JPH04246193A - Galvanized metal material excellent in resistance to heat and corrosion - Google Patents

Abstract

PURPOSE:To obtain a galvanized metal material excellent in resistance to heat and corrosion by attaching the oxide or hydroxide of magnesium on a galvanizing film in a specified amt. CONSTITUTION:At least one kind between the oxide and hydroxide of magnesium is attached on a galvanizing film or zinc-based plating film contg. >=70% Zn at 10-5000mg/m<2> expressed in terms of Mg, and at least one kind between the oxide and hydroxide of chromium is attached, as required, at 5-2000mg/m<2> expressed in terms of Cr. These films are attached by applying a soln. of the compds. of Mg and Cr and heat-treating the compds. or by cathodic electrolysis. A galvanized metal material excellent in resistance to heat and corrosion is obtained in this way.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to galvanized metal materials, and more particularly to steel materials having surface treatment applied to the galvanized surface.

0002

[Prior Art] Zinc plating is the most common and widely used rust prevention method for iron-based materials, but if a galvanized iron-based material is used as is with the zinc plating exposed, the zinc will rust. Because some zinc oxides readily form white rust, chromate treatment has usually been performed as a post-treatment to prevent white rust. Chromate treatment is a method in which the material to be treated is immersed in an aqueous solution of hexavalent chromic acid, and different types of films are produced depending on the film appearance, such as glossy, yellow, green, etc., but all of them are extremely weak against heat and have an 80% When heated to a temperature of about 0.degree. C. or above, defects such as cracks and peeling occur in the chromate film, resulting in a significant decrease in corrosion resistance.

0003

For this reason, sufficient corrosion resistance cannot be obtained by chromate treatment for parts used in applications where the influence of heat is large, such as parts used in the engine room of automobiles. Furthermore, the color of the chromate film generally exhibits the yellow or interference color of hexavalent chromium, which often poses problems in terms of appearance. As a countermeasure to this problem, reducing the amount of chromium deposited would make the film nearly colorless, but this also had the problem of reducing the corrosion resistance of the chromate film. For these reasons, there is a need for a post-treatment film for galvanized materials that has excellent heat resistance and corrosion resistance, and also has good appearance.

[0004] The present inventors have conducted basic research on the composition of the zinc anticorrosive film formed on the galvanized film and the method for forming the film, and have clarified the effects of various metal ions and their compounds on the corrosion of zinc. At the same time, we searched for a zinc anti-corrosion coating that solved the problems of conventional methods.

[0005]

[Means for solving the problem] As a result, high corrosion resistance can be obtained even after heating to a temperature of 300°C or higher by forming a magnesium oxide or hydroxide film layer on the surface of a galvanized metal material. I found it. In addition, as a result of further detailed research, we found that by forming a film containing both magnesium oxide or hydrated oxide and chromium oxide or hydrated oxide on the surface of galvanized metal material, It was discovered that excellent corrosion resistance could be obtained, and the present invention was completed.

Therefore, the first aspect of the present invention is to provide at least one of magnesium oxide and hydrated oxide on a zinc plating film or a zinc-based plating film containing 70% or more of zinc, in an amount of 10 to 5000 mg in terms of magnesium. /m2
It is a galvanized metal material with excellent heat resistance and corrosion resistance, and the second aspect of the present invention is a zinc-plated metal material that has excellent heat resistance and corrosion resistance. At least one type of hydroxide oxide is 10 to 5000 mg in terms of magnesium
/m2, and at least one of chromium oxide and chromium hydrated oxide is 5 to 2000 m2 in terms of chromium.
g/m2 Galvanized metal material with excellent heat resistance and corrosion resistance. The configuration of the present invention will be explained below.

[0007] The galvanized metal material in the present invention is not limited to its shape, but it may be steel or cast iron on the surface of which a plating layer containing 70% or more of zinc, such as zinc plating, alloy galvanization, or composite plating, is formed. , iron-based materials such as stainless steel are typical. Examples of alloy plating include zinc-nickel alloy plating, zinc-iron alloy plating,
Typical examples include zinc-tin alloy plating, and examples of composite plating include those in which compounds such as alumina, silica, and chromium oxide are combined.

[0008] The composition of zinc plating is 70% or more of zinc.
% or less (the remainder being added alloying elements or complexing elements and impurities).The reason why the corrosion resistance of magnesium (hydrated) oxide is quite selective with respect to the other metal; This is because it has no effect on the material. Further, the most suitable thickness of the galvanized layer is between 1 and 30 μm, which is between these two limits, because if it is too thin, the rust prevention effect will be low, and if it is too thick, it will be uneconomical.

[0009] It is necessary that magnesium oxide or magnesium hydroxide be deposited on the galvanized film in an amount of 10 to 5000 mg/m2 in terms of magnesium. If even better performance is desired, in addition to the corresponding amount of magnesium oxide or hydrated oxide, chromium oxide or chromium hydrate may be added to the galvanized film.
~2000mg/m2 must be attached. From the viewpoint of corrosion resistance and adhesion, the coating weight of these films is preferably 50 to 1000 in terms of magnesium.
mg/m2, and chromium is 10-500mg/m2
It is. In particular, when the film is required to be colorless, it is desirable that the amount of chromium deposited is in the range of 10 to 200 mg/m2. The adhesion amount of magnesium and chromium compounds is 10 to 5000 mg/m2 as Mg.
The reason for limiting the amount of Cr to 5 to 2000 mg/m2 is that below the lower limit, the film has little effect on improving corrosion resistance, so it is not practical, and above the upper limit, the adhesion of the film to the base material deteriorates and the film may peel off. This is because The chromium compound film can be formed as a separate film from the magnesium compound film, below or above it, or above and below it, or the magnesium compound and the chromium compound may coexist in a single film.

[0010] Magnesium oxide, hydrated oxide film is
It can be easily formed on a plating film by coating or by applying a cathode electric field. The coating method is to form a magnesium oxide film by coating a magnesium compound solution on the zinc plating and thermally decomposing it at 200 to 300°C.The compounds to be coated include, in particular, magnesium chloride, magnesium nitrate, and magnesium carbonate. Solutions such as are suitable. Similarly, the galvanized metal material to which both the chromium compound and the magnesium compound are attached can be produced by thermally decomposing a coating solution containing a trivalent chromium compound in addition to the magnesium compound. can. The chromium compounds used in this case include chromium nitrate (trivalent), chromium chloride (trivalent),
Chromium sulfate (trivalent) is suitable. The amount of magnesium and chromium deposited can be freely controlled by changing the concentration of each metal ion in the coating solution.

Another method suitable for forming the magnesium oxide or hydrated oxide film of the present invention is cathodic electrolysis. In this method, in an electrolytic bath consisting of an aqueous solution containing magnesium ions and depolarizing agent ions such as nitric acid, nitrous acid, bromic acid, and iodic acid,
By electrolyzing a galvanized metal material as a cathode,
This is a method of depositing magnesium ions as hydroxide or hydrated oxide on the galvanized surface to form a film. This hydrated oxide can be used as it is as a film, or an oxide that has been dehydrated by heating at a low temperature of room temperature to 120°C can also be used as a film. The hydroxide is also heated and used in the coating in the form of a hydrated oxide or oxide. This method has the advantage that it does not require thermal decomposition at high temperatures, unlike the coating method, and that a uniform amount of adhesion can be obtained even on molded products. In this method as well, a mixed film of magnesium and chromium hydrated oxide as described in claim 2 can be produced by using an electrolyte containing magnesium ions and chromium ions (however, chromium must be a valent ion). It can be deposited on galvanized films. Another method for forming a chromium oxide film is chromate treatment. In this case, the drawbacks of the chromate film are alleviated by the action of the magnesium compound, but methods other than chromate are more preferred. Furthermore, since hexavalent chromium, which is essential for chromate treatment, requires waste water treatment, it is preferable to use the above-mentioned coating method or cathode electrolysis method, since there are no problems with waste water treatment.

0012

[Operation] The galvanized metal material having a magnesium compound attached to its surface according to the present invention exhibits high heat resistance and corrosion resistance due to the effects of the magnesium compound attached to its surface. The reason for this is that magnesium oxide or hydrated oxide has high electrical insulation properties, prevents corrosion current from flowing when the galvanized film corrodes, and also prevents oxygen from permeating, so it is resistant to zinc corrosion. By having a protective effect. This protective effect is also possessed by conventional chromate films, and is not an effect unique to magnesium. However, 10
When compared after being heated to 0°C to 300°C, the chromate film has many cracks, and in some cases the film peels off, so the defective parts of the film become the starting point for corrosion, which accelerates after the film is formed. In contrast, when a magnesium compound is present on the galvanized surface, corrosion resistance hardly decreases even after heating, and a clear difference is observed. This is because the magnesium compounds present on the galvanized surface become anodized by corrosion, the acid generated by the corrosion reaction dissolves the magnesium compound, and the magnesium solution covers the galvanized surface again in the early stages of corrosion, resulting in formation of a protective film by heating. Magnesium compounds have the ability to repair defects.

[0013] This action provides effective protection at high temperatures because these magnesium compounds have the property of being easily soluble in acids and their acid solubility does not decrease even when heated. On the other hand, it is thought that the chromate film has almost no restorative action due to its low solubility in acids.

The reason why the galvanized material to which both magnesium and chromium compounds are attached, as described in claim 2, has better corrosion resistance than when the magnesium compound is attached alone is that the magnesium compound is close to neutral. pH
This is because it has a certain degree of solubility in water, so coexisting with a chromium compound that does not dissolve even near neutrality provides better water resistance and higher corrosion resistance than a film made only of magnesium compounds.

[0015]

[Example] As a material, a cold rolled steel plate (SPCC) plated with electrogalvanization, electrolytic zinc-nickel alloy (Ni 11%) plating, or alloyed hot-dip galvanization is used, and magnesium, chromium compounds (oxides or A test piece was prepared by forming a hydrated oxide film. The magnesium and chromium compound film was formed by a coating method and a cathode electrolysis method under the following conditions.

[0016] In Comparative Examples 1 to 4 and Examples 1 to 14, test pieces were prepared by a coating method. For coating, an aqueous coating solution containing magnesium chloride and chromium chloride was prepared so that the amounts of magnesium and chromium deposited were as shown in the table, and the coating was carried out using a bar coater on a plated steel plate so that the film was uniform. The coated test piece was heated at 250° C. for 1 hour in a heated open oven.

Comparative Examples 5 to 8 and Examples 15 to 28 are as follows:
It was produced by cathodic electrolysis. The Mg and Cr ion temperatures were varied in the range of 50 to 5000 ppmn depending on the target deposition amount, the electrolysis current density was 0.3 to 2.5 A/dm2, and the electrolysis time was 5 to 600 seconds. Nitrate ions were added to the electrolytic bath at a total concentration of 10 g/l so that Mg, Cr hydrated oxides were deposited on the plating surface. Magnesium and chromium were added in the form of nitrates, and in an electrolytic bath composition in which the amount of nitrate ions was excessive, magnesium and chromium were added in the form of chlorides, and the pH was adjusted to 3 to completely dissolve them. After electrolysis is finished, immediately wash with water and heat at 120℃ for 10 minutes.
Dry for a minute.

[0018] In addition, for evaluation of heat resistance and corrosion resistance, all the prepared test pieces were heated in the open at 300°C for 3 hours, and then subjected to a salt spray test (JIS Z2371), the time until red rust appeared was measured, and the following standards were evaluated. It was evaluated by Heat resistance and corrosion resistance evaluation rating Heat resistance and corrosion resistance Time until red rust appears 5 Very good 300 hours or more 4 Excellent 150
~299 hours 3 good
50-149 hours 2 Slightly inferior
25-49 hours 1 Poor
Plating for less than 24 hours 1 Electrogalvanizing
(20g/m2) 2 Electrolytic zinc nickel alloy plating (20
g/m2) 3 Alloyed hot-dip galvanizing
(45g/m2)

[0019]

[Table 1]

[Table 2]

[Table 3]

[Table 4]

[0020]

[Effects of the Invention] As explained above, galvanized metal materials subjected to chromate treatment, which have been most widely used in the past, cause defects such as rust when used in parts that are affected by heat, but the material of the present invention can be used as heat-resistant parts without such problems. That is, since the surface treatment film according to the present invention maintains its underlying material protection effect even at high temperatures, it provides a material with uses that cannot be realized with conventional chromate-treated galvanized materials.

Claims (2)

[Claims] Claim 1: At least one of magnesium oxide and hydrated oxide is deposited on the zinc plating film or the zinc-based plating film containing 70% or more of zinc in an amount of 10 to 5000 mg/m2 in terms of magnesium. Galvanized metal material with excellent heat resistance and corrosion resistance. 2. On the zinc plating film or the zinc-based plating film containing 70% or more of zinc, at least one of magnesium oxide and hydrated oxide is deposited in an amount of 10 to 5000 mg/m2 in terms of magnesium, and chromium A galvanized metal material having excellent heat resistance and corrosion resistance, characterized in that at least one of an oxide and a hydrous chromium oxide is deposited in an amount of 5 to 2000 mg/m2 in terms of chromium.