SPECIFICATIONCorrosion inhibitors, method of producing them and protective coatings containing themThis invention relates to corrosion inhibitors suitable for incorporation into protective coatings, e.g. paints.
It is well known that certain anions, e.g. phosphate, chromate and benzoate anions have corrosion inhibiting properties and that compounds containing such species can be included in protective coatings.
The compounds are usually in the form of sparingly water-soluble salts. The coatings themselves have a limited permeability to water and it is believed that the mechanism of corrosion inhibition involves the gradual dissolving of the compounds in water releasing the anions as the active inhibitors. For such systems to be effective over a long period the solubility of the compound is particularly important. If the compound if too soluble, blistering of the coating may occur and the compound will be rapidly depleted; if it is insufficiently soluble the compound will be ineffective.
The present invention is concerned with corrosion inhibitors which depend for their effectiveness on ion exchange rather than solubility.
According to the present invention, a corrosion inhibitor comprises particles of an inorganic oxide having corrosion inhibiting anions chemically bound to the particles.
The preferred corrosion inhibiting anions are those mentioned above, i.e. phosphate, chromate and benzoate anions but other known corrosion inhibiting anions may also be used provided, as discussed hereafter, they are stable in acid media.
The preferred inorganic oxide is alumina. Other oxides which may be suitable include zirconia, iron oxides (Fe2O3 and Fe304), and tin oxide. Mixed metal oxides may also be suitable as may naturally occurring clays such as kaolinite. As is well known, particles of alumina and other oxides may be prepared which have a proportion of hydroxyl groups on their surface, e.g. the so-called activated aluminas of commerce used, inter alia, as packing for chromatographic columns.
It has been found that the hydroxyl groups can be replaced by contacting the oxide (e.g. alumina) with an acidic solution containing phosphate, chromate or benzoate anions. The uptake of anions tends to increase as the pH is decreased below p7, and at a relatively low pH, (e.g. a pH of from 2 to 5), uptake will occur at ambient temperature relatively quickly (contact time of, for example, up to 5 hours). Elevated temperatures are not damaging, however, and may be used if required, e.g. with benzoic acid to increase its solubility in water. The uptake of anions on the oxide can be measured by standard analytical technicals, e.g. x-ray fluoresence for phosphate or chromate anions, and carbon analysis for benzoate anions. The minimum uptake will depend on the proportion of replaceable hydroxyl groups and, clearly, oxides with a high proportion of such groups are preferred.Examples of suitable aluminas are the commercially available activated aluminas sold under the name "Camag" and defined as having a Brockman Activity I for chromatography, and F1 aluminas sold by the Aluminium Company of America.
Depending on the proportion of hydroxyl groups on the inorganic oxide it has been found that up to 5% wt of anion can be combined with the oxide (i.e. up to 0.7 millimoles/g). Since, as indicated above, the technique of ion-exchange is relatively simple the selection of preferred inorganic oxides and the treatments to give maximum uptake of corrosion inhibiting anions can be determined by simple comparative experiments. The preferred lower limit is 1% wt.
The corrosion inhibiting particles may be included in protective coatings and the present invention includes protective coatings containing corrosion inhibiting particles as described above. The protective coatings may be any of the known types of protective coatings based on film forming polymers or resins, e.g. paints, varnishes and lacquers. It may, in particular, be primer paints based on epoxy resins, vinyl resins, alkyd resins or chlorinated rubbers.
The corrosion inhibiting particles may act as a filler for the coating and may be included in relatively large amount of up to 40% wt, based on the composition to be applied and up to 80% wt based on the dry film weight.
Having regard to the quantity of anions which can be combined with the oxide as discussed previously it will be seen that the coatings may contain up to 4% wt of corrosion-inhibiting anions based on the dry film weight.
Preferably the quantity of corrosion-inhibiting anions is at the upper end of the range, preferred amounts of particles being 30-80% wt based on the dry film weight giving from 1.5 up to 4.0% wt of corrosion inhibiting anions.
When used in protective coatings the particles should be suitably small so as to remain in suspension in the composition before application and so as not to substantially affect the ease of application or the smoothness of the dry coating. Suitable particles sizes may be up to 100 micron diameter.
The corrosion inhibiting particles act to release the anion into solution by ion exchange with an anion which exists in the environment in which the particles are used. Thus the invention is particlarly useful for protecting structures in or above the sea, the sea providing chloride anions for exchange with the corrosion inhibiting anions. The structures will normally be metal structures and the corrosion inhibiting particles will normally be in a protective coating. Unlike present paints which act by the solubilisation of corrosion inhibiting salts, it is the permeability of water which controls the rate of release of the corrosion inhibiting ions. Thus the corrosion inhibiting anions will be preferentially released from the alumina in those areas where the desired barrier properties of the coating are weakest.
Particular structures which may be protected are the hulls and superstructures of ships, and rigs and platforms used for oil or gas exploration or production.
The invention may, however, have application for protecting structures on land where potentially corrosive anions may be present in the atmosphere, e.g. structures subject to atmospheres with relatively high concentrations of SO2, SO3 or Cl-.
In addition to control of the release of the corrosion inhibiting anions by control of the ion permeability of the protective coating, control may also be exercised by the type of anion and the type of oxide.
Thus, in otherwise identical conditions, it has been found that phosphate anions are released less easily than chromate anions which, in their turn, are released less easily than benzoate anions. There may also be differences in the rate of release as between different types of alumina.
The invention is illustrated by the following examples.
EXAMPLE 1Preparation of ion-exchanged aluminasThe aluminas used were activated aluminas sold under the designations F1 by the Aluminium Company ofAmerica and "CAMAG" M.F.C. Brockmann Activity I (Neutral) by Hopkin and Williams. The same treatment was given to both aluminas. The F1 alumina was in the form of 14-28 mesh granules and the "CAMAG" alumina was also in similar granular form. Chromate, phosphate and benzoate anions were combined with the aluminas as follows:1. 500 g of the activated aumina were treated with 1 litre of an aqueous solution of 40 g K2CrO4 at 25"C.
Concentrated nitric acid (approximately 40 ml) was added to give a pH of 3.5 after 2 hours stirring. The alumina was then separated on a sieve and washed thoroughly with distilled water.
2. 500 g of the activated alumina were treated with 1 litre of water and approximately 50 ml orthophosphoric acid (90%) at 250C to give a pH of 3.2 after 3 hours stirring. The alumina was then separated on a sieve and washed thoroughly with distilled water.
3. 500 g of the activated alumina were treated with 1 litre of a 70:30 water: isopropanol solutioncontaning 429 benzoic acid for 2 hours at 25go. The alumina was then separated on a sieve and washed thoroughly, first with 70:30 water:isopropanol and finally with distilled water.
4. The alumina used as a control was simply washed thoroughly with distilled water.
The amounts of anion incorporated by these treatments were 0.3 mmole/g chromate, 0.7 mmolelgphosphate and 0.4 mmole/g benzoate for the F1 alumina. For the "CAMAG" alumina the figures were 0.2mmole/g chromate and 0.6 mmole/g phosphate. The benzoate anion content was not measured but waspredicted to be 0.3 mmole/g.
EXAMPLE 2Preparation and testing ofpaintsThe chromate exchanged "CAMAG" alumina of Example 1, which had a maximum particle size f 100micron (after dry grinding in a porcelain ball mill for 9 hours), was incorporated into a paint by ball milling for15 minutes. Untreated alumina was also incorporated, in the same amount, into another sample of the samepaint as control.
The composition of the paint was:"Alloprene" R10 15g "Cereclor" 70 lOg "Cereclor" 42 5 9 Alumina 30 g Xylene 25g White Spirit 6 9 Alloprene R10 is a chlorinated rubber sold by ICI Limited. Cereclor 70 and 42 are chlorinated paraffins sold byICI Limited.
Paints 2a (chromate exchanged alumina) and 2b (un-exchanged alumina), were applied by brush topolished mild steel plates (15cm x 10cm size) ata thickness of 80 microns. When dry, each paint film wasscored through to the metal with diagonal lines and each paint tested by immersing the plates in salt waterfor 384 hours and by placing the plates in a salt spray cabinet conforming to ASTM B1 17173 for 360 hours.
After testing each paint film was assessed visually for corrosion using a scale from 1 (= little corrosion) to 5 (= severe corrosion). The results are shown in Table 1 below.
TABLE 1Salt waterPaint immersion Salt spray2a 1 12b 2 2It will be seen that the paint containing chromate anions had better corrosion inhibiting properties than the paint containing unexchanged alumina.
EXAMPLE 3Chromate, phosphate, benzoate and unexchanged F1 aluminas prepared as described in Example 1 were prepared for incorporation into paints by grinding in distilled water in a porcelain ball mill for 60 hours and drying for 16 hours at 100"C under vacuum. The maximum particle size was 10 m. The ground particles were incorporated into paints by ball milling for 16 hours. The composition of the paints was:"Synolac 76W" 309 36% Lead Octoate 0.25 g12% Cobalt Octoate 0.75 g"Nuodex Exkin" 2 0.15gSoya Lecithin 0.04 gAlumina (each ofsamples a to d) 21.1 g"Microdol" Extra 7.2 gWhite Spirit 5.0 gSynolac 76W is a long-oil alkyd solution in white spirit sold by Cray Valley Products Ltd. The lead and cobalt octoates are driers sold by Manchem Ltd.Nuodex Exkin 2 is an anti-skinning agent sold by Durham RawMaterials Ltd. Soya Lecithin is a pigment dispersant sold by BOCM Silcock. Microdol Extra is a micronised talc sold by AIS Norwegian Talc.
Paints 3a (chromate exchanged alumina), 3b (phosphate exchanged alumina), 3c (benzoate exchanged alumina) and 3d (unexchanged alumina), were brush-applied to polished mild steel plates (15 cm x 10cm size) at a thickness of approximately 35 cm. The plates were allowed to dry for one week at room temperature before testing by placing the plates in a salt spray cabinet (conforming to ASTM B1 17-73) for 336 hours.
Aftertesting each plate was assessed visually for corrosion as in example 2. The results are shown in Table 2 below:TABLE 2Paint Salt Spray3a 13b 13c 23d 5It will be seen that the paints containing ion exchanged alumina had better corrosion inhibiting properties than the paints containing unexchanged alumina.