GB2384782A

GB2384782A - A paint system comprising an agent that reduces the leach rate of an anticorrosive additive from the system

Info

Publication number: GB2384782A
Application number: GB0202365A
Authority: GB
Inventors: John Damian Smart; Glynn Potts
Original assignee: BAE Systems PLC
Current assignee: BAE Systems PLC
Priority date: 2002-02-01
Filing date: 2002-02-01
Publication date: 2003-08-06
Also published as: GB0202365D0

Abstract

A paint system which includes a paint, an anticorrosive additive that leaches from the system over time and an agent that reduces this leach rate in the presence of an organic acid. The paint system is particularly suitable for use in environments where resistance to microbiologically influenced corrosion is required, such as in aircraft fuel tanks, with the organic acid being a substance excreted from micobial growth. A method of reducing the leach rate of an anticorrosive additive from a paint system in the presence of microbial growth comprises determining a substance excreted from the microbial growth that increases the leach rate and adding an agent to the system to counteract the excreted substance. In particular, the anticorrosive additive is strontium chromate and the agent that reduces its leach rate is zinc oxide, titanium dioxide, aluminium oxide or calcium carbonate. A method of prolonging the biocidal effect of a paint system having an anticorrosive additive comprises adding the agent which reduces the rate at which the additive leaches from the system.

Description

A PAINT SYSTEM The present invention relates to paints that include in their composition constituents that act as corrosion inhibitors i. e. will act to slow down or prevent corrosion of a substrate to which the paint has been applied. In particular, the invention relates to paints that are specified for environments in which a resistance to microbiologically influenced corrosion may be required.

It is well known that a substrate may be protected from interaction with the environment by coating it with a protective coating such as paint. The paint is generally formulated to have a high ionic resistance to slow down or prevent ion movement and hence any corrosion reactions with the environment. Anticorrosive additives such as pigments are also often incorporated which will dissolve in any liquid layer that may form under the paint and act as a corrosion inhibitor.

In some environments, microbial growth i. e. the growth of fungii, bacterial or yeasts may be an issue. Such growth may result in rapid breakdown of the protective treatment and severe localised corrosion of the substrate. Such corrosion is termed Microbiologically Influenced Corrosion (MIC). in order to control or prevent MIC, water-soluble biocides or pigments which have biocidal or biostatal properties may be included in the composition of the paint.

In the aircraft industry, epoxy primers containing chromate pigments are generally used to protect aircraft structures, commonly made from aluminium alloys, from corrosion. Strontium chromate, zinc chromate and barium chromate, either individually or in combination, in epoxy primers has been the conventional protective treatment for over forty years. The chromates are intended to act as corrosion inhibitors but the leachate from chromated epoxy primers are also known to have biocidal properties. Any water film that forms on the surface of such primers should therefore contain sufficient dissolved chromate to prevent or control microbial growth for the lifetime of an aircraft, provided adequate maintenance procedures are carried out during this period.

However, it has been found that in some environments, particularly the inside of aircraft fuel tanks, microbial contamination can result in severe corrosion of the structure.

It is an object of the present invention to provide a paint system that offers greater resistance to MIC than conventional paint systems such as chromated epoxy primers.

Through testing and other investigations it has surprisingly been found that chromates have a higher leach rate from conventional aircraft primers into an aqueous solution when an organic acid is present than when the acid is not present. Such acids are produced during microbial growth by the Tricarboxylic acid cycle ; for example a fungus commonly found in aircraft fuel tanks, Cladosporium Resinae, (also known as Hormoconis resinae), was found to produce citric acid as its major waste product. The leached chromates are also rapidly reduced from their hexavalent to their trivalent state (chromic compounds) when the acid is present, particularly in the presence of readily oxidisable compounds, such as those present in aviation fuel and in paints, In contrast with chromates, chromic compounds do not have significant biocidal properties.

In the first aspect of the present invention there is provided a paint system including a paint and an anticorrosive additive that leaches from the system over time, having an agent that reduces the leach rate and reduction of the additive in the presence of an organic acid, from that of the said system without the agent.

The said agent may be in the form of an additive added to the conventional paint system or as a constituent in a top coat that is applied over a surface previously coated with a conventional paint. Preferably the agent would have act as a buffer in contact with the acidity produced by microbial growth.

The agent may also usefully have the following properties:

i) Unable to be utilised by microbes as a source of nutrition. ii) Stable as a constituent in an epoxy primer or as the major constituent in a top coat iii) Galvanically compatible with an aluminium alloy substrate. iv) Sparingly soluble in aqueous solutions v) Non-corrosive to an aluminium alloy substrate when in aqueous solution vi) Possess intrinsic anti-microbial properties The agent may be selected from; zinc oxide, titanium dioxide, aluminium oxide or calcium carbonate.

Zinc oxide is a well-known pigment for paints. It possesses all of the properties detailed above, being insoluble in water but soluble in acids and has a recognised ability to act as a mould growth inhibitor in paints.

The pigment may be one of the chromates that is commonly used in epoxy primers by the aircraft industry and as such the agent that leaches fron the paint system.

According to a second aspect of the invention there is provided a method of reducing microbiologically influenced corrosion of a substrate whereby the substrate is treated with a paint system as described above. The method may be used to reduce microbiologically influenced corrosion of aircraft structures such as fuel tanks.

According to a third aspect of the invention there is provided a method of prolonging the biocidal effect of a paint system having an anticorrosive additive, by the addition of an agent to the system that reduces the leach rate of the additive in the presence of an organic acid in comparison with the leach rate of the paint system without the agent. Preferably the agent acts as a buffer in contact with the organic acid. The agent may be selected from the following: zinc oxide, titanium dioxide, aluminium oxide or calcium carbonate.

According to a fourth aspet of the invention there is provided a method of reducing the leach rate of an anticorrosive additive from a paint system in the presence of microbial growth by determining a substance excreted from the microbial growth that increases the leach rate and then adding an agent to the paint system to counteract the substance excreted.

The invention will now be further explained by way of example only with reference to the accompanying drawings of which :- Figure 1 is a graphical representation of the leach rate of Strontium Chromate from Epoxy Primer over time.

Figure 2 is a graphical representation of the total amount of Chromate leached from Epoxy Primer at a given time.

Figure 3 is a graphical representation of the amount of Chromate reduced to chromic ions in three weeks.

Figure 4 is a graphical representation of the leach rate of Chromate in Citric Acid at pH 5.0.

Figure 5 is a graphical representation of the leach rate of Chromate in Citric Acid at pH 4.5.

Figure 6 is a graphical representation of the leach rate of Chromate in Citric Acid at pH 4.0.

Figure 7 is a graphical representation of the leach rate of Chromate in Citric Acid at pH 3.5.

Figure 8 is a graphical representation of the leach rate of Chromate in Citric Acid at pH 3.0.

Figure 9 is a graphical representation of the leach rate of Chromate in Citric Acid at pH 2. 5.

Figure 10 is a graphical representation of the leach rate of Chromate in Citric Acid at pH 2.0.

Figure 11 is a graphical representation of the comparison of leach rates in Citric and Hydrochloric Acids at pH 3.5.

Figure 12 is a graphical representation of the comparison of leach rate test results from coupon and fuel pipe samples (deionised water).

Figure 12 is a graphical representation of the extracellular acid production during growth of Cladosporium resinae in aviation kerosene.

A test programme was carried out to investigate methods of controlling microbiologically influenced corrosion (MIC). As part of this programme, the effectiveness of chromated epoxide primer was assessed by its influence on the sporulation and growth of cultures of Cladosporium resinae. Tests were also carried out to determine if there were some mechanism by which the effectiveness of the primer as a biocide was reduced due to the growth of Cladosporium resinae.

EXPERIMENTAL PROCEDURES Growth Medium For all of the tests undertaken, the growth medium used had the composition: tap water + O. 1g/litre ammonium nitrate + O. 1g/litre di-ammonium hydrogen phosphate (adjusted to pH 6 with sulphuric acid). Medium then sterilised by autoclaving.

Inoculant For all of the tests employed, the inoculant used was a growing culture of Cladosporium resinae in the growth medium defined above. The Cladosporium resinae had been removed from a contaminated aircraft fuel tank and cultured in the laboratory in the growth medium. The spore count was measured using the Miles and Misra drop count method.

ASSESSMENT OF THE BIOCIDAL EFFECTIVENESS OF EPOXY PRIMER CONTAINING STRONTIUM CHROMATE PIGMENT

Tests on coated specimens The tests were carried out on coupon specimens, approximately 50 x 40mm, of aluminium alloy 2014-T6. These had been alocromed and then coated with different thicknesses of chromated epoxy primer 37092 (manufactured by Akzo Nobel) : "Uncoated" (2014-T6) "Standard" (34 to 41 microns) "Thin" (12 to 17 microns) The effect of degradation due to loss of chromate by leaching out under prolonged immersion in water was also investigated by immersing a"thin" coated specimen in deionised water for three weeks prior to test.

The specimens were laid flat in individual glass jars which were approximately 100 mm diameter, 500 cm3 capacity. Fuel Jet A 1 was then saturated with the growth medium by vigorous agitation followed by draining off the excess growth medium. 200 cm3 of the saturated fuel plus 1 cm3 of inoculant was then added to the jar. After 60 days, the test was altered in that once per week approximately 3 cm3 of growth medium was sprayed on to the surface of the fuel using an aerosol. Excess growth medium at the base of the jar was removed with a pipette at the same time.

Tests on the effectiveness of strontium chromate"leachate"as a biocide Sections of aluminium alloy 2014-T6, coated with a standard thickness of epoxy primer 37092 were immersed in deionised water for approximately 15 days. The water gradually became a bright yellow colour as the strontium chromate pigment leached from the primer. The"leachate"was then drained off.

The level of chromate in the leachate was then estimated using spectrophotometry ie 1 cm3 of dilute sulphuric acid was added to 10 cm3 of the leachate to convert the chromate to dichromate. The absorbance peak at

I

350nm, corresponding to Cr (VI) was then compared with those of standards made using potassium dichromate solutions. A level of 34 ppm was determined in this manner.

The leachate was then diluted with different quantities of growth medium, maintaining the overall nutrient content at 0.1 g/litre, and the chromate levels determined for each level of dilution as:

100 cm3 of leachate 34.0 ppm chromate 50 cm3 of leachate + 50 cm3 of growth medium 17. 3ppm chromate 20 cm3 of leachate + 80 cm3 of growth medium 6.9 ppm chromate 10 cm3 of leachate + 90 cm3 of growth medium 3.4 ppm chromate 0 cm3 of leachate + 100 cm3 of growth medium 0.0 ppm chromate

Each of these solutions was then added to 30 cm3 of Fuel Jet A 1 which had been saturated with growth medium and contained 1 cm3 of inoculant. The samples were then stored for 40 days at 28 +2 C.

ASSESSMENT OF THE LEACH RATE OF THE STRONTIUM CHROMATE PIGMENT Tests on the leach rate in deionised water of the strontium chromate from the primer.

Tests were carried out on three samples : a) A"control"sample.

A sample of aluminium alloy 2014-T6, approximately 55 x 55 mm, coated with a standard thickness of epoxy primer 37092. b) A sample from a fuel pipe exhibiting MIC on the upper surface.

A section approximately 55 x 55 mm taken from an area apparently unaffected by MIC i. e. still possessing a bright yellow appearance.

c) A sample from a fuel pipe exhibiting MIC on the upper surface.

A section approximately 55 x 55 mm taken from an area apparently affected by MIC i. e. with a dull brown, blistered appearance.

Each of these samples were placed in separate glass jars, with the painted surfaces uppermost. 250 cm3 of deionised water was added to the jars, covering each specimen completely. After 1,2, 6,12, 21,28 and 64 days, the "leachate"was stirred and 10 cm3 was removed; 1 cm3 of dilute sulphuric acid was added to the removed sample of leachate and the chromate content was then measured using spectrophotometry as described in section 2.1. 2. The average leach rate, in g/m2/day was then calculated for each period.

Tests on the leach rate of the strontium chromate from the primer in dilute citric acid. deionised water and dilute hydrochloric acid.

Citric acid solutions were prepared with the following pH values : 2,2. 5,3, 3.5, 4,4. 5,5. The dilute hydrochloric acid solution had a pH of 3.5. Specimens 60 x 60 mm, painted on one side only with epoxide primer 37092, were immersed in 250 cm3 of the solutions. 10 cm3 samples were removed at intervals of 1,2, 5,24, 48,72, 168 hours, 336 and 504 hours. The level of chromate in each of the samples were then measured using spectrophotometry as described in section 2.1. 2.

There was a concern, particularly with some of the solutions at the lower values of pH, that some of the Cr (VI) may have been reduced to Cr (lit) before the level of chromate was measured. 0.8 g of ammonium persulphate, together with 0.5 cm3 of 3% silver nitrate solution to act as a catalyst, were therefore added to each of the samples. The samples were then boiled for ten minutes in order to oxidise any Cr (\lI) back to Cr (VI) and the measurements were repeated.

TESTS ON THE STABILITY OF THE CHROMATE Tests on the stabilitv in deionised water and in dilute citric acid Sections of aluminium alloy 2014- T6, coated with a standard thickness of epoxy primer 37092 were immersed in either deionised water or 3% citric acid and the colour changes noted with time i. e. the degree of yellow colouration due to leaching out of the chromate and the subsequent changes in colour due to reduction of the chromate to Cr (I H).

Tests on the stability in fuel and dilute citric acid To test the stability of the chromate in a simulated tank environment, the undiluted"leachate", produced as described in section 2.1. 2, was added to various mixtures of fuel Jet A 1 and dilute citric acid. The colour changes were noted with time:

100 cm3 of fuel Jet A 1 + 50 cm3 of leachate 100 cm3 of fuel Jet A 1 + 50 cm3 of leachate + 1 % citric acid 50 cm3 of leachate + 1% citric acid RESULTS ASSESSMENT OF THE BIOCIDAL EFFECTIVENESS OF EPOXY PRIMER CONTAINING STRONTIUM CHROMATE PIGMENT Tests on coupons coated with epoxy primer After 18 days of immersion in fuel saturated with growth medium there were numerous fungal deposits on the uncoated specimen, typically 2 mm diameter. In contrast, all of the coated specimens exhibited light brown spots.

After 60 days, the deposits on the uncoated specimen had almost doubled in size, but there was no evidence of corrosion occurring. The coated specimens

all displayed numerous brown spots, typically 7 mm in diameter, but there was no obvious growth.

The test was then altered, with weekly spraying of the surface of the fuel with growth medium. This had a noticeable effect in increasing the rate of growth on all of the specimens. Within ten days the coated specimens were generally covered in a light growth and there was a increase in the number of deposits on the surface of the uncoated specimen. Pitting on the surface of the uncoated specimen was observed after a further 50 days. The specimens with the standard thickness of primer appeared to exhibit the least growth.

After a total of 165 days test, small sections were cut from each coupon and microsections were prepared. Numerous corrosion pits were observed on the uncoated specimen. The largest of these was 215 microns deep. No corrosion was observed on any of the other specimens, even though it was noted that the"thin"coatings of primer (12 to 17 microns thick) contained numerous breaks in the coating, leaving a large number of small areas of the substrate exposed. There was therefore no evidence of microbiological corrosion on any of the microsections taken from the coated specimens. The only effect visible was light brown and dark brown staining of the surface of the primer.

Tests on the effectiveness of strontium chromate"leachate"as a biocide Within six days of the test, a large number of fungal colonies were observed at the fuel/growth medium interface in the samples with the 0.0 ppm and 3.4 ppm chromate levels. These colonies were typically 2 to 3 mm in diameter. The growth in the three samples with the higher levels of chromate was confined almost entirely to the growth medium and was typically 1 to 2 mm in diameter.

Within 18 days of the test, the growth in the unchromated sample was quite extensive and covered all of the fuel/growth medium interface. There was,

however, only slight growth observed at the fuel/growth medium interface in the samples with 3.4 ppm and 6.9 ppm chromate, and no significant growth in the samples with 17.3 ppm and 34.0 ppm chromate.

After 40 days at 28 + 2 C, the samples had the appearance summarised below : leachate containing 0.0 ppm chromate-massive growth at the fuel/growt medium interface leachate containing 3.4 ppm chromate-significant growth at the fuel/growth medium interface. No yellow colouration due to the chromate. leachate containing 6.9 ppm chromate-slight growth at the fuel/growth medium interface. No yellow colouration due to the chromate. leachate containing 17.3 ppm chromate-no significant growth. Slight yellow colouration due to the chromate. leachate containing 34.0 ppm chromate-no significant growth. Still retained a deep yellow colouration due to the chromate.

ASSESSMENT OF THE LEACH RATE OF THE STRONTIUM CHROMATE PIGMENT Tests on the leach rate in deionised water The measurements of leach rate of the strontium chromate from different samples of primer are shown in Figure 1. These show that the initial leach rates from the"control"sample of fresh primer and from the primer on the fuel pipe in an area apparently unaffected by MIC were very similar, reducing from over 0.4 g/m2/day to less than 0.014 g/m2/day after 64 days. The leach rate from the

primer on the fuel pipe in an area having a dull brown, blistered appearance was significantly lower, reducing from just over 0.1 g/m2/day to 0.003 g/m2/day after 64 days. It was noted that the leach rates from all three samples comfortably exceeded the specification values of an average of 0.03 g/m2/day between 7 and 10 days exposure and an average of 0.0007 g/m2/day between 29 and 36 days exposure.

Tests on the leach rate in dilute citric acid of the strontium chromate from the primer The total amounts of chromate leached from the primer were measured at intervals during the test, with the results are shown in Figure 2. The leach rates were initially faster in solutions of pH 2.0 and 2.5. The leach rates at increased pH values were significantly slower but, as the tests proceeded, the leach rate in the solution of pH 3.5 was faster than the leach rates in the solutions of pH 3.0 or 4.0 and above.

The average amount of chromate reduced to chromic ions in the 1,2 and 5 hour samples over the three week test period are shown in Figure 3. This displayed a similar trend to the leach rates, with reduction being fastest in the solutions of pH 2.0 and 2.5 ; the rate reduced significantly to an approximately constant level at pH values between 3.0 to 5.0. There appeared to be no significant reduction of the chromate in the deionised water. It was noted, however, that there was an increase in the rate of reduction at pH 4.0 compared with the rates in the other solutions of pH 3.0 and above.

The net effects of the two processes are shown in Figures 4 to 10, in which the total amount of chromate leached out of the primer and the total amount of chromate remaining after three weeks in solution are compared with the amount of chromate leached into deionised water.

At pH values of 5.0, 4.5 and 4.0, there was little difference in leach rate or degree of chromate reduction compared with deionised water. At pH 3.5,

however, there was a significant increase in the amount of chromate leached out of the primer. As this was combined with a slow rate of reduction of the chromate, this resulted in higher levels of chromate in solution compared with deionised water. At pH 3.0, a slightly lower amount of chromate leached out of the primer and was only slowly reduced, resulting in a slightly higher level of chromate in solution compared with deionised water.

The overall effect of the citric acid at pH values down to 3.0 therefore appeared to be relatively benign, but this changed at lower pH values. At pH 2.5, there was a significantly faster leach rate of the chromate, but the relatively rapid rate of reduction meant that the overall level of chromate in solution was less than that of the deionised water. At pH 2.0, the chromate leach rate increased even further, but the increase in the rate of reduction of the chromate more than counteracted this, giving very low levels of chromate in solution. The primer exposed to the citric acid solution of pH 2.0 would therefore have become the most rapidly depleted in chromate and the solution would have contained the least amount of chromate..

In order to determine if the leach rate of the chromate was merely a function of pH or was particularly affected by citric acid, tests were also carried out using a hydrochloric acid solution with pH 3.5. A comparison of the results for both acids at this pH is shown in Figure 11. The leach rate in the citric acid was significantly faster than that in the hydrochloric acid.

A comparison of these results with those of the leach rate tests on the fuel pipes detailed in section 3.2. 1 is shown in Figure 12. This shows that the leach rates from the control sample of fresh primer and from the primer on the fuel pipe in an apparently undamaged area were slightly higher than that from the specimen immersed in deionised water; the leach rates were significantly higher than that from the brown, discoloured area of primer.

TESTS ON THE STABILITY OF THE CHROMATE

Tests on the colour changes of the"leachate"in deionised water and in dilute citric acid The leach rate of the chromate from the sample immersed in the 3% citric acid was much faster than that from the sample immersed in the deionised water. Within two hours, the citric acid solution was a deep yellow colour, whereas only a faint yellow colouration of the water was visible. After five days, sufficient chromate had leached out into the water to give it a distinct yellow colour, but the citric acid solution had only a faint violet hue, resulting from reduction of the chromate to chromic ions. This reduction of the chromate implies that it is no longer acting as a biocide and so some fungal growth later occurred due to unintentional contamination.

Tests on the colour changes of the"leachate"in mixtures of fuel and dilute citric acid The leachate added to the fuel Jet A 1 maintained its yellow colouration, indicating that the chromate was stable in the presence of fuel on its own i. e. there was no significant reduction of chromate. The leachate added to the citric acid lost its colour over a period of approximately two weeks, indicating a slow reduction of chromate. However, the leachate added to the fuel plus 1% citric acid lost its colour within 30 hours, indicating rapid reduction of the chromate.

As for the previous test, this allowed fungal growth to occur later occurred due to unintentional contamination.

DISCUSSION THE BIOCIDAL EFFECT OF THE LEACHABLE STRONTIUM CHROMATE PIGMENT The biocidal effect of the leachable strontium chromate pigment was investigated using laboratory tests. Tests on coupons coated with epoxy primer showed that Cladosporium resinae could grow on the surface of the primer

within days of immersion. This appeared to contradict the results of the tests on the effectiveness of the strontium chromate"leachate"as a biocide. In these tests, leachate containing only 6.9 ppm chromate resulted in a significant reduction in growth, and 17.3 ppm completely prevented growth. It has also been reported previously that a level of 10 ppm chromate was required for biocidal effectiveness. However, this effect has been studied previously. The leach rate of the chromate was reported to fall off rapidly with time as the outer part of the primer became depleted in strontium chromate, thus allowing growth to occur. This was confirmed by tests on the leach rate of the chromate in deionised water, which showed that the amount of chromate leaching from an area affected by MIC was significantly lower than that from an adjacent, unaffected area (Figures 1 and 12).

It was considered that the effectiveness of the primer as a biocide could be further reduced in two possible ways: i) If the chromate exhibited a faster leach rate than expected, resulting in more rapid depletion in the primer ii) If there was some mechanism for breakdown of the chromate into a form that was ineffective in preventing growth (either in aqueous solution or in the primer).

Tests were therefore carried out to determine if the leach rate of the chromate could be influenced by the increase in acidity resulting from the growth of Cladosporium resinae. The extracellular acid production during growth of Cladosporium resinae on aviation kerosene has been investigated and the results are reproduced in Figure 13. These results indicate that citric acid and isocitric acid are the principal acids produced. It was therefore decided to carry out tests on the leach rate of the chromate in the presence of dilute citric acid. The results showed a much faster leach rate of the chromate in the presence of dilute citric acid, with the leach rate being pH dependent (Figure 2).

However, citric acid at pH 3.5 resulted in a higher leach rate than did

hydrochloric acid at pH 3.5 (Figure 11), showing that the effect was not only due to pH.

The chromate was also reduced to chromic ions in the presence of citric acid, with the rate of reduction also varying with pH (Figure 3). The net effect of citric acid alone was the most deleterious at a pH of 2.0 (Figure 10), resulting in the most rapid depletion of chromate from the primer and the fastest rate of reduction of the chromate in solution.

The colour variations in the leachate also indicated that there was also a reduction reaction occurring, with the bright yellow colour associated with chromium in the soluble hexavalent state transforming to the violet colour associated with the soluble trivalent state. This indicated that there may be some constituent of the primer which could be being oxidised.

A further test, assessing the effect of fuel and citric acid on the leach rate and rate of reduction of the chromate showed a synergistic effect on chromate.

The chromate leachate in contact with only fuel Jet A 1 appeared stable, maintaining its bright yellow appearance. The leachate in contact only with dilute citric acid lost its yellow colouration over a period of approximately two weeks, demonstrating a slow reduction of the chromate. In contrast, the leachate in contact with both fuel and dilute citric acid lost its yellow colouration within hours. The latter case could be considered to represent more closely the environment in a fuel tank in which there is fungal growth occurring and so could explain the poorer biocidal performance than expected of the chromated primer.

This synergistic effect could be attributable to the presence in the fuel of a readily oxidisable substance which, in combination with the increased leach rate of the strontium chromate from the primer and with the increased oxidising ability of the dichromate in an acidic solution, would result in rapid depletion and reduction of the strontium chromate from the epoxy primer.

Chromium, in its hexavalent state exists as chromium trioxide which dissolves in water to give a strongly acidic, red-orange solution :

This can be made to precipitate bright orange dichromate salts such as SrCr207 or, when made basic, the solution turns yellow and chromate salts such as SrCr04 can be obtained. This, like all soluble chromates, is toxic and will act as a biocide.

Both of the hexavalent species are powerful oxidising agents, especially in acidic solutions, where the dichromate ion, Cor207, is reduced to the chromic ion Cr :

The oxidising ability of the dichromate ion is strongly pH dependant i. e. it increases at lower pH values.

In basic solutions, Cr (VI) exists as the chromate ion, CrCaq), which is a much less powerful oxidising agent:

The reduced chromic compounds mostly exist in two forms: A violet form in hydrated crystals or in solution. This contains chromic ions Cor3', or probably [Cr (H20) 6] 3+ A green modification in which all or part of the Cr is present as a complex ion. In the green solutions, hydrolysis generally proceeds. With very weak acids, trivalent chromium forms complex salts in which it exists as highly stable anions.

These chromic compounds do not have biocidal properties.

It is therefore considered possible that the effect of the citric acid is to convert the strontium chromate to dichromate and hence significantly increase its oxidising ability. This results in more rapid reduction to the trivalent state in the presence of a readily oxidised substance in either the primer or the fuel.

The approximate composition of a typical epoxy primer, PR143, is:

n-butyl alcohol approximately 10% Xylene approximately 30% Strontium chromate approximately 15% Titanium dioxide approximately 15% Resin plus extender approximately 30%

This is mixed in the ratio of two parts primer to one part activator to one part thinner.

The composition of the activator, 143, is: Amine adduct

1 methoxy propanol 1 to 10% Butan-1-01 10 to 25% Xylene 25 to 30% The thinners contain:

4 methyl pentan-2-one 25 to 50% Butan-1-ol 25 to 50% Xylene 25 to 50%

Thus in each of these constituents, there is a high proportion of alcohols.

Although a large proportion of these, together with the xylene, are removed on curing, any remaining after curing would be readily oxidised. Amines can also be oxidised to aldehydes or ketones, and the ketones may undergo oxidative cleavage to carboxylic acids.

Constituents of kerosene which can be oxidised include : Aromatics These comprise approximately 17% of the composition of kerosene and can be oxidised to quinones or carboxylic acids.

Olefins

These comprise approximately 1 % of the composition of kerosene and can be oxidised to aldehydes and ketones.

Sulphur compounds Mercaptans and other sulphur compounds, such as H2S, can be oxidised to sulphonic acids. The specification limit for total sulphur in kerosene is 0.2%.

Fuel additives It is also possible that some of the fuel additives may be oxidised.

Additives to kerosene include gum inhibitors, anti-static inhibitors, anti-icing inhibitors, anti-corrosion inhibitors and metal deactivators.

It was therefore considered that the citric acid had two major effects on the performance of the chromate in resisting growth of Cladosporium resinae : i) Accelerating the leach rate of the chromate from the primer ii) In conjunction with a constituent of the primer and/or the fuel, increasing the rate of reduction of the Cr (VI) to Cr (III).

Organic acids are also known to be effective constituents of paint strippers and so the combination of these properties could have a major effect on the overall degradation of the primer in service.

It was concluded that the effectiveness of the chromated epoxy primer in preventing MIC was greatly reduced by the effect of citric acid produced during fungal growth, resulting in an increased leach rate and, combined with some readily oxidised constituents in the primer and the fuel, resulting in rapid reduction of the Cr (VI) to Cr (ill). Therefore, if this process could be slowed down or prevented, it should increase the effectiveness of the primer in preventing MIC.

It was therefore considered that a possible solution would be to introduce an agent that could have an effect on the leach rate or the chromate reduction reaction, either introduced as an additive to the epoxy primer itself or as a thin, semi-permeable coating on top of the primer. Such an agent would need to possess the following properties : i) Act as a buffer in contact with the acidity produced by microbial growth ii) Inability to be utilised by microbes as a source of nutrition. iii) Stable as a constituent in an epoxy primer or as the major constituent in a top coat iv) Galvanically compatible with an aluminium alloy substrate. v) Sparingly soluble in aqueous solutions vi) Non-corrosive to an aluminium alloy substrate when in aqueous solution It would also be desirable to have intrinsic anti-microbial properties.

CONCLUSIONS

DETERMINATION OF THE BIOCIDAL EFFECT OF THE LEACHABLE STRONTIUM CHROMATE PIGMENT IN THE EPOXY PRIMER The biocidal effect of the chromate in the primer was found to be less than that expected from tests carried out using chromate solutions. One reason for this was the rapid depletion of the chromate from the primer. The leach rate of the chromate from the primer was significantly increased in the presence of dilute citric acid, which is the principal acid produced during fungal growth.

However the chromate was also rapidly reduced in the presence of dilute citric acid and fuel, so reducing its effectiveness as a biocide.

Claims

Claims 1. A paint system including a paint and an anti corrosive additive that leaches from the system over time, having an agent that reduces the leach rate of the additive in the presence of an organic acid, from that of the said system without the agent.
2. A paint system as claimed in claim 1 wherein the said agent is a substance within a coating that is applied over the surface of the paint.
3. A paint system as claimed in any previous claim wherein the agent acts as a buffer in contact with the organic acid.
4. A paint system as claimed in any previous claim wherein the agent is selected from the following substances; zinc oxide, titanium dioxiide, aluminium oxide and calcium carbonate.
5. A paint system as claimed in any previous claim wherein the additive is a pigment.
6. A paint system as claimed in any preceding claim wherein the said pigment is chromate.
7. A paint system as claimed in any preceding claim wherein the paint system comprises an epoxy primer.
8. A paint system as claimed in any preceding claim wherein the organic acid, is produced as a waste product of microbial growth.
9. A paint system as claimed in claim 8 wherein the microbial growth is

Cladosporium resinae (Hormoconis resinae).
10. A paint system as claimed in any preceding claim wherein the paint system is exposed to fuel.
11. A paint system as claimed in claim 10 wherein the fuel is kerosene.

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12. A method of reducing microbiological influenced corrosion of a substrate whereby the substrate is treated with a paint system according to any preceding claim.
13. A method of reducing microbiological influenced corrosion of a substrate according to claim 12 wherein the substrate is part of an aircraft fuel tank.
14. A method of prolonging the biocidal effect of a paint system having an anticorrosive additive, by the addition of an agent to the system that reduces the leach rate of the additive in the presence of an organic acid in comparison with the leach rate of the paint system without the agent.
15. A method of prolonging the biocidal effect of a paint system as claimed in claim 14 wherein the agent acts as a buffer in contact with the organic acid.
16. A method of prolonging the biocidal effect of a paint system as claimed in claim 15 or claim 16 wherein the agent is selected from the following: zinc oxide, titanium dioxide, aluminium oxide or calcium carbonate.
17. A method of reducing the leach rate of an anticorrosive additive from a paint system in the presence of microbial growth by determining a substance excreted from the microbial growth that increases the leach rate and then adding an agent to the paint system to counteract the substance excreted.