DE1796353B1 - FERROUS METAL ARTICLE WITH A ZINC-ALUMINUM ALLOY COATING AND PROCESS FOR ITS MANUFACTURING - Google Patents

Description

The invention relates to, and particularly relates to, zinc-aluminum coatings on iron surfaces Zinc-aluminum protective coatings on steel surfaces that are supple and corrosion-resistant and on adhere firmly to the surfaces.

To protect the surfaces of ferrous metals against corrosive media it is common to coat the iron surface with a non-ferrous metal. These coatings should not only protect, but also be supple, firmly adhering and create a smooth, have a pleasing appearance.

As a protective coating for surfaces made of ferrous metals, the metals tin, zinc, White metal, nickel, chromium, cadmium, copper, aluminum, bronze and lead are used. Most for The metal used in coatings is zinc, partly because of its relatively low cost and partly because of its high point in the series of voltages opposite the iron. In numerous cases the coatings are zinc however not sufficiently resistant to corrosion. Therefore one is constantly looking for metallic coatings, where the protection, the suppleness, the adhesion, good looks and low manufacturing costs are combined. In other words In other words, you need a metallic coating that is extremely corrosion-resistant on a ferrous surface is. In addition, it must be ensured that such coatings metallurgically to the metal Document are bound. Furthermore, the iron-bearing alloy layer at the interface between the coating and the metal should be thin and uniform, and the coating should look good and be good Have deformability. After all, it should be pliable and stuck firmly.

From IT-PS 5 65 969 it is known engine valves with an aluminum-zinc coating applied in powder form to provide, the zinc content is only about 5 wt .-%. From DT-PS 2 86 939 and GB-PS 8 64 532 is the coating of iron objects known with aluminum or aluminum or copper powder.

So far, a general prejudice existed in the production of aluminum-zinc coatings, to use more than about 12% zinc, when the coating consists essentially of aluminum, or more than a very small percentage of aluminum, for example, 2 '/ _2% use if the coating consists essentially of zinc.

The subject of the invention is the metal object characterized in claims 1 and 2 as well as that in Claim 3 characterized method.

In the method for producing the object according to the invention is thus on a ferrous Document, e.g. B. a strip, the surface of which is from Oxides and other impurities are free, a coating of 25 to 70% aluminum and zinc as the remainder applied in such a way that at the interface between the substrate and the coating a very results in a thin, uniform, iron-bearing alloy layer.

The method according to the invention is particularly useful for coating steel strips and wires. Experience has generally shown that a thick layer of alloy at the interface between the steel strip and a non- ferrous protective coating metal promotes the formation of cracks in the coating if the coated article is later deformed. In addition, a strong alloy with the clad metal results in a relatively large loss of base metal in thin iron strips. Also, a strong alloy at the interface sometimes results in an uneven coating, so that the appearance of the surface leaves much to be desired.

The surface of a steel sheet with a low carbon content is used to roll the coating

roughened by acid etching. It is provided with a thin layer of tridecyl alcohol. Then it will be the sheet metal covered with a mixture of zinc powder (75%) and aluminum powder (25%) because the alcohol is the promotes initial adhesion of the powder to the sheet metal.

The sheet metal carrying the powder is pulled through pressure rollers to force the powder onto the sheet metal compact and then held at 400 ° C for about 5 minutes to bond the coating to the panel.

The coated article according to the invention

is explained in more detail with reference to the drawings:

F i g. 1 to 4 show photomicrographs of cross-sections through coated steel strips according to FIG Invention wherein the coatings consist of about 25%, 35%, 54% and 70% aluminum with essentially zinc as Remainder exist in the individual examples. The ferrous ones coated with the zinc-aluminum alloy Metal articles of the invention contain a thin intermediate alloy layer between the ferrous one Core and the coating. The coating is characterized by zinc-rich regions (those in the coating in FIGS appear as lighter patches) and regions rich in aluminum (those in the coating in Figs. 1 to 4 as darker parts of the surface appear). Such zinc rich regions and aluminum rich regions result in a metallurgical interaction with the alloy interlayer, so that one coated metal object with superior corrosion resistance.

The photomicrograph of FIG. 1 shows in 50 times Enlargement of a cross-section through a coated article according to the invention, the The coating consists of 25% aluminum, 74% zinc and 0.77% silicon, the remainder being impurities. In this For example, there is no visible alloying of the iron with the coating metal.

In Fig. Figure 2 is a photomicrograph, magnified 500X, of another coated film of the present invention Object reproduced. The alloy layer between the coating and the metal the base is with a composition of the coating of 35% aluminum, 64% zinc and 1% Silicon so thin that it is almost indistinguishable at 500x magnification

When the aluminum content of the coating is increased beyond 35%, a small amount of material occurs as Alloy boundary layer in appearance. Even if the aluminum represents 70% of the alloy coating, the ferrous alloy is uniform and thin at the interface. This with larger amounts of aluminum Occurring thin dimension and uniformity of the interface alloy can be seen in the F i g. 3 and 4, which are enlarged 500 times, show the cross-section through a similar metal object show according to the invention. In the sample according to FIG. 3, the coated object consists in the coating from approximately 54% aluminum, 44.5% zinc and 1.5% silicon, in the sample according to FIG. 4 from approximately 70% aluminum, 28% zinc and 2% silicon.

The thin iron-bearing alloy layer is intended to denote a layer at the interface, the one average thickness of no more than about 0.00635 mm excluding local protrusions Has. However, the invention is not limited in a broader sense to an alloy layer of this thinness, because in some applications it is possible to allow a layer of greater thickness.

The term "uniform" is intended to denote an alloy layer which, in the case of a flat base, is in the is essentially flat, but which may contain localized protrusions in the zinc-aluminum coating, such as this z. B. from Fig. 3 and F i g. 4 emerges

Typical results (averages of 4 10.1 cm 15.2 cm specimens with protected Edges) are in the following table for salt spray tests on a hot-dip galvanized sheet steel and on Steel sheets specified according to the invention with zinc-aluminum coatings according to the above described procedures are provided.

Table I.

Type of coating

Time in hours

(up to 1% rust per 0.025 mm
Coating thickness)

325
2290

3234

> 3805 *)

> 3405 *)

940

1) hot-dip galvanized

2) Zinc-aluminum coating
with approximately 25% Al

3) as 2) with about 35% Al

4) as 2) with about 45% Al

5) as 2) with about 54% Al
6) as 2) with 70% Al

*) Attempt not completed.

In the so-called exhaust test, the corrosion resistance of the products according to the invention is determined with

a content of 25 to 70% aluminum in the coating is far better than that of hot-dip galvanized objects Fulfills

In the exhaust test, two coated samples measuring 5.08 cm χ 10.1 cm are immersed in an aqueous solution of 0.05 normal sulfuric acid and 0.01 normal hydrobromic acid for 8 seconds at a solution temperature of 82 ° C. At the end of the immersion time of Samples are removed for eight seconds and suspended in the fumes of the immersion solution for one hour. This procedure is repeated every hour (total of 20 χ X after which the samples in a laboratory oven for 2 hours maintained at 260 ⁰ C for at the cycle of 24 h to complete. Thereafter the samples to the beginning of the roasting examined wherein the rusting of the cut edges of the samples are not taken into account. The entire experiment lasts 13 cycles of one day each.
The results of the exhaust test are shown in Table II

Table II

45 Art of the coating Cycles by
beginning of
Roasting the
Steel base *) 1) galvanized 6th 50 2) Zinc-aluminum coating
with about 25% Al 12-13 3) like 2) with about 35% Al 13th 4) like 2) with about 45% Al 13th 55 5) like 2) with about 54% Al 13th 6) like 2) with about 70% Al 13th

*) The rusting of the cut edges of the samples is not taken into account.

The articles according to the invention, the coating of which contains 25 to 70% aluminum, meet Bending tests meet the requirements of A.S.T.M.-Norm A93-59T. Furthermore, the objects according to FIG Invention of flanging the edge flat in a corresponding standard test at all aluminum concentrations no peeling in the coating from 25 to 45%. The 70% aluminum coating shows a slight local flaking.

Furthermore, the articles according to the invention show their coating 25 to 45% aluminum contains little cracking when bent back 180 ° flat on itself.

The suppleness and the adhesion of the coatings are with the articles according to the invention with an aluminum content of 25 to 70% in such a way that this allows for the customary deformation, e.g. B. the training of beads, bending, tolerated without forming significant cracks or scales. ι ο

In summary, the ferrous objects have been coated with zinc-aluminum are, the aluminum content being 25 to 70% of the total coating, a corrosion resistance which is superior to that of the hot-dip galvanized objects. Draw within the wide area the coatings, the aluminum content of which is 25 to 40%, are characterized by their flexibility and freedom from flaking or peeling when subjected to severe deformation processes; the coatings with an aluminum content of 40 to 60% of the total coating are characterized by their remarkable resistance to corrosion, although the ductility is at the upper end of the range of the Aluminum content is lost somewhat.

Even if the coatings for the objects according to the invention are essentially made of zinc and metals Aluminum exist, so can others that do not adversely affect the basic properties There may be additives or impurities. For example, the coating can also be up to 0.3% Contain chromium to influence corrosion between grain boundaries. Such substances should not be excluded if the term "consists essentially" is previously used.

All percentages relating to the components of the coating are based on weight.

For this purpose 2 sheets of drawings

Claims (3)

Patent claims: 1. Ferrous metal article with an aluminum-zinc alloy coating consisting of 25 to 70% by weight aluminum and the remainder consists essentially of zinc and between that and that ferrous core an intermetallic intermediate layer is formed, characterized in that that the intermediate layer at the lowest value for the aluminum content in the coating is so is thin in that it is invisible when the cross-section of the metal object is enlarged 500 times remains and that silicon is only present in the coating, if at all, in amounts that are normally ι occur as an impurity. 2. Metal object according to claim 1, characterized in that the coating for influencing the intergranular corrosion contains up to 0.3% chromium. 3. Process for the manufacture of a powder coated metal article according to claim 1 or 2, according to which the object is first cleaned and roughened, then with Aluminum-zinc powder coated with the addition of an organic bonding agent and then is heated to bond the coating to the article, characterized in that the After acid etching, apply a thin layer of tridecyl alcohol to the object Powder mixture of 25 to 70% aluminum, the remainder zinc, applied and the object for compaction of the powder is passed through pressure rollers, whereupon the coated object is about Is exposed to a temperature of around 400 ° C for 5 minutes.