GB2283492A - Protection of metal from rust or other corrosion - Google Patents

Abstract

A method of protecting a metal substrate against corrosion, comprising applying to the substrate a coating of a hardenable composition comprising a liquid carrier medium, halogenated rubber as binder, an acid acceptor, a filler, a curing or hardening agent and an accelerator, and allowing or causing the coating to harden. The invention also provides a composition comprising a solution of halogenated rubber in organic solvent, the rubber, prior to being dissolved in the solvent, being compounded with one or more inert inorganic fillers and optionally one or more pigments.

Description

PROTEcTION OF METAL FROM RUST OR OTHER CORROSION The present invention relates to protection of metal from rust or other corrosion. In particular, but not exclusively, the present invention relates to protection of steel substrates.

It is an object of the present invention to seek to provide means whereby metal substrates, in particular but not exclusively steel substrates, can be protected against rust or other corrosion especially when such substrates are subjected to flue gas desulphurisation (FGD) or similar harsh or aggressive chemical environments.

According to a first aspect of the present invention there is provided a method of protecting a metal substrate against corrosion, comprising applying to the substrate a coating of a hardenable composition (hereinafter referred to as "rubber-based composition") comprising, in a liquid carrier medium, halogenated rubber as binder, an acid acceptor, a curing or hardening agent and an accelerator, and allowing or causing the coating to harden.

The liquid carrier medium may comprise a solvent as referred to below.

The halogenated rubber is preferably a butyl rubber such as bromobutyl rubber.

The acid acceptor is preferably a metal oxide and may be red lead oxide, magnesium oxide or zinc oxide.

The curing or hardening agent may be ethylene thiourea.

The accelerator may be based on an amine compound. Suitable amine compounds include the condensation product of aniline and butyraldehyde, the condensation product of butylamine and butyraldehyde, hexamethylene tetramine and 3-methyl-thiazolidine-thione- 1.

The composition is preferably prepared shortly before application to the substrate by mixing together three components as described below.

The first or base component is a solution of halogenated rubber in organic solvent. Prior to being dissolved in the solvent the rubber is compounded with one or more inert inorganic fillers and optionally one or more pigments.

Suitable organic solvents include xylene, toluene, hexane, commercial rubber solvent and blends of nvo or more of such solvents. The solvent may also include one or more other solvents such as isopropyl alcohol and diacetone alcohol to facilitate spray application.

The inorganic filler or fillers may be selected from: (a) carbon black to impart good physical and mechanical properties to the hardened coating on the substrate; (b) mica and waxes to enhance water vapour permeability resistance of the hardened coating; (c) barium sulphate and silica as bulking fillers; (d) processing oils based on petroleum residue.

The rubber, filler and pigments are, after compounding together, mixed with the organic solvent to dissolve the rubber and the acid acceptor is then added.

The second component is a solution or dispersion of the curing agent and an inert elastomer. The second component may be prepared by adding the curing agent to the elastomer in a rubber mill, in which the curing agent and elastomer are milled together, and then dissolving or dispersing the resulting mixture in solvent. Suitable solvents or dispersion mediums for the second component are those mentioned above as solvents for first component.

Preferably the solvent or dispersion medium for the second component is the same as that used as the solvent for the first component.

The coating of the rubber-based composition may be applied to the substrate by brush and/or roller or by airless spray. Preferably the coating is applied by airless spray. It may be desirable to apply a primer to the substrate before applying the coating of the rubber-based composition.

Preferably the coating of the rubber-base composition has a thickness, when hardened. of at least 1 mm. Such a thickness may usually be attained by application of four coats of the composition.

The third component is the accelerator.

The rubber-based composition may be formulated so that the hardened coating has the following properties: (i) excellent resistance to permeation of water vapour over a significant temperature range and gradient; (ii) resistance to acid gas condensate vapour and vapour of such condensate; (iii) resistance to de-ionised water at temperatures up to 85"C; (iv) general all round chemical resistance as would be encountered in an FGD environment and similar harsh or aggressive chemical environments; (v) capability of being applied practically and economically e.g.

capability of providing adequate corrosion resistance when only two or three coats are applied and capability of being applied using commercially available cold or heated airless spray equipment.

In order to increase the coating performance characteristics it is preferable to heat the applied coating to a temperature of 80 to 1200C for a period of 16 to 24 hours.

The organic solvent in which the halogenated rubber is dissolved constitutes the carrier medium for the composition.

Coating of the filler with a silane may facilitate incorporation of the filler into the first or base component. Furthermore certain silanes can beneficially alter water absorption characteristics of the filler. A suitable silane for use in the present invention is B15-(3-(triethoxvsilvl)-propyl)-tetrasulphane.

The first or base component may comprise as binder, in addition to halogenated rubber. one or more other organic film-formers. The presence of such additional film-formers may impart to the composition specialised properties, e.g. suitability for use in a highly defined area. Suitable organic film formers include (a) paraffinic hydrocarbon polymers composed of long straight chain molecules having terminal unsaturation only, e.g.

polyisobutylenes; (b) phenolic resins e.g. heat reactive, oil soluble, alkyl substituted phenolic resins.

Such film-former (a) and (b) may be incorporated into the first or base component as solutions in any organic solvent e.g. xylene.

The paraffinic hydrocarbon polymers may be incorporated into the first or base component by blending them with the halogenated rubber on a rubber mill prior to incorporating the halogenated rubber into the base component.

According to a second aspect of the present invention there is provided a composition comprising or consisting of the first or base component as described above.

The invention is illustrated by the following Example.

Example The following ingredients in the amounts stated, were compounded on a rubber mill.

bromobutyl rubber, grade 2030 made by Polysar 8.10 parts by weight N550 carbon black 3.20 " " barytes (barium sulphate) 37.10 " " paraffin oil 0.80 " " " Vaseline (petroleum jelly) 0.80 " " The resulting compounded product was granulated and added to 51.25 parts by weight of xylene in a container under a high speed mixing machine. the high speed mixing machine comprises a rotating disc blade with a toothed edge and the blade is immersed in the xylene in the container. The granulated product dissolved in the xylene.

To the resulting solution are added the following ingredients in the amounts stated below.

Mica 5.0 parts by weight Micronised polyethylene wax 5.0 Thixotrope (Thixatrol GST) 0.5 Red lead dispersion 1.25 " " " The resulting product. obtained in an amount of 113 parts by weight, is referred to as the "base component".

The base component is. just before use mixed with firstly 0.5 parts by weight of a dispersion of ethylene thiourea and elastomer in xylene (the dispersion containing 30% bv weight of ethvlene thiourea and 10% by weight of the elastomer) and secondlv with 0.25 parts bv weight of the condensation product of aniline and butyraldehyde as accelerator.

A coating of the resulting composition may be applied to a substrate by brush and/or roller but the preferred method is bv airless spray. A final dry film thickness of 1 mm is desirable and this may be attained in four coats.

In order to maximise the coating performance characteristics it is recommended that the coating be subjected to temperature of 100 C for a period of 20 hours.

Tests were carried out on the coating to determine: (a) its water vapour permeability; (b) its changes in mass, hardness and appearance (of free film) when exposed to water and water vapour and acid condensate and acid condensate vapour; (c) its adhesion to a carbon steel substrate and its ability to protect the substrate from corrosion when subject to thermal cycling.

Details of the tests are given below.

(a) Water vapour permeabilitv Water vapour permeability of free films of the coating was determined in accordance with DIN 53122, Part 2.

The coating showed good resistance to water vapour permeability. The water vapour permeability of the coating at 40 C, was 0.10 g/cm3 dry.

(b) Exposure to water acid condensate and vapours thereof Exposure to water and water vapour and to acid condensate and acid condensate vapour was effected in accordance with DIN 53521. The tests were carried out on free films of the coating. Test conditions were as follows: Test temperature: 85"C Test medium: deionized water or acid condensate (Siemens SK2) with agitation in the liquid zone.

Test duration: 8 weeks with weekly evaluation The test medium was replaced after 4 weeks.

It was found that the coating tends to markedly absorb water (Rubbel test).

After 8 weeks testing with deionized water an increase in mass of about 15% occurred and after 8 weeks testing with acid condensate an increase in mass of about 14% occurred. There is a tendency for mass increase to stabilize after a test duration of about two weeks.

(c) Thermal cvcling Test specimens of carbon steel of dimensions 130 x 130 x 10mm coated with he coating of the above Example were used.

The test specimens were clamped tightly with the coating side towards a test container half-filled with test medium. The load on the coating was half in the liquid and half in the vapour zone. The back of the test specimen was exposed to room temperature (20 C).

Each test cvcle consisted of: - heating in 20 min. from 40"C to 85"C - holding for 5 hours at 85"C - cooling in 7 hours from 850V to 400C (12 hours 40 min) - holding for 20 min at 40"C Test medium: deionized water (conductivity = s 2 S/cm) Test duration: 30 cycles The test specimens subjected to thermal cycling are compared with a similar specimen not subjected to the thermal cycling in accordance with the following standard test methods: Results are given in the table below.

TABLE

before thermal after thermal after thermal cycling cycling liquid cycling zone vapour zone Dry film thickness [mm] to DIN 50981 max 1.38 1.44 1.40 min 1.30 1.36 1.30 mean 1.34 1.39 1.34 Hardness (Buchholz) to DIN 53153 32 29 30 Degree of gloss to DIN 67530 0.6 0.5 0.4 (60 measurement angle) Degree of blistering to DIN 53209 m 0/go m O/g 0 m O/g 0 Adhesion to DIN iso 4624 [MPa]2) x (3) = 1.53) x (2) = 0.9 x (2) = 1.0 100 Gc CB 100 57c 100 % CB Visual assessment to DIN 53230 Black, matt. Slight swelling, No cracks or very soft surface No cracks or blisters. No - RAL 7021 blisters. No corrosion blackish grey corrosion under the under the coating coating In the above Example, Thixomen may be used instead of Thixatrol GST as the thixotrope.

Thixatrol GST and Thixomen are reaction products of 12-hydroxy-stearic acid and ethylene diamine.

Claims (17)

1. A method of protecting a metal substrate against corrosion, comprising applying to the substrate a coating of a hardenable composition (hereinafter referred to as "rubber-based composition") comprising, in a liquid carrier medium, halogenated rubber as binder, an acid acceptor, a curing or hardening agent and an accelerator, and allowing or causing the coating to harden.

2. A method according to claim 1, wherein the liquid carrier medium comprises xylene, toluene, hexane or commercial rubber solvent.

3. A method according to claim 1 or 2, wherein the halogenated rubber is a butyl rubber.

4. A method according to any preceding claim, wherein the acid acceptor is a metal oxide.

5. A method according to any preceding claim, wherein the curing or hardening agent is ethylene thiourea.

6. A method according to any preceding claim, wherein the accelerator is an amine compound.

7. A method according to any preceding claim, wherein the composition is prepared by mixing together: a first or base component which comprises a solution of halogenated rubber in organic solvent to provide at least part of the liquid carrier medium, the rubber, prior to being dissolved in the solvent, being compounded with one or more inert inorganic fillers and optionally one or more pigments, and the acid acceptor; a second component which is a solution or dispersion of the curing agent and an inert elastomer; and the accelerator.

8. A method according to claim 7, wherein the first component is prepared by mixing the rubber, the filler and any pigment, after compounding together, with the organic solvent to dissolve the rubber and then adding the acid acceptor.

9. A method according to claim 7 or 8. wherein the second component is prepared by adding the curing agent to the elastomer in a rubber mill, in which the curing agent and the elastomer are milled together, and then dissolving or dispersing the resulting mixture in solvent.

10. A method according to any preceding claim, wherein the hardenable composition is applied to the substrate by brush and/or roller or by airless spray.

11. A method according to any preceding claim, wherein the applied coating is heated to a temperature of 80 to 1200C for a period of 16 to 24 hours.

12. A method according to any preceding claim, wherein the substrate is a steel substrate.

13. A composition comprising a solution of halogenated rubber in organic solvent, the rubber, prior to being dissolved in the solvent, being compounded with one or more inert inorganic fillers and optionally one or more pigments.

14. A composition according to claim 13, wherein the organic solvent comprises xylene, toluene, hexane or commercial rubber solvent.

15. A composition according to claim 13 or 14, wherein the halogenated rubber is a butyl rubber.

16. A method of protecting a metal substrate against corrosion, substantially as described herein in the Example.

17. A composition according to claim 13, substantially as described herein in the Example.