DE102018132318A1 - Gel pad for quality inspection of aluminum surfaces - Google Patents

Abstract

Gel pad for testing the quality of a passivation layer of an aluminum or an aluminum alloy, comprising the components: a hydrogel; a protonating agent; and an indicator which shows a color change with aeline, the protonating agent and the indicator being homogeneously distributed in the hydrogel and the gel pad being transparent on a closed section of the passivation layer.

Description

Technical field

The invention is in the field of the aluminum industry, the aerospace and automotive industries, and the coating industry. It concerns surface technology, corrosion protection and quality assurance for surfaces made of aluminum and aluminum alloys.

Prior art

Known test methods for quality assurance of surface passivation in the areas mentioned are chamber test methods, for example the salt spray test, a potential measurement and immersion test methods. With the exception of the potential measurement, the named methods typically include a visual inspection of the surface, or the visual preselection of sections of a component surface with expected corrosion damage, or the complete inspection of a component under conditions of accelerated aging (corrosion). A standard industry requirement for checking the quality of a pretreatment is to conduct filiform corrosion tests.

However, the previously known solutions are only partially satisfactory. In particular, the tougher corrosion conditions in the salt spray test overwhelm the tested surfaces and lead to test results with the possibility of misinterpretation. Another disadvantage of the salt spray test is high costs and a comparatively long test time. A visual inspection of component surfaces for immediate corrosion damage is inaccurate and subjective. In addition, the known methods are carried out on fully applied protective systems due to the high testing effort, which means that quality testing after individual pretreatment steps to describe their benefits is not possible.

Brief description of the invention

• - ein Hydrogel,
• - ein Protonierungsmittel,
• - einen Indikator, der mit Al³⁺ einen Farbumschlag zeigt,

wobei das Protonierungsmittel und der Indikator homogen im Hydrogel verteilt sind und das Gelpad auf einem geschlossenen Abschnitt der Passivierungsschicht transparent ist.Against this background, a gel pad for the quality inspection of passivated component surfaces made of aluminum and aluminum alloys is proposed. It includes the components:

• - a hydrogel,
• - a protonating agent,
• an indicator which shows a color change with Al ³⁺ ,

the protonating agent and the indicator being homogeneously distributed in the hydrogel and the gel pad being transparent on a closed section of the passivation layer.

• - Bereitstellen eines Gelpads wie vorstehend und nachfolgend beschrieben;
• - Herstellen und Aufrechterhalten eines Kontaktes zwischen dem Gelpad und der Passivierungsschicht und Anzeigen einer lokalen Korrosionsreaktion im Falle frei diffundierender Aluminium-Ionen durch einen lokal begrenzten Farbumschlag im Gelpad;
• - optional, Quantifizieren der angezeigten Korrosion durch:
  1. i. eine Anzahl und/oder
  2. ii. einen Durchmesser und/oder
  3. iii. eine Fläche

von Abschnitten des Gelpads, die den Farbumschlag aufweisen.Furthermore, a method for testing a passivation layer, a conversion layer and a corrosion protection coating on aluminum or an aluminum alloy with regard to a defect thereof is proposed. The process includes:

• Providing a gel pad as described above and below;
• - Establishing and maintaining contact between the gel pad and the passivation layer and indicating a local corrosion reaction in the case of freely diffusing aluminum ions through a locally limited color change in the gel pad;
• - optional, quantify the displayed corrosion by:
  1. i. a number and / or
  2. ii. a diameter and / or
  3. iii. an area

portions of the gel pad that have the color change.

A method for identifying an alloy that is corrosion-resistant under given natural environmental conditions is also proposed. This procedure involves the use of the above. Gel pads on surfaces comprising aluminum or an aluminum alloy.

Detailed description of the invention

The proposed embodiments relate to the testing of passivated aluminum surfaces and the testing of passivated surfaces of aluminum alloys. According to the invention, it is also possible to test zinc-aluminum spray coatings with a varying aluminum content for atmospheric corrosion protection using the gel pad proposed here.

• - die sich unter natürlich Umgebungsbedingungen auf der Oberfläche ausbildende Passivschicht aus Aluminiumoxid;
• - die sich unter Einwirkung verschiedener Medien (Elektrolytlösungen) auf der Oberfläche ausbildende Passivschicht;
• - eine eloxierte Aluminiumoberfläche; und
• - jegliche Art von Konversions- und Vorbehandlungsschichten auf Aluminiumoberflächen.

Passivation is to be understood in the context of the present description:

• - The passive layer of aluminum oxide that forms on the surface under natural environmental conditions;
• - The passive layer that forms on the surface under the influence of various media (electrolyte solutions);
• - an anodized aluminum surface; and
• - any kind of conversion and pre-treatment layers on aluminum surfaces.

• - ein Hydrogel,
• - ein Protonierungsmittel, und
• - einen Indikator, der mit Al³⁺ einen Farbumschlag zeigt,

wobei das Protonierungsmittel und der Indikator homogen im Hydrogel verteilt sind, und das Gelpad auf einem geschlossenen Abschnitt der Passivierungsschicht bzw. der untersuchten Oberfläche farblos transparent oder einfarbig transparent ist.According to one embodiment, this gel pad comprises the following components:

• - a hydrogel,
• - a protonating agent, and
• an indicator which shows a color change with Al ³⁺ ,

wherein the protonating agent and the indicator are homogeneously distributed in the hydrogel, and the gel pad is colorless transparent or monochrome transparent on a closed section of the passivation layer or the examined surface.

The hydrogel advantageously acts as an inert carrier material and provides a polymer matrix for the components present in solution and for an essentially unhindered diffusion of any Al ³⁺ ions present in this matrix and the reaction with the indicator. The hydrogel stabilizes the color change caused by the interaction of aluminum ions and indicator. The intensity and extent of sections of the gel pad that show a color change enable the quantification and qualification of injuries to the passivation layer. Regions with a color change are also referred to here as an advertisement. In particular, imperfections that can be attributed, for example, to a surface that was insufficiently cleaned prior to the passivation or that result from faulty pickling or faulty anodizing can be identified. The identified defects in the passivation layer can advantageously be localized by the color change of the indicator in the gel pad, since they occur immediately above those surface sections which are susceptible to corrosion.

According to one embodiment, the gel pad has a corrosion stimulator as a further component, the corrosion stimulator being selected from an alkali and / or an alkaline earth chloride. The chloride is preferably selected from the list consisting of NaCl, KCl, MgCl ₂ , CaCl ₂ and LiCl.

The corrosion stimulator is advantageously selected in such a way that corrosion is provoked at locations of a missing or inadequate passivation. Furthermore, the corrosion stimulator can stimulate and trigger local corrosion in the case of naturally formed passive layers on aluminum which are insufficiently designed. Sulphates can also advantageously be used as corrosion stimulators, e.g. B. to check the resistance of conversion layers under the influence of sulfates. For pure aluminum surfaces, however, chlorides have the greatest influence on local corrosion (pitting corrosion) and are therefore preferred according to the invention.

According to one embodiment, the indicator homogeneously distributed in the gel pad is dissolved in the aqueous phase of the hydrogel. The indicator substance is typically selected from at least one member of the group consisting of aluminon or the ammonium salt of aurintricarboxylic acid, Chromazurol S - or Mordant Blau 29, and eriochromcyanin R - or Mordant Blau 3, alizarin, alizarin sulfonic acid sodium, quinalizarin, hematoxylin, Morin, benzopurpurin, Congo red and a gold sol.

The indicators mentioned either form intensely colored complexes (colored lacquers) with aluminum ions, which are clearly visible in the otherwise transparent gel pad over locations where Al ^{3+ is released,} or other clear color changes. Due to an ongoing corrosion of the surface progressive diffusion of Al ³⁺ into the gel pad, the region with a color change typically grows concentrically, so that the diameter of a corrosion indicator allows conclusions to be drawn about the extent of a disturbance in the passivation layer. By integrating the indicator substance into the gel matrix of the gel pad, the color change reaction takes place locally. The location and number of defects in passive layers and conversion layers can thus be detected and assessed.

According to one embodiment, the polymer which forms the hydrogel is selected from the list consisting of: agar or agar agar; Agarose; Gelatin; Polyvinyl alcohol; partially hydrolyzed polyvinyl alcohol; Polyvinyl alcohol slime; Acrylate, mowiol; and polyacrylamide. In principle, all materials are considered that form a solid gel film.

The polymers mentioned are advantageously available commercially at low cost; hydrogels formed therefrom are sufficiently mechanically stable at a concentration known to the person skilled in the art, even at higher temperatures, for example at temperatures up to 60 ° C. The polymer selected advantageously has the ability to syneresis (phase separation). This means that during gel formation, the corresponding polymer forms a thin wet film on the gel surface. This wet film is advantageous for the reliable electrolytic coupling to the aluminum surface to be examined.

According to one embodiment, the protonating agent is selected and a concentration of the protonating agent in the gel pad is adjusted such that at the temperature of the component surface to be tested with the gel pad, typically at room temperature, a pH value in the wet film of the gel pad between pH 2 and pH 6, in particular is adjustable between pH 3 and pH 5.

For example, the protonating agent acetic acid is used in a concentration of 0.01 mol / 1 to 0.05 mol / l in the gel pad. This gives: 0.01 mol / l a pH in the wet film of the gel pad of 5 (pH 5); 0.05 mol / l a pH in the wet film of the gel pad of 3 (pH 3). Concentrations deviating from this too, e.g. in the range of 0.0025 mol / l to 0.1 mol / l can be set.

According to one embodiment, the protonating agent is selected from a member of the group consisting of a single-proton or poly-proton, water-soluble, organic or inorganic acid, in particular from formic acid, acetic acid and citric acid.

Corresponding acids are typically commercially available and inexpensive. The latter three acids are advantageous in the context of the concentrations and exposure options used here comparatively harmless from the point of view of user protection.

Agar Agar:

2-5 g auf 100 ml Wasser (2 - 5 %-ige Lösung bzw. Gel);

NaCl:

0,005 Mol/l bis 1 Mol/l;

Essigsäure:

0,01 Mol/1 bis 0,05 Mol/l;

Aluminon:

0,2 mMol/1 bis 0,4 mMol/l.

According to one embodiment, the hydrogel is agar, the protonating agent is acetic acid, the indicator is aluminon, and the corrosion stimulator is NaCl, the concentration of the components mentioned in the gel pad being within a range:

Agar Agar:

2-5 g to 100 ml of water (2-5% solution or gel);

NaCl:

0.005 mol / l to 1 mol / l;

Acetic acid:

0.01 mol / 1 to 0.05 mol / l;

Aluminon:

0.2 mmole / 1 to 0.4 mmole / l.

Agar is available inexpensively and in gel form, for example as a 2%, as a 3%, as a 4% or as a 5% gel at least up to 50 ° C, and as a natural product completely harmless with regard to possible health risks for the user. Acetic acid and table salt are easily accessible and inexpensive, aluminon has proven itself through an intensive color change reaction.

According to one embodiment, the shape of the gel pad is arbitrary at a constant height, e.g. circular, rectangular, triangular. The gel pad preferably has a circular shape with a diameter between 10 mm and 100 mm, more preferably between 15 mm and 50 mm and a thickness between 1 mm and 10 mm, for example 2 to 5 mm, in particular approximately 3 mm.

Advantages of this embodiment result from the possibility of being able to provide differently shaped gel pads for the most varied of test tasks.

• - Bereitstellen eines Gelpads gemäß zumindest einem der Ansprüche 1 bis 8;
• - Herstellen und Aufrechterhalten eines Kontaktes zwischen dem Gelpad und der Passivierungsschicht und Anzeigen einer üblicherweise lokalen Korrosion an Hand frei diffundierender Aluminium-Ionen durch einen Farbumschlag im Gelpad; und
• - Quantifizieren einer Korrosion durch Ermitteln
  • - einer Anzahl und/oder
  • - eines Durchmessers und/oder
  • - einer Fläche
  eines oder mehrerer Abschnitte des Gelpads, der/die den Farbumschlag zeigen.

According to one embodiment, a method for evaluating a passivation layer on an aluminum or an aluminum alloy is proposed. The process includes the steps:

• - Providing a gel pad according to at least one of claims 1 to 8;
• - Establishing and maintaining contact between the gel pad and the passivation layer and indicating a usually local corrosion by means of freely diffusing aluminum ions by a color change in the gel pad; and
• - quantify corrosion by determining
  • - a number and / or
  • - a diameter and / or
  • - an area
  one or more sections of the gel pad that show the color change.

Advantages of the proposed method are that it can be carried out easily. No complex devices or systems are required. Any color advertisements are evaluated visually with an unarmed eye. The gel pad can also be scanned in after the test has been carried out and its digitized image can be evaluated using suitable image processing software and a corrosion signal can be quantified.

According to previous experience, the size of a corrosion indicator depends, i.e. of a section of the gel pad with the respective color change, only to a lesser extent from the pore diameter of a passivation layer, but is determined under optimal diffusion conditions in the wet film of the gel pad and in the gel pad itself by the number of aluminum ions escaping and by the surface area of an injury to the passivation layer. Against this background, according to the invention, a rapid test for checking the quality of the execution of passivation processes or the success of a passivation is provided.

According to one embodiment, the use of the gel pad described above for the identification of an alloy comprising aluminum is proposed which is or should be corrosion-resistant under specifically selected environmental conditions.

Based on the findings, it is possible to optimize the composition of an aluminum alloy with regard to the proportion of alloy components for a specific application scenario. The environmental conditions can be selected from those of the application scenario intended for an aluminum alloy component. For example, the stability of an alloy against corrosive conditions can be checked and evaluated under the influence of a defined humidity and a certain cyclical temperature profile. At different times when a test specimen of the alloy to be tested is removed, its corrosion resistance can be assessed and documented reproducibly (e.g. using digital photography). In this way, different alloys can advantageously be tested in parallel under identical aging conditions and assessed in parallel.

The embodiments described above can be combined with one another as desired. However, the invention is not limited to the specifically described embodiments, but can be modified and modified in a suitable manner. It is within the scope of the invention to suitably combine individual features and combinations of features of one embodiment with features and combinations of features of another embodiment in order to arrive at further embodiments according to the invention.

Before the exemplary embodiments are explained in more detail below with reference to figures, it is pointed out that the same elements in the figures are provided with the same or similar reference symbols and that a repeated description of these elements is omitted. Furthermore, the figures are not necessarily to scale, the focus is rather on the explanation of the basic principle.

Figure list

• 1 illustrates the formation of a color display in the proposed gel pad in the presence of aluminum ions.
• 2nd shows the contrast ratios of color displays to be observed when the passivation layer is anodized aluminum with different concentrations of acetic acid and aluminon in the gel pad.
• 3rd shows the influence of the pH value in the wet film of the gel pad on the test result for different types of passivation.

In particular shows 1 Results of the reaction of aluminum ions with the indicator aluminon in the gel pad 3rd depending on the presence of a surface protective layer acting as a diffusion barrier 2nd the aluminum sample 1 .Picture A shows schematically the diffusion of Al ³⁺ ions into the gel pad containing the indicator dye in acetic acid medium 3rd and the formation of the colored precipitation used for detection 4th . The colored precipitation shown hatched 4th only forms from an injury 5 the passivation layer 2nd from where aluminum ions can diffuse into the gel pad while an undamaged passive layer 2nd a chemical attack on the aluminum 1 prevents, consequently, no aluminum ions from occurring and not in the gel pad 3rd can diffuse. In the drawing file B is an anodized aluminum sample 1a with a passivation layer formed during anodizing and in section C a chemically passivated aluminum sample 1b shown with a conversion layer formed thereon. The undamaged passivations lead to no coloring in the gel pad 3rd (see drawing B and drawing C, each above) during a mechanical injury 5 the conversion layer 2a , 2 B by reacting leaking aluminum ions with the indicator aluminon in the gel pad 3rd cross-shaped scratches deliberately introduced at locations 5 with the help of clearly visible color displays 4th is detectable.

In particular shows 2nd in the drawing file A the color display 4th on a 3% agar-agar gel pad 3rd in the presence of 1 mol / 1 chloride (NaCl), 0.01 mol / l acetic acid, and 0.2 mmol / 1 aluminon on a surface of pure aluminum with a naturally formed passive layer. The drawing file B shows the color displays for an identical passive layer with a gel pad, which in a 3% agar-agar gel at 1 mol / l NaCl a 5-fold higher concentration of acetic acid (0.05 mol / l) and twice the concentration of aluminon ( 0.4 mmol / l)) contained. It can be seen that in the case B the displays have less contrast. In order to ensure a good contrast and the associated good evaluability, it is therefore important to ensure a balanced relationship between protonating agent and indicator.

3rd shows the influence of the pH value in the wet film of the gel pad 3rd on the test result for different types of passivation.

Embodiments

The following is a standard recipe for testing anodized aluminum and aluminum that has been provided with a conversion layer. The indicator aluminon (C ₂₂ H ₂₃ N ₃ O ₉ ) used here reacts with aluminum ions according to the following formula with a color change from transparent to red-orange / red: Al ³⁺ + 3 C ₂₂ H ₂₃ N ₃ O ₉ → 3 NH ⁴⁺ + Al (C ₂₂ H ₁₉ O ₉ ) ₃

The components used to manufacture the gel pads are manufactured according to a typical embodiment as follows:

1% aluminon solution: 1.00 g aluminon (p.a.) or ammonium salt of aurintricarboxylic acid (p.a.) are dissolved in approx. 80 ml demineralized water and this made up to 100 ml.

1 molar NaCl solution: 58.44g NaCl (p.a.) are dissolved in approx. 800 ml deionized water (demineralized water) and this made up to 1 liter.

Agar-agar: 3.00 g of agar (very pure) are stirred into 100 ml of the 1 molar NaCl solution.

The homogeneous mixture of agar-agar and the sodium chloride solution is boiled at approx. 95-99 ° C. with constant stirring until it is completely clear and cooled to approx. 85 ° C. Then be 2 ml of the 1% aluminon solution and 0.2 ml of a 30% acetic acid are added. The solution is then poured out on a plexiglass plate and allowed to solidify. After the gel has solidified and formed, the gel pads are cut out in the desired shape. The cut-out pads can be stored in the refrigerator at 5-8 ° C for at least 4 months.

Embodiment 1

According to a first practical embodiment, a circular gel pad is provided as a test device for the task described in the introduction. It has a diameter of 20 mm and a height of approx. 3 mm. However, the shape of the base area of the gel pad is arbitrary (eg rectangular, square, etc.) and can be adapted to the geometry of the areas to be tested. A circular shape has proven itself for better handling and an optimal assessment of the color advertisements that appear. Likewise, the height of the gel pad is freely selectable, but is chosen so that the ads still show through the gel pad due to the color change reaction, so they are visible from above. Formula embodiment 1 component Amount / concentration Agar Agar 3 g to 100 ml CH ₃ COOH 0.01 mol / l Aluminon (1% aqueous solution) 0.4 mmol / l NaCl 1 mol / l

The composition according to embodiment 1 is advantageously suitable for testing chemically passivated or anodized aluminum surfaces. It is not suitable for testing naturally formed passive layers on aluminum, as it overwhelms the naturally formed passive layer due to the high chloride content in the pad and in any case leads to locally limited color change reactions on the surface, even if there is no specific mechanical damage to this passivation. It is no longer possible to distinguish between naturally passivated and poorly passivated surfaces. This is explained in embodiment 2.

Embodiment 2

For naturally formed passive layers, a chloride content of <0.1 mol / l chloride should be selected in order to be able to test them specifically. Gelpade variants with a neutral pH value in the wet film and, accordingly, a lower level of protonating agent have also proven successful in naturally formed passive layers. Formula embodiment 2 component Amount / concentration Agar Agar 3 g to 100 ml CH ₃ COOH without acetic acid (pH 6 - 7) Aluminon (1% aqueous solution) 0.4 mmol / l NaCl 0.01 mol / l

Embodiment 3

1. 1) Reinigen und Entfetten der Aluminiumoberfläche. Hierbei ist, insbesondere bei Konversionsschichten darauf zu achten, dass sich das verwendete Reinigungsmittel mit der Konversionsschicht verträgt und nicht zu einer ungewünschten chemischen Reaktion vor der Prüfung mit dem Gelpad führt. Das Reinigen erfolgt beispielsweise mit Ethanol oder Ethylacetat, Entfetten mittels Aceton oder Waschbenzin. Die zur Reinigung und Entfettung verwendeten Lösungsmittel können an die jeweilige Zusammensetzung der Konversionsschicht angepasst werden. Es werden Mittel ausgewählt, die selbst keine korrosive Wirkung haben und sich mit der Konversionsschicht vertragen.
2. 2) Gelpad auf die zu prüfende Aluminiumoberfläche mittels eines Kunststoffspatels auflegen. Dabei lässt man sich ggf. zuvor im Kühlschrank gelagerte Gelpads vor der Prüfung auf Raumtemperatur erwärmen.
3. 3) 25 min - 30 min warten (in der Zwischenzeit visuell inspizieren: wann kommen erste Anzeigen, wo?)
4. 4) Gelpad von der Oberfläche mit einem Kunststoffspatel abnehmen und auf einen Kunststoffträger, beispielsweise eine durchsichtige Plastikfolie übertragen. (Metalloberflächen sind nicht zulässig!).
5. 5) visuelle Auswertung, Dokumentation des Ergebnisses, ggf. Digitalisieren.
6. 6) Unter Umständen muss die Aluminiumoberfläche nach der Prüfung noch einmal kurz mit einer wässrigen Ammoniaklösung (NH₄-Lösung) abgewischt werden, um mögliche kleinere Rückstände der Farbreaktion zu beseitigen. Hierbei ist im Falle von Konversionsschichten auf die Verträglichkeit der Konversionsschicht mit Ammoniak zu achten.

The method proposed according to a third practical embodiment comprises the steps:

1. 1) Clean and degrease the aluminum surface. It is important to ensure that the cleaning agent used is compatible with the conversion layer and that it does not lead to an undesired chemical reaction before the test with the gel pad, especially with conversion layers. Cleaning takes place, for example, with ethanol or ethyl acetate, degreasing with acetone or white spirit. The solvents used for cleaning and degreasing can be adapted to the respective composition of the Conversion layer can be adjusted. Agents are selected that have no corrosive effect and that are compatible with the conversion layer.
2. 2) Place the gel pad on the aluminum surface to be tested using a plastic spatula. If necessary, gel pads previously stored in the refrigerator can be warmed to room temperature before the test.
3. 3) Wait 25 min - 30 min (in the meantime, visually inspect: when are the first ads coming, where?)
4. 4) Remove the gel pad from the surface with a plastic spatula and transfer it to a plastic carrier, for example a transparent plastic film. (Metal surfaces are not permitted!).
5. 5) visual evaluation, documentation of the result, digitization if necessary.
6. 6) After the test, the aluminum surface may have to be wiped briefly with an aqueous ammonia solution (NH ₄ solution) in order to remove any small residues of the color reaction. In the case of conversion layers, attention must be paid to the compatibility of the conversion layer with ammonia.

Other embodiments

Through a targeted variation of the parameters, the gel pad can be adjusted to test different types of passivation on aluminum surfaces. In particular, by varying the proportion of acetic acid in the gel, the pH of the wet film can be adjusted in the pH range from 3 (0.05 mol / l acetic acid) to 5 (0.01 mol / l acetic acid). Without the acetic acid component, the pad has a pH of approx. 7. The proportion of the protonating agent means that the gel pad can be adapted for the respective test task or passivation layer. A neutral pH value in combination with a low chloride content in the pad is more suitable for testing naturally formed passive layers, as these should not be destroyed by the influence of the gel pad, whereas a pH value of 3 is advantageous for testing anodized ones and chemically passivated surfaces.

Is in 3rd shown. The gel pads 3rd in the left frame A comprise 3% agar with 0.4 mmol / l aluminon, 1 mol / l chloride and 0.01 mol / l acetic acid. The pH of the gel pads 3rd was pH 5. In contrast, the gel pads had 3rd in the right drawing B - due to the fact that acetic acid was not added here - a pH of pH 7. The upper picture shows gel pads 3rd on anodized aluminum 1a . The pictures below show gel pads 3rd on aluminum after chemical passivation, ie with a conversion layer 1b . It becomes clear that through a targeted adjustment of the pH value of the gel pad 3rd Misinterpretation of a color display can be prevented. It has been shown that a lower pH value is advantageous for the detection of defects in anodized layers 1a and chemical passivation 1b is. Using the example of anodized aluminum 1a this is clearly shown. If you choose a pad variant without protonating agent, the anodized aluminum shows 1a Despite a defined mechanical injury, no displays (pad on the top right) - it would therefore be considered good, even though the layer is damaged and has imperfections that can lead to signs of corrosion. With a gel pad with pH 5 (partial image A , Gel pads top left and bottom left) the injury is clearly recognizable. If the pH was lowered even further, the display would become clearer.

A higher proportion of aluminon results in a more intense red coloring of the corrosion indicators (color change reaction).

With an increased content of acetic acid (0.05 mol / l acetic acid) and aluminone (0.4 mmol / l) in the gel pad, the pad experiences a more intense red color in the initial state (without Al ³⁺ ), which can lead to a lower contrast between the gel pad background and corrosion indicators caused by the color change reaction (Al ³⁺ with aluminon). Adequate contrast is important for optimal evaluation.

With an increased chloride content, the redox potential of the gel pads increases, which means that more stringent test conditions can be set. An increased chloride content tends to provide more displays than a lower chloride content with the same aluminum surface. This could be demonstrated on naturally formed passive layers on pure aluminum.

• - Qualität und Stabilität der natürlich ausgebildeten Passivschicht auf Aluminium;
• - Prüfung von eloxierten Aluminiumoberflächen auf Fehlstellen und Verletzungen der eloxierten Schicht;
• - Prüfung von Konversionsschichten auf Aluminiumoberflächen auf Fehlstellen, die verfahrensbedingt vorhanden sein können und auf Verletzungen durch äußere mechanische Einflüsse zur Beantwortung der Fragestellungen: „Ist die Konversionsschicht dicht oder besitzt sie Poren und Defekte?“, „Reicht eine Verletzung bis zum Aluminiumsubstrat?“

The passivation of the respective aluminum surface is tested with the gel pad proposed here with regard to:

• - Quality and stability of the naturally formed passive layer on aluminum;
• - Checking anodized aluminum surfaces for defects and injuries to the anodized layer;
• - Checking conversion layers on aluminum surfaces for defects that may be present due to the process and for injuries caused by external mechanical influences to answer the questions: "Is the conversion layer tight or does it have pores and defects?", "Is an injury sufficient to the aluminum substrate?"

However, the gel pad proposed here can also be used, a temporal degradation of a conversion layer or a pretreatment layer, under defined environmental conditions such. B . under the influence of different atmospheric humidity. Results of this use of the gel pad show whether the protective effect of such layers becomes better or worse over time and whether the quality of the layers concerned changes.

The contact time of the gel pad with the passivation layer to be tested until the evaluation of the optional color display (s) was the same for all exemplary embodiments. It is typically 20 to 30 minutes at room temperature.

For example, the contact time can be 25 min as standard, and should not be less than 20 min for a good contrast of the color displays with the gel pad background.

According to the proposed test method of the gel pads described, defective surfaces differ from sufficiently passivated layers on the basis of color advertisements, in particular with regard to their intensity, number and size:

Surfaces with insufficient passivation, where defects extend to the aluminum substrate and are based on a mechanical damage to the passivation, show deep red, permanent, localized displays in the gel pad.

Surfaces with a homogeneous passive layer without defects or pores up to the substrate show no indications in the gel pad, since the passivation (aluminum oxide layer, anodized aluminum oxide layer, conversion layer) prevents the passage of aluminum ions into the gel pad and thus the reaction of aluminum ions with the indicator (aluminon).

The nature of the proposed gel pads and their application for the qualitative testing of passivation layers on surfaces of aluminum and aluminum alloys can be summarized as incorporating a chemical color change reaction in a gel pad. This test approach is suitable for in-house quality assurance in the aluminum processing industry due to short test times and significantly reduced costs compared to more complex technical procedures.

Essential aspects of the proposed method therefore relate to the provision of a gel pad (CorrAl-Pad), the use of this gel pad on aluminum surfaces, the detection of corrosion-sensitive aluminum surfaces and the assessment and detection of corrosion protection measures on aluminum surfaces.

With the gel pad described, it is also possible to compare different aluminum alloys with regard to their susceptibility to pitting corrosion if all the alloys to be examined have undergone the same passivation. In this case, the same passivation means the conditions under which the natural passive layers can form. In particular, this includes the atmospheric conditions such as temperature, air humidity, dry-wet cycles and the time factor. In addition, the effect of heat treatment and the influence of individual alloy elements on the corrosion resistance can be checked. It should be noted, however, that the factors "alloying elements", "heat treatment" and "atmospheric formation conditions" often interact and cannot always be separated properly, as they can all have a positive or negative effect on passive layer formation. Injuries to the described passive layers and defects and defects in the passive layers resulting from the manufacturing process can advantageously be detected with the aid of the gel pad proposed here.

Although specific embodiments have been shown and described herein, it is within the scope of the present invention to appropriately modify the shown embodiments without departing from the scope of the present invention. The following claims represent a first, non-binding attempt to define the invention in general.

Reference symbol list

1

Aluminum sample

1a

anodized aluminum

1b

Aluminum with a conversion layer

2nd

Passive layer, passivation layer

3rd

Gel pad

4th

colored precipitation, color display

5

Violation of the passive layer or passivation

Claims (12)

Gel pad for testing the quality of a passivation layer of an aluminum or an aluminum alloy, comprising the components: - a hydrogel, - a protonating agent, and - an indicator which shows a color change with Al ³⁺ , the protonating agent and the indicator being homogeneously distributed in the hydrogel and the gel pad is monochrome transparent on a closed section of the passivation layer. Gel pad after Claim 1 , further comprising the component: - a corrosion stimulator, the corrosion stimulator being selected from an alkali and / or an alkaline earth chloride. Gel pad after Claim 1 or 2nd , the indicator being selected from: aluminon, chromazurol S and eriochrome cyanine R, mordant blue 3, alizarin, alizarin sulfosine sodium, quinalizarin, hematoxylin, morin, benzopurpurine, congo red and a gold sol. Gelpad after at least one of the Claims 1 to 3rd , wherein the hydrogel comprises: an agar agar, an agarose, a gelatin, a polyvinyl alcohol, a partially hydrolyzed polyvinyl alcohol, an acrylate, a Mowiol or a polyacrylamide. Gelpad after at least one of the Claims 1 to 4th , wherein the protonating agent and a concentration of the protonating agent in the hydrogel are adapted to set a pH value in a wet film of the gel pad between pH 2 and pH 6, in particular between pH 3 and pH 5. Gel pad after Claim 5 , wherein the protonating agent is selected from a single-proton or poly-proton, water-soluble, organic or inorganic acid, in particular from formic acid, acetic acid and citric acid. Gelpad after at least one of the Claims 2 to 6 , wherein the hydrogel comprises agar, the protonating agent is acetic acid, the indicator is aluminon, and the corrosion stimulator is NaCl, the concentration of said components in the gel pad being within a range of: agar agar: 2-5 g per 100 ml; NaCl: 0.005 mol / l to 1 mol / l; Acetic acid: 0.01 mol / l to 0.05 mol / l; Aluminon: 0.2 mmol / l to 0.4 mmol / l. Gelpad after at least one of the Claims 1 to 7 , wherein the gel pad has a circular shape with a diameter between 10 mm and 100 mm, more preferably between 15 mm and 50 mm and a thickness between 1 mm to 10 mm, for example 2 to 5 mm, in particular approximately 3 mm. A method for evaluating a passivation layer on an aluminum or an aluminum alloy comprising: Providing a gel pad according to at least one of the Claims 1 to 8th ; - Establishing and maintaining contact between the gel pad and the passivation layer and indicating a corrosion reaction by a color change in the gel pad; Quantifying corrosion of the aluminum or the aluminum alloy on the basis of a number and / or a diameter and / or an area of at least a section of the gel pad which has the color change. Rapid test to check the quality of the execution of passivation processes and / or the success of a passivation of a surface of a material comprising aluminum using a gel pad according to at least one of the Claims 1 to 8th . Use of a gel pad according to at least one of the Claims 1 to 8th for the identification of an alloy comprising aluminum, which is corrosion-resistant under specifically selected environmental conditions. Use of a gel pad according to at least one of the Claims 1 to 8th to optimize an aluminum alloy with regard to a composition of the aluminum alloy for a defined application scenario of a component, comprising the aluminum alloy.

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