EP3211113A1

EP3211113A1 - Galvanized product & method

Info

Publication number: EP3211113A1
Application number: EP16157254.0A
Authority: EP
Inventors: Ville Saarima; Aaretti Kaleva; Pasi VÄISÄNEN; Antti Markkula; Erkki LEVÄNEN; Saara Heinonen; Juha-Pekka NIKKANEN
Original assignee: Rautaruukki Oyj
Current assignee: Rautaruukki Oyj
Priority date: 2016-02-25
Filing date: 2016-02-25
Publication date: 2017-08-30
Also published as: DK3420114T3; PL3420114T3; WO2017144721A1; ES2853205T3; EP3420114A1; EP3420114B1

Abstract

A galvanized product (10) having a galvanized surface (14) comprising a zinc carbonate layer (24) that comprises at least one type of metallic ion of a metal that is more noble than zinc in a delivery condition of said galvanized product (10).

Description

TECHNICAL FIELD

The present invention concerns a galvanized product having a surface comprising zinc carbonate and a method for manufacturing such a galvanized product.

BACKGROUND OF THE INVENTION

Patina is a thin layer that naturally forms on the surface of copper, bronze and ferrous metals i.e. steels and irons, on environmental exposure to atmospheric elements such as air and precipitation, in the form of rain, snow, dew, ice, humidity, condensation, etc. Patinas, which may comprise various chemical compounds such as oxides, carbonates, sulfides or sulfates, can provide a protective covering to materials that would otherwise be damaged by corrosion or weathering.
Galvanization is the process of applying a protective zinc coating to a ferrous metal, i.e. iron or steel, to prevent rusting. The most common galvanization method is hot-dip galvanization, in which ferrous metal parts are submerged in a bath of molten zinc. Galvanizing protects the ferrous metal surface in two ways: it forms a coating of corrosion-resistant zinc which prevents corrosive substances from reaching the more delicate part of the metal, and the zinc serves as a sacrificial anode so that even if the coating is scratched, the exposed metal will still be protected by the remaining zinc.
A patina layer (i.e. a zinc carbonate layer) forms naturally on a galvanized ferrous metal that is exposed to atmospheric elements during its use in the following stages: the zinc in the galvanized ferrous metal surface reacts with oxygen in the air to form zinc oxide, ZnO, water or moisture from atmospheric precipitation reacts with the zinc oxide to form zinc hydroxide, Zn(OH)₂, and oxygen and carbon dioxide in the air reacts with the zinc hydroxide to form zinc carbonate, 2ZnCO₃·Zn(OH)₂.
Usually, the natural formation of patina layer on a galvanized ferrous metal can take up to several years depending on environmental conditions. This natural formation of a patina layer is disadvantageous since it is necessary to wait until a patina layer has formed before a galvanized ferrous metal can be painted in order to ensure that the paint will properly adhere to the surface of the ferrous metal steel. Normally, galvanized ferrous metal is passivated to protect the relatively sensitive zinc layer from corrosion. Paint applied to a surface of a passivated galvanized ferrous metal may lose its adhesion over time since such a freshly applied passivation layer is not an optimum surface for painting. A naturally formed patina layer may not be uniform as regards its thickness and/or structure, and it may contain impurities, i.e. contaminants picked up from its surroundings, such as dust, dirt, oil, grease and/or sea salt or trace elements from air pollution caused by industry or traffic for example. However, a naturally formed patina does provide a good substrate for painting. Furthermore, the continuous colour coating of galvanized ferrous metal usually requires several chemical pre-treatments, which may at least partly be rendered unnecessary by allowing a patina layer to form and applying paint directly on the patina layer.
US patent no. US 3,767,478 discloses the formation of a smooth, even, corrosion resistant patina on a zinc surface by abrading or otherwise mechanically graining the surface, and thereafter forming an even zinc phosphate Zn₃(PO₄)₂ coating thereon, preferably in the presence of a nickel ion. This solution involves the formation of a protective layer having a different chemical composition and structure than a naturally formed patina layer.

SUMMARY OF THE INVENTION

An object of the invention is to provide an improved galvanized product.
This object is achieved by a galvanized product having a galvanized surface comprising a zinc carbonate layer that comprises at least one type of metallic ion of a metal that is more noble than zinc and that excludes avoidable impurities in a delivery condition of the galvanized product, i.e. the zinc carbonate layer is not a naturally formed zinc carbonate layer but it has been artificially formed in under controlled conditions before it is delivered to an end user and put into use. The expression controlled conditions as used herein means that impurities such as dust, dirt, oil, grease, sea salt, and/or any impurities from air pollution caused by industry or traffic for example, will not be incorporated into the zinc carbonate layer during its formation, as would be the case if the zinc carbonate layer was naturally formed on environmental exposure during its use.
The galvanized product according to the present invention therefore comprises a zinc carbonate layer that is more uniform and of a higher quality, i.e. more pure and ideal, than a naturally formed zinc carbonate layer. This artificially formed zinc carbonate layer is advantageously formed much more quickly than a naturally formed zinc carbonate layer.
The artificially formed zinc carbonate layer will protect the galvanized product from corrosion during the transportation, the storage and the use of the galvanized product and it serves as a good substrate for post-patination operations such as painting. Post-patination operations may be carried out, without pre-treatment or cleaning in many cases, as soon as the zinc carbonate has been artificially formed or as soon as the galvanized product is delivered to an end user, or as soon as the galvanized product has been mounted ready for use. Such an artificially formed zinc carbonate layer may also at least partly eliminate the need for chemical pre-treatments in post-patination operations such as continuous colour coating.
The artificially formed zinc carbonate layer will have the same chemical composition as a naturally formed patina layer (but will exclude avoidable impurities, such as dust, dirt, oil, grease, sea salt, and/or any impurities from air pollution caused by industry or traffic for example) and the same structure as a naturally formed patina layer (but will exclude structural irregularities). It is non-toxic, non-combustible and thereby environmentally friendly.
The word "layer" as used in this document is intended to mean any amount of material of any uniform or non-uniform thickness that is uniformly or non-uniformly spread over an area of a substrate, i.e. over at least a part of a galvanized surface. A zinc carbonate layer may namely be artificially formed on a galvanized surface of a galvanized product in any desired manner or pattern depending on the end use of the galvanized product, or even for aesthetic reasons.
The galvanized product according to the present invention may be of any shape, weight, or size. It may be in the form any flat, tubular or shaped product, such as strip, sheet, plate, wire or profile, it may be a hot-rolled or cold-rolled product.
The galvanized product may be used for any suitable application, such as applications in the automotive, construction, marine, engineering or metal-producing industries. The galvanized product may for example constitute at least one part of a vehicle, a civil engineering construction, such as a bridge, part of a building, such as a roof, a façade, a silo, a trailer, a monument, etc.
According to an embodiment of the invention the galvanized product is a galvanized ferrous metal product, i.e. zinc-coated iron or steel.
According to an embodiment of the invention the galvanized product is a hot-dip galvanized product, an electrogalvanized product or a product with a thermal spray zinc coating.
According to an embodiment of the invention the at least one type of metallic ion of a metal that is more noble than zinc is preferably an ion of at least one of the following: copper, iron, silver, palladium, platinum, gold, titanium, molybdenum, tungsten, tin, lead, steel, brass, tantalum, nickel, indium, aluminium, cadmium or beryllium.
The present invention also concerns a method for manufacturing a galvanized product comprising an artificially formed zinc carbonate layer (artificial patination) according to any of the embodiments of the invention. The method comprises the step of treating a galvanized surface of a galvanized product with carbon dioxide (in any form, i.e. in solid, liquid, gaseous or supercritical form, or in any combination of these forms), water and at least one type of metallic ion of a metal that is more noble than zinc under controlled conditions, for example in a controlled environment, such as under controlled temperature, pressure and time, in order to provide an artificially formed zinc carbonate layer on the galvanized surface of the galvanized product. This method is performed before the galvanized product leaves the factory, i.e. before it is delivered to an end user, or before it undergoes a subsequent manufacturing or finishing step, such as painting.
According to an embodiment of the invention, the method is performed in a continuous line manufacturing process in which the galvanized product is produced without interruption, whereby a material is substantially continuously in motion while it undergoes galvanizing and/or artificial patination and/or one or more chemical reactions, mechanical treatments and/or heat treatments. Alternatively, or additionally at least part of the method according to the present invention may be carried out using batch production in which one or more galvanized products are created stage by stage over a series of workstations.
According to an embodiment of the invention the step of treating the galvanized surface of a galvanized product involves subjecting the galvanized surface to a temperature of at least 293 K, or a temperature of 293-343 K, 300-340 K or 310-330 K, and/or to a pressure of 6-30 MPa, or 6-20 MPa or 10-20 MPa for a predetermined exposure time.
According to an embodiment of the invention the predetermined exposure time is up to 60 minutes, or 5-50 minutes or 5-30 minutes or 10-30 minutes or until a zinc carbonate layer having the desired properties has formed on at least part of the galvanized surface. In a continuous line manufacturing process an exposure time of a fraction of one second or up to 10 seconds may be sufficient.
According to an embodiment of the invention the method comprises the step of decreasing the pressure at a rate, such as 6-30 MPa per minute, after the predetermined exposure time has elapsed. This may be done by decreasing the pressure in a pressure chamber in batch production, or by merely moving a galvanized product out of an artificial patination section of a continuous line manufacturing process at a predetermined rate.
According to an embodiment of the invention the galvanized product is a galvanized ferrous metal product.
According to an embodiment of the invention the product is a hot-dip galvanized product, an electrogalvanized product or a product with a thermal spray zinc coating.
According to an embodiment of the invention the at least one type of metallic ion of a metal that is more noble than zinc is preferably an ion of at least one of the following: copper, iron, silver, palladium, platinum, gold, titanium, molybdenum, tungsten, tin, lead, steel, brass, tantalum, nickel, indium, aluminium, cadmium or beryllium.
It is essential that at least one type of metallic ion that is more noble than zinc is used to artificially form the zinc carbonate layer since a galvanic pair must be created on the galvanized surface, as this leads to the electrochemical dissolution of zinc during the formation of a zinc carbonate layer. Zinc will namely corrode preferentially to metallic ion in the metal precursor that is used, i.e. the compound containing at least one type of metallic ion of a metal that is more noble than zinc. According to an embodiment of the invention, the precursor is preferably non-soluble in water, although it can alternatively be soluble in water.
According to an embodiment of the invention the carbon dioxide is in at least one of the following forms: gaseous, liquid phase, solid form, supercritical, i.e. at a temperature and pressure above its critical point (304.25 K and 7.39 MPa), where distinct liquid and gas phases do not exist. The carbon dioxide may namely be in any form or a mixture of forms depending on the reaction speed and zinc carbonate layer structure which are desired. Different reaction speeds and different zinc carbonate layer structures, such as needle-like or ball-like structures, may namely be obtained depending on the form of carbon dioxide that is used.
According to an embodiment of the invention the water is de-ionized water, tap water, sea water, lake water or any other suitable aqueous electrolyte which allows a zinc hydroxide layer to form during the artificial formation of a zinc carbonate layer.
According to an embodiment of the invention the method preferably comprises at least one of the following steps: cleaning and/or roughening a galvanized surface of the galvanized product prior to artificially forming the zinc carbonate layer on the galvanized surface. It is not however always necessary to clean and/or roughen the galvanized surface of the galvanized product.
According to an embodiment of the invention the method comprises the steps of dissolving a metal precursor comprising the at least one type of metallic ion of a metal that is more noble than zinc, which may be soluble or non-soluble in water, in a solvent, and then mixing the solvent containing the metal precursor with the water to treat the galvanized surface of a galvanized product. The solvent can be an organic solvent, such as ethanol, methanol or another alcohol or an aldehyde or a ketone. It should be noted that the expression "solvent" is intended to include a single solvent, or any mixture of solvents.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended figures where;

Figure 1: shows the natural formation of a zinc carbonate layer on a galvanized surface of a galvanized product that is exposed to the atmosphere, and
Figure 2: is a flow diagram showing the steps of a method according to an embodiment of the invention in which an artificially formed zinc carbonate layer is formed on at least part of a galvanized surface of a galvanized product.

It should be noted that the drawings have not necessarily been drawn to scale and that the dimensions of certain features may have been exaggerated for the sake of clarity.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1a) shows a galvanized product 10 comprising a metal substrate 12, such as steel or iron, having a galvanized surface 14. When the galvanized product 10 is exposed to the atmosphere during its use oxygen in the air 16, illustrated as a free-flowing air flow, will react with the zinc in the galvanized surface 14 to form zinc oxide (ZnO) 18.
Figure 1b) shows that water and/or moisture from atmospheric precipitation 20 reacts with the zinc oxide 18 to form zinc hydroxide 22 (Zn(OH)₂).
Figure 1c) shows that oxygen and carbon dioxide in the air 16 reacts with the zinc hydroxide 22 to form zinc carbonate (2ZnCO₃·Zn(OH)₂) 24.
The process of naturally forming a zinc carbonate layer 24 as illustrated in figures 1 a) to c) can take years depending on the atmospheric conditions to which a galvanized product 10 is subjected during its use, transportation and/or storage. Furthermore, a naturally formed zinc carbonate layer 24 may not be uniform and it may contain impurities from its surroundings, such as dust, dirt, oil, grease, sea salt and/or trace elements of contaminants present in its surroundings, since a zinc carbonate layer 24 exposed to atmospheric elements is not formed under controlled conditions.
A galvanized product 10 according to the present invention, such as a hot-dip galvanized iron or steel sheet, will, on the contrary, have a galvanized surface comprising a zinc carbonate layer 24 of better quality than a naturally formed zinc carbonate layer since it has been artificially formed under controlled conditions. The zinc carbonate layer 24 will however comprise at least one type of metallic ion of a metal that is more noble than zinc, such as copper (Cu²⁺), iron (Fe²⁺) or silver (Ag²⁺) which remain as a consequence of the artificial formation. The artificially formed zinc carbonate layer 24 will not however contain avoidable impurities, such as dust, dirt, oil, grease, sea salt etc. when it is delivered to an end user.
The steps of a method for manufacturing a galvanized product 10 comprising an artificially formed zinc carbonate layer 24 according to an embodiment of the invention are illustrated in figure 2, wherein non-essential method steps are shown in rectangles drawn with dashed lines and essential method steps are shown in rectangles drawn with continuous lines. The method comprises the steps of optionally cleaning and/or roughening at least part of a galvanized surface 14 of the galvanized product 10 prior to artificially forming the zinc carbonate layer 24 thereon in order to provide a more adhesive surface for subsequent treatments, such as painting. One or more impurities, such as an aluminium oxide, which may be formed on top of the zinc layer after galvanization, may for example be removed from the metallic zinc layer prior to artificial patination.
The galvanized surface 14 may then be placed or moved into in a controlled environment. For example the galvanized surface 14 may be placed into a pressure chamber or an artificial patination workstation during batch production or into it may be moved into an artificial patination section at a controlled feed rate during a continuous line manufacturing process. During artificial patination, the galvanized surface 14 is exposed to carbon dioxide in any suitable form, such as carbon dioxide having a purity of at least 99.8%, and may optionally be heated to a temperature of at least 293 K, such as 293-343 K and/or subjected to a pressure of 6-30 MPa.
Once the desired temperature and pressure has been reached, water containing a metal precursor, i.e. any compound comprising at least one type of metal ion that is more noble than zinc, may be applied and/or sprayed and/or injected onto the galvanized surface 14 either separately or together with carbon dioxide, whereupon a zinc carbonate layer 24 will begin to form on the galvanized surface 14. The metal precursor may be Cu(hfac)₂ or Ag(cod)(hfac) or Cu(NO₃)₂ or Fe(NO₃)₃. Preferably, the precursor is non-soluble in water, such as Cu(hfac)₂ or Ag(cod)(hfac), and may firstly be dissolved in a solvent, such as an organic solvent, such as ethanol and then mixed with water before it is applied and/or sprayed and/or injected onto the galvanized surface 14. This process is easily adapted to production environments and is especially suited to complicated surfaces found in grooved couplings and fittings of galvanized products 10.
The galvanized surface 14 may remain in the controlled environment for a predetermined exposure time, such from a fraction of a second (which may be sufficient during a continuous line manufacturing process) up to 60 minutes. If the galvanized product 10 is being manufactured using batch production, in a pressure chamber for example, the pressure may be decreased at a predetermined rate, such as at a rate of 6-30 MPa per minute, after the predetermined exposure time has elapsed. The rate at which the pressure is decreased will affect the structure of the artificially formed zinc carbonate layer 24. The zinc carbonate layer 24 will continue to form during the entire method until the galvanized surface 14 is removed from the pressure chamber, or from an application section of a continuous line manufacturing process, where carbon dioxide at a desired temperature and/or pressure, water and at least one metallic ion of a metal that is more noble than zinc is applied and/or injected and/or sprayed onto the galvanized surface 14.
After this treatment, the galvanized product 10 may leave the factory where it is manufactured and be delivered to an end user or placed in storage. The artificially formed zinc carbonate layer 24 is not water soluble and does not wash off the galvanized surface 14.
The at least one type of metallic ion acts as a catalyst in the formation of a zinc carbonate layer 24, it speeds up the reaction time and will remain in the artificially formed zinc carbonate layer 24, which makes an artificially formed zinc carbonate layer 24 distinguishable from a naturally formed zinc carbonate layer. For example, the artificially formed zinc carbonate layer 24 may contain one or more copper compounds if a metal precursor containing copper ions has been used in its manufacturing method. The artificially formed zinc carbonate layer 24 will have superior properties as compared to a naturally formed zinc carbonate layer since its structure will be more uniform (rather than randomly compact or porous) and it will adhere better to the underlying galvanized surface. Paint can be applied directly to the artificially formed zinc carbonate layer 24 and no adhesion problems have been encountered.
The method according to the present invention bypasses the different stages of the natural formation of a zinc carbonate layer 24 as illustrated in figures 1a) to c) and will form a more pure and ideal and more tightly adhered layer as compared to a naturally formed zinc carbonate layer. A naturally formed zinc carbonate layer 24 may namely be non-uniform as regards its thickness and/or properties, and it may be loosely adhered to the underlying galvanized layer, it may be porous and contaminated with impurities.

EXAMPLE

The method according to the present invention may comprise the step of treating a galvanized surface 14 of a hot-dip galvanized steel sheet 10 with supercritical carbon dioxide and 0.53 m-% copper solution (including water) at a temperature of 323 K and at a pressure of 30 MPa for 30 minutes, and then decreasing the pressure at a rate of 10 MPa per minute.
Further modifications of the invention within the scope of the claims would be apparent to a skilled person.

Claims

A galvanized product (10) having a galvanized surface (14) comprising a zinc carbonate layer (24), characterized in that said zinc carbonate layer (24) comprises at least one type of metallic ion of a metal that is more noble than zinc in a delivery condition of said galvanized product (10).
A galvanized product (10) according to claim 1, characterized in that it is a galvanized ferrous metal product.
A galvanized product (10) according to claim 1 or 2, characterized in that it is a hot-dip galvanized product, an electrogalvanized product or a product with a thermal spray zinc coating.
A galvanized product (10) according to any of claims 1-3, characterized in that said at least one type of metallic ion of a metal is an ion of at least one of the following: copper, iron, silver, palladium, platinum, gold, titanium, molybdenum, tungsten, tin, lead, steel, brass, tantalum, nickel, indium, aluminium, cadmium or beryllium.
A method for manufacturing a galvanized product (10) comprising zinc carbonate, characterized in that said method comprises the step of treating a galvanized surface (14) of a galvanized product (10) with carbon dioxide, water and at least one type of metallic ion of a metal that is more noble than zinc under controlled conditions in order to provide an artificially formed zinc carbonate layer (24) on said galvanized surface (14) of said galvanized product (10).
Method according to claim 5, characterized in that it is carried out in a continuous line manufacturing process, or using batch production.
A method according to claim 5 or 6, characterized in that said step of treating said galvanized surface (14) of a galvanized product (10) involves subjecting said galvanized surface (14) to a temperature of at least 293 K and/or a pressure of 6-30 MPa for a predetermined exposure time.
A method according to claim 7, characterized in that said predetermined exposure time is a fraction of a second up to 60 minutes.
A method according to claim 7 or 8, characterized in that it comprises the step of decreasing said pressure at a rate of 6-30 MPa per minute after said predetermined exposure time.
A method according to any of claims 5-9, characterized in that said galvanized product (10) is a galvanized ferrous metal product.
A method according to any of claims 5-10, characterized in that said galvanized product (10) is a hot-dip galvanized product, an electrogalvanized product, or a product with a thermal spray zinc coating.
A method according to any of claims any of claims 5-11, characterized in that said at least one type of metallic ion is an ion of at least one of the following: copper, iron, silver, palladium, platinum, gold, titanium, molybdenum, tungsten, tin, lead, steel, brass, tantalum, nickel, indium, aluminium, cadmium or beryllium.
A method according to any of claims any of claims 5-12, characterized in that said carbon dioxide is in at least one of the following forms: gaseous, liquid, solid, supercritical.
A method according to any of claims 5-13, characterized in that said water is at least one of the following de-ionized water, tap water, sea water, lake water.
Method according to any of claims 5-14, characterized in that it comprises at least one of the following steps: cleaning and/or roughening a galvanized surface (14) of said galvanized product (10) prior to artificially forming said zinc carbonate layer (24) on said galvanized surface (14) of said galvanized product (10).
Method according to any of claims 5-15, characterized in that it comprises the steps of dissolving a metal precursor comprising said at least one type of metallic ion of a metal that is more noble than zinc and which is non-soluble in water in a solvent and then mixing said solvent containing said metal precursor with said water to treat said galvanized surface (14) of a galvanized product (10).