DE3787347T2 - Colored zinc coating. - Google Patents

Description

Field of the invention

This invention relates to a method of applying a colored zinc coating to the surface of an iron or steel material, and more particularly to a colored zinc coating method using Ti-Zn or Ti-Mn-Zn system alloys, which enables the development of new colors not achieved by conventional techniques and clearer color formations compared with conventional ones. According to this invention, golden and dark red colors not yet achieved can be made possible, and at the same time, yellow color, green color, blue color, purple color, etc. can be developed more clearly. Thus, this invention provides colored zinc coated materials which can be applied to a wide variety of fields and which have a coloration which better suits the environment in which they are used.

Background of the invention

Hot-dip galvanized iron and steel materials coated by immersion in molten zinc are used for corrosion protection purposes in a wide range of applications, forming parts and equipment in the fields of building and construction, civil engineering, agriculture, fishery, chemical plants, electric power and communication, and so on.

For pillars and other masts, lightning rods, railings, temporary scaffolding and frames for various works and projects, casings and decking and the like, there has been a growing demand in recent years for colored hot-dip galvanized materials that have an attractive appearance that matches the surrounding environment, and are preferred over the classic hot-dip galvanized steels with metallic luster. With the spread of aesthetic awareness, colored hot-dip galvanized products are promising and have considerable demand potential in architecture, civil engineering, industrial plants, in electricity and communications, transportation, agriculture, marine products and other industries.

The coloring of hot-dip galvanized steels has traditionally been done by applying paints. This process has the disadvantage that the paint film may eventually peel off the coated surface. This results from the activity of Zn in the coating of the hot-dip galvanized steel, which causes a gradual alkaline decomposition of the fatty acids that make up the oily material of the paint, leading to the formation of zinc soap, which hinders the adhesion of the paint film to the underlying surface.

In an effort to eliminate this disadvantage, a complex procedure had to be followed. A steel product is first galvanized by immersing it in a molten zinc bath. The coated steel is exposed to air for one to three weeks so that corrosion products such as Zn(OH)2, ZnO, ZnCO3, ZnCl2 and the like are deposited on the coated steel surface. The surface is then cleaned and painted.

In addition to the coating process described above, another approach is known in the art which depends on the color developing effect of the oxide film during hot dip galvanizing. In particular, US-A-3 630 792 discloses a process for producing zinc coatings which uses a hot dip bath of a zinc alloy containing at least one element selected from manganese, titanium and vanadium and optionally also containing one or more elements selected from niobium, zirconium, thorium, mischmetal, cadmium, arsenic, copper, lead and chromium, the amount of which is sufficient to form a colored oxide film on the coating upon reaction of the surface of the coating with oxygen. It is stated therein that the preferred range for the compositions of the alloy is 0.1 to 0.45% for both manganese and titanium. In actual reported tests, the manganese content of zinc-manganese alloys which provide yellow, red and blue colored coatings is 0.02 to 0.15% and the titanium content of zinc-titanium alloys which provide yellow, bronze, red, blue or silver-blue colored coatings is 0.008 to 0.15%. When a zinc-titanium alloy containing 0.15% Ti and a bath temperature of 500ºC were used, the final color of the coating was yellow or red, depending on the thickness of the coated sheet. At a bath temperature of 550ºC, an alloy of zinc and 0.15% Ti resulted in a bronze or yellow colored coating of the immersed plate. The coated article can be post-heat treated, for example by induction heating, after which the coated article is allowed to cool in air or quenched with cold air blowers. It is pointed out that the tests were carried out with commercial special high grade zinc and that the other commercial grades of zinc, e.g. Prime Western, containing up to 1.5% Pb, can be used provided the aluminium content is limited to less than 0.002%. However, it has been found that the hot-dip galvanised coatings obtained by the disclosed process are generally very thin and light and are prone to rapid colour fading and film separation over time. It is difficult to precisely control the desired colour development, often producing pale and undefined colours. For such reasons, hot-dip galvanising colouring techniques have not been put into practical use, although many years have passed since their development.

FR-A-1 115 121 discloses a method for coating ferrous metallic objects, in which a layer of aluminium, zinc or another non-ferrous metal is applied, e.g. by means of a gun. Onto this metallization is applied a layer of a water glass-like plastic material, e.g. potassium silicate, or glycerophthalate. The coated object is then heat treated at a temperature of about 350°C, the final temperature depending on the melting point of the applied non-ferrous metal.

There is still a demand for many improvements in technology such as:

(a) the development of new colours not achieved in the past ;

(b) the achievement of colour formations which are more beautiful and clearer than those previously achieved;

(c) improved stability of colour formation;

(d) the aspect that the inherent corrosion resistance of the galvanized zinc coating is not sacrificed;

(e) less change over a period of time; and

(f) to provide a simple and consistent procedure.

Object of the invention

The object of this invention is to provide a method for producing colored zinc coatings in which the above-mentioned improvements can be achieved by using Ti-Zn or Ti-Mn-Zn zinc alloys.

Summary of the invention

According to one aspect of the invention, there is provided a method of forming a gold-colored zinc coating on an iron or steel surface, comprising coating the iron or steel surface in a hot dip bath of an electroplating zinc alloy containing 0.10 to 0.5 wt.% Ti in a distilled zinc at a bath temperature of 450 to 470°C, allowing the coated surface to cool in air for 5 to 20 seconds and then cooling it with cold or warm water.

According to another aspect of the invention, there is provided a method of forming a purple zinc coating on an iron or steel surface, which comprises coating an iron or steel base metal using a hot-dip zinc alloy having a composition consisting essentially of 0.10 to 0.5 wt.% Ti, balance Zn at a bath temperature of 500 to 550°C for at least one minute, and either allowing the coated surface to cool in air for 10 to 50 seconds or heating the coated surface in an atmosphere at 500 to 520°C for 10 to 20 seconds and then cooling it with cold or warm water.

The present invention further provides a method for forming a zinc coating having a color selected from the group consisting of yellow, dark red and green on an iron or steel surface, which comprises coating the iron or steel surface in a hot dip bath of an electroplating zinc alloy containing 0.2 to 0.7 wt.% Ti at a temperature of 590 to 620°C for one to three minutes, and cooling the coated surface after heating to a temperature of 450 to 550°C, wherein the color of the coating is selected by controlling the degree of oxidation of the coating.

The present invention also provides a method for forming a zinc coating having a color selected from the group consisting of gold, purple, blue, yellow, dark red and green on an iron or steel surface, comprising: immersing the iron or steel surface at a temperature of 490 to 620°C in a hot dip bath of

(a) a zinc electroplating alloy containing 0.2 to 0.7 wt.% Ti and 1.3 to 5.9 wt.% Pb

(b) an electroplating zinc alloy containing 0.2 to 0.7 wt.% Ti, 1.2 to 1.3 wt.% Pb and 0.1 to 0.2 wt.% Cd, or

(c) an electroplating zinc alloy containing 0.2 to 0.7 wt.% Ti, 1.0 to 1.2 wt.% Pb, 0.05 to 0.2 wt.% Cd and 0.01 to 0.05 wt.% of at least one element selected from the group consisting of Cu, Sn, Bi, Sb and In,

and in which the coated surface is cooled after heating to a temperature of 450 to 550ºC, the colour of the coating being selected by controlling the extent of oxidation of the coating.

Another aspect of the invention is a method for forming a dark red colored zinc coating on an iron or steel surface, which comprises coating an iron or steel base metal using a hot dip zinc alloy having a composition consisting essentially of 0.2 to 0.5 wt.% Ti, 0.05 to 0.15 wt.% Mn, balance Zn at a bath temperature of 580 to 600°C, and heating the coated workpiece in an atmosphere at 500 to 520°C for 30 to 70 seconds and then cooling it with cold or warm water.

The present invention also provides a method for forming a green colored zinc coating on an iron or steel surface, which comprises coating a base metal of iron or steel using a hot-dip zinc alloy containing 0.2 to 0.5 wt.% Ti and 0.05 to 0.15 wt.% Mn at a bath temperature between 600 and 620°C, and heating the coated workpiece for 50 to 60 seconds in an atmosphere at 500 to 520°C and then cooling it with cold or warm water or with a cooling gas.

According to a further aspect of the invention, there is provided a method for forming a yellow-colored zinc coating on an iron or steel surface, in which a base metal of iron or steel is coated using a hot-dip zinc alloy containing 0.2 to 0.5 wt.% Ti and 0.05 to 0.15 wt.% Mn at a bath temperature between 580 and 600°C, and in which the coated workpiece is heated for 20 to 30 seconds in an atmosphere at 500 to 520°C and thereafter cooled with cold or warm water or with a cooling gas.

Furthermore, according to the present invention, in a method for forming a blue-colored zinc coating on an iron or steel surface, a base metal of iron or steel is coated using a hot-dip zinc alloy having a composition consisting essentially of 0.10 to 0.5 wt.% Ti, 0.05 to 0.15 wt.% Mn, balance Zn at a bath temperature of 530 to 550°C, and the coated surface is allowed to cool in air for 15 to 25 seconds and thereafter cooled with cold or warm water.

The present invention further provides a method for forming a colored zinc coating on an iron or steel surface, in which a zinc alloy containing 0.1 to 2.0 wt.% Ti and optionally 0.01 to 4.0 wt.% of at least one other element selected from the group consisting of Mn, Cu, Cr and Ni, in the form of a sprayable wire or rod or powder by means of a spray gun, and then heating the thermally sprayed surface until the colour in question is developed.

In color hot-dip galvanizing, the composition of the coating bath and the conditions for forming an oxidized film skillfully cooperate to produce color effects by light interference. By skillfully controlling these factors, the invention successfully achieved the development of gold color in Ti-Zn alloys, which was previously thought to be beyond the realm of possibility, and was successful in developing dark red color, which was highly desired. In addition, it became possible to permanently achieve the development of clearer yellow, purple, green or other colors than those previously achieved. Furthermore, the process of the present invention enables the development of a much-needed dark red color in Ti-Mn-Zn alloys, and stable yellow, green and blue colors which are clearer than those previously achieved.

It was also found that a colored zinc coating can be applied by a thermal spray process and that change of the colored zinc coating over time can be suppressed by a paint application.

Detailed description of the invention

Hot dipping in zinc alloys is carried out by melting a zinc alloy in a plating bath and by dipping a component to be coated into it.

A-1) The development of golden colour with a Ti-Zn alloy

It is possible to form a colored coating with a golden color on an iron or steel surface by coating the base metal using a hot-dip zinc alloy bath, the composition of which is 0.1-0.5 wt.% Ti, balance Zn, at a bath temperature of 450-470ºC, allowing the coated workpiece to rest in air for 5-20 seconds, and then cooling it with cold or warm water.

The metallic zinc ingot used to form the zinc alloy for hot dipping typically has one of the purity grades according to JIS H2107, e.g. distilled First grade zinc (at least 98.5% pure), ultra pure zinc (at least 99.99% pure) and special grades of zinc purity. The unavoidable impurities contained in these zinc materials are, for example, in the case of first grade distilled zinc, a total of up to 1.2% by weight Pb, 0.1% by weight Cd and 0.020% by weight Fe. For the purposes of the invention, a metallic zinc is desired whose total impurity content is less than 1.5% by weight. Distilled zinc is particularly preferred because it allows the coating to be carried out using a normal flux and because the color intensity produced is greater.

In this embodiment, the coating is effected using a molten zinc alloy bath containing 0.1-0.5 wt.% Ti, balance Zn, which is obtained by adding 0.1-0.5 wt.% Ti to the above-mentioned zinc. A bath of the molten zinc alloy containing 0.3 wt.% Ti is particularly desirable.

To produce the golden colored coating from the hot-dip zinc alloy bath of the above composition, a base metal of iron or steel is immersed in the coating bath at 450-470ºC for at least one minute, the base metal is withdrawn from the bath and allowed to cool in air for about 5-20 seconds, and then it is immediately cooled with cold or warm water to form an oxide film having a golden color thereon.

Thus, in producing a gold colored coating, it is essential to immerse the iron or steel base metal in the bath of molten zinc alloy containing 0.1-0.5 wt.% Ti, balance Zn, the temperature of the bath being 450-470ºC, and then to allow it to cool in air for a very short period of time of 5 to 20 seconds, preferably 10-20 seconds. If the conditions are outside the specified ranges, the desired golden color will not result. For example, if the heating temperature is above 470ºC, and if the time for which the coated workpieces are allowed to cool in air exceeds 20 seconds, the color of the coating will become purple.

As previously mentioned, a colored coating having a uniform, permanent, golden color can be formed on a base metal of iron or steel by coating the base metal under certain conditions using a molten zinc alloy of the specified composition. Thus, a corrosion-resistant material is provided for those components and devices which, from an aesthetic point of view, are required to have a golden color. The iron or steel products with colored coatings according to the invention are highly corrosion-resistant and are of value for a wide range of applications.

A-2) The development of clear purple color with a Ti-Zn alloy

It is possible to form a colored coating having a purple color on an iron or steel surface by coating the base metal using a hot-dip zinc alloy bath containing 0.1-0.5 wt.% Ti, balance Zn, at a bath temperature of 500-550ºC, either allowing the coated workpiece to cool in air for 10-50 seconds, or heating it in an atmosphere at 500-520ºC for 10-20 seconds, and then cooling it with cold or warm water. Regarding a zinc ingot, the same explanation as in A-1) is applied here.

The coating is effected using a zinc alloy melt bath containing 0.1-0.5 wt.% Ti, balance Zn, and which is obtained by adding 0.1-0.5 wt.%, preferably 0.3 wt.% Ti to the above-mentioned zinc

To produce the purple colored coating from the bath of hot-dip zinc alloy of the above composition, a base metal of iron or steel is immersed in the coating bath at 500-550ºC, preferably 500-520ºC, for at least one minute, the base metal is withdrawn from the bath and allowed to cool in air for 10-50 seconds, preferably 40-50 seconds, and then immediately cooled with cold or warm water to form an oxide film having a purple coloration thereon. Alternatively, the workpiece taken out of the bath is heated in an atmosphere at 500-520ºC for 10-20 seconds, and then cooled with cold or warm water to form a purple colored oxide film thereon.

Thus, in producing a purple colored coating, it is essential to immerse the iron or steel base metal in the bath of molten zinc alloy containing 0.1-0.5 wt.% Ti, balance Zn, the temperature of the bath being 500-550ºC, preferably 500-520ºC, and then either allow it to cool in air for a very short period of time of 10 to 50 seconds, preferably 40-50 seconds, or heat it for 10-20 seconds in an atmosphere at 500-520ºC, and then cool it with cold or warm water. If the conditions are outside the above ranges, the desired purple coloration will not result.

As previously mentioned, a colored coating having a uniform, permanent, purple color can be formed on a base metal of iron or steel by coating the base metal under certain conditions using a molten zinc alloy of the specified composition. Thus, a corrosion-resistant material is provided for those components and devices which, from an aesthetic point of view, require a purple color.

The iron or steel products with colored coatings according to the invention are highly corrosion resistant and they are of value for a wide range of applications.

A-3) Selective development of yellow, dark red, green color with Ti-Zn alloy

A zinc alloy for colored hot-dip galvanizing has been created which can selectively develop yellow, dark red and green colors as desired, and which contains 0.2-0.7 wt.% Ti, the balance zinc and unavoidable impurities. An iron or steel surface is coated at a temperature of 590 to 620ºC in a hot-dip bath of such an alloy. The coated surface is cooled after heating to 450 to 550ºC. The color of the coating is selected by controlling the degree of oxidation of the coating.

It has also been found that the following alloys, prepared by adding the ingredients as follows to the above Ti-Zn alloy, are useful for uniform coloring in yellow, dark red and green:

(a) A zinc alloy for colored hot-dip galvanizing, which can develop yellow, dark red and green colors selectively as desired, consists of 0.2-0.7 wt% Ti, 1.3-5.9 wt% Pb, the balance being zinc and unavoidable impurities.

(b) A zinc alloy for colored hot-dip galvanizing, which can develop yellow, dark red and green colors selectively as desired, consists of 0.2-0.7 wt% Ti, 1.2-1.3 wt% Pb, 0.1-0.2 wt% Cd and the rest is zinc and unavoidable impurities.

(c) A zinc alloy for coloured hot-dip galvanising capable of developing yellow, dark red and green colours as desired, consisting of 0.2-0.7 wt% Ti, 1.0-1.2 wt% Pb, 0.05-0.2 wt% Cd, 0.01-0.05 wt% of at least one element selected from the group consisting of Cu, Sn, Bi, Sb and In and the balance being zinc and unavoidable impurities.

A base material of iron or steel is galvanized by immersion in a bath of molten zinc of such an alloy at a temperature of 490 to 620ºC, and the coated metal is cooled in air after heating to 450 to 550ºC. By appropriate control of the conditions, it is possible to produce yellow, dark red and green colors as desired. Even with an alloy based on a very pure metallic zinc (at least 99.995% pure) or special zinc (at least 99.99% pure) Galvanization with good wettability and uniformity of color can be achieved.

Zinc alloy hot dipping is carried out by melting a zinc alloy into a plating bath and dipping a workpiece to be plated into the bath. The zinc alloy is prepared by adding a special alloying additive to a metallic zinc. In the practice of the invention, a metallic zinc ingot with a high purity of at least 99.9% is used, which is referred to as ultrapure zinc (99.995% pure) and specialty zinc (at least 99.99% pure) as defined in JIS H2107. This prevents any adverse effect that the variable introduction of impurities (Pb, Cd, Fe, etc.) may have on the controllability of color development. However, the use of such high purity zinc involves disadvantages while eliminating changes in plating conditions due to the presence of impurities. For example, when an iron or steel material is electroplated by immersion in a (Fe-saturated) plating bath containing predetermined amounts of Ti and Mn, the formation of an oxide film on the bath surface occurs rapidly and extensively. These and other factors tend to cause color shades such as partial two-tone mixing of the colored oxide film of the plating layer.

The present inventors have found that under these conditions, the addition of 0.2-0.7 wt% of Ti is effective to give a yellow, dark red or green color clearly and brightly without partial lack of coating and without unevenness of coloring.

If the Ti content in the plating bath is less than 0.2 wt.%, the formation of a colored oxide film in the plating layer of the electroplated metal is insufficient, and the coloration is poor and uneven, thereby reducing the sales value of the colored electroplated product. If the Ti content is greater than 0.7 wt.%, the oxide film is formed too quickly and the change in color of the colored oxide film becomes too fast to be controlled.

In addition, excessive oxide formation on the coating bath reduces the wettability of the bath with respect to the base metal to be galvanized.

In order to further improve the wettability of the coating, various alloys were investigated which were prepared by adding Pb, Cd, Sn, Bi, Sb, In, and/or similar to the alloy of 0.2-0.7 wt.% Ti, balance Zn. It was found that the zinc alloys (a), (b) and (c) described above were particularly suitable. These three alloys are described below.

a) Alloy containing 1.3-5.9 wt.% Pb in addition to Ti:

If the Pb content is less than 1.3 wt%, the wettability improvement effect is limited. When colored coating is carried out at a bath temperature of 470-500ºC, no coating will be formed in part. Particularly within the bath temperature range of 470-490ºC, slag deposition on the coating film will often occur. Also in the range of 500-600ºC, vacancies and color shades will appear in the colored oxide film. Pb addition has been shown to be increasingly effective up to its solubility limit. Since the Pb solubility in molten zinc at a bath temperature of 600ºC is 5.9 wt%, this value is taken as the upper limit.

b) Alloy containing in addition to Ti 1.2-1.3 wt.% Pb and 0.1-0.2 wt.% Cd:

When Pb and Cd are used together, small amounts can prove effective. If the Pb content is less than 1.2 wt.%, partial non-plating will occur in the colored coating at a bath temperature of 470-600ºC even if Cd is present. In the temperature range of 470-490ºC, the possibility of slag deposition on the coating film will be greater. Even if the Pb content is in the specified range, similar problems will occur if the Cd content is less than 0.1 wt.%. If the Pb content exceeds 1.3 wt.%, or if the Cd content is over 0.2 wt.%, the oxide formation on the coating bath will be so great that the non-plating rate will increase.

c) Alloy containing, in addition to Ti, 1.0-1.2 wt.% Pb, 0.05-0.2 wt.% Cd and 0.01-0.05 wt.% of one or more elements selected from Cu, Sn, Bi, Sb and In:

The addition of at least one element selected from Cu, Sn, Bi, Sb and In promotes the wettability improving effect of Pb and Cd. If the Pb content is less than 1.0 wt% and the Cd content is below 0.05 wt%, partial plating by colored electroplating will not occur at a bath temperature of 470-600ºC. In particular, at a bath temperature in the range of 470-490ºC, the deposition of slag on the plating film will increase. On the other hand, if the Pb content is greater than 1.2 wt% and the Cd content exceeds 0.2 wt%, severe oxide formation will be observed on the surface of the plating bath. The addition of 0.01-0.05 wt% of at least one element of Cu, Sn, Bi, Sb and In retards the formation rate of the oxide film. on the surface of the bath and improves the wettability of the workpiece to be galvanized.

The additive elements thus prevent non-coating, color shading, slag deposition and other problems, facilitate the control of the coloring of the colored oxide film and increase the color depth or intensity.

In hot-dip galvanizing with such a zinc alloy, the workpiece to be galvanized is degreased, for example by using an alkaline bath, descaled by pickling or similar, and then treated with a flux to make it ready for galvanizing. The flux treatment is carried out, for example, by briefly immersing it in a ZnCl₂-KF solution, in a ZnCl₂-NH₄Cl solution or in another known flux solution.

After pretreatment, the workpiece is immersed in a coating bath at a special, controlled temperature for 1 to 3 minutes. The coated metal is pulled out of the bath and, by appropriately controlling the degree of oxidation, a yellow, dark red or green color is obtained as desired.

After the coated workpiece is withdrawn from the bath, it is kept in an atmosphere at 450-550ºC for a predetermined period of time so that the degree of oxidation can be controlled. The holding temperature, holding time and subsequent cooling process are selected as desired.

When the degree of oxidation is increased, yellow, dark red and green colors are successfully formed in the order mentioned here.

An example of controlling the degree of oxidation is as follows:

Yellow: After the workpiece is pulled out of the plating bath with a bath temperature of 590ºC, it is kept in an atmosphere of 500ºC for 15-20 seconds, and then cooled with warm water.

Dark red: The bath temperature is increased by 5-10ºC and either the temperature of the atmosphere is increased or the residence time is extended by 5-10 seconds.

Green: The bath temperature is increased by 5-10ºC and either the atmosphere temperature is increased further or the residence time is extended by a further 5-10 seconds.

With the alloys of the invention, ie (a) the alloy with Ti, 1.3-5.9 wt.% Pb, balance Zn, (b) the alloy with Ti, 1.2-1.3 wt.% Pb, 0.1-0.2 wt.% Cd, balance Zn, and (c) the Alloy with Ti, 1.0-1.2 wt.% Pb, 0.05-0.2 wt.% Cd, 0.01-0.05 wt.% (Cu, Sn, Bi, Sb and/or In), balance Zn, the color development is controllable in the order of golden, purple and blue colors. With increasing degree of oxidation, golden, purple, blue, yellow, dark red and green colors are produced.

B-1) The development of dark red color with a Ti-Mn-Zn alloy

It is possible to form a colored coating having a dark red coloration on a base metal of iron or steel by coating the base metal using a bath of molten zinc alloy containing 0.2-0.5 wt.% Ti, 0.05-0.15 wt.% Mn, balance Zn at a bath temperature of 580-600ºC, heating the coated workpiece for 30-70 seconds in an atmosphere at 500-520ºC, and then cooling it with cold or warm water.

The metallic zinc to be used to form the zinc alloy for hot dipping is typically of one of the purity grades specified in JIS H2107, such as first-grade distilled zinc (at least 98.5% pure), ultra-pure zinc (at least 99.99% pure) and special zinc purity grades. The unavoidable impurities contained in these zinc materials are, for example, for first-grade distilled zinc, a total of up to 1.2 wt.% Pb, 0.1 wt.% Cd and 0.020 wt.% Fe. For the purposes of the invention, a metallic zinc having a total impurity content of less than 1.5 wt.% is desirable. Among these forms of zinc, distilled zinc is preferred, in part because it can be coated with a normal flux and the concentration is high.

In this embodiment, the coating is carried out using a bath of molten zinc alloy prepared by adding 0.2-0.5 wt.%, preferably 0.3 wt.% of Ti and 0.05-0.15 wt.%, preferably 0.1 wt.% of Mn to the zinc mentioned above.

To produce the dark red colored coating from the hot-dip zinc alloy bath of the above composition, a base metal of iron or steel is immersed in the coating bath at 580-600ºC for at least one minute, the base metal is withdrawn from the bath and kept in an atmosphere of 500-520ºC (for example, in a furnace) for 30-70 seconds, and then it is immediately cooled with cold or warm water to form an oxide film with a dark red coloration thereon.

Thus, when producing a colored coating with a special dark red color, it is important to heat the iron or steel base metal using the bath of the molten zinc alloy of the specified composition at the special bath temperature to dip it, heat it under specific temperature conditions, and then cool it with cold or warm water. . If the conditions are outside the ranges given above, a coating with the desired dark red color will not result.

B-2) The development of green color with a Ti-Mn-Zn alloy

Using a hot-dip zinc alloy to form a green-colored coating on a base surface containing 0.2-0.5 wt% Ti and 0.05-0.15 wt% Mn, it is possible to form a green-colored coating on an iron or steel surface by coating the base metal with the hot-dip zinc alloy at a bath temperature of 600-620ºC, heating the coated workpiece for 50-60 seconds in an atmosphere at 500-520ºC, and then cooling it with cold or warm water or with a cooling gas.

The zinc to be used is consistent with B-1).

The coating is carried out using a molten zinc alloy bath of the previously mentioned zinc, with 0.2-0.5 wt.% Ti and 0.05-0.15 wt.% Mn added. The use of a hot dip bath of a zinc alloy containing 0.3 wt.% Ti and 0.1 wt.% Mn is particularly desirable for forming a green colored coating.

To produce the green colored coating from the hot dip bath of zinc alloy containing the above percentages of Ti and Mn, a base metal of iron or steel is immersed in the molten zinc alloy bath at 600-620ºC for at least one minute, the base metal is then withdrawn from the bath and it is heated in an atmosphere (for example in a furnace) at 500-520ºC for 50-60 seconds. After heating, the workpiece is cooled with cold or warm water or with a cooling gas to form thereon a colored coating of an oxide having a green coloration.

As described above, a colored coating with a uniform, stable green color can be achieved by carrying out the coating using a hot dip bath of a molten zinc alloy containing 0.2-0.5 wt% Ti and 0.05-0.15 wt% Mn under the specified conditions. If the Ti and Mn contents in the zinc alloy are outside the specified ranges, the green color of the resulting colored coating will be uneven and the oxide film will exhibit poor wettability with respect to the coated base metal.

Likewise, if the bath temperature and the temperature and duration of subsequent heating as the hot dip conditions are not within the specified ranges, other colors may mix in, making it impossible to obtain a coating with a uniform green color.

Thus, in order to produce a green colored coating with a uniform color, the Ti content and the Mn content in the molten zinc alloy play an important role in the hot dipping bath, the hot dipping conditions and the conditions of the subsequent heating. It is only through the combination of such special conditions that it is possible to obtain the intended green colored coating.

The resulting colored coating provides excellent resistance to corrosion attacks, with the so-called corrosion weight loss being far lower than that of coatings using normal molten zinc alloys.

B-3) The development of yellow color with a Ti-Mn-Zn alloy

It is possible to form a colored coating with a yellow coloration on an iron or steel surface by coating the base metal with a hot-dip zinc alloy containing 0.2-0.5 wt.% Ti, 0.05-0.15 wt.% Mn at a bath temperature of 580-600 ºC, heating the coated workpiece for 20-30 seconds in an atmosphere at 500-520 ºC, and then cooling it with cold or warm water or with a cooling gas.

The zinc to be used is B-1).

The coating is carried out using a molten zinc alloy bath of the previously mentioned zinc with 0.2-0.5 wt% Ti and 0.05-0.15 wt% Mn added. A bath of molten zinc alloy containing 0.3 wt% Ti and 0.1 wt% Mn is particularly desirable.

To produce the yellow colored coating from the hot dip bath of the zinc alloy containing the amounts of Ti and Mn specified above, a base metal of iron or steel is immersed in the coating bath at 580-600ºC for at least one minute, the base metal is then withdrawn from the bath and it is heated in an atmosphere (for example in a furnace) at 500-520ºC for 20-30 seconds. After heating, the workpiece is water cooled for about 10 seconds to form thereon a colored coating of an oxide having a yellow coloration.

Thus, in the manufacture of a yellow-colored coating, it is particularly important to carry out the coating using the bath of molten zinc alloy with the specified composition under the specified conditions, and then to heat the coated workpiece for 20-30 seconds in an atmosphere at 500-520ºC. If the heating after coating is carried out under conditions outside the above-mentioned ranges, a uniform yellow coloration will not be obtained. For example, if the duration of heating exceeds 30 seconds, the coloration will be mixed with green and the desired yellow-colored coating will no longer be obtained.

The resulting colored coating is excellent in terms of its corrosion resistance.

B-4) The development of blue colour with a Ti-Mn-Zn alloy

It is possible to form a colored coating with a blue tint on an iron or steel surface by coating the base metal using a hot-dip zinc alloy bath, the composition of which is 0.1-0.5 wt.% Ti, 0.05-0.15 wt.% Mn, balance Zn, at a bath temperature of 530-550ºC, allowing the coated workpiece to cool in air for 15-25 seconds, and then cooling it with cold or warm water.

The zinc to be used is consistent with B-1).

The coating is carried out using a bath of molten zinc alloy prepared by adding 0.1-0.5 wt.%, preferably 0.3 wt.% of titanium (Ti) and 0.05-0.15 wt.%, preferably 0.1 wt.% of manganese (Mn) to the zinc mentioned above.

To produce the blue colored coating from the hot dip bath of the zinc alloy of the above composition, a base metal of iron or steel is immersed in the coating bath at 530-550ºC for at least one minute, the base metal is pulled out of the bath and allowed to cool in air for about 15-25 seconds, and it is then immediately cooled with cold or warm water to form thereon an oxide film having a blue coloration.

Thus, in the manufacture of a blue colored coating, it is essential to coat the iron or steel base metal at a bath temperature of 530-550ºC using the bath of a molten zinc alloy whose composition consists of 0.1-0.5 wt.% Ti, 0.05-0.15 wt.% Mn, balance Zn, and then to evaporate it in air for a short period of time of 15-25 seconds. If the conditions are outside the ranges given above, a coating with the desired blue color will not result.

The resulting colored coating is excellent in terms of its corrosion resistance.

C) Aftercare

The colored oxide film formed on the colored hot-dip galvanized material tends to discolor or fade over time, with the coloration changes occurring due to the progression of deterioration depending on the environmental conditions including sunlight, temperature and humidity. Although the deterioration of the colored oxide film naturally does not adversely affect the corrosion resistance of the hot-dip galvanized steel itself, the original beautiful appearance will inevitably be disfigured.

As a simple measure to protect the colored oxide film on the colored hot-dip galvanized material to prevent discoloration or fading over time, painting has surprisingly proven to be a useful way to accomplish this task. As mentioned above, painting the coated surface of normal (uncolored) hot-dip galvanized steel presents problems of inadequate adhesion or separation of the paint film during short-term exposure. Partly responsible for this is the deposit of oxides (zinc white rust) and flux used for galvanizing, such as ammonium chloride, on the galvanized steel surface. Also probably responsible is the dissolution of basic zinc product formed between zinc and the water that has penetrated the paint film. It is believed that this product causes the decomposition of the resinous ingredients (oleaginous fatty acids) of an oil paint or long oil alkyd paint, causing the decomposition product to react with the zinc to form zinc soap along the interface between the zinc surface and the paint film, thereby significantly reducing the adhesion of the paint.

A common assumption was that the colored oxide film layer formed on the surface of colored hot-dip galvanized steel did not provide an adequate barrier between the zinc surface and the surrounding air. The pessimistic view prevailed that applying paint over the oxide film would have the same overall effect as applying paint directly to the galvanized surface. Contrary to these predictions, it was now found that the colored oxide film had good affinity and adhesion to paints. which allows the applied paint to penetrate the film to show a high resistance to stripping, and that the film is sufficiently capable of preventing the penetration of water to counteract the reaction of the zinc layer with water and thus the formation of zinc soap.

According to the invention, the hot-dip galvanized materials thus colored can be coated with a paint having excellent adhesion, weather resistance, durability and environmental protection properties.

For painting ordinary hot-dip galvanized steels, pretreatment is essential and the types of paint that can be used are limited. In contrast, for colored hot-dip galvanized steels, there is no need for pretreatment and various colors can be used. Since the heating for the oxidation following the galvanizing step produces an oxide film of, for example, TiO2 or MnO on the galvanized surface, the coating on the galvanized steel is so clean that there is no need to treat the surface before painting.

The paint to be used can be of any type that does not adversely affect the colored oxide film layer to be painted but protects it. Typically, a synthetic resin paint is used. Among the synthetic resin paints, those with excellent protective effect are polyurethane resin, acrylic resin, epoxy resin and chlorinated rubber paints. The paint is suitably selected taking into account the price, the expected environments, the ease of application and other factors.

If the color of the colored oxide film is to be shown as it is, a clear paint is the best choice, and if the color tint 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, multiple coats are not impractical. For example, if the environmental conditions are very harsh or unfavourable, multiple coats of paint should be considered. For example, a water-based paint can be applied as the base coat and a clear paint as the intermediate and finishing coats. Alternatively, an epoxy paint resistant to alkaline attack resulting from zinc elution can form the bottom coat and a chlorinated rubber or polyurethane paint having excellent resistance to water, chemicals and weather can form the intermediate and finishing coats.

Even if the color degrades over time, causing the coating to fall off or peel off, the beautiful appearance of the galvanized steel remains thanks to the colored Oxide film on the steel surface is unaffected. According to the invention, such falling off or flaking rarely occurs because the paint has penetrated the colored oxide film and is firmly bonded to it. The paint that has penetrated the oxide film keeps water and the like away through its water-repellent effect and thereby protects the film.

D) Syringes

For coloured hot-dip galvanising, it is a prerequisite that the workpiece to be coated is immersed in a bath of molten zinc alloy. In practice, the following limitations sometimes occur:

(1) The process is difficult to carry out with moulds that are too large to be immersed in the bath.

(2) Coating parts and structures of an assembly is sometimes difficult.

(3) Local dyeing is difficult. Although masking and other techniques can be used, they involve many complexities and difficulties. The techniques are difficult to reconcile with the trend towards increasingly frequent situations where the formation of patterns is required for decorative purposes.

(4) In the case of repairs to equipment and the like, the procedure is difficult to carry out on site.

(5) There are tendencies that the greater the content of those alloying elements such as Ti and Mn, the worse the wettability of the bath and the greater the number of vacancies and other coating defects. Although an increase in the content of the additional elements correspondingly improves the durability of the resulting coating, such addition is sometimes difficult from the standpoint of coating technology.

(6) The process sometimes produces coating voids and other coating defects.

The colored zinc coating produced by metal spraying basically involves spraying a zinc alloy, which is otherwise used for a plating bath and which can be in the form of a wire, rod or powder, over the object. Surprisingly, it was found that the oxidation reaction of the additional element proved to be more suitable than expected during the spraying process and an effect was achieved that was at least as satisfactory as with colored hot dip galvanizing.

Thus, in the present invention, a colored zinc coating can be produced by spraying a coloring, oxidizing zinc alloy onto a base surface by a metal spraying process, whereby a colored oxide film is formed on the base surface. After spraying, the color formation of the colored oxide film can be controlled by cooling and/or heating.

Metal spraying involves heating a sprayable material to a semi-molten state and spraying it over a base surface to form a coating that is firmly bonded to the surface. The sprayable material may be in the form of a wire, rod or powder, any of which forms may be used in the invention.

The sprayable material can be one of the zinc alloys commonly used for colored hot dip galvanizing. It can be, for example, a Ti-Zn or a Ti-Mn-Zn alloy, with or without a further addition of Cu, Ni and/or Cr. In the case of hot dipping, a workpiece with a high content of Ti, Mn or similar is not sufficiently wetted when immersed in the bath, leaving color-free areas on the surface. The possibility of non-overcoating creates limitations on the amounts of additive components. In metal spraying, the wetting problem does not exist and larger proportions of the additive elements can be used. Accordingly, the area of color formation is larger and the colors have a longer durability.

The desired sprayable material is a zinc alloy containing 0.1-2.0 wt.% Ti and optionally 0.01-4.0 wt.% of at least one element selected from Mn, Cu, Cr and Ni. Due to the good processability of the zinc alloy, it can easily be made into a wire, rod or powder by crushing or dripping the melt.

The spraying device commonly used is the type known as a gas flame spray gun. An arc spray gun may also be used.

The sprayable material is melted by the spraying device and sprayed over the base surface to be coated. The corners and intricate parts of the workpiece that are difficult to coat by hot dipping can be completely coated by directing the spray gun at those places. Local coatability allows easy creation of shapes and other patterns. Another major advantage of metal spraying is the ability to coat iron and steel structures or similar on site.

After spraying, the degree of surface oxidation is controlled to develop the desired color. A variety of colors, such as yellow, dark red, green, gold, purple and blue colors can be selectively developed as desired depending on the degree of oxidation. To control oxidation, the cooling rate of the sprayed coating can be adjusted using natural cooling in air or forced cooling with water or air. Likewise, the sprayed coating can be heated for a variable period of time with a flame, infrared lamp, oven (where applicable) or the like and the subsequent cooling can be controlled. A suitable combination of the composition of the sprayable material with the conditions of surface oxidation enables a desired coloration to be produced.

In this way, a sprayed zinc coating is produced with both corrosion resistance and color capability.

The paint described above can be applied to the spray coating.

The functional effects of spraying are listed as follows:

1. Applicable to large components that cannot be hot dipped.

2. Suitable for easy coating of parts and structures of assemblies that are difficult to hot dip.

3. Allows local colour formation and the creation of a desired shape or other pattern, thus improving the decorative value of the coating.

4. Possibility of covering on site.

5. Ability to use refractory alloys.

6. Ease of forming a thick coating capable of providing long-term corrosion protection.

7. A high Ti content in the alloy improves corrosion resistance and increases coloration.

8. The coating film with a rough and porous surface is suitable as a base layer to be painted, and painting with a clear color or various colored dyes can improve the durability of the colored oxide film of the coating.

In addition to the spraying method, vapor deposition, sputtering, ion plating or other surface coating methods can be used in this invention.

The examples are described below: Examples A to D concern points A to D explained in the detailed description.

Example A-1 (Development of golden color with a Ti-Zn alloy)

A test piece made of steel sheet 5541, 50 mm wide, 100 mm long and 3.2 mm thick, was degreased by immersion in an alkaline bath for 30 minutes at 80ºC. It was washed with warm water and then derusted by immersion for 30 minutes in a 10% hydrochloric acid bath at normal temperature.

The steel sheet was then washed with warm water and fluxed by immersing it in a solution containing 35% ZnCl2-NH4Cl for 30 seconds at 60°C.

The steel sheet thus pretreated was coated by immersion for one minute at 450-470ºC in a plating bath, the composition of which was 0.3 wt.% Ti, the remainder Zn. It was withdrawn from the bath, allowed to cool in air for 10-20 seconds, and immediately cooled with water at a normal temperature. The steel surface thus obtained had an oxide coating with a shiny, uniform, golden color.

The test piece made of sheet steel with the resulting paint coating was subjected to a salt spray corrosion test for 240 hours. The corrosion weight loss was 72 g/m².

For comparison, normally coated steel sheets that were hot-dip galvanized with distilled zinc were also tested. The corrosion weight loss was 120-150 g/m².

Example A-2 (Development of purple color with a Ti-Zn alloy)

The steel sheet, pretreated in the same manner as in the previous example, was coated by immersion for one minute at 500-520ºC in a plating bath, the composition of which was 0.3 wt.% Ti, balance Zn. It was withdrawn from the bath, allowed to cool in air for 40-50 seconds, and was immediately cooled with water at a normal temperature.

The resulting steel surface had an oxide coating with a uniform, purple color.

The test piece made of sheet steel with the resulting paint coating was subjected to a salt spray corrosion test for 240 hours. The corrosion weight loss was 63 g/m².

For comparison, normally coated steel sheets that were hot-dip galvanized with distilled zinc were also tested. The corrosion weight loss was 120-150 g/m².

Example A-3: (Development of yellow, dark red, green color and additional development of golden purple, blue color)

The individual pieces, pretreated as described above, were immersed for one minute in plating baths having the compositions shown in Table I and then they were withdrawn at a rate of about 6 meters per minute. The steel pieces thus removed from the baths were heated for the times shown in an atmosphere at 500ºC and cooled with warm water to produce the following colored oxide films.

The treatment conditions were as follows

Yellow: bath temperature 590ºC

↓

Dwell time 15-20 seconds at 500ºC.

Dark red: bath temperature 600ºC

↓

Dwell time 25-30 seconds at 500ºC.

Green bath temperature 610ºC

↓

Dwell time 35-40 seconds at 500ºC. Table 1 This invention Comparative example Alloy No. Zinc alloy addition (wt.%) Colorless area Color shade Slag deposit Assessment acceptable good very good unacceptable No Yes

Using alloys Nos. 2 to 5 of the examples, golden, purple and blue colors were successfully formed under the following conditions:

Gold: bath temperature 490ºC (1 minute)

↓

Dwell time 1-2 seconds at 500ºC.

Purple: bath temperature 500ºC (1 minute)

↓

Dwell time 10-15 seconds at 500ºC.

Blue: bath temperature 520ºC

↓

Dwell time 15-20 seconds at 500ºC.

Thus, in the same manner as in the examples, the oxidation conditions were gradually increased to successfully form a wide variety of colors, i.e., six colors, golden → purple → blue → yellow → dark red → green, in a controllable manner. No colorless spots and no color shades were obtained.

Example B-1 (Development of dark red color with a Ti-Mn-Zn alloy)

A test piece of steel sheet SS41, 50 mm wide, 100 mm long and 3.2 mm thick, was degreased by immersing it in an alkaline bath for 30 minutes at 80ºC. It was washed with warm water and then derusted by immersing it in a 10% hydrochloric acid bath at normal temperature for 30 minutes. The steel sheet was then washed with warm water and fluxed by immersing it in a solution containing 35% ZnCl₂-NH₄Cl for 30 seconds at 60ºC.

The steel sheet thus pretreated was coated by immersion for one minute at 580-600ºC in a plating bath, the composition of which was 0.3 wt.% Ti, 0.1 wt.% Mn, the balance Zn. It was withdrawn from the bath, kept for 30-70 seconds in an oven at 500-520ºC, removed from the oven and immediately cooled with warm water at 40 - 60ºC.

The steel surface thus obtained had an oxide coating with a dark red color.

The test piece made of sheet steel with the resulting paint coating was subjected to a salt spray corrosion test for 240 hours. The corrosion weight loss was 60 g/m².

For comparison, normally coated steel sheets that were hot-dip galvanized with distilled zinc were also tested. The corrosion weight loss was 120-150 g/m².

Example B-2 (Development of green color with a Ti-Mn-Zn alloy)

The steel sheet pretreated as described was coated by immersion for one minute at 600-620ºC in a plating bath of the composition given below. It was withdrawn from the bath, kept in an oven at 500-520ºC for 50-60 seconds, removed from the oven, and cooled with warm water by immersion in the bath for 10 seconds.

Composition of the bath:

0.3 wt% Ti, 0.1 wt% Mn, balance Zn.

The zinc used was first-grade distilled zinc.

The successive steps of coating, heating and cooling with warm water resulted in a uniformly colored coating layer with a bright green color on the steel sheet.

The test piece made of sheet steel with the resulting paint coating was subjected to a salt spray corrosion test for 240 hours. The corrosion weight loss was 61 g/m².

For comparison, normally coated steel sheets that were hot-dip galvanized with distilled zinc were also tested. The corrosion weight loss was 120-150 g/m².

Example B-3 (Development of yellow color with a Ti-Mn-Zn alloy)

The steel sheet pretreated as described was coated by immersion for one minute at 580-600ºC in a plating bath whose composition consisted of 0.3 wt.% Ti, 0.1 wt.% Mn, balance Zn. It was withdrawn from the bath, kept for 20-30 seconds in a furnace at 500-520ºC, removed from the furnace and immediately cooled by immersion in warm water at 40-60ºC for 10 seconds.

The steel surface thus obtained had an oxide coating with a bright yellow color.

The test piece made of sheet steel with the resulting paint coating was subjected to a salt spray corrosion test for 240 hours. The corrosion weight loss was 48 g/m².

For comparison, normally coated steel sheets that were hot-dip galvanized with distilled zinc were also tested. The corrosion weight loss was 120-150 g/m².

Example B-4 (Development of blue color with a Ti-Mn-Zn alloy)

The steel sheet, pretreated as described above, was coated by immersion for one minute in a plating bath at 530-550ºC, the composition of which was 0.3 wt.% Ti, 0.1 wt.% Mn, balance Zn. It was withdrawn from the bath, allowed to cool in air for 15-25 seconds, and immediately cooled with water at normal temperature.

The steel surface thus obtained had an oxide coating with a uniform, blue color.

The test piece made of steel sheet with the resulting paint coating was subjected to a salt spray corrosion test for 240 hours. The corrosion weight loss was 70 g/m².

For comparison, normally coated steel sheets that were hot-dip galvanized with distilled zinc were also tested. The corrosion weight loss was 120-150 g/m².

Example C (aftercare)

Test pieces made of steel sheet, 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, water-based acrylic resin paint by brushing or dipping. The coated pieces were subjected to outdoor weathering tests together with uncoated pieces. These tests were carried out in a plant belonging to the applicant. The degree of degradation was visually inspected after test periods of three months, six months and one year. The results are listed in Table 2.

Conventional hot-dip galvanized pieces failed in as little as three months after painting. Among the colored hot-dip galvanized pieces, the gold-colored piece had a thinner oxide film than the rest due to undeveloped oxidation. Without a paint film, therefore, the gold-colored piece was worn away after three months and the blue-colored after one year. Painting could retard the wear. Needless to say, increasing the thickness of the paint coating, multiple coats, or other similar steps would prove effective in further retarding the wear. Table 2 Test piece condition Outdoor weathering test 3 months Water-based acrylic resin hot-dip galvanized colored galvanized blue gold Clear polyurethane resin yellow green olive Unpainted good fairly poor poor

Example D-1 (syringes)

A zinc alloy rod containing 1.9 wt% Ti and 0.3 wt% Mn was used as a sprayable material. It was sprayed over a steel material using an acetylene-oxygen gas flame spray gun. The sprayed surface was allowed to cool, heated to 500ºC for 30 seconds and allowed to air cool again.

The result was a green colored coating.

Example D-2 (syringes)

Under the same conditions as in Example 1, but using a zinc alloy rod containing 1.0 wt% Ti, the spraying and post-treatment were carried out.

The result was a blue colored coating.

Claims (12)

1. A method for forming a gold-colored zinc coating on an iron or steel surface, which comprises coating the iron or steel surface at a bath temperature of 450 to 470°C in a hot dip bath of an electroplating zinc alloy containing 0.10 to 0.5 wt.% Ti in a distilled zinc, and allowing the coated surface to cool in air for 5 to 20 seconds and then cooling it with cold or warm water. 2. A method of forming a purple zinc coating on an iron or steel surface, which comprises coating an iron or steel base metal using a hot-dip zinc alloy having a composition consisting essentially of 0.10 to 0.5 wt.% Ti, balance Zn, at a bath temperature of 500 to 550°C for at least one minute, and either allowing the coated surface to cool in air for 10 to 50 seconds or heating the coated surface in an atmosphere at 500 to 520°C for 10 to 20 seconds and then cooling it with cold or warm water. 3. A method for forming a zinc coating having a color selected from the group consisting of yellow, dark red and green on an iron or steel surface, which comprises coating the iron or steel surface for one to three minutes at a temperature of 590 to 620°C in a hot dip bath of an electroplating zinc alloy containing 0.2 to 0.7 wt.% Ti, and cooling the coated surface after heating to a temperature of 450 to 550°C, wherein the color of the coating is selected by controlling the degree of oxidation of the coating. 4. A method of forming a zinc coating of a color selected from the group consisting of gold, purple, blue, yellow, dark red and green on an iron or steel surface, comprising: immersing the iron or steel surface at a temperature of 490 to 620ºC in a hot dip bath of (a) a zinc electroplating alloy containing 0.2 to 0.7 wt.% Ti and 1.3 to 5.9 wt.% Pb (b) an electroplating zinc alloy containing 0.2 to 0.7 wt.% Ti, 1.2 to 1.3 wt.% Pb and 0.1 to 0.2 wt.% Cd, or (c) an electroplating zinc alloy containing 0.2 to 0.7 wt.% Ti, 1.0 to 1.2 wt.% Pb, 0.05 to 0.2 wt.% Cd and 0.01 to 0.05 wt.% of at least one element selected from the group consisting of Cu, Sn, Bi, Sb and In, and in which the coated surface is cooled after heating to a temperature of 450 to 550ºC, the color of the coating being selected by controlling the extent of oxidation of the coating. 5. A method for forming a dark red colored zinc coating on an iron or steel surface, which comprises coating a base metal of iron or steel using a hot-dip zinc alloy having a composition consisting essentially of 0.2 to 0.5 wt.% Ti-0.05 to 0.15 wt.% Mn - balance Zn at a bath temperature of 580 to 600°C, and heating the coated workpiece for 30 to 70 seconds in an atmosphere at 500 to 520°C and then cooling it with cold or warm water. 6. A method for forming a green colored zinc coating on an iron or steel surface, which comprises coating a base metal of iron or steel using a hot-dip zinc alloy containing 0.2 to 0.5 wt.% Ti and 0.05 to 0.15 wt.% Mn at a bath temperature between 600 and 620°C, and heating the coated workpiece for 50 to 60 seconds in an atmosphere at 500 to 520°C and then cooling it with cold or warm water or with a cooling gas. 7. A method for forming a yellow-colored zinc coating on an iron or steel surface, which comprises coating a base metal of iron or steel using a hot-dip zinc alloy containing 0.2 to 0.5 wt.% Ti and 0.05 to 0.15 wt.% Mn at a bath temperature between 580 and 600°C, and heating the coated workpiece for 20 to 30 seconds in an atmosphere at 500 to 520°C and then cooling it with cold or warm water or with a cooling gas. 8. A method for forming a blue-colored zinc coating on an iron or steel surface, which comprises depositing a base metal of iron or steel using a hot-dip zinc alloy having a composition consisting essentially of 0.10 to 0.5 wt.% Ti-0.05 to 0.15 wt.% Mn-balance Zn is coated at a bath temperature of 530 to 550ºC, and in which the coated surface is allowed to cool in air for 15 to 25 seconds and then cooled with cold or warm water. 9. A method for forming a colored zinc coating on an iron or steel surface, which comprises thermally spraying a zinc alloy containing 0.1 to 2.0 wt.% Ti and optionally 0.01 to 4.0 wt.% of at least one other element selected from the group consisting of Mn, Cu, Cr and Ni in the form of a sprayable wire or rod or powder by means of a spray gun, and then heating the thermally sprayed surface until the color in question is developed. 10. A method according to any one of claims 1 to 9, wherein the colored zinc coating is directly coated with a color without applying any pretreatment. 11. A method according to claim 10, wherein the paint used is a synthetic resin paint. 12. The method of claim 11, wherein the synthetic resin paint is selected from the group consisting of polyurethane resin, acrylic resin, epoxy resin and chlorinated rubber paints.