Patent Publication Number: US-2007119715-A1

Title: Corrosion Resistant Wire Products and Method of Making Same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a Continuation-In-Part of U.S. patent application Ser. No. 11/287,051 which was filed on Nov. 25, 2005. 
    
    
     FIELD OF THE INVENTION  
      This invention relates to wire products and in particular to fabricated wire products, such as welded or woven mesh, stucco corner reinforcements, barbed wire and wire fencing.  
     BACKGROUND OF THE INVENTION  
      Using zinc to protect steel from corrosion has been practiced for over 150 years. The zinc can be applied by hot galvanizing methods, by electro-galvanization, by sherardizing, by spraying, or by cold galvanizing methods. The zinc protects the steel since it is less noble in the galvanic series of metals and hence will be sacrificed in a corrosive environment in relation to the steel. Since zinc is sacrificial, it will be consumed with time. The rate of zinc loss or consumption over time depends on the severity of the corrosive environment. Zinc also has the further benefit that since it protects the steel by being sacrificial, small localized areas of bare steel are still protected by the adjacent zinc. This differs from the behavior of other protective coatings such as paint or chrome whereby any non-coated or scratched areas corrode quickly.  
      However, as the zinc is consumed, protection of the steel substrate is ultimately lost and corrosion of the steel begins to occur. Zinc consumption rates can vary under different general atmospheric exposures, but have been evaluated by various researchers to be generally 0.06 oz./sq. ft. per year in rural atmospheres, 0.07 oz./sq. ft. per year in marine atmospheres, and 0.1 to 0.4 oz./sq. ft. per year in industrial areas.  
      The amount of zinc coating can be increased for specific applications or environments to provide a longer effective life. Ranges of zinc coating weights are given in specifications such as ASTM A 641/A 641M-03 ‘Standard Specification for Zinc-Coated (Galvanized) Carbon Steel Wire’. Coating weights range from a low of 0.15 oz./sq. ft. for Class  1  wires up to a maximum of 3.00 oz./sq. ft. for Class C coating. Similarly, ASTM A653/A653M-05a ‘Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) by the Hot-Dip Process” and provides standards for various zinc or zinc-iron alloy coating weights or coating designations.  
      The coating weight can be selected from such standards and specifications to best suit the intended application and expected life. Nonetheless, in the case of wire coatings, even at maximum zinc weights, life expectancy may range from 50 years to as little as 7.5 years.  
      Therefore, in the prior art, there have been various efforts to improve or increase the corrosion resistance of zinc coatings. One approach has been to coat steel substrates with aluminum-zinc coatings. Aluminum-zinc coatings, and the products of such coatings have been disclosed in U.S. Pat. Nos. 3,343,930, 3,393,089 and 3,952,120. It has been said that optimum corrosion resistance for such coatings occurs with a combination of 55% aluminum and 45% zinc and suppliers claim that 55% Al—Zn sheets will have a life from 30 to 40 plus years, except in severe marine applications, and at least twice the life of equal galvanized steels.  
      The Zn-55 Al alloy exhibits good corrosion resistance, but does not provide satisfactory sacrificial protection of the steel substrate because of the high aluminum content. Further, weldability is very poor due to the high aluminum content. The process is further complicated by the need for expensive preliminary surface treatments, and even so has a tendency to form bare spots and similar defects. These deficiencies led to the development of improved coatings as disclosed in U.S. Pat. No. 4,448,748. The coating bath contains zinc, 5% aluminum, and a mischmetal addition. The term mischmetal refers to a variety of known rare earth alloys, such as cerium and lanthanum. This coating was developed by ILZRO (The International Lead-Zinc Research Organization), and is known in the trade as Galfan®. Corrosion resistance of the Galfan® products is not quite as good as the original 55% Al—Zn coatings but the sacrificial aspect of the coating was improved. However, weldability is still a problem and surface preparation is still difficult. Normal zinc ammonium chloride fluxes cannot be used in the surface preparation, as would be the case with conventional hot galvanizing.  
      Further, it has been found that aluminized wire coatings, when formed into products such as welded mesh for concrete reinforcement, showed very rapid corrosion in concrete, compared to pure zinc coatings, with the aluminized coating losing 95% of its coating weight in five years. In the same period, galvanized wire mesh loses only 40%. Cinders, fly ash, and lime found in concrete, mortar or stucco, in direct contact with aluminized steels is extremely corrosive.  
      Another prior art approach to enhance corrosion resistance of galvanized surfaces is the application of a chromate surface treatment. Chromate conversion coatings are produced on various metals by chemical or electrochemical treatment with mixtures of hexavalent chromium and certain other compounds. These treatments react with the metal surface to provide a superficial layer containing a complex mixture of chromium compounds. Chromate conversion of zinc or cadmium coatings was first patented in  1936  by E. J. Wilhelm (U.S. Pat. No. 2,035,380).  
      Chromate coatings are usually applied to electroplated zinc coatings. A range of distinctive colors such as clear-blue, yellow, green or black can be obtained. The color differences are obtained by varying the chemical formulation and pH values. The darker colors indicate greater chromate thickness. Coating thickness can vary from 0.2 to 0.7 mils. Greater thickness also provides the greatest corrosion resistance. For example, salt spray tests show the following hours to red rust on zinc plated steel with and without chromate: 150 to 400 hours with no chromate, 250 to 750 hours with clear or bluish chromate, 250 to 1000 hours with yellow chromate, and 500 to 1500 hours with olive/khaki chromate. As can be seen, chromate coatings can enhance corrosion resistance by a factor of 2 to 4.  
      Chromate coatings are relatively inexpensive, can be applied quickly and can provide good corrosion enhancement as shown above. Protection is due to both the corrosion inhibiting effect of chromium compounds contained in the film, and to the physical barrier presented by the film itself. Even scratched or abraded films retain a great deal of their protective value, since the chromium content is slowly leachable in contact with moisture, providing a self healing effect.  
      The disadvantages of chromate coatings are that they soft and easily damaged while wet. They become reasonably hard when dried, and can withstand normal handling and part assembly. However, they will not withstand continued scratching, abrasion, stamping or cold forming, such as is encountered in fabrication processes. In this disclosure and in the claims, the terms “fabrication” and “fabricated” refer to mechanical steps that involve the manipulation of strands to change their relative positions, or their shapes or mechanical interrelationship. Examples of fabrication steps as understood herein are welding, weaving, rolling, reshaping, bending, cutting and twisting.  
      A second disadvantage of chromate coatings is that they only convert zinc coatings well when the zinc coatings are pure, with few contaminants.  
      The two most common methods for commercial galvanization are hot galvanizing and electro-galvanizing.  
      In general, a zinc coating obtained by hot dip galvanizing consists of several layers: an internal alloy of iron and zinc which adheres to the surface of the ferrous material, and an external layer, consisting almost entirely of pure zinc called the Eta phase. In the interior layer, formed by the diffusion of zinc into the ferrous material, up to three zones or sub-layers can be distinguished, identified by their different iron contents. The sub-layer closest to the base material is called the Gamma phase and contains 21 to 28% iron. The next layer is the Delta phase, which contains from 6% to 11% iron, and finally the Zeta phase which contains approximately 6% iron.  
      In hot galvanizing of wire, it is common to wipe the wires as they exit the zinc bath to remove excess zinc and meet the lower coating weights that customers have specified. This wiping action removes the outer phases leaving primarily the Gamma phase, which is iron rich.  
      As pointed out in U.S. Pat. No. 4,171,231, iron contamination of the zinc coating causes black staining of the zinc coating when chromated. The cause of this black staining is postulated to be the formation of black iron oxides as a result of reaction of hexavalent chromium compounds and iron deposits. Not only is the appearance of the black staining not attractive, but some of the corrosion enhancement is lost as well  
      As a result, chromate conversion is usually not performed on hot galvanized surfaces, and is restricted to steel parts that have been electro-galvanized since the zinc coating is very pure.  
      Many products such as fasteners, hardware, stamped parts, nails, etc, are made from uncoated steel. Once fabricated, the product is then degreased, electro-galvanized and chromated. Since there is no post galvanizing fabrication, there is no damage to the galvanizing or chromating. In addition, there are typically no ungalvanized areas, since there is no further punching, cutting, welding that would result in ungalvanized surfaces.  
      However, post-fabrication galvanizing is not considered to be practical for many products where the bulkiness or shape of the finished product would make it uneconomical or impractical to galvanize after fabrication. Classic examples are welded and woven wire mesh such as is used for stucco lath. This problem is exacerbated in the global economy where semi-finished steel may be produced overseas, and shipment of unprotected steel would be difficult and costly.  
      Such products, such as metal flashings, metal roofing, wire fencing, barbed wire, lath, welded wire fabrics and others, are fabricated from pre-galvanized sheet or wire. However, the galvanizing may be damaged during fabrication. Zinc may be burned off during welding, and cut edges such as on barbed wire results in non galvanized areas. Such products are nonetheless manufactured from pre galvanized feed stock since it would be extremely difficult and expensive to galvanize them after fabrication. However, it would be ineffective to use pre-chromated galvanized steel since the chromate surface would itself be damaged during fabrication, and hence the chromating properties could not be assured on the finished products.  
      Further, the economics of galvanizing have been dramatically affected by recent rises in the price of zinc. To compound this problem, the hot galvanizing process inherently results in the wastage of zinc through the formation of zinc dross and zinc oxides. These losses can amount up to 50% of the zinc usage, especially with lighter zinc coating weights such as Class  1  wire. Further losses of zinc are experienced with difficulty in controlling coating weights. A certain amount of over galvanization can occur which increases the cost of production.  
      Despite the drawback that chromate conversion is viewed as not applicable to hot galvanized surfaces, hot galvanizing offers distinct advantages. These include the fact that hot galvanizing is fast resulting in high production speeds, requires low capital investment and requires relatively little space. Adhesion of zinc is normally very good as a result of the alloying effect of the hot galvanizing process.  
      Conversely, electro-galvanizing is a very efficient process. There is no dross or oxide formation and therefore virtually 100% of the zinc is plated onto the product. Further, coating weights can be exactly controlled to specification and there is no give away of zinc. With rapidly rising zinc costs, economics have swung in favor of electro-galvanizing. Unfortunately, the electro-galvanizing process is slow and coating weights are inversely proportional to line speed. In order to achieve a two-fold increase in coating weight, a production line needs to operate at half the speed. Further, with increased coating weight, zinc adhesion becomes more difficult such that zinc flaking may occur with mechanical deformation during fabrication of finished products. Therefore, electro-galvanizing can not take full economic advantage of the rising zinc costs in relation to hot galvanizing, and consequently the costs of all galvanized products are rising.  
      Although the present invention may be applied to many fabricated wire products, the primary application is for wire mesh stucco lathing and stucco wire corner reinforcement. Stucco lath and stucco wire reinforcement products may be of the welded type or the woven type. These products are described in specifications ASTM C933-05 ‘Standard Specification for Welded Wire Lath’, ASTM C1032-06 ‘Standard Specification for Woven Wire Plaster Base’ and ASTM C1063-03 ‘Standard Specification for Installation of Lathing and Furring to Receive Interior and Exterior Portland Cement -Based Plaster’. According to ASTM A641-03 ‘Standard Specification for Zinc-Coated (Galvanized) Carbon Steel Wire, the wire must be Class  1  zinc coated (galvanized) soft temper steel material. The wire sizes usually incorporated in these wire laths are 0.035 inch diameter to 0.053 inch diameter. Class  1  coatings for these size ranges specify a minimum 0.15 oz./sq. ft.  
      Corrosion resistance of stucco laths has become of increasing concern in the building industry over the last 10 years because of premature failures. The Class  1  coating as specified is still considered to be a light zinc coating and is not intended for years of service in an outdoor environment. The galvanized coating is intended to provide short term protection after the lath is produced but still not applied, while the lath is on the wall prior to stucco application, and while the stucco is wet and curing. Long term protection is provided by the stucco keeping the majority of moisture away from the wire. Installation instructions caution the installer to ensure that wire is covered with at least ⅛ inch of stucco.  
      However, there are a number of changes that have occurred that have resulted in rapid deterioration of the stucco laths. First has been the advent of acrylic finish coats. These finishes have the ability to diffuse water vapor but are much slower than previous cementitious finishes. Therefore, if any moisture gets behind the surface of the stucco, it takes a longer time to dry which results in available moisture for accelerated corrosion. Acrylic finishes are not as thick as cementitious and do not result in ⅛ inch of covering, especially at corners. A second contributor to accelerated corrosion is the shift to one coat stucco systems. These systems are all thin systems resulting in stucco thickness down to ⅜ inch. Further, these proprietary stuccos have various additives such as fly ash and a myriad of other chemical admixtures which can result in more aggressive stuccos when in contact with moisture. Thirdly are environmental exposures. More stucco clad structures are being built in non-typical stucco climates such as coastal areas subject to wind driven rains and salt air, and high humid areas such as Texas and Florida. Therefore, there is a growing focus of building officials, architects, engineers, contractors and home owners to build improved stucco claddings.  
      Therefore, there is a need to provide a method for improved corrosion resistance on galvanized steel products whilst minimizing the amount of zinc used. In particular it is an object of the present invention to provide improved corrosion resistance on galvanized fabricated wire mesh products such as lath and corner reinforcements.  
      It is further an object of the present invention to provide a method for enhanced corrosion protection of wire products such as welded and woven wire fabrics, welded and woven meshes, barbed wire, and woven wire fencing.  
      These and other objects of the invention will be better understood by reference to the detailed description of the preferred embodiments which follows.  
     SUMMARY OF THE INVENTION  
      In one aspect the invention is a method of manufacturing fabricated wire products comprising welded or reverse twist woven wire mesh, comprising the steps of galvanizing wire strands prior to fabricating said mesh, fabricating the mesh from the galvanized wire strands and applying a chromate treatment to the fabricated wire mesh.  
      In another aspect, the wire strands are galvanized by hot galvanizing followed by electrogalvanizing. In a more specific aspect, the electrogalvanized layer is less than 0.10 oz./sq. ft.  
      In another aspect the invention consists of welded or woven wire mesh manufactured according to the foregoing methods.  
      In another aspect, the invention consists of welded wire mesh comprising perpendicular strands of wire welded together at their intersections, wherein said strands comprise a first hot galvanized layer of zinc or zinc alloy and a second electrogalvanized layer, said second layer having been subjected to a chromate treatment.  
      The foregoing was intended as a broad summary only and of only some of the aspects of the invention. It was not intended to define the limits or requirements of the invention. Other and more particular aspects of the invention will be appreciated by reference to the detailed description of the preferred embodiment and to the claims.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The preferred embodiments will be described by reference to the drawings thereof in which:  
       FIG. 1  is a diagram of the first embodiment of the method of the invention;  
       FIG. 1A  is an enlarged cross-sectional view of a strand made according to the invention;  
       FIG. 2  is a diagram of the alternative embodiment of the method of the invention;  
       FIG. 2A  is an enlarged cross-sectional view of a strand made according to the alternative embodiment of the invention; and,  
       FIG. 3  is illustrates the method of applying a chromate treatment to rolls of fabricated product according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Referring to  FIGS. 1 and 1 A, in a first embodiment of the present invention, stucco lath  10  consists of welded wire mesh formed from strands  12  welded at their intersections  13 . Prior to the fabrication steps  16  of forming the mesh and welding, the strands  12  have been galvanized with a galvanized zinc or zinc alloy coating  14  of less than 0.10 oz./sq. ft. Preferably the galvanized coating is less than 0.05 oz./sq. ft. In the preferred embodiment, the zinc is applied by electro-galvanization.  
      After the lath has been fabricated (at  16 ), a chromate treatment  18  coating  16  is applied to provide a chromate coating  20 .  
      Although lath  10  has been exemplified as a welded wire mesh stucco lath, the invention applies equally to woven wire mesh lath, woven wire plaster base, or welded stucco corner reinforcement, wherein the fabrication may involve formation of the structure and bending, rolling, twisting or weaving steps.  
      Testing on products produced in accordance with the invention has shown that corrosion resistance is increased by 6 to 10 times in relation to products with a Class  1  coating. These tests were conducted in salt spray chambers in accordance with ASTM B117-03 ‘Standard Practice for Operating Salt Spray (Fog) Apparatus’. The onset of red rust failure is at damaged areas. In the case of the welded products, this is at the weld points where the zinc has been burned off. In the case of woven products, rust failure occurs at the twisted areas which have been in contact with the twister gears.  
      By applying the chromate conversion coating on the finished product after fabrication as described herein, a zinc-chromate layer is applied to areas that have been damaged during welding or mechanical twisting. It has been found that despite the presence of localized ungalvanized areas (due to trauma during fabrication), a post-fabrication chromate treatment seems to somehow repair the damaged areas, resulting in a significant increase in corrosion resistance even for such areas. Further to this benefit is the opportunity for cost savings through the use of less zinc to obtain equal or better corrosion resistance in comparison to higher zinc coating weights. This offers significant economic benefits.  
       FIGS. 2 and 2 A illustrate the steps according to an alternative embodiment. According to this embodiment, welded wire stucco lath  22  is fabricated from strands  24  that were first hot galvanized ( 26 ) to produce a hot galvanized layer  28 , then treated ( 30 ) to provide an electro-galvanized coating  32 . The lath is then fabricated ( 34 ) by forming the mesh into perpendicular strands  36  and  38 , and welding them at their intersections  13  as illustrated in  FIG. 1 . Other fabrication steps such as rolling to deform the nominally round shape of some of the strands, and bending may also be involved. Once the lath has been fabricated, it is subjected to a chromate conversion process  40  to produce a chromate layer  42 . This embodiment has the advantage of utilizing the hot galvanization process which can rapidly apply the base zinc with good adhesion. A very thin electro-galvanized coating is then applied. The benefits of this embodiment are that both the hot galvanizing and the electro-galvanizing can be accomplished in line at the wire galvanizing plant at high speed. This utilizes the positive attributes of each type of galvanizing. The hot galvanization can be applied at high speeds, and since only a very thin electro-galvanized layer is called for in the invention, it can also be achieved at that same high speed. Since the outer layer is pure zinc as a result of the electro-galvanization, the chromate conversion is of good quality.  
      According to this alternative embodiment, the zinc or zinc alloy coating applied by hot galvanization is between 0.08 and 0.20 oz/sq.ft. and the electro-galvanized layer is less than 0.10 oz/sq.ft. or less. Preferably, the hot galvanization is less than 0.10 oz./sq.ft. and the electro-galvanized layer isbetween 0.02 and 0.1 oz./sq.ft. and preferably about 0.05 oz./sq.ft.  
      In each of the above embodiments, the wires are generally round in cross section. In other embodiments, some or all of the wires could be shaped such as flattened, oval, square, rectangular, or fluted. Further, these shaped areas could be parallel with the longitudinal axis of the wire, or they may be twisted to form spirals. Shaped wire adds rigidity or stiffness to the products, and reduces overall weight These wires would also be formed from pre galvanized round wires, either hot galvanized, electro-galvanized, or a combination of hot and electro galvanized type (for the first alternative embodiment). Shaped wires within the various stucco products would have an even greater benefit with this new invention since the wires have to undergo primary deformation to create the shape prior to final fabrication into the finished product. During this primary deformation, damage to the zinc surface may occur. As well, thinning of the zinc coating occurs since the surface area of the wire is increased, whilst the available zinc quantity remains the same. Therefore, this new invention is of even greater benefit to shaped wires as compared to round wires.  
      Although the preferred embodiments have been described in relation to welded wire mesh, the invention is also applicable to reverse twist woven wire mesh.  
      The production of the various stucco laths is accomplished on high speed machinery. The products are packaged either in rolls, in sheets or in bundles. In each of the embodiments, the chromating treatment can be applied in line as the products are being produced, or in a secondary line after the product is in the packaged form. In the former case, a web of welded lath or woven lath passes through the chromating process before final packaging. With corner reinforcement, single lengths of the product pass through the chromating process. In the secondary line case, rolls  44  of welded or woven lath pass through the chromating process  46 - 52  in batches after fabrication. In the case of sheet products, groups of sheets could similarly be processed. Corner reinforcements would pass through the chromate process in groups of individual corners stacked together. Since the corners are normally marketed in groups of ten, it would be desirable to process groups of ten through the chromating process.  
      In either the inline or secondary line approach, the chromating process consists of a series of steps. The first step  46  is an aqueous dip for removal of lubricants or corrosion inhibitors that have been applied either during the galvanizing process or during fabrication of the product. This dip bath is a heated alkaline solution, preferably 160 to 180 deg. F. The time in this bath can range from 30 to 60 seconds. This is followed by a cold water rinse  46   a.    
      The second step  48  is removal of zinc oxides. This dip bath is a nitric acid solution, with acid concentrations preferably in the 0.25% to 0.5% by volume and at temperatures in the 70 deg.F to 80 deg.F. range. The time in this bath is 1 to 5 seconds, to limit loss of zinc. This step is also followed by a cold water rinse ( 48   a ).  
      The third step is the actual chromating step  50  in which the roll is submerged in a bath of an aqueous solution of either a hexavalent sodium chromate or a trivalent sodium chromate. The temperature is from 70 deg.F. to 110 deg.F., but the preferred temperature would be 90 deg.F. The preferred concentration is 1% by volume. The time in this bath can range from 5 to 60 seconds. The preferred time is 25 to 30 seconds. This is followed by a cold water rinse  50   a.    
      The preceding step is followed by a hot water dip  52 . This bath aids in curing the coating and provides heat to aid in flash drying the product. For drying, temperatures above 200 deg.F. are advantageous. However, above 150 deg.F., the iridescent yellow coating will start to bleach and fade. Therefore, the preferred temperature of the hot water rinse is 150 deg.F. and the time in this bath can range from 1 to 2 minutes.  
      The cold water rinses serve to minimize the contamination of subsequent solutions. They also reduce the quantity of contaminated rinse solutions, which would have to be treated in a waste water treatment facility prior to discharge.  
      Another feature of the chromating treatment is the relative movement of the solutions and the product itself. It was found that when there was very little relative motion, the chromate coating was very uneven. Satisfactory coatings were obtained at the outer edges of the rolls or bundles, but there was no coating within the package. Conversely, when there was high relative motion or agitation, virtually no chromate coating was achieved since it was washed off as it was being formed.  
      Agitating the solution does not produce satisfactory results, since solution velocities will be higher around the outside of the roll or bundle and virtually zero through the product itself. This is a result of the density of wire creating a high resistance to liquid flow through the product. Therefore, again the chromate is not consistent throughout the product package.  
      Therefore, in the preferred embodiment, the product is moved within each of the baths. The preferred rate of motion is 3 inches per second to achieve optimum results. The rolls or bundles are mounted on hooks or racks, which are transported from tank to tank by a mechanized crane or conveyor  56  before being collected on a pallet  54 . At each tank, the crane or conveyor stops for the appropriate dip time and oscillates the product either up and down or from side to side at the desired velocity.  
      It will be appreciated by those skilled in the art that the preferred and alternative embodiments have been described in some detail but that certain modifications may be practiced without departing from the principles of the invention.