EP0269006B1

EP0269006B1 - Colored zinc coating

Info

Publication number: EP0269006B1
Application number: EP87117098A
Authority: EP
Inventors: Masatoshi Tomita; Susumu Yamamoto; Chikara Tominaga
Original assignee: Nikko Aen KK; Nippon Mining Co Ltd
Current assignee: Nikko Aen KK; Eneos Corp
Priority date: 1986-11-21
Filing date: 1987-11-19
Publication date: 1992-08-26
Anticipated expiration: 2007-11-19
Also published as: DE3781375T2; DE3781375D1; EP0269006A3; EP0269006A2

Description

Field of the invention

This invention relates to a colored zinc coating technique applied onto the surface of an iron or steel material, and particularly to a colored zinc coating method with the use of Mn-Zn, Mn-Cu-Zn, Mn-Ti-Zn system alloys by which the development of new colors not obtained by conventional techniques and clearer color developments compared to conventional ones are permitted. According to this invention, the developments of gold, olive gray and iridescent colors which could not have yet obtained are permitted and simultaneously blue color, purple color etc. may be more clearly developed. Thus, this invention provides colored zinc coated materials which are applicable to wider variety of fields and have coloring more suitable to the environment where they are placed.

Background of the invention

Hot-dip galvanized iron and steel materials, coated by dipping in molten zinc, are used for corrosion protection purposes in a wide range of application, forming parts and facilities in the fields of building and construction, civil engineering, agriculture, fisheries, chemical plants, electric power supply and communications, and so forth.
For pylons and other towers, lighting poles, guardrails, temporary stands and frames for various operations and displays, shells and planks, and the like, there has been growing demand in recent years for colored hot-dip galvanized materials that present attractive appearances matching the environments involved, in preference to the classic hot-dip galvanized steels with metallic lusters. With the spread of the aethetic sense the colored hot-dip galvanized articles show promise, with extensive potential demand in architecture, civil engineering, industrial plants, electric power supply and communications, transportations, agriculture, marine products and other industries.
Coloration of hot-dip galvanized steels has usually been by the application of paints. The method has the disadvantage of the paint film eventually coming off the coated surface. This results from the activity of Zn in the coating of the hot-dip galvanized steel that causes gradual alkali decomposition of the fatty acid constituting the oily matter in the paint, leading to the formation of zinc soap that hampers the adhesion of the paint film to the underlying surface.
In an effort to eliminate the disadvantage, a complex procedure has had to be followed. A steel article is first galvanized by dipping in a molten zinc bath. The coated steel is exposed to the air for one to three weeks so that corrosion products such as Zn(OH)₂, ZnO, ZnCO₃, ZnCl₂ and the like deposit on the coated steel surface. The surface is then cleaned and colored.
Aside from the coating method described above, another approach that depends on the color-developing action of the oxide film in the hot-dip galvanizing is known in the art. Particularly, US-A-3 630 792 discloses a process for the production of zinc coatings that uses a hot-dip bath of a zinc alloy containing at least one element selected from manganese, titanium and vanadium, and optionally also one or more elements selected from columbium, zirconium, thorium, mischmetal, cadmium, arsenic, copper, lead and chromium, in an amount sufficient to form on the coating, upon reaction of the surface of the coating with oxygen, a colored oxide film. It is indicated that the preferred range for the alloy compositions is from 0.1 to 0.45 % both for manganese and for titanium. In experiments actually described the manganese content of zinc-manganese alloys providing for yellow-, red- and blue-colored coatings is from 0.02 to 0.15 %, and the titanium content of zinc-titanium alloys providing for yellow-, red- or blue-colored coatings is from 0.017 to 0.15 %. When using a zinc-manganese alloy containing 0.1 % Mn and a bath temperature of 475°C the final coating color was yellow or blue dependent on the gauge of the coated sheet material. At a bath temperature of 500°C an alloy of zinc and 0.15 % Mn resulted in a red color of the dipped panel. The coated article may be post-heated, for example by induction heating, whereupon the article is allowed to air cool or is quenched with cold air blasts. Also disclosed is the use of a zinc-copper allow containing 2.5 % Cu, which alloy, at a bath temperature of 650° to 700°C, results in a light gold color. Generally, a copper-containing zinc alloy is said to require bath temperatures of at least 625°C. It is indicated that the tests were made with commercial Special High Grade zinc and that the other commercial grades of zinc, e.g. Prime Western which contains up to 1.5 % Pb, can be used provided that the aluminum content is limited to below 0.002 %.
FR-A-1 115 121 discloses a method for coating ferrous metal objects in which a layer of aluminium, zinc or another non-ferrous metal is applied, e.g. by a gun. Upon this metallization a layer of a waterglass-type plastic material, e.g. potassium silicate, or of glycerophthalate is applied. Then the coated object is heat-treated at a temperature of about 350°C, the final temperature being dependent on the melting point of the applied non-ferrous metal.
There is still a steady demand in the art for many improvements such as

(a) the development of new colorings which have not yet been obtained in past;
(b) the obtainance of the color developments which are more beautiful and clearer than ones previously obtained;
(c) the enhanced stability of color development;
(d) that the inherent corrosion resistance of galvanized zinc coating is not sacrified;
(e) less change with the lapse of time; and
(f) to provide easy and stable operation.

Object of the invention

The object of this invention is to establish colored zinc coating technique by which the above mentioned improvements may be attained using Mn system zinc alloys.

Summary of the invention

In conformity with one aspect there of the invention provides for a method of forming an olive gray colored zinc coating on an iron or steel surface, comprising coating a base metal of iron or steel by hot dipping in a zinc alloy of a composition consisting of 0.2 to 0.8 wt % Mn - bal. Zn and unavoidable impurities at a bath temperature of 490 to 530°C, heating the coated surface in an atmosphere at 500 to 520°C, for 50 to 150 seconds, and thereafter either cooling it with warm water or cooling it first in air forcibly and then with warm water.
According to another aspect the invention provides for a method of forming an olive gray colored zinc coating on an iron or steel surface, comprising coating a base metal of iron or steel by hot dipping in a zinc alloy of a composition consisting of 0.2 to 0.8 wt % Mn and 0.05 to 1.0 wt % Cu - bal. Zn and unavoidable impurities at a bath temperature of 490 to 530°C, heating the coated surface in an atmosphere at 500 to 520°C for 50 to 150 seconds, and thereafter either cooling it with warm water or cooling it first in air forcibly and then with warm water.
The present invention further provides for a method of forming a zinc coating exhibiting a color selected from the group consisting of gold, purple and blue, on an iron or steel surface comprising using a galvanizing zinc alloy containing 0.2 to 0.8 wt % Mn and 0.01 to 0.1 wt % Ti, the balance of the alloy being zinc and unavoidable impurities with impurity Pb being limited to 0.005 wt % or less, wherein said iron or steel surface is coated at a bath temperature of 480 to 550°C and the coated surface obtained is cooled or is cooled after heating to a temperature of 450 to 550°C, the color of the coating being selected by controlling the extent of the oxidation of the coating.
The subject invention also provides for a method of forming an iridescence-colored zinc Coating exhibiting a blend of golden, purple, blue, green and other colors by hot-dip galvanizing, comprising hot-dipping a base metal of iron or steel in a coating bath of a zinc alloy consisting of 0.1 to 0.8 wt % Mn and 0.05 to 1.0 wt % Cu and the balance Zn and inevitable impurities at a bath temperature of 450 to 550°C, and then cooling the galvanized metal with warm water.
Another aspect of the invention is a method of forming an iridescence-colored zinc coating exhibiting a blend of golden, purple, blue, green and other colors by hot-dip galvanizing, comprising hot-dipping a base metal of iron or steel in a coating bath of a zinc alloy consisting of 0.1 to 0.8 wt % Mn and the balance Zn and inevitable impurities at a bath temperature of 450 to 550°C, wherein the metallic zinc bullion to be used in the alloy is either distilled zinc first grade (at least 98.5 % pure), purest zinc bullion (at least 99.995 % pure), or special zinc bullion (at least 99.99 % pure) conforming to JIS H 2107, the total impurity content being below 1.5 wt %, and then cooling the galvanized metal with warm water.
In conformity with a further aspect thereof the invention provides for

a zinc alloy for hot dipping to form on a base surface an olive gray colored coating of a composition comprising 0.2 to 0.8 wt % Mn and 0.05 to 1.0 wt % Cu, the balance Zn and inevitable impurities;
a zinc alloy for hot dipping to form an iridescence-colored coating exhibiting a blend of golden, purple, blue, green and other colors, consisting of 0.1 to 0.8wt % Mn and 0.05 to 1.0 wt % Cu, the balance Zn and inevitable impurities; and
a zinc alloy for colored hot-dip galvanizing capable of developing golden, purple and blue colors selectively as desired, containing 0.2 to 0.8 wt % Mn and 0.01 to 0.1 wt % Ti, the balance of the alloy being zinc and unavoidable impurities with impurity Pb being limited to 0.005 % or less.

In the colored hot-dip galvanizing, the composition of the plating bath and the conditions of producing an oxidized film delicately combine to present coloring effects by light interference. By ingeniously controlling these factors, this invention succeeds in developing an olive-gray color strongly demanded using Mn-Zn and Mn-Cu-Zn alloys. In Mn-Zn and Mn-Cu-Zn alloys, an iridescent color the development of which had never thought was successfully obtained. By using a Mn-Ti-Zn alloy with impurity Pb content controlled, selective color development of purple and blue colors markedly clearer than ones obtained heretofore was successfully attained. Surprisingly, even golden color which had never thought possible could be successfully developed.
This invention also found that colored zing coating may be applied by spraying method.
The change of the colored zinc coating with the lapse of time may be suppressed by painting thereon.

Detailed explanation of the invention

Zinc alloy hot dipping is carried out by melting a zinc alloy in a coating bath and immersing a member to be coated thereinto.

1) The development of olive gray color with Mn-Zn alloy

Using a zinc alloy for hot dipping to form on a base surface an olive gray colored coating of a composition composed of 0.2 - 0.8 wt % Mn - bal. Zn, it is possible to form a colored coating with an olive gray hue on a base metal of iron or steel by plating the base metal using a bath of a the above zinc alloy for hot dipping at a bath temperature of 490 - 530°C, heating the plated work in an atmosphere at 500 - 520°C for 50 - 150 seconds, and thereafter either cooling it with warm water or first forcibly air-cooling and then cooling it with warm water.
The plating is carried out using a bath of molten zinc alloy made by adding 0.2 - 0.8 wt % Mn to a purest metallic zinc bullion (at least 99.995 % pure) or special zinc bullion (at least 99.99 % pure) conforming to JIS H2107 and used primarily as molten zinc alloy. The metallic zinc bullion for use in making the molten zinc alloy is desired to have a Pb content of 0.005 wt % or less.
In order to produce the olive gray colored coating from the hot-dip zinc alloy bath of the above composition, an iron or steel material as the base metal is immersed in the plating bath at 490 - 530°C for at least one minute. The base metal is pulled out of the bath and heated in an atmosphere at 500 - 520°C for 50 - 150 seconds, and then is either cooled with warm water or first air-cooled forcibly in air and then is cooled with warm water.
Thus, in producing a colored coating with an olive gray hue by the use of the molten zinc alloy bath of a composition comprising 0.2 - 0.8 wt % Mn - bal. Zn, it is important to heat the plated metal in an atmosphere at 500 - 520°C.
If the composition of the molten zinc alloy bath or the plating conditions deviate from the range specified above, the resulting colored coating can become uneven in hue or lose its hue, or the colored oxide film formed by the plating tends to come off, rendering it impossible to obtain the desired olive gray colored coating.
As stated hereinbefore, a colored coating with a uniform orange gray hue can be formed on an iron or steel material by plating it under the specific conditions using the molten zinc alloy bath of the specific composition, heating the plated metal, and then either cooling it with warm water or first air-cooling forcibly and then cooling it with warm water. It thus provides a corrosion-resistant material for the components and facilities for uses where they are required to be olive gray in color from the aethetic viewpoint. Since the color-coated metal thus obtained is highly corrosion-resistant, the iron and steel products with such colored coatings according to the invention can be effectively used in a wide range of applications.

2) The development of olive gray color with Mn-Cu-Zn alloy

Using a zinc alloy for hot dipping to form on a base surface an olive gray colored coating of a composition comprising 0.2 - 0.8 wt % Mn - 0.05 - 1.0 wt % Cu - bal. Zn, it is possible to form a colored coating with an olive gray hue on a base metal of iron or steel by plating the base metal using a bath of a the above zinc alloy for hot dipping at a bath temperature of 490 - 530°C, heating the plated work in an atmosphere at 500 - 520°C for 50 - 150 seconds, and thereafter either cooling it with warm water or first forcibly air-cooling and then cooling it with warm water.
The zinc to be used in making the molten zinc alloy is according to 1).
In order to produce the olive gray colored coating on an iron or steel material, the base metal is immersed in the plating bath of the molten zinc alloy of the above zinc containing 0.2 - 0.8 wt % Mn and 0.05 - 1.0 wt % Cu at 490 -530°C for at least one minute. The metal is pulled out of the bath and heated in an atmosphere at 500 - 520°C for 50 -150 seconds, and then is either cooled with warm water or first air-cooled forcibly in air and then is cooled with warm water. In this way a colored coating of oxide film olive gray in hue is formed on the iron or steel surface.
Thus, in producing a colored coating with an olive gray hue it is important to use the molten zinc alloy bath of the specific composition, and carry out the plating, heating, and other after treatments under the specific conditions.
If the composition and the plating conditions deviate from the ranges specified above, the resulting colored coating can mix with some other hue or lose its hue, or the colored oxide film tends to come off, rendering it impossible to obtain the desired olive gray hue.
The colored zinc coated steel obtained is excellent in its corrosion resistance.

3) The development of iridescent color with Mn-Zn or Mn-Cu-Zn alloy

Iridescent, multicolored coating which exhibits a blend of golden, purple, blue, and green colors was found in an epochal way of color development that is not mere coloration of the ordinary metallic-colored hot-dip galvanized articles but a breakthrough in the traditional concept of hues with ordinarily colored galvanized products. This is accomplised, under the use of a zinc alloy comprising either 0.1 - 0.8 wt % Mn alone or 0.1 - 0.8 wt % Mn and 0.05 - 1.0 wt % Cu and the balance Zn and inevitable impurities, by hot-dipping a base metal of iron or steel into a bath at a temperature of 450 - 550°C, and then cooling the galvanized metal with warm water.
The zinc alloy is made by adding a specific alloying additive or additives to metallic zinc bullion. The metallic zinc bullion to be used in making the molten zinc alloy under the invention is typically one of the grades conforming to JIS H2107, for example, distilled zinc 1st grade (at least 98.5 % pure), purest zinc (at least 99.995 % pure), and special zinc grades (at least 99.99 % pure). The impurities inevitably contained in these zinc materials are, for example in the distilled zinc 1st grade, all up to 1.2 wt % Pb, 0.1 wt % Cd, and 0.020 wt % Fe. For the present invention a metallic zinc with a total impurity content below 1.5 wt % is employed when using the aforementioned Mn-Zn alloy and also is desirable when using the above Mn-Cu-Zn alloy.
According to this invention, a molten zinc alloy bath of the above metallic zinc containing

(1) 0.1 - 0.8 wt %, preferably 0.2 - 0.8 wt %, Mn or
(2) 0.1 - 0.8 wt %, preferably 0.2 - 0.8 wt %, Mn and 0.05 - 1.0 wt % Cu

Hot dipping is effected by the use if the above molten zinc alloy bath at a bath temperature of 450 - 550°C. The immersion time is about 1 to 3 minutes. After the immersion the coated work is cooled with warm water. The cooling is done by dipping the work in warm water at 40 - 60°C for 3 -30 seconds. If the bath composition and treating conditions are outside the specified ranges, the desired iridescent color development will not be attained.
Experiments revealed that too thin sheets sometimes cannot be colored in blended iridescent hues, presumably due to high cooling rates. The workpieces to be galvanized are desired to be 1.6 mm or more in thickness.
Before being galvanized, the work is pretreated in the usual way. It is degreased, for example by the use of an alkaline bath, descaled by pickling or other treatment, and then fluxed by a quick dip in a flux solution such as ZnCl₂-KF solution or ZnCl₂-NH₄Cl solution.
The simple procedure described above yields an iridescent multicolored coating which exhibits a blend of golden, purple, blue, and green colors. The articles galvanized in this way are resistant to corrosive attacks and are capable of extensive use in the fields where both beautiful appearance and corrosion resistance are required.

4) The development of gold-purple-blue color with Mn-Ti-Zn alloy

By maintaining a relative high Mn level and low Ti level with the restriction of the impurity lead level in Mn-Ti-containing zinc alloy, namely by using a hot-dip galvanizing zinc alloy containing 0.2 - 0.8 wt % Mn and 0.01 - 0.1 wt % Ti, with impurity Pb limited to 0.005 % or less, it is possible to develop colors in the series of golden-purple-blue hues with a substantial reduction of the holding time in the heating atmosphere following the galvanizing. The galvanized surface is outstandingly smooth to the beauty of the appearance. The bath temperature may be lower than usual.
The metallic zinc bullion to be used in making the zinc alloy of the invention must be such that its impurity Pb content is limited to 0.005 wt % or less. For this reason the use of the purest zinc bullion (at least 99.995 wt % pure) defined in JIS H2107 is desirable. Special zinc bullion (at least 99.99 wt % pure) may also be used provided its Pb content is confined within the limited 0.005 wt % or below. If more than 0.005 wt % lead is present in the coating bath, the colors of the golden-purple-red series will not develop within short periods of time.
In accordance with the invention, 0.2 - 0.8 wt % Mn and 0.01- 0.1 wt % Ti are added to the metallic zinc of high purity. These ranges of additions are based on the fact that a relatively small amout of Ti and a relatively large amount of Mn in the zinc alloy have been found helpful in shortening the period of time for which the galvanized work is held in the heating atmosphere. Thus, the upper limit of Ti is fixed to be 0.1 wt %. If the Ti content is less than 0.01 wt %, there is no beneficial effect of the Ti addition and coloring in desired hues becomes impossible. A large Mn content of 0.2 wt % or above is necessary to obtain desired hues rapidly, but if the content exceeds 0.8 wt % the adjustment of hues becomes difficult and the work is not adequately wetted with the bath.
In the hot-dip galvanizing with the zinc alloy, the work to be galvanized is degreased, for example by the use of an alkaline bath, descaled by pickling or the like, and then treated with a flux to be ready for galvanizing. The flux treatment is effected, for example, by a dip for a short time in a ZnCl₂-KF solution, ZnCl₂-NH₄Cl solution, or other known flux solution.
After the pretreatment, the work is immersed in a coating bath at a specific controlled temperature for 1 to 3 minutes. The coated metal is pulled out of the bath and, through proper control of the degree of oxidation of the coating film, a golden, purple, or blue color is selectively obtained at will. As the degree of oxidation increases, golden, purple, and blue colors are brought out successively in the order of mention.
The galvanizing bath temperature is generally 480 -550°C, preferably 490 - 520°C, or lower than the usual bath temperatures. This means a substantial reducation of energy cost in the case of mass treatment.
After the coated work has been taken out of the bath, its degree of oxidation is changed through control of the cooling rate by cooling the work in a variety of ways, including natural cooling in the air, cooling with cold or warm water, forcible cooling, and slow cooling in an oven. A desirable practice consists in holding the galvanized metal in an atmosphere at 450 - 550°C for a predetermined period of time and changing the rate of subsequent cooling so as to control the degree of oxidation. If the alloy layer comes up to the surface no color will develop, and therefore it is important to thicken the oxide film in preference to the growth of the alloy layer. The holding temperature, holding time, or cooling rate is so chosen as to cause appropriate color development. Under the invention the heating time can be shortened.
Thus, within shorter periods of time than in the past, colors of the golden-purple-blue series are brought out. The repid color development combines with great smoothness of the coated surface to give a fine-looking colored hot-dip galvanized material.
This embodiment produces the following effect:

1. Because of the short heating time in the heating atmosphere, the process involving the zinc alloy of the invention is adapted for continuous hot-dip galvanizing lines.
2. The lower bath temperature and shorter heating time than heretofore permit reduction of energy cost and provide favorable conditions for quantity production.
3. The zinc alloy gives very smooth, fine-looking galvanized surfaces with bright hues in the golden-purple-blue series.

After-treatment

The colored oxide film formed on the colored, hot-dip galvanized material tends to discolor or fade with time, with changes in hue due to the progress of deterioration, depending on the environmental conditions including the sunlight, temperature, and humidity. Although the deterioration of the colored oxide film, of course, does not adversely affect the corrosion resistance of the hot-dip galvanized steel itself, the original beautiful appearance is unavoidably marred.
As a simple measure for protecting the colored oxide film on the colored hot-dip galvanized material to suppress the discolor or fade with time, surprisingly, painting has been found appropriate for realizing the object. As noted already, painting of the coated surface of ordinary (uncolored) hot-dip galvanized steel poses the problems of inadequate adhesion or separation of the paint film on short-period exposure. Partly responsible for them is the deposits on the galvanized steel surface of oxides (zinc white rust) and flux such as ammonium chloride used for the galvanizing. Presumably responsible too is the basic zinc dissolution product formed between zinc and the water that has permeated through the paint film. It is presumed that this product acts to decompose the resinous content (oily fatty acid) of an oily paint or long oil alkyd resin paint, causing the decomposition product to react with the zinc to produce zinc soap along the interface between the zinc surface and the paint film, thereby substantially reducing the adhesion of the paint.
A common belief has been that the colored oxide film layer formed on the surface of the colored hot-dip galvanized steel does not provide an adequate barrier between the zinc surface and the surrounding air. The pessimistic view that painting over the oxide film would, after all, be the same as direct paint application to the galvanized surface has been predominant. Contrary to these predictions, it has now been found that the colored oxide film has good affinity for and adhesion to paints, allowing the applied paint to permeate through the film to show high separation resistance, and is sufficiently capable of preventing water permeation to inhibit the reaction of the zinc layer with water and therefore the formation of zinc soap.
In accordance with the invention, the hot-dip galvanized materials thus colored may be coated with a paint having excellent adhesion, weather resistance, durability, and environmental barrier properties.
For the painting of ordinary hot-dip galvanized steels, pretreatment is essential and the types of paints that may be employed are limited. With colored, hot-dip galvanized steels, by contrast, there is no need of pretreatment and various paints may be used. Since the heating for oxidation that follows the galvanized step produces a film of oxide such as TiO₂ or MnO on the galvanized surface, the coating on the galvanized steel is so clean that there is no necessity of treating the surface before painting.
The paint to be used may be any type which does not unfavorably affect, but protect, the colored oxide film layer to be painted. Typically a synthetic resin paint is used. Among synthetic resin paints, those superior in protective effects are polyurethane resin, acrylic resin, epoxy resin, and chlorinated rubber paints. The paint is properly chosen in consideration of the price, environments to be encountered, ease of application, and other factors.
Where the color of the colored oxide film is to be shown as it is, a clear paint is the best choice, and where the color tone is to be modified, an aqueous paint is the easiest to handle. In any case, the paint can be applied by brushing, spraying, or dipping.
In certain situations multicoating is not impractical. For instance, where the environments are very severe or adverse, multiple painting may be taken into account. An example is the application of an aqueous paint as the base coat and a clear paint as the intermediate and top coats. Alternatively, an epoxy resin paint durable against the alkali attacks that result from zinc elution may form the undercoat and a chlorinated rubber or polyurethane paint excellently resistant to water, chemicals, and weather may form the intermediate and surface coats.
Even if the paint degrades with time, leading to chipping or flaking of the coat, the beautiful appearance of the galvanized steel will remain unaffected thanks to the colored oxide film on the steel surface. Under the invention, such chipping or flaking seldom takes place because the paint permeated through and binds solidly with the colored oxide film. The paint that had permeated the oxide film keeps off water and the like by its water-repelling action and thereby protects the film.

Spraying

For the colored hot-dip galvanizing it is prerequisite that the work to be coated be dipped in a molten zinc alloy bath. In the practice, therefore, there are sometimes met the following limitations:

(1) The process is difficult to apply to shapes too large to be dipped in the bath.
(2) The coating of assembly parts and structures is sometimes difficult.
(3) Localized coloring is cumbersome. Although masking and other techniques may be resorted to, they involve much complexities and difficulties. The techniques are difficult to cope with the trend toward more frequent situations requiring pattern drawing for decorative purposes.
(4) For repairs of installations and the like the process is difficult to practice at sites.
(5) There are tendencies that the larger the content of such an alloying element as Ti and Mn, the worse the wettability of the bath and the more the number of holidays and other coating defects. Although an increase in the content of the additive element improves the durability of the resulting coating accordingly, such addition is sometimes difficult from the standpoint of the coating technology.
(6) The process sometimes brings failure of coating and other coating defects.

The colored zinc coating by metal spraying basically involves spraying a zinc alloy, which is otherwise used for a coating bath, in the form of wire, rod, or powder, over the object. Surprisingly, the oxidation reaction of the additional element had been found to proceed more favorably than expected during the spraying process, achieving at least as satisfactory effects as the colored hot-dip galvanizing.
Thus, in the present invention, a colored zinc coating may be attained by spraying a coloring, oxidizing zinc alloy over a base surface by a metal spraying process, whereby a colored oxide film is formed on the base surface. After the spraying, the color development of the colored oxide film may be controlled by cooling and/or heating.
Metal spraying comprises heating a sprayable material to a half-molten state and spraying it over a base surface to form a coating tightly bonded to the surface. The sprayable material takes the form of a wire, rod, or powder, any of which may be employed under the invention.
The sprayable material may be any of the zinc alloys in common use for colored hot-dip galvanizing. It may, for example, be a Mn-Zn, Mn-Cu-Zn or Mn-Ti-Zn alloy with or without the further addition of Cu, Ni and/or Cr. In the case of hot dipping, a work high in Ti, Mn or the like is not readily wetted when dipped in the bath, leaving holidays on the surface. The possibility of uncoating puts limitations to the amounts of the additive ingredients. Metal spraying is free from the wettability problem, and larger proportions of the additional elements can be used. Accordingly, the range of color development is wider and the hues have longer life. An example of desirable sprayable material is a zinc alloy selected from the group of

(a) a composition consisting of 0.1 to 0.8 wt % Mn - bal. Zn and unavoidable impurities;
(2) a composition consisting of 0.1 to 0.8 wt % Mn and 0.05 to 1.0 wt % Cu - bal. Zn;
(3) a composition consisting of 0.2 to 0.8 wt % Mn and 0.01 to 0.1 wt % Ti, the balance of the alloy being zinc and unavoidable impurities with impurity Pb being limited to 0.005 % or less.

With good workability the zinc alloy can be easily made into a wire or rod or powdered by crushing or melt dropping.
The sprayer that may usually be used is of the type known as a gas flame spray gun. An arc type spray gun may be employed as well.
The sprayable material is melted by the sprayer and sprayed over the base surface to be coated. The corners and intricate portions of the work difficult to coat by hot dipping can be completely coated by aiming the spray gun to those portions. Localized coatability permits figures and other patterns to be made easily. Another major advantage of metal spraying is the ability of coating iron and steel structures or the like at the sites.
After the spraying, the degree of surface oxidation is controlled so as to develop a desired color. A variety of colores, e.g., yellow, dark red, green, golden, purple, and blue colors, can be selectively developed as desired, depending on the degree of oxidation. For the oxidation control, the cooling rate of the sprayed coat can be adjusted by the use of natural cooling in the air or forced cooling with water or air. Also, the spray coat may be heated for a variable period with a flame, infrared lamp, oven (where usable) or the like, and the subsequent cooling may be controlled. A proper combination of the sprayable material composition and surface oxidation conditions renders it possible to bring out a desired hue.
In this way a zinc sprayed coating with both corrosion resistance and colorability is produced.
The painting described above may be applied onto the sprayed coating.
The functional effects of the spraying are summerized as follows:

1. Applicable to large components that cannot be hot-dipped.
2. Capable of easily coating the portions of assembly parts and structures difficlut to hot-dip.
3. Permits localized color development and display of a desired figure or other pattern thus enhancing the decorative value of the coating.
4. Possibility of coating at the site.
5. Ability to use high-melting alloys.
6. Ease of forming a thick coat suited for providing long-term corrosion protection.
7. A high Ti content in the alloy enhances the corrosion resistance and enriches the color hue.
8. The coating film, with a rough and porous surface, is suited as a base to be painted, and painting with a clear paint or various colored dyes can improve the durability of the colored oxide film of the coating.

Other than spraying process, vapor deposition process, sputtering process, ion plating process or other surface coating process may be applied in this invention.
The Examples will be described below: The Examples 1 to 4 correspond to the items 1 to 4 described in the detailed explanation.

Example 1 (development of olive-gray color with Mn-Zn alloy)

A test piece of steel sheet, SS41, 50 mm wide, 100 mm long, and 3.2 mm thick, was degreased by immersion in an alkaline bath at 80°C for 30 minutes. It was washed with warm water, and then descaled by immersion in a 10 % hydrochloric acid bath at ordinary temperature for 30 minutes. Next, the steel sheet was washed with warm water and was fluxed by a dip in a solution containing 35 % ZnCl₂-NH₄Cl at 60°C for one minute.
The steel sheet thus pretreated was plated by the use of a plating bath of the following composition under the following conditions:
The plated steel sheet surface had a colored coating with a uniform olive gray hue.

Example 2 (development of olive gray color with Mn-Cu-Zn alloy)

The steel sheet pretreated as previously described was plated by immersion in a plating bath of the following composition at 490 - 530°C for one minute. The sheet was then pulled out of the bath and held in an oven at 500 -520°C for 50 - 150 seconds. The plated sheet taken out of the oven was either cooled with warm water or forcibly air-cooled in air and then cooled with warm water.
The plated steel sheet surface had a colored coating with a uniform olive gray hue.

Example 3 (development of iridescent color with Mn-Zn or Mn-Cu-Zn alloy)

Test pieces of steel sheets, grade SS41, measuring 50 mm wide, 100 mm long, and 1.6 - 6.0 mm thick, were degreased by immersion in an alkaline bath at 80°C for 30 minutes. They were washed with warm water, and then were descaled by immersion in a 10 % hydrochloric acid solution at ordinary temperature for 30 minutes. Next, the steel pieces were washed with warm water fluxed by immersion in a 35 % ZnCl₂-NH₄Cl solution at 60°C for one minute. The steel pieces so pretreated were galvanized by immersion in the baths of compositions shown in Table 1 at 450 - 550°C for one minute, and then cooled with warm water. The cooling was done by a dip in a bath of warm water at 40°C for 5 seconds. The results are shown in Table 1.

Example 4 (development of gold - purple - blue with Mn-Ti-Zn alloy)

The steel pieces treated as described in Example 1 were immersed in a bath of molten zinc alloy containing 0.5 wt % Mn and 0.08 wt % Ti, with the Pb content restricted to 0.004 wt %, at 500°C for one minute. They were then held in a heating atmosphere at 500°C and cooled. The relations between the treating conditions and coloring are shown in the following Table 2. Golden and purple colors came out very rapidly and even blue color developed in 30 seconds. The galvanized surfaces were quite smooth and beautiful in appearance.

Example 5 (After-treatment)

Test pieces of steel sheet, measuring 50 mm wide, 100 mm long, and 3.2 mm thick, were either conventionally hot-dip galvanized or colored, hot-dip galvanized (with a Zn-Ti alloy). The galvanized pieces were coated with a clear polyurethane resin (resin : hardener = 5 : 1) or a colored, aqueous acrylic resin paint by brushing or dipping. The coated pieces, together with uncoated ones, were subjected to outdoor weathering tests. The tests were conducted within a plant under the possession of the present applicant. The degrees of degradation after test periods of three months, six months, and one year were visually inspected. The results are tabulated below in Table 3.
Conventionally hot-dip galvanized pieces became defective in only three months after the painting. Among the coloured, hot-dip galvanized pieces, the golden-colored piece had a thinner oxide film than the rest because of the immature oxidation. Without a paint coat, therefore, the golden-colored piece degraded in three months and the blue-colored in one year. Painting could retard the degradation. Needless to say, an increase in the thickness of the paint coat, multicoating, or other similar step would prove effective in further retarding the degradation.

Example 6 (Spraying)

A rod of zinc alloy containing 0.3 wt % Mn was used as a sprayable material. It was sprayed over a steel material by means of an oxy-acetylene gas flame type spray gun. The sprayed surface was allowed to cool, heated to 500°C for 30 seconds, and again allowed to cool in the air.
A olive gray colored coating was obtained.

Claims

A method of forming an olive gray colored zinc coating on an iron or steel surface, comprising coating a base metal of iron or steel by hot dipping in a zinc alloy of a composition consisting of 0.2 to 0.8 wt % Mn - bal. Zn and unavoidable impurities at a bath temperature of 490 to 530°C, heating the coated surface in an atmosphere at 500 to 520°C, for 50 to 150 seconds, and thereafter either cooling it with warm water or cooling it first in air forcibly and then with warm water.
A method of forming an olive gray colored zinc coating on an iron or steel surface, comprising coating a base metal of iron or steel by hot dipping in a zinc alloy of a composition consisting of 0.2 to 0.8 wt % Mn and 0.05 to 1.0 wt % Cu - bal. Zn and unavoidable impurities at a bath temperature of 490 to 530°C, heating the coated surface in an atmosphere at 500 to 520°C for 50 to 150 seconds, and thereafter either cooling it with warm water or cooling it first in air forcibly and then with warm water.
A method of forming a zinc coating exhibiting a color selected from the group consisting of gold, purple and blue, on an iron or steel surface comprising using a galvanizing zinc alloy containing 0.2 to 0.8 wt % Mn and 0.01 to 0.1 wt % Ti, the balance of the alloy being zinc and unavoidable impurities with impurity Pb being limited to 0.005 wt % or less, wherein said iron or steel surface is coated at a bath temperature of 480 to 550°C and the coated surface obtained is cooled or is cooled after heating to a temperature of 450 to 550°C, the color of the coating being selected by controlling the extent of the oxidation of the coating.
A method according to any one of claims 1 to 3 wherein the colored zinc coating is directly coated with a paint without using any pretreatment.
A method according to claim 4 wherein the paint is selected from among synthetic resin paints.
A method according to claim 5 wherein the synthetic resin paint is selected from among polyurethane resin, acrylic resin, epoxy resin, and chlorinated rubber paints.
A method of forming an iridescence-colored zinc coating exhibiting a blend of golden, purple, blue , green and other colors by hot-dip galvanizing, comprising hot-dipping a base metal of iron or steel in a coating bath of a zinc alloy consisting of 0.1 to 0.8 wt % Mn and 0.05 to 1.0 wt % Cu and the balance Zn and inevitable impurities at a bath temperature of 450 to 550°C, and then cooling the galvanized metal with warm water.
A method of forming an iridescence-colored zinc coating exhibiting a blend of golden, purple, blue, green and other colors by hot-dip galvanizing, comprising hot-dipping a base metal of iron or steel in a coating bath of a zinc alloy consisting of 0.1 to 0.8 wt % Mn and the balance Zn and inevitable impurities at a bath temperature of 450 to 550°C, wherein the metallic zinc bullion to be used in the alloy is either distilled zinc first grade (at least 98.5 % pure), purest zinc bullion (at least 99.995 % pure), or special zinc bullion (at least 99.99 % pure) conforming to JIS H 2107, the total impurity content being below 1.5 wt %, and then cooling the galvanized metal with warm water.
A zinc alloy for hot dipping to form on a base surface an olive gray colored coating of a composition comprising 0.2 to 0.8 wt % Mn and 0.05 to 1.0 wt % Cu, the balance Zn and inevitable impurities.
A zinc alloy according to claim 9 wherein the metallic zinc bullion to be used in the alloy is either the purest zinc bullion (at least 99.995 % pure) or special zinc bullion (at least 99.99 % pure) conforming to JIS H 2107 and which contains 0.005 wt % or less Pb.
A zinc alloy for hot dipping to form an iridescence-colored coating exhibiting a blend of golden, purple, blue, green and other colors, consisting of 0.1 to 0.8 wt % Mn and 0.05 to 1.0 wt % Cu, the balance Zn and inevitable impurities.
A zinc alloy for colored hot-dip galvanizing capable of developing golden, purple and blue colors selectively as desired, containing 0.2 to 0.8 wt % Mn and 0.01 to 0.1 wt % Ti, the balance of the alloy being zinc and unavoidable impurities with impurity Pb being limited to 0.005 % or less.
A method of forming a colored zinc coating on iron or steel surface comprising thermally spraying a zinc alloy as defined in one of claims 9 to 12 in the form of sprayable wire, rod or powder, and heating the thermally sprayed surface to develop a given color.