Method for evaluating the corrosion resistance of a surface

Abstract

A method for evaluating the corrosion resistance of at least one surface, comprising: a) carrying out one or more sequences of n cyclic electrochemical test cycles on the surface, where n is an integer greater than or equal to 1, each cycle comprising the following three successive steps: a first step of measurement by electrochemical impedance spectroscopy of one or more electrochemical quantities reflecting the corrosion resistance of the surface; a second step of cathodic polarization; a third step of potential relaxation; and b) carrying out a visual inspection at the end of each cyclic electrochemical test sequence for the detection of surface degradation, and as soon as the appearance of surface degradation is detected by visual inspection during a sequence, the cyclic electrochemical test cycles being stopped at the end of that sequence;and c) the evaluation of the surface's corrosion resistance from the study of measured electrochemical quantities.

Description

Title of the invention: Method for evaluating the corrosion resistance of a surface Technical field

[0001] The present invention relates, in general, to the corrosion resistance of a surface.

[0002] More specifically, the invention relates to a method for evaluating the corrosion resistance of a surface, such as the surface of an aircraft part. State of the art

[0003] In the field of aeronautics, protection against corrosion of the constituent parts of an aircraft is an important issue in order to guarantee their reliability and longevity.

[0004] Several means of protection against corrosion can be obtained by treating the surface of the aircraft part.

[0005] An existing solution consists of applying a coating to the surface of the part made of a material having corrosion resistance properties superior to the properties of the part receiving the coating. According to one example, it is possible to deposit a layer of paint.

[0006] Conventionally, salt spray tests are carried out in order to discriminate between different means of protection according to their ability to resist corrosion and to detect the best candidate according to the role and location of the part in the aircraft.

[0007] A salt solution is sprayed onto the surface of each of the materials to be evaluated under certain predefined conditions of pressure, pH and temperature.

[0008] However, the process can require up to thousands of hours of operation, particularly for surface treatments that are particularly resistant to corrosion, such as certain paints. Consequently, the implementation of salt spray tests is long and tedious. Statement of the invention

[0009] The invention therefore aims to remedy these drawbacks and to propose a simple method to implement making it possible to quickly evaluate the corrosion resistance of a surface.

[0010] A method is therefore proposed for evaluating the corrosion resistance of at least one surface, comprising:

[0011] a) carrying out one or more sequences of n cyclic electrochemical test cycles on the surface, where n is an integer greater than or equal to 1, each cycle comprising the following three successive steps:

[0012] a first step of measurement by electrochemical impedance spectroscopy of one or more electrochemical quantities reflecting the corrosion resistance of the surface;

[0013] a second cathodic polarization step; and

[0014] a third stage of potential relaxation;

[0015] b) performing a visual inspection at the end of each cyclic electrochemical test sequence for the detection of surface degradation and, upon detection by visual inspection of the appearance of surface degradation during a sequence, the cyclic electrochemical test cycles being stopped at the end of said sequence; and

[0016] c) the evaluation of the corrosion resistance of the surface from the study of the measured electrochemical quantities.

[0017] Other objects, characteristics, aspects and advantages of the invention will appear even more clearly on reading the description of the appended drawings in which:

[0018] [Fig-1] is a photograph of a metal sample coated with a layer of non-chromated solvent-based paint A obtained following a corrosion resistance evaluation method according to the invention.

[0019] [Fig.2] is a photograph of a metal sample coated with a layer of chromated paint B obtained following a corrosion resistance evaluation process according to the invention.

[0020] [Fig.3] is a photograph of a metal sample coated with a layer of non-chromated water-dilutable C paint obtained following a corrosion resistance evaluation process according to the invention.

[0021] In what follows, the limits of a domain of values ​​are included in this domain, in particular in the expression “between”.

[0022] Furthermore, the expression “at least one” used in the present description is equivalent to the expression “one or more”.

[0023] The surface whose corrosion resistance is evaluated may be, for example, the surface of an aircraft part.

[0024] It does not depart from the scope of the invention if the method relates to the evaluation of the corrosion resistance of a surface of a part from a different sector.

[0025] The surface studied may be the surface of a part having undergone a surface treatment.

[0026] According to one example, the surface studied is the surface of a coating applied to the part, such as a paint.

[0027] During a first phase of the method for evaluating the corrosion resistance of the surface, one or more sequences of n cyclic electrochemical test cycles are carried out on the surface.

[0028] The cyclic electrochemical test is an accelerated cyclic electrochemical test or "ACET", abbreviation of the English terms Accelerated Cyclic Electrochemical Test, otherwise known as the AC-DC-AC test method, is conventionally used to theoretically characterize the corrosion resistance properties of a coating.

[0029] Each cycle comprises three successive stages.

[0030] The first step is a measurement step by electrochemical impedance spectroscopy making it possible to characterize the surface by obtaining one or more electrochemical quantities from which it is possible to determine the corrosion resistance of the surface.

[0031] The measurements can be carried out using a potentiostat comprising an electrochemical impedance spectroscopy component.

[0032] Preferably, the first step of measurement by electrochemical impedance spectroscopy comprises the establishment of one or more graphical representations from among: a phase diagram, an impedance modulus diagram and a Nyquist diagram.

[0033] The second step is a cathodic polarization step, allowing the surface to be cathodically stressed in order to degrade it.

[0034] Preferably, the cathodic polarization step comprises monitoring the evolution of the intensity as a function of time by establishing a graphical representation.

[0035] The third stage of potential relaxation is a resting stage which allows the surface to regain its equilibrium after stress.

[0036] Preferably, the potential relaxation step comprises monitoring the evolution of the abandonment potential, also called open circuit potential (OCP) as a function of time by establishing a graphical representation.

[0037] According to the invention, the accelerated cyclic electrochemical test (ACET) sequences, and in particular the interpretation of the data from the first step, are correlated with a visual inspection of the surface.

[0038] Visual inspection means the detection of degradation with the naked eye by an operator.

[0039] Indeed, a visual inspection is carried out at the end of each accelerated cyclic electrochemical test (ACET) sequence in order to detect surface degradation.

[0040] As soon as the appearance of degradation is detected by visual inspection during a sequence, the accelerated cyclic electrochemical test cycles are stopped.

[0041] More specifically, the accelerated cyclic electrochemical test cycles are stopped at the end of the sequence during which the appearance of surface degradation has been detected by visual inspection.

[0042] This correlation makes it possible to simplify and optimize the characterization of the corrosion resistance properties of a surface and to define them with greater precision.

[0043] The corrosion resistance of the surface is thus determined from the study of the results obtained during the first stage of accelerated cyclic electrochemical testing, i.e. from the measured electrochemical quantities.

[0044] According to one embodiment, after detection of degradation by visual inspection, it will be possible to provide, during the last cycle of the last sequence, a fourth step similar to the first step, for the measurement by electrochemical impedance spectroscopy of one or more electrochemical quantities reflecting the corrosion resistance of the surface.

[0045] Preferably, the method for evaluating the corrosion resistance of the surface comprises determining the cycle during which the degradation, observed by visual inspection, appeared. This determination is carried out by studying the electrochemical quantities measured during the last sequence which was carried out, that is to say before stopping the accelerated cyclic electrochemical test cycles.

[0046] Preferably, determining the cycle in which the degradation occurred includes studying established graphical representations, such as a phase diagram, an impedance modulus diagram, and a Nyquist diagram.

[0047] Preferably, the second cathodic polarization step is carried out at a voltage lower than -5V in order to create defects on the surface, preferably between -5V and -10V, more preferably between -7V and -9V.

[0048] In addition, the number n of cycles of an accelerated cyclic electrochemical test sequence may be between 2 and 10, preferably between 4 and 8, more preferably equal to 6.

[0049] The number n of cycles of a sequence may be adjusted according to the nature of the surface, for example the surface of a coating, studied.

[0050] The second cathodic polarization step can preferably be carried out for a time of between 10 min and 60 min, preferably between 15 min and 25 min.

[0051] A duration of between 15 and 25 min is particularly advantageous for obtaining visible degradation without being too severe.

[0052] Prior to carrying out the sequences of n accelerated cyclic electrochemical test (ACET) cycles, a step of verifying the absence of surface defects may be carried out in order to ensure that no defects are likely to occur. to distort the results of the evaluation of the corrosion resistance of the surface, in particular by the formation of preferential corrosion at the level of this potential defect.

[0053] According to one embodiment, it will be possible to provide for the evaluation of the corrosion resistance of a plurality of surfaces. The method for evaluating the corrosion resistance may then comprise a step of comparing the corrosion resistance of a plurality of surfaces.

[0054] Evaluating the corrosion resistance of a plurality of surfaces can, for example, make it possible to discriminate between different coatings based on their ability to resist corrosion and to identify the best candidate based on the role and location of the coated part.

[0055] It is then possible to determine and classify the different surfaces according to their resistance to corrosion. The different surfaces can be classified by considering the number of sequences of n cycles to which each of these surfaces resists.

[0056] According to one embodiment, upon detection by visual inspection of the appearance of degradation of one of the surfaces during a sequence, the accelerated cyclic electrochemical test cycles of the plurality of surfaces are stopped at the end of this sequence.

[0057] Advantageously, the different coatings are of the same thickness.

[0058] Advantageously, prior to carrying out the sequences of n accelerated cyclic electrochemical test (ACET) cycles, the method for evaluating corrosion resistance may include a step of verifying the equality of the thicknesses of the different coatings.

[0059] According to one example, the equality of the thicknesses of the different coatings can be carried out by a method based on the principle of eddy currents. Detailed description of an embodiment Example#:

[0060] Three samples of aircraft parts were prepared from 7050 aluminum alloy metal substrates coated, respectively, with a layer of non-chromated solvent-borne paint A, a layer of chromated paint B and a layer of non-chromated water-borne paint C and the corrosion resistance of the three samples is studied using a corrosion resistance evaluation method according to one embodiment of the invention.

[0061] The corrosion resistance of these three coatings is evaluated using a three-electrode system comprising an Ag / AgCl reference electrode, a counter electrode graphite, and a NaCl electrolyte solution with a mass concentration of between 20 and 40g.L

[0062] The studied surface area of ​​the coatings of the three samples, exposed to the NaCl solution, is 7.07cm2.

[0063] A sequence of six successive accelerated cyclic electrochemical test (ACET) cycles is carried out on the surface of the three samples.

[0064] Each cycle comprises successive first, second and third stages.

[0065] A first step of measuring electrochemical quantities comprising the establishment, by electrochemical impedance spectroscopy, of Bode diagrams, in particular phase diagram and impedance modulus diagram, and of a Nyquist diagram.

[0066] Electrochemical impedance spectroscopy is performed for a time t equal to 7 min and at a voltage equal to ±10 mV vs OCP, where OCP is the open circuit potential or dropout potential.

[0067] A second cathodic polarization step is carried out for a time between 15 and 25 min and at a voltage between -7 and -9V vs OCP, allowing the surface to be cathodically stressed in order to degrade it. A graphical representation of the intensity as a function of time is produced.

[0068] A third potential relaxation step is carried out. This resting step allows the surface to return to its equilibrium after having been heavily stressed. A graphical representation of the evolution of the open-circuit potential as a function of time, over a total time equal to 3 hours, is produced.

[0069] The number n of cycles per sequence is chosen equal to 6.

[0070] At the end of each sequence of 6 cycles, a visual inspection of the surface is carried out in order to detect the appearance of degradation on the three coatings.

[0071] When degradation of one of the surfaces is detected during a sequence, the accelerated cyclic electrochemical test (ACET) cycles of the three coatings are stopped at the end of this sequence.

[0072] Visual inspection revealed the appearance of degradation during the first sequence of 6 cycles. A blister appeared on the surface of the non-chromated water-borne paint C and blistering and pitting appeared on the surface of the chromated paint B, as seen, respectively, in Figures 3 and 2.

[0073] No degradation appeared on the surface of the non-chromated solvent-based paint A as can be seen in [Fig.l].

[0074] The accelerated cyclic electrochemical test (ACET) cycles are then stopped at the end of this first sequence of 6 cycles.

[0075] Interpretation of results for the electrochemical system with non-chromated solvent-based paint A after 6 ACET test cycles

[0076] The phase diagram shows the presence of capacitive behavior during the 6 cycles and the absence of degradation. The impedance modulus diagram shows a high overall corrosion resistance of 1010 Q.cm2. This reflects good corrosion resistance according to the graph.

[0077] The Nyquist diagram shows a negligible reduction in the arcs of circles and an absence of degradation.

[0078] The graphical representation of the intensity as a function of time during cathodic polarization makes it possible to observe a very low charge transfer of 10 9A and therefore the absence of degradation.

[0079] The graphical representation of the abandonment potential as a function of time during potential relaxation makes it possible to observe the obtaining of high potentials and the presence of low variations, reflecting the absence of degradation.

[0080] No degradation appeared on the surface of the non-chromated solvent-based paint A and the study of the electrochemical cycles makes it possible to validate this visual observation.

[0081] All these interpretations lead to the conclusion that the non-chromated solvent-based paint 1 is an effective protective coating against corrosion, not undergoing degradation after a sequence of 6 ACET cycles.

[0082] Interpretation of the results for the electrochemical system with non-chromated water-dilutable C paint after 6 ACET cycles

[0083] The phase diagram shows the transition from a capacitive character to a resistive character during the 6 ACET test cycles, indicating degradation.

[0084] The impedance modulus diagram shows a high overall corrosion resistance, but this decreases over the 6 ACET test cycles, with a change from 1010 Q.cm2 to 108 Q.cm2. This reflects good corrosion resistance and low or localized degradation.

[0085] The Nyquist diagram allows us to observe a negligible reduction in the arcs of circles, reflecting little or no degradation.

[0086] The graphical representation of the intensity as a function of time obtained during cathodic polarization shows a very low charge transfer of 10 7A, but which increases for cycles 5 and 6 with intensities of the order of 10 4A. This reflects a degradation during the cycles.

[0087] The graphical representation of abandonment potential as a function of time obtained during potential relaxation shows that potentials equal to the corrosion potential of the 7050 aluminum alloy are obtained. This reflects a risk infiltration of the electrolyte to the metal substrate which could lead to degradation of the coating.

[0088] All these interpretations lead to the conclusion that, according to the impedance modulus diagram, the non-chromated water-based paint C has good overall corrosion resistance. However, according to the graphical representations obtained during cathodic polarization and potential relaxation during the ACET test cycles, degradation appears to appear. Indeed, a blister appeared on the surface of the non-chromated water-based paint C and the study of the electrochemical cycles makes it possible to validate this visual observation.

[0089] Interpretation of the results for the electrochemical system with chromated paint B after 6 ACET cycles

[0090] The phase diagram shows the transition from a capacitive character to a resistive character during the 6 cycles, and this in two stages. This reflects the appearance of degradation.

[0091] The impedance modulus diagram shows an overall resistance to corrosion that is initially high, but which decreases over the 6 cycles, with a change from 109 Q.cm2 to 105 Q.cm2. This reflects a significant degradation of the system, 105 Q.cm2 being a low value for a paint.

[0092] The Nyquist diagram allows us to observe a significant reduction in the arcs of circles, reflecting a degradation.

[0093] The graphical representation of the intensity as a function of time obtained during cathodic polarization reflects a charge transfer which increases during the 6 ACET test cycles up to 40mA. This significant current indicates the appearance of corrosion phenomena.

[0094] The graphical representation of the abandonment potential as a function of time obtained during the potential relaxation makes it possible to observe the obtaining of potentials equal to the corrosion potential of the 7050 aluminum alloy. This reflects a risk of infiltration of the electrolyte to the metal substrate which could lead to degradation of the coating.

[0095] Blistering and pitting appeared on the surface of the chromated paint B and the study of the electrochemical cycles makes it possible to validate this visual observation.

[0096] All these interpretations lead to the conclusion that the chromated B paint coating may exhibit defects after 6 ACET test cycles.

[0097] In conclusion, after 6 ACET test cycles, it is expected, according to the interpretations of the results, no degradation of the non-chromated solvent-based paint A, a localized degradation of the non-chromated water-based paint C and a generalized degradation of the chromated paint B. These interpretations made by the study of the electrochemical cycles allow the validation of visual observation of the surface of the samples.

[0098] It is thus possible to classify the three paints according to their resistance to corrosion by studying the electrochemical measurements recorded during the cycles correlated with the visual appearance recorded on the surface of the three coatings.

Claims

Claims

1. Method for evaluating and comparing the corrosion resistance of a plurality of surfaces of at least one aircraft part, comprising: a) carrying out one or more sequences of n cyclic electrochemical test cycles on the surfaces, where n is an integer greater than or equal to 1, each cycle comprising the following three successive steps: a first step of measurement by electrochemical impedance spectroscopy of one or more electrochemical quantities reflecting the corrosion resistance of the surfaces; a second step of cathodic polarization carried out at a voltage lower than -5V, preferably between -5V and -10V, more preferably between -7V and -9V; and a third step of potential relaxation;(b) carrying out a visual inspection at the end of each cyclic electrochemical test sequence to detect surface degradation and, upon detection by visual inspection of the appearance of degradation of one of the surfaces during a sequence, the cyclic electrochemical test cycles being stopped at the end of said sequence; and (c) evaluating the corrosion resistance of the surfaces based on the study of the electrochemical quantities measured.;

2. Method according to claim 1, comprising determining the cycle during which the degradation appeared by studying the electrochemical quantities measured during the last sequence carried out.

3. The method of claim 2, wherein the first step of measuring by electrochemical impedance spectroscopy comprises establishing one or more graphical representations among: a phase diagram, an impedance modulus diagram and a Nyquist diagram.

4. Method according to any one of the preceding claims, in which the number n of cycles of a cyclic electrochemical test sequence is between 2 and 10, preferably between 4 and 8, more preferably equal to 6.

5. A method according to any preceding claim, wherein the second cathodic polarization step is carried out

6. for a time between 10 min and 60 min, preferably between 15 min and 25 min. Method according to any one of the preceding claims, comprising a step of verifying the absence of defects in the surface prior to carrying out the sequences of n cyclic electrochemical test cycles.

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