TEST DEVICE AND METHOD FOR DETERMINING THE CONCENTRATION OF VOLATILE CORROSION INTHIBITORS

Description

[0001] The invention relates to a test device and a test method for determining the concentration of a volatile corrosion inhibitor in an infrared-transmissive sample containing the volatile corrosion inhibitor by means of transmission infrared spectroscopy.

[0002] One of the established methods for protecting semi-finished products or components made of metallic materials from corrosion during storage or transport is the application of volatile corrosion inhibitors (VCIs). The corrosion protection method based on volatile corrosion inhibitors is accordingly called the VCI method. The volatile corrosion inhibitors are typically added to the storage or transport packaging together with the goods to be protected, in the form of so-called VCI materials. These materials contain the volatile corrosion inhibitors on or in carrier materials such as paper, cardboard, film, or foam, or in carrier substances like powders, sprays, or oils.Volatile corrosion inhibitors exert their corrosion-inhibiting effect by creating an atmosphere containing these volatile corrosion inhibitors, the so-called VCI atmosphere, through evaporation or sublimation within the packaging compartment. This atmosphere then forms a protective film on the surface of the packaged goods. In this context, the term VCI atmosphere refers to the air environment surrounding the semi-finished products or components within the packaging compartment, enriched with volatile corrosion inhibitors.

[0003] For effective corrosion protection, the VCI materials and their quantity are compiled according to the specific application, whereby the type of metallic packaged goods to be protected with regard to their corrosion sensitivity and value are just as crucial selection criteria as the intended transport or storage scenario.

[0004] Since standard specifications are virtually impossible to establish for the multitude of conceivable scenarios, the selection of the type and quantity of VCI materials in practical application is often based on the experience of the respective users. Therefore, even when using the VCI method, corrosion damage to the packaged goods can sometimes occur due to incorrect handling of the VCI materials. Sources of error when applying the VCI method include, for example, incorrect or excessively long storage of the VCI materials before their actual use, or a quantity of VCI material that is not adapted to the packaging system. The generally lacking knowledge of the current inhibitor concentration of a specific VCI material or the VCI atmosphere in a packaging system makes it difficult to predict the corrosion protection effect and makes an accurate prediction of the corrosion-free storage period almost impossible.

[0005] Apart from comparatively complex laboratory testing methods, primarily based on chromatography, which regularly require testing times of several days, there are currently no testing procedures and devices available that allow the determination of the concentration of the volatile corrosion inhibitor of a VCI material in a production-related environment – ​​for example, in warehouse logistics or incoming goods inspection – and thus permit a quick, uncomplicated and cost-effective prediction of the corrosion protection effect of the VCI material when used in a specific transport or storage scenario.

[0006] The same applies to goods packaged using the VCI method. During extended storage and / or transport times, the corrosion-inhibiting effect of the volatile corrosion inhibitors can diminish. Therefore, especially with goods stored for long periods, there is a need to detect a decrease in the protective effect of the VCI atmosphere within the packaging early on, or even before corrosion begins to appear. Currently, there are no applicable testing methods or devices available for determining the concentration of the volatile corrosion inhibitor in the VCI atmosphere of packaging systems – for example, for direct use in storage.

[0007] DE 10 2012 200 203 A1 discloses a method and a device for the spectroscopic investigation of a test object consisting of a material sample surrounded by an outer shell, wherein the test object is irradiated with measurement radiation of a predetermined spectral composition. Two different individual spectra of the test object are measured, with the measurement radiation transmitting through the outer shell twice in each case. The investigation of VCI materials is not described in DE 10 2012 200 203 A1.

[0008] US 2010 / 264315 A1 describes a device for measuring hydrocarbon concentration that can reliably determine the concentration of hydrocarbons even when the concentration and composition of the hydrocarbons in the sample gas fluctuate. A corresponding measurement method is also disclosed. In this method, the sample gas is irradiated with infrared light whose wavelength range encompasses a common absorption region that is absorbed by one or more chemical species. The light passing through the sample gas is detected by a line sensor. Based on the detected light, an analyzer calculates the absorption in the common absorption region. Subsequently, using this data, the total concentration of the chemical species that absorb in the relevant wavelength range within the sample gas is determined.

[0009] US 2018 / 113025 A1 discloses an optical spectroscopy system for monitoring chemical reactions in a liquid reactor system. The system comprises a sample liquid line connected to the reactor system, which carries the sample liquid to an optical spectroscopy device, and a laser beam source. The spectroscopy device receives the laser beam, directs it onto the sample liquid, and generates an optical spectroscopy signal. A detector captures this signal and generates a corresponding detection signal. A processor processes the detection signal to determine the identity and concentration of at least one chemical species in the liquid.

[0010] Saberion, M. et al.: "Synergistic Corrosion Inhibition of Benzotriazole and Thiourea for Refineries and Petrochemical Plants", Protection of Metals and Physical Chemistry of Surfaces, Vol. 58, No. 1 (2022-02-01), XP037805627, ISSN: 2070-2051, DOI: 10.1134 / S2070205122010178 describes the combined effect of benzotriazole and thiourea as corrosion inhibitors. Infrared spectroscopy is used to characterize the structure and properties of the protective layers formed on the metal surface.

[0011] The object of the invention is to provide a testing device suitable for quality control in the production-related environment and a corresponding testing method for the quantitative determination of the concentration of a volatile corrosion inhibitor of a VCI material or of a VCI atmosphere present in a packaging system - and thus to be able to assess the currently existing corrosion protection effect.

[0012] This problem is solved by a testing device according to claim 1 and a testing method according to claim 5. Advantageous embodiments of the invention are set out in claims 2 to 4 and 6 to 7.

[0013] The test device set up for determining the concentration of a volatile corrosion inhibitor in an infrared-transmissive VCI sample containing the volatile corrosion inhibitor by means of transmission infrared spectroscopy comprises at least one optical unit, which in turn has a light source for emitting infrared radiation and an infrared detector for detecting the infrared radiation emitted by the light source after passing through the VCI sample.

[0014] The VCI sample can be a geometrically defined element made of a VCI material, for example, a carrier material containing the volatile corrosion inhibitor, such as paper, cardboard, film, or foam. The infrared-transmissive VCI sample does not necessarily have to be transparent to visible light as well.

[0015] The volatile corrosion inhibitor is typically dispersed throughout the VCI sample. Examples of volatile corrosion inhibitors include nitrites, carboxylic acids, triazoles, and organic carboxylic anhydrides such as maleic anhydride or 3-methylphthalic anhydride.

[0016] According to the invention, the light source and the infrared detector are fixed within the respective compact optical unit at a predetermined distance from each other, which is variably adjustable. The light source-infrared detector distance is the free distance between the light source and the infrared detector along the beam path or in the direction of radiation. The direction of radiation is understood to be the axis of the symmetrical light emission of the light source, for example, the axis of the light source's beam cone.

[0017] To allow for variable adjustment of the predetermined light source-infrared detector distance, the light source can be fixed longitudinally relative to the infrared detector with respect to the infrared radiation path, i.e., in the direction of radiation. This means that the distance between the light source and the infrared detector can be set or adjusted to the desired distance before the test is performed. With the infrared radiation path vertically aligned within the optical unit, the light source is therefore height-adjustable relative to the infrared detector. For the test, the light source and the infrared detector are fixed relative to each other at the desired or predetermined distance. This makes it possible to set the optimal light source-infrared detector distance for a specific test situation, aiming for the smallest possible gap while maintaining good illumination.For example, the light source-infrared detector distance can also be readjusted as the light sources age.

[0018] The test device is preferably designed such that the VCI sample can be passed or transmitted through the optical unit with a relative movement transverse to the beam path of the infrared radiation between the light source and the infrared detector.

[0019] According to the invention, the light source is a mid-infrared light source that emits infrared radiation in the wavelength range of 2.5 µm to 50 µm, particularly in the wavelength range of 3 µm to 15 µm. The light source can, for example, be a light-emitting diode, also known as an LED emitter. The infrared detector is a spectrophotometer configured to quasi-continuously detect several electromagnetic spectra of infrared radiation in the infrared wavelength range, i.e., in rapid succession or at regular intervals. The detection range of the infrared detector lies in the wavelength range of 2.5 µm to 50 µm, preferably in the wavelength range of 3 µm to 15 µm. Suitable infrared detectors, for example, are those for the wavelength range of 2.5 µm to 5 µm and the wavelength range of 3 µm to 11 µm. The light source and the infrared detector are or are synchronized for quasi-continuous acquisition of the spectra, preferably by means of a clock signal.

[0020] The light source of the test device can have multiple individual emitters, for example, several LED emitters arranged in series on an emitter strip. Similarly, the infrared detector can consist of several individual sensors.

[0021] To achieve the compact design, small, i.e., miniaturized, yet robust mid-infrared components (MIR components) are used as the light source and infrared detector. These components are mounted, for example, on a printed circuit board or circuit board. The test device has no infrared components that move during the test – i.e., they are exclusively fixed in space. For the precise positioning of a shape-specific VCI sample, i.e., for example, when using film as the substrate, the test device may include a sample holder, a sample carrier, or a sample guide that only allows lateral movement of the VCI sample, perpendicular to the path of the infrared radiation.

[0022] The testing device is designed in particular as a mobile testing device, for example as a stand-alone device or handheld device, making it flexible to use in production-related environments.

[0023] According to the inventive test method performed with the test apparatus described above, the concentration of the volatile corrosion inhibitor in the infrared-transmissive VCI sample containing the volatile corrosion inhibitor is determined by transmission infrared spectroscopy by introducing the VCI sample transversely to the path of the infrared radiation into the at least one optical unit of the test apparatus. Preferably, the VCI sample is passed or moved through the optical unit. Such lateral movement of the VCI sample in the optical unit can be effected either by moving the VCI sample relative to the test apparatus or by moving the test apparatus relative to the VCI sample.

[0024] After the VCI sample is placed in the optical unit, or during its passage through the optical unit, a plurality of infrared radiation spectra in the infrared wavelength range of 2.5 µm to 50 µm are acquired quasi-continuously, i.e., in rapid succession. The concentration of the volatile corrosion inhibitor in the VCI sample is then determined by multivariate data analysis of the acquired majority of infrared radiation spectra. The algorithms for multivariate data analysis of electromagnetic spectra are generally known.

[0025] The concentration of the volatile corrosion inhibitor in the VCI sample can be provided as a number of individual values ​​or—preferably—as an average value. Individual values ​​can also be assigned to specific locations on the VCI sample, and based on this, the spatial distribution, for example, the area distribution, of the volatile corrosion inhibitor in the VCI sample can be determined. Furthermore, it is also possible to perform a moving average calculation of the concentration as the VCI sample passes through the optical unit.

[0026] With the proposed test device and the associated test procedure, it is possible to non-destructively determine the concentration of the volatile corrosion inhibitor in the VCI sample at different points within a single test cycle. Depending on the desired accuracy of the concentration determination, the required test duration is in the single- to triple-digit second range – and thus significantly shorter than the test duration required for currently standard laboratory tests.

[0027] The advantageous areas of application – not least due to the short testing times – lie in the field of incoming goods inspection, long-term storage of packaging systems, quality assurance in packaging assembly, and quality control during the production of VCI materials. Furthermore, the testing device can be used to determine residual concentrations of the VCI material in the VCI sample in cases of corrosion-related damage.

[0028] The test device and test method according to the invention make it possible, among other things, to determine the optimal amount of VCI materials for a specific packaging system. This allows the amount of VCI materials used to be limited to the essential minimum – advantageously from an economic, ecological, and occupational safety perspective; in particular, this eliminates the need for the method of overdosing VCI materials, which is currently frequently used for safety reasons.

[0029] Ultimately, the testing equipment and procedures contribute to determining corrosion protection measures that are optimally adapted to the respective storage or transport scenarios, thus avoiding corrosion damage.

[0030] The test device can further include a processor unit and a database unit for storing spectra of known volatile corrosion inhibitors. The database unit containing the stored spectra of known volatile corrosion inhibitors is also referred to as a model library. The processor unit is configured to identify the volatile corrosion inhibitor under test by comparing the acquired spectra with those stored in the database unit or model library. This version of the test device can also be used, among other things, to identify the manufacturer—for example, in the case of particularly specific inhibitor combinations—or to detect and reject counterfeit VCI materials.

[0031] According to one embodiment of the test device, several light sources, for example, multiple LED emitters arranged on an emitter strip, can be combined with an infrared detector, with each light source forming an optical unit with the infrared detector. Conversely, several infrared detectors can also be combined with one light source to form several optical units. By individually controlling the components, measurement data from multiple optical units can be acquired and processed simultaneously.

[0032] Furthermore, several optical units, each consisting of a light source and an infrared detector, can be arranged side by side. According to an exemplary embodiment of such a testing device, it comprises two optical units aligned such that the infrared radiation directions of both optical units are parallel to each other but oppositely directed. Both optical units are also spaced apart from each other perpendicular to the path of the infrared radiation. They preferably have the same light source-infrared detector distance and are aligned with each other. The opposing transmission of light through the VCI sample allows direction-dependent measurement effects to be detected or suppressed.

[0033] The test device described above, featuring two optical units, is specifically designed for testing VCI materials in film form. For this purpose, the test device can be combined with a roller system, which, for example, unwinds and rewinds a web of film containing the volatile corrosion inhibitor. As the pre-tensioned film web, conveyed by the roller system, passes through the two optical units, the film containing the volatile corrosion inhibitor is irradiated from both sides at each specific position, sequentially. The roller system thus enables the film to be transported at a constant speed and ensures that the film is precisely positioned and tensioned within the test area, i.e., as it passes through the optical units.Impairments to the measurement signals, caused, for example, by waves, kinks, folds, or similar imperfections in the film, are reduced by pre-tensioning the film. With this design of the testing device, film strips can be tested continuously in transit.

[0034] According to the invention, the VCI sample to be tested is in a form-bound state.

[0035] According to a preferred embodiment, the VCI sample is a VCI film consisting of an infrared-transparent plastic film containing the volatile corrosion inhibitor. The film thickness of the VCI films is typically in the range of approximately 25 µm to 120 µm.

[0036] To fix the shape-bound, i.e., geometrically defined, film-like VCI sample, the testing device further comprises a sample holder or film holder designed to tension the film-like VCI sample across its entire surface, i.e., to stretch or pull it smooth and wrinkle-free in one plane. For this purpose, the sample holder is designed, for example, as a frame comprising a clamping frame and a receiving frame serving as a counter-support. The film-like VCI sample is clamped and simultaneously tensioned between the clamping frame and the receiving frame. For this purpose, the clamping frame can have a circumferential rib that engages in a geometrically corresponding circumferential groove on the receiving frame. During the insertion and clamping of the film-like VCI sample between the clamping and receiving frames, the film is stretched across its entire surface, i.e., pulled in one plane, as it is drawn into the groove.By clamping the film-like VCI sample across its entire surface, the measurement accuracy during transmission is improved; in particular, light deflections that can occur with uneven, for example, corrugated, VCI films are avoided. The described sample holder design can be a fully enclosed frame or a partially enclosed frame. The sample holder can, for example, be designed as a clamp with which the VCI sample can be grasped, inserted into the optical unit of the testing device, and fixed within the optical unit.

[0037] The sample holder enables fixed clamping within the optical unit or, as a guiding element for the VCI sample, allows lateral movement perpendicular to the beam path, enabling both linear and arc-shaped movements. In terms of time, movement can be rasterized or continuous.

[0038] According to one embodiment of the frame-type specimen holder, in addition to the clamping frame and the receiving frame serving as a counter-support, it also includes a fixing frame that completely or partially encloses the clamping frame. This fixing frame, like the clamping frame, has a circumferential web. The receiving frame, which also serves as a counter-support for the fixing frame, has a circumferential groove that geometrically corresponds to the web of the fixing frame and encloses the corresponding groove of the clamping frame. The web height of the fixing frame is preferably greater than or equal to the web height of the clamping frame web; the grooves of the receiving frames are shaped accordingly.With this design of the sample holder, the film-like VCI sample is first clamped between the fixing frame and the receiving frame; subsequently, the film-like VCI sample is stretched onto the already fixed VCI sample by pressing the clamping frame. This pre-fixation of the film-like VCI sample further improves the uniform, flat, and even tensioning of the film-like VCI sample.

[0039] The invention is explained in more detail below with reference to exemplary embodiments and the schematic drawings, wherein identical or similar features are provided with the same reference numerals; to this end, the following are shown. Fig. 1: a first embodiment of the test device in longitudinal section, Fig. 2: a second embodiment of the test device in longitudinal section, Fig. 3: a first embodiment of the sample holder with VCI sample in top view, Fig. 4: the open sample holder of the first embodiment in longitudinal section, Fig. 5: the closed sample holder of the first embodiment in longitudinal section, Fig. 6: a third embodiment of the test device in longitudinal section, Fig. 7: a second embodiment of the sample holder in longitudinal section, and Fig. 8: the second embodiment of the sample holder during the closing process in longitudinal section.

[0040] The one in Fig. 1 The optical unit shown, according to the first embodiment of the test device, comprises the light source 1 and the infrared detector 2. The light source 1 emits infrared radiation 4 in the direction of the infrared detector 2, which detects the infrared radiation 4 in the wavelength range of 5 µm to 15 µm in the form of an electromagnetic spectrum.

[0041] The light source 1 and the infrared detector 2 are fixed at a predetermined distance 6 between the light source and the infrared detector. The infrared radiation 4 emitted by the light source 1 passes through the infrared-transparent VCI sample 3, which contains the volatile corrosion inhibitors VCI, along the transmission length 5 and strikes the infrared detector 2, which is located opposite the light source 1. The concentration of the volatile corrosion inhibitors VCI in the VCI sample 3 can be determined from the spectrum influenced by the interaction of the infrared radiation 4 with the volatile corrosion inhibitors VCI by multivariate data analysis.

[0042] The VCI sample 3 is positioned transversely between the light source 1 and the infrared detector 2, thus enabling the relative movement 7 of the VCI sample 3 through the optical unit or the path of the infrared radiation 4. As the VCI sample 3 is moved through the optical unit, multiple spectra can be acquired quasi-continuously in different local regions of the VCI sample 3. Multivariate data analysis of such a plurality of spectra allows the determination of the mean concentration of the volatile corrosion inhibitors VCI in the VCI sample 3 or the spatial concentration distribution of the volatile corrosion inhibitors VCI in the VCI sample 3.

[0043] The test device according to the second version after the Fig. 2It comprises two optical units that are spatially arranged in a way that is not directly related to each other. The direction of the infrared radiation 4 from the two optical units is parallel to each other, but oppositely directed.

[0044] The light source 1, 1.1 of the first optical unit and the infrared detector 2, 2.2 of the second optical unit are mounted on a common circuit board 8; the infrared detector 2, 2.1 of the first optical unit and the light source 1, 1.2 of the second optical unit are installed together on another circuit board 8. The VCI sample 3, in this embodiment a polyethylene-based VCI film, is guided through the two optical units perpendicular to the path of the infrared radiation 4, according to the arrow indicating the relative motion 7. The VCI film is advanced at a constant speed. For this purpose, the VCI film, which is designed as a film strip, is conveyed through the two optical units by means of a roller system (not shown).The roller system ensures the constant feed of the VCI film and simultaneously tensions it, so that the VCI sample 3 runs through the gap between the optical units in a positionally accurate and wrinkle-free manner.

[0045] Both optical units have the same light source-infrared detector distance 6 and are aligned with each other. The VCI probe 3 passes through both optical units centrally between the light sources 1, 1.1, 1.2 and the infrared detectors 2, 2.1, 2.2, so that the transmission length 5 is located at the same position in both optical units with respect to the light source-infrared detector distance 6. Only the direction of transmission differs between the two optical units.

[0046] The execution according to the Fig. 2This allows the identical position of a VCI sample 3 to be tested in rapid succession using two different radiation directions, thereby minimizing direction-dependent measurement effects during radiation exposure. Furthermore, foil strips can be continuously monitored during transmission. In addition to determining the average concentration of the volatile corrosion inhibitors (VCI), this also allows for the determination of their surface distribution within the foil strip and any concentration fluctuations.

[0047] In Fig. 3 The sample holder 9, designed as a surrounding frame, with the VCI sample 3, designed as a VCI film, clamped therein, is shown in a top view. The corresponding views of the Figs. 4 and 5Figure 1 shows the arrangement and clamping of the VCI sample 3 between the clamping frame 9.1 and the receiving frame 9.2 of the sample holder 9. When the VCI film is clamped between the clamping frame 9.1 and the receiving frame 9.2, the VCI film is pressed by the web of the clamping frame 9.1 into the groove of the receiving frame 9.2 and simultaneously tensioned in the surface area within the frame. This tensioning puts the VCI film under tension in one plane. Fig. 6 The VCI sample 3, which is inserted into the optical unit between the light source 1 and the infrared detector 2 in the beam path of the infrared radiation 4 and is fixed in the sample holder 9, is shown.

[0048] A further embodiment of the sample holder 9 with the clamping frame 9.1, the fixing frame 9.3 and the receiving frame 9.2, which serves as a counter-holder for both, is described in the Figs. 7 and 8The fixing frame 9.3 encloses the tensioning frame 9.1 and is freely movable relative to it. Otherwise, the design corresponds to that shown. Fig. 4 To clamp the VCI sample 3, which is designed as a VCI film, it is first clamped between fixing frame 9.3 and receiving frame 9.2 by – as in Fig. 8 As shown, the fixing frame 9.3 is pressed against the receiving frame 9.2. The web of the fixing frame 9.3 engages in the corresponding groove of the receiving frame 9.2. In the following step, the fixed VCI film is pressed against the receiving frame 9.2 by pressing the tensioning frame 9.1 against the receiving frame 9.2, whereby the web of the tensioning frame 9.1 engages in the groove of the receiving frame 9.2. Reference symbol list

[0049] 1 Light source 1.1 Light source of the first optical unit 1.2 Light source of the second optical unit 2 Infrared detector 2.1 Infrared detector of the first optical unit 2.2 Infrared detector of the second optical unit 3 VCI sample 4 Infrared radiation 5 Transmission length 6 Light source-infrared detector distance 7 Relative movement 8 Circuit board 9 Sample holder 9.1 Clamping frame 9.2 Mounting frame 9.3 Fixing frame VCI Volatile corrosion inhibitor

Claims

1. Test device, adapted for determining the concentration of a volatile corrosion inhibitor, VCI, in an infrared-light-transmissive VCI sample (3) containing the volatile corrosion inhibitor, VCI, by means of transmission infrared spectroscopy, wherein the test device comprises at least one optical unit having a light source (1) for emitting infrared radiation (4) and an infrared detector (2) for detecting the infrared radiation (4) emitted by the light source (1) after passage of the radiation through the VCI sample (3), wherein - the light source (1) and the infrared detector (2) are fixed relative to one another within the respective optical unit, which is constructed in a compact design, with a predetermined, variably adjustable light-source / infrared-detector distance (6), wherein, for variable adjustment of the predetermined light-source / infrared-detector distance (6), the light source (1) is fixable so as to be longitudinally displaceable relative to the infrared detector (2) with respect to the beam path of the infrared radiation (4), - the light source (1) is a mid-infrared light source which emits infrared radiation (4) in the infrared wavelength range from 2.5 µm to 50 µm, and - the infrared detector (2) is a spectrophotometer which is adapted to detect spectra of the infrared radiation (4) in the infrared wavelength range from 2.5 µm to 50 µm quasi-continuously, characterised in that the test device further comprises a sample holder (9) for fixing a film-like VCI sample (3) formed as a geometrically defined shaped element, wherein the sample holder (9) is adapted to tension the film-like VCI sample (3) flat in one plane without folds.

2. Test device according to claim 1, characterised in that it comprises a processor unit and a database unit for storing spectra of known volatile corrosion inhibitors, VCI, wherein the processor unit is adapted to identify, in terms of substance, the volatile corrosion inhibitor, VCI, to be tested by comparing detected spectra with the known spectra stored in the database unit.

3. Test device according to claim 1 or 2, characterised in that it comprises two optical units, wherein the two optical units are arranged spaced apart from one another transversely to the beam path of the infrared radiation (4), with oppositely directed infrared radiation (4).

4. Test device according to any one of claims 1 to 3, characterised in that the test device is designed such that the VCI sample (3) can be guided through the optical unit between the light source (1) and the infrared detector (2) with a relative movement (7) transversely to the beam path of the infrared radiation (4).

5. Test method for determining the concentration of a volatile corrosion inhibitor, VCI, in an infrared-light-transmissive, film-like VCI sample (3), formed as a geometrically defined shaped element and containing the volatile corrosion inhibitor, VCI, by means of transmission infrared spectroscopy, carried out by means of the test device according to any one of claims 1 to 4, wherein a plurality of spectra of the infrared radiation (4) in the infrared wavelength range from 2.5 µm to 50 µm are detected quasi-continuously while the VCI sample (3) is introduced into the at least one optical unit of the test device transversely to the beam path of the infrared radiation (4), wherein the film-like VCI sample (3) is tensioned flat in one plane without folds by means of the sample holder (9), and wherein the concentration of the volatile corrosion inhibitor, VCI, in the VCI sample (3) is determined by multivariate data analysis of the detected plurality of spectra of the infrared radiation (4).

6. Test method according to claim 5, characterised in that the VCI sample (3) is guided through the beam path of the infrared radiation (4) of the two optical units.

7. Test method according to claim 5 or 6, characterised in that the VCI sample (3) is a VCI film consisting of an infrared-light-transmissive plastics film containing the volatile corrosion inhibitor, VCI.