Method and device for quantitatively detecting salt deposits on a metal surface by means of laser-induced plasma spectroscopy

Abstract

The invention relates to a device for quantitatively detecting salt deposits on a metal surface by means of laser-induced plasma spectroscopy. The invention also relates to a method for uniformly applying salts to a metal surface (13) or for producing a reference metal surface. The invention further relates to the use of laser-induced plasma spectroscopy for quantitatively detecting salt deposits on a metal surface and the use of a corresponding device for carrying out laser-induced plasma spectroscopy for quantitatively detecting salt deposits on a metal surface.

Description

[0001] Method and device for the quantitative detection of salt deposits on a metal surface using laser-induced plasma spectroscopy

[0002] The present invention relates to a method and a device for the quantitative detection of salt deposits on a metal surface using laser-induced plasma spectroscopy. The invention further relates to a method for uniformly applying salts to a metal surface or for producing a reference metal surface. The invention also relates to the use of laser-induced plasma spectroscopy for the quantitative detection of salt deposits on a metal surface, as well as to the use of a device according to the invention for carrying out a method according to the invention.

[0003] The invention is defined in the appended claims. Preferred aspects of the present invention will become apparent from the following description, including the examples.

[0004] To the extent that certain embodiments are designated as preferred for one aspect of the invention, the corresponding statements also apply to the other aspects of the present invention, mutatis mutandis. Preferred individual features of aspects of the invention (as defined in the claims and / or disclosed in the description) can be combined with one another and are preferably combined with one another, unless otherwise apparent to the person skilled in the art from the present text.

[0005] Metal surfaces, especially steel surfaces, must be free of salts, among other contaminants, before coating, as otherwise blistering may occur in the coating to be applied, leading to premature failure of the coating's protective function. Consequently, in practice, before coating a metal surface, it is usually necessary to demonstrate that the limit value of acceptable salt deposits on the metal surface to be coated (often a maximum of 20 mg / m) assigned to the respective coating material to be used is met. 2 NaCI equivalent).

[0006] In practice, the quantitative content of salt deposits on a metal surface to be coated is determined randomly using the so-called Bresle test. In this test, the (water-soluble) salts present on a defined area of ​​the metal surface to be tested are first dissolved by adhering a chamber patch to the metal surface to be tested in accordance with the standard DIN EN ISO 8502-6:2020-08. A known amount of distilled water is injected into the patch using a syringe. After waiting for a defined extraction time, the water is withdrawn from the chamber patch. The content of extracted salt in the water sample is then determined by conductivity measurement in accordance with the standard DIN EN ISO 8502-9:2020-12 and is usually expressed in NaCl equivalents.

[0007] The application of the Bresle test to determine the salt content on metal surfaces has several disadvantages. Due to the procedure outlined above, the Bresle test is comparatively time-consuming and cannot be automated. Furthermore, the Bresle test contaminates the surface to be tested with adhesive residues due to the necessary application of a chamber patch, which requires repeated local surface cleaning after the Bresle test before applying a coating to the surface. Furthermore, the Bresle test does not allow for the differentiation of individual salt components on the metal surface to be tested. In addition, the Bresle test requires comparatively large surface areas (approximately 3-5 cm) per "measurement point." 2), which is usually at the expense of the resolution of the overall measurement result when several individual measurements are carried out (in other words, the Bresle test cannot detect point-like salt deposits, but rather any salt deposits can only be limited to a specific larger surface area).

[0008] Against this background, it was a primary object of the present invention to provide a simple and efficient method for the quantitative detection of salt deposits on a metal surface, which overcomes the above-mentioned disadvantages of the methodology known from the prior art.

[0009] A further object of the present invention was to provide a device for the simple and rapid quantitative detection of salt deposits on a metal surface, wherein the device should be particularly suitable for carrying out the method to be provided for the quantitative detection of salt deposits on a metal surface.

[0010] Further objects arise from the following description and the patent claims.

[0011] The primary object of the present invention is achieved by a method for the quantitative detection of salt deposits (or salt-forming chemical elements) on a metal surface by means of laser-induced plasma spectroscopy (LIPS or LIBS for short), comprising the following steps:

[0012] I) Focusing a laser beam onto a point on the metal surface to be examined, so that a local plasma is created;

[0013] II) spectroscopic analysis of the radiation emitted by the local plasma during cooling, so that a spectrum for the radiation emitted by the local plasma is obtained;

[0014] III) identifying (at least) one characteristic spectral line for (at least) one chemical element contained in the salt deposit from the spectrum obtained in step II) and determining the area under the spectral line;

[0015] IV) Quantifying the content of salt deposits (or salt-forming chemical elements) on the metal surface to be examined by comparing the area determined in step III) with the area obtained from calibration measurements, whereby the calibration measurements are carried out on reference metal surfaces each with known contents of salt deposits.

[0016] Laser-induced plasma spectroscopy can be used to analyze the elemental composition of a surface. A high-energy laser is focused onto the surface under investigation, creating a local plasma that emits radiation upon cooling. The emitted radiation can be analyzed spectroscopically, and the wavelengths of the spectral lines detectable in the resulting spectrum provide information about the element types present on the surface (since the spectral lines of each chemical element occur at known element-specific wavelengths). The intensity of a spectral line, expressed as the area under the respective spectral line, also provides an indication of the quantity of a chemical element at the area under investigation.

[0017] However, absolute concentrations (e.g., atomic % or ppm) cannot be directly determined from the areas under the spectral lines of a spectrum obtained by laser-induced plasma spectroscopy. However, it has been shown that quantification of salt deposits or salt-forming chemical elements on a metal surface using laser-induced plasma spectroscopy is possible by comparing or comparing the determined areas of one or more characteristic spectral lines of the metal surface under investigation with reference areas of spectral lines of the same species determined on reference metal surfaces with known salt deposit contents.

[0018] In other words, for the quantitative detection (i.e., for the determination of absolute concentration values) of salt deposits (or salt-forming chemical elements) on a metal surface by means of laser-induced plasma spectroscopy, calibration measurements on reference metal surfaces with known contents of salt deposits are required, provided that there is a linear correlation between the salt content or the content of salt-forming chemical elements on a metal surface and the intensity of spectral lines detectable in the LIBS spectrum.

[0019] It is precisely this aspect that has so far prevented experts from considering the methodology of laser-induced plasma spectroscopy for the quantitative detection of salt deposits or salt-forming chemical elements on a metal surface, since a linear correlation between the intensity of spectral lines of salt-forming chemical elements and the salt content on the metal surface could not be readily expected and thus a successful application of laser-induced plasma spectroscopy for the quantitative detection of such salt contents on metal surfaces could not be predicted.

[0020] Another hurdle to date has been the provision of reference metal surfaces with defined levels of salt deposits (which are required for the calibration measurements to be performed), since such reference metal surfaces with homogeneous and known concentrations of salt contamination are not readily producible. For example, based on current knowledge, when producing such salt-contaminated reference metal surfaces, it has always been assumed that the deliberate salt contamination can simultaneously cause corrosion of the metal surface, which can negatively influence or falsify the results of the calibration measurement.

[0021] However, the inventors discovered in their investigations that there is indeed a linear correlation between the content of salt deposits or the content of salt-forming chemical elements on metal surfaces and the intensity (area area) of spectral lines of salt-forming chemical elements identifiable in LIBS spectra. Furthermore, by spraying (fogging) metal surfaces with a salt-containing aerosol (fog) under predefined conditions, the inventors succeeded in a surprisingly simple and efficient way in creating reference metal surfaces with homogeneous and known amounts of salt deposits, which are suitable for the calibration measurements of the method according to the invention.

[0022] Through the considerations and efforts made, the inventors have succeeded in providing, with the method according to the invention, a possibility for the quantitative detection of salt deposits or salt-forming chemical elements on metal surfaces, which, in comparison to the Bresle test known from the prior art for the present purposes, is faster and easier to carry out and less complex (the determination can be controlled purely instrumentally if necessary and is completely unproblematic and can be repeated at any number of points on the metal surface to be examined without great expenditure of time), leaves no or less surface contamination on the metal surface to be examined (with the method according to the invention, in comparison to the Bresle test, no extensive moistening of the metal surface to be examined is required;also, with the method according to the invention there is no risk of adhesive residues remaining after the measurement has been carried out), not only allows a statement to be made about the total salt content on the metal surface to be examined, but at the same time enables a differentiation of the salt-forming chemical elements present on the metal surface to be examined, also allows the detection and quantitative determination of small concentrations of salt deposits (the accuracies for the determination of salt contents obtained with the method according to the invention can be achieved with the accuracies known from the Bresle test and can even be exceeded), allows a very precise analysis of the metal surface to be examined (which means that when multiple measurements are carried out on the metal surface to be examined, a better resolution for the overall measurement result can be obtained compared to the Bresle test).

[0023] The term “metal surface” in the sense of the present invention also includes surfaces which have a thin metal oxide layer (e.g. caused by oxidation / corrosion).

[0024] Preferably, in step I) of the method according to the invention, a pulsed laser beam is focused onto a location on the metal surface to be examined. The use of a pulsed laser beam has the advantage that the cooling of the resulting plasma can be better or faster, which in turn is beneficial for the implementation of the subsequent step II) of the method according to the invention, in which the

[0025] Cooling radiation emitted by the local plasma is analyzed spectroscopically.

[0026] To determine the total salt content on a metal surface, it may be sufficient to determine the area under just a single characteristic spectral line in step III) of the method according to the invention and to derive the total content of salt deposits from this in step IV). This is particularly the case if the type or composition of the expected salt deposit is known. If the expected salt deposit is sea salt, for example, the sodium content on the surface can be determined using a spectral line characteristic of sodium or salt.Sodium ions can be used to determine the total content of sea salt deposits on the metal surface, since the ratio of the main ions in seawater is approximately always the same and thus the quantification of a single salt-forming chemical element simultaneously allows conclusions to be drawn about the content of the other salt-forming chemical elements in the sea salt.

[0027] In other cases, however, it may be useful to determine the quantitative content of salt deposits on a metal surface under investigation based on the surface areas of several characteristic spectral lines, especially when the type of salt deposits to be expected is not sufficiently known. The subject matter of the present invention therefore expressly also includes methods in which the quantitative detection of salt deposits or salt-forming chemical elements on a metal surface is carried out based on the determination of the surface area under more than one characteristic spectral line.

[0028] For the purposes of the present invention, characteristic spectral lines for chemical elements contained in the salt deposit are generally understood to mean the spectral lines of all salt-forming elements that can be detected on the respective metal surface to be examined.

[0029] To determine the content of a salt-forming element on the metal surface to be examined, the most intense spectral line of the respective element in the spectrum is preferably used. However, within the scope of the invention, any other spectral line of a salt-forming element can also be used for quantitative detection, as long as the intensity of the respective spectral line is sufficiently high so that the area beneath it can be determined.

[0030] The use of laser-induced plasma spectroscopy offers the advantage that with each measured spectrum, the spectral lines of all elements present on the metal surface can be simultaneously shown or identified, and thus the recording of a single spectrum is sufficient to carry out the quantitative detection of salt deposits using several characteristic spectral lines.

[0031] As already explained above, the quantification of the salt deposit content on the metal surface to be examined is carried out within the framework of the method according to the invention by comparing the area(s) determined in step III) with the area(s) of the characteristic spectral lines selected for the examination obtained from calibration measurements. The calibration measurements are each carried out on reference metal surfaces with known salt deposit contents. The (calibration) area(s) obtained by calibration measurements are also determined from spectra obtained by laser-induced plasma spectroscopy. Preferably, a measuring device of the same type and / or the same measurement conditions for laser-induced plasma spectroscopy are used for the analysis of the metal surface to be examined and for carrying out the calibration measurements.

[0032] The quantitative content of salt deposits on the reference metal surfaces is additionally verified or determined using a reference method. The quantitative determination of the content of salt deposits on the reference metal surfaces is preferably carried out by applying the Bresle test and / or by differential weighing of the metal substrate used for the reference before and after exposure to a salt of known composition.

[0033] The determination of the surface area under one or more characteristic spectral lines (of a spectrum obtained by means of laser-induced plasma spectroscopy) in combination with the simultaneous determination of the salt content on the same reference metal surface in each case makes it possible to assign a specific content of salt deposits or a specific content of the respective salt-forming chemical elements present to the determined surface area under the respective characteristic spectral line. By comparing the surface area of ​​the spectral lines determined on the reference metal surfaces with the surface areas of the characteristic spectral lines (of the same species) on the metal surface to be examined, the existing linear correlation between the size of the surface area under a characteristic spectral line and the content of salt deposits (orthe content of the respective salt-forming chemical element under consideration) the quantitative amount of salt deposits (or the amount of the respective salt-forming chemical element) on the metal surface to be examined.

[0034] Calibration measurements for the method according to the invention are carried out on at least one reference metal surface with a known salt deposit content. Calibration measurements for the method according to the invention are preferably carried out on two or more reference metal surfaces, each with a different known salt deposit content. From the measurement points thus obtained (for each characteristic spectral line identified or used for quantitative detection), a calibration line can then be created by interpolation, which can be used for comparison with the measurement results from the metal surface under investigation.

[0035] Preferably, the calibration measurements for the method according to the invention are carried out on reference metal surfaces each having a similar or identical chemical composition to the metal surface to be examined.

[0036] The term "similar chemical composition" refers to the fact that the main chemical element of the reference metal surface and the metal surface being examined is identical, although the two metal surfaces may differ in any additional chemical element content present in the metal surface. For the examination of steel surfaces, for example, the calibration measurements should also be performed on steel surfaces as reference metal surfaces. However, it is fundamentally irrelevant for the calibration measurements which exact alloying additives the steel surfaces used for the calibration measurements contain.

[0037] Furthermore, separate calibration is not required for each steel type. Rather, for example, it is sufficient to perform one-time calibration measurements on a specific type of (unalloyed) structural steel for the analysis of (unalloyed) structural steel. These calibration measurements can then also be used for the quantitative detection of salt deposits on the surfaces of various other types of (unalloyed) structural steel.

[0038] A method according to the invention for the quantitative detection of salt deposits (or salt-forming chemical elements) on a metal surface by means of laser-induced plasma spectroscopy is preferred, comprising as additional steps:

[0039] 111.1) Identifying a spectral line characteristic of the metal surface to be examined from the spectrum obtained in step II) and determining the area under the spectral line;

[0040] III.2) Forming an area ratio from the area contents determined in steps III) and III.1); wherein in step IV) the content of salt deposits (or salt-forming chemical elements) on the metal surface to be examined is quantified by comparing the area ratio obtained in step III.2) with the area ratios obtained from the one or more calibration measurements.

[0041] The use of an area ratio (formed from the area under a characteristic spectral line for a chemical element contained in the salt deposit and the area under a spectral line characteristic of the metal surface to be examined) improves the accuracy and reproducibility of the quantification of the content of salt deposits or salt-forming chemical elements on the metal surface to be examined.

[0042] When using an area ratio for quantification, the comparison of the resulting area ratios in step IV) is naturally carried out with (calibration) area ratios, which are each calculated from the area contents of spectral lines of the same species. Accordingly, especially when using an area ratio for quantification, it is advisable to perform the calibration measurements on reference metal surfaces with a similar or identical chemical composition to the metal surface to be examined.

[0043] For the purposes of the present invention, characteristic spectral lines for the metal surface to be examined are generally understood to mean the spectral lines of all elements from which the metal surface is formed.

[0044] Preferably, a spectral line of the main chemical element of the metal surface is used as the characteristic spectral line for the metal surface under investigation. For example, an iron spectral line is chosen when examining steel surfaces, and an aluminum spectral line is chosen when examining an aluminum surface.

[0045] Furthermore, a high-intensity, particularly the most intense, spectral line of an element of the metal surface is preferably selected as the characteristic spectral line for the metal surface to be examined. However, lower-intensity spectral lines of elements of the metal surface to be examined are also suitable, as long as the intensity of the respective spectral line is sufficiently high so that the area beneath it can be determined. When selecting the characteristic spectral line for the metal surface to be examined, care is usually taken to avoid selecting a spectral line of an element that could also be part of a salt deposit to be detected.

[0046] A method according to the invention for the quantitative detection of salt deposits (or salt-forming chemical elements) on a metal surface by means of laser-induced plasma spectroscopy is preferred, wherein steps I) to IV) are repeated at one or more further locations on the metal surface to be examined.

[0047] In other words, a method according to the invention is preferred in which the content of salt deposits or salt-forming chemical elements is determined at several locations on the metal surface to be examined. By measuring or checking the salt content at several locations on the metal surface to be examined, a more meaningful statement is obtained regarding the presence of any salt contamination on the metal surface to be examined.

[0048] The measured values ​​obtained at multiple locations can be averaged (to obtain a statistically reliable result) or accumulated for further evaluation. Furthermore, by measuring at multiple locations, a scan or mapping of the surface under investigation can be realized.

[0049] Furthermore, multiple measurements are preferably performed at one and the same location on the metal surface to be examined. Such multiple measurements at one location can be advantageous for checking whether and to what extent the individual measurements are subject to possible fluctuations.

[0050] A method according to the invention for the quantitative detection of salt deposits (or salt-forming chemical elements) on a metal surface using laser-induced plasma spectroscopy is preferred, wherein steps III) to IV) are repeated for one or more additional spectral lines characteristic of the salt deposits. Regarding the advantages of considering several spectral lines characteristic of the salt deposits, reference is made to the above explanations.

[0051] Also preferred is a method according to the invention for the quantitative detection of salt deposits (or salt-forming chemical elements) on a metal surface using laser-induced plasma spectroscopy, wherein the metal surface to be examined is a steel surface, a galvanized steel surface, or an aluminum surface, preferably a steel surface of a steel for steel construction or a galvanized steel surface. Preferably, the aluminum surface to be examined is a thermally sprayed aluminum surface.

[0052] Particularly preferably, the metal surface to be examined is a steel surface of a steel for steel construction, as specified in Table 1 of the standard DIN EN 10027-1:2017-01.

[0053] A method according to the invention for the quantitative detection of salt deposits (or salt-forming chemical elements) on a metal surface by means of laser-induced plasma spectroscopy is preferred, wherein the spectral line identified in step III) and characteristic of the salt deposits is a spectral line of an element selected from the group consisting of sodium, calcium, magnesium, potassium, chlorine, sulfur, nitrogen, phosphorus, carbon, bromine, iodine and oxygen, preferably a spectral line of an element selected from the group consisting of sodium, calcium, potassium and magnesium, wherein the spectral line identified in step III) and characteristic of the salt deposits is preferably selected from the group consisting of sodium spectral lines with a wavelength in the range of 300 to 600 nm, calcium spectral lines with a wavelength in the range of 200 to 600 nm,Potassium spectral lines with a wavelength in the range of 400 to 800 nm and magnesium spectral lines with a wavelength in the range of 200 to 550 nm, wherein the spectral line identified in step III) which is characteristic of the salt deposits is particularly preferably selected from the group consisting of a sodium spectral line at a wavelength of 589.0 nm, a sodium spectral line at a wavelength of 589.6 nm, a calcium spectral line at a wavelength of 393 nm, a calcium spectral line at a wavelength of 396.8 nm, a potassium spectral line at a wavelength of 766.5 nm, a potassium spectral line at a wavelength of 769.9 nm, a magnesium spectral line at a wavelength of 279.6 nm and a magnesium spectral line at a wavelength of 280.3 nm, wherein the spectral line identified in step III) which is characteristic of the salt deposits characteristic,Spectral line is most preferably selected from the group consisting of a sodium spectral line at a wavelength of 589.0 nm and a sodium spectral line at a wavelength of 589.6 nm.,

[0054] As already stated above, the use of high-intensity spectral lines is preferred. However, the method can also be carried out using less intense spectral lines, such as sodium spectral lines at a wavelength of 330 nm.

[0055] It is also possible to quantify carbonaceous salt deposits on a steel surface using the method according to the invention, since a clean steel surface (i.e., one without salt deposits) exhibits less intense carbon spectral lines in the LIBS spectrum compared to a steel surface containing carbonaceous salt deposits. Using, for example, calibration measurements on reference metal surfaces or reference steel surfaces with the same carbon content as the steel surface under investigation, the intensity of the carbon spectral lines originating from the steel surface can then be calculated.

[0056] A method according to the invention for the quantitative detection of salt deposits (or salt-forming chemical elements) on a metal surface by means of laser-induced plasma spectroscopy is preferred, wherein the salt deposits on the metal surface to be examined are salt deposits caused by seawater, road salt, industrial exhaust gases and / or emissions from the agricultural sector.

[0057] Also preferred is a method according to the invention for the quantitative detection of salt deposits (or of salt-forming chemical elements) on a metal surface by means of laser-induced plasma spectroscopy, wherein the salt deposits on the metal surface to be examined or a part of the salt deposits on the metal surface to be examined are salt deposits caused by blasting media used in a blasting process (e.g. for cleaning the metal surface to be examined before applying a coating).

[0058] A preferred method according to the invention is for the quantitative detection of salt deposits (or salt-forming chemical elements) on a metal surface using laser-induced plasma spectroscopy, wherein the quantitative detection of salt deposits is carried out before coating the metal surface to be examined. As already mentioned above, by checking the salt content on a metal surface to be coated before the coating process, subsequent blistering (caused or promoted by salt deposits) in the coating to be applied can be reduced or completely prevented.

[0059] A method according to the invention for the quantitative detection of salt deposits (or salt-forming chemical elements) on a metal surface by means of laser-induced plasma spectroscopy is preferred, comprising as an additional step:

[0060] Va) Repeating steps I) to IV) in the immediate vicinity of those previously analysed areas of the metal surface to be examined where a previously defined limit value for the content of salt deposits was exceeded, and / or

[0061] Vb) Cleaning those areas of the metal surface to be examined where a previously defined limit for the content of salt deposits is exceeded, whereby the previously defined limit for the content of salt deposits is preferably 20 mg / m 2 of sodium chloride equivalents.

[0062] If the measurement result obtained by means of the method according to the invention shows that a limit value of still acceptable salt deposits on the metal surface to be examined is exceeded, it is advisable to limit the extent of the salt contamination by further measurements and then to (re)clean the salt-contaminated area before coating the metal surface.

[0063] Preferably, one or more further measurements should be carried out after the (re-)cleaning to check the result of the cleaning.

[0064] A method according to the invention for the quantitative detection of salt deposits (or salt-forming chemical elements) on a metal surface by means of laser-induced plasma spectroscopy is preferred, wherein the one or more reference metal surfaces are each

[0065] - exposure of a cleaned metal surface to a salt-containing aerosol, preferably to a salt-containing mist, over a defined period of time - and optionally a drying step taking place after exposure to the salt-containing aerosol, wherein the salt-containing aerosol comprises one or more salts (in solid or dissolved form) and the chemical composition of the one or more salts comprised by the salt-containing aerosol is known in each case and, in the case of several salts, the quantitative ratio of the salts to one another is known, and the content of salt deposits on the one or more reference metal surfaces is referenced in each case using at least one measuring method other than laser-induced plasma spectroscopy, preferably using differential weighing before and after exposure to the salt-containing aerosol and / or using the Bresle method according to DIN EN ISO 8502-6:2020-08 and DIN EN ISO 8502-9:2020-12.

[0066] Exposure to a salt-containing aerosol has proven particularly advantageous for obtaining reference metal surfaces with defined and homogeneous (uniform) contents of salt deposits.

[0067] By changing, for example, the exposure time and the concentration of salt in the aerosol, the final concentration of salt deposits on the reference metal surfaces can be adjusted.

[0068] The application of a saline aerosol has the advantage, among other things, that no liquid (when using suspended dust, i.e. a mixture of finely distributed particles in a gas, as an aerosol) or only a small amount of liquid (when using a saline mist, i.e. finely distributed liquid droplets of a saline solution in a gas, as an aerosol) comes into contact with the metal surface to be treated, thus eliminating or greatly minimizing the risk of corrosion of the reference metal surface.

[0069] In principle, drying at room temperature (20 °C) and atmospheric pressure is sufficient when applying a saline mist. However, when using a saline mist, the drying step is preferably carried out with the addition of heat (e.g., using a hot plate or inductive heating) and / or by reducing the ambient pressure (e.g., by drying in a desiccator). If a drying step takes place after application, and the applied salt content is referenced by differential weighing, the second weighing takes place only after the end of the drying step.

[0070] In the case of referencing the applied salinity using the Bresle method (the Bresle test), the reference metal surface must be at least large enough to allow for at least one chamber patch (Bresle patch) for the Bresle test to be applied, in addition to the required space (i.e. the required measuring point) for the laser-induced plasma spectroscopy to be carried out.

[0071] A method according to the invention for the quantitative detection of salt deposits (or of salt-forming chemical elements) on a metal surface by means of laser-induced plasma spectroscopy is preferred, wherein in step II) of the spectroscopic analysis an exposure time of a detector (gate width) is selected in the range of 1 ps to 100 ps, ​​preferably in the range of 1 ps to 10 ps, ​​particularly preferably 10 ps, ​​and / or a distance between a laser pulse and the measurement of a spectrum (gate delay) is selected in the range of 0.5 ps to 5 ps, preferably in the range of 0.5 ps to 2 ps, particularly preferably 2 ps.

[0072] As already mentioned, an essential part of the invention was to develop a methodology with which metal surfaces can be homogeneously exposed to a defined content of salt deposits in a simple and efficient manner, which can then be used as reference metal surfaces for calibration measurements.

[0073] Part of the invention is therefore also a method for uniformly applying salts to a metal surface or for producing a reference metal surface (as defined above and in the claims), comprising the following steps:

[0074] - Providing a metal substrate with a cleaned metal surface or cleaning the metal surface of a metal substrate, - Providing an (aerosol) chamber or an (aerosol) container comprising an aerosol inlet and an aerosol outlet,

[0075] - Positioning the metal substrate with cleaned metal surface in the chamber or container and / or near the aerosol outlet,

[0076] - Exposure of the metal surface to salts by introducing a salt-containing aerosol into the chamber or container, wherein the salt-containing aerosol comprises one or more salts (in solid or dissolved form) and the chemical composition of the one or more salts comprised by the salt-containing aerosol is known in each case and, in the case of several salts, the quantitative ratio of the salts to one another is known.

[0077] This method of producing reference metal surfaces has the advantage (as already mentioned above) that the risk of corrosion of the reference metal surface is eliminated or greatly minimized. The use of an (aerosol) chamber or container serves the purpose of minimizing the influence of ambient air movement on the exposure process. By changing, for example, the exposure time and the salt concentration in the aerosol, the final concentration of salt deposits on the reference metal surfaces can be adjusted.

[0078] The formulation of positioning the metal substrate near the aerosol outlet includes both positioning the metal substrate near the aerosol outlet inside the chamber or container and positioning the metal substrate near the aerosol outlet outside the chamber or container.

[0079] Preferred is a method for uniformly applying salts to a metal surface or for producing a reference metal surface (as defined above and in the claims), wherein the metal substrate with cleaned metal surface is positioned near the aerosol outlet, preferably in the immediate vicinity of the aerosol outlet outside the chamber or container (or directly below the aerosol outlet), and / or the chamber or container additionally comprises a cover for indirect aerosol application.

[0080] To create test series, the metal substrates are preferably always positioned in the same place and at the same distance from the aerosol outlet.

[0081] For the purposes of the method according to the invention for uniformly applying salts to a metal surface or for producing a reference metal surface (as defined above and in the claims), the metal substrate can be positioned, for example, by laying or hanging.

[0082] In principle, reference metal surfaces for the calibration measurements of the method according to the invention for the quantitative detection of salt deposits on a metal surface can also be obtained by other methods known to the person skilled in the art; e.g., by immersing a metal substrate in a salt-containing solution or by coating a metal substrate with a salt-containing solution.

[0083] Furthermore, it is possible to achieve a uniform application of salts to a metal surface or the production of a reference metal surface (as defined above and in the claims) by a method comprising the following steps:

[0084] - Providing a metal substrate with a cleaned metal surface or cleaning the metal surface of a metal substrate,

[0085] - Applying a salt deposit of known composition using an inkjet printer.

[0086] Part of the invention is also a device for the quantitative detection of salt deposits (or salt-forming chemical elements) on a metal surface by means of laser-induced plasma spectroscopy according to a method according to the invention (as defined above and in the claims) comprising

[0087] - a laser for generating a (local) plasma on the metal surface to be examined; - a spectrometer configured to detect a spectrum of plasma emission radiation, and

[0088] - an evaluation unit, wherein the evaluation unit comprises one or more stored calibration data, preferably one or more area contents formed on one or more spectra from calibration measurements and using step III) of the method according to the invention for the quantitative detection of salt deposits (or salt-forming chemical elements) on a metal surface by means of laser-induced plasma spectroscopy, particularly preferably one or more area contents formed on one or more spectra from calibration measurements and using steps III), III.1) and III.2) of the preferred method according to the invention for the quantitative detection of salt deposits (or salt-forming chemical elements) on a metal surface by means of laser-induced plasma spectroscopy, and the evaluation unit is configured to quantify salt deposits on a metal surface by comparing a detected spectrum of a plasma emission radiation with the one or more stored calibration data.

[0089] Preferably, a device according to the invention additionally comprises a focusing unit for focusing a laser beam on the metal surface.

[0090] Also preferred is a device according to the invention, wherein the laser is configured to scan the metal surface and / or is an Nd:YAG laser and / or emits radiation having a wavelength of 266 nm, 532 nm or 1064 nm and / or is a pulsed laser, preferably having a pulse energy in the range of 10 mJ to 250 mJ and / or having a pulse length of 1 ps to 10 ns, particularly preferably having a pulse energy of 45 mJ and a pulse length of 8 ns.

[0091] Scanning the metal surface means that the laser is configured so that it can be moved across the metal surface. Additionally or alternatively, the device can also be configured to allow movement (change of position) of the metal surface being examined.

[0092] Also preferred is a device according to the invention, wherein the spectrometer is an echelle spectrometer, preferably with a CCD sensor as a detector, and / or the device is portable and / or the device is designed in the form of a handheld device or in the form of an automatic measuring device, preferably a robot-assisted automatic measuring device.

[0093] In a further preferred embodiment, the device according to the invention can also be a stationary device (tied to a fixed location). The stationary device configuration is particularly preferred for laboratory applications.

[0094] Part of the invention is also the use of laser-induced plasma spectroscopy for the quantitative detection of salt deposits on a metal surface.

[0095] The use of laser-induced plasma spectroscopy is preferred for the quantitative detection of salt deposits on metal surfaces to be coated, particularly preferably on metal surfaces to be coated for steel construction, steel water construction, shipbuilding, offshore technology, energy and plant engineering, the automotive sector and aviation.

[0096] Part of the invention is also the use of a device according to the invention (as defined above and in the claims) for carrying out a method according to the invention for the quantitative detection of salt deposits (or salt-forming chemical elements) on a metal surface by means of laser-induced plasma spectroscopy (as defined above and in the claims).

[0097] The invention is explained in more detail below using examples and the accompanying figures. The examples provided below are intended to describe and explain the invention in more detail without limiting its scope.

[0098] They show:

[0099] Fig. 1 : Schematic representation of an aerosol chamber for uniformly applying salts to a metal surface or for producing a reference metal surface.

[0100] Fig. 2: Image (side view) of an aerosol chamber for uniformly applying salts to a metal surface or for creating a reference metal surface. A tray for transporting and positioning the metal substrates below the aerosol chamber can be seen in front of the aerosol chamber.

[0101] Fig. 3: Image of the aerosol outlet of an aerosol chamber for uniformly applying salts to a metal surface or for producing a reference metal surface.

[0102] Fig. 4: Comparison of the area ratio obtained by laser-induced plasma spectroscopy from the area under the sodium spectral line at 588.995 nm and the area under the iron spectral line at 275 nm with the salt content quantified by the Bresle test (expressed in NaCl equivalent) on reference metal surfaces of steel specimens with varying levels of salt deposits on the surface. Fig. 5: Comparison of the area ratio obtained by laser-induced plasma spectroscopy from the area under the calcium spectral line at 396.847 nm and the area under the iron spectral line at 275 nm with the salt content quantified by the Bresle test (expressed in NaCl equivalent) on reference metal surfaces of steel specimens with varying levels of salt deposits on the surface.

[0103] Fig. 6: Comparison of the area ratio obtained by laser-induced plasma spectroscopy from the area under the magnesium spectral line at 280,271 nm and the area under the iron spectral line at 275 nm with the salt content quantified by the Bresle test (expressed in NaCl equivalent) on reference metal surfaces of steel specimens with different contents of salt deposits on the surface.

[0104] Preparation of reference metal surfaces with known salt deposit content and correlation between the salt content determined by the Bresle test and the area ratios of characteristic spectral lines obtained by laser-induced plasma spectroscopy:

[0105] A metal substrate in the form of a steel specimen was cleaned and positioned beneath an aerosol chamber in close proximity to the aerosol outlet. The aerosol chamber used is shown schematically in Fig. 1 and illustrated in Figs. 2 and 3.

[0106] After positioning, the steel specimen was exposed to a saline mist for a defined period of time. The saline mist was introduced into the aerosol chamber through the aerosol inlet and, after exiting the aerosol outlet, landed on the steel specimen positioned below. A cover for indirect aerosol exposure was located above the aerosol outlet in the aerosol chamber, ensuring even distribution of the saline mist across the entire span of the aerosol outlet.

[0107] Synthetic seawater was used as the liquid component to produce the saline fog. The synthetic seawater was obtained by preparing two individual solutions from the components listed in Table 1 and then mixing the two solutions 1 and 2 together.

[0108] Table 1: Compositions of solutions 1 and 2 for the production of synthetic seawater (according to DIN 50905).

[0109] To generate a saline mist from the synthetic seawater, the synthetic seawater was fed into an inhalation system and nebulized. An inhalation system commonly used for medical purposes was used for this purpose. The inhalation system used (Compact 2) from Pari consisted of a compressor, an atomizer, and an outlet.

[0110] After the predefined time for fogging the steel specimen, it was removed from the aerosol outlet. After drying, the salt content on the salt-treated surface of the steel specimen was quantified using the Bresle test according to DIN EN ISO 8502-6:2020-08 and a conductivity measurement according to DIN EN ISO 8502-9:2020-12. In addition, measurements were performed at another location on the salt-treated surface of the steel specimen using laser-induced plasma spectroscopy.

[0111] The salt content determined by the Bresle test was converted into NaCl equivalent and compared with the area ratios obtained by laser-induced plasma spectroscopy, each formed from the area under a characteristic spectral line for a chemical element contained in the salt deposit and the area under a spectral line characteristic of the steel surface.

[0112] The procedure described above was repeated for additional steel specimens, varying the exposure time to a saline mist in each case to obtain a set of reference metal surfaces with different levels of salt deposits on the metal surface. The steel specimens to be exposed were each positioned at the same location and at the same distance from the aerosol outlet under the aerosol chamber.

[0113] Figures 4, 5, and 6 graphically illustrate the comparison of the salt contents of the manufactured reference metal surfaces obtained by the Bresle test with selected area ratios (obtained by laser-induced plasma spectroscopy). Figures 4, 5, and 6 each show a linear correlation between the salt content on the reference metal surfaces and the size of the area ratio obtained by laser-induced plasma spectroscopy.

[0114] The calibration measurements obtained in this way were then used to quantitatively detect salt deposits on the surface of a steel substrate by comparing the calibration data with LIBS measurements on a surface of a steel substrate (exposed to seawater).

[0115] List of reference symbols:

[0116] I Aerosol chamber for uniformly applying salts to a metal surface or for producing a reference metal surface

[0117] II Aerosol inlet

[0118] 12 Cover for indirect aerosol application

[0119] 13 Metal substrate with cleaned metal surface

[0120] 14 Aerosol outlet

Claims

1. Method for the quantitative detection of salt deposits on a metal surface using laser-induced plasma spectroscopy, comprising the following steps: I) Focusing a laser beam onto a point on the metal surface to be examined, so that a local plasma is created; II) Spectroscopic analysis of the radiation emitted by the local plasma during cooling, so that a spectrum for the radiation emitted by the local plasma is obtained; III) Identify (at least) one characteristic spectral line for (at least) one chemical element contained in the salt deposit from the spectrum obtained in step II) and determine the area under the spectral line; IV) Quantifying the content of salt deposits on the metal surface to be investigated by comparing the area determined in step III) with the area obtained from calibration measurements, the calibration measurements being carried out on reference metal surfaces with known salt deposit contents.

2. The method of claim 1, comprising as additional steps: 111.1) Identifying a spectral line characteristic of the metal surface to be examined from the spectrum obtained in step II) and determining the area under the spectral line; 111.2) Forming an area ratio from the area ratios determined in steps III) and 111.1); wherein in step IV) the content of salt deposits on the metal surface to be examined is quantified by comparing the area ratio obtained in step III.2) with the area ratios obtained from the one or more calibration measurements.

3. Method according to one of the preceding claims, wherein steps I) to IV) are repeated at one or more further locations of the metal surface to be examined.

4. Method according to any of the preceding claims, wherein steps III) to IV) are repeated for one or more further spectral lines characteristic of the salt deposits.

5. Method according to any of the preceding claims, wherein the metal surface to be examined is a steel surface, galvanized steel surface or aluminium surface, preferably a steel surface of a steel for structural steelwork or a galvanized steel surface.

6. A method according to any one of the preceding claims, wherein the spectral line identified in step III) and characteristic of the salt deposits is a spectral line of an element selected from the group consisting of sodium, calcium, magnesium, potassium, chlorine, sulfur, nitrogen, phosphorus, carbon, bromine, iodine, and oxygen, preferably a spectral line of an element selected from the group consisting of sodium, calcium, potassium, and magnesium, wherein the spectral line identified in step III) and characteristic of the salt deposits is preferably selected from the group consisting of sodium spectral lines with a wavelength in the range of 300 to 600 nm, calcium spectral lines with a wavelength in the range of 200 to 600 nm, potassium spectral lines with a wavelength in the range of 400 to 800 nm, and magnesium spectral lines with a wavelength in the range of 200 to 550 nm, wherein the spectral line identified in step III) isThe spectral line characteristic of the salt deposits is particularly preferably selected from the group consisting of a sodium spectral line at a wavelength of 589.0 nm, a sodium spectral line at a wavelength of 589.6 nm, a calcium spectral line at a wavelength of 393 nm, a calcium spectral line at a wavelength of 396.8 nm, a potassium spectral line at a wavelength of 766.5 nm, a potassium spectral line at a wavelength of 769.9 nm, and a magnesium spectral line. at a wavelength of 279.6 nm and a magnesium spectral line at a wavelength of 280.3 nm, wherein the spectral line identified in step III) and characteristic of the salt deposits is particularly preferably selected from the group consisting of a sodium spectral line at a wavelength of 589.0 nm and a sodium spectral line at a wavelength of 589.6 nm.

7. A method according to any one of claims 2 to 6, wherein the spectral line identified in step 111.1) and characteristic of the metal surface to be investigated is a spectral line of an element selected from the group consisting of iron, zinc and aluminum, preferably a spectral line of the element iron, wherein the spectral line identified in step 111.1) and characteristic of the metal surface to be investigated is preferably selected from the group consisting of iron spectral lines with a wavelength in the range of 200 to 600 nm, zinc spectral lines with a wavelength in the range of 200 to 650 nm and aluminum spectral lines with a wavelength in the range of 200 to 400 nm, wherein the spectral line identified in step 111.1) The identified spectral line, characteristic of the metal surface to be investigated, is particularly preferably selected from the group consisting of an iron spectral line at a wavelength of 275.6 nm, a zinc spectral line at a wavelength of. 481.1 nm and an aluminum spectral line at a wavelength of 396.2 nm, wherein the spectral line identified in step 111.1) which is characteristic of the metal surface to be investigated is most preferably an iron spectral line at a wavelength of 275.6 nm.

8. Method according to any of the preceding claims, wherein the salt deposits on the metal surface to be examined are salt deposits caused by seawater, road salt, industrial exhaust gases and / or emissions from the agricultural sector.

9. Method according to one of the preceding claims, wherein the quantitative detection of salt deposits is carried out before coating the metal surface to be examined.

10. Method according to any of the preceding claims, comprising as an additional step: Va) Repeat steps I) to IV) in the immediate vicinity of those previously analyzed areas of the metal surface under investigation where a previously defined limit value for the salt deposit content was exceeded, and / or Vb) Cleaning those areas of the metal surface to be examined where a previously defined limit value for the content of salt deposits is exceeded, wherein the previously defined limit value for the content of salt deposits is preferably 20 mg / m² 2 in sodium chloride equivalents.

11. Method according to any one of the preceding claims, wherein the one or more reference metal surfaces are each defined by - Exposure of a cleaned metal surface to a salt-containing aerosol, preferably a salt-containing mist, for a defined period of time - and, if necessary, a drying step taking place after exposure to the salt-containing aerosol, are obtained, wherein the saline aerosol comprises one or more salts and the chemical composition of the one or more salts comprising the saline aerosol is known in each case, and in the case of several salts, the ratio of the salts to each other is known, and the content of salt deposits on the one or more reference metal surfaces is referenced in each case by at least one measurement method other than laser-induced plasma spectroscopy, preferably by differential weighing before and after exposure to the saline aerosol and / or by the Bresle method according to DIN EN ISO 8502-6:2020-08 and DIN EN ISO 8502-9:2020-12.

12. Method according to one of the preceding claims, wherein in step II) of the spectroscopic analysis an exposure time of a detector in the range of 1 ps to 100 ps is selected, preferably in the range of 1 ps to 10 ps, ​​particularly preferably in the range of 10 ps, ​​and / or a distance between a laser pulse and the measurement of a spectrum in the range of 0.5 ps to 5 ps is selected, preferably in the range of 0.5 ps to 2 ps, particularly preferably in the range of 2 ps.

13. A method for uniformly applying salts to a metal surface or for producing a reference metal surface, as defined in any of the preceding claims, comprising the following steps: - Providing a metal substrate with a cleaned metal surface (13) or cleaning the metal surface of a metal substrate, - Providing a chamber (1) or container comprising an aerosol inlet (1 1) and an aerosol outlet (14), - Positioning the metal substrate with a cleaned metal surface (13) in the chamber (1) or container and / or near the aerosol outlet (14), - Applying salts to the metal surface by introducing a saline aerosol into the chamber (1) or container, wherein the saline aerosol comprises one or more salts and the chemical composition of the one or more salts comprised by the saline aerosol is known and, in the case of several salts, the ratio of the salts to each other is known.

14. Method according to claim 13, wherein the metal substrate with a cleaned metal surface (13) is positioned near the aerosol outlet (14), and / or the chamber (1) or container additionally comprises a cover for indirect aerosol application (12).

15. Device for the quantitative detection of salt deposits on a metal surface by means of laser-induced plasma spectroscopy according to a method as defined in any one of claims 1 to 12, comprising - a laser to generate a plasma on the metal surface to be examined; - a spectrometer that is set up to detect a spectrum of plasma emission radiation, and - an evaluation unit, wherein the evaluation unit comprises one or more stored calibration data sets, preferably one or more area areas formed on one or more spectra from calibration measurements and by applying step III) of claim 1, particularly preferably one or more area ratios formed on one or more spectra from calibration measurements and by applying steps III), 111.1) and III.2) of claim 2, and the evaluation unit is configured to quantify salt deposits on a metal surface by comparing a recorded spectrum of plasma emission radiation with the one or more stored calibration data.

16. Device according to claim 15, further comprising a focusing unit for focusing a laser beam on the metal surface.

17. Device according to claim 15 or 16, wherein the laser is configured to scan the metal surface and / or is an Nd:YAG laser and / or Radiation emitted with a wavelength of 266 nm, 532 nm or 1064 nm and / or is a pulsed laser, preferably with a pulse energy in the range of 10 mJ to 250 mJ and / or with a pulse length of 1 ps to 10 ns, particularly preferably with a pulse energy of 45 mJ and a pulse length of 8 ns.

18. Device according to any one of claims 15 to 17, wherein the spectrometer is an echelle spectrometer, preferably with a CCD sensor as a detector, and / or the device is portable, and / or the device is designed in the form of a handheld device or in the form of an automatic measuring device, preferably a robot-assisted automatic measuring device.

19. Use of laser-induced plasma spectroscopy for the quantitative detection of salt deposits on a metal surface.

20. Use of a device as defined in any one of claims 15 to 18 for carrying out a method defined in claims 1 to 12.

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