JP5169357B2 - Pinhole evaluation method - Google Patents

Description

  The present invention relates to a pinhole evaluation method in which pinholes existing in a coating layer provided on a metal substrate are quantitatively determined.

  Conventionally, a laminated structure including a coating layer made of an inorganic material such as another metal or ceramics or an organic material such as a resin on a base material made of metal has been widely used for industrial products. For example, iron is used as a housing material for electronic equipment because it is excellent in rigidity, workability, economy, etc., but it is easy to corrode (easy to rust) in a normal environment, thus improving corrosion resistance. For this purpose, alloying or the above-mentioned coating layer is often applied. A typical iron alloy with improved corrosion resistance is stainless steel. Examples of the coating layer include those made of plating with a metal having high corrosion resistance such as gold, silver, chromium, and nickel. In addition, there is a thin film in which a ceramic such as TiN is formed by a PVD method or a CVD method (Non-patent Document 1, Patent Document 1).

  For example, if a defect such as a pinhole is present in a coating layer made of a metal that is more precious than the base metal, dissimilar metal contact corrosion in which the base metal is corroded at an accelerated rate may occur. Although ceramics and organic materials generally have better corrosion resistance than metals, the presence of pinholes in the coating layer made of these corrodes the underlying metal. Therefore, it is desirable that the number of pinholes in the coating layer be as small as possible.

  Conventionally, salt spray tests (for example, MIL-STD-202-101D) and ferroxyl tests (JIS H 8617) have been widely used for the evaluation of the pinhole. In these methods, after exposing the measurement object to a predetermined test solution such as salt water, the corrosion state is visually confirmed, and the number of pinholes is estimated based on the degree of corrosion. Alternatively, the pinhole may be actually visually observed using a microscope such as SEM (Scanning Electron Microscope).

On the other hand, Patent Document 1 and Non-Patent Document 1 disclose that the pinhole area ratio of a TiN film coated on stainless steel is obtained based on the critical passivation current density method (dynamic potential anodic polarization method). Yes. This method involves immersing the measurement object in an acid solution obtained by adding KSCN (potassium thiocyanate) to H ₂ SO ₄ (sulfuric acid), and measuring the current density when applied while changing the potential in this state. Then, the area of the pinhole is obtained using the passivating current (peak current) at the transition from the active band to the passive band.

JP-A-6-027075 "Current Status of Defect Evaluation of Corrosion Resistant Dry Coating Films", Katsuhisa Sugimoto, Materials and Environment Vol.44, No.5, pp.308-313 (1995)

  In order to perform quality control of industrial products with higher accuracy, it is desirable to quantitatively evaluate pinholes. For example, the degree of quality improvement can be quantitatively grasped by the amount of pinholes.

  Patent Document 1 and Non-Patent Document 1 disclose methods for quantitatively evaluating pinholes. However, when the present inventors investigated, when this technique is applied to a laminated structure composed of a metal whose base metal (base material) is hardly immobilized under acidic conditions such as iron, it cannot be evaluated with high accuracy, that is, It was found that the peak potential capable of detecting pinholes may not be obtained by the potentiodynamic anodic polarization method. In addition, these documents are directed to those in which the coating layer is made of ceramics, and disclose a specific method for quantifying pinholes in a laminated structure in which both the base and the coating layer are made of metal. Not.

  On the other hand, only qualitative evaluation can be performed in a salt spray test or the like. Further, since the salt spray test usually requires 48 hours for measurement, it is desired to develop a method capable of quantitative measurement in a short time.

  When a microscope is used, observation is difficult unless the magnification is very large because the pinhole is very small, and only local evaluation is possible at best. In order to grasp the state of the coating layer more accurately, it is desired that the measurement object can be evaluated over a wider range.

  Therefore, an object of the present invention is to provide a pinhole evaluation method capable of quantitatively measuring pinholes existing in a coating layer in a laminated structure having a coating layer on a metal substrate. It is in.

  As a result of various studies by the present inventors, the following findings were obtained. In a laminated structure of dissimilar metals, for example, when the difference in ionization tendency between these metals is small, in addition, when the base metal is difficult to generate passivation, or when the base metal is extremely susceptible to oxidation reaction in the electrolyte For example, when the potentiodynamic anodic polarization measurement is performed, it is difficult to obtain the peak current of the base metal, and it is difficult to obtain sufficient information for quantifying pinholes. Further, even if the peak current can be acquired by the measurement of the dynamic potential anodic polarization, the oxidation current flowing until the acquisition is large, the base metal is greatly damaged, and the covering layer may be damaged. In contrast, when constant potential anodic polarization measurement is applied, information for quantifying pinholes can be accurately measured. The present invention is based on these findings.

  According to the pinhole evaluation method of the present invention, a pinhole of the coating layer is formed by electrochemical measurement on a laminated structure including a metal substrate and a coating layer formed on the surface of the substrate. Assess quantitatively. Specifically, in this evaluation method, the laminated structure is immersed in an electrolytic solution, a change in current over time is measured with a constant potential applied to the laminated structure, and the obtained measurement result is Based on this, the amount of pinholes present in the coating layer is determined.

  According to the above configuration, even when the base material hardly generates a passive state or when the difference in ionization tendency between different metals constituting the laminated structure is small, the pinhole of the coating layer is quantitatively determined. And it can be evaluated with high accuracy. Further, the evaluation method of the present invention can easily evaluate pinholes over a wide range of the laminated structure to be measured by using an electrochemical technique, and has a short measurement time. Hereinafter, the configuration of the present invention will be described in more detail.

  The evaluation method of the present invention is to produce an electrochemical measurement cell using a laminated structure (measurement object) and an electrolytic solution in which a base material exposed from a pinhole of a coating layer of the structure can cause an oxidation reaction, The oxidation reaction rate or reaction amount of the substrate (metal) in the liquid is measured electrochemically, and this measured value is evaluated as the amount of pinholes. In particular, the evaluation method of the present invention utilizes a constant potential anodic polarization (amperometry) method for measuring a change in current over time when a constant potential is applied to the measurement object while the measurement object is immersed in an electrolyte solution. To do. When pinholes are present in the coating layer to be measured, when the base material (metal) is exposed from the pinholes and this exposed portion undergoes an oxidation reaction in the electrolyte, the reaction rate appears as a change in current. Therefore, the change in current is measured and this result is used.Specifically, the current value (mA) after a predetermined time has elapsed, or the integrated value of the current value in a predetermined time range (electricity (C) = reaction By using (quantity), pinholes can be quantified.

  The evaluation method of the present invention can be used for a laminated structure capable of evaluating pinholes by dynamic potential anodic polarization measurement, typically, a stainless steel having a ceramic coating layer. The present invention can be used for a laminated structure including a base material in which a peak current by dynamic potential anodic polarization measurement hardly appears. Examples of such a substrate include those made of a metal such as iron and an iron alloy (excluding stainless steel). In particular, iron is less likely to generate a passivity than stainless steel and reacts with an acid, so that evaluation by the method of the present invention is preferable. The coating layer may be made of an inorganic material containing a metal or an organic material such as a resin.

  The evaluation method of the present invention is particularly suitably used for a laminated structure in which both the base material and the coating layer are made of metal and the difference in ionization tendency of each metal constituting the base material and the coating layer is small. be able to. In such a laminated structure, for example, a Ni / Fe structure (from the surface side of the structure) in which the base material is composed of iron or an iron alloy and the coating layer is composed of one or more metals of nickel and a nickel alloy. Listed in order).

  The coating layer is made of a metal inorganic material having a composition different from that of the substrate (e.g., nickel, chromium, silver, gold, and an alloy of each element), an organic material such as a resin, or a non-metallic inorganic material such as ceramics or DLC (diamond-like carbon). In any case, the evaluation method of the present invention can be used. Under normal circumstances, chromium forms a dense passive film on the surface by an oxidation reaction and loses its conductivity. Therefore, when the substrate is made of iron, it is continuously oxidized at a lower potential than iron. However, the evaluation method of the present invention can be used. Further, the coating layer may be a single layer or a multilayer. That is, the laminated structure may be two layers or three or more layers. The method for forming the coating layer includes, when the coating layer is made of metal, plating methods such as electrolytic plating and electroless plating, as well as vapor deposition methods such as CVD and PVD methods. And PVD method, and in the case of resin, it may be applied. In general, the thinner the coating layer, the more likely to have defects such as pinholes. Therefore, quantifying pinholes by the evaluation method of the present invention is information for quality control (for example, an index for quality improvement). Information) is expected to contribute.

  It is preferable to use an electrolytic solution that causes the base material to undergo an oxidation reaction in a state where a certain potential is applied to the laminated structure. For example, when the substrate is made of iron or an iron alloy, an acid solution such as sulfuric acid or hydrochloric acid is preferable.

  The concentration of the electrolytic solution is not particularly limited. However, when the concentration is high, it becomes difficult to control the oxidation reaction by causing a spontaneous reaction without applying a voltage. It becomes difficult to do. Therefore, it is preferable to adjust the concentration of the electrolytic solution to an appropriate concentration corresponding to the measurement target. For example, when the base material to be measured is made of iron or an iron alloy and a sulfuric acid solution is used, about 0.1 to 5M is considered appropriate.

In the evaluation method of the present invention, when a measurement object is immersed in the electrolyte solution, a constant potential is applied to the measurement object, and a change in current with time is measured. The potential to be applied can be appropriately set such that the substrate exposed from the pinhole is oxidized and the constituent material of the coating layer is not oxidized. When the applied potential is too high, the base material is excessively oxidized, and the base material may be damaged to affect the measurement result. Therefore, it is preferable that the damage is assumed to be negligible. The smaller one (for example, 1 A / cm ² or less) within the measurable range based on the oxidation reaction of the substrate is considered preferable.

  The pinhole evaluation method of the present invention can quantitatively measure pinholes existing in the coating layer with respect to a laminated structure including a coating layer on a metal substrate.

  The Ni / Fe structure with nickel plating (coating layer) on a substrate made of iron is the object of measurement, and quantification of pinholes in the coating layer is performed by anodic polarization measurement using an acid solution as the electrolyte. Do. First, a basic procedure for anodic polarization measurement will be described.

  The measurement is performed by configuring a three-electrode electrochemical measurement cell 1 as shown in FIG. The cell 1 includes a container 10 into which an electrolyte solution BL is injected, a reference electrode (RE) 11 and a counter electrode (CE) 12 that are immersed in the electrolyte solution BL, and a measurement target (WE) 13, and both electrodes 11, 12, and One end of the measurement object 13 is connected to the potentiostat / galvanostat device 20. Here, Ag / AgCl is used for the reference electrode 11, Pt is used for the counter electrode 12, and a commercially available device 20 is used. The device 20 is set in a potentiostat mode, and a change in current is measured by applying a constant potential or sweeping the potential at a predetermined sweep rate. The apparatus 20 is connected to a control device (not shown) having input means, storage means, calculation means, comparison means, judgment means, display means, etc., and applies a constant potential, sweeps potential, and measures. Obtain results (polarization curve) automatically.

<Test Example 1 Measurement of potentiodynamic anodic polarization>
Using the cell shown in FIG. 1, potentiodynamic anodic polarization measurement was performed on iron and nickel.

In this test, an iron plate (Co. Nilaco Ltd., FE-223512, 99.5% purity), nickel plate (Co. Nilaco Ltd., NI-three hundred and thirteen thousand five hundred and eleven , purity 99%) was prepared, each plate to expose the 5 mm ² The other parts were masked with an epoxy resin and measured.

  When the device 20 in FIG. 1 is in potentiostat mode and each measurement object is immersed in an electrolyte solution (1M sulfuric acid solution), a potential sweep is started at a sweep rate of 10 mV / s, and a change in current is measured while sweeping. . FIG. 2 shows a graph in which the measurement results (kinetic potential anodic polarization curves) of each measurement object are superimposed. In FIG. 2, the horizontal axis represents the applied potential (V), and the vertical axis represents the current (mA) flowing during the measurement (the same applies to FIGS. 3 and 4 described later).

  As shown in Fig. 2, iron flows at a lower potential than nickel and is easily oxidized in an acid solution. Nickel is oxidized with little current flowing up to a certain potential (about 1.5 V). I find it difficult to do. In addition, when the potential is less than 1.5 V, only the iron oxidation current can be measured, and it can be seen that the iron oxidation current is hardly affected by nickel. However, the iron oxidation current increases with time, and it is difficult to observe a peak (a point at which the increase changes from a decrease). In other words, in the potentiodynamic anodic polarization measurement, information that can separate iron and nickel can be acquired, but it is difficult to accurately determine the pinholes of nickel plating using this information (iron oxidation current). Conceivable.

  Next, the concentration of the electrolytic solution (sulfuric acid solution) was changed to 0.1 to 5 M, and iron dynamic potential anodic polarization measurement was performed in the same manner as described above. The results are shown in FIG.

  As shown in FIG. 3, when the concentration of the electrolytic solution is 0.1 to 5 M, it can be seen that the current based on the oxidation reaction can be measured in the same potential region (−0.5 to 0.5 V). However, even when the concentration of the electrolytic solution is the same, there are cases where the peak can be observed and cases where the peak cannot be observed, and it is difficult to obtain a peak having a sharp shape. Even if such a peak current value is used, it is considered that pinholes cannot be appropriately quantified. On the other hand, when a high concentration electrolyte (for example, a sulfuric acid solution of about 5M) was used, a relatively sharp peak of iron oxidation current could be observed. However, since a large amount of current flows until the peak value is detected, when the measurement target is a laminated structure including a base material and a coating layer, the part other than the exposed part from the pinhole in the base material is also electrolyzed. It is considered that the base material is greatly damaged by reacting in the liquid. Further, this damage may cause the coating layer to peel off or fall off. Not only such damage, but also the measurement results are not accurate, so the error increases and pinholes cannot be properly quantified. Also from these points, it is considered that the potentiodynamic anodic polarization measurement is not preferable in the determination of pinholes in the coating layer in a laminated structure of a metal having a small difference in ionization tendency such as iron and nickel.

  Next, the oxidation current was measured in the same manner as above for the iron plate having the same model number as the iron plate prepared in Test Example 1 (electrolytic solution: 1M sulfuric acid solution). The results are shown in FIG.

  As shown in FIG. 4, if the materials to be measured are the same, it can be seen that the current value in the potential region (−0.1 V or less) where the oxidation current starts flowing is reproducible. However, reproducibility is not recognized in the peak current value of the oxidation current. From this result, it is considered that high-accuracy pinhole evaluation is difficult for the potentiodynamic anodic polarization measurement in determining the pinholes in the coating layer in the laminated structure based on iron.

<Test Example 2 Constant Potential Anode Polarization Measurement>
Using the cell shown in FIG. 1, iron constant potential anodic polarization measurement was performed.

  In this test, a plurality of pieces having the same model number as the iron plate prepared in Test Example 1 were prepared, a plurality of masks with different exposure areas were prepared for each plate, and these were measured.

  When the apparatus 20 of FIG. 1 is set in a potentiostat mode and each measurement object is immersed in an electrolytic solution, a constant potential is applied for a predetermined time, and a change in current over time is measured. Here, the upper limit of the oxidation current is set to 10 mA from the graphs shown in FIGS. 2 to 4 so that the substrate does not react significantly and the reproducibility of the oxidation current value is taken into account. A 1 M sulfuric acid solution was used as the liquid. In addition, as a potential that is not easily affected by the oxidation current of nickel, which is a constituent material of the coating layer, and that the oxidation current of iron can be measured appropriately (here, a potential with high reproducibility of the oxidation current value), FIG. -0.3V was selected from the graph shown in ~ 4. Thus, the dynamic potential anodic polarization measurement performed in Test Example 1 can be used as a preliminary test for setting a constant potential. Further, since the damage to the measurement object increases as the potential application time becomes longer, the potential application time is set to 20 seconds here. The measurement results (constant potential anodic polarization curve) are shown in FIG. In the graph of FIG. 5, the horizontal axis represents the elapsed time (sec) from the start of potential application, and the vertical axis represents the current (mA) flowing during measurement.

  Although a slight current appears after application (after 20 seconds), this current is considered to be based on a reaction other than the oxidation reaction of iron because the voltage application is canceled at the time of 20 seconds. Also, in this test, the current increases smoothly as time passes immediately after application, but a curve reflecting the difference in surface properties such as the presence of contaminants (oxides, etc.) on the substrate surface is drawn. There is. For example, a peak current may appear immediately after application. However, the appearance of such a peak is temporary, and after a few seconds, a smooth curve as shown in FIG. 5 is stably drawn. Furthermore, even if such a contaminant is present in a small amount, it is considered that it does not affect the measured value because it is completely electrolyzed and removed in a short time as described above. When it seems that there are many pollutants, it is preferable to perform measurement after performing surface treatment such as removal of pollutants on the measurement object, so that more accurate quantification can be performed.

  In the constant potential anodic polarization measurement, an oxidation current corresponding to the set potential is observed. Table 1 shows the current values for each hour. Further, a relationship line (calibration curve) between the exposed area (x) and the oxidation current (y) was obtained using the current value for each time. The results are also shown in Table 1. Further, a calibration curve at the time of 10 sec is shown in FIG.

  As shown in FIG. 5, it can be seen that, at the same time, the larger the exposed area, the larger the oxidation current, and there is a correlation between the exposed area and the oxidation current. In addition, as shown in Table 1 and FIG. 6, a good calibration curve is obtained with respect to the exposed area, and as shown in FIG. 6, the exposed area and the oxidation current are generally in a proportional relationship. I understand.

  From this test, it can be said that the constant potential anodic polarization measurement can be used for pinhole quantification of the coating layer for a laminated structure of dissimilar metals having a small difference in ionization tendency such as Ni / Fe structure. In particular, it can be said that the evaluation method of this example in which the constant potential anodic polarization measurement is performed in a potential region where the current value is relatively small is superior in reproducibility compared to the conventional dynamic potential anodic polarization measurement using the peak current value. Moreover, even if the concentration of the electrolytic solution is changed in the range of 0.1 to 5M, it can be said that pinholes can be appropriately quantified by performing constant potential anodic polarization measurement in a potential region where the current value is relatively small.

  For example, a sample made of the same metal as the constituent metal of the base material, and a plurality of samples with different exposed areas as described in Test Example 2 are used as measurement targets for verification, and an electrolyte solution corresponding to each target is used. The cell shown in FIG. 1 is constructed, current is measured over time with a predetermined potential applied, and a calibration curve between the exposed area and the current value is created. Then, using the laminated structure as a measurement object, constant potential anodic polarization measurement was performed under the same conditions as when the calibration curve was obtained, and the current value at a desired time was collated with the calibration curve, corresponding to the current value. The pinhole can be quantified by evaluating the exposed area as the area of the pinhole. In preparing a calibration curve, pinholes can be quantified with higher accuracy as the number of verification measurement objects increases.

<Test Example 3 Measurement of actual sample>
Using the cell shown in FIG. 1, the pinhole ratio existing in the coating layer was examined for a Ni / Fe structure in which nickel plating (coating layer) was applied on a base material made of iron.

In this test, the same iron plate prepared in Test Example 1 above is prepared and used as a base material, and the surface of the base material is subjected to nickel plating to produce a plurality of samples No. 11 to No. 16 having different plating thicknesses. did. Table 2 shows the substrate area (mm ² ) and plating time (min) of each sample. Nickel plating was performed by electrolytic plating using a general Watt bath (matte). The plating thickness was changed by changing the plating time. The longer the plating time, the thicker the plating thickness.

1 is set in potentiostat mode, and each measurement object is immersed in the electrolyte, then a constant potential (-0.3V) is applied (potential application time: 20 sec) to measure the change in current over time. . The measurement results (constant potential anodic polarization curve) are shown in FIG. In the graph of FIG. 7, the horizontal axis represents the elapsed time (sec) from the start of potential application, and the vertical axis represents the current per area (mA / cm ² ) that flowed during the measurement. In FIG. 7, the graphs of sample Nos. 15 and 16 overlap the horizontal axis (0 mA / cm ² ). Since the potential was set to -0.3 V, the nickel oxidation current was hardly measured as described in Test Example 2-1.

  The pinhole ratio was obtained using the obtained measurement result and the calibration curve at the time of 10 sec shown in FIG. Specifically, in the graph shown in FIG. 7, the current value at the time of 10 seconds from the start of potential application is read, and this current value is collated with the calibration curve shown in FIG. The corresponding exposed area was read (determined from the relational expression), this exposed area was evaluated as the pinhole area, and (pinhole area / sample area) × 100 was defined as the pinhole ratio (%). The results are shown in FIG. Table 3 shows the current value and pinhole area of each sample at 10 sec. Sample Nos. 15 and 16 having negative current values were evaluated as having no pinholes (pinhole ratio 0%).

  As shown in Table 3 and FIG. 8, it can be seen that the pinhole ratio is reduced as the plating time increases, that is, as the plating thickness increases. That is, it can be said that the improvement of the plating quality can be quantitatively evaluated by performing the constant potential anodic polarization measurement.

  By performing constant potential anodic polarization measurement in this way, for industrial products such as structures in which different types of metals with a small difference in ionization tendency are laminated, a layer (coating layer) made of metal disposed on the surface side thereof Pinholes present in the can be measured quantitatively. And by using this result information for quality control etc., it is expected that it can contribute to the improvement of commercial value.

  In the above method, the pinhole ratio can be quantified up to about 0.01%. In addition, the above method requires about several tens of seconds for pinhole measurement, and can easily perform measurement over a wide range. Furthermore, as a control device to be connected to the potentiostat / galvanostat device, the storage means for storing the calibration curve, the calibration curve called from the storage means and the obtained measurement result (current value at a predetermined time) are collated. Then, using collation means for obtaining the area of the pinhole, the area of the obtained pinhole, and an arithmetic means for calculating the pinhole ratio from the entire area of the measurement object inputted in advance, The pinhole rate is automatically calculated. A separately obtained calibration curve is input to the storage means.

  The present invention is not limited to the above-described embodiment, and can be appropriately changed without departing from the gist of the present invention. For example, in the above embodiment, the current is used as the measurement value, but the amount of electricity (current × elapsed time) can be used instead of the current value. Moreover, although the said embodiment demonstrated the example from which a coating layer was comprised from one layer, a multilayered structure may be sufficient. Furthermore, in the above embodiment, the laminated structure of different metals has been described as an example. However, the evaluation method of the present invention can be used even when the coating layer is made of an organic material such as a resin, a ceramic such as TiN, or a non-metallic inorganic material such as DLC. can do.

  The pinhole evaluation method of the present invention can be suitably used for quantitative determination of pinholes present in a coating layer in a laminated structure including a metal substrate and a coating layer formed on the surface thereof. Laminated structures include, for example, Ni / Fe structures used in various electronic equipment casings, Cr / Fe structures used for industrial bolts and nuts, decorative Au / Fe structures and Ag / Fe structure, ceramic film such as TiN / iron or iron alloy structure.

It is a schematic block diagram of a three-electrode type electrochemical measurement cell. It is a potentiodynamic anodic polarization curve of iron (Fe) and nickel (Ni). 6 is a potentiodynamic anodic polarization curve of iron (Fe) when a plurality of electrolytic solutions having different concentrations are used. It is a potentiodynamic anodic polarization curve of the same measurement object (iron). It is a constant potential anodic polarization curve of a measurement object (iron) with different exposed areas. It is a calibration curve showing the relationship between the exposed area and the current at 10 seconds. It is a constant potential anodic polarization curve of a Ni / Fe structure. It is a graph which shows the relationship between plating time and a pinhole rate.

Explanation of symbols

1 cell 10 container 11 reference electrode 12 counter electrode 13 object to be measured
20 Potentiostat / galvanostat BL electrolyte

Claims (4)

Iron or iron alloy (excluding stainless steel) and substrate that consists of, immersed in the electrolyte solution a laminated structure comprising a coating layer formed on the substrate surface,
Measure the change in current over time associated with the reaction of the base material in a state where a constant potential within a specific potential region is applied to the laminated structure,
Based on the current value measured for a predetermined time after the elapse of 4 seconds from the start of potential application , to determine the amount of pinholes present in the coating layer ,
The specific potential region is obtained by immersing a metal material of the same material as the base material in an electrolyte solution similar to the electrolyte solution, sweeping a potential applied to the metal material, measuring a change in current, and starting a current flow evaluation of pinholes and said range and to Rukoto potential a reproducible current value from the potential.   The pinhole evaluation method according to claim 1, wherein the specific potential region is −0.1 V or less. Before Symbol coating layer is composed of nickel, chromium, silver, and of one or more metals of gold,
Wherein as the electrolyte, the evaluation method of the pinholes according to claim 1 or 2, characterized in that an acid solution. Before Symbol coating layer is composed of an organic material, or non-metallic inorganic material,
Wherein as the electrolyte, the evaluation method of the pinholes according to claim 1 or 2, characterized in that an acid solution.

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