The term metal components intends to encompass both components comprising pure metals and components comprising metal alloys.

The invention is preferably applied in association with a fixing element, such as a screw or bolt connection, which comprise a more noble metal than the metal component or the metal components as the fixing element should be in electrical contact with upon mounting.

Advantageously, the invention is particularly applied at vehicle components, but may also be used for metal components in other applications where a risk for galvanic corrosion exists.

The background of the invention Chemical attacks of among others the oxygen of the air on a metal is often denominated atmospheric corrosion. Gold and platinum are examples of metals almost being resistant ("noble") against atmospheric corrosion, while other metals such as aluminium, chrome and stainless steel alloys will be covered of a protecting layer in air preventing further attacks. For instance in iron the corrosion products do not give such a corrosion protecting oxide layer.

In the case that two different metals are brought in electrical contact with each other in a humid environment, a so-called galvanic corrosion may occur.

The effects of so-called galvanic corrosion may be studied for instance by launching a motorboat with a propeller shaft of stainless steel and a propeller of bronze in salt water. In this case, the galvanic corrosion leads to considerable attacks on the lesser noble propeller, already after a short time. Consequently, in the case of motorboats, it is customary to attach a

so-called sacrifice anode of an even lesser noble metal than those present in the propeller under the water line, which leads to the result that the sacrifice anode corrodes instead of the propeller. Thus, the sacrifice anode gives the propeller a protection against galvanic corrosion until the anode has been consumed and must be replaced by another one.

The galvanic corrosion mav be explained by an electrochemical discussion, resulting from that metals can be divided in a so-called electrolytic electromotive chain. If pieces of two different metals, being in different positions in the electromotive chain, are located in an electrolyte and are in electrical contact with each other, a galvanic element appears. Thereby, the metal at the lowest position in the electromotive chain becomes anode or solution pole, while the second metal becomes cathode or precipitant pole.

The location of the metals in the electromotive chain may be stated as so-called normal-electrode potentials at 25°C, whereby for instance iron (Fe) is often stated to have the value of-0,440 V and magnesium (Mg) a value of-2,37 V. In the case that pieces of magnesium and iron are brought into electrical contact with each other in a electrolyte, a short-circuited galvanic element is obtained, where the magnesium functions as a anode/solution pole and dissolves during formation of ions and delivering electrons. At the iron-piece functioning as cathode/precipitant pole, hydrogen gas (H2) will instead be precipitated out of the electrolyte, or oxygen gas will be reduced.

It should be noted that also the normally present air humidity contains sufficient of dissolved elements, to be able to function as an electrolyte. Therefore, galvanic corrosion may appear without the actual metals being immersed in any liquid.

It is previously known a number of methods to protect metals being susceptible to corrosion against atmospheric corrosion. Amongst such methods, coating may for instance be mentioned using different anti-corrosive agents, galvanization, and chromium plating etcetera.

Regarding protection against atmospheric corrosion and/or galvanic corrosion, it is also known using particular sacrifice coatings, i. e. that a corrodible first metal is coated with an even more

corrodible second metal with the ability to form a protecting oxide layer. Thus, it is previously known, to coat a metal substrate by means of steam dumping with a sacrificial coating of aluminium to protect the substrate from corrosive attacks.

Further it is previously known that different cathode surfaces have different kinetic properties regarding disintegrating water and reducing hydrogen ions and oxygen gas.

Further a method is described in US-A-4,980,195 to prevent atmospheric corrosion of steel, in which the steel substrate, which should be protected, is coated with aluminium by means of ionic steam dumping coating. The steel coated with aluminium is then immersed in an aqueous acid solution of an indium acid salt, which is said to give small surface deposits of metallic indium. These surface deposits is stated to be able to activate"the sacrificial properties"of the aluminium of the coating, so that the coating provides a better corrosive protection than a pure aluminium coating.

Further US-A-4,844,943 describes a process to protect a metallic surface against corrosion depending on the contact with the vanadium and/or sodium oxides at gas combustion temperatures coming up as a consequence of combustion of heavy fuels. The process is stated to comprise deposition of a layer of at least one protecting metal chosen from the group containing of alkaline earth metals, boron, aluminium, gallium, indium, thallium, silicon, zirconium and titanium on the metal surface. The protective metal is stated to oxidize in situ to form an protective oxide layer reacting with vanadium oxide, sodium oxides or solutions thereof, to form a composition with a softening temperature being higher than the softening temperature of vanadium or sodium oxides.

The international patent application no. PCT/DK95/00421 of one of the applicants describes a coated aluminium material containing a substrate of aluminium or aluminium alloy at least with a surface section thereof, and a first layer containing oxides of the substrate material, and a second layer substantially containing indium, tin and/or gallium. The coated aluminium material is particularly useful in cars, aircrafts and buildings, and has very good properties with regard to corrosion resistance, abrasion resistance and low friction.

Among others with purpose to be able to decrease the fuel consumption by means of a lower vehicle weight, the use of different lightweight materials has increased even more in the vehicle manufacturing industry. This has lead to an increased use of materials such as aluminium, magnesium and alloys of these. For different reasons it is still necessary or desirable to be able to use more traditional materials such as steel alloys for certain vehicle components. The increased amounts of different metals and metal alloys, which are used in modem engine vehicles, have lead to an increased risk that galvanic corrosion should appear between different components.

The risk for galvanic corrosion may often be particularly great in association with attachment means, fasten articles or consoles, since for instance bolts and screws for reasons of strength often contain a steel alloy, while the components which should be joined together often contain light weight materials. Further the use of attachment means is often particularly great in positions, for instance of engine vehicles, where a high air humidity provides good prerequisites for galvanic corrosion, such as in association with wheel suspensions of the engine vehicle or inside the engine bay.

In order to reduce the problems of galvanic corrosion of attachment means, for instance for use in engine vehicles, an intermediate aluminium plate is often mounted between, for instance, a bolt or a screw of steel at the attachment means, and a component of a light weight metal or light weight metal alloy. This is particularly common in the case of magnesium alloys.

Even if the solution of such an electrochemically plate being more compatible, reduces the problem of galvanic corrosion between the attachment means and the actual metal component, it gives an undesired additional weight, an undesired cost, a more complicated mounting, and in some cases a greater risk for settlements in the joint.

Description of the invention Hence, the present invention has as a first purpose to provide an arrangement for decreasing galvanic corrosion between metal components, removing the need of corrosion decreasing

plates or spacers, and thus, admits both cost and weight savings, a more simple mounting and more stable joints.

This first purpose is achieved, in accordance with patent claim 1, as the arrangement comprises at least a first component in which a first metal is a part, and at least a second component in which a second metal is a part, whereby the first metal has a higher normal-electrode potential co than the second metal, and whereby the first component is intended after mounting to be in electrical contact with the second component. According to the invention, the first component is thereby coated with a substantially continuous surface layer, which is adjusted to provide the second component an insignificant galvanic corrosion velocity after the mounting.

A second purpose of the present invention is to provide an arrangement for decreasing the galvanic corrosion between metal components, which may be accomplished with a proportionally low environmental influence, both regarding work environment and surrounding ecosystems.

This second purpose is accomplished, according to patent claim 5, as the surface layer of the arrangement, according to the invention, substantially consists of indium (In), an indium alloy or an indium compound.

Further purposes of the present invention will be apparent from the following description, while the features enabling that the additional purposes are achieved are indicated in the enclosed sub claims.

Brief description of the figures The invention will in the following be described in greater detail with reference to the enclosed drawing, in which Fig. 1 shows a strongly schematically cross-sectional view of a preferred embodiment of an arrangement according to the invention.

Detailed description of preferred embodiments In the following a number of different embodiments of an arrangement according to the invention will be described, whenever applicably with reference to fig. 1.

The arrangement 1 according to the invention is intended for decreasing the galvanic corrosion between metal components and comprises at least a first component 2 in which a first metal is a part, and at least a second component 3 in which a second metal is a part. Thereby it has to be understood that other metals or metal alloys also may be a part of the respective component than the above-mentioned first or second metals.

Thereby the first metal has a higher normal-electrode potential e° than the second metal. In this respect, with normal-electrode potential is herein intended to have been obtained in measurements using a normal hydrogen gas electrode at 25°C, in the way as it is stated for instance in G. Hagg,"Allman och oorganisk kemi" (eng."General and inorganic chemistry"), Stockholm, 1963, according to which, for instance, e° of the redox reaction of magnesium (Mg) is stated to be-2,37 V, while e° of the redox reaction of iron (Fe) is stated to be-0,440 V.

In the arrangement according to the invention, the first component 2 is intended to be in electrical contact with the second component 3 after mounting.

Thereby according to the invention the first component 2 is coated with a substantially continuous surface layer 4, being adapted to give the second component 3 an insignificant galvanic corrosion velocity after the mounting. Thereby it is of great importance that the surface layer 4 is substantially continuous if the intended effect should be obtained. The term "insignificant corrosion velocity"intends to mean that the second component galvanically corrodes with a velocity being insignificant in proportion to the commonly expected lifetime of the component.

According to the preferred embodiment the insignificant corrosion velocity, expressed as a current density Io at a cathode reaction under formation of hydrogen gas in a 1M strong acid, is

lesser than 10-12 A/cm2. The term Io intends to mean the current density as is stated, for instance, at p. 236 in"Instrumental Methods in Electrochemistry", Ellis Hirwood Limited, 1990, Chicester, England. By this very slow kinetic corrosion velocity, the surface layer 4, i. e. the cathode, reassures that the galvanic corrosion attacks on the second component 3, i. e. the anode, only remains insignificant during a long period of time with regard to practical purposes.

In another preferred embodiment the surface layer 4 is adapted to form, at increasing pH due to the corrosion, a hydroxide with a solubility which gives a lower content of free hydroxide ions than 10-5 M in distilled water at 25°C. In this embodiment, the substantially insoluble hydroxide formed in the surface layer will make it difficult for the underlying metal layer to form a solution as a side reaction due to alkalisation.

In a particularly preferred embodiment the surface layer 4 comprises indium (In) or an indium compound.

Inter alia from environmental reasons, the surface layer 4 mainly consists of indium, an indium alloy or an indium compound. In comparison with other corrosion inhibitors, indium is namely considered to be proportionally harmless, both from the point of view of labour welfare and when it comes to contaminations to the external environment, since indium has not shown to have any obviously negative environmental consequences.

Further in another preferred embodiment the surface layer 4 comprises a sealer, which may be a previously known type. such as alkali silicate/silicate of sodium. In this embodiment it is reassured that possible pores of the surface layer 4 are sealed, and that an electrically insulating layer is formed.

Further according to another embodiment, the surface layer 4 comprises an addition of tin, zinc, bismuth or gallium. This embodiment makes it possible to create the properties of the surface layer based on the requirements of the actual application.

The surface layer 4 of the first component 2 is preferably coated by means of hot dipping process, electrolysis, steam dumping coating, mechanical coating or chemical coating. In that mechanical or chemical plating may for instance be used. It is also possible using other appropriate coating processes for this purpose, per se previously known.

In a preferred embodiment the surface layer 4 has a thickness in the range of 0,5-100 um.

However, the thickness must be adapted to the actual application depending on the actual porosity of the surface layer, expected mechanical abrasion, required length of life, etcetera.

In another preferred embodiment the second component 3 is principally composed of a metal alloy of magnesium (Mg) or aluminium (Al). As should be well-known to the person skilled in the art, such metal alloys may under certain conditions, be sensitive to galvanic corrosion, and thus, the arrangement according to the invention will particularly be right in this embodiment.

In the particularly preferred embodiment of the arrangement, according to the invention, the second metal is magnesium (Mg). It is well-known that magnesium may show great problems with galvanic corrosion, whereby the increased use of magnesium, for instance in the vehicle manufacturing industry, makes this embodiment particularly preferred.

In another preferred embodiment of the arrangement, according to the invention, the first component 2 is principally composed of an untreated or surface finished steel alloy.

In a further preferred embodiment of the arrangement, according to the invention, the first component 2 is coated with zinc or tin-plated below the surface layer 4.

In the particularly preferred embodiment of the invention, the first component 2 is an attachment element such as a bolt or a screw. In this embodiment the corrosive protecting properties of the arrangement, according to the invention, will be particularly useful, since the attachment element is often located in particularly corrosion exposed positions for instance of a motor vehicle, and nevertheless must be able to be remounted even after a long time after the

original mounting, something which may easily be impossible with previously known attachment elements.

In still another preferred embodiment, the first component is a fixed item or a console (not shown in the figures), which are usually frequent components, for instance, of the vehicle industry and is often exposed for galvanic corrosion.

In a particularly preferred embodiment of the arrangement according to the invention, the first and the second component (2) of vehicle components are intended to be used for instance in a passenger car or a truck.

The preferred invention should not be seen as limited to what has been described above in association to the preferred embodiments, or to what is shown in the enclosed drawings, but the scope of the invention is defined of the following patent claims.