CS255554B1 - Method of metallic structure's cathodic protection efficiency determination in aqueous electrolyte - Google Patents

Abstract

The present invention relates to a method for determining efficacy cathodic protection of a metal structure in aqueous electrolyte, In cases when the reference electrode cannot be used for example, metal-metal salt, is used with a metal measuring electrode whose potenial however, it changes over time, e.g. deposition. However, this metal electrode makes it possible to determine with sufficient accuracy polarization component of tripping potential or switching technology, respectively. using special apparatus with the possibility of elimination the ohmic components of the potential can then be determined degree of cathodic protection of a defended structure under given conditions. The essence of the solution it consists in measuring the potentials at power source on and off, ie. the potential turns on Ug and trip Uy as well i potential after depolarization of Un, and sought The electrical values are determined from the above relations. To achieve the desired accuracy the measurement cycle may be repeated. Searched values ie polarization component PotentialZlUpol, ohmic slog of potential / jUjR characterize the degree of electrochemical protection, respectively. current field at protected surface. Potential polarization component is determined by the relationship: Ήοΐ ■ »2 -» 1, - 4Ula> / V / taking the ohmic component of the potential in general determined by the relationship: UIR «Uz - Uv / v /.

Description

The invention relates to a method for determining the efficiency of cathodic protection by means of a polarizing component of the potential of a metal structure in an aqueous electrolyte. The potential shift value due to polarization gives us the degree of electrochemical protection of the metal surface in an aggressive environment and is used as a protection criterion where it is not possible to accurately determine the polarization potential value using a reference electrode). of the metal-metal salt type.

The cathodic polarization of the metal surface of the structure exposed to the corrosive action of an aqueous electrolyte is caused by both a charge transfer overvoltage (eg, a hydrogen surge) and a crystallization overvoltage . Over time, in connection with the formation of a mineral coating on the cathode, diffuse overvoltage (transport, concentration changes at the metal surface) predominates. These overvoltages constitute the total polarization component of the potential, which is spatially located in the electrical bilayer at the metal-electrolyte interface. It is this component that determines the kinetics of the electrode reactions and is a criterion determining the condition of the device in terms of corrosion. The efficiency of cathodic protection can be determined by electrical measurements, especially potentials. Under operating conditions, however, an ohmic component is present in the measured potential value, which does not affect the kinetics of electrochemical processes and therefore these measurements lead to erroneous evaluation of the corrosion state of the structure.

Therefore, the following components are included in the measured value of the switch-on potential:

^U2 = ^U S + AU pol au

IR / V / (1) where Ug is stationary, wound current corrosion potential).

Δϋροί polarization potential component AU _ir ohmic potential component (before switching ON / V / / V /

Furthermore:

^U pol = ^U S ⁺ Δ ^υ ρο1 (1)

235 354 where U _pol is the polarization potential in volts, the value of which (usually determined using a metal-metal salt reference electrode) is the most important protection criterion. This polarization potential can be very accurately measured under laboratory conditions using, for example, a Haber-Luggin capillary. Under operating conditions, the U _pol value is determined indirectly, most often by the cut-off or change-over technique. For metal containers containing an aqueous electrolyte, a reference metal-metal salt reference electrode such as Ag / AgCl often cannot be used. In both marine and freshwater environments with a higher solute content at specific electrical conductivity /”^ 500^<.S.cm'···, measuring electrodes of pure zinc with a purity of at least 99,95% are used which are in terms of stationary potential relative to other metals from the electrochemical point of view relatively stable (at temperature λ9> ^ 50 ° ο). These measuring electrodes, most often in the form of a cylinder or circular disk, are usually fixed near the cathodically protected surface and connected by cable to the measuring box terminal. The disadvantage of all metal measuring electrodes, including zinc, is the change in their stationary potential over time, mainly due to the build-up of deposits on their surface. it is very difficult to deduce from these values the degree of suppression of the corrosion process in a given corrosion environment and hence the effectiveness of cathodic protection. Incorrect determination of the value of ϋ _ρθ 2 ^m1 ^ can lead on one side to sagging ”, ie to a larger loss of auxiliary anodes, to higher consumption of el. energy and thus increase the operating costs of cathodic protection. On the other hand, however, the lack of corrosion protection may result. as a result of corrosion damage to the metal structure, thereby reducing its service life and operational reliability. Thus, in both cases, economic losses occur.

The above drawbacks are overcome by a method of determining the cathodic protection efficiency of a metal structure in an aqueous electrolyte, by measuring the potential difference between a polarized metal surface and a measuring electrode that is fixed at the same location during the measurement. The essence of the method according to the invention is that the switching potential is measured after a polarization time of ≥ 10 minutes, preferably several days, by the input current of the source I, determined

255 554, the ohmic component of the potential AUjp, from the current difference I and I = 0, is measured after depolarization U-θ after depolarization time T ^ 3 minutes, preferably Τ '= 5 to 30 minutes, according to polarization time and flow rate from the moment of switching off the current I and the polarization potential component is determined from the relation:

^ ^U pol = ^U Z - ^U D * ^AU Ifi U 'all values are in volts.

The ohmic component of the potential AUj ^ is determined in one of the following ways:

After measuring U _z , the protective current is switched off and the breaking potential Uy is measured at I = 0 until T * ^ s 5 s, preferably 7Γ = 2s, from the moment the current is switched off, then the protective current is switched _on and time 6 ^ 5 s from the moment of switching on the power supply, then the protective current is switched off and the tripping potential Uy is measured at I = 0 and the measurement is repeated periodically as described, with the measurement cycle U _z - Uy performed at least once. Advantageously two times, the mean value of the ohmic component is determined from:

(U _Z -Uy) + (U _Z -Uy) + ...

AU _ir = - - (4) where n is the number of cycles, the values of U _z , Uy, U _z , Uy, AUj _r are in volts.

The second method is that after measuring U _z , the protective current is switched off and the tripping potential Uy is measured at 1 = 0 time s, preferably Τ = 2 s, from the moment the current is switched off, then the protective current is switched on and the switching potential is measured. UI until t I

45 s, preferably T = 2 s from the moment the power is turned on and the measurement is repeated periodically, with the changeover being performed at least three times and the mean ohmic (U _Z - Uy) + (-Uy + U ') + ( U '- Uy) + ... (5) m

once, preferably components from the relation:

AU

IR where m is the number of switches and all other values are in volts.

2S8 354

The ohmic component of the potential can also be determined by measuring the switching potential U _z at current I, then measuring the polarization potential Up _Q ^ immediately after switching off the current, eg at time (microseconds), preferably using a commutator apparatus and the ohmic component is determined from the relation:

* ^U IR = ^U Z - ^U pol (6) where all values are in volts.

The method of determining the effectiveness of cathodic protection according to the invention has the following advantages:

For pressurized metal containers containing an aqueous electrolyte, it is not possible to use a reference electrode, such as a metal-metal salt type. Any metal measuring electrode changes over time the potential value at the metal-electrolyte interface, so that instead of the polarization potential value Up _Q ^, it is possible to monitor the potential shift value due to cathodic polarization Up ₀ ^ which can be determined very precisely. thus increasing the accuracy of the measurement and thus achieving an optimum degree of protection, which brings significant economic benefits. The knowledge of the dependence of the degree of cathodic protection) on the value of the polarizing component UUp _Q ^ is used.

The advantages of the method according to the invention can be demonstrated by way of practical application. Cathodic protection parameters for a given device exposed to aggressive electrolyte can be determined on a laboratory model under conditions of prolonged (eg 20 days) potentiostatic cathodic polarization, including simulation of the original corrosion rate (at stationary corrosion potential). In imitation of operating conditions, such as the chemical composition of the electrolyte, temperature, flow rate, etc. Corrosion rate was monitored in mm.a. K ^'1 according to the values of the polarization potential Up ^ ₀ volts relative to a reference electrode, e.g. Ag / AgCl. Furthermore, the degree of cathodic protection 3 in% and the corresponding protective current density in mAm are simultaneously monitored. Thus, for example, for corrosion in the cooling water of power plants, the following parameters have been established for the steel tube-brass condenser tube system for an initial average corrosion rate K = 0,2 mm.a. ' ¹ :

- 5 255 554

polarization component of potential AU _pol / mV / degree of cathodic protection 3 /% / Protective current density j /mA.m ^-2 / 10 23 10 30 49 20 May 50 64 30 80 78 45 100 ALIGN! 82 60 150 89 90 200 92 120 250 94 155 300 96 185

E.g. at a potential shift due to polarization of AUp _O = 60mV, a degree of protection of 5 = 70% is obtained at an average protection current density of j = 35 mA.m. The original, average corrosion rate K = 0.2 mm.a / is reduced to K _Q = θχ1ίϊθθ? 2θ.2 ^ 0mm.a. ^1, wherein pitting and crevice corrosion are practically suppressed.

The practical application of the method according to the invention is shown by way of example of cathodic protection of the condenser tubes in a circulating cooling system with additional river water of specific el. conductivity y = 350 to 520 µS.cm ^-1 (at λ = 20 ° C); for the cooling water in the circuit, the value of y = 1400 ^ S.cm ^- ^ (at 20 ° C) f the temperature of the cooling water fluctuates during the year between · $ = 15 and 42 ° C.

After long-term polarization (ca 1 year) at output voltage U = 12 V and rectifier output current I = 7.5.A (protective current density j = 153 mA.m ^-2 ) and at water temperature $ = 19.8 ° C (lower chamber) and specific el. The conductivity of the cooling water y = 620 S.cm was measured by the Unigor 6e with a switch-on potential value ^U Z ⁼ “560 mV (plus the pole connected to the steel measuring electrode, minus the pole to the steel structure). After the protective current was switched off, the tripping potential Uy = -420 mV was measured in the interval up to 'TT = 3 s. The current was switched on again and measured = 550 mV and similarly Uy = -410 mV. After 7Γ = 20 minutes after switching off the current, the potential after depolarization of U ^ = -340 mV was measured, whose value did not change over time.

Ohmic component according to formula (4):

AND

(ϋ _ζ - D _in ) + (U '- Up n

255 554 = - ?, 6 ° - t 4 | Or..5tŽQ ⁺ ', 4io ₌ _ _{140 nV} *

Polarization component of the potential according to the relation. (3):

ŮU _pQ1 = ^u z ^u d " ^AU IR ^{= '56} ° ^{+ 340 + 140 =} ~ ^{80 mV}

The minus sign indicates that the shift is in the negative (cathodic) direction. The polarization shift of AU 80 mV corresponds to a degree of cathodic protection of at least 78%, which in the given conditions (elimination of pitting and crevice corrosion) fully satisfies, ie for a tube sheet of thickness.

mm is a negligible corrosion rate (in 25 years the maximum corrosion would be 1 mm).

The ohmic potential component can also be determined according to formula (5) as follows: The value of the switching-on potential U _z = -560 mV and the breaking-off potential Uy = -420 mV are measured according to the previous procedure: the switch-on is measured with U _z = -550 mV and again the current is switched off and Uy = -410 mV · is measured until ^ = 33 after switch-off. The mean value of the ohmic component is calculated according to (5):

_Λ ττ ^ ^u z - ^u v ^ ^{+ ( u} v " ^u z ^ ⁺ ( ^u z ^u v ^

A ^u ir = -s- = _-560 + 420 + 420-550-550 + 410 ₌ . Thus, _{7 mV} is approximately the same as in the previous case.

Finally, the ohmic component can also be determined according to formula (6): The switching potential value is U _z = -560 mV. Polarization potential value measured by the apparatus using a commutator over a time range of 50 ^ 3 ^ 5 ^^ 10 ^ 3 (microseconds): = = - 430 mV. Thus, AUi _R = -560 + 430 = -130 mV. Polarization shift according to (3) AUp _ol = -560 + 340 + 130 = -yp my. The method according to the invention can be used in the cathodic protection of capacitors. system sea water-fresh water, generally u

255 554 metal containers containing water electrolyte, ie boilers, some water supply systems (pressure filters, clarifiers) in some waste water treatment plants and hydrotechnical equipment such as weirs and the like.

Claims (4)

A method for determining the effectiveness of cathodic protection of an aqua structure in an aqueous electrolyte, wherein a potential / difference between a polarized metal surface and a measuring electrode is measured at the same location during the measurement, characterized in that the switching potential U _z after polarization is measured 97> 10 minutes, preferably several days, at the output current of source I, the ohmic component of the AU _IR potential is determined _{from the current difference} I and I = 0, after which the potential after depolarization U _n is measured after a depolarization time T ≥ 3 minutes, preferably 97 = 5 to 30 minutes, according to the polarization time and the flow rate of the aqueous electrolyte, from the relationship: * ^U pol = ^U Z - ^U D - ! ^υ ! Η Method according to claim 1, characterized in that after the measurement of U 1, the protective current is switched off and the trip potential Uy is measured at I = 0 to dPbyT '≥ 5 s> preferably 97 = 2 s. the protective current is switched on and the switching potential U% is measured after time ¹ ! > 5 s from the moment the source is switched on, after which the protective current is switched off and the tripping potential Uy at I = 0 is measured and the measurement is repeated periodically in such a way that the measurement cycle U _z - Uy is performed at least once, preferably twice; the mean value of the ohmic component is determined from the relation: AU IR ^<U Zu _v) + (Ux - UX) + where n is the number of cycles, the value _of U, U _y, U _z, uy, AU _{IR data are in} volts. Method according to claim 1, characterized in that, after measuring U _z , the protective current is switched off and the tripping potential Uy is measured at I = do for a time T 4 of 5 s, preferably 97 = 2 s, current is measured and the switching potential U _z is measured up to the time T <5 s, preferably 97 ^Z = 2 s from the moment of switching on the source and the measurement is repeated periodically with the switching - 9 2S5 354 is performed at least once, preferably three times, and the mean value of the ohmic component is determined from the relation: AU (U _z - U _v ) + (-U _v + u ') + (u' - up + .. m where m is the number of switches and all other values are in volts. 4. Method according to claim 1, characterized in that the switching potential U _z at current I is measured, then the polarization potential U _pol is measured immediately after switching off the current, preferably using a commutator apparatus, and the ohmic component is determined from: A ^U IR = ^U Z- ^U pol ^{/ V /}