JPH0242906B2 - - Google Patents

Abstract

This invention relates to a pretreatment process for galvanized metal which, unlike existing processes, was specifically developed for galvanized metal. Because of the natural corrosion protection properties of the zinc cladding of galvanized metal, no additional passivation is necessary as is required with existing processes. Also, the invention relates to the galvanized metal which has been pretreated by the process of this invention prior to application of an organic powder coating. Galvanized metal treated by this pretreatment process prior to coating leads to a uniform coating which is impervious to acid, base and salt solutions, and which will not scratch or flake off on impact.

Description

[Detailed description of the invention]

As background to the present invention, a general description of the method of manufacturing powder coating metals is provided. This makes it possible to compare the conventional metal manufacturing method with the method of the present invention. Those skilled in the art will recognize that a number of steps are required to achieve ultimate corrosion protection of steel by powder coating. First, the surface must be thoroughly cleaned of all dirt, oil, oxidation products, and other foreign matter. Next, the surface must be provided with junction points to which the powder can adhere. These junctions are typically made by depositing some type of crystal onto the surface during the manufacturing process. Thirdly, the coating must be of a functional type. It is different from decorative coatings and has a special composition that can impart corrosion resistance to steel when coated to a thickness typically less than 0.010 inches (0.25 mm). Field of Powder Coatings Experts recommend that organoleptic coatings be applied to mild steel or "black" steel. For mild steel, exotic surface treatment methods have been developed. These methods first clean the steel and
This is then characterized by a solution that deposits a very thin layer of a crystalline material such as zinc phosphate. The purpose of this ultra-thin layer is to passivate the surface against corrosion and to form a junction for the functional powder. Galvanized steel is not commonly used as a substrate, despite the excellent corrosion resistance afforded by the zinc coating. This is because experience has shown that functionalized powders have poorer adhesion to zinc coatings than properly prepared mild steel. This erroneous conclusion was reached because previous studies were carried out on galvanized steel, surface treatment methods were developed for mild steel, and then the adhesion of functionalized powder coatings was poor compared to galvanized steel. This is because it was considered unfavorable as a base material. Research leading to the present invention has shown that it is possible to prepare zinc surfaces such that the adhesion of functionalized powders to zinc coatings is comparable to the adhesion to suitably prepared mild steel. In other words, it is now possible to use galvanized steel as the base material, benefit from the excellent corrosion resistance of the zinc coating material, and obtain the desired excellent adhesion between the functional powder coating and the base material. Can be done. The main advantage of using galvanized steel over mild steel is that physical failure of the powder coating may occur. This often occurs, for example, during the transportation, assembly, and installation of industrial equipment. In this case, corrosion resistance is determined only by the base material;
Zinc coatings on galvanized steel provide much greater corrosion resistance than can be obtained using passivating rinse treatments such as those used on mild steel. As further background to the present invention, the following describes an eight-step or eight-step process currently used as a pretreatment prior to organic powder coating of plated metals. Thereafter, the improved four-step method of the present invention will be explained. That is, the 8-step or 8-step method is as follows. First Step The first step is the step of removing dirt such as grease and dirt present on the surface of the zinc base material, which is plated metal. Such grease and/or dirt is removed using an alkaline cleaning solution whose pH is adjusted so as not to soak the zinc. 2nd step Water rinsing is performed to remove the alkaline cleaning solution. Due to the alkaline cleaning solution carried over from the first step, this rinse solution is slightly alkaline. Third step Following the second step, a second water rinse is performed to remove any weakly alkaline liquid remaining on the surface. In the subsequent fourth step, a delicate acid balance is required for the zinc phosphate solution, so it is important to thoroughly remove all alkaline residues. Any alkali left on the substrate will affect the acid balance of the zinc phosphate solution. Fourth Step In this step, substrate passivation is achieved by spraying a zinc phosphate solution onto the substrate, and is the essence of the eight-step method. The reaction between this acidic solution and the zinc base material forms and deposits water-insoluble zinc phosphate crystals on the surface. It is important that this aqueous zinc phosphate solution is kept at a pH of about 3, otherwise a powdery precipitate will deposit on the substrate. This precipitate is undesirable because it significantly reduces the adhesion of the coating. The purpose of the zinc phosphate crystals is to passivate the substrate and thus form irregular molecular junctions to which the powder coating mechanically adheres. Fifth Step Step 5 is a water rinse necessary to remove excess zinc phosphate as well as any water-soluble salts (chlorides, sulfates or nitrates) that may be present on the substrate surface. If these water-soluble salts are not removed from the surface, the adhesion of the coating will be reduced. Sixth Step The sixth step is acidification using a chromium compound such as chromic acid. The purpose of this rinsing is to remove any remaining salts with low solubility that were not removed by the water rinsing in the fifth step. The chromium compound is further used to deposit a barrier coating to add corrosion resistance to the substrate. Chromium also improves metal passivation by filling the pores present in the zinc phosphate crystalline film and increases molecular junctions for powder coating. Seventh Step The seventh step is water rinsing for the purpose of removing foreign salts or minerals. Eighth Step The final step is to sufficiently dry the plated metal by applying heat. Furthermore, prior art plating metal pretreatment methods include so-called six-stage or six-step methods. The method consists of a third rinsing step, each consisting of two water rinsing steps.
This method is the same as the 8-step method except that the step and the seventh step are omitted. The present invention features a pretreatment method, particularly a four-step pretreatment method performed before coating the galuanized metal with an organic powder. This pretreatment method was developed specifically for galvanized metals that do not require passivation to ensure corrosion protection of the steel. It is an object of the present invention to provide a pretreatment method which, when coated with a functionalized organic powder, makes it possible to obtain adhesion forces that are superior to those of coatings obtained with conventional pretreatment methods. be. Another object of the present invention is to directly utilize the zinc metal layer on the steel material as a sacrificial anti-corrosion layer for the metal in a powder coating method for plating metal, and to replace the less effective zinc phosphate layer. It is an object of the present invention to provide a method for depositing a corrosion-resistant layer on a surface without using chemicals such as rinse solutions. Another object of the present invention is to provide a four-step process for pre-treating plated metal prior to powder coating, which provides savings in material, time and labor compared to conventional pre-treatment methods. It is to provide law. Another object of the invention is to provide a method for pretreating plated metals prior to powder coating without using chromium compounds and thus without hazardous waste problems. . Still another object of the present invention is that minimal zinc phosphate is deposited on the metal surface (other methods would deposit significant amounts of zinc phosphate) and the adhesion of the powder coating is improved. It is an object of the present invention to provide a method for pre-treatment of ferrous metal. In the present invention, the adhesion of the organic powder coating layer to the plated metal is superior to that achieved by conventional pretreatment methods when only a minimal amount of zinc phosphate is present on the metal. It was discovered that That is, by applying the organic powder coating directly to the metal, the surface treatment effect of zinc phosphate is not required. The four-step method of the present invention is as follows. First Step The first step of the four-step pretreatment method of the present invention is stain removal and phosphoric acid etching. In this process,
As grease and dirt are removed, the plated metal is etched with phosphoric acid to form small amounts of zinc phosphate crystals randomly distributed over the surface of the substrate. Second Step The second step is a water rinse performed to remove excess cleaning from the first step, the etching solution, and any salts (chlorides, nitrates, sulfates, etc.) that may be present on the substrate. be. Because of the acid brought in from the first step, this rinse solution is slightly acidic. 3rd step The 3rd step is the weak acid remaining after the 2nd step,
A water rinse is used to completely remove salts or mineral residues. This step can shorten the time and simplify the equipment by heating the water. Fourth Step The final step is to completely dry the plated metal by applying heat. Below, a detailed explanation of the implementation method for each of the above steps will be given. The first step in preparing plated metal for coating is cleaning and etching to obtain a clean, oxide-free surface. The treatment liquid used for this purpose is a typical biodegradable solution (liquid acid solution) containing phosphoric acid, a solvent, and a surfactant. Such solutions are commercially available in various formulations, such as from Orakite Products, Inc. (Berkeley Heights, New Jersey). The formulation is intended to remove soft grease, factory debris, weld flux, oxides, mill scale, etc. from the plated metal, and is usually preceded by a fine etching of the metal prior to organic powder coating. Since this cleaning etching agent contains phosphoric acid, a small amount of zinc phosphate crystals produced by the reaction of phosphoric acid and zinc are randomly distributed on the surface of the metal substrate. Specifically, a solution of 3 to 20% by volume of this acid compound was prepared.
Use in the temperature range of 110~150〓. Precise temperature and concentration adjustments must be made to minimize zinc erosion and ensure thorough cleaning. The treatment solution is usually placed in a vat large enough to immerse the entire plated metal. The minimum immersion time for this treatment solution is 1.5 to 3.5 minutes; however, if the plated metal is heavily soiled with dirt or oil, or if it has been treated with a special rinsing solution by the metalworker, A longer period of time is required. In some cases, instead of dipping, the plated metal may be sprinkled with a cleaning etching solution. The second step includes the addition of water necessary to remove excess entrainer from the first step and any other salts that may be present on the substrate surface, such as chlorides, nitrates, or sulfates. It's gi. Specifically, tap water is used, but if the water contains a lot of minerals, it is more appropriate to use deionized water. The water used in this step is slightly contaminated by the acidic detergent used in the first step, which remains attached to the plating metal surface. The third step is also a water rinsing step. As mentioned above, this water rinse is required to completely remove any diluted detergent and salts and minerals remaining after the second step. In the fourth step, the treated plated metal must be dried before applying the organic powder coating. This drying is usually accomplished by inserting the pretreated and rinsed standing metal into a 130°-400° oven for about 2 to 10 minutes until completely dry. As a preferable condition, specifically, the pretreated metal is heated to 250㎓
Just insert it into the oven for about 5 minutes. The powder coating of the plated metal must be applied within a short time after the above four-step pretreatment method is completed, or at least before there is much zinc oxide left as a result of exposure to other oxidizing agent atmospheres. This is particularly important since, unlike conventional pretreatment methods, no passivation coating is provided in the present invention.
Ideally, the powder coating should be applied immediately after the plated metal has dried. However, in reality, the time lag between drying (the fourth step) and powder coating of the gilded metal is in the range of about 5 minutes to 6 hours, due to equipment limitations and layout, labor conditions, transportation needs, etc.
The average time is about 10 minutes. Although 6 hours was mentioned as an upper limit, this is only an approximation. It was decided that if there is a gap of 2 or 3 days between drying (the fourth step of the invention) and powder coating, the coating will be more difficult to coat than with a time difference of 5 minutes to 6 hours. This is due to poor adhesive strength. The effect of time difference on coating adhesion varies depending on the temperature and humidity to which the plated metal is exposed (ie, whether stored inside or outside a building). The coating materials used include typical organic powders, such as epoxy, polyester, acrylic, or a mixture of polyester and epoxy. Among these, epoxy-coated powder is extremely suitably used in the present invention, and specific examples of products obtained from various companies are listed below.
pulualure Scotchkote, Corvel or Vedoc
etc. These typical epoxy coating powders are homogeneous, melt-blended, 100% solids for metal applications. Organic powder coatings are usually carried out by electrostatic spraying, but it is also possible to coat the treated, rinsed and dried glazed metal of the invention by immersing it in a fluidized bed of the powder. When coating with an electrostatic spray, the powder is fired from a gun that charges it positively. Since the plating metal is negatively charged, the powder is attracted and adheres to the metal. The thickness of the coating is not particularly limited, but the ideal thickness is usually 0.004 inch (0.1 mm) ± 0.001 inch (0.0254 mm). This is an approximation of the thickness as applied to the pretreated, rinsed and dried glazed metal before being put into the oven to harden. Suitable curing conditions include heat treating the powder coated metal in a furnace at temperatures between 300° and 550° for about 1 to 20 minutes. Temperature and curing time are given by the manufacturer of the organic powder used. Although the present invention has been described above with reference to preferred embodiments, various modifications and changes can be made within the spirit and scope of the present invention as defined in the claims. A 5% salt water spray test and PH4 and PH11 immersion tests were conducted on the organic resin coating produced by the method of the present invention and the organic resin coating produced by the conventional method. The results are shown in Table 1. According to the test results, the coating according to the present invention exhibited durability comparable to that obtained using conventional methods. Therefore, the present invention can provide a durable organic film more easily and at a lower cost than conventional methods.

[Table] a: Slight blistering occurred in the scribe area. <1/
16" diameter. A: In the case of the present invention B: In the case of the prior art 1 Pre-treatment of the present invention; phosphoric acid cleaning, water rinsing, water rinsing in this order. 2 Pre-treatment of the prior art; alkaline cleaning, water rinsing, conditioner in that order. 3. All test samples were mill galvanized steel (mill galvanized steel).
galvanized steel substrate) was used. 4 All samples were thermoset epoxy powder coated. 5 All samples were scribed in an X-shape before testing. 6. Corrosion resistance was judged by peeling and/or blistering of the epoxy coating near the X scribe as the test progressed.

Claims (1)

[Claims] 1. Contacting the galvanized metal with a dilute acid solution containing phosphoric acid at a concentration of 3 to 20% by volume so that an excessive amount of crystal binding sites is not deposited on the galvanized metal. The galvanized metal is cleaned by acid corrosion without damaging it, washed with water, washed again with water, dried, and coated with an organic resin powder within 6 hours after drying, and then the temperature is raised to harden the organic resin powder. A method for coating galvanized metal, which includes the step of applying a uniform coating by coating the galvanized metal. 2. The method according to claim 1, wherein the dilute acid solution further contains a surfactant. 3. The method of claim 1, wherein the organic resin powder coating is an epoxy powder coating. 4 Claim 1 in which the plating metal is zinc
The method described in section.