NO138410B - HEAT-CURTAINABLE, CORROSION-PROTECTIVE COATING OR COATING PASS ON THE BASIS OF AN ACID-WATER SOLUTION, CONTAINING A POWDER-SHAPED AL / MG ALLOY - Google Patents

Description

The present invention relates to a heat-curable coating or coating compound, consisting of a dispersion of an inorganic solid material in particle form with a grain size of less than 44 µm in an aqueous solution of phosphoric acid, chromic acid or molybdic acid, respectively the metal salts of these acids, with at least one mole per liter of dissolved phosphate, at least o.3 mol per

liter of dissolved chromate and/or molybdate and at least o.5 mol per

liter of dissolved metal, as the solid material in particulate

form is present in an amount of lo to 2ooo g per liters of the solution, "and the distinctive feature of the heat-curable coating or coating compound according to the invention is that at least 10% by weight of the solid material is an alloy of aluminum and magnesium, containing at least 3% by weight of aluminum and at least 3 weight percent magnesium, the sum of aluminum and magnesium being at least 80 weight percent.

These and other features of the invention appear in patent

the requirements.

In the case of a previously known heat-curable coating compound of this

type, known from US patent 3,248,251, aluminum powder was used as solid material in particulate form. The known coating compound was used to provide better corrosion resistance for parts in jet engines.

The task underlying the invention is to

increase the achievable corrosion protection in previously known coating compounds of this type. Such an elevation of

the corrosion protection could be achieved by adding magnesium which is higher in the electrochemical voltage series and would therefore have better properties with regard to corrosion protection. If, however, pure magnesium powder is added to the aqueous solution of the aforementioned acids or their metal salts, a strong reaction and dissolution of the magnesium powder takes place. The fact that magnesium is higher in the electrochemical voltage series cannot therefore be exploited,

in that magnesium reacts with the solution to bind and disperse it. However, the aforementioned advantages are achieved and the aforementioned disadvantages are avoided by the present invention in that at least 10% by weight of the solid material is an alloy of aluminum and magnesium, which contains at least 3% by weight of aluminum and at least 3% by weight of magnesium, the sum of aluminum and magnesium being at least 80 percent by weight.

It is known per se to use a magnesium-aluminium alloy in coating agents of an otherwise organic type. It is also well known to use zinc dust in corrosion-protective paints. Thus, it is proposed in GDR patent document 38,316 to replace Zn with the alloy here called "Al^ Mg^"

(magnesium content: 37.5%). Al/Mg alloys in powder form are thus in and of themselves known additives for corrosion-protective paints. However, there are problems with using Zn dust in purely inorganic paints, and then the alloy "Al2Mg2" - according to the table in the aforementioned GDR patent document -

is even more "impure" than Zn, one would then have expected that the problems would be even greater with this alloy. In the GDR patent document, only examples of organic binders are given.

With the heat-curable coating or coating compound according to the invention, a reaction between magnesium and the aqueous solution of the aforementioned acids or their metal salts is prevented by using, as mentioned, a specified aluminium-magnesium alloy, so that the coating compound contains magnesium and thus achieves significantly better corrosion properties at the same time as better adhesion.

It has been established that the preferred coating material according to the above-mentioned US patent - where aluminum powder is used as particulate material - has the desired property

cabinet to effect sacrificial corrosion protection. That is, when the substrate for the coating is ferrous metal, the aluminum powder in the coating, due to the electrochemical reactions that occur, gradually sacrifices itself with the result that little or no corrosion of the ferrous metal occurs.

It has been shown that this sacrificial quality of the familiar

coating is greatly increased by treating the coating so that it becomes electrically conductive. The best way to effect this is to harden the coating to water insolubility at a temperature equal to or higher than the sintering temperature of the aluminum powder in the coating. In other words, it has been shown that a curing temperature of approximately 538° C is desirable.

When using the coating material according to the invention, where the particulate material is an alloy containing magnesium and aluminium, even greater corrosion protection is achieved, even when using a curing temperature as low as 315°C.

It is possible that this may be due to magnesium having even stronger sacrificial properties than aluminium. Furthermore, and this is of equal importance for achieving the overall benefits, the alloys of magnesium and aluminum have a lower sintering temperature than powdered loo% aluminium, and it is therefore that the increased corrosion protection can be achieved even if a significantly lower curing temperature. One would expect that this alloy, due to magnesium's high reactivity, would dissolve very quickly in the aqueous solution, as the pH of the solution is around 1.6 - 2.7. However, it has been found that when using the specified Al/Mg alloy, there is plenty of time between the mixing of the alloy powder in the solution and the time when the coating is heat cured. Nevertheless, it is desirable that the alloy powder be mixed

with the water solution a relatively short time before application and heat curing.

The above-mentioned and other purposes, features and advantages of the invention shall be illustrated and clarified in the following more detailed description.

The alloy powder should preferably contain from 3-95 weight percent aluminum and the rest magnesium. From the point of view of achieving the lowest possible sintering temperature and consequently curing temperature for the coating material, it is the most preferred

alloy lo weight percent aluminum and 30 weight percent magnesium, which corresponds to the eutectic composition. Achieving a low hardening temperature is, however, only one factor in the choice of the alloy to be used, and another factor is e.g. that the reaction between the alloy and the acidic water solution must be inhibited for as long as possible. Taking all factors into account, a 5o-5o alloy is excellent, and the preferred range is from 3o-7o weight percent aluminum and the rest magnesium.

However, alloys which predominantly consist of magnesium and aluminium, but which also contain one or more other metals, can also be used. Typical examples of such other metals are zinc, chromium, tin, nickel silicon, titanium and copper. It can be advantageous to mix such other metals into the alloy in that an alloy with a low melting point and consequently a low sintering temperature is obtained. However, the essential thing is that such other metals can be present without eliminating the advantages achieved by the use of the Al/Mg combination. In general, therefore, alloys which are usable for the invention can contain from 3-95 weight percent aluminium,

from 3-95 weight percent magnesium, although at least 80 weight percent of the alloy consists of aluminum and magnesium.

In particular to achieve a relatively even coating, which is often necessary especially for jet engine components where unevenness could cause turbulence, the alloy's particle size should be small, preferably less than lo ^m. The alloy powder can also be used in combination with other powder materials. For certain applications, it can e.g. be desirable to use a 5o-5o

. mixture of aluminum powder and Al/Mg alloy powder. The alloy particles, and also the aluminum particles when a mixture thereof is used, should preferably be of spherical shape, often referred to as "atomized" powder - this in contrast to the flat aluminum particles that are usually used in pigmented coating agents on an organic basis. Mixtures containing as little as 10% by weight Al/Mg alloy and the remainder other powders can be used when special properties are desired. When other powders are used in combination with the alloy powder, the other powder should in any case not dissolve quickly in the acidic solution. Besides aluminum powder, examples can be mentioned of calcium carbonate, zinc, silicon dioxide, aluminum oxide, chromium oxide (i.e. C^O^)

and metal carbides and silicides.

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The total amount of particulate material included in the coating can be from lint to as much as 2ooo g per liters depending on the application for which the coating is intended, and also depending on the atomic or molecular weight of the particulate material. For most applications and particularly when the particulate material is entirely or at least predominantly made up of the alloy powder, however, the amount of particulate material mixed into the coating should be 250-100 g per -liter solution.

In US patent document 3,248,251, the formulation and composition of the water solution is explained in detail, also with regard to the details of the mixing. It should be pointed out here that although it can be expected that the +6 valence of chromium in the solution will be present as dichromate rather than as chromate, due to the solution being acidic, the term "chromate" used here should also include dichromate , and for the purpose of achieving uniformity in the definition of the molar concentrations, the chromate is assumed to be present as chromate and not as bichromate, although in reality chromium with +6 valence. available in the form of bichromate.

In all cases, the solution must contain at least o.3 mol

per liter of dissolved chromate or molybdate, at least o.5 mol per liter of dissolved metal and at least 1 mol per liters of dissolved phosphate.

The preferred ranges are 1-4 mol per liter of dissolved phosphate, o.5-3 mol per liter of chromate or molybdate and 1-4 mol per

liters of dissolved metal. When specifying these concentrations, reference is made to the specified radicals or ions, instead of to the salt or acid to which the component can be added in the form of or present in the solution, and in this context the term "ion" is used, e.g. .ex. "phosphation" for convenience to make it clear that an ion is being referred to, even if (depending on the ionization constant of the individual constituent) it should happen that part of the solute referred to may not actually be present in ionized or dissociated form.

The phosphate cation, i.e. the anion, can be introduced into the water solution in the form of phosphoric acid or in the form of phosphates of the metal or metals that are desired mixed in as a metal cation, or it can be added in both forms. The term "phosphate" shall include not only the PO^ ion, but also the HPO^ and ^PO^ ions. All three write, e.g. from ionization of H^PO^ and the hydrogen phosphate ions are usually present, at least to a certain extent, in the mixtures. When phosphoric acid is used as an additive in the preparation of the mixtures, it is highly desirable to use the ortho acid, H^PO^ , although the other forms can also be used, such as e.g. the meta- or pyroacids, all of which ionize in water.

The chromate cation can be added either as chromic acid or as its anhydride CrO^, or as a chromate or bichromate of the metal or metals that one wishes to add as a metal cation. Combinations of acid and metal salts can of course also be used. If molybdate anion is used, this can be added as molybdic acid or as the anhydride MoO^ or as metal molybdate. For most applications, no particular advantage is gained by incorporating molybdate, and molybdate has the disadvantage of being expensive. It also does not give the mixtures as good properties as those that can be achieved with chromate.

The metal cation can be added either as a metal phosphate, chromate, bichromate or molybdate. The metals can also be added in whole or in part in the form of oxide, hydroxide or carbonate, which are dissolved in the phosphoric acid or chromic acid. If carbonate is used, of course carbon dioxide is evolved. It is clear that when the metals are added in this way, e.g. as oxide, at least part of the phosphate, chromate or molybdate should be added as acid or as acid anhydride. The metal cations with valences +2 and +3 are preferred, and for most applications of the mixtures magnesium ion is particularly advantageous.

In the following, exemplary preferred mixtures will be illustrated.

Example 1

The prepared mixture of Al/Mg alloy and the phosphate chromate metal ion solution is applied, e.g. by spraying, dipping, rolling or brushing on the required surface(s).

coated, and then hardened by heating to at least 315°C.

The optimum hardening temperature varies depending on the exact composition of the alloy. Generally, however, the preferred annealing temperature is about 19o-427°C when the preferred alloy is used (ie an alloy containing 3o-7o weight percent aluminum and the balance magnesium). After complete curing, the coating is insoluble in water, even if it is exposed to water for a long time and the obtained coating has a sacrificial effect in relation to the steel substrate. The time required for curing depends on the temperature used, and the higher the temperature, the shorter the time required. For an average curing of a given coating at low temperature, e.g. a curing time of 15-60 minutes at 315°C would be desirable. When rapid hardening is required, 5-lo minutes at 427°C should be adequate. For slow curing, 24 hours at 19o°C should be sufficient. For even faster curing, radiation heating at higher temperatures can be advantageously used. Generally, the mixtures cannot be overcured, within relatively wide limits, and apart from costs and possible temperature limitations with regard to the particulate material or the substrate materials used, it is not associated with any special

disadvantage of curing, for longer periods of time or at higher temperatures than is absolutely necessary.

The thickness of the coating after curing should preferably be about 0.125-0.50 mm, and consequently the exact concentration of the mixture applied and the amount applied should be regulated accordingly. In some cases, it is desirable to apply

apply two or more coatings, with or without curing between the individual coatings. The mixtures according to the invention wet the substrate well. There is therefore little need to

organic wetting agents are used to achieve an even coating.

The following further examples illustrate the invention.

Example 2 N

Example 3

Example 4

Example 5 Example 6

Example 7 Example 8

Example 9

Example lo

MgCr20?6H20 174 g Al2 (Cr207)3 59 g MgO 4o g H3P04 196 g H20 to looo ml

Alloy of 65% by weight Al 25% by weight Mg

and lo weight percent Zn 8oo g Coated on compressor blade {for jet engines)

of iron-based metal and hardened at 371° C for 3o minutes.

Claims (3)

1. Heat-curable coating or coating compound, consisting of a dispersion of an inorganic solid material in particle form with a grain size of less than 44 /a. m in an aqueous solution of phosphoric acid, chromic acid or molybdic acid, respectively the metal salts of these acids, with at least one mole per liter of dissolved phosphate, at least o.3 mol per liter of dissolved chromate and/or molybdate and at least o.5 mol per liter of dissolved metal, the solid material in particulate form being present in an amount of 1 to 2ooo g per liters of the solution, characterized by at least a weight percent of of the solid material is an alloy of aluminum and magnesium, which contains at least 3 weight percent aluminum and at least 3 weight percent magnesium, the sum of aluminum and magnesium being at least 80 weight percent. 2. Heat-curable mass as specified in crawl, characterized in that the alloy essentially consists of 30 to 10% aluminum by weight, while the rest is magnesium. 3. Heat-curable mass as specified in claim 1 or 2, characterized in that the solid material in addition to the aluminium-magnesium alloy also contains a further metal, preferably aluminium.