NL8203248A - Anticorrosive pigment for ferrous metal - is glass of oxide(s) of zinc, calcium and phosphorus - Google Patents

Abstract

The pigment is a glass with a compsn. falling within zones 1, 2 and 3 where zone 1 comprises ZnO 49.8-50.4, CaO 18.0-18.5 and P2O5 31.4-31.7; zone 2 comprises ZnO 31.8-32.3, CaO 29.0-29.8 and P2O5 38.2-38.7; and zone 3 comprises ZnO 23.9-26.3, CaO 33.6-35.0 and P2O5 40.0-41.3. Pref. the pH of an aq. soln. of 0.5g of the glass with an ave. particle size of 500-710 microns in 75 ml. deionised water at 25 deg. C. is greater than 3.2, esp. greater than 4.0. The corrosion inhibiting compsns. comprise a resin carrier and the above water soluble glass with a particle size of less than 60 microns. The compsn. may also contain an alkaline material such as a water soluble glass or CaCO3 to neutralise acid substances released by the dissolution of the glass. The resin carrier is esp. a short oil alkyd resin. Glass pigments have good anti-corrosion action, are non-toxic, do not cause loss of metal as do known zinc phosphate pigments, and can be used at particle sizes of less than 10 microns to give finishes with good cosmetic effect. The compsns. motor vehicle body panels, etc.

Description

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Improvements in paint compositions

The invention relates to compositions for inhibiting corrosion of a ferrous metal surface to which these compositions are applied and to paint formulations containing such compositions.

One of the main problems associated with the use of metals as construction materials is that of the corrosion of the metal to which ferrous metals are particularly susceptible. The corrosion mechanism is not fully understood, but it is well known that the process is accelerated under aggressive conditions, which are characteristic in industrial environment and the environment of sea influence. The standard technique for reducing corrosion is to apply a base coat containing one or more corrosion inhibiting materials to the metal surface. Such basecoats generally comprise a resinous binding medium in which finely ground pigments are dispersed. The purpose of these pigments is to provide either opacity and color or to provide corrosion inhibition, the latter pigments being known as active pigments. The most commonly used active pigments are lead menia and calcium plubate, but these materials are highly toxic. Zinc chromate is also used as a corrosion inhibitor, but this does not have the efficacy level of the lead pigments and can also cause color bleeding from a subsequent coat of paint. In addition, hexavalent chromium salts are believed to have carcinogenic activity.

Recently zinc phosphate has been used as a non-toxic alternative to 25 lead and chromate pigments. Compositions using this material are described in British Pat. Nos. 904,861 and 915,512. This material is said to be nearly as effective as the previously used toxic pigments, but its effectiveness is poor in certain binder media and when exposed to an atmosphere of low sulfur dioxide, such as an environment where the influence of the sea rules. In addition, if welding is to be effected on a primed steel surface, the use of zinc phosphate paints should preferably be avoided. The significant heat generated in the welding process can cause evaporation of the paint, releasing toxic fumes of zinc oxide and / or free zinc.

In the aforementioned British Pat. Nos. 904,861 and 915,512, the use of calcium phosphate (tricalcium phosphate, calcium water 8203248

Ir * «Λ.

* 2 hydrogen phosphate and mono-calcium dihydrogen phosphate) which avoids the toxicity problem encountered in zinc based paints. However, these calcium salts do not have the optimum solubility values in water and pH for effective corrosion inhibition in a range of paint media and environmental conditions. Since the compounds are stoichiometric, it will also be apparent that their properties cannot be controlled.

British patent applications 23790/77 and 7939544 disclose the use of various zinc alumino-phosphate glass pigments as anti-corrosion materials. These materials are more effective than conventional zinc orthophosphate pigments in that they provide zinc and phosphate ions in a predetermined optimum degree and ratio under a variety of corrosion conditions. Further corrosion inhibiting% compositions based on the calcium oxide / phosphorus pentoxide glass system are described in British patent application 810776.

Although the paints incorporating these corrosion-inhibiting glass compositions are considerably more effective than conventional zinc phosphate paints, it has been found that under certain conditions the slightly acidic nature of these glasses dissolves a thin layer of the metal on the surface to which the paint is applied. is caused. For many applications, if the paint is applied to a relatively thick metal body, this small metal loss is not important. However, if the surface to be protected is a thin plate, for example a panel of the body of a vehicle, the metal loss can become considerable and therefore undesirable.

It is the object of the present invention to limit or overcome this drawback.

Certain glass materials have been found to provide even more effective anti-corrosion pigments than those previously described.

It has also been found that a paint formulation can be adapted to complete a particular glass composition to provide pH conditions which increase corrosion inhibition despite the danger of acid formation and subsequent attack on the metal surface.

It has also been found that the glass compositions of the invention can be used in a very finely divided form, usually less than 10 microns in average diameter, to prepare a paint which has a sufficiently high finish quality for use in additives. ·· '·. "-S"; ·. .- ··. · --W- '.. · 3 fits where a cosmetic effect is required in addition to the corrosion protection provided by the paint. Such applications include the corrosion protection of vehicle bodies.

According to a feature of the present invention there is provided a corrosion inhibiting paint composition comprising a resin support and a water soluble corrosion inhibiting glass dispersed in the resin and characterized in that the pH of the solution of the glass as defined below is greater than 3.2.

According to another feature of the invention, a corrosion-in-vibrating material is provided comprising a glass composition selected from the zone 1, zone 2 or zone 3 glasses as defined below.

According to a further feature of the invention there is provided a corrosion inhibiting paint composition comprising a resin support, a water-soluble corrosion inhibiting acid glass dispersed in the resin and characterized in that an alkaline material is dispersed or incorporated in the resin which maintains the acidity of the glass-resin combination below 3.2.

It is believed that the rate at which a surface too low of a ferrous metal is dissolved by anti-corrosive agents of the phosphate type is dependent on the pH at the metal surface. The mechanism by which corrosion inhibition is effected is not entirely clear, but it is believed that the first step is the reaction of the metal surface with phosphate ions, which ions are released by dissolving the glass to form a coherent layer of an iron phosphate. . This phosphate layer then protects the metal surface against subsequent corrosive attack. The integrity of this protective film increases due to the presence of Group IIA and / or IIB metal ions which are also released by the dissolution of the glass. These ions are believed to form complex phosphates on the surface of the ferrous metal.

The effectiveness of this phosphate layer in preventing corrosion is determined by a number of factors, many of which are interdependent. In particular, the pH and the phosphate concentration in the vicinity of the metal surface together determine the rate at which the phosphate film is dissolved and the rate at which it is replenished by the conversion of more ferrous metal into the corresponding phosphate. In particular, it has been found that the solution of the film with subsequent attack of the ferrous metal 40 becomes measurable for active pigments with a pH value of the solution 8203248

f I

Less than 4.3, preferably less than 4.0, and that this attack occurs quickly if this pH value is less than 3.2.

The pH value of the solution is defined as the pH of the solution, which is obtained when 0.5 g of the active pigment, 5 a glass, with a particle size in the range between 500 and 710 microns is dispersed in 75 ml deionized water at 25 ° C. The pH of the resulting solution is measured at 24 hour intervals until a constant value is obtained. This period can last from 50 to 60 days.

The pH-controlling surface of a metal in contact with a type of paint in which the active corrosion inhibitor is a water-soluble glass can be controlled in a number of ways. First, the glass composition can be selected so that the final pH of the solution remains below the initial value of 3.2. This takes place by adapting the various glass-forming and glass-modifying components, and in particular it has been found, for example, that glasses containing the zinc oxide, calcium oxide and phosphorus pentoxide as the main components meet this requirement if the phosphorus pentoxide content is maintained below 40 mol%.

A further method is the choice of the paint medium in which the glass is dispersed. An alkaline material dispersed in, or incorporated into the paint base, allows the use of relatively acidic glasses which, in the absence of such a neutralizing material, would provide an insufficient degree of corrosion inhibition.

According to an embodiment of the invention, anti-corrosive paints can be prepared, the active pigment of the paint comprising a glass whose pH of the solution is above 3.2 and preferably above 4.0. The glass may comprise phosphorus pentoxide as the glass-forming oxide together with one or more glass-modifying oxides selected from the oxides of the Group IIIa and Group IIB metals. The pH of the glass is determined by the proportions of the oxides in question, and it has been found in particular that glasses containing less than 40 mole percent phosphorus pentoxide have suitable pH values of the solution.

The dissolution rate of the glass is also determined by the interrelationships of the glass-forming oxides and the glass-modifying oxides. This rate can be further controlled by incorporating further glass-modifying oxides such as aluminum oxide. The various techniques for controlling the dissolution rate of the glass 8203248 5 * -9 * \ and the pH of the solution are described in more detail in British Patent Application 7930041.

The glasses can be formed by fusing mixtures of the oxides, or precursors thereof, for a sufficient time to obtain a homogeneous melt which is then poured into cold water or a cold steel sheet. The solidified material is crushed and ground to a particle size normally in the range of 10 to 100 microns and then dispersed in a paint resin as part of the total solid pigment. Preferably, the amount of solid pigment of the paint is, for example, 40% by volume, of which a total of 20% by weight is the corrosion-inhibiting glass material

In particular, it has been found that certain areas of the glass compositions, as depicted in Zone 1 and Zone 2 in Figure 1 of the accompanying drawings, have a solution / time curve pH which has been found to provide corrosion protection without acid encroachment. Furthermore, some types of glass from a different zone, namely zone 3, also fall within this pH limit. When these glasses are contacted with water, the value of the pH of the solution drops rapidly to a value between 6 and 4 and then decreases less rapidly, approaching a value of 3.2 in some cases. It has been found that glass compositions whose final pH of the solution is above this limit provide corrosion protection of metal surfaces without the associated problem of deterioration of that surface by the dissolved product. The pH of the solution / time curve of the glasses falling within zones 1, 2 and 3 are shown in Figure 2. For comparison purposes, glasses from a further zone, zone 4, are included.

A number of glasses are made from each of the zones shown in Figure 1. It is noted that evaporation of some components during the melting process, especially phosphorus pentoxide, makes it difficult to predefine a composition. Preferred compositions, however, are defined in each of these zones, the preferred compositions being (mole%): 8203248.

4's% 6

ZnO CaO ** 2®5

Zone 1 50.0 18.0 32.0

Zone 2 32.0 30.0 38.0 5 Zone 3 24.5 35.0 40.5

The molar composition ranges in each zone are as follows: Zone 1 ZnO 49.8 to 50.4

CaO 18.0 to 18.5 P2O5 31.4 to 31.7 10

Zone 2 ZnO 31.8 to 32.3

CaO 29.0 to 29.8 P2O5 38.2 to 38.7 «15 Zone 3 ZnO 23.9 to 26.3

CaO 33.6 to 35.0 P205 40.0 to 41.3

The different compositions are tabulated together with the dissolution rates in Tables A, B and C below.

8203248

N

TABLE A

ZONE 1 (pigment P) 7

5 PREPARATION ANALYTICAL COMPOSITION SOLUTION RATE

mol% eC hr ZnO CaO cm “^ hr- ^ 950 1 50.4 18.0 31.6 0.023 10 1050 1 49.8 18.5 31.7 0.032 1050 1 50.1 18.5 31.4 0.034 1050 1 49.8 18.5 31.7 0.042 950 4 50.3 17.6 32.1 0.028 15 950 4 49.7 18.6 31.7 0.021 1050 4 49.6 18.4 32.0 0.027 1050 4 51.0 17.2 31.8 0.036 950 16 50.1 17.9 32.0 0.021 20 1050 16 50.1 17.9 32.0 0.027 950 64 50.2 17.8 32.0 0.023 1050 64 49.9 18.6 31.5 0.024 25 8203248

5 PREPARATION ANALYTICAL COMPOSITION SOLUTION RATE

mol.%

TABLE B

ZONE 2 (pigment N) 8 ° C hr ZnO CaO P2O5 mg cm ^ hr-1 950 1 32.3 29.5 38.2 0.060 10 1050 1 32.1 29.2 38.7 0.098 1050 1 31.8 29.8 38.4 0.055 1050 1 32.3 29.0 38.7 0.098 950 4 31.9 29.7 38.4 0.056 15 1050 4 32.6 29.3 38.1 0.043 1050 4 31.6 30 .7 37.6 0.073 1050 4 32.0 30.4 37.6 0.052 1050 4 31.8 30.7 37.5 0.052 20 950 16 32.3 29.7 38.0 0.062 950 16 31.8 30, 5 37.7 0.053 1050 16 31.8 30.5 37.7 0.057 950 64 32.0 29.6 38.4 0.83 25 1050 64 31.5 30.2 38.3 0.050 1050 64 32.1 30 .3 37.6 0.78 * 8203248

TABLE C

ZONE 3 (pigment R)

5 PREPARATION ANALYTICAL COMPOSITION SOLUTION RATE

mol ·% 9 »° C hr ZnO CaO - ^ 2 ^ 5 mg cm” "^ hr" "^ 950 1 23.9 34.8 41.3 0.056 10 950 1 24.9 34.4 40.7 0.078 1050 1 25.0 35.0 40.0 0.067 1050 1 26.3 33.6 40.1 0.055 1050 1 24.7 34.8 40.5 0.073 15 950 4 24.8 34.4 40.9 0.061 950 4 24.6 35.4 40.0 0.090 950 4 25.2 33.9 40.9 0.073 1050 4 24.9 34.5 40.6 0.087 20 950 16 25.8 33.6 40.6 0.050 1050 16 25.1 34.6 40.3 0.063 950 64 24.5 34.7 40.8 0.074 1050 64 25.6 34.0 40.4 0.070 25 _________

It has been found that the corrosion inhibiting effect of a glass composition can be electrochemically tested.

A standard test procedure has been used in which the glass 30 can dissolve in a test cell, which cell contains aqueous potassium chloride (to provide electrical conductivity) and is provided with a mild steel electrode and an unresponsive reference electrode.

Corrosion-inhibiting ions released by dissolving the glass produce an insulating protective film on the mild steel electrode. The quality of this film, that is, the degree of corrosion inhibition, is determined by applying an increasing voltage to the mild steel electrode and determining the voltage at which the film breaks and current begins to flow. . A 100% inhibition is defined as a theoretically perfect film 40 that makes the flow impossible to flow.

8203248 V v * 10

Measurements of glasses from zones 1, 2 and 3 have been made and the results thereof are summarized in Fig. 3. Two other relatively acidic glass pigments, zones 4 and 5, and zinc orthophosphate (a common corrosion inhibitor) are included for comparison. As can be seen from 5, zone 1 and zone 2 glasses provide a very high level of inhibition, while zone 3 glasses which do not provide the initial high degree of corrosion protection caused by zone 1 and 2 glasses , the inhibitory effect increases over time to a satisfactory level.

In a further embodiment of the invention, a paint can be formulated by using a corrosion inhibiting glass whose pH of the solution is less than 3.2, while the paint also includes a neutralizing agent to reduce this relatively high acidity of offset the active material. This neutralizing agent is alkaline in nature and may comprise a sparingly soluble stoichiometric compound such as calcium carbonate or a second water-soluble glass whose pH of the solution is relatively high. It is the function of the neutralizing agent to raise the pH in the vicinity of a metal surface on which the paint is applied and in the presence of moisture. Moisture penetration over a longer period of time therefore does not even result in a pH of the solution in the vicinity of the metal surface of less than 3.2.

The glasses can be formed and dispersed in a paint resin as described above, the neutralizing agent then being a further part of the total solid pigment. This use of a neutralizing agent has the further advantage that the tendency of the paint to flake can be reduced. It is known that if the concentration of the dissolved material in a humid environment below a coat of paint is excessive, osmosis occurs, whereby more water will diffuse through the paint film. In some cases, the osmotic pressure will be high enough to cause the paint film to flake. It has been found that the electrolyte concentration under such a paint film can be kept so low that it is compatible with an effective corrosion protection by pairing the nature and rates of dissolution of the active pigment and neutralizing agent in the paint. Thus, the dissolution rate of the active glass pigment can be adjusted using techniques described in British Patent Application 7930041 to the dissolution rate of the neutralizing agent so that the rate at which the active glass dissolves corresponds to the rate of 8203248. with which the dissolved products are removed by the neutralizing agent.

Preferably, alkyd resins (normally a "short oil" alkyd in xylene) are used to prepare the paints from the glasses of the invention, but it is noted that other conventional resins or binders known in the art may also be used , for example epoxy resins, acrylates, or chlorinated rubbers.

For applications in structures using thick coatings 10, that is, 50-100 micron or more in thickness, the glass should be pulverized to a final size with the largest weight fraction of the particles having an average diameter of less than 60 microns. In other applications, for example, in the corrosion protection of vehicle bodies, the 15 'particle size of the glass may have an average diameter of less than 15 microns and preferably less than 10 microns.

Unlike previous corrosion resistant compositions, the glass compositions described herein are substantially colorless. Thus, they can be used in paint compositions as the sole pigment or together with the desired final color determining pigment. As such, a single coat of the paint composition will in many cases suffice both for corrosion resistance and for giving the final color. Thus, the paint compositions described herein may be the only paint layer on a metal surface.

8263248

Claims (19)

Corrosion-inhibiting material, characterized in that it comprises a glass composition selected from the group comprising the glasses of zone 1, zone 2 and zone 3, which zones have the following 5 limit values: Zone 1 ZnO 49.8 up to 50.4 CaO 18.0 to 18.5 P205 31.4 to 31.7 10 Zone 2 ZnO 31.8 to 32.3 CaO 29.0 to 29.8 P205 38.2 to 38.7 Zone 3 ZnO 23.9 to 26.3 15 CaO 33.6 to 35.0 × 20 ^ 40.0 to 41.3 Corrosion-inhibiting material according to claim 1, characterized in that the pH of the described solution of the material is greater than 3.2, the pH of the solution being understood to mean the constant pH obtained if 0.5 g of the active pigment, a glass with a particle size in the range between 500 and 710 microns, is dispersed in 75 ml of deionized water at 25 ° C. Corrosion-inhibiting material according to claim 1 or 2, characterized in that the phosphorus pentoxide content of the glass is less than 40 mol%. Corrosion-inhibiting material according to claim 1, 2 or 3, characterized in that it comprises a powder with an average particle size of less than 60 microns. Corrosion-inhibiting paint composition comprising a resin 30 as support and a water-soluble corrosion-inhibiting glass dispersed in the resin, characterized in that the pH of the solution, as described in claim 2, of the glass is greater than 3.2. The paint composition according to claim 5, characterized in that the pH of the solution of the glass is greater than 4.0. * Corrosion-inhibiting paint composition, characterized in that it comprises a resin as a support together with a glass composition according to any one of claims 1 to 4. 8. Corrosion-inhibiting paint composition comprising a resin as a support, a water-soluble corrosion-inhibiting acid glass dispersed in the resin, characterized in that an alkaline material 8203248 1 W * i * rial dispersed or incorporated in the resin thereby substantially neutralizing acidic substances released by dissolving the glass. Paint composition according to claim 8, characterized in that the alkaline material is a water-soluble glass. Paint composition according to claim 8, characterized in that the alkaline material is calcium carbonate. 11. Corrosion-inhibiting paint composition according to any one of claims 5 to 10, characterized in that the phosphorus pentoxide content in the corrosion-inhibiting glass is less than 40 mol%. A paint composition according to any one of the preceding claims, characterized in that the corrosion-inhibiting glass comprises aluminum oxide. Paint composition according to any one of claims 5 to 12, characterized in that the glass has an average particle size of less than 10 microns » Paint composition according to any one of claims 5 to 13, characterized in that the resin is a "short oil" alkyd resin. 15. Method for treating a metal surface, characterized in that it comprises coating the surface with a paint composition according to any one of claims 5 to 14. Steel construction, characterized in that it is covered with a paint according to any one of claims 5 to 14. Vehicle body, characterized in that it is coated with a paint according to any one of claims 5 to 14. 1111 M lll-l 11-1 + 8203248