Method and device for decontaminating a metallic surface

Abstract

The present invention concerns a method and device for oxidative erosion or for decontamination of a metallic surface, comprising a step consisting of intermittently polarizing the metallic surface to be eroded or decontaminated, placed in contact with a solution containing manganese VII, at a more anodic electric potential than the corrosion potential of said surface.

Claims

1. A method for oxidative erosion or for decontamination of a metallic surface, comprising: polarizing the metallic surface to be eroded or decontaminated by chemical etching using oxidative erosion; and placing said polarized metallic surface in contact with an electrolytic solution containing an oxidant at a more anodic electric potential than a corrosion potential of said polarized metallic surface whereby electrolysis of the electrolytic solution is prevented, the electrolytic solution containing the oxidant including manganese VII, wherein said polarization is intermittent and is generated by at least one electric pulse; an anodic overpotential between an electric potential at which the metallic surface is polarized and the corrosion potential of the polarized metallic surface is between 0.005 and 0.800 V; and said method is conducted in the presence of ozone; and current densities on the metallic surface to be eroded or decontaminated lie between 0.5 and 5.0 A.Math.m.sup.2.

2. The method according to claim 1, wherein the anodic overpotential between the electric potential at which the metallic surface is polarized and the corrosion potential of said surface is between 0.010 and 0.500 V.

3. The method according to claim 2, wherein the anodic overpotential between the electric potential at which the metallic surface is polarized and the corrosion potential of said surface is between 0.020 and 0.200 V.

4. The method according to claim 3, wherein the anodic overpotential between the electric potential at which the metallic surface is polarized and the corrosion potential of said surface is between 0.050 and 0.100 V.

5. The method according to claim 1, wherein a duration of each said at least one electric pulse is between about 1 sec and about 1 h.

6. The method according to claim 5, wherein the duration of each said at least one electric pulse is between about 10 sec and about 45 min.

7. The method according to claim 6, wherein the duration of each said at least one electric pulse is between about 1 min and about 30 min.

8. The method according to claim 7, wherein the duration of each said at least one electric pulse is between about 100 sec and about 1000 sec.

9. The method according to claim 1, wherein said at least one electric pulse comprises multiple electric pulses, and wherein a frequency of said multiple electric pulses ranges from 250 h.sup.1 to 0.05 h.sup.1.

10. The method according to claim 9, wherein said frequency of said multiple electric pulses ranges from 100 h.sup.1 to 0.1 h.sup.1.

11. The method according to claim 10, wherein said frequency of said multiple electric pulses ranges from 50 h.sup.1 to 0.5 h.sup.1.

12. The method according to claim 1, wherein said electrolytic solution contains nitric acid.

13. The method according to claim 1, wherein the manganese is initially added to said solution in the form of manganese II, manganese IV, manganese VII, or a mixture thereof.

14. The method according to claim 13, wherein the manganese is initially added to said solution at a concentration of less than 500 mg/L.

15. The method according to claim 14, wherein the manganese is initially added to said solution at a concentration of between 10 and 400 mg/L.

16. The method according to claim 15, wherein the manganese is initially added to said solution at a concentration of between 20 and 200 mg/L.

17. The method according to claim 16, wherein the manganese is initially added to said solution at a concentration of between 50 and 100 mg/L.

18. The method according to claim 1, further comprising: stabilizing manganese VII to manganese II.

19. The method according to claim 18, wherein said stabilizing comprises adding oxygenated water (H.sub.2O.sub.2) to said solution containing manganese VII.

20. The method for oxidative erosion or for decontamination of a metallic surface according to claim 1, wherein said method further comprises subjecting the metallic surface to be eroded or decontaminated to at least one non-corrosive rinsing, prior to said polarizing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the variation in density of the corrosion current of AISI 304L stainless steel with [HNO.sub.3]=0.5 M and different initial concentrations of Mn II at a temperature of 37 C.

(2) FIG. 2 shows the influence of nitric acid concentration on the density of the corrosion current of AISI 304L stainless steel at different initial Mn II concentrations and at a temperature of 37 C.

(3) FIG. 3 shows the influence of temperature on the density of the corrosion current of AISI 304L stainless steel at different initial Mn II concentrations and with [HNO.sub.3]=0.5 M.

(4) FIG. 4 shows the influence of Mn II concentration on the rate of corrosion of AISI 304L stainless steel over an attack time of 24 h, in 0.5 M nitric acid medium, at a temperature of 37 C. and with a surface/volume ratio of 64 m.sup.1.

(5) FIG. 5 shows the influence of initial Mn II concentration on time-related changes in the polarisation resistance of AISI 304L stainless steel measured at rest, with [HNO.sub.3]=0.5 M and at a temperature of 37 C.

(6) FIG. 6 shows the influence of Mn II concentration on time-related changes in the polarisation resistance of AISI 304L stainless steel measured at E=+0.1 V/E.sub.rest, where [HNO.sub.3]=0.5 M and at a temperature of 37 C.

(7) FIG. 7 shows the influence of the applied potential on the corrosion rate (1/Rp) of AISI 304L stainless steel, in which [Mn II]=100 mg/L, [HNO.sub.3]=0.5 M and at a temperature of 37 C.

(8) FIG. 8 is a SEM photograph of the surface of AISI 304L stainless steel after 24 h corrosion in a 0.5 M solution of HNO.sub.3, where [MnII]=100 mg/L, at a temperature of 37 C. and where E=E.sub.rest.

(9) FIG. 9 is a SEM photograph of the surface of AISI 304L stainless steel after 24 h corrosion in a 0.5 M HNO.sub.3 solution, where [MnII]=100 mg/L, at a temperature of 37 C. and where E=+0.1 V/E.sub.rest.

(10) FIG. 10 is a SEM photograph of the surface of AISI 304L stainless steel after 24 h corrosion in a 0.5 M HNO.sub.3 solution, where [MnII]=100 mg/L, at a temperature of 37 C. and where E=+0.2 V/E.sub.rest.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(11) The objective of the study is to present the optimal operating conditions allowing limited formation of manganese IV oxide (MnO.sub.2), thereby obtaining maximum corrosion of AISI 304L stainless steel with a minimum addition of manganese to the nitric acid medium held under constant ozone flushing (ozonated oxygen or air).

(12) In a first part, the behaviour of AISI 304L steel was studied by setting the Surface/Volume (S/V) ratio and causing the different parameters to vary such as the initial concentration of manganese II (Mn.sup.2+), the concentration of nitric acid and temperature. The optimal conditions for corrosion of AISI 304L stainless steel can thereby be determined. This study was conducted in an electrochemical cell with three electrodes. The saturated calomel electrode provided with double junction was used as reference electrode, and the platinum electrode as auxiliary electrode.

(13) In the second part, the corrosion of steel was studied in a 2 liter-reactor under the optimal conditions previously determined in the electrochemical cell. The results obtained allow validation of the operation conditions of the method. Subsequently, an anodic overpotential relative to the corrosion potential was applied to the steel. This anodic overpotential must avoid precipitation of manganese IV on the surface of the steel and hence keep the concentration of Mn VII constant in the electrolyte over time, thereby maintaining the corrosion rate of the stainless steel throughout the decontamination process. The non-precipitation of MnO.sub.2 on the surface of the stainless steel was verified under these conditions.

(14) The morphology of the surface after corrosive attack on the stainless steel was examined under scanning electron microscopy (SEM) and mass loss was determined using a micro-balance.

(15) I. Electrochemical Cell Study

(16) An electrochemical cell study allowed the experimental conditions to be determined (concentrations of Mn, nitric acid and temperature).

(17) I.1. Influence of the Initial Concentration of Manganese and of Nitric Acid Concentration on the Corrosion Rate of AISI 304L Stainless Steel at 37 C.

(18) The ozone flow rate was set at 1.5 g/h/l and the S/V ratio at 34 m.sup.1.

(19) The polarisation curves were plotted for different initial concentrations of Mn II [Mn.sup.2+] with a nitric acid concentration set at 0.5 M.

(20) The initial concentrations of Mn II used in this experiment were respectively: 0, 25, 50, 100, 150 and 200 mg/L.

(21) From the polarisation curves, the corrosion current density of the stainless steel was determined graphically (via the step value of cathodic diffusion-limited current) in relation to the initial concentration of Mn II [Mn.sup.2+]. The results are given in FIG. 1.

(22) The corrosion current density of AISI 304L stainless steel increases linear fashion with the initial concentration of Mn II in solution ([Mn II]).

(23) The linear equation characteristic of this trend is:
i.sub.corr (mA.Math.cm.sup.2)=5*10.sup.4 [Mn II] (mg/L)

(24) With the decontamination method, the nitric acid concentrations are generally higher than 0.5 M and may reach 2.5 M. Under these conditions, the rate of corrosion of stainless steel at 37 C. varies differently in relation to the initial Mn II concentration. The results comparing the two acidities are recorded in FIG. 2.

(25) For the two different concentrations of nitric acid, the corrosion current density increases globally with the initial concentration of Mn II. However, with a nitric acid concentration of 2.5 M, the corrosion current density remains constant over and above an initial Mn II concentration of 100 mg/L.

(26) Therefore, for initial concentrations of Mn II equal to or higher than 100 mg/L, the corrosion current density of stainless steel for a nitric acid concentration of 0.5 M becomes higher than that obtained with a nitric acid concentration of 2.5 M.

(27) To conclude, at 37 C., the corrosion rate of stainless steel increases with the initial concentration of Mn II present in solution in a nitric acid medium of low concentration (0.5 M). However, corrosion does not appear to be helped by an increase in the initial concentration of Mn II when the nitric acid concentration reaches 2.5 M.

(28) When measurements were taken, a deposit of MnO.sub.2 was seen on the surface of AISI 304L steel. This deposit is much greater with a nitric acid concentration of 2.5 M. It is the formation of this deposit which reduces the efficacy of the oxidant. A high initial concentration of manganese would be of no use under these conditions.

(29) I.2. Influence of Temperature and Initial Manganese Concentration on the Corrosion Rate of AISI 304L Stainless Steel.

(30) The changes in the corrosion current density of AISI 304L stainless steel in relation to temperature and to initial Mn II concentration (FIG. 3) are comparable with those observed with a nitric acid concentration of 0.5 M and at a temperature of 37 C.

(31) The corrosion current density increases with the initial Mn II concentration but in different manner depending upon temperature. A rise in temperature to 60 C. has an adverse effect on the corrosion rate of stainless steel. At this latter temperature, the black deposit of MnO.sub.2 is much greater than in all other cases. It increases further with an increase in the concentration of nitric acid.

(32) Temperature is a kinetic factor which benefits the rate of MnO.sub.2 formation, and this (at 60 C.) leads to a reduction in the corrosion rate of stainless steel compared with temperatures of 25 C. and 37 C.

(33) At 25 C., the formation of manganese oxide remains negligible, but the temperature is not high enough to allow corrosion of stainless steel that is comparable with that observed at 37 C.

(34) I.3. Conclusion.

(35) According to our results, in an electrochemical cell, the corrosion rate of stainless steel increases with the initial concentration of Mn II over the range 0-200 mg/L and with a 0.5 M concentration of nitric acid and an ozonated medium (ozonated air or oxygen).

(36) However, an increase in temperature to 60 C. and/or in the concentration of nitric acid promotes the formation of a black deposit of MnO.sub.2. The formation of this deposit reduces the efficient concentration of oxidant (MnO.sub.4.sup.), which leads to a decrease in the corrosion rate of stainless steel.

(37) II. Reactor Study

(38) Study in a reactor allows the corrosion rate of AISI 304L stainless steel to be evaluated under real decontamination conditions, and allows examination of the conditions to avoid the formation of MnO.sub.2.

(39) II.1. Experimental Conditions

(40) Two 2-liter reactors are mounted in series. To the first reactor designated reactor 1 hereunder, are added the nitric acid solution and the manganese nitrate (Mn II). This reactor 1 is supplied with ozone (ozonated air or oxygen) by means of an air-lift and is filled with the solution to be oxidized. The inlet flow rate of ozone (ozonated oxygen) into the reactor 1 is set at 10 L/h. The ozone concentration at the input to the reactor 1 is 72 g/Nm.sup.3. It is in this reactor 1 that complete oxidation takes place of colourless Mn II (Mn.sup.2+) to purple Mn VII (MnO.sub.4.sup.).

(41) Once the reaction of Mn VII formation is completed, the oxidizing solution is decanted into the second reactor (reactor 2) in which the sheets of AISI 304L stainless steel have already been placed with a Surface/Volume ratio of 64 m.sup.1. The constant, continuous flow of ozone is maintained throughout the experiments to ensure regeneration of Mn VII.

(42) The temperature of the electrolyte inside the reactor 2 is held constant by means of a thermostat and the double jacket provided on this reactor.

(43) A saturated calomel reference electrode and a platinum electrode were used for all electrochemical measurements.

(44) The reactor study used:

(45) 40 cm.sup.2 sheets of AISI 304L stainless steel;

(46) initial Mn II concentrations in solutions of 100 and 200 mg/L, for a nitric acid concentration of 0.5 M;

(47) a temperature of 37 C.;

(48) a time of 24 h.

(49) II.2. Influence of Initial Mn II Concentration on the Rate of Corrosion of AISI 304L Steel After 24 h Attack at the Corrosion Potential.

(50) The corrosion rate of stainless steel expressed in m/h as a function of the initial Mn II concentration is given in FIG. 4. The results were obtained after an attack time of 24 h and with a 0.5 M nitric acid medium at 37 C.

(51) The attack rate is proportional to the Mn II concentration. For an initial concentration of Mn II in solution of 100 mg/L, the mean corrosion rate of the stainless steel after 24 h was 0.07 m/h. This value is already sufficient to ensure surface decontamination of installations.

(52) II.3. Monitoring the Formation of MnO.sub.2 Deposit. Measurement of Stainless Steel Polarisation Resistance During Treatment.

(53) During the 24 h treatment of stainless steel in reactor 2, a deposit of MnO.sub.2 was formed on the surface of the steel.

(54) To evaluate the extent of this phenomenon, the changes in polarisation resistance R.sub.p (.Math.cm.sup.2) of the steel was measured by electrochemical impedance spectroscopy (EIS) over the time period. This value was obtained from Bode or Nyquist diagrams determined at the corrosion potential of stainless steel, and by extrapolating the curve at low frequencies.

(55) The variations in polarisation resistance throughout the duration of the process (i.e. 24 h) are given (FIG. 5) for: a temperature of 37 C., a nitric acid concentration of 0.5 M, and the two concentrations of Mn II: 100 mg/L and 200 mg/L.

(56) With an initial Mn II concentration of 100 mg/L, polarisation resistance increases from 2000 ohm.Math.cm.sup.2 to 8000 ohm.Math.cm.sup.2 over the course of time.

(57) This phenomenon translates the formation of a MnO.sub.2 deposit on the surface of the stainless steel. This deposit causes slowing of the corrosion rate of stainless steel over the 24 h period.

(58) Similar results were obtained with an initial Mn II concentration of 200 mg/L. Polarisation resistance also increased over time on and after 3 h but nevertheless remained lower than that measured with [Mn II]=100 mg/L. Therefore, in this case also there was formation of MnO.sub.2 deposit. This medium is more oxidizing, which accounts for Rp ([Mn II]=200 mg/L)<Rp ([Mn II]=100 mg/L). It would be equivalent to say that the corrosion rate increases when the concentration of oxidant is increased twofold.

(59) I.4. Conclusion.

(60) Measurements of the polarisation resistance (Rp) of stainless steel throughout the 24 h treatment in a reactor evidence a progressive increase in Rp even in cases when the nitric acid concentration is low (0.5 M) and the temperature scarcely high (37 C.). This increase in polarization resistance is also observed with a higher concentration of oxidant. This increase in Rp over time slows down the corrosion rate of stainless steel.

(61) It is therefore necessary to propose a further element for corrosive attack of the medium, to limit the slowing effect of the MnO.sub.2 deposit. This step must restrict or eliminate the formation of the MnO.sub.2 deposit and not act as further attacking element for the oxidizing medium.

(62) III. Anti-MnO.sub.2 Deposit Electrochemical Treatment. Intermittent Oxidation of AISI 304L Stainless Steel.

(63) To eliminate the formation of MnO.sub.2 on the surface during attack of the stainless steel, the present inventors propose applying a slightly higher potential than the corrosion potential (E.sub.rest) of stainless steel. This holding of the steel at a more anodic potential is intended solely to cause dissolution of the MnO.sub.2 deposit. The corrosion rate of the metal does not need to be modified through this modification in potential since the oxidizing medium is already highly efficient. It is the maintaining of its efficacy that must be ensured over 24 h.

(64) Therefore an overpotential of 0.1 V/E.sub.rest and of 0.2 WE.sub.rest was tested on sheets of 304L steel for a period of 10 min every hour, for example, and at the two previously cited Mn II concentrations. The time during which the electrode is held at a higher potential than the corrosion potential must be limited to re-solubilisation of the MnO.sub.2 spontaneously formed in the reactor.

(65) The time monitoring of polarisation resistance over 24 h for these different conditions allows determination of the efficacy imparted to the method if a potential is applied in addition to chemical corrosion.

(66) III.1. Application of a Potential. Influence of Initial Mn II Concentration on Polarisation Resistance.

(67) The changes in polarisation resistance of AISI 304L stainless steel brought for 10 min/h to the potential +0.1 V/E.sub.rest in 0.5 M nitric acid medium at a temperature of 37 C., and with initial Mn II concentrations of 100 mg/L and 200 mg/L, are given in FIG. 6.

(68) Contrary to the measurements made with the corrosion potential (FIG. 5), polarisation resistance remains constant over time for both initial Mn II concentrations of 100 mg/L and 200 mg/L. The intermittent application (10 min/h) of an anodic potential of +0.1 V/E.sub.rest to the stainless steel is sufficient and effectively eliminated the passivating effect of the MnO.sub.2 deposit.

(69) In addition, in both cases, the polarisation resistance values are very close. The initial Mn II concentration therefore appears to have little influence on polarisation resistance and hence on corrosion rate.

(70) III.2. Influence of the Applied Potential on Corrosion Rate.

(71) The influence of the applied potential on the corrosion rate (1/Rp) of stainless steel was compared, FIG. 7, under the same operating conditions: T=37 C., nitric acid concentration=0.5 M, initial Mn II concentration=100 mg/L, intermittent schedule=10 min/h. Three cases were envisaged:

(72) no application of potential E=E.sub.rest,

(73) E.sub.applied=+0.1 V/E.sub.rest,

(74) E.sub.applied=+0.2 V/E.sub.rest.

(75) The intermittent application of an anodic potential of +0.2 V/E.sub.rest or +0.1 V/E.sub.rest to stainless steel significantly increases the corrosion rate (increase in 1/Rp ratio) compared with the tests using the corrosion potential (E.sub.rest).

(76) The corrosion rate also increases with application of a higher potential, but this is not the targeted objective. It is sufficient that the corrosion rate should remain constant throughout the decontamination treatment period with minimum perturbation. Therefore, according to these results, and notably under the experimental conditions used, intermittent application of a potential of +0.1 V/E.sub.rest is preferable.

(77) III.3. Conclusion.

(78) The application of an intermittent signal (e.g. 10 min/h) bringing the potential of stainless steel to a more anodic value E.sub.applied=0.1 V/E.sub.rest prevents the formation of MnO.sub.2 deposit, and allows the corrosion rate to be maintained constant throughout the entire duration of the decontamination treatment (24 h). The initial manganese concentrations can therefore be limited to 100 mg/L and will remain efficient 24 h without any additional manganese.

(79) With intermittent oxidations and low anodic overpotentials, the current densities on stainless steel are very low. There is therefore no release of hydrogen to take into account on the auxiliary electrode in an aqueous medium that is saturated with ozonated oxygen or air and high in oxidant.

(80) IV. Morphology of Corrosion.

(81) IV.1. Mass Loss

(82) Table 1 summarises the mass losses of the stainless steel plates after corrosion. The plates were subjected to: either attack in a reactor at 37 C. for 24 h in 0.5 M nitric acid medium, with an initial concentration of Mn II=100 mg/L; or attack in a reactor under the same conditions but, in addition, with intermittent polarisation for 10 min/h at +0.1 V/SHE; or attack in a reactor under the same conditions but, in addition, with intermittent polarisation for 10 min/h at +0.2 V/SHE.

(83) The mass losses obtained allow evaluation of the mass of formed MnO.sub.2 deposit, and of the corrosion rates of stainless steel under these conditions (Table 1).

(84) TABLE-US-00001 TABLE 1 Influence of the treatment on the corrosion rate of AISI 304L stainless steel (SS). Initial Mass Mass after Mass of m mass after H.sub.2O.sub.2 MnO.sub.2 (by of SS treatment rinsing deposit corrosion) V.sub.corr Treatment (g) (g) (g) (g) (g) (m/j) E = E.sub.rest 31.5498 31.5445 31.4795 0.065 0.07 2.3 0.1 V/E.sub.rest 31.1362 31.0282 31.0251 0.003 0.111 3.6 0.2 V/E.sub.rest 31.8369 31.6681 31.6661 0.002 0.171 5.5 T = 37 C., [HNO.sub.3] = 0.5M, [Mn II] = 100 mg/L, Intermittent oxidation 10 min/h.

(85) These results confirm the formation of a MnO.sub.2 deposit on the surface of the steel during corrosion of the stainless steel at the rest potential, and almost complete disappearance thereof when intermittent anodic oxidation is applied (10 min/h) in addition to corrosion. The application of low polarisation of +0.1 V/E.sub.rest appears to be sufficient. It allows a 56% increase in the mean daily corrosion rate of stainless steel.

(86) IV.2. Comparison of Surfaces After Attack

(87) The surfaces of the stainless steel sheets subjected to the different types of treatment (with or without intermittent anodic overpotential) were observed under SEM. The results are given FIGS. 8-10.

(88) The SEM photographs (FIGS. 8-10) of the three surfaces of AISI 304L stainless steel evidence corrosion of intergranular type for the three treatments.

(89) The corrosion appears to be more marked in cases when the steel sheets were subjected to a potential higher than the rest potential, and more particularly after application of a potential of +0.2 V/E.sub.rest. This result is in keeping with the mass losses of the samples after corrosion.

(90) Observations made on cross-sections of the steel confirmed these results.

REFERENCES

(91) 1. Patent application FR 2 792 763 (Commissariat l'Energie Atomique) published on 27 Oct. 2000.

(92) 2. Patent application FR 2 641 895 (Commissariat l'Energie Atomique) published on 20 Jul. 1990.

(93) 3. Patent application FR 2 644 618 (Commissariat l'Energie Atomique) published on 21 Sep. 1990.

(94) 4. Patent application FR 2 850 673 (Electricit de France) published on 6 Aug. 2004.