Field of the Invention

The present invention relates to a non-destructive chemical probe for the detection of corrosion and a method of detecting corrosion at a metal substrate, and in particular although not exclusively, to the detection of metal ions generated at the metal substrate.

Background to the Invention Metal corrosion represents a significant problem within the automotive and aviation industries. Corrosion, if undetected and untreated, leads to a considerable decrease in the integrity and strength of metal structures which in turn have significant economic and safety implications particularly within these types of industries.

One of the more primitive forms of corrosion detection is by visual inspection in which a person is required to visually inspect all parts of the metal structure. This type of detection method is disadvantageous for a number of reasons including for example the time taken to carefully inspect all parts of the metalwork including joints, welds and relatively inaccessible locations which make visual detection difficult. Additionally, it is extremely difficult to detect corrosion visually at a very early stage. Accordingly, a number of different types of inspection methods have been developed in an attempt to facilitate the ease of early corrosion detection.

US 2003/0068824 discloses a method of detecting corrosion involving the application of a removable corrosion-detecting substance that changes appearance in response to corrosion occurring on the surface of the metal structure. The indication method disclosed is cathodic reaction based in that cathodic reactions, that are integral to the corrosion process, increase the local pH at the point of corrosion. The detection coating is pH sensitive whereby

corrosion detection is indicated by an associated coating colour change in the region of pH change.

US 4,278,508 also discloses a method of detecting a cathodic corrosion site on a metallic substrate in which a pH sensitive fluorescent dye is deposited onto the substrate and configured to fluoresce in response to the hydrogen generated from the cathodic corrosion reaction.

Similarly, Sibi and Zong Progress in Organic and Coatings 47 (2003) 8-15 disclose a method using fluorescent probes for the determination of metal ions in corrosion processes of AL 2024T3. Fluorescent probes, including lumogallion and Phen Green™ are used being responsive to aluminum, magnesium and copper ions in aqueous solution and corrosion processes. The fluorescent probes are doped into epoxy/polyamide primers deposited on aluminum alloy surfaces. Fluorescence microscopy is then used to reveal localised corrosion.

An alternative inspection method is disclosed in US 4,044,253 which discloses a method for detecting cracks or defects in a composite skin bonded to a metal substrate. The method comprises the steps of applying a solution of 8- hydroxyquinoline in a solvent to the composite coating and then exposing the coating to ultraviolet radiation. The presence of defects or cracks in the coating is indicated by a fluorescent glow. The 8-hydroxyquinoline reacts with the metal substrate forming a metal chelate which fluoresces in response to exposure to ultraviolet light.

The inventors have identified a number of disadvantages with the detection method of US 4,044,253 including for example the method of application which can lead to contamination of the uncorroded metal substrate. Additionally, the time taken for the formulation to detect flaws in the coating is unacceptable in addition to the ease with which the formulation may be removed from the inspection area in turn providing a perceived lack of corrosion when in fact corrosion exists.

What is required therefore is a corrosion detection method and apparatus that addresses the above problems.

Summary of the Invention

The inventors provide an inexpensive corrosion detection probe and convenient method of corrosion detection at both a bare metal substrate and a coated metal substrate.

Through experimental investigations, the inventors have identified numerous problems with prior art early corrosion detection methods. In particular, by spray coating the metal substrate with the corrosion detection solution of the present invention the problems associated with conventional brush or roller coating methods are avoided. It has been discovered that these prior art application methods cause contamination of uncorroded areas of the substrate surface coated with a plastic or polymer based coating.

Through the choice of chelating agent and by the action of spray coating the detection solution onto the substrate an instantaneous detection system is provided in contrast to the detection method disclosed in US 4,044,253 which specifies a time period of between 10 seconds to 30 minutes to elapse after application of the detection formulation to enable faults in the coating to be detected. This significantly lengthens the time required for a corrosion investigation particularly where large structures are involved.

By including a binder within the detection solution/formulation configured to increase adhesion of the applied corrosion detection coating to the substrate, the detection formulation is capable of remaining in position at the substrate for a considerable period of time following the spray application. Therefore, once the substrate has been coated with the formulation of the present invention, the step of detecting corrosion using ultraviolet radiation may be carried out at some later

stage that may be more practical and convenient whilst still enabling reliable corrosion identification.

According to a first aspect of the present invention there is provided a method of detecting corrosion at a surface of a metal substrate, said method comprising: spray coating a portion of said surface of said metal substrate with a solution comprising a binder and a metal ion chelating agent capable of fluorescence; and exposing the coated metal surface to ultraviolet radiation.

The chelating agent may comprise any one or a combination of the following: alizarin; lumogalliol; morin; o.o'-dihydroxyazobenzene (dhab); 8- hydroxyquinoline; 8-hydroxyquinoline-5-sulphonic acid hydrate (8-HQS).

The binder compound configured to provide adhesion of the detection solution to the metal substrate is preferably a polymer compound in particular polyvinylalcohol. The binder is configured to increase the viscosity of the corrosion detection solution, thereby inhibiting removal once applied to the substrate. The binder is selected to provide the required viscosity whilst permitting the solution to be applied to the substrate in the form of a particulate spray or mist. The binder may alternatively include a natural or synthetic gum.

Preferably, the solution comprises a composition of 0.5-1.5 weight % of the chelating agent and 3.5-5.5 weight % of the binder.

The present method and apparatus for corrosion detection is particularly suitable for use with aluminum based substrates and for the chelation of trivalent aluminum ions.

The solution of the present invention comprises a solvent suitable for solvating the chelating agent wherein the solvent is preferably dimethylsulphoxide. Preferably, the solution is allowed to dry or partially dry after application on the substrate surface prior to exposing the coated metal surface to

ultraviolet radiation. However, the present invention is equally suitable for the instantaneous detection of corrosion immediately after spray application and exposure to ultraviolet radiation.

Preferably, the formulation solution of the present invention is applied to the substrate in aerosol form according to conventional aerosol spray techniques.

According to a third aspect of the present invention there is provided a corrosion indicator formulation configured to indicate the presence of metal ions generated at a surface of a metal substrate, said formulation comprising: a metal ion chelating agent configured to chelate with metal ions generated at said substrate and to fluoresce in response to exposure to ultraviolet radiation; and a binder configured to inhibit removal of said chelating agent from said substrate following a spray coating of said formulation onto said substrate.

Brief Description of the Drawings

Fora better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which:

Figure 1 herein shows an aluminum test panel exposed to UV light indicating corrosion where a protective film has been scored by a scalpel.

Detailed Description

There will now be described by way of example a specific mode contemplated by the inventors. In the following description numerous specific details are set forth in order to provide a thorough understanding. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the description.

The present invention provides a corrosion detection probe suitable for investigating corrosion at metal substrates and in particular large metal structures particularly within the aviation and automotive industries. The detection probe, in the form of a solution, is capable of being sprayed onto the metal substrate in the form of a temporary coating. According to further specific implementations of the present invention the detection probe may be formed as a permanent or semipermanent coating, in the form of a paint. Both the rudimentary 'spray applied' coating and the corrosion detecting paint are configured to fluoresce when exposed to ultraviolet radiation and, in the presence of metal ions generated at the metal substrate resulting from the corrosion process.

Studies reported below include investigations of the ion transport properties and light transmission of three polymer coatings applied to an aluminium based substrate. Fluorescence measurements in solution were carried out in addition to the preparation and investigation of aluminium test panels in which a polymer coated aluminium surface was first scratched and then exposed to salt solution so as to induce corrosion.

Example 1

The corrosion detection formulation configured specifically for corrosion detection at aluminium based substrates comprises 8-hydroxyquinoline-5- sulphonic acid hydrate (8-HQS). 8-HQS is capable of chelating with trivalent aluminium ions to form a complex having a maximum excitation wavelength of 367 mn, an emission maximum at 395 nm and a stability constant pK of 20.3.

8-hydroxyquinoline-5-sulphonic acid hydrate (8-HQS).

Studies of ion transport and light transmission

Polyester samples were prepared from a polyester based resin in styrene with peroxide catalyst P2 in the ratio 197:3 by weight. Epoxy samples were prepared from an epoxy based resin cured with an amine hardener in the ratio 69:31 by weight. Polyurethane samples were prepared from a water based polyurethane allowed to dry in air. Films of each polymer were prepared by spreading the polymer on a PTFE base using a draw-bar, and removing the films when hardened. Light transmission was measured at various wavelengths in a UV/visible spectrophotometer. Ion transport was measured in a container fashioned from polycarbonate, with two wells joined by a tunnel and with the sample placed in the tunnel to form a barrier between the solutions in each well. One well was filled with the test solution and the other with de-ionised water. The increase in ion concentration of the de-ionised water was measured as a function of time as ions passed through the polymer film.

Fluorescence measurements

A series of aqueous solutions were made from dilutions of a stock solution of aluminium nitrate mixed with an equimolar solution of 8-HQS (supplied by Lancaster Synthesis). These solutions were scanned in a Shimadzu single- beam fluorimeter.

Preparation of aluminium test panels for corrosion

A solution in dimethyl sulfoxide was prepared by dissolving 1% by weight of 8-HQS and 4% by weight of polyvinyl alcohol (Aldrich Chemicals) at 70 ⁰ C. The solution was allowed to cool and kept in a stoppered bottle. A thin film was deposited on a standard aluminium test panel using a draw-bar set at 100 microns gap. The solution was allowed to dry in air for 48 hours and then further drying carried out at 60 ⁰ C in an oven. The aluminium surface had an even, yellow colouration. The surface was then coated with epoxy resin to provide a protective coating for the bare aluminium substrate. The exposed edges were coated with beeswax and the film was scored with a scalpel to provide a corrosion site and the panel exposed to a salt spray. The test panel was visualised using a hand-held UV lamp with a maximum output centred at 373 nm.

Studies of ion transport and light transmission.

The transmission of light, necessary for inspection of painted surfaces using UV sources, shows considerable variation across the three polymer types chosen. Table (1 ) gives the percentage transmission at wavelengths selected to represent excitation and emission wavelengths of fluorophores of interest.

Table 1. Light transmission of paint films at selected wavelengths.

The aluminium complex of 8-HQS has an excitation maximum at 360 nm and emission maximum at 495 nm. From a comparison of absorbances it can be shown that an epoxy film could be about 4 times as thick as polyester or polyurethane for similar sensitivity. Initial studies of ion transport in which 0.1 M NaCI solutions were separated from deionised water by films of 600 microns were conducted and the deionised water tested for the presence of chloride using silver nitrate solution (0.01M). After 5 days exposure of an epoxy film there was the unequivocal presence of chloride in the receiving chamber and after 12 days a heavy precipitate was registered. The polymer had distorted into a domed shape due to the ingress of salt. Polyurethane gave a similar result but polyester showed no distortion after 12 days exposure and chloride was not detected in the receiving chamber. This would infer that polyester coatings would be the most effective to inhibit corrosion. However for the study of fluorescent probes, the polymer of choice is epoxy due to its good light transmission and ion transport characteristics.

Fluorescence measurements of 8-HQS

The fluorescent output of the aluminium complex of 8-HQS is shown in Table 2 below. In the absence of aluminium, 8-HQS is only weakly fluorescent at 0.01 M concentration.

Table 2. Concentration dependence of 8-HQS fluorescence

The semi-quantitative data in Table 2 show the detection limit and linear response range of the fluorescent probe in solution.

Corrosion of aluminium test panels

Figure 1 herein shows an aluminium test panel prepared in accordance with the preparation procedure detailed above. The region of the test panel scored with a scalpel is identified by the lightly shaded horizontal line positioned just above centre of the test panel. The salt solution inducing corrosion at the scored region also induced corrosion at various regions of the test panel, being represented by circular light spots. These circular regions correspond to bubbles formed in the epoxy film that allow penetration of the corrosive species through the film in contact with the aluminium substrate.

Figure 1 shows the test panel illuminated by UV light whereby corrosion is easily identified by the lightly shaded regions according to figure 1.