Protective metal coating

The invention relates to a protective metal coating adhering to a substrate - generally steel sheet, which coating comprises for the most part zinc.

The use of a zinc-containing protective coating has been known for many years. These zinc coatings are applied by hot dipping, either continuous or discontinuous, to various steel products for corrosion protection.

In order to achieve better corrosion protection as well as other advantages (for example better steel protection with slight metal loss, improved formability, weldability and paint capability), efforts have been made to develop improved zinc alloys for continuous or discontinuous application of these zinc coatings to substrates. Attempts in this direction have resulted in the development of new types of coatings, such as Zn-55 alloy, Al-1.5 Si alloy or other zinc-based alloys having low (i.e. content of Al and containing 1.5 wt. Si. The Zn-55 Al alloy coating developed by Bethlehem Steel (see, for example, U.S. Patent Nos. 3,343,930 and 3,393,089) appears to have good corrosion resistance, but due to its high aluminum content, it does not provide satisfactory protection for the steel substrate at low loss. metal.

Further efforts have been directed to adjusting the composition of the molten metal baths in order to form a (hot dipping) coating that enhances corrosion resistance in a variety of environments. One of the problems addressed by these investigations was the effect of surface treatment intended for coating on the quality of the resulting coating. It is evident from the results of these studies that, in order to obtain a high-quality coating, the surface needs to be costly pretreated using the prior art alloys to form the coating, which requires expensive equipment. This was the case, for example, with zinc coatings containing typically about 5% by weight of aluminum and the addition of other elements such as antimony, lead 4-magnesium and lead 4-magnesium 4-copper as suggested by Inland Steel (see, for example, U.S. Pat. (4,029,478, 4,056,366 and 4,152,472). There is evidence that alloys of these types exhibit a pronounced tendency to form uncovered spots and similar defects even on carefully treated surfaces.

In view of the above, there remains a need for a protective metal coating that does not exhibit exposed areas or other defects that can be produced by hot dipping the substrate into a metal bath having a composition such that it is not necessary to perform a special expensive surface treatment.

Accordingly, according to the invention, a protective metal coating adhering to the substrate and containing zinc has been prepared by hot dipping the substrate into a metal bath which is free of quality, such as uncoated. Mixtures of rare defect elements, spots, and spotted coatings represent an improvement over zi by soils.

The subject of the invention is therefore a protective metal coating adhering to the substrate, characterized in that it comprises zinc in an amount of 85 to 97% by weight, aluminum in a quantity of 3 to 15% and a mixed metal in an amount of 5 ppm to 1.0% by weight.

The preferred mixed metal content of the protective metal coating according to the invention is 0.01 to 0.1%.

The mixed metal contained in the protective metal coating of the present invention is cerium-based metal or lanthanum-based mixed metal.

The cerium-based mixed metal consists of 45-60% by weight, cerium, 50-35% by weight, other rare earths, and an iron, magnesium, aluminum, silicon, and impurity dwelling. Cerium-based mixed metal comprises 99.3% by weight, rare earth, 0.04% by weight, iron, 0.28% by weight, magnesium, 0.08% by weight, aluminum, 0.27% by weight, silicon and the rest impurities, wherein the rare earths include cerium at 52.7% by weight and other rare earths at 47.3% by weight.

The lanthanum-based mixed metal comprises lanthanum in an amount of 60 to 90% by weight, cerium in an amount of up to 8.5% by weight, neodymium in an amount of up to 6.5%, praseodymium in an amount of up to 2%, and iron. magnesium, aluminum, silicon and impurities. The lanthanum-based mixed metal comprises 98% rare earths, 0.2% iron, 0.03% magnesium, 0.18% aluminum, 0.43% silicon and the impurity residue, with rare earths containing 83% by weight of lanthanum, cerium at 8.5%, neodymium at 6.5% and praseodymium at 2%.

It has been found that uncoated areas can be avoided, as commonly found in zinc alloy coatings with a small amount of aluminum, by adding a rare earth alloy, preferably a mixed metal, to the zinc-aluminum mixture. The cause of this surprising result is that the addition of the mixed metal is likely to improve the wettability of the liquid mixture of zinc, aluminum and mixed metal when the substrate is hot soaked by reducing the surface tension of the liquid alloy. This achieves an improved, truly perfect adhesion of the coating to the substrate.

The metal baths for applying the coatings according to the invention by hot dipping and hence the coatings obtained by applying them can be considerably different, as well as different known baths based on zinc and aluminum

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6.5; Pr 2; the rest as well as any

Pr 2; Fe 0.2; The Mg residue consists of the coatings obtained therefrom. In all cases, however, it is essential that the mixed metal is added in an amount sufficient to achieve improved results as described herein. Generally, it is considered sufficient to add the mixed metal to the zinc-aluminum bath which provides a protective metal coating containing the mixed metal in an amount of from about 5 ppm to about 1.0%, preferably from about 0.01% to about 0, 1%.

As can be seen from the foregoing, the term "mixed metal" refers to a series of known rare earth alloys.

For example, two typical cerium-based mixed metals may have this weight composition in% by weight:

1) Ce 45–00; other rare earths 35-50, the remainder being Fe, Mg, Al, Si and impurities;

2) Ce 52.7; other rare earths 47.5, Fe 0.04, Mg 0.28, Al 0.08, Si 0.27; the rest is impurities.

Typical lanthanum based mixed metals can be defined by the following weight composition in% by weight:

1) La 60-90; Ce 8.5; Nd forms Fe, Mg, Al and Si, impurities;

2) La 83; Ce 8.5; Nd 6.5;

0.03; Al 0.18; Si 0.43; purity.

Thus, the term "mixed metal" as used herein refers primarily to the aforementioned mixtures, but also to other similar mixtures.

As mentioned above, the preferred alloy to which the mixed metal is to be added is a zinc-aluminum alloy containing from about 3 to about 15% aluminum. Such alloys typically contain about 5% aluminum. Although these alloys may contain components other than the mixed metal, such as Fe. Pb, Sb, Mg, Su, Cu and Si have been found that these additives, in particular Pb, Sb and Sn, generally do not improve, and in turn may deteriorate the quality of the coating formed. It is therefore preferred that the alloy be limited to substantially pure zinc, aluminum and mixed metal. In other words, the levels of antimony, lead and tin should not exceed those of the following metals as they appear in the following starting materials:

Zn - especially high quality (99,99 ° / o)

AI - commercially pure (99.9 ° / o) mixed metal - with iron content (total rare earth content 96%, Fe - 4%)

Thus, one embodiment of the invention comprises a zinc coating with a low (i.e. 3- to 15%) aluminum content containing lead or tin as well as a mixed metal. Lead and tin are known additives for plating baths to adjust the flowability of a liquid metal or shimmer of a solidified coating.

The addition of antimony to the plating bath is described in U.S. Patent No. 4,056,366 to improve the zinc and aluminum coating capability of the coating, similar to the addition of lead, but without the deleterious effect that lead has on the intergranular corrosion of the coatings formed. The invention therefore contemplates the addition of antimony to the mixed metal compositions of the invention. In addition, a zinc-aluminum coating comprising lead together with antimony is within the scope of the invention. A typical coating according to the invention may comprise 3-15% aluminum, 0.03-0.15% antimony, at most 0.02% lead and the remainder of zinc to which a mixed metal has been added.

Zinc-aluminum alloys containing lead as well as magnesium and copper are said not to be subject to intergranular corrosion. It has been found that in this type of coating alloys, the addition of mixed metal has a beneficial effect on the integrity and uniformity of the coating. Zinc-aluminum based alloys containing magnesium, lead, copper and mixed metal are therefore within the scope of the invention. Typically, an alloy of this type may contain 3 to 15% aluminum, 0.02 to 0.15% magnesium, 0.02 to 0.15 why, lead and optionally 0.1 to 0.3% copper, the remainder being zinc added of mixed metal.

According to the invention, various mixed metals, including mixed metal mixtures, can be advantageously used in a single zinc coating. For example, a lanthanum mixed metal and a cerium-based mixed metal may be added simultaneously, preferably in an amount such that the total weight of the mixed metal is in the range described above, i.e. from about 5 ppm to about 1.0%, preferably from about 0.01 to about 0.1%.

To facilitate the addition of the mixed metal to the plating bath, a master alloy can first be prepared and then added to the zinc bath in an amount to achieve the desired mixed metal concentration. Such master alloys may consist of 20% zinc and 80% mixed metal or 85-95% aluminum and 15-5% mixed metal.

The following examples illustrate the invention.

Example 1

Samples of non-restrained 68 x 120 x 0.7 mm steel sheet are galvanized in a device simulating a continuous galvanizing bath. They are first preheated in an atmosphere containing 95% N₃ and 5% H₂, at various temperatures ranging from 750 to 800 ° C for 1 to 10 minutes. After this preheating stage, the samples are removed from the hot zone of the furnace, cooled to approximately 430 ° C, and then introduced into the molten zinc 261856 alloy maintained at 430 ° C and protected by a 95% N atmosphere.₂ 5% H₂. The samples remain in the zinc bath for 5 to 60 seconds before being removed from the bath and cooled with a 95% N gas stream.₂ and 5% H₂.

Such experiments are carried out using baths of different composition. The galvanized samples are inspected to determine the integrity of the coating, particularly as regards the appearance of bare spots and uncoated areas.

When using baths containing in wt. 5 to 8% of aluminum without any other additives have a high proportion of uncoated areas and bare spots on the samples. This is the case for samples that have been preheated at the highest temperature indicated and have been left in the reducing atmosphere for the longest. Addition of 0.15% antimony to the 5% aluminum zinc bath results in a reduction in the number of bare spots, yet up to 33% of the galvanized surface has bare spots.

By using an additional, third bath containing 5% aluminum and 0.02% cerium added as a cerium-based mixed metal, 100% good coatings are achieved under various heat treatment conditions.

By using a zinc bath containing 5% aluminum, 0.03% lanthanum and 0.025% cerium, added as mixed metals based on both lanthanum and cerium, 100% good coatings are obtained even at preheating temperatures as low as 750 ° C .

Example 2

This example relates to experiments carried out in a pilot plant continuously operating annealing and galvanizing plant. In these experiments, coils of untreated steel sheet having a coil weight of 800 kg and a strip width of 150 mm and a thickness of 0.25 mm are first heated in a Selas-type furnace at temperatures ranging from 680 ° C to 860 ° C. The sheet is then cooled in a controlled atmosphere to a temperature of approximately 430 [deg.] C. and then introduced into a seven-ton galvanizing bath. Upon leaving the bath, the excess molten metal is wiped off the sheet with a stream of nitrogen gas, then the sheet is cooled with a stream of gas and then rolled into a roll. Depending on the test conditions, the gradual speed of the sheet metal varies and is 10 to 30 m. min^-1.

Several coils of sheet metal are galvanized in a zinc bath containing 5% aluminum and 0.05 to 0.001% of cerium-based mixed metal. The cerium content ranges from 0.04 why, to 0.0008%, and the lanthanum content from 0.02 why, to 0.0002%. A bright, light coating having a grain size in the range of 1 to 5 µm, depending on the cooling conditions and a thickness in the range of 5 to 35 µm, is obtained depending on the conditions in which the excess metal is wiped off with a gas stream. The coating is uniform and does not contain bare spots, uncovered areas, or any other defects.

Further, a zinc bath containing 5% aluminum, 0.13% tin and as above 0.05% cerium-based mixed metal is used in said pilot galvanizing plant. The resulting coatings have properties similar to those described above, the coatings being somewhat less glossy due to different shimmering. In a further experiment, a bath containing zinc, 5% aluminum, 0.13% tin, 0.05% lead and about 0.05% cerium + lanthanum (added as cerium-based mixed metal or mixed metal) was used in the pilot plant. lanthanum-based metal, optionally added as a master alloy containing about 20% zinc and 80% lanthanum-based and / or cerium-based mixed metals, or added as master-alloy containing about 90 ° / o aluminum and 10 ° / o lanthanum base metal and / or cerium). The resulting coatings have a wide range of thickness, are homogeneous and also do not contain bare spots and uncovered areas.

It is clear that the conditions in said pilot plant are given by way of example only, and that other conditions such as those used in continuously operating annealing and galvanizing plants, such as the type of furnace, gas composition, velocity, , methods of wiping off excess metal, etc. Moreover, the bath and coating composition as described above can also be used in batchwise working (i.e. batch) galvanizing methods.

Example 3

Samples of the galvanized sheet in the pilot plant described in Example 2 were subjected to various tests to evaluate formability and adhesion, corrosion resistance in various environments, galvanic protection and microstructure.

The formability and the adhesion are evaluated by the buckling test and the Erichsen test. In both of these tests, the coatings obtained in the bath containing the mixed metal exhibit adhesion and formability that are equivalent to both of these properties for standard zinc coatings. For example, there is no cracking at 180 ° bend and, in the Erichsen test, a 9 mm depression is formed in the 0.25 mm sheet without peeling off the coating.

The corrosion resistance of aluminum-containing zinc coatings is more than twice that of a standard zinc coating of the same thickness as determined by a test in which the sheet samples are sprayed with brine. For example, in the coatings of the present invention, the time to first rust appearance is approximately 900 hours, as opposed to 350 hours in conventional zinc coatings of the same thickness.

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Similarly, the corrosion resistance in an environment containing 10 ppm sulfur dioxide is at least 50% greater than that of a conventional zinc coating. In addition, the galvanic protection afforded by the zinc coating containing aluminum and mixed metal was determined according to a corrosion attack procedure around scratches made by mechanical scratching on samples exposed to a sulfur dioxide containing environment. The galvanic protection afforded by the zinc coating containing 5% aluminum and the mixed metal is the same as that provided by the pure zinc coating and much better than the protection provided by the zinc coating containing 55% Al and 1.5% Si.