GB2289691A

GB2289691A - Coated metal

Info

Publication number: GB2289691A
Application number: GB9504712A
Authority: GB
Inventors: Ii Jay F Carey; Mehrooz Zamanzadeh
Original assignee: Louis Berkman Co
Current assignee: Louis Berkman Co
Priority date: 1994-03-14
Filing date: 1995-03-09
Publication date: 1995-11-29
Anticipated expiration: 2015-03-09
Also published as: GB2289691B; GB9504712D0

Abstract

Metal sheet has thereon a two-phase metallic coating comprising a large weight percentage of zinc and a relatively large weight percentage of tin. The tin-zinc coating may also include nickel and/or copper. The tin-zinc alloy coating provides for both a highly corrosion-resistant coating which protects the surface of the metal from oxidation and also produces a grey, earth tone colour, which is not highly reflective. The metal substrate may be carbon steel, stainless steel, copper, tin, aluminium, bronze or titanium. The metal may be plated with nickel prior to applying the alloy coating. Preferred alloys comprise:- a) 15.0 - 70.0% Sn, 30.0 - 85.0% Zn, 0 - 5% Ni, 0 - 1.7% Bi, 0 - 7.5% Sb, 0 - 5% Cu, 0 - 0.1% Fe, 0 - 0.05% Pb; b) 15 - 35.0% Sn, 65.0 - 35.0% Zn, 0 - 1% Ni, 0 - 1.7% Bi, 0 - 7.5% Sb, 0 - 0.1% Fe, 0-0.05% Pb.

Description

Coated Metal The present invention relates to coated metals, such as sheet steel, suitable for use as, for example, architectural materials, for the production of fuel tanks or the like.

As background material, to avoid the need to specify in detail what is known in the art, U.S. patents 4987716 and 4934120 illustrate metal roofing systems of the type for which coated metal according to the invention can be used (these documents are incorporated herein by reference). GB-A-2265389, which illustrates a process of hot-dip coating roofing materials, is also incorporated herein by reference.

Over the years, architectural materials, such as metal roofing systems and metal guttering systems, made of pliable metals in various sheet gauge thicknesses, have been used.

Metals such as carbon steel, stainless steel, copper and aluminium are the most popular metals for such purposes, these metals being frequently treated with corrosion-resistant coatings to prevent rapid oxidation of the metal surface.

A popular corrosion-resistant coating for carbon steel and stainless steel is of terne alloy. Terne alloy, which is an alloy typically containing about 80% lead and the remainder tin, has been the predominant and most popular coating for roofing materials due to its relatively low cost, ease of application, excellent corrosion-resistant properties and desirable colorisation during weathering. The coating is generally applied to architectural metals by a hot-dip process wherein the metal is immersed into a molten bath of terne alloy.

Automobile fuel (petrol or gasoline) tanks are also commonly coated with terne alloy. However, this may cause problems when, at the end of the life of vehicles equipped with such tanks, the vehicles are partially dismantled so that the metal parts can be melted for re-use. This is because the fuel tanks have to be removed from the remaining metal structure because of their lead content. They must then be disposed of in landfill or the like. This causes concern at the possibility of lead leaching from the terne coating into the landfill site and potentially contaminating the surrounding area and underground water reservoirs.

Although terne coated sheet metals have excellent corrosion resistance, and have been used for a variety of uses (such as coatings for architectural materials and for fuel tanks as described above), the impact on the environment of terne coating has been questioned. Environmental and public safety laws have been recently proposed and/or passed prohibiting the use of materials containing lead. Because the terne alloy contains a very high percentage of lead, materials coated with terne alloy have been prohibited for some applications such as aquifer roofing systems. The concern that lead might leach from the terne alloy coating has made such coated materials inadequate and/or undesirable for several types of building applications.

A further disadvantage of terne coated architectural materials is that the newly applied terne alloy is very shiny and highly reflective; such a highly reflective coating cannot be used on buildings or roofing systems in certain locations, such as at airports and military establishments. The terne coating eventually loses its highly reflective properties as the components within the terne coating are reduced (weathered); however, the desired amount of reduction takes approximately six months to 2 years when the terne coating is exposed to the atmosphere, thus requiring the terne metals to be stored over long periods of time before use in these special areas.

The necessary storage time is significantly prolonged if terne-coated metals are stored in rolls and the rolls are protected from the atmosphere. However, once the terne alloy has properly weathered, the colour of the weathered coating is a very popular grey-earth tone colour.

The use of terne coating on fuel tanks was originally intended to protect the stainless steel or carbon steel tank from corroding. However, the corrosion resistance provided by the terne coating is commonly defective due to the limited coating thickness and coating processes. Standard terne coated tanks have a 6-8 lb terne coating (0.0003 - 0.0004 inch), which is thin and results in pinholes in the coating (small uncoated areas.on the gasoline tank surface). Because the terne coating is so thin, and because of the pinholes, drawing the coated sheet by a die so as to form a tank component has a tendency to tear or shear the coating, thereby exposing the metal surface.

Furthermore, the pinholes in the coating are enlarged as the coated sheet is drawn, thereby further exposing more of the metal surface. These exposed surfaces, when subjected to environmental attack, readily begin to corrode and over time compromise the structural integrity and safety of the tank and may ultimately result in the leaking of fuel from the tank. The non-uniform coating of the tank with the terne coating is especially evident when stainless steel is used since terne does not bond as well to the stainless steel as it does to carbon steel.

Tin coating of carbon steel is a well-known process for use in the food industry. However, in the specialised fields of architectural materials and fuel tanks, a tin coating for architectural materials has not been previously proposed.

The most popular process for applying a tin coating to carbon steel for use in the food industry is by an electrolysis process. In an electrolysis process, the coating is very thin, typically between 3.8 x 104 to 20.7 x 104mum (1.5 x 10-5 to 8.15 x 105 in.). Furthermore, the equipment and materials needed to properly electroplate metals are very expensive and relatively complex to use. The expense of applying an electroplated-tin coating and the limited thicknesses of tin coating obtainable are a disadvantage for using such a process for building and roofing materials.

A hot-dip process for applying the tin coating may be used; however, if the metal substrates are not properly prepared and the coating is not properly applied, minute areas of discontinuity in the tin coating may occur resulting in nonuniform corrosion protection. This is especially a problem when the tin is applied to stainless steel by a hot-dip process. Tin is not electroprotective to steel under oxidising conditions, so that discontinuities in the tin coating result in corrosion of the exposed metal. Tin coatings have the further disadvantage of having a highly-reflective surface. Tin coatings are, however, very stable and resist oxidation, so that the highly reflective surface of the tin remains on the coated materials for many years. Even when the tin coating does begin to oxidise, the oxidising coating forms a white texture (tin oxide) and does not turn the colour of the popular grey, earth tone colour found on weathered terne coatings. As a result, architectural materials having a tin coating cannot be used in an environment where highly-reflective materials are undesirable until the coated materials are further treated (i.e. painted) or the tin is allowed time to oxidise.

Coating architectural materials with zinc metal, commonly known as galvanization, is another popular metal treatment to inhibit corrosion. Zinc is a highly desirable metal for coating architectural materials, because of its relatively low cost, ease of application (for example, by hot-dip application) and excellent corrosion resistance. Zinc is also electroprotective to steel under oxidising conditions, and prevents corrosion of the exposed metal resulting from discontinuities in the zinc coating. This electrolytic protection extends away from the zinc coating over exposed metal surfaces for a sufficient distance to protect the exposed metal at cut edges, scratches, and other coating discontinuities.

Despite all of the advantages of zinc coatings, they have several disadvantages that make them undesirable for many types of building applications. Although zinc coatings will bond to many types of metals, the bond is not strong and can result in the zinc coating flaking off the building materials. Zinc does not bond well to standard stainless steel; neither does it form a uniform and/or thick coating in a hot-dip process on stainless steel, so that discontinuities of the coating are usually found on the stainless steel surface. Zinc is also a very rigid and brittle metal, which tends to crack and/or flake off when building or roofing materials are formed, for example, by press fitting. When zinc begins to oxidise, the zinc coating forms a white powdery texture (zinc oxide). The popular grey, earth tone colour cannot be obtained from pure zinc coatings.

Because of environmental concerns and problems with corrosion-resistant coatings applied to architectural metals, there has been a demand for a coating which can be easily and successfully applied to protect such metals from corrosion, does not have a highly-reflective surface subsequent to application, can be applied by a standard hot-dipping process, weathers to the popular grey, earth tone colour, and allows the materials to be formed at the building site.

In the context of fuel tanks, electroplating of tin or zinc onto metal sheets to be formed into such tanks has not proved to be a reliable and cost effective substitute for terne coatings. Electroplating of tin or zinc results in coatings which are much thinner than hot-dipped terne coatings, thereby making electroplated coatings much more susceptible to tearing or shearing when the electroplated sheet is drawn on a die.

Electroplating is also much for expensive and time consuming than a hot-dipped terne process.

Fuel tanks of plastics materials have been used but with limited success. Although the use of such tanks eliminates the environmental concerns associated with lead, the plastics materials themselves are not environmentally-friendly because they do not readily degrade, and therefore must be disposed of in a landfill. The plastics used to make fuel tanks are usually not the type that can be recycled. Plastics have also been found to be less reliable than metal fuel tanks. Specifically, plastics fuel tanks have a tendency to rupture upon impact, such as from a car accident, whereas a metal fuel tank would absorb much of the shock by bending and slightly deforming.

Furthermore, the plastics tanks are more susceptible to being punctured from roadside debris since the plastics skin is not as strong or malleable as the skin of a metal fuel tank. Plastics tanks also require new materials, special tools and new assembly methods to fix and install the tanks due to the nature of plastics and their physical properties.

These additional costs and shortcomings of plastics tanks have resulted in very little adoption of plastics tanks in present day motor vehicles. Due to the environmental concerns and problems associated with terne coated petrol tanks and with the shortcomings of plastics tanks, there has been a demand for a corrosion-resistant, environmentally-friendly fuel tank which can be easily and safely instaiaed into a vehicle without requiring additional tools and assembly methods and which can be subsequently recycled with a vehicle once the useful life of the vehicle has ended.

The present invention relates to a corrosion-resistant, environmentally friendly coated metal substrates, wherein the coating has a low lead content and weathers to form a not highly reflective, desirable surface which resembles the grey, earth tone colour of weathered terne.

In accordance with a principal feature of the invention, there is provided a metal substrate, typically of stainless steel, carbon steel or copper, coated with a tin-zinc alloy. Other materials can also be coated by the tin-zinc alloy such as nickel alloys, aluminium, titanium, tin, bronze etc. The tin-zinc alloy is a multiple phase alloy coating mainly comprising zinc and tin.

The tin-zinc alloy may contain small amounts of other metals to modify the physical properties of the tin-zinc, multiple phase alloy; such metals may contribute primarily to the colouring of the coating and to the corrosion-resistant properties of the coating.

For example, nickel may be added to the tin-zinc alloy in amounts up to 5 weight %, preferably less than 1 weight 8, to increase the corrosion resistance of the tin-zinc alloy.

Nickel has also been found to increase the corrosion resistance of the tin-zinc alloy, especially in alcohol- and halogen-containing environments. The nickel content of the tinzinc alloy is preferably not more than 5.0 weight percent.

Greater nickel concentrations can make the coated materials difficult to form. Typically, the nickel content is less than 1.0 percent by weight, such as from 0.3 - 0.9 percent by weight, preferably about 0.7 percent by weight.

As a second example, copper or other colouring agents may be added to the tin-zinc alloy, typically in amounts of up to 5 percent by weight of copper, more preferably up to 2.0 percent by weight of copper. If copper is added, it is usually in amounts from 0.1 to 1.6 percent by weight and preferably from 1.0 to 1.5 percent by weight. The addition of copper dulls the colour of the tin-zinc alloy (as newly applied) thereby making the alloy less reflective. Both nickel and copper are believed to be metallic stabilisers in the alloy coating.

The tin-zinc alloy forms a multiple phase alloy coating (that is, an alloy comprising at least two primary components).

Surprisingly, we have found that the tin-zinc alloy provides a protective coating with a higher corrosion resistance as compared to a coating primarily of tin.

The amount of zinc within the alloy coating is preferably not more than 85% by weight, so that the alloy coating remains relatively pliable for use in a press-fit roofing system and can be applied by standard hot-dipped processes.

We have discovered that the use of large weight percentages of zinc in the tin-zinc alloy does not cause the coating to be too rigid or brittle which might otherwise prevent forming of the coated material or bending, which would result in a cracked coating. Extensive experimentation was performed on tin-zinc coatings having a zinc content above 30 percent by weight. Surprisingly, we have found that a tin-zinc coating containing 30-85 percent by weight of zinc, the balance being essentially tin, produced a malleable metallic coating which resisted cracking when bent or formed. We believe that the unique characteristics of the multiple phase metallic tin-zinc alloy modifies the rigid characteristics of zinc to allow the tin-zinc coating to be malleable. In addition to the surprisingly malleability of the tin-zinc coating, we have found that the coating provides comparable and/or superior corrosion resistance to tin, zinc or terne coatings.

The tin-zinc alloy contains a large percentage by weight of zinc and tin. Preferably, the alloy contains zinc in amounts of at least 30 percent by weight (typically 30-65%, such as 45 to 55%) and tin at least 15 percent by weight, and a tin plus zinc content of at least 80 percent by weight, whereby the corrosion resistance of the multiple phase metallic alloy is significantly increased as compared to a protective coating essentially composed of tin.

The tin essentially makes up the balance of the alloy coating. The tin content typically ranges between 15-70 percent by weight of the tin-zinc alloy.

The tin and zinc content of the tin-zinc alloy is preferably at least 80 percent by weight of the alloy more preferably at least 90 percent by weight (such as at least 95 percent, up to about 100 percent) of the alloy. The tin-zinc alloy provides a corrosion-resistant coating that protects the surface of the metal substrate from oxidation, a coating which is environmentally friendly and therefore immune from the prejudices associated with lead-containing materials, and a coating which forms a grey, earth tone coloured surface which is very similar to weathered terne and which is also not highly reflective. It is new to the art of metal coating to provide a tin-zinc alloy on a stainless steel substrate to form a low lead coloured protective coating on the stainless steel.

Although the exact reasons for this physical phenomenon of increased corrosion resistance due to the addition of zinc to tin is not known, we have found that by adding zinc to tin, the multiple phase metallic alloy coating has corrosion-resistant properties which exceed that of tin coatings and, in some environments, that of a terne coating.

The coating may contain bismuth and antimony to increase the strength thereof and also to inhibit crystallisation of the tin at lower temperatures. The amount of bismuth in the coating may range from 0 to 1.7 percent by weight and the amount of antimony may range from 0-7.5 percent by weight of the coating. Antimony and/or bismuth can be added to the coating in amounts as low as 0.05 percent by weight; such low amounts are sufficient to prevent the tin from crystallising at low temperatures, which might result in the coating flaking off the metal substrate. It is believed that high levels of zinc also help stabilise the tin within the tin-zinc alloy.

Thus, antimony and/or bismuth may be present in amounts lower than 0.05 percent by weight, and still help prevent crystallisation of the tin. Antimony and/or bismuth in amounts greater than 0.58 by weight may be added primarily to harden and/or strengthen the alloy coating and/or to increase its wear resistance. Small amounts of other metals such as iron may be added to the alloy coating. If iron is added to the tin-zinc metallic coating, the iron content is preferably not more than 0.1 percent by weight.

In accordance with another feature of the present invention, the tin-zinc coating is preferably essentially lead free. The lead content may be maintained at extremely low levels, typically not exceeding 0.05 percent by weight.

Preferably, the lead content is maintained at much lower amounts, typically less than 0.01 percent by weight, so as to avoid environmental concerns associated with the tin-zinc alloy coating.

A typical alloy composition for use according to the invention is as follows (in percentages by weight): Tin 15-70 Zinc 30-85 Nickel < 5.0 Antimony < 7.5 Bismuth < 1.7 Copper < [2.0] 5 Iron < 0.1 Lead < 0.05 A few examples of the tin-zinc, two-phase alloy having the desired characteristics as mentioned above are as follows: Alloy Ingredient A B C D E Tin 15 30 35 45 50 Nickel < 1.0 < 1.0 < 1.0 < 1.0 < 1.0 Antimony < 0.5 < 0.5 < 0.5 < 0.5 < 0.5 Bismuth < 0.5 < 0.5 < 0.5 < 0.5 < 0.5 Copper < 2.0 < 2.0 < 2.0 < 2.0 < 2.0 Iron < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 Lead < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 Zinc Bal. Bal. Bal. Bal. Bal.

Alloy Ingredient F G H Tin 55 60 70 Nickel < 1.0 < 1.0 < 1.0 Antimony < 0.5 < 0.5 < 0.5 Bismuth < 0.5 < 0.5 < 0.5 Copper < 2.0 < 2.0 < 2.0 Iron < 0.1 < 0.1 < 0.1 Lead < 0.01 < 0.01 < 0.01 Zinc Bal. Bal. Bal.

Preferably, the tin-zinc alloy coating includes, in weight percentages: 30-65% zinc, 0-0.5% antimony, 0-0.5% bismuth, 35-70% tin, up to 1.0% nickel, 0.0-2% copper and less than 0.05% lead and more preferably 45-55% zinc, 45-55% tin, 0.3-0.9% nickel, 0.0-0.5% bismuth and/or antimony, 1.0-1.5% copper, less than 0.01% lead, the tin content plus zinc content exceeding 95% of the coating.

Examples of further tin-zinc alloys having the desired properties are as follows: Alloy Ingredient I J K L Tin 15 20 30 20 Antimony < 7.5 < 7.5 < 7.5 < 7.5 Bismuth < 1.7 < 1.7 < 1.7 < 1.7 Iron < 0.1 < 0.1 < 0.1 < 0.1 Copper < 2.0 < 2.0 < 2.0 < 2.0 Lead < 0.05 < 0.05 < 0.05 < 0.05 Nickel < 1.0 < 1.0 < 1.0 < 0.7 Zinc Bal. Bal. Bal. Bal.

Generally, the zinc-tin alloy includes, in weight percentage amounts; 65-85% zinc, 0-0.5% antimony, 0-0.5% bismuth, 15-35% tin, 0.0-1.0% nickel and less than 0.01% lead.

By coating metals with a tin-zinc alloy according to the invention, the life of the coated metals may extend beyond the life of the structure in which they are used due to the corrosion-resistance of the tin-zinc alloy coating.

The high zinc content of the multiple phase tin-zinc alloy coating has not been previously used on architectural materials such as metallic building and roofing materials, or on metal fuel tanks. The bonding of the tin-zinc coating to carbon steel and stainless steel is surprisingly strong and forms a durable protective coating which is not easily removable, thereby resisting flaking of the coating. The surfaces of the metal may be pre-treated prior to the coating to improve the bonding between the zinc-tin coating and the surface of the metal. For stainless steel substrates, a special pre-treatment process may be used which includes aggressively pickling and chemically activating the surface of the stainless steel to activate the stainless steel surface to provide significantly greater bonding of the tin-zinc coating.

The life of such metals may be significantly extended by applying a tin-zinc alloy according to the invention. The tin-zinc coating acts as a barrier to the atmosphere which prevents the metallic coating from oxidising and/or reducing in the presence of oxygen, carbon dioxide or other reducing agents in the environment. Although the tin-zinc coating oxidises in the presence of various reducing agents in the atmosphere, the rate of oxidation is significantly slower than that of the metal substrate. Furthermore, the tin and zinc oxide which forms on the coating surface provides corrosion resistance to the tin-zinc coating itself which further enhances the corrosion protection provided by the tin-zinc coating. The tin-zinc oxide also lowers the reflectivity of the tin-zinc alloy, and colours the alloy.

The present invention further comprises a petrol tank comprising a coated metal according to the invention.

According to this aspect of the invention, there is provided a corrosion-resistant, environmentally-safe fuel tank comprising at least one shell member, said at least one shell member including a cavity surrounded by a peripheral edge, and forming an inner chamber, said at least one shell member comprising a carbon steel sheet having thereon a coating of a zinc-tin alloy.

The tank preferably comprises first and second sheet metal shell members, each member including a drawn cavity surrounded by a peripheral edge, said members being placed together such that said edges of said members abut each other and said cavities combine to form an inner fuel receiving chamber, means for joining said abutting edges, spout means for introducing fuel into said inner chamber of said tank, draining means for removing fuel from said inner chamber; said sheet metal of each member comprising a steel sheet having a thickness of less than 0.2 inch having deposited thereon a hot-dipped coating of a zinc-tin alloy on the exposed surface of said sheet.

The two shell members may be joined in any of a number of ways that will securely prevent the shells from separating and prevent fuel from leaking from the interior chamber (such as welding, soldering and/or bonding the edges together). The fuel tank may include a spout which communicates with the inner chamber of the tank so that the inner chamber can be filled with fuel. The tank may also include a drain which can be connected to the fuel system of a vehicle.

The zinc-tin coating provides excellent protection to fuel tanks containing petroleum products which include alcohols.

Alcohol additives such as methanol or ethanol are commonly added to fuels such as petrol to reduce emission problems. These additives are highly corrosive to metals such as carbon steel and stainless steel. The zinc-tin coating provides superior corrosion protection against alcohol additives as compared to terne coated materials.

When nickel is added to the zinc-tin coating it has been found to provide additional corrosion protection, especially against alcohol products or alcohol additives. The amount of nickel added to the zinc-tin coating should be controlled so as not to make the zinc-tin coating too difficult to form in a die.

A coating thickness of 0.002 inch (40 lbs) can have virtually no pinholes in the coating and can resist tearing when the coated metal sheet is drawn into a fuel tank shell member.

The thickness of the zinc-tin coating on the metal sheet essentially eliminates uncovered areas on the surface of the metal sheet that are commonly found with thinner coatings. The additional coating thickness of the zinc-tin coating allows for greater elongation characteristics as compared to thinner coatings, resulting in the maintaining of a coating on the surface of the metal sheet during and after the metal sheet has been drawn by a die to form the respective members of the tank.

The thicker coating also provides for additional corrosion resistance and a higher quality product.

Although other methods may be used the coating of tinzinc alloy is preferably applied to the metal substrate by a hotdip process. If the tin-zinc coating is to be applied to stainless steel the coating is preferably applied by a special process which removes the oxides from the surface of the stainless steel and activates the stainless steel surface so that a strong bond is formed between the stainless steel surface and the coating of tin-zinc alloy; such a method is disclosed in our GB-A-2265389.

"Stainless steel" in the present application means one of a large variety of alloy metals containing chromium and iron.

The alloy may also contain other elements such a nickel, carbon, molybdenum, silicon, manganese, titanium, boron, copper, aluminium, nitrogen and various other metals or compounds.

Elements such as nickel can be flashed (electroplated) onto the surface of the chromium-iron alloy or directly incorporated into the chromium-iron alloy. The special pre-treatment process may also be used for other metal substrates such as carbon steel, copper, titanium, aluminium, bronze and tin to remove oxides from the substrate surface prior to applying the coating of tin-zinc alloy. The special pre-treatment process includes aggressive pickling and chemical activation of the substrate surface.

Prior to aggressive pickling and chemical activation of the substrate, the substrate may be treated with an abrasive and/or absorbent material and/or subjected to a solvent or other type of cleaning solution to remove foreign materials and oxides from the substrate surface.

The aggressive pickling process is designed to remove a very thin surface layer from the substrate surface. The removal of a very thin layer from the surface of the substrate effects the removal of oxides and other foreign matter from the substrate surface thereby activating the substrate surface prior to applying the tin-zinc coating. The activation of a stainless steel substrate is important in order to form a strong bonding and uniformly coated tin-zinc coating. The activation of stainless steel substrates removes the chromium oxide film on the stainless steel which is formed when the stainless steel is passivated by the manufacturer or is formed naturally in the presence of an oxygen-containing environment. Testing of stainless steel substrates has revealed that the chromium oxide film interferes with the bonding of the tin-zinc coating and does not allow for thick and/or uniform tin-zinc coatings to be formed. The aggressive pickling process may also slightly etch the substrate surface to remove a very thin layer of the surface.

The rate of etching is not the same throughout the surface of the substrate thereby forming microscopic valleys on the substrate surface which increases the surface area available on the substrate to which the tin-zinc coating can bond.

The aggressive pickling generally includes the use of a pickling solution which removes and/or loosens the oxide from the substrate surface. The pickling solution contains various acids or combinations of acids such as hydrofluoric acid, sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid and/or hydrobromic acid.

A specially formulated pickling solution is preferably used if the substrate is stainless steel, since the activation of a stainless steel surface is not properly accomplished by use of prior art pickling solutions containing only sulfuric acid, nitric acid or hydrochloric acid. Such a specially formulated pickling solution may contain a combination of hydrochloric acid and nitric acid. This dual acid formulation has been found to be surprisingly effective in the rapid removal of chromium oxide from stainless steel substrates. The dual acid composition of the pickling solution contains 5-25% hydrochloric acid and 1-15% nitric acid and preferably about 10% hydrochloric acid and 3% nitric acid. The temperature of such a pickling solution should be controlled to maintain the proper activity of the pickling solution. The temperature of the pickling solution is generally above 800F and usually between 120-1400F, more preferably 1281330F.

The pickling solution may be agitated to prevent the solution from stagnating or varying in concentration and/or to remove gas pockets which form on the substrate surface. The typically between 50 to 115 ft/min thereby subjecting the substrate to the pickling solution in each pickling vat for less than one minute. The sheet strip thickness is usually less than 0.1 inch and preferably less than 0.03 inch so that the sheet strip can be properly guided through the continuous process.

Once the substrate has been aggressively pickled, the substrate may further be treated in a chemical activation process. The chemical activation process further removes oxides and foreign material from the substrate by subjecting the substrate surface to a deoxidising agent. Due to the difficulty in removing oxides from stainless steel substrates, a stainless steel substrate should be treated in the chemical activation process after the stainless steel substrate has been treated in the aggressive pickling process. Various types of deoxidising solutions can be used. For the treatment of stainless steel substrates, zinc chloride has been found to be an excellent deoxidising solution.

The zinc chloride acts as both a deoxidiser and a protective coating for the substrate. The zinc chloride solution is generally kept at ambient temperature such as (60 90 F) and may be agitated to maintain a uniform solution concentration. Small amounts of hydrochloric acid may be added to the deoxidising solution to further enhance oxide removal.

Preferably, hydrochloric acid is added to the zinc chloride when treating a stainless steel substrate. The substrate is generally subjected to the deoxidising solution for less than 10 minutes. If the substrate is in strip form and is processed in a continuous manner, the deoxidisation solution tanks are usually 25 feet in length, thereby subjecting the substrate to the deoxidation solution for less than one minute.

The special pre-treatment process may also include the maintaining of a low oxygen environment prior to and/or subsequent to subjecting the substrate to the aggressive pickling process and/or chemical activation process. The maintenance of a low oxygen environment inhibits the formation and/or reformation of oxides on the substrate surface. The low oxygen environment may take on several forms. Two examples of low oxygen environments are a low oxygen-containing gas environment about the substrate or a low oxygen-containing liquid environment in which the substrate is immersed. Such environments act as shields against atmospheric oxygen and prevent and/or inhibit oxide formulation. If the substrate is stainless steel, the low oxygen environment should be maintained throughout the pretreatment process of the stainless steel substrate to just prior to the coating of the substrate with the coating of tin-zinc alloy. The non-oxidised surface of a stainless steel substrate is highly susceptible to rapid reoxidation when in contact with oxygen. By creating a low oxygen environment about the stainless steel strip, new oxide formation is inhibited and/or prevented.

Examples of low oxygen gas environments include nitrogen, hydrocarbons, hydrogen, noble gases and/or other nonoxidising gases. Generally, nitrogen gas is used to form the low oxygen gas environment. Examples of low oxygen liquid environments include non-oxidising liquids and/or liquids containing a low dissolved oxygen content. An example of the latter is heated water sprayed on the surfaces of the substrate; however, the substrate may also be immersed in the heated water.

Heated water contains very low levels of dissolved oxygen and acts as a shield against oxygen from forming oxides with the substrate. The spray action of the heated water may also be used to remove any remaining pickling solution or deoxidising solution from the substrate. Generally, the temperature of the heated water is maintained above 1000F and typically about 110 F or greater so as to exclude the unwanted dissolved oxygen.

In accordance with yet another aspect of the present invention, the tin-zinc alloy may be applied to the substrate by a hot-dip process, which may be operated in a batch or continuous manner. The substrate may be coated in the hot-dip process by passing the substrate through a coating vat which contains the special (molten) tin-zinc formulation, typically after passing through a flux box. Such a flux box typically contains a flux which has a lower specific gravity than the molten tin-zinc, so that the flux floats on the surface thereof.

The flux within the flux box acts as the final surface treatment of the substrate, removes residual oxides from the substrate surface, and shields the substrate surface from oxygen until the substrate is coated with the tin-zinc alloy. The flux preferably contains zinc chloride and may contain ammonium chloride. The flux solution typically contains approximately 30-60 percent by weight of zinc chloride and up to about 40 percent by weight of ammonium chloride and preferably about 50% zinc chloride and 8% ammonium chloride; however, the concentrations of the flux agents may be varied accordingly.

Once the substrate passes through the flux, the substrate enters the molten tin-zinc alloy formulation. The temperature of the molten tin-zinc can range from 4490F to over 8000F. The tin-zinc alloy must be maintained above its melting temperature to avoid improper coating. Tin melts at 2320C (4500F) and lead melts at 3280C (6220F). Zinc melts at 4200C (7880F). The larger the content of zinc, the closer the melting point of the tin-zinc coating approaches 4200C. In order to accommodate for the temperatures, the coating vat should be of a material which can withstand the higher temperatures. Palm oil is generally located on the surface of the molten tin-zinc in the coating vat, but this degrades at temperatures above about 6500F, so that special oils and/or special cooling procedures for the palm oil must be employed for high zinc content alloys. A zinc content of not more than 65 percent by weight has a low enough melting point temperature that a modified coating vat is not needed, and palm oil can be used.

The time period for applying the tin-zinc alloy to the substrate is usually less than 10 minutes. If the substrate is in sheet strip form and is being processed in a continuous process, the time period for applying the tin-zinc coating is typically less than two minutes and usually from 10 to 30 seconds. After the substrate has been coated, the coated substrate is usually cooled. The cooling of the coated substrate can be accomplished by spraying a cool fluid such as ambient temperature water. The cooling of the coated substrate usually is less than one hour and preferably is less than two minutes.

The thickness of the tin-zinc coating is usually regulated by coating rollers. The thickness of the tin-zinc coating may vary, depending upon the environment in which the coated metals are to be used, typical thicknesses being 0.0001 to 0.005 inch, such as 0.003 to 0.002 inch. The tin-zinc alloy has better corrosion-resistance than tin coatings. Preferably, the coating thickness is between 0.001 to 0.002 inch. Such a coating thickness has been found to be adequate to prevent and/or significantly reduce the corrosion of the metal substrate in virtually all environments. Coatings having thicknesses greater than 0.002 inch can be used in harsh environments to provide added corrosion protection.

Spray jets which spray the tin-zinc alloy onto the substrate may be used to ensure a uniform and continuous coating on the substrate. In the case of a fuel tank, the first and second sheet metal shell members may be joined together by welding or soldering the two members together. The electrode and/or solders used to join the shell members are preferably lead free so as not to introduce lead to the tank.

According to the invention, the metal substrate may be coated with a thin nickel barrier layer, before applying the tinzinc alloy. This provides additional corrosion resistance, especially against halogens such as chlorine or fluorine.

Although the tin-zinc alloy provides excellent protection against most corrosion-producing materials, elements such as chlorine can eventually penetrate the tin-zinc alloy and attack and oxidise the surface of the metal substrate, thereby weakening the bond between the metal and the tin-zinc alloy. A nickel barrier layer (preferably applied electrolytically) has been found to provide an almost impenetrable barrier to halogens or the like, which penetrate the tin-zinc alloy. Therefore, a thin nickel barrier layer can be employed, while still preventing halogens or the like from attacking the metal substrate. The tin-zinc alloy and the thin nickel barrier coating effectively complement one another to provide superior corrosion resistance.

The thickness of the nickel layer is preferably not more than 3 microns (1.18 x 10A in), more preferably from 1 to 3 microns. The bond between the tin-zinc alloy and the nickel layer is surprisingly strong and durable, and thereby inhibits the tin-zinc alloy from flaking, especially when the metal substrate is preformed or formed during installation.

The coated metal according to the invention can be formed and sheared to, for example, various building and roofing components that can be subsequently assembled on site without the alloy coating flaking off, chipping, and/or cracking. The coated sheet can, when used as a building material, further be preformed into roof pans and subsequently seamed on site either by pressed seams or soldered seams into waterproof joints.

The tin-zinc alloy is electroprotective under oxidising conditions, which inhibits oxidation of exposed metal near the tin-zinc alloy. As a result, minor discontinuities in the tinzinc alloy do not result in oxidation of the exposed metal, contrary to what happens if only a tin coating is used.

The tin-zinc coating can be welded with standard lead solders and no-lead solders. Preferably, no-lead solders are used to avoid concerns associated with the use of lead.

Preferred features of the present invention, particularly the aspect of the invention relating to fuel tanks, will now be described with reference to the accompanying drawings, in which: Figure 1 is an exploded sectional view of an exemplary petrol tank according to the present invention, prior to assembly; Figure 2 illustrates the joining of the first and second shell members of the tank of Figure 1, at the peripheral edges; Figure 3 is an enlarged partial cross-sectional view of a petrol tank illustrating the zinc-tin coating on the metal shell after the coated metal shell has been drawn; Figure 4 is a further enlarged view of the zinc-tin coating thickness of the present invention on a metal sheet; and Figure 5 is an illustration of prior art metal coating thickness on a metal sheet.

Referring now to the drawings, which are for the purpose of illustrating preferred embodiments of the invention only and not for the purpose of limiting the same, reference is first made to Figure 1 which, as noted above, illustrates an exemplary petrol tank according to the present invention.

Petrol tank 10 is made up of two shell members 12 and 14. The shell members are each shaped in a die by placing a sheet of zinc-tin coated metal sheet on the die and drawing the sheet over the die. The shells are preferably formed in a cylindrical shape and each have a peripheral edge 16.

The two shells are joined together at the respective peripheral edges to form an inner petrol receiving chamber 18 wherein the petrol is stored within the tank. Petrol tank 10 also contains a spout 20 which communicates with interior chamber 18 of the petrol tank so that the inner chamber can be filled with petrol. The spout is inserted at the top portion of shell 12 for easy insertion of petrol into the tank. Petrol tank 10 also has a drain hole 22 which communicates the interior of the tank chamber with the fuel system of a motor vehicle. Drain hole 22 is located at the top of the tank, on shell 12. Generally, a fuel pump is located in the inner chamber of the tank and pumps the petrol through the vehicle's fuel system.

As illustrated in Figure 2, shell members 12 and 14 are joined together by abutting peripheral edges 16 of the respective shell members together and subsequently soldering the shell members together with a lead-free solder 30. Spout 20 and drain hole 22 are also soldered to the shell member with a lead-free solder. The lead-free solder is used so as not to add any lead to the tank thereby maintaining the environmental safeness of the petrol tank and not compromising the recyclability of the tank.

The lead-free solder is comprised essentially of tin, so that the solder complements the zinc-tin coating on the tank, thereby assisting in the corrosion protection of the tank.

Referring to Figure 3, each shell member includes an outer zinc-tin coating 36 and an inner zinc-tin coating 38, which coatings have essentially the same thickness. Prior to applying the zinc-tin coating to metal sheet 34, the exposed surface of the sheet is preferably pre-treated, as described above.

The zinc-tin coated metal sheets are formed into shell members 12 and 14 by placing the coated sheet on a die and then drawing the coated sheet over the die to form a metal shell member with a drawn cavity. As illustrated in Figure 3, when the coated sheet is drawn over the die, the forming of the coated sheet forces the zinc-tin coating to become elongated about the outer peripheral edge corner 40. When the zinc-tin coating is elongated, the coating begins to reduce in thickness. If the coating is too thin, the coating will tear or shear and expose the unprotected surface of the metal sheet. The coating on the metal sheet 34 is at least 0.01 inch thick so that when the zinctin metallic coating is elongated while being shaped in the die, coating 36 will not shear at peripheral corner 40 and expose the surface of the sheet.

As illustrated in Figure 5, prior metal coatings on metal sheets having the equivalent of about 6-8 lb. coating on the metal sheets (0.0004 inch thick coating) have a tendency to tear apart or shear at peripheral edge 40 when the shell members are drawn on the die. In addition, the thicknesses of the prior metal coatings were inadequate to prevent pinholes 42 from existing on the metal sheet after coating. These pinholes significantly enlarged when the coating began to elongate at peripheral edge 40 while being drawn on the die. As a result of the expansion of the pinholes and/or tearing or shearing of the coating at peripheral edge 40 of prior metal coated sheets, various surfaces of the coated sheets were directly exposed to the atmosphere, resulting in accelerated corrosion of the tank at the exposed areas.

As illustrated in Figure 4, the zinc-tin coating on the metal sheet of the present invention is significantly thick to overcome such shortcomings of thinner coatings. The coating is preferably at least 0.01 inch thick to eliminate pinholes. As a result, the problems associated with the expansion of pinholes and the shearing or tearing of the zinc-tin coating during the forming of a tank in a die are essentially eliminated thereby increasing the corrosion protection to the tank.

Claims

CLAIMS:

1. A coated metal, which comprises a metal substrate having thereon a low-reflecting, highly corrosion-resistant coating, comprising a multiple-phase tin-zinc alloy mainly comprising tin and zinc.

2. A coated metal according to claim 1, wherein said alloy contains tin and zinc in an amount in excess of 80 percent by weight of said coating.

3. A coated metal according to claim 1 or 2, wherein said alloy includes nickel and/or copper.

4. A coated metal according to any of claims 1 to 3, wherein said coating comprises at least 15 percent by weight of tin, at least 65 percent by weight of zinc and up to 1.0 percent by weight of nickel.

5. A coated metal according to claim 4, wherein said nickel content is 0.3 to 0.9 percent by weight, such as about 0.7 percent by weight.

6. A coated metal according to any of claims 1 to 5, wherein the zinc content of said alloy is less than 85 percent by weight.

7. A coated metal according to any of claims 1 to 6, wherein said alloy contains at least 30 percent by weight of zinc.

8. A coated metal according to any of claims 1 to 7, wherein said alloy contains 35 to 65 percent by weight of zinc.

9. A coated metal according to any of claims 1 to 8, wherein said alloy includes up to 5.0 percent by weight of nickel and/or up to 5.0 percent by weight of copper.

10. A coated metal according to claim 9, wherein said alloy includes at least 0.3 percent by weight of nickel and/or at least 0.1 percent by weight of copper.

11. A coated metal according to any of claims 1 to 10, wherein said alloy includes at least 0.05 percent by weight of a metallic stabiliser.

12. A coated metal according to any of claims 1 to 11, wherein said alloy contains lead in an amount of less than 0.05 percent by weight, such as less than 0.01 percent.

13. A coated metal according to any of claims 1 to 12, wherein said alloy comprises: Tin 15.0 - 70.0% Zinc 30.0 - 85.0% Nickel 0.0 - 5.0% Bismuth 0.0 - 1.7% Antimony 0.0 - 7.5% Copper 0.0 - 5.0% Iron 0.0 - 0.1% Lead 0.0 - 0.05%

14. A coated metal material according to claim 1, wherein said alloy comprises: Tin

15.0 - 35.0% Zinc 65.0 - 85.0% Nickel 0.0 - 1.0% Bismuth 0.0 - 1.7% Antimony 0.0 - 7.5% Iron 0.0 - 0.1% Lead 0.0 - 0.05% 15. A coated metal according to any of claims 1 to 14, wherein said metal substrate comprises carbon steel, stainless steel or copper.

16. A coated metal according to any of claims 1 to 15, wherein the surface of said substrate is plated with a thin nickel layer prior to applying said alloy coating.

17. A coated metal according to claim 16, wherein said nickel layer has a thickness of up to three microns.

18. A coated metal according to any of claims 1 to 17, wherein said alloy is applied to said substrate by continuously passing said metal substrate through a molten bath of said alloy.

19. A method of applying a coating to a sheet of metal which comprises passing said sheet through a molten bath of an alloy comprising zinc, tin, less than 0.05% lead, 0.0-5% nickel and 0.0-5.0% copper, until the resulting coating has a thickness of 0.001 to 0.05 inch.

20. A method according to claim 19, wherein said alloy has a zinc content of 30 to 85 percent by weight.

21. A method according to claim 20, wherein said alloy comprises: Tin 15.0 - 70.0% Zinc 30.0 - 85.0% Bismuth 0.0 - 1.7% Antimony 0.0 - 7.5% Iron 0.0 - 0.1% Lead 0.0 - 0.05% Nickel 0.0 - 5.0% Copper 0.0 - 5.0%

22. A method according to claim 21, wherein said alloy comprises: Tin 35.0 - 70.0% Zinc 30.0 - 65.0% Bismuth 0.0 - 0.5% Antimony 0.0 - 0.5% Lead Less than 0.05% Nickel 0.0 - 1.0% Copper 0.0 - 1.5%

23. A method according to any of claims 19 to 22, wherein said alloy contains at least 0.3 percent by weight of nickel.

24. A method according to any of claims 19 to 23, wherein said alloy contains 0.1 to 1.6 percent by weight of copper.

25. A method according to any of claims 19 to 24, wherein said alloy contains 45 to 55 percent by weight of tin.

26. A method according to any of claims 19 to 25, wherein said alloy includes at least 0.05 percent by weight of a metallic stabiliser.

27. A method according to any of claims 19 to 26, wherein said sheet of metal is of carbon steel, stainless steel, copper, tin, aluminium, bronze or titanium.

28. A method according to any of claims 19 to 27, including an initial step of applying a thin layer of nickel onto said metal sheet prior to passing said sheet through said molten bath.

29. A method according to claim 28, wherein said nickel layer has a thickness not exceeding three microns.

30. A corrosion-resistant, environmentally-safe fuel tank for motor vehicles comprising first and second sheet metal shell members, each member including a drawn cavity surrounded by a peripheral edge, said members being placed together such that said edges of said members abut each other and said cavities combine to form an inner fuel receiving chamber, means for joining said abutting edges, spout means for introducing fuel into said inner chamber of said tank, draining means for removing fuel from said inner chamber; said sheet metal of each member comprising a steel sheet having a thickness of less than 0.2 inch having deposited thereon a hot-dipped coating of a zinc-tin alloy on the exposed surface of said sheet.

31. A tank according to claim 30, wherein said means for joining said edges includes welding said edges together with a lead-free electrode or soldering said edges with a lead-free solder.

32. A corrosion-resistant, environmentally-safe fuel tank comprising at least one shell member, said at least one shell member including a cavity surrounded by a peripheral edge, and forming an inner chamber, said at least one shell member comprising a steel sheet having thereon a coating of a zinc-tin alloy.

33. A tank according to any of claims 30 to 32, wherein said coating of said zinc-tin alloy has a thickness of at least 0.01 inch.

34. A tank according to any of claims 30 to 33, wherein said tin-zinc alloy comprises at least 15 percent by weight of tin and at least 7 weight percent zinc.

35. A tank according to claim 34, wherein said alloy contains zinc at least 65 percent of weight of zinc.

36. A tank according to claim 34, wherein said alloy contains tin 15 to 35 percent by weight of tin.

37. A tank according to claim 34, wherein said alloy comprises: Tin 15.0 - 35.0% Zinc 65.0 - 85.0% Bismuth 0.0 - 1.7% Antimony 0.0 - 7.5% Iron 0.0 - 0.1% Lead 0.0 - 0.05%

38. A tank according to any of claims 30 to 36, wherein said alloy includes at least 0.05 percent by weight of metallic stabiliser.

39. A tank according to any of claims 30 to 38, wherein the or each shell member is of carbon steel, stainless steel, copper, tin, aluminium, bronze or titanium.

40. A tank according to any of claims 30 to 39. wherein the surface of the or each of shell members is plated with a thin nickel layer prior to applying said coating of zinc tin alloy.

41. A tank according to claim 40, wherein said nickel layer thickness does not exceed three microns.