auxiliary specimen 1 and the other, 3, to the principal specimen 2. The cur— rent source 6> is employed here to set the auxiliary specimen 1 to its potential of repose, and the priπci— pal specimen 2 is controlled with the aid of the cir— cuit 3 to carry the potential which it is desired to have, whereby substantially the sole currents flowing between the specimens, that iε between the auxiliary specimen 1 and the principal specimen 2, are true cor— rosion currents, which thus can be accurately measured. As shown with dotted lines in Fig. 2, a plurality of separate circuits 3 can be connected to this system, each such circuit connected to its own principal speci¬ men 2, or to a specific group of principal specimens, the corrosion currents always flowing between the prin¬ cipal specimen and the auxiliary specimen. In Fig. 3 is presented another embodiment of the invention, which also comprises a plurality of principal specimens 2 at different potential levels but only one circuit 4 for establishing these potentials. The different potential levels, or principal specimens 2, are here interconnected through resistances 5, and these resistances are advantageously furthermore step¬ lessly adjustable rheostats, whereby the principal specimens at different potential levels can be step¬ lessly adjusted to be at desired voltage levels rela¬ tive to each other and to the auxiliary specimen 1, in order to obtain such corrosion propertieε as are de¬ sired in each instance. In Fig. 4 is presented a set-up conforming to the invention, comprising a supporting structure de- signed to be immersed in the electrolyte and comprising three potential levels a, b, c, one above the other. Each potential level consists of a frame 11 of conduc¬ tive material and of holders 12 provided therein, in which the specimens 13 to be examined can be placed. The frames are interconnected with resistance wire 14, whereby owing to the voltage drop produced in said re¬ sistance wire the different frames are at different voltage levels and therefore specimens requiring dif¬ ferent corrosion currents can be simul aneously stud- ied, or mutually similar specimens can be simultaneous¬ ly studied with corrosion currents of different magni¬ tudes. In the foregoing the invention has been de¬ scribed in detail, referring to some advantageous em- bo imentε thereof. However, these do not in any way de¬ limit the invention, of which the different embodiments may vary within the scope of the inventive idea defined by the claims following below.

Claims: CLAIMS 1. A specimen testing procedure wherein a specimen in an electrolyte is arranged to carry a given potential , for studying the corrosion properties of the specimen, characterized in that in the procedure are employed at least two specimens at different poten¬ tials, one of them being used as auxiliary specimen (1) which is controlled to carry its potential of repose, and the potentials of the other specimenε, or principal specimens (2), being controlled relative to the auxili¬ ary specimen in such manner that the true corrosion current flows between the principal specimen and the auxiliary specimen, substantially free from any excita— tion current required to establish said potentials. 2. Procedure according to claim 1, character— ized in that several principal specimenε (2) , or groups of principal specimenε, at different potentials are simultaneously measured. 3. Procedure according to claim 1 or 2, char— acterized in that the principal specimens (2), or groups of principal specimens, at different potentials are controlled with circuitε (3) specific to each spec¬ imen or group, respectively. 4. Procedure according to claim 1 or 2, char— acterized in that the principal specimens (2), or groups of principal specimens, at different potentials are controlled with one circuit (4), the principal specimens, or groups of principal specimens, being in- terconπected through resistances (5). 5. A potentiostat for studying the corrosion properties of a specimen immersed in an electrolyte, characterized in that the potentiostat comprises an auxiliary specimen (1) connected to a current source (6) and at least one principal specimen (2) connected to another current source (3,4) to control the princi¬ pal specimen to be at desired potential level relative to the auxiliary specimen. 6. Potentiostat according to claim 5, charac¬ terized in that the potentiostat compriseε a plurality of different potential levelε to which the principal specimens (2), or principal specimen groups, can be allocated. 7. Potentioεtat according to claim 5 or 6, characterized in that the potentioεtat comprises a sep¬ arate current source (3) for each potential level that is being used. 8. Potentiostat according to claim 5 or 6, characterized in that the potentiostat compriseε for the principal specimens (5) only one current source (4) , the principal specimens, or groups of principal specimens, at different potentials being interconnected through resistances (5).

Inventors: AALTONEN PERTTI, KEKKONEN TIMO, SELIN LAURI, SILEN LARS, AALTONEN, PERTTI, KEKKONEN, TIMO, SELIN, LAURI, SILEN, LARS
Applicants: VALTION TEKNILLINEN, VALTION TEKNILLINEN TUTKIMUSKESKUS
Application year: 1989
Main classification: G01N17
IPC codes: G01N17, G01N27