NZ202685A - Corrosion inhibiting composition and paint - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £02685 202 HUE COOT Priority Date(s): ''''I'x'iok's Complete Specification Filed: ■ ess: Publication Date: P.O. Journal, No: "ti'nioi86[" HO QUA NEW ZEALAND THE PATENTS ACT , 1953 COMPLETE SPECIFICATION "ANTI-CORROSION PAINT" WE, INTERNATIONAL STANDARD ELECTRIC CORPORATION, a Corporation of the State of Delaware, United States of Americaa of 320 Park Avenue, New York 22, New York, United States of America, hereby declare the invention, for which we pray that a; patent may be. granted to us^ and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to coating compositions for inhibiting corrosion of a metal surface to which they may be applied and to processes for the preparation and use of the compositions.

.One of the major problems involved in the use of metals as structural materials is that of corrosion of the metal, • ferrous metals being particularly susceptible. The mechanism of corrosion is incompletely understood, but it is well known that the process is accelerated under hostile conditions, typically in industrial and marine environments. The standard technique for reducing corrosion is to apply to the metal surface a primer coating containing one or more corrosion inhibiting materials. Such primer coatings generally comprise a resinous binding medium in which finely ground pigments are dispersed, the purpose of these pigments being either to provide opacity and colour or to provide corrosion inhibition, these latter being known as active pigments. Commonly used active pigments include red lead and calcium plumbate. Zinc chromate is also employed as a corrosion inhibitor, but it does not possess the level of performance of the lead pigments and can also cause colour bleeding of a subsequent paint coat.

More recently zinc phosphate has been employed as a nontoxic alternative to lead and chromate pigments. Compositions employing this material are described in U.K. Patent Specifications Nos. 904,861. and 915,512. It is claimed that this material is almost as effective as the previously employed toxic pigments, but its performance is poor in certain binder media and under conditions of exposure where the atmospheric sulphur dioxide level is low, typically marine conditions.

The aforementioned U.K. Patent Nos. 904,861 and 915,512 also describe the use of calcium phosphate (tricalcium phosphate, calcium hydrogen phosphate and mono-calcium dihydrogen phosphate) which avoid the toxicity problem experienced with some zinc based paints. However these calcium salts do not possess the optimum values of water solubility and pH for effective corrosion inhibition. Also it will be clear that, as the compounds are stoichiometric, these properties are not subject to control.

Other commonly used corrosion inhibitors include zinc metal, chromium oxide, and lead chromate, strontium chromate. Generally corrosion inhibitors fall into the classes of chromate, plumbates, phosphates, molybdates and tungstates.

A particularly important group of corrosion inhibiting material are those provided by water soluble glass compositions.

The specifications of UK Patent 1,604,3 83 and Australian Patent Nos. 541,638 and 542,583 describe the use, as anti-corrosion materials, of various zinc/phosphorus oxide and calcium/phosphorus oxide glass pigments. These materials are more effective than the conventional zinc orthophosphate pigments in that they provide zinc or calcium and phosphate ions at predetermined optimum rates and ratios under a variety of corrosion conditions.

It will be appreciated that the effectiveness of any corrosion inhibitor material is influenced considerably by the pH of the local aqueous environment in which it is disposed. It has been found that the efficiency of a corrosion inhibitor may be optimised by control of the pH of this environment to a value at which the inhibitor provides its optimum inhibition effect. In particular it was found that where such a material forms the active constituent of a paint composition the corrosion inhibition effect of the active constituent can be optimised by suitable selection of the nature and proportion of the other constituents of the paint composition.

According to one aspect of the invention there is provided a corrosion inhibiting composition comprising a corrosion inhibitor together with one or more additives, in which the composition is such that the pH of an aqueous solution produced by interaction between the composition and an aqueous environment is below pH7.

According to another aspect of the invention there is provided a corrosion inhibiting paint composition comprising a resin vehicle and a water soluble corrosion inhibiting pigment dispersed in the resin together with a blend of additives, in which the composition is such that the pH of the aqueous solution produced by the interaction between the paint and an 202685 /"""s 10 aqueous environment is below pH 7.

The term 'additives' as employed herein is understood to include such materials as extenders, further pigments and the like.

It is well known to those skilled in the art, whose corrosion inhibiting compositions are incorporated in paints, that paints should be formulated such that no adverse reactions occur between the various constituents otherwise the properties of the paint may be adversely affected, for example the paint may thicken up or gel. The various paint constituents should therefore be selected with this consideration in mind.

The following description is directed towards corrosion inhibiting compositions wherein the active constitutent is a water soluble glass. However the techniques described herein are equally applicable to corrosion inhibiting compositions wherein the active material is non-vitreous. Furthermore, it will be appreciated that although the techniques are immediately applicable to paint compositions they may also be used with other corrosion inhibiting devices, e.g. monolithic composite blocks.

In the case of paints containing water soluble corrosion inhibiting glass pigments, it may, under certain circumstances, 2.026 8 5 be desirable to formulate compositions such that the inhibiting species generated when the pain film comes into contact with water should react with one or more' of the other constituents of the paint film, e.g. where relatively highly acidic glass anticorrosive pigments are used. Alkaline extenders, such as dolomite, may be incorporated in order to buffer or compensate for the relatively high acidity of the water soluble corrosion inhibiting glass.

However if reactive alkaline materials are used in this way the corrosion inhibiting properties of a rather less acidic glass pigment may be diminished or lost altogether. In order to ensure that these corrosion inhibiting properties are not reduced, it is necessary, to formulate the cesins, pigments, extenders and additives, together with the water soluble corrosion inhibiting glass pigment with such constituents , in such proportions that the pH of the aqueous solution produced in an aqueous environment by interaction of the aqueous medium with the glass pigment and other constituents is less than 7 and preferably less that 6. Typically this pH value should be greater than 3.

The ultimate solution pH of the combination of the glass material and the other paint constituents is measured by the "following technique: lOg of the water soluble anticorrosion pigment is mixed together with such other solid materials as are present in the paint with 1 litre of distilled water and allowed to stand at 20° for 24 hours. The pH of the aqueous phase is then measured.

It is important to be able to adjust and control the dissolution irate of the glass pigments in order to optimise other corrosion inhibiting properties. It has been found that when the pH of the aqueous environment in contact with glass pigment is made more acid the dissolution rate is increased (see Table I). For the determination of solution rate of the pigment lg thereof having a particle size in'-.the range. 500 to 710 microns was dispersed in deionised water at 25°C for one hour. The water was adjusted in pH using phosphoric acid and the corresponding dissolution rates were measured.

Table 1 pH Dissolution rate 4 0.14mg/cm^/hr .5 0.067mg/cm^/hr It is therefore desirable to adjust the other^ingredients of the paint such that the pH of the agueous environment generated by the various constituents of the paint is either more acid or at least not significantly more alkaline from that -present if the glass pigment alone was in contact with v/ater. However in dissolving, these glasses generate acid species and it has been found that under certain extreme conditions D 20268 of pH these glasses cause dissolution of a small thickness of metal from the surface to which the paint is applied (Drake 80). It is therefore important that the total paint composition should not generate such acidic species as would 5 result in significant attack of the metal substrate. For this reason the pH of the paint composition as defined should not be less than 3 and desirably not less than 4.

Examples of resins that can be employed for preparing the paint compositions described herein include alkyds, epoxy 10 resins, acrylics, chlorinated rubbers, vinyl copolymers or other conventional resins or binders.

Examples of pigments and extenders include barytes, yellow iron oxide, red iron oxide, rutile, talc and china clay.

In particular we have found that anti-corrosion paint composition incorporating yellow iron oxide as an additive show enhanced performance over similar paint compositions lacking this material. The loading of yellow ion oxide in the paint may comprise 1 to 50 weight % of the total non-20 volatile material and preferably 20-40 weight %. Preferably the principle extender is barytes, china clay, or a mixture thereof.

A variety of corrosion inhibiting glass compositions may be employed. Typically phosphate glasses are employed such 25 as those described in the aforementioned U.K. and Australian Patents. Typical of the glasses that we employ are those incorporating phosphorous pentoxide as a glass former and one or more Group IIA or IIB metal oxides as glass modifications.

To illustrate the techniques described herein three paint compositions were prepared, the first and second (Compositions A and B) having balanced constituents and the third (Composition C) for comparison purposes having unbalanced constituents. The compositions are set out in Table 2 below. All proportions are by weight percentage.

Table 2 Composition Composition Composition C Vinyl chloride-vinyl ether copolymer Hordaflex LC 60 Acronal 4F Yellow iron oxide (Bayferrox 1420) Rutile (Tioxide RTC4) Talc (Microtalc AT extra) Dolomite (Microdol Ester) Glass Pigment Bentone paste "(10% NV) Xylene Butyl Acetate A 22. 96 2.29 .41 B. 23. 8 2. 32 2.7 2.7 11.03 23.56 2.35 .24 .20 3.83 27.55 12.76 6.02 4.22 47.8 13.09 10. 47 3.93 28.27 13.09 202685 tJT Two glasses were used in these evaluations. They had the following molar- compositions: Table 3 I ZnO fl CaO Glass 1 I34.4%| 2 8.3% 3 37.4% Glass 2 1 49.2% | 20.3% | 30.5% The combined constituent pH was measured as previously described for each paint.

The paints were applied each to a steel panel at a film thickness of 100 microns, allowed to dry and then scribed through to bare metal. The panels were exposed to salt spray in accordance with ASTM Bli7. They were examined after 200 hours for the degree of rusting occurring in the scribe region. The results of the pH measurements and salt spray tests are summarised in Table 4.

Table 4 Composition G Glass 1 '""A B . c • pH .9 6.9 • 7.05 rusting very slight rusting moderate rusting bad rusting Glass 2 ' pH 6.6 8.1 rusting slight rusting very bad rusting

Claims (15)

202 These examples illustrate the feasibility of preparing corrosion inhibiting paints using the techniques described herein and also■illustrate the efficiency of the techniques fox* enhancing or optimising the corrosion inhibiting action of an inhibitor material. The techniques described herein optimise the performance of an active paint pigment and may thus allow a reduction in the loading of that pigment in a paint composition. Where an expensive pigment is employed a significant cost-saving may be obtained. The paints described herein.may be employed in a variety of corrosion prevention applications. Typically they may be applied to freight containers or to vehicle bodies. ;-'v * V'1**"- | ^ j"\ — \ I fi"*!^**1, 'nf~*Tr ~Vla)t"liiT.liji|itin~i 1 1 ■ J*. - 202685 What we claim is -

1. A process for inhibiting corrosion of a substrate in a particular aqueous environment comprising forming a composition of a corrosion inhibitor together with one or more additives the composition being such that the pH of a solution of ten grams of the composition in one litre of distilled water when tested after the solution has been allowed to stand at 20°C for 24 hours is below pH7, and applying the composition as a coating to the substrate or as one or more monolithic composite blocks connected to the substrate.

2. A process as claimed in claim 1 in which the corrosion inhibitor is a water soluble glass, the glass including phosphorus pentoxide as a glass former together with one or more Group IIA or IIB metal oxides as glass modifiers.

3- A process for inhibiting corrosion of a substrate as claimed in claim 1 or claim 2 as herein described with reference to the examples.

4. A composition for inhibiting corrosion in a process as claimed in any one of claims 1 to 3, the composition comprising a corrosion inhibitor together with one or more additives, in which the composition is such that the pH of an aqueous solution produced by interaction between ten grams of the composition dissolved in one litre of distilled water when tested after the solution has been allowed to stand at 20°G for 24 hours is below pH7, and wherein the corrosion inhibitor is a water soluble glass. 202685

5- A composition as claimed in claim 4, wherein said glass includes phosphorus pentoxide as a glass former together with one or more Group IIA or IIB metal oxides as glass modifiers.

6. A paint composition for inhibiting corrosion of a substrate the composition comprising a resin vehicle and a corrosion inhibiting pigment dispersed in the resin together with a blend of additives, in which the composition is such that the pH of the aqueous solution produced by the interaction between ten grams of the pigment and one litre of distilled water when tested after the solution has been allowed to stand at 20°C for 24 hours is below pH7, and wherein said pigment is a water soluble glass.

7. A paint composition as claimed in claim 6, wherein said pigment includes a chromate, a phosphate, a plumbate, a molybdate or a tungstate.

8. A paint composition as claimed in claim 6 including a resin vehicle, a corrosion inhibiting water soluble glass pigment dispersed in the resin, and yellow iron oxide comprising 1 to 50% by weight of the total non-volatile material.

9- A paint composition as claimed in any one of claims 6, 7 and 8 wherein said glass includes phosphorus pentoxide as a glass forming oxide together with one or more Group IIA or IIB metal oxides as glass modifiers.

10. A paint composition as claimed in any one of claims 6 to 9, which includes 20 to 40 wt % yellow iron oxide. 202685

11. A paint composition as claimed in any one of claims 6 to 10, wherein said additives include barytes, red iron oxide, rutile, china clay, talc or mixtures thereof.

12. A paint composition as claimed in any one of claims 6 to 11, wherein the principal extender is barytes, china clay, or mixtures thereof.

13. A paint composition as claimed in any one of claims 6 to 12, wherein said solution pH is between 3 and 6.

14. A corrosion inhibiting paint composition as claimed in claim 8, and substantially as described herein.

15. A vehicle body or a freight container coated with a paint as claimed in any one of claims 6 to 14. INTERNATIONAL STANDARD ELECTRIC CORPORATION P.M. Conrick Authorized Agent P5/1/1466 i PAT&rf off icg [ -2 MAY 1936