WO2009127707A1

WO2009127707A1 - Anticorrosive process for fluoroaluminate brazed aluminium parts

Info

Publication number: WO2009127707A1
Application number: PCT/EP2009/054564
Authority: WO
Inventors: Placido Garcia-Juan; Andreas Becker; Alfred Ottmann; Thomas Born; Hans-Walter Swidersky
Original assignee: Solvay Fluor Gmbh
Priority date: 2008-04-18
Filing date: 2009-04-16
Publication date: 2009-10-22

Abstract

Aluminium parts, e.g. heat exchangers, with improved resistance towards corrosion caused by contact with water for an extended period of time can be obtained by applying a heat treatment or a flux which provides after brazing a high content of K₃AlF₆. The corrosive effect can also be suppressed if at least one anticorrosive agent selected from the group of water-soluble calcium containing salts with the exception of CaF₂ and salts containing the AlF₄, AlF₅ or AlF₆ ion and hydrates thereof is contacted with the contact water which contains leached fluoride ion.

Description

ANTICORROSIVE PROCESS FOR FLUOROALUMINATE BRAZED ALUMINIUM PARTS

The invention concerns brazed aluminium parts with improved anticorrosivity and a process for reducing the corrosive influence of fluoride ions on aluminium parts brazed using potassium fluoroaluminate.

It is well known in the art that brazing of aluminium parts can be performed utilizing fluxes based on alkali metal fluoroaluminates, especially potassium fluoroaluminate and cesium fluoroaluminate. Fluxes of this type are generally considered to be noncorrosive. See for example, US patent 3,971,501 which applies a flux based on KAlF₄ and K₃AlF₆, or US patent 4,689,092 which applies a flux based on potassium fluoroaluminate and cesium fluoroaluminate. One of the advantages compared to chloride fluxes which had been applied for brazing is that the latter must be removed after the brazing process to prevent corrosion induced by chloride residues to take place. As is described in US-A 3,971,501 the potassium fluoroaluminate based fluxes leave no water- soluble residue. The use of these fluxes, as is stated in said US patent, obviates the necessity to remove flux residue and the possibility of corrosion resulting from unremoved flux residue.

US patent 6,949,300 discloses kinetic spraying onto metal substrates of a brazing composition that comprises corrosion protector, brazing filler and/or non-corrosive flux. Contrary to belief in the brazing industry concerning the non-corrosive properties of potassium fluoroaluminate fluxes, it was observed that under certain circumstances, flux residues may cause a certain level of corrosion. Corrosion was especially observed if water or an aqueous composition (i.e. a composition which contains water) remained in contact long enough with brazed aluminium so that a part of the flux, though the flux is generally only very sparely soluble in water, was dissolved to a certain concentration. This was observed even if water or an aqueous composition circulated in brazed aluminium parts.

If contacted for extended times with water or aqueous liquids, aluminium parts brazed with potassium fluoroaluminate based fluxes show signs of corrosion. This is disclosed by Bo Yang et al. in Journal of ASTM International, VoI. 3, Issue 10 (2006). The corrosion can be recognized by the appearance of turbidity in the water or liquid. This corrosion seems to be caused by fluoride ions dissolved in the water which are leached from brazing residues if the brazed parts are in contact with water for extended periods of time, e.g. for at least one day or longer. Bo Yang et al. demonstrate that "Hybrid" coolants, "Organic acid" coolants and conventional "High silicate" coolants provide a certain degree of protection against corrosion.

The occurrence of corrosion can be recognized by the formation of a white precipitate or turbidity assumed to be aluminium oxide, aluminium hydroxide and/or aluminium oxide hydroxide. Such corrosion is, of course, undesired, and the present invention provides a solution to this problem.

Consequently, the invention provides a process for reducing corrosion of potassium fluoroaluminate flux-brazed aluminium parts caused by fluoride ions leached from flux residues during the extended period of time in contact of the parts with water or aqueous compositions by applying, for brazing of the parts, a potassium fluoroaluminate flux which forms a flux residue with reduced leachable fluoride ion content, or by subjecting the brazed parts to an oxidative heat treatment after brazing, or by applying to the brazed parts at least one anticorrosive agent selected from fluoride ion scavengers and one or more compounds reducing the solubility of the flux residue. Two or more of these alternatives can be combined, for example, in one embodiment, a potassium fluoroaluminate flux can be applied which forms a flux residue on the brazed parts with reduced leachable fluoride ion content, and then subjecting the brazed parts to a heat treatment after brazing. In a preferred embodiment, the invention provides a process for reducing corrosion of potassium fluoroaluminate flux-brazed aluminium parts caused by fluoride ions leached from flux residues during an extended period of time of the parts with water or aqueous compositions comprising a step of applying, for brazing of the parts, a potassium fluoroaluminate flux which forms a flux residue with reduced leachable fluoride ion content, or of subjecting the brazed parts to an oxidative heat treatment after brazing, or of applying to the brazed parts at least one anticorrosive agent selected from fluoride ion scavengers and one or more compounds reducing the solubility of the flux residue, and a step of contacting the brazed parts with water for an extended period of time. If a flux high in dipotassium pentafluoroaluminate or an oxidative heat treatment are applied for reducing corrosivity, then the step of contact of the brazed part with water occurs after brazing and/or heat treatment, respectively. If at least one anticorrosive agent selected from fluoride ion scavengers and one or more compounds reducing the solubility of the flux residue are applied to the brazed parts, then the step of applying these compounds and the step of contact of the parts with water can be performed simultaneously, or the contact with water occurs afterwards.

In another aspect, the present invention provides a process for brazing of aluminium parts to aluminium parts wherein a potassium fluoroaluminate flux is applied to the parts, the parts are joined, brazed and, after brazing, are contacted with water or aqueous compositions for an extended period of time with the proviso that, for brazing of the parts, a potassium fluoroaluminate flux is used which forms a flux residue with reduced leachable fluoride ion content, and/or by subjecting the brazed parts to a heat treatment after brazing, and/or by applying to the brazed parts at least one anticorrosive agent selected from fluoride ion scavengers and one or more compounds reducing the solubility of the flux residue.

The term "aluminium parts" denotes in the present invention parts made from pure aluminium and parts made from aluminium alloys. Aluminium often contains manganese (up to 1.5 % by weight), silicon, magnesium (often up to 0.3 % by weight), iron (up to 0.7 % by weight), copper, zinc, titanium, and sometimes low amounts of chromium or nickel. The content of alloy metals usually lowers the melting point. The melting point of aluminium alloys in the frame of the present invention should not be too low because a too low melting point may have negative effects in brazing processes. A melting point of equal to or greater than 630⁰C gives good results. If magnesium is present, it may be advantageous to apply a flux which contains cesium fluoroaluminate. The invention can be applied with good results to Al AA3003 alloy. Preferred brazed aluminium parts are heat exchangers. The term "corrosion" preferably denotes the formation of white turbidity or a white precipitate, which is explainable by the formation of aluminium hydroxide, aluminium oxide or aluminium oxide hydroxide or hydrates thereof. In the present invention, the term "caused by fluoride ions" is used. The reason is that the greatest corrosive influence is allocated to the fluoride ion. It is considered possible that other species originating from the dissolution of the flux residue may also have corrosive properties. Thus, the term "caused by fluoride - A -

ions" does not exclude the possibility that corrosion is caused by other species present in the water or aqueous solution.

The term "water" as used in the present invention includes water from natural sources, e.g. rain water, water formed as dew, and water formed after snow melt. It includes artificial water sources, e.g. tap water. The term

"aqueous compositions" in its broadest sense includes any composition which contains water and at least one additional component, e.g. an inorganic or organic salt or an organic liquid, e.g. a monobasic or dibasic alcohol, and comes into contact with brazed aluminium parts. It includes, for example, cooling liquid which, besides water, usually additionally contains antifreeze compounds, especially glycol, and additives, for example, anticorrosives or colorants, such as those used in water coolers for stationary refrigeration equipment or stationary heat exchangers or in cooling water for vehicles. Preferably, the aqueous composition is a cooling liquid or a cleaning composition. In a preferred embodiment, the terms "water" and "aqueous composition" in the frame of the present invention do not include artificially manufactured salt water or salt water from natural sources (e.g. sea water) when used for test purposes to identify properties of the brazed parts, especially salt water from a natural source or manufactured artificially to be used according to the AST-M G43 or G85 SWAT test. In another preferred embodiment, the terms

"water" and "aqueous composition" do not include water with a sodium chloride salt concentration equal to or greater than 4 % by weight; such a composition is used in the SWAT test. In still another preferred embodiment, the term "water" and "aqueous compositions" does not include sodium chloride solutions containing added acetic acid, or when contact between brazed parts and a sodium chloride solution is performed in a chamber which contains added sulfur dioxide, or water shich contains dissolved sodium chloride and added ammonium sulfate. Such solutions are used in the ASTM G 85-94 tests. Consequently, the deliberate treatment of brazed parts with sodium chloride containing solutions, especially when applied for testing, is disclaimed.

Preferably, brazing is performed in an inert gas atmosphere. This is also called Controlled Atmosphere Brazing ("CAB" process). Usually, the inert gas consists of nitrogen, argon or their mixtures. The oxygen content in the inert gas generally is lower than 2 % by volume, preferably lower than 1 % by volume, and especially equal to or lower than 100 ppm by volume. The oxidative heat treatment, the application of a scavenger for fluoride ions and the application of one or more compounds reducing the solubility of the flux residue can be applied to brazed parts wherein any flux based on potassium fluoroaluminate had been used. For example, the oxidative treatment can be performed after brazing using fluxes based on potassium fluoroaluminate, for example as described in US 3,951,328 (fluxes containing mainly KAlF₄ and K₃AlF₆), US 4,428,920 (describes how to prepare a potassium fluoroaluminate composition from tetrafluoro aluminium acid and potassium salts), US 4,579,605 (describes how to prepare fluxes with a variable content OfKAlF₄ and K₂AlF₅ or its hydrate), US 5,318,764 (describes several methods for the preparation of KAlF₄ or K₃AlF₆), or US 5,980,650 (mentions, i.a., how reversibly dehydrated flux is prepared - by drying at comparatively low temperatures; describes the preparation and uses of a flux comprising irreversibly dehydrated K₂AlF₅). The process can also be applied for aluminium parts brazed with fluxes which additionally comprise cesium ions; such fluxes are especially suitable to braze aluminium alloys with relatively high magnesium content. For example, parts brazed using fluxes described in EP-B-O 239642 (describes the use of cesium fluoroaluminate for brazing of Al-Mg alloys, optionally in admixture with conventional fluxes), or US4,670,067 (describes mixtures of cesium fluoroaluminates and potassium fluoroaluminates and their use for brazing of Al-Mg alloys).

The process can also be applied to aluminium parts brazed with fluxes based on fluoroaluminates modified by certain other metal compounds. For example, it can be applied to parts brazed by a flux modified by zirconium fluoride or titanium fluoride, optionally in the additional presence of certain metal oxides, for example, oxides of zirconium, titanium, manganese, hafnium or rhenium as described by US 2004/0163734.

It also can be applied to aluminium parts brazed with a flux based on alkali metal fluoroaluminates modified by metal salts of metals of the main group or sub groups of the periodic system of the elements, for example, halides, nitrates, carbonates or oxides of zirconium, niobium, lanthanum, yttrium, cerium, titanium, strontium, indium, tin, antimony, or bismuth. It can also be applied to fluxes based on potassium and cesium fluoroaluminates comprising salts of cerium, bismuth and lanthanum as described in WO 2007/131993. The process can also be applied to aluminium parts brazed with fluxes based on alkali metal fluoroaluminate comprising additives. For example, it can be applied to flux comprising filler metal, e.g. Al-Si filler metal, e.g. in powder form. Such a flux can be applied to aluminium parts which must not be plated by filler alloy. It can also be applied to alkali metal fluoroaluminate flux which comprises a filler alloy precursor. This is, for example, described in US 5,100,048. As filler alloy precursor, for example, silicon, germanium or copper can be contained. Alternatively, or additionally, silicon dioxide or alkali metal fluoro silicates can be contained as filler metal alloy precursor. The process can be applied to parts having been brazed using fluxes per se; e.g. fluxes applied as dry flux using electrostatic forces. The process can also be applied to parts having been brazed using flux compositions which comprise a flux, optionally together with the modifying agents mentioned above, and additives which improve the applicability. For example, it can be applied to parts brazed with flux compositions comprising a solvent and/or a binder and/or thickeners. For example, water or organic liquids, e.g. alcohols, compounds with two or more hydroxyl groups, e.g. monoalcohols, diols, for example, ethylene glycol, polyols, for example, glycerine, ketones, pyrrolidones, nitriles, or ethers including hydroxysubstituted ethers, e.g. glycol ethers, for example, ethylene glycol, can be contained as solvent. Isopropanol is, for example, a very suitable solvent. As a binder, many organic compounds are useful. For example, flux compositions can be used which contain polyurethanes, organic resins, phthalates, acrylates, vinyl resins, epoxy resins, nitrocellulose, polyolefines or hydro xyethyl cellulose as a binder. The binder may also be partially or completely be present in the form of a thixotropic agent, for example, in the form of gelatine, pectines, acrylates or polyurethanes.

Suitable compositions are described, for example, in US 3,951,328, EP-A 1287941, WO 2005/092563, and WO 2007/131993.

The process can be applied to parts brazed with fluxes of a very variable particle size. For example, the particles may have the particle size distribution as disclosed in US-A 6,733,598. A flux comprising particles with such a particle size distribution can be applied for example in a dry flux method. In dry fluxing, the flux is applied without solvent to the surface of the metal parts to be joined by brazing. This can be supported by electrostatic power. The particles of the flux may be of a coarser nature than the finer particles disclosed in said US 6,733,598. Such coarser fluxes are very suitable for the application in the form of a flux composition including the flux dispersed a solvent; they can, for example, be applied by painting, printing or spraying onto the parts.

Preferably, the process is applied to potassium fluoroaluminate brazed parts which have a contact with water or aqueous compositions for an extended period of time with a respective higher level of fluoride leaching. The term "extended period of time" denotes a period of contact which lasts at least one day, preferably at least 2 days. The term "extended period of time" has no specific upper limit. It may last for one week or longer. In the case of water containing cooling liquid, for example, the contact between the liquid and aluminium can last for years. Often, the upper limit is equal to or shorter than 1 year, preferably, equal to or shorter than 6 months, more preferably, equal to or shorter than 2 months.

Reduction of corrosive influence of fluoride ions leached from the brazed parts during extended contact with water or aqueous compositions can be achieved according to the invention by two alternatives : by using a specific flux for brazing which provides low content of leachable fluoride ion, or by treating the brazed parts. The latter alternative includes a heat treatment, the application of a scavenger for fluoride ions or by applying a compound which reduces the solubility of the flux residue.

In the following, applying a heat treatment, or applying fluxes which provide low contents of leachable fluoride will be explained in detail.

In one preferred embodiment, the brazed parts are post-treated by an oxidative heat treatment after brazing. It was observed that fluoride leached out of the flux residues after extended periods of time of contact of the aluminium parts with water or aqueous solutions has less corrosive impact on the brazed parts compared to brazed parts without heat treatment after brazing.

Brazing is preferably performed in an inert gas atmosphere, preferably, in an atmosphere which contains at least one inert gas selected from the group consisting of nitrogen or argon, or in an atmosphere which consists of nitrogen, argon or their mixtures. The inert gas is essentially oxygen free. The term "essentially" denotes preferably an atmosphere during brazing which contains less than 2 % by volume of oxygen, more preferably less than 1 % by volume, and most preferably, equal to or less than 100 ppm per volume. After brazing, the brazed parts are subjected to a heat treatment in an oxygen containing atmosphere. If desired, the brazed aluminium part can be treated as a whole. In a technical view, this can be performed in a very simple manner. If desired, it can be foreseen that only those parts are treated with the oxygen containing atmosphere which can be expected to be contacted with water or aqueous liquids for extended times. For example, if only the inner walls of the heat exchanger tubes through which cooling liquid circulates are to be treated, an oxygen containing gas atmosphere is allowed to be present within the tubes but the outer walls are kept under inert gas.

Preferably, the temperature of the brazed parts at heat treatment is equal to or higher than 400⁰C. Preferably, it is equal to or lower than 530⁰C. If desired, the temperature may be higher, e.g. up to 550⁰C or even more. The preferred range for the oxidative heat treatment is equal to or higher than 400⁰C and equal to or lower than 530⁰C.

A preferred oxygen containing atmosphere is air.

The duration of the heat treatment is preferably equal to or longer than 10 seconds, especially preferably equal to or longer than 30 seconds. It is preferably equal to or shorter than 1 hour, especially equal to or shorter than 15 minutes. A preferred range for the oxidative heat treatment is equal to or longer than 30 seconds, and equal to or shorter than 15 minutes.

An oxidizing heat treatment to improve corrosion resistance is already known from EP-A-O 347106. However, it is not clear from the description of said EP application relative to which kind of corrosion or corrosion caused by what corrosive agent the treated aluminium parts might be protected. A reference to the examples indicates that the protection is intended against corrosion caused by salt water. Said EP application is not addressing problems caused by fluoride ions leached from the flux after contact with water for an extended period of time, for example, for one day or longer.

It must be considered very surprising that a simple heat treatment can reduce corrosivity caused by contact with water containing fluoride ions leached from flux residues after an extended period of time. According to another embodiment of the present invention, a specific flux is used for brazing which provides low content of leachable fluoride ion after brazing is performed. In this embodiment, a potassium fluoroaluminate-based flux is applied which comprises equal to or less than 80 % by weight of KAlF₄. It was found by the inventors that dipotassium pentafluoroaluminate has an anticorrosive effect because, compared to other potassium fluoroaluminate fluxes, less fluoride ions are leached during extended water contact of the brazed parts. It is known that dipotassium pentafluoroaluminate and its hydrates are converted during brazing to form a flux residue containing mainly monopotassium tetrafluoroaluminate and tripotassium hexafluoroaluminate. The formed tripotassium hexafluoroaluminate, if contacted with water, forms dissolved potassium fluoride and almost insoluble dipotassium pentafluoroaluminate. The surprising effect is that - though corrosive potassium fluoride is formed - the anticorrosive effect of the resulting dipotassium pentafluoroaluminate overcompensates the corrosive effect of the fluoride ions and renders an anticorrosive effect to the brazed aluminium. As a consequence of the knowledge gained during investigation of the underlying problem, it was found to be advantageous to apply a potassium fluoroaluminate flux the residues of which, after brazing, contain as much OfK₃AlF₆ as possible. While it might be possible to apply a brazing flux high in K₃AlF₆, this is not preferred because this compound has disadvantages in brazing (for example, its melting point is rather high). Instead, it is preferred to apply a flux comprising monopotassium fluoroaluminate and dipotassium pentafluoroaluminate or its hydrates containing equal to or less than 70 % by weight of KAlF₄, preferably equal to or less than 65 % by weight, especially preferably equal to or less than 60 % by weight OfKAlF₄. Preferably, the remainder to 100 % by weight is K₂AlF₅ and/or its hydrate. The content OfK₂AlF₅ and/or its hydrate is preferably equal to or greater than 20 % by weight.

In one preferred embodiment, the flux contains or consists of 20 to 45 % by weight OfK₂AlF₅ and/or its hydrate, more preferably, contains or consists of 25 to 45 % by weight OfK₂AlF₅ and/or its hydrate; the remainder KAIF₄. In another very preferred embodiment, the content of dipotassium pentafluoroaluminate and/or its hydrate in the flux is as high as possible, and is equal to or greater than 95 % by weight, especially 98 % by weight ± 2 %. The remainder is KAlF₄ and/or K₃AlF₆.

IfK₃AlF₆ is present, its content is preferably equal to or lower than 5 % by weight.

The weight per area of the flux coating on the parts before brazing can vary. For example, it can be in a range of 1 to 40 g/m². Preferably, it is in a range between 1 and 20 g/m². More preferably, it is in a range between 2 and 12 g/m². It was found that the anticorrosive effect is especially remarkable in a range between 7 and 12 g/m². For practical purposes, a range between 3 and 10 g/m² is preferred because the flux residues on parts with such a flux loading are technically preferred.

Additionally, as will be later described, further treatments can be performed to reduce corrosivity of brazed parts, especially the addition of a fluoride scavenger or reducing the solubility of the flux residues.

The dipotassium pentafluoroaluminate can be produced as described in US-A 4,579,605. According to that patent, aluminium hydroxide is dissolved in hydrofluoric acid, and the resulting product is then neutralized with a potassium compound. The concentration of hydrofluoric acid is in the range of 5 to 40 % by weight, the ratio of A1:F:K of the starting material is in the range of 1 :4-4.5:1-1.5, the pH at the time of completion of the reaction is below 4 and the reaction temperature is in the range of 30⁰C to 100⁰C. To obtain essentially pure K2A1F5, the ratio needed is 1 :5-5,l :2-2,l. The potassium compound may be potassium carbonate or a KOH solution with a concentration of 5 to 50 % by weight. Reaction conditions which favor the production of a flux with high content of dipotassium fluoroaluminate can be taken from the many examples of that US patent. For example, the reaction temperature advantageously is lower than 60⁰C (see examples 7 to 10), and/or the content of Al and F is advantageously in the upper range, see example 17, and/or the concentration of HF and/or KOH solution is advantageously in the lower range, see examples 18, 10 and 19 and 20, 10 and 21, respectively. If desired, the hydrate may be converted to irreversibly dehydrated dipotassium pentafluoroaluminate as described by US-A 5,980,650 by heating to sufficiently high temperature, e.g. shortly to 375°C or higher. According to two further embodiments of reducing the corrosivity caused by fluoride ions on potassium fluoroaluminate-brazed aluminium in water or in aqueous compositions wherein at least one anticorrosive agent selected from fluoride ion scavengers and one or more compounds reducing the solubility of the flux residue is applied. The anticorrosive agent is applied as a coating on the brazed aluminium.

Alternatively or additionally, the anticorrosive agent is added to the water or aqueous composition.

"Scavenger compounds for fluoride" are compounds which react with the fluoride ion preferably under the formation of salts with lower solubility than the leached fluoride salt originally present in the water or aqueous composition. For example, calcium salts or salts, especially alkali metal salts, very preferably the potassium salt, containing the tetrafluoroaluminate anion are very suitable. KAlF₄ can be applied as scavenger for fluoride ions because the tetrafluoroaluminate anion reacts with the fluoride anion under formation of pentafluoro aluminate . Compounds reducing the solubility of the flux residue (which, as will be explained later, contains predominantly of KAlF₄ and K₃AlF₆) shift the solubility equation, e.g. by adding potassium salts or by adding salts with the AlF₄, AlF₅ or AlF₆ anion, especially the respective potassium compounds. In a preferred embodiment, the present invention provides a process for reducing the corrosivity of fluoride ions on potassium fluoroaluminate-brazed aluminium in water or in aqueous compositions wherein at least one anticorrosive agent selected from the group of water-soluble Ca containing salts and salts shifting the solubility equation of flux residues, preferably alkali metal salts, especially the potassium salts, containing the AlF₄, AlF₅ or AlF₆ ion and hydrates thereof with the exception Of CaF₂, is contacted with the fluoride containing aqueous solution. This contacting is performed after brazing is terminated. Preferably, the water-soluble calcium salt is the salt of an organic acid.

The term "water-soluble" denotes those salts which have a solubility in at 20⁰C of equal to or greater than 1 g/liter.

The term "Ca containing salts" includes inorganic and organic salts. Preferably, CaCl₂ is not applied. Especially preferably, the anion is selected from the group of organic acids. The anion can be a monobasic, dibasic or polybasic acid. The organic residue attached to the carboxyl group can be composed of carbon and hydrogen, and it can be linear or branched. It can be substituted by one or more oxo groups, hydroxy groups, nitro groups or halogen atoms. The calcium salts containing calcium cationsions and anions of alkyl carboxylic acids with, in total, 2 to 12 carbon atoms are very suitable. Calcium salts containing calcium cations and anions of alkyl carboxylic acids with, in total, 2 to 12 carbon atoms which are substituted by one or more hydroxy groups are likewise very suitable. Highly suitable are the calcium salts of acetic acid, propionic acid, lactic acid and gluconic acid.

Suitable inorganic calcium salts are, for example, the nitrates, iodides and thio sulfates. In the following, it is described how the solubility of the flux residues is lowered. The flux residue essentially consists of KAlF₄ and K₃AlF₆. Thus, adding compounds which influence the solubility product or solubility of these constituents to the water or aqueous composition in contact with the brazed parts is advantageous. In principle, addition of potassium compounds (but preferably not potassium chloride) is feasible. It is preferred to add KAlF₄, K₂AlF₅ or K₃AlF₆, their hydrates or mixtures thereof to the water or aqueous composition because addition of these compounds (which have a relatively low solubility in water) yield water or aqueous compositions saturated with these compounds, and thus reduce the dissolution of the flux residue. In some cases, addition of K₂AlF₅ or K₃AlF₆ is preferred. The fluoride ion scavenger or the compound(s) reducing the dissolution of flux residues, especially K₂AlF₅ or K₃AlF₆, but also KAlF₄, can be applied dry, as an aqueous solution or suspension. For example, they can be sprayed onto the brazed parts, or the brazed parts can be immersed into a solution or suspension, and dried thereafter so that a coating results. If desired, a water-soluble binder, e.g. a water-soluble polyurethane or polyvinyl alcohol, can be used. The scavenger or the preferred complex aluminium salts K₂AlF₅ or K₃AlF₆ can be added to the liquid fluid, e.g., to cooling water, if desired. They also can be added dry, as solution or as a suspension to the water or aqueous composition which is in contact with the brazed parts. If desired, up to 30 % by weight of monopotassium tetrafluoroaluminate can be contained in the penta- or hexafluoroaluminate applied after brazing to shift the solubility equation of the flux residues.

The suitability of the complex fluoroaluminate salts is very surprising. It is well known that potassium fluoroaluminate brazing flux usually contains or consists of monopotassium tetrafluoroaluminate and dipotassium pentafluoroaluminate. The content of tripotassium hexafluoroaluminate in the flux itself before brazing is often very small, and often, the flux is produced free of that compound. It is further known that dipotassium pentafluoroaluminate transforms to monopotassium tetrafluoroaluminate and tripotassium hexafluoroaluminate in the flux melt. Based on the inventor's analytical results, tripotassium hexafluoroaluminate, in contact with water, forms dipotassium pentafluoroaluminate which is sparely soluble, and dissolved potassium fluoride. Thus, it must be assumed that tripotassium hexafluoroaluminate can potentially be a source of corrosive fluoride ions. Contrary to that, it was found that, dipotassium pentafluoroaluminate and tripotassium hexafluoroaluminate, as well as KAIF4, act anticorrosive if added to the aqueous solution or composition. The calcium salts or the fluoroaluminate salts can be applied in many ways. For example, they can be applied to the surface of the brazed aluminium parts as dry coating, for example, by applying it in the form of a water-soluble lacquer using water-soluble binder, or by applying a suspension or solution to the surface of the aluminium parts, for example, by spraying, and then drying the aluminium parts.

According to an alternative embodiment, the calcium salt or fluoro aluminium salt is added to the water or aqueous composition, as a preventive measure or when the first signs of corrosion are observed. The concentration is variable. The amount applied should be sufficient to guarantee the desired anticorrosive effect. Often, a concentration of equal to or greater than 1 g/1 water or aqueous composition is sufficient. Generally, a concentration of equal to or less than 5 g/1 is sufficient, but could be higher if desired, usually up to the saturation concentration. If desired, a solid deposit especially of the fluoroaluminate salts which have a relatively low solubility in water can be present. Such solid deposit serves as a reserve.

It was already mentioned that two or more of the embodiments of the invention can be combined. Thus, the brazed parts can be subjected to an oxidative heat treatment, and after the heat treatment, scavengers for the fluoride ion or compounds shifting the solubility equation of constituents of the flux residue are applied. Or a specific flux high in dipotassium pentafluoroaluminate is used for brazing, and subsequently, a heat treatment as described above is performed. Or, a flux high in dipotassium pentafluoroaluminate is applied for brazing, and subsequently, a scavenger for fluoride ions as described above is applied. Or, a flux high in dipotassium pentafluoroaluminate is used for brazing, and subsequently, the brazed parts are subjected to a heat treatment, and at the same time or afterwards, scavengers for the fluoride ion or compounds shifting the solubility equation of constituents of the flux residue are applied.

For example, a heat treatment at a temperature of equal to or higher than 400⁰C and preferably equal to or lower than 530⁰C in an oxygen containing atmosphere, e.g. air, can be performed after brazing of the parts but before applying the anticorrosive agents of the present invention or when the specific advantageous flux high in dipotassium pentafluoroaluminate described above was used having the effect that less leachable fluoride ion is present in the flux residue. The way of heat treatment is described above. The respective disclosure of line 23 of page 7 to line 29 of page 8 of this description is incorporated here by reference to avoid repeating those paragraphs.

The embodiment of applying a scavenger for fluoride ions or of compounds, especially those with a tetrafluoro- or pentafluoroaluminate anion, shifting the solubility equation of flux residues, can be applied to any aluminium parts brazed with fluxes based on potassium fluoroaluminate. To avoid repeating the respective paragraphs, aluminium parts can be treated as described in lines 17 to 28 of page 3 of this description which are incorporated here by reference. As to the fluxes suitable for brazing the parts which are then treated with fluoride ion scavengers or compounds shifting the solubility equation of flux residues, referral is made the paragraphs of page 5, line 1 to page 7, line 3 which are incorporated here by reference to avoid double citation.

A technical field where the process of the invention (to reduce the corrosive influence of fluoride ions by applying a specific flux, heat treatment, adding scavengers for fluoride ions or applying compounds shifting the solubility equation of flux residues) can especially be applied are large heat exchangers which are stored outside of weather-protected rooms and are in contact with rain water. The corrosivity of such heat exchangers can be reduced if a heat treatment is performed as described above, a specific flux is used for brazing as described above, and/or water-soluble calcium salts or KAlF₄, dipotassium pentafluoroaluminate or tripotassium hexafluoroaluminate are applied to the surface of these parts before and/or during contact of these parts with rain water.

Another technical field concerns brazed parts in contact with an aqueous composition in the form of the cooling water of fuel-driven motors, e.g. motors operated with gas, gasoline or diesel. Soluble calcium salts, for example, calcium gluconate or calcium acetate, and/or monopotassium tetrafluoroaluminate, dipotassium pentafluoroaluminate and/or tripotassium hexafluoroaluminate are added to the cooling water to reduce and even prevent fluoride-induced corrosion, and/or a heat treatment of the brazed parts is performed and/or a specific flux as described above is applied for brazing.

Another aspect of the present invention concerns aqueous compositions suitable as cooling liquid especially for motors fuelled by gas, gasoline or diesel. These compositions contain water and an antifreeze compound, especially glycol and optionally additives like colorants, lubricants, and one or more compounds with anticorrosive properties. The water-soluble calcium salt or the KA1F4, dipotassium pentafluoroaluminate or tripotassium hexafluoroaluminate are contained in an amount equal to or greater than 1 g/liter. Preferably, the are contained in an amount of equal to or lower than 5 g/liter. The cooling liquid preferably contains an anticorrosive agent selected from the group consisting of calcium acetate, calcium propionate, calcium gluconate or calcium lactate, K₂AlF₅, K₂AlF₅ H₂O or K₃AlF₆.

The invention also provides potassium fluoroaluminate-brazed aluminium parts with enhanced protection against corrosion. The parts (which comprise brazing residues) comprise a coating applied after brazing containing at least one anticorrosive agent selected from the group of water-soluble calcium containing salts and salts containing the AlF₄, AlF₅ or AlF₆ ion and hydrates thereof with the exception of CaF₂. The coating can, for example, be applied by spraying or painting an aqueous or organic solution or suspension onto the parts and drying them. If desired, a water soluble binder can be used, for example, gelatine, a water-soluble polyurethane, a polyvinylalcohol or a strippable varnish.

Another aspect of the present invention concerns a method of handling or using potassium fluoroaluminate flux-brazed aluminium parts wherein during handling or use, these parts come into contact with water or aqueous compositions the improvement which comprises the selection of potassium fluoroaluminate flux-brazed aluminium parts which have been obtained by selecting and applying a potassium fluoroaluminate flux forming a flux residue with reduced leachable fluoride ion and/or by subjecting the brazed parts to a heat treatment after brazing, and/or by applying to the brazed parts at least one anticorrosive agent selected from fluoride ion scavengers and one or more compounds reducing the solubility of the flux residue.

The term "handling" includes the steps of cleaning the parts using water or aqueous compositions, for example, cleaning compositions, for example, after the production of the parts, after transport or before or during their use, or storing the parts in a place which offers no complete protection against weather conditions like dew, snow or rain water. The term "use" especially includes the use of the parts for providing heat exchange, e.g. by removing heat from or transferring heat to water-containing cooling liquids in motors.

Preferred embodiments concerning the brazed parts to be selected are those given above. The following examples are intended to explain the invention further without the intent to limit it. Examples

Example 1 : Soaking test

Angle-on-coupon samples were prepared by brazing aluminium (AA3003) angles on coupons clad with aluminium- silicium alloy 4343. Nocolok^® flux was applied in a load of 10 g/m². Then, the assembled angle/coupons parts were brazed. They were brought into a polypropylene bottle, and the pH value was measured. It was found that after 5 minutes contact between brazed part and water, the pH was between 5.4 and 5.8 for different sets of parts. After 18 days contact, the pH was between 8.4 and 8.8 for the samples. A dummy part (not brazed aluminium AA3003) had an initial pH of 6.3 and a final pH after 18 days of 7.3. The test shows that the pH of brazed samples becomes higher, i.e. the water becomes basic.

Example 2 : Soaking test

In this example, several fluxes were brazed, and the change of the pH value was observed :

1. A dummy sample heated without flux

2. A sample brazed with pure KAlF₄,

3. Samples 3. a ( flux load 5 g/m²) and 3.b (flux load 10 g/m²), brazed with Nocolok^® Drystatic flux, a flux commercially available from Solvay Fluor GmbH in Hannover, specifically suited for dry static application,

4. A flux essentially consisting OfK₂AlF₅-H₂O

5. A flux essentially consisting of reversibly dehydrated K₂AlF₅

6. A flux essentially consisting of irreversibly dehydrated K₂AlF₅

7. A sample brazed with Nocolok^® LM (a flux essentially consisting OfK₂AlF₅ with a low melting point

8. A sample brazed with a flux containing 30 mol- % K₂AlF₅ where some milligrams of KF were added to the water after one hour contact with water.

9. A sample brazed containing 30 mol.- % K₂AlF₅ where one drop of HF was added to the water after one hour contact with water. After 166 hours of contact with water, the pH of the water was measured.

Also, the mass difference was determined by washing the samples with de- ionized water and subsequent drying and forming the difference to the weight of the sample before the soaking tests. The result is given in brackets. Results : Sample 1 : 6.4 (+ 0.2 mg) Sample 2 : 8.0 (- 2.4 mg) Sample 3.a : 8.1 (- 1.3 mg) Sample 3. b : 6.9 (- 1.8 mg)

Sample 4 6.9 (- 2.4 mg) Sample 5 7.0 (- 2.1 mg) Sample 6 7.1 (- 2.6 mg) Sample 7 6.8 (- 2.4 mg) Sample 8 7.6 (- 1.9 mg) Sample 9 6.7 (+ 10.2 mg)

These tests show that KAlF₄ flux (sample 2) is very corrosive, while fluxes rich in K₂AlF₅ (samples 4 to 7 - 9) are not corrosive; and that the fluoride ion, especially when entered in the form of potassium fluoride, seems to bring about a rise in the pH. High pH was accompanied with a formation of a white turbidity.

Example 3 : Tests with samples brazed with fluxes with varying K₂AlF₅ content As described above, angle-on-coupons were prepared by brazing, using fluxes with 20mol- % 30 mol- %, 40 mol- %, 50 mol- % and 100 % K₂AlF₅ (in % by weight : 34 % ; 37 %; 48 % ;58 % and 100 %); the remainder to 100 % being KAlF₄. The pH of the samples with 100 % K₂AlF₅ and with 30 % K₂AlF₅ was determined to be 6.8. The pH of the other samples was higher. Example 4 : Soaking test

Angle-on-coupon samples were prepared by brazing aluminium (AA3003) angles on coupons clad with aluminium-silicon alloy 4343. Nocolok^® flux was applied in a load of 10 g/m². Then, the assembled angle/coupons parts were brazed. They were brought into a polypropylene bottle, and the pH value was measured. It was found that after 5 minutes contact between brazed part and water, the pH was between 5.4 and 5.8 for different sets of parts. After 18 days contact, the pH was between 8.4 and 8.8 for the samples. A dummy part (not brazed aluminium AA3003) had an initial pH of 6.3 and a final pH after 18 days of 7.3. The test shows that the pH of brazed samples becomes higher, i.e. the water becomes basic.

Example 5 : Soaking test

In this example, a dummy sample, a sample brazed with pure KAlF₄ and two samples brazed with a flux comprising about 30 mol- % K2AIF5 were tested by contacting them with water. To one of these samples, some milligrams of KF were added after one hour contact with water, to the other, one drop of HF was added after one hour of contact. After 166 hours of contact with water, the pH of the water being in contact with the dummy was 6.4, the pH of the water in contact with the sample brazed with pure KAlF₄ was 8.0, the pH of the sample with added KF was 8.1, and the pH of the water of the sample with added HF was 7.6. These tests show that the fluoride ion, especially when entered in the form of potassium fluoride, seems to bring about a rise in the pH. High pH was accompanied with a formation of a white turbidity. Example 6 : Addition of an anticorrosive additive Examples 6.1 : Parts brazed using standard Nocolok^® flux 6.1.1 Angle-on-coupons were brazed using standard Nocolok^® flux. One thus produced assembly was contacted with 10 ml water without additive for comparison. The resulting aqueous solution became turbid after 10 days, and the pH was measured to be 7.7.

6.1.2. Another assembly thus produced was contacted with water in the presence of 6 mg KF (concentration = 600 mg/1 =10^"2M) for comparison to the soaking water. The solution became turbid, and the pH was measured to be 8.9.

6.1.3. Still another assembly thus produced was contacted with water in the presence of 6 mg OfK₃AlF₆. Even after 15 days, no turbidity and no corrosion was observed. The pH was measured to be 7.0. It is known that K₃AlF₆ dissociates in water under the formation of dissolved KF and almost undissolved K₂AlF₅. Accordingly, the addition of K₂AlF₅ instead of (or together with K₃AlF₆) will provide comparable results of anticorrosive activity. Example 6.2 : Parts brazed using KAlF₄ 6.2.1. An angle-on-coupon was brazed using pure KAlF₄ flux. The formed assembly was contacted with water. The solution became turbid after 5 days, the pH was then 7.9. 6.2.2. Example 6.2.1 was repeated, but this time, 6 mg KF were added to the soaking water. The pH of the solution was measured to be 9.3. 6.2.3. An angle-on-coupon was brazed using standard KAlF₄ flux. The formed assembly was contacted with water under the addition of 6 mg of K₃AlF₆. Once again, no turbidity or corrosion was observed, the pH was measured to be 7.2.

Example 7 : Anticorrosive activity of calcium acetate AlO* 10 cm brazed section composed of tubes and fins brazed with

Nocolok^® (flux load 8 g/m²) resembling that of a heat exchanger was immersed in a 2 M Ca-gluconate solution for 30 seconds. Excess adhering liquid was blown off with compressed air and the sample was dried at 90⁰C. It was then kept in 90 ml water for one week. Almost no turbidity developed during that time.

Claims

C L A I M S

1. A process for reducing corrosion caused by fluoride ions leached from flux residues during an extended period of time of contact of water or aqueous compositions with potassium fluoroaluminate-brazed aluminium parts by applying, for brazing of the parts, a flux forming a flux residue with reduced leachable fluoride ion and/or by subjecting the brazed parts to a heat treatment after brazing, and/or by applying to the brazed parts at least one anticorrosive agent selected from fluoride ion scavengers and one or more compounds reducing the solubility of the flux residue.

2. The process of claim 1 comprising a step of applying, for brazing of the parts, of a flux forming a flux residue with reduced leachable fluoride ion and/or of subjecting the brazed parts to a heat treatment after brazing, and/or of applying to the brazed parts at least one anticorrosive agent selected from fluoride ion scavengers and one or more compounds reducing the solubility of the flux residue, and a step of contacting the brazed parts with water or an aqueous composition for an extended period of time.

3. The process according to claim 1 wherein flux forming a flux residue with reduced leachable fluoride ion is high in K₂AlF₅ or its hydrates is applied as brazing flux.

4. The process according to claim 3 wherein a flux consisting essentially from K₂AlF₅ or its hydrates is applied, or a flux containing 20 to 45 % by weight OfK₂AlF₅ and/or its hydrate.

5. The process according to claim 1 wherein the anticorrosive agent which is contacted with the fluoride containing water or aqueous composition, is selected from the group of water-soluble calcium containing salts and salts, preferably the potassium salts, containing the AlF₄, AlF₅ or AlF₆ ion and hydrates thereof, with the exception Of CaF₂.

6. The process of claim 4 wherein the water-soluble calcium salt is calcium acetate, calcium propionate, calcium gluconate or calcium lactate.

7. The process of claim 1 wherein a potassium pentafluoroaluminate or hexafluoroaluminate or the hydrates thereof are applied as compounds reducing the solubility of the flux residue.

8. The process of claim 7 wherein K₂AlF₅, K₂AlF₅-H₂O or K₃AlF₆ are applied.

9. The process of claim 1 wherein the water in extended contact with the brazed parts is rain water or water applied to clean the brazed aluminium.

10. The process of claim 1 wherein the aqueous composition is a cooling liquid or heat-exchanger liquid in contact with potassium fluoroaluminate-brazed aluminium parts.

11. The process of claim 10 wherein the aqueous composition is cooling liquid of gas-fueled, gasoline-fueled or diesel- fueled motors.

12. A cooling liquid in the form of an aqueous composition comprising water, an antifreeze agent and optionally additives, e.g. colorants, which additionally comprises at least one anticorrosive agent selected from the group of water-soluble calcium containing salts and salts containing the AlF₄, AlF₅ or AlF₆ ion and hydrates thereof, with the exception of CaF₂.

13. The cooling liquid of claim 12 wherein the anticorrosive agent is selected from the group consisting of calcium acetate, calcium propionate, calcium gluconate or calcium lactate, K2AIF5, K2AIF5Η2O or K3AIF6.

14. Potassium fluoroaluminate-brazed aluminium parts with enhanced protection against corrosion which comprise a coating applied after brazing comprising at least one anticorrosive agent selected from the group of water- soluble calcium containing salts and salts containing the A1F4, AlF₅ or AlF₆ ion and hydrates thereof, with the exception of CaF₂.

15. A method of handling or using potassium fluoroaluminate flux-brazed aluminium parts wherein during handling or use, these parts come into contact with water or aqueous compositions the improvement which comprises the selection of potassium fluoroaluminate flux-brazed aluminium parts which have been obtained by selecting and applying a potassium fluoroaluminate flux forming a flux residue with reduced leachable fluoride ion and/or by subjecting the brazed parts to a heat treatment after brazing, and/or by applying to the brazed parts at least one anticorrosive agent selected from fluoride ion scavengers and one or more compounds reducing the solubility of the flux residue, according to the process of anyone of claims 1 to 11.

16. The method of claim 15 wherein the handling includes storing the parts not completely protected against rain, dew or snow, or wherein the use includes applying the brazed parts for removing heat from or transferring heat to water- containing cooling liquids.

17. A process for brazing of aluminium parts to aluminium parts wherein a potassium fluoroaluminate flux is applied to the parts, the parts are joined, brazed and, after brazing, are contacted with water or aqueous compositions for an extended period of time with the proviso that, for brazing of the parts, a potassium fluoroaluminate flux is used which forms a flux residue with reduced leachable fluoride ion content, and/or by subjecting the brazed parts to a heat treatment after brazing, and/or by applying to the brazed parts at least one anticorrosive agent selected from fluoride ion scavengers and one or more compounds reducing the solubility of the flux residue.