Brazing pre-flux coating with improved corrosion performance

Abstract

A pre-flux coating for the manufacturing of heat exchanger components of aluminum, wherein the coating comprises a combination of fluxes in the form of potassium aluminum fluoride K.sub.1-3AlF.sub.4-6, potassium trifluoro zincate, KZnF.sub.3, lithium aluminum fluoride Li.sub.3AlF.sub.6, a filler material in the form of metallic Si particles, Al—Si particles and/or potassium fluoro silicate K.sub.2SiF.sub.6, an additive in the form of aluminum oxide and at least one other oxide selected from the group consisting of zinc oxide, titanium oxide and cerium oxide forming a post braze ceramic layer, and a solvent and a binder containing at least 10% by weight of a synthetic resin which is based, as its main constituent, on a methacrylate homopolymer or a methacrylate copolymer.

Claims

1. A pre-flux coating for the manufacturing of heat exchanger components of aluminum, wherein the coating comprises a combination of fluxes in the form of potassium aluminum fluoride K.sub.1-3AlF.sub.4-6, potassium trifluoro zincate, KZnF.sub.3, lithium aluminum fluoride Li.sub.3AlF.sub.6, filler material in the form of metallic Si particles, Al—Si particles and/or potassium fluoro silicate K.sub.2SiF.sub.6, an additive in the form of aluminum oxide and at least one other oxide selected from the group consisting of zinc oxide, titanium oxide and cerium oxide forming a post braze ceramic layer, and a solvent and a binder containing at least 10% by weight of a synthetic resin which is based, as its main constituent, on a methacrylate homopolymer or a methacrylate copolymer.

2. The pre-flux coating according to claim 1, wherein the coating is blended as (i) a one layer coating comprising the flux component, the filler material and the additive mixed with the binder and the solvent, or (ii) a multi-layer coating comprising the flux component, the additive and the filler material mixed as separate coatings with the binder and the solvent.

3. The pre-flux coating according to claim 2, wherein the multi-layer coating comprises 2, 3 or 4 individually blended coating layers, wherein each layer comprises the binder and the solvent with at least one of (i) the flux component, (ii) the additive, and (iii) the filler material or a filler generating material.

4. The pre-flux coating according to claim 1, wherein the potassium aluminum fluoride, K.sub.1-3 AlF.sub.4-6, is at least one compound selected from the group consisting of KAlF.sub.4, K.sub.2AlF.sub.5, and K.sub.3AlF.sub.6.

5. The pre-flux coating according to claim 1, wherein the aluminum component is based on an aluminum alloy with a high Mg content, and the coating further comprises an additional flux component of cesium aluminum fluoride CsAlF.sub.4.

6. The pre-flux coating according to claim 1, wherein a ratio of the particles and the binder is between 3:1 to 4:1.

7. The pre-flux coating according to claim 1, wherein a ratio of particles of the different components of the coating corresponds to a load of 0-5.2 g/m.sup.2 Si, 1.41-16 g/m.sup.2 Zn flux (KZnF.sub.3), 2.2-9.2 g/m.sup.2 potassium flux (KAlF.sub.4/K.sub.3AlF.sub.6) and 0.1-5 g/m.sup.2 additive/oxide.

8. A method of applying the pre-flux coating according to claim 1 on an aluminum component as a one layer coating or a multi-layer coating, wherein the method comprises (i) applying in one operation a mixture of the flux component, the filler material, the binder and the solvent on the aluminum component to obtain the one layer coating, or (ii) applying a first layer comprising a mixture of the flux component, the filler material and a second layer comprising the binder and the solvent, and optionally applying an intermediate curing between the first and second layers, on the aluminum component to obtain the multi-layer coating.

9. The method according to claim 8, wherein the coating is applied on the aluminum component by roll coating or dip coating.

Description

(1) The invention will now be further described in the following by way of examples. The major focus within the applicant's work on combining a braze material with additives was, as mentioned above, on non-automotive applications.

(2) In particular, applications relating to where varying conditions might cause influence to the heat exchanger, additional corrosion protection, beyond sacrificial Zn protection is desired. In the applicants former Norwegian patent application No. 20101172 the inventors further improved the applicant's HYBRAZ® coated products by introducing an Li-containing flux into the flux coatings for Al tubes used in heat exchangers since this flux after brazing provides flux residues on the surface of the product that show limited water solubility and therefore reduced attack from dissolved fluorides to the aluminum surface. This former invention thus provided a novel pre-flux coating which provides both sacrificial and passive protection and which, at the same time provides braze (filler) material for the joint formation and flux for removal of oxide layer.

(3) With the present invention the inventors have come a step further in their attempt to improve corrosion resistance against impact from varying conditions in industrial applications. Thus, the inventors got the idea to create and strengthen a post-braze ceramic layer on the substrate (product) to be brazed which is able to withstand better the requirements in HVAC&R applications. The idea was not to stop at the stage of a layer from standard post-braze flux residues but maintain the stability of this ceramic layer with additives.

(4) Different materials were added to HYBRAZ® coatings and the corrosion behaviour of brazed minicores were tested by SWAAT exposure.

(5) Initially, aluminum oxide was used as additive and a clear improvement was seen which is further shown as discussed below.

(6) In a second step other materials will be investigated. This work is ongoing and will be finished within 2011. Other materials are zinc oxide, titanium oxide, cerium oxide and common pigments used as corrosion protection pigments in lacquers/paints

(7) The HYBRAZ® pre-flux coating according to the present invention is based on a mixture including flux particles from one or more fluxes with different properties, as well as Si particles as filler material and suitable oxide or material forming a post braze ceramic layer and further including a solvent and binder. More precisely the present invention may be composed of fluxes such as potassium aluminum fluoride (K.sub.1-3 AlF.sub.4-6), potassium trifluoro zincate (KZnF.sub.3), lithium aluminum fluoride Li.sub.3AlF.sub.6, filler material in the form of metallic Si particles, Al—Si particles and/or potassium fluoro silicate K.sub.2SiF.sub.6, and aluminum oxide and/or other suitable oxide or material forming a post braze ceramic layer, the coating further including solvent and binder containing at least 10% by weight of a synthetic resin which is based, as its main constituent, on methacrylate homopolymer or methacrylate copolymer.

(8) The potassium aluminum fluoride (K.sub.1-3Al.sub.F4-6) as mentioned above can be KAlF.sub.4, K.sub.2AlF.sub.5 and K.sub.3AlF.sub.6 or a combination of these. This is a product from a real synthesis.

(9) Potassium trifluoro zincate, KZnF.sub.3 can be added for corrosion protection by forming a Zn diffusion gradient into the brazed product.

(10) The potassium fluoro silicate K.sub.2SiF.sub.6 reacts with Al and generates Si metal, which forms AlSi12 as filler metal.

(11) Further, lithium aluminum fluoride Li.sub.3AlF.sub.6 may as an option be added for limiting water solubility of flux residues and therefore limited attack from stationary water.

(12) As to the particles added to form the post braze ceramic layer, beyond the aluminum oxide, other suitable oxides such as zinc oxide, titanium oxide, cerium oxide or combination of these may be added. Further, common pigments used as corrosion protection pigments in lacquers/paints may be added.

(13) Correct composition is required to obtain the desired effect from post-braze flux residues. For alloys with high Mg, optionally potassium aluminum fluoride (see above) plus cesium aluminum fluoride CsAlF.sub.4, mechanically blended, may be added.

(14) As to the composition of the coating materials, the content of solvent may preferably be approximately 30 wt % depending on the desired application properties. Further the ratio of particles and binder may vary from 3:1 to 4:1.

(15) Additional thickener might be added to the coating material (cellulose), content approx. 14 wt % related to acrylic binder.

(16) The ratio of particles of the different fluxes may vary as is apparent from the table below.

(17) The coating as applied on an aluminum component may further vary with different total load between 8 g/m.sup.2 and 16 g/m.sup.2. See as well in this connection the table below.

(18) TABLE-US-00001 TABLE (particle content): Silicon Zn Flux Flux Additive (Si) (KZnF.sub.3) (KAlF.sub.4/K.sub.3AlF.sub.6) (oxide/pigment) g/m.sup.2 g/m.sup.2 g/m.sup.2 g/m.sup.2 Ratio 0-4.5   2-16 4-8 0.01-5 (coating) Ratio (load) 0-5.2 1.41-16 2.2-9.2 0.01-5

(19) The coating is produced by mixing based on the following sequence: blending of solvent and binder by stirring in a suitable blender, and adding of the flux particles and oxide or other material particles forming the ceramic layer to the solvent and binder composition under continuous stirring, thorough mixing of the composition until desired quality with respect to specified parameters of the coating material is obtained.

(20) Upon application of the coating on the components to be brazed, the coating is again subjected to stirring to guarantee a homogenous coating material. During the stirring operation viscosity of the coating is adjusted according to the application process and equipment.

(21) Drying of coated components may take place in a separate drying process, e.g. using IR light or other heating sources.

(22) It should be stressed that the invention as defined in the claims is not restricted to the example as described above. Thus, the coating may be blended and applied as a one layer coating or a multi layer coating.

(23) One layer coating represents the preferred embodiment of the invention and implies that all flux components are mixed with binder and solvent and are applied in one step to the aluminum surface.

(24) As a multi layer coating is understood that the coating is mixed as separate coatings with binder and solvent and can be applied in 2, 3 or 4 layers as follows: 2 layer coating: In a first layer flux, potassium aluminum fluoride, and filler material or filler generating material are applied to the aluminum surface. In a second layer the additive is applied. The coating applied either in the first or in the second layer might contain other components such as potassium trifluoro zincate or lithium aluminum fluoride. The opposite direction of the two layers is possible too, with the additive/oxide as first layer. 3 layer coating: Each component is applied as a single coating layer. Flux coating layer Filler material or filler generating material coating layer. Potassium trifluoro zincate coating layer. The additive can be applied within each of the coating layers. Each layer might con lithium aluminum fluoride. 4 layer coating: Each component is applied as a separate coating layer as with the 3 layer above.

(25) In the case of a multi layer coating it will be important to control the total amount of binder to avoid any trouble from too high content of organic resin and therefore trouble in brazing. In case of a multi layer coating some of the layers might be discontinuously applied.

(26) As to how the pre-flux coating may be provided on an aluminum component, any technique may be used such as roll coating, dip coating, spray coating or even screen printing.