CS248779B1 - A method for indirectly determining the crack propagation rate resulting from corrosion cracking in low alloy steels - Google Patents

Abstract

The solution concerns a method of indirect determination of the crack propagation rate resulting from corrosion cracking in low-alloy steels based on determination of the electrode potential distribution inside the crack under conditions of exposure to sodium hydroxide solutions of different concentrations and temperatures, in which, for a given system characterized by the type of steel and the concentration and temperature of sodium hydroxide, a series of voltammetric curves are measured with a gradual change in the potential rate, from whose cathodic and anodic parts the electrochemical parameters are evaluated and from which the electrode potential distribution inside the crack tip is evaluated for the given mechanical parameters of the materials.

Description

The invention relates to a method for indirect determination of the crack propagation rate resulting from corrosion cracking in low-alloy steels, based on determination of the electrode potential distribution inside the crack under conditions of exposure to sodium hydroxide NaOH solutions of various concentrations and temperatures.

Low alloy steels are resistant to corrosion cracking in chlorides, but caustic cracking can be observed in alkalis. An assessment that allows for quantitative processing of the kinetics of material property degradation by this process therefore requires the determination of the crack propagation velocity on NaOH fracture mechanics specimens.

Since measuring the crack propagation rate on fracture mechanics samples in NaOH solutions of high parameters requires a combination of a loading system with an autoclave made of a special high-alloy nickel alloy, these devices are very expensive. In contrast, for measuring crack growth on fracture mechanics samples under atmospheric pressure and exposure to NaOH solutions of various concentrations up to a maximum temperature of 100 °C, a combination of a loading system with a Teflon cell is sufficient.

However, the crack propagation speed is very low for the mentioned system under these conditions, so its measurement cannot be performed as a few-hour experiment. This situation is due to the fact that in the mentioned system the anodic dissolution reaction proceeds by a rather complicated mechanism, the knowledge of which has not yet been used in the application of the model derived for calculating the potential distribution inside a corrosion crack growing during corrosion cracking of low-alloy steels in NaOH solutions.

This mechanism, which was experimentally verified by combining cyclic voltammetry with Mössbauer spectroscopy, assumes that with a continuous change in potential in the anodic direction, Fe(OH) ₂ is first formed and then beta-FeOOH, while in the reversible process, beta-FeOOH is first reduced to Fe(OH) ₂ and then reduced to Fe.

Since Fe(OH) is highly porous and not very adhesive, it takes over the interphase ^from 2+

Fe/Fe(OH) ₂ functions as an Fe/Fe interface, i.e. an interface on which the anodic dissolution reaction of the bare metal surface in acidic solutions takes place. In contrast, beta-FeOOH is, depending on the concentration and temperature of NaOH solutions, a passive layer with good protective effects, i.e. a surface on which the cathodic depolarization reaction takes place, which is the reduction of hydrogen ions in NaOH solutions.

For this reason, the state of a crack growing in the mentioned system by corrosion cracking can be described by the equilibrium between the anodic partial reaction - anodic dissolution of the bare surface, occurring at the crack tip, i.e. at the Fe/Fe(OH) ₂ interface - and the cathodic partial reaction - depolarization of hydrogen ions occurring at the crack walls, i.e. at the FeOOH/NaOH interface.

The model derived for calculating the potential distribution inside a corrosion crack, growing during corrosion cracking of low-alloy steels in NaOH solutions, assumes that for given mechanical parameters - stress intensity factor K, yield strength Gy and elasticity model Ey - the crack width W can be determined according to the relationship

W Sř 0.49 K ² / /GyEy/ (1) and use them in the complex expression for the mentioned equilibrium, i.e. in the equation

Wi _F βχρ{(Ε _χ=θ - E _p /stV [ 4i _c C(w - 2t)j ^1/2 [(Ε _χ=0 - E _c )' ₊ ACexp{(E _c - Ε _χ=θ )/β}- ljJ ^1/2 (2) in which i _p is the exchange current density of the reactions Fe—>.Fe(OH) ₂ and Fe (OH) ₂ -►Fe, at equilibrium potential E _p , alpha is the Tafel parameter of the reaction Fe-j»Fe(OH) ₂ , beta = Tafel parameter of the reaction H ⁺ —.—►Η, Ε _χ _θ is the mixed potential at the tip of the growing crack, i _c is the exchange current density of the reactions FetOH)^—** FeOOH and FeOOH—Fe (OH) at equilibrium potential E _c , C is the specific conductivity of the solution, t is the limiting thickness of the FeOOH layer on the crack walls.

If we know the values of w and t (for solutions of 1-8 MNaOH at temperatures of 20-100 °C, the values can be used

2.10 m) to determine the value of Ε _χ _θ it is necessary that the values of E _c< E _F , i , i _F al:fa and beta are known or accessible for measurement. Only under these assumptions can the value of Ε _χ= θ be calculated from equation (2) and with its help determine the potential gradient inside the growing crack according to the relationship ^dE x ₌₀ / ^dx = ⁽⁴ⁱ c/ ^WC,1/2 t ^(E x=0 ' ^E x> ⁺ /fexp{< ^E c - ^Ε χ=θ'/Ί} - lj ^1/2 O)

Since the anodic current flow per unit crack length increasing due to anodic dissolution of the crack tip during the failure of the FeOOH layer can be determined from the equation ⁱ x=0 = WC(dE _x=0 /dx) (4), it is possible to quantitatively evaluate the current density associated with the course of the reaction of anodic dissolution of the crack tip during corrosion cracking and, using Faraday's law, convert this to the speed of crack propagation. .

Although the described model seems simple to apply, this has not yet been possible, because experimental procedures for determining the values of E , ·Ε _ρ , l _c , and _F alpha and beta in the low-alloy steel-NaOH system have not yet been designed and formulated.

This deficiency of the current state of the art is largely eliminated by the method of indirect determination of the crack propagation rate arising from corrosion cracking in low-alloy steels based on determination of the electrode potential distribution inside the crack under the conditions of action of sodium hydroxide NaOH solution of various concentrations and temperatures. The method of repeated formation and disappearance of thin layers by cyclic voltammetry according to the invention.

Its essence lies in the fact that for a given system characterized by the type of steel and the concentration and temperature of sodium hydroxide (NaOH), a series of voltammetric curves are measured with a gradual change in the potential rate, from whose anodic and cathodic parts the electrochemical parameters are evaluated and from which the distribution of the electrode potential inside the crack tip is evaluated for the given mechanical parameters of the material.

It is advantageous if the evaluation of the electrochemical equilibrium parameters is carried out from the intersection of the linear dependences of the current maxima of the anodic and cathodic parts of the voltammetric curves at the potentials corresponding to these maxima, and the evaluation of the kinetic parameters of electrochemical reactions is carried out from the ratio of the value of the current maximum associated with the formation of the relevant layer to the total value of the current maxima associated with the formation and disappearance of this layer during the reversible process.

The method of evaluating the distribution of electrode potential inside a crack growing during corrosion cracking of low-alloy steels in NaOH solutions allows, compared to measurements on fracture mechanics samples, to determine the crack propagation rate more quickly and in a simpler experimental setup.

Another advantage of this method is the fact that for the above materials which require the transfer of results obtained from thick-walled fracture mechanics samples to thin-walled tubes of paragenerators, it allows the determination of the crack propagation velocity based on measurements made on thin-walled samples. In addition, it allows the determination of very low crack velocities, including information on the size of the localization of the electrochemical effect associated with the breakdown of the oxide film at the crack tip during corrosion cracking.

The method according to the invention is expediently carried out using a Teflon measuring cell equipped with a Teflon cooler and two platinum wires between which a current from an external source passes,

248779 4 so that the negative of these electrodes can be used as a reference. This prevents the negative influence of the measured data by the reaction products of NaOH solutions with glass, as well as hydrolysis of the calomel electrode in the case of its use as a reference electrode.

P. example.

Samples of the 21/.4CrlMo material, which was in the state after normalizing annealing and tempering, were ground with metallographic papers and degreased. These were samples in the form of sheets with dimensions of 2x30x50 mm, which were mounted in a thermostatic Teflon cell equipped with a Teflon cooler and Teflon-insulated leads to the sample, the platinum grid surrounding the sample and to the platinum wires. A galvanostatic connection was used to polarize the platinum wires. A potentiostat without a sawtooth generator served as a current source. A potential with a triangular pulse generator was used to measure the voltammetric curves, to which the sample was connected, the platinum grid = polarizing electrode and the platinum wire polarized cathodically = reference electrode. The voltammetric curves were registered with a coordinate recorder.

orientation measurements showed that after obtaining reproducible results, it is appropriate to include several voltammetric scans at a potential change rate of dE/dt = 0.01 V/s before the actual measurement, until reproducible results are obtained, usually 20 min. The potential change rate is then reduced in subsequent steps of 0.01—0.005 —».0.0025—ι»0.001 V/s. In this way, stationary voltammetric curves are obtained, from the anodic and cathodic parts of which the values of current and potential corresponding to the maxima characterizing the reactions Fe—».(OH) ₂ , Fe(OH) ₂ .-FeOOH, FeOOH—a» Fe (CH) ₂ and Fe (OH) ₂ _^.Fe are evaluated.

By plotting these values in semilogarithmic coordinates, the values of E _p and _F are determined for the data characterizing the reactions Fe—»*Fe(OH) ₂ ^and Fe(0H) ₂ —>- Fe, and the values of E _c and _c for the data characterizing the reactions Fe(OH) ₂ —» FeOOH and FeOOH—»-Fe(OH) _2. At the same time, the values of the charge contribution coefficients in the reactions Fe—j»Fe(OH) ₂ and Fe(OH) ₂ —«—FeOOH are determined from the current values corresponding to the maxima of the anodic and cathodic parts, and the values of the Tafel parameters for the anodic dissolution of the bare metal surface, taking place in the Fe/Fe (OH) ₂ (oW) interface and for the cathodic depolarization of hydrogen ions on the FeOOH(A) layer are calculated from them. For a given type of material, the values of E _c , E _p , i _c , i _p , alpha and beta are different for NaOH solutions and different concentrations and temperatures - see Table 1 - and therefore must be determined for various combinations of these parameters, which affect both the specific conductivity of the solution C and the values of the parameters dE _x _g/dx, ί _χ _θ and the crack propagation velocity v at a given K - see Table 2.

Table 1

Comparison of the values of the parameters E _c< E _p , i _c , ip, alpha and beta determined for 21/4CrlMo steel samples under the conditions of exposure to 2MNaOH solution at a temperature of 40 °C and IMNaOH solution at a temperature of 60 °C

Parameter E _c (in) E _F (in) i _c (Am ² ) iplA.m ^-2 ) oOv) 2MNaOH,40 °C - 0.820 - 0.974 3,03.10 ^-3 1.73.10 ^-1 0.0308 0.0499 IMNaOH,60 °C - 0.868 - 0.978 9,09.10 ^-1 3.03.10 ^-2 0.0430 0.0597

Table 2

Comparison of the values of the parameters άΕ _χ _θ/άχ ₍ ί _χ _θ, vav _LM for 21/4CrlMo steel for the same mechanical parameters (K = 30MN.m ^-3/2 , Gy = 3.83.10 ² MN.m ^-2 and Ey = 2.10 ⁵ MN.m ^-2 ) in the conditions of immersion of 2MNaOH solution at a temperature of 40 °C and IMNaOH solution at a temperature of 60 °C

Parameter dE _x _ _Q ldx(vm ^-1 ) i _x=0 (Am ^-2 ) in(ms ^-1 ) in _LM (ms ^-1 ) 2MNaOH,40 °C 1,24.10 ^-2 2.43.10 ^-5 1.68 1.7 IMNaOH,60 °C 2,21.10 ^-2 3.25.10 ^-5 2*, 16 2.2

Fig. 1 and 2 compare the dependences of the current maxima of the anodic and cathodic parts of the voltammetric curves on the potentials corresponding to these maxima for both the reactions Fe ► Fe(OH) and Fe(OH) ₂ ► Fe, as well as for the reactions FeťOH)^ ► FeOOH and FeOOH with^FeíOH)^ in these different solutions. The values of the parameters E _c , E^., i _c , i _F , alpha and beta evaluated by the method according to the invention for both solutions are in Table 1. The values of the parameters ΰΕ _χ _θ/άχ, ί _χ _θ and the crack propagation rates in determined by the aforementioned procedure for the same mechanical parameters (K, Gy and Ey) are given for both solutions in Tab. 2. At the same time, the crack propagation rates measured under the same conditions on LM fracture mechanics samples with a pre-prepared crack are given in the same table for comparison.

In addition to determining the rate of crack propagation for a given value of K, the invention can be used to evaluate the size of the localization of the electrochemical effect associated with the breakdown of the oxide film at the crack tip, increasing during corrosion cracking. The degree of localization of a similar effect can be evaluated based on the size of the parameter άΕ _χ _θ/ΰχ in relation (3), which, with the same mechanical parameters and the same specific conductivity of the solution, is the only parameter determining the rate of crack propagation and can be influenced by the chemical composition of the material.

The above procedure can provide information necessary to understand the degradation kinetics of low-alloy steels for fast reactor steam generators under conditions of corrosion cracking induced by the presence of NaOH in the steam generator water. The prediction of the effects of NaOH, which may arise from the decomposition of Na^CO^ present in the cooling river water penetrating the condenser tubes, is in this way less uncertain with respect to the prediction of permissible NaOH concentrations in the steam generator water.

Another possibility of the invention is the evaluation of the possible influence of the chemical composition of the surface in the production process or the determination of its influence on the localization of the electrochemical effect associated with the breakdown of the oxide film at the tip of a crack growing during corrosion cracking.

Claims (2)

1. Method of indirectly determining the crack propagation rate of corrosion cracking in low alloy steels by determining the distribution of electrode potential within a crack under conditions of sodium hydroxide solution of varying concentration and temperature by the method of repeated formation and dissolution of thin layers by cyclic voltammetry. For a given system characterized by steel type and sodium hydroxide concentration and temperature, NaOH measures a series of voltametric curves as the potential velocity changes, from which both the anodic and cathodic sections are evaluated to determine electrochemical parameters and assess the electrode potential distribution within the crack tip. 2. The method according to claim 1, wherein the evaluation of the electrochemical equilibrium parameters is performed from the intersection of the line dependencies of the current maxima of the anodic and cathodic part of the voltammetric curves on the potentials corresponding to these maxima. layer to the total value of current peaks associated with the formation and disappearance of this layer in the return process.