GB2118977A - Corrosion inhibiting coatings for non-ferrous metal surfaces - Google Patents

Abstract

The coating incorporates a water soluble glass having corrosion inhibiting properties. Typically the coating comprises a paint incorporating a water soluble phosphate glass. In a preferred application of the process a steel surface is coated with a layer of zinc to which the corrosion inhibiting paint is applied.

Description

SPECIFICATION Corrosion inhibiting coatings This invention relates to compositions for inhibiting corrosion of a metal surface and to paint formulations containing such compositions. The invention also relates to metal articles or metallic structures provided with such corrosion protection.

One of the major problems involved in the use of metals as structural materials and in fabricated articles is that of corrosion of the metal. Ferrous metals are particularly susceptible. Other metals may be used as alternatives to steel, such as aluminium or employed as protective layers to protect steel.

Examples of materials which may be used in this way include zinc, aluminium, and cadmium.

Zinc coatings can be employed, for example, to protect steel from rusting. Zinc is used in this way as it offers relatively good atmospheric corrosion resistance and because it is anodic to steel can provide sacrificial protection. These coatings can also be used to prevent accelerated corrosion when two dissimilar metals, e.g. copper and stainless steel, are in contact.

Under very mild corrosive conditions many non-ferrous coatings have a long life and it may not be necessary to treat them further. When however the coatings are subjected to more aggressive environments such as industrial or marine conditions corrosion is accelerated. This is particularly the case with strongly electrodepositive coatings, e.g. zinc, when there are discontinuities in the film either as a result of porosity inherent in the method of application or because of coating removal by abrasion or corrosion, then the steel may be exposed, in which case sacrificial corrosion of the coating may occur.

It is important that the coating be preserved from corroding so that it may continue to give protection by physical exclusion. The life of a metal coating may be prolonged by surface treatment, e.g. with chemical conversion coatings such as phosphate coatings, but the corrosion protection provided by such coatings, without a sealing treatment is of a low order. Alternatively or in addition one or more coats of paint can be applied.

The mechanism of corrosion of zinc is incompletely understood but it is thought that in clean moist atmospheres the metal corrodes to form mainly basic zinc carbonate which acts as a partly insulating barrier between zinc and the atmosphere. Under mild atmospheric conditions the corrosion rate is low. However under aggressive conditions such as in industrial or marine environments the corrosion rate can be much higher.

Steel fabricated articles such as car bodies and components and steel structures often require very long term protection and in aggressive conditions a composite coating of paint over zinc may be used. In addition even under less agressive conditions one or more coats of paint may be required to provide a decorative finish to articles which have been zinc coated.

Problems may be encountered in the adhesion of the paint to the metal if the surface has not been suitably prepared.

Paint failures on galvanised steel are frequently adhesion failures of the total system applied.

Even with well prepared surfaces there is frequently loss of adhesion of paints especially under more aggressive conditions, e.g. high humidity. This loss of adhesion may be manifested as blistering or flaking or other related film deficiences. In addition unsightly white deposits may appear on the surface of the paint.

Even in the absence of oxygen, zinc can react with water to form zinc hydroxide and hydrogen and in humid conditions corrosion products can form underneath the paint coating. This process may account for the loss of paint adhesion. Also if the paint film becomes damaged or contains discontinuities corrosion can be accelerated.

In order to minimise such problems, conventionally pretreatment primers (known as etch or wash primers) are used. Alternatively chemicai conversion coatings such as phosphate or chromate coatings may be employed. The use of these materials willimprove adhesion particularly with newly galvanised or bright plated surfaces. However this requires an extra process before the main paint system can be applied and there is still frequently a problem of loss of adhesion of the paint film under aggressive conditions. Alternatively primer paints containing calcium plumbate may be used but this pigment is highly toxic.

Aluminium and its common alloys show a high degree of corrosion resistance in normal environments due to the formation of a thin oxide film which forms on any freshly exposed surface of the metal. However by means of conversion coatings the normal oxide may be modified to produce advantageous properties-chromate and phosphate treatments have been employed or the technique of 'anodising' may be used but when there is damage to these surfaces there is no further protection and serious corrosion can ensue under aggressive conditions.

Also like all metals aluminium may be protected by a suitable paint system-but again the problem is adhesion and it is necessary to pretreat with conversion coatings or use an etch primer initially.

Copper is not very active and oxidises slowly in air and in the electrochemical series of elements is near the less electropositive end, but in practice this non reactive behaviour is dependent on the maintainence of the protective oxide layer (or other insoluble corrosion product) on the surface of the copper. However its corrosion is still generally low. Several of the copper alloys are however liable to undergo a selective type of corrosion in certain circumstances e.g. dezincification of brasses.

The object of this invention is to minimize and overcome these disadvantages.

Our copending British application No. 23790/77 (C.F. Drake 58) describes and claims a paint composition adapted to inhibit corrosion of a metal surface to which it is applied, the composition including a glass material dispersed in a resin carrier in which said glass comprises as its major constituents phosphorus pentoxide and zinc oxide which together provide the glass forming oxide and the glass modifying oxide of the glass together with a minor proportion of one or more oxides of group IIA or IIIB (the boron group) of the periodic table. The composition of the glass being such that zinc and phosphate ions leach into solution when contacted with water.

Other paint and glass compositions are described in our copending applications No. 803671 8C.

F. Drake. Maries-P. F. Bateson 74-3-2).

According to one aspect of the present invention there is provided a method of inhibiting corrosion of a non ferrous metal, said method including treating the metal surface with a corrosion inhibiting composition incorporating an acidic water soluble glass.

According to another aspect of the invention there is provided a method of protecting a ferrous metal surface from corrosion, said method including coating the surface with a sacrificial metal or alloy, and applying to the sacrificial metal a corrosion inihibiting coating incorporating or comprising a water soluble phosphate glass composition.

According to a further aspect of the invention there is provided a paint composition for inhibiting corrosion of a non-ferrous metal surface, said paint incorporating a corrosion inhibiting water soluble glass.

We have found surprisingly that water soluble glass compositions and coating compositions containing the glasses as described above may be employed to prevent the corrosion of non ferrous such as zinc, aluminium etc, particularly under certain aggressive conditions. In particular coating compositions containing these water soluble glass compositions as described above may be employed in combination with a protective layer of zinc to provide a corrosion resistant protective 'system' for steel without the disadvantages of loss of adhesion of the paint layer after exposure to humid or aggressive conditions found with conventional systems. This effect of corrosion protection of nonferrous surfaces was totally unexpected. Although the mechanism of corrosion is not well understood it is established that the corrosion procedures for ferrous and non-ferrous metals differ considerably.In addition we have found that when incorporated into paint compositions the corrosion inhibiting glass compositions promote adhesion of the paint to the metal substrate. We have found this adhesion promotion to be particularly advantageous when galvanised steel is coated. (Metals coated with zinc whether applied by hot dipping or electroplating are known as 'galvanized').

We have found that coating, e.g. paint, compositions containing a corrosion inhibiting agent comprising a water soluble phosphate glass exhibit enhanced adhesion to non-ferrous surfaces under aggressive conditions in comparison with conventional coatings and provide a significant degree of corrosion inhibition.

In a particularly advantageous application of the techniques described herein a ferrous metal surface is protected against corrosion by first coating the surface with a non ferrous metal layer comprising either a continuous plated or deposited layer or a paint containing the metal at a high loading in dust form. This metal layer is then protected by a coating composition, e.g. a paint containing a corrosion inhibiting water soluble glass. Typically we employ phosphate glasses such as those described in our co-pending applications Nos. 23790/77,8036718 and 810776. The corrosion inhibiting coating prevents corrosion of the underlying metal layer and, in particular, prevents both sacrificial corrosion of the metal coating and normal corrosion of the underlying metal in the event of abrasive damage exposing one or both metals.In some applications the non-ferrous metal layer may be passivated by conventional passivation treatments prior to the application of the corrosion inhibiting coating.

In particular a system consisting of a coating of zinc over steel followed by a further coating of paint as described above provides a good corrosion resisting system under aggressive conditions, furthermore this system may receive further coats of paint of a conventional nature in order to achieve a satisfactory appearance.

We have also found that when the paint is applied over galvanised steel which has been pretreated with a Bonderite pretreatment, enhancement of performance is observed compared to a control using a conventional coating composition.

A coating can be applied by hot dip galvanizing, zinc spraying, zinc plating, sheradizing, the use of zinc dust paints or by processes such as those described in U.K. patent No. 1 424,465 (Diamond Shamrock). Zinc is used to protect steel as it offers good atmospheric corrosion resistance and is anodic to steel and protects it sacraficially.

The kind of zinc coating chosen for any application will depend on the mechanical properties required and the corrosion conditions it will encounter.

Typically corrosion inhibiting glass compositions are applied in the form of a paint coating by any of the conventional techniques including spray coating, dip coating, flow coating, or roller coating. The paint may either comprise a primer for subsequent application of further paint coatings or it may be the sole coating on the surface.

For many applications suitable glasses comprise, but are not limited to, 30 to 60 mole % zinc oxide. 0 to 2 mole % alumina, 0 to 40 mole % calcium oxide and 35 to 45 mole % phosphorous pentoxide. In some applications these glasses may also incorporate vanadium pentoxide. Glasses of this type are described in our copending application No. 8138285 (C. F. Drake 84). It will of course be appreciated by those skilled in the art that the discrete oxides are not necessarily present in the glass in their free form. It is however both conventional and convenient to describe the composition of a glass in terms of the constituent oxides.

To illustrate the techniques described herein we have determined the corrosion rate of various non-ferrous metal specimens using the method described by M. Stern ("Corrosion", Vol. 14 pp 440 444, 1 958).

Cylindrical samples of zinc, aluminium and copper of 0.8 cm diameter, and of cadmium of 0.6 cm diameter, were employed. The metals were of 99.999% purity or greater. Cylindrical sections of 1 cm length were drilled, tapped and wired on one circular face so as to permit subsequent electrical connection. The samples were then metallographically mounted in cold-cure polyester resin so that the opposite circular face of the sample was exposed. The exposed sample face were abraded with 220 grit abrasive paper immediately before placing 4 cm below the surface of test solutions.

Test solutions were prepared from aqueous 0.1 M potassium chloride with pigment additive, where present, added at a concentration of 10 g L-1. Glass pigments had average particle sizes of less than 10 microns. Test solutions were placed in a 250C thermostat bath.

Linear polarisation data was obtained using an H. B. Thompson and Associates "Ministat" potentiostat. The reference and auxiliary electrodes were calomel and platinum respectively. The "polarisation resistance", Rp, was determined 4 hours after the test electrodes had been placed in the test solutions. This was done by measuring the current resulting from the application of a potential of no more than +25 mV with respect to the corrosion potential, Ecorr, of the test electrode. The potential was scanned at a rate of 5 mV mild' from the cathodic to the anodic region.

The inhibitive effect of a pigment is approximately inversely proportional to Rp, and the efficiency of inhibition I, expressed as a percentage is thus given by: (1/Ro-1/Ri) 1(%)= 1 00 1/Ro (Ri-Ro) = 100 Ri where Ro is the polarisation resistance in the absence of inhibitor and Ri is the polarisation resistance with inhibitor present.

Three glasses were used in these evaluations. They had the following Molar % compositions: ZnO At203 CaO P205 Glass 1 56.2 1.3 - 42.5 Glass 2 50.9 - 12.8 36.3 Glass 3 31.6 - 30.6 37.8 The results are summarised in the Table of Fig. 1 below, with Examples 1, 5, 8 s 10 representing freely corroding controls, i.e. no added inhibitive pigment.

Electrode Corrosion Example metal inhibitor % lnhibition 1 Zn None O 2 Zn Glass 1 89 3 Zn Glass 2 99 4 Zn Glass 3 99 5 Al None O 6 Al Glass 1 67 7 Al Glass 3 100 8 Cu None O 9 Cu Glass 3 44 10 Cd None O 11 Cd Glass 1 80 12 Cd Glass 3 80 To further illustrate the techniques described herein 3 paint compositions were prepared.

All parts are by weight percentages.

1 2 3 Rutile 8.29 8.58 8.61 Winnofil SP 4.13 4.21 4.31 Micronised dolomite 28.97 20.75 12.28 red iron oxide 1.38 1.40 1.43 Glass pigment 2 - 8.53 17.81 Synolac 150S 27.49 27.16 26.68 Dynomin M1-1 1 6.89 6.80 6.68 Bentone 38 0.10 0.10 0.10 Aromasol H 18.67 18.44 18.13 n butanol 4.08 4.03 3.97 The paints were applied by spraying each to a small spangle galvanised steel panel at a film thickness of 25 microns, allowed to stand at room temperature for 30 minutes and then stoved at 1 50CC for 20 minutes (metal temperature). The panels were stored at room temperature for 24 hours then scribed through to ferrous metal and exposed to salt spray in accordance with ASTM B1 17-73.

They were examined after 150 hours. It was found that composition (1) exhibited bad blistering and considerable white deposits occurred all over the panel and were particularly noticeable in the vicinity of the scribe line. Composition (2) exhibited slight blistering and slight whitening on the face of the panel.

Composition (3) exhibited virtually no blistering and slight whitening within the scribe itself.

These examples demonstrate the feasibility of the techniques described herein.

Claims (18)

Claims

1. A method of inhibiting corrosion of a non ferrous metal or alloy, said method including treating the metal surface with a corrosion inihibiting composition incorporating an acidic water soluble glass.

2. A method of protecting a ferrous metal surface from corrosion, said method including coating the surface with a sacrificial metal or alloy, and applying to the sacrificial metal a corrosion inhibiting coating incorporating or comprising a water soluble phosphate glass composition.

3. A method of promoting paint adhesion to a zinc, cadmium or aluminium metal or alloy surface, said method including applying to the surface a corrosion inhibiting coating incorporating or comprising a water soluble phosphate glass composition.

4. A method of inhibiting corrosion of a paint containing a particulate non ferrous metal said method including treating the metal surface with a corrosion inhibiting composition incorporating an acidic soluble water soluble glass.

5. A method of inhibiting corrosion of a non ferrous metal or alloy which metal or alloy has been pretreated to passivate the surface, said method including treating the pretreated surface with a corrosion inhibiting composition incorporating an acidic water soluble glass.

6. A method as claimed in any one of claims 1 to 5 wherein said glass is a phosphate glass.

7. A method of inhibiting corrosion of a ferrous metal body, said method including coating the surface of said body with a layer of zinc, and applying to the zinc layer a surface coating of a corrosion inhibiting paint composition incorporating a water soluble phosphorus glass.

8. A method as claimed in claim 7, wherein the zinc coating is applied via a galvanising process.

9. A method as claimed in claim 7 or 8, wherein said ferrous method body comprises a steel sheet.

1 0. A method as claimed in claim 7 or 8 wherein said ferrous metal body comprises a vehicle body.

11. A method as claimed in any one of claims 1 to 10, wherein said glass comprises 30 to 60 mole % zinc oxide, 0 to 2 mole % alumina, 0 to 40 mole % calcium oxide and 35 to 45 mole % phosphorus pentoxide.

12. A method as claimed in any one of claims 1 to 11 wherein said glass incorporates vanadium pentoxide.

13. A method of inhibiting corrosion of a non ferrous metal, which method is substantially as hereinbefore described.

14. A paint composition for inhibiting corrosion of a non-ferrous metal surface, said paint incorporating a corrosion inhibiting water soluble glass.

15. A paint as claimed in claim 9, wherein said glass comprises 30 to 60 mole % zinc oxide, 0 to 2 mole % alumina, 0 to 40 mole % calcium oxide and 35 to 45 mole % phosphorus pentoxide.

1 6. A paint as claimed in claim 9 or 10, wherein said glass comprises 31.6 to 56.2 mole % zinc oxide, 0 to 1.3 mole % alumina, 0 to 30.6 mole % calcium oxide and 37.8 to 42.5 phosphorus pentoxide.

1 7. A body having a non-ferrous metal surface and provided with corrosion protection by a method as claimed in any one of claims 1 to 8.

18. A body as claimed in claim 16 and comprising a vehicle body.