MXPA98009457A - Soldier, corrosive resistant coating compositions - Google Patents

Abstract

An anticorrosive coating based on zinc is described. The coating includes zinc, iron phosphide, and potassium silicate having a molar ratio of potassium silicate from 4.1 to 6.

Description

WELDABLE COATING COMPOSITIONS. RESISTANT TO CORROSION FIELD OF THE INVENTION The invention relates to an anticorrosive coating composition that does not interfere with welding. The composition includes zinc, iron phosphide and an aqueous solution of potassium silicate containing a high proportion of silicate to potassium.

BACKGROUND OF THE INVENTION In industrial and marine construction, a pre-painted steel with zinc-rich initiator is generally desirable prior to manufacture, and many such coating compositions are known. Many of these compositions include zinc powder in an organic resin, such as an epoxy resin, a chlorinated gum, a polystyrene resin or a silicone resin. Coatings based on these organic binders are not very suitable for coating steel which must ultimately be welded because the binder tends to decompose from the heat of the weld, resulting in damaged coatings and loss of adhesion. As an alternative for coatings based on organic binders, zinc coatings based on inorganic binders are also known. For example, Schutt (US Pat. No. 3,620,784) describes a coating composition containing zinc powder carried in a silicate vehicle. It is said that the particular formulation described by Schutt avoids many of the problems commonly encountered with inorganic coatings: weathering, cracking, cracking and non-uniform adhesion before and after baking However, even if one overcomes the problems associated with the physical characteristics of the applied silicate coating, as Schutt claims to have done, one still faces the problem of welding steel that has been coated with zinc dust initiator in a silicate vehicle. During welding, vaporized zinc can penetrate the root of the weld and become trapped, creating a porous weld. Sometimes it is possible to partially overcome the porosity problem by reducing the welding speed enough to allow the zinc to volatize completely in front of the weld. However, even when this technique is successful, it results in increased manufacturing costs due to the reduced welding speed. It is known that a portion of zinc in the zinc-rich initiators can be replaced by iron phosphide of suitable particle size. The resulting zinc-coated steel, while maintaining its anti-corrosive properties, can be welded more easily and efficiently. As an example of this approach, Makishima et al. (US Patent 4,011,088) have proposed an anticorrosive coating composition comprising from 5 to 80% of a binder, which is either potassium silicate or ammonium silicate, and 20 to 95% of a pigment mixture. of zinc dust and iron phosphide or zinc powder or nickel phosphide. In one embodiment, in which potassium silicate is used as the binder, Makishima et al. indicate that the ratio of silicate to potassium should be between 2.5 and 4.0. If the molar ratio is less than 2.5, it is said that the film-forming property is insufficient and if the molar ratio is greater than 4.0, it is said that the stability of the binder is reduced. I have now discovered an improved zinc-based initiator that provides a corrosion-resistant, durable coating combined with excellent welding capability. Moreover, the composition is water based and therefore avoids the environmental problems associated with solvent based initiators.

BRIEF DESCRIPTION OF THE INVENTION The invention relates to an anticorrosive coating composition comprising: (a) from 8 to 40 parts by weight of a pigment, said pigment comprising a mixture of from 35 to 90 percent by weight of zinc and from 65 to 10 by weight. percent by weight of iron phosphide; (b) from 2 to 5 parts by weight of potassium silicate of formula K 0 «nSi02, wherein n is from 4.1 to 6.0; and (c) from 7 to 12 parts by weight of water. Preferably, zinc and iron phosphide are of average particle size between 2 μm and 20 μm; very preferably 3 to 8 μm in average particle size. The ratio of silicate to potassium in the potassium silicate of the inventive composition can be from 4.5 to 6.0; more preferably 5.3 to 6.0; and most preferably 5.0 to 5.3. The compositions according to the invention may further include lithium silicate Li20 »pSi02, wherein p is from 3.0 to 8.0 or sodium silicate Na20« mSi02, wherein m is from 2.0 to 4.0, or a combination of the two. The compositions according to the invention may also include a filler such as mica. In a preferred embodiment, the invention comprises (a) from 23 to 28 parts by weight of a pigment; (b) from 3 to 4 parts by weight of potassium silicate, wherein n is from 5.0 to 5.5; and (c) from 9 to 10 parts by weight of water. The pigment in this case is a mixture of from 45 to 55 percent by weight of zinc and from 45 to 55 percent by weight of iron phosphide.

DETAILED DESCRIPTION OF THE INVENTION The coating composition of the invention comprises three basic elements: (a) a zinc-iron phosphide pigment; (b) a potassium silicate binder; and (c) an amount of water to provide the appropriate rheology. The compositions may also contain compatible colorants and fillers, which should not, however, adversely affect the durability and solderability of the resulting composition. The zinc-iron phosphide pigment consists of a mixture of metallic zinc in the form of a powder or flakes and particles of approximately the same size as the iron phosphide. Iron powder is preferred, and is commercially readily available. I have found that U P6 zinc dust from North American Oxide (Clarksville, Tennessee), having an average particle size of 6 to 7 microns, is suitable for the composition of the invention. The iron phosphide is also commercially available, and I have found that the Grade 2131 Ferrophos®, available from Occidental Chemical Corporation (Niagara Falls, New York), having an average particle size between 3 and 5 microns is suitable for the compositions of the invention. Ferrophos® is mainly iron phosphide (Fe2P) with lower amounts of SiO2. Details of its preparation and constitution can be found in US Patent 3,562, 124 (see column 3, line 45 to column 4, line 33), the full disclosure of which is incorporated herein by reference. The proportion by weight of zinc in the pigment should be greater than % in order to provide adequate protection against corrosion, but can not be greater than 90% without giving an increase to porous welds. Other pigments and fillers may be added to the composition of the invention, as long as the percentages of zinc and iron phosphide are maintained within the established range. Examples of fillers that could be added include diatomaceous earth, bentonite clay, kaolinite, wollastonite, talc and mica. The binder is potassium silicate having a molar ratio of silica to potassium oxide greater than 4.0. Aqueous solutions of potassium silicate are commercially available and are prepared by the method described in US Patent 4,162,169, the full disclosure of which is incorporated herein by reference. I have found that commercially available K-silicate ™ from Polyset Chemical Co. (Mechanicville, New York) is suitable for the compositions of the invention. This material is approximately 23-28% by weight of potassium silicate in water (ie, 23-28% solids content) and the molar ratio of silicate to potassium is approximately 5.2. Although it is not necessary to produce coating compositions of the invention, other alkali metal silicates may be added to the binder. In this regard, one can add either lithium silicate (Li20 «pSiO2, where p is from 3.0 to 8.0) or sodium silicate (Na2O« mSiO2 in which m is from 2.5 to 4.0), or both. A composition of the invention is prepared by mixing 3.17 kilograms of UP6 zinc powder to homogeneity with 3.17 kilograms of 2131 Ferrophos® (iron phosphide) in a high speed mixer. The homogeneous mixture is combined with 3.31 kilograms of K-silicate MR (potassium silicate) and stirred to produce 3.78 liters of a homogeneous mixture according to the invention. As discussed above, if a filler such as mica is to be added, it would be mixed together with the zinc and the iron phosphide before being mixed with the silicate vehicle. Similarly, if a lithium silicate or sodium silicate were to be added to the composition, they would be mixed with the aqueous vehicle of potassium silicate before being combined with the pigment. The substrates to which the compositions of the invention are applied will commonly be metals. Actually, the main advantage of the present compositions, the capacity for welding, is observed when the substrate is steel. For steel substrates, the substrate is generally cleaned with phosphoric acid or sanded immediately before the application of the coating. The coating is sprayed, brushed or laminated to the substrate by conventional means. It spreads well and adheres evenly. The resulting coating resists corrosion and abrasion; it does not crack or peel and it resists the weather well.

Claims (12)

1. An anticorrosive coating composition comprising: (a) from 8 to 40 parts by weight of a pigment, said pigment comprising a mixture of from 35 to 90 percent by weight of zinc and from 65 to 10 percent by weight of iron phosphide; (b) from 2 to 5 parts by weight of potassium silicate of formula K2O "nSiO2, wherein n is from 4.1 to 6.0; and (c) from 7 to 12 parts by weight of water.

2. A composition according to claim 1, wherein said zinc and said iron phosphide are of average particle size between 2 μm and 20 μm.

3. A composition according to claim 2, wherein said zinc and said iron phosphide are of average particle size of 3 to 8 μm.

4. A composition according to claim 1, wherein n is from 4.5 to 6.0.

5. A composition according to claim 1, wherein n is from 5.0 to 6.0.

6. A composition according to claim 1, wherein n is from 5.3 to 6.0.

7. A composition according to claim 1, wherein n is from 5.0 to 5.3.

8. A composition according to claim 1, further comprising lithium silicate Li 0 < »PSi? 2, where p is from 3.0 to

9. A composition according to claim 1, further comprising sodium silicate Na2O "mSiO2, wherein m is from 2.0 to 4.0.

10. A composition according to claim 1, further comprising a filler. eleven . A composition according to claim 10, wherein said filler is mica. 12. A composition according to claim 1, comprising: (a) from 23 to 28 parts by weight of a pigment, said pigment comprising a mixture of from 45 to 55 percent by weight of zinc and from 45 to 55 percent by weight of iron phosphide; (b) from 3 to 4 parts by weight of potassium silicate of formula K20"nSi02, wherein n is from 5.0 to 5.5; and (c) from 9 to 10 parts by weight of water.