TWI527932B - Corrosion protection with al/zn-based coatings - Google Patents

Description

Aluminium/zinc based coating for corrosion protection Field of invention

The present invention generally relates to the production of a product having an alloy coating containing aluminum and zinc as a main component of an alloy (hereinafter referred to as "a product coated with an Al/Zn-based alloy"). .

The term "a product coated with an Al/Zn-based alloy" is understood herein to include, as an example, a product in the form of a ribbon, a tube, and a structural segment having Al/Zn as The coating of the master alloy is on at least a portion of the surface of the products.

Although not exclusively exclusive, the present invention is more particularly directed to Al/Zn in the form of a metal, such as steel, ribbon, and having an Al/Zn based alloy coating on at least one surface of the ribbon. A product coated with a predominantly alloy and a product made from a ribbon coated with an Al/Zn based alloy.

The metal ribbon coated with the Al/Zn based alloy may be a ribbon coated with inorganic and/or organic compounds for protection, aesthetics or other reasons.

Although not exclusively exclusive, the present invention is more particularly directed to a steel strip coated with an Al/Zn-based alloy having a coating having an alloy other than Al and Zn. There is more than one element, such as Mg and Si.

Although not exclusively excluded, the present invention more particularly relates to a steel strip coated with an Al/Zn-based alloy having an Al/Zn-based alloy containing Mg and Si. 20-95% Al, up to 5% Si, up to 10% Mg, and the remaining Zn and minor amounts of other elements are typically less than 0.5% for each other element, with all percentages being by weight. It is to be noted that, unless otherwise explicitly mentioned, all percentages of elements discussed are in this specification and percentages by weight.

Background of the invention

Thin (i.e., 2-100 μm thick) Al/Zn based alloy coatings are typically formed on several surfaces of the steel strip to provide protection against corrosion.

The Al/Zn-based alloy coating is generally, but not exclusive, the alloy elements Al and Zn and Mg, Si, Fe, Mn, Ni, Sn and a small amount such as V, Sr. a coating of one or more of the other elements of Ca, Sb.

The Al/Zn based alloy coating is generally, but not exclusive, formed on the steel strip by hot dip coating the ribbon through a bath of molten alloy. The steel ribbon is typically, but not necessarily exclusive, heated prior to soaking to promote alloy bonding to the ribbon. The alloy then cures to the ribbon and forms a cured alloy coating as the ribbon emerges from the molten bath.

The Al/Zn based alloy coating typically has a microstructure consisting essentially of a mixture of Al-rich alpha phases in the form of dendrites and Zn-rich eutectic phases located between the dendrimers. When the rate of cure of the molten coating is suitably controlled (for example, as described in US Pat. No. 3,782,909, incorporated herein by reference), the Al-rich alpha phase solidifies into a sufficiently fine dendritic body to allow It defines a continuous network of channels in the inter-branched region, and the Zn-rich eutectic phase mixture solidifies in the inter-branched region.

The properties of these coatings depend on the following combinations: (a) sacrificial protection of the steel substrate, initially by a Zn-rich inter-eutectic eutectic mixture, and (b) barrier protection, supported by a rich Al phase Branches. The Zn-rich inter-branched interphase mixture is preferentially corroded to provide sacrificial protection against the steel substrate, and in some environments, once the Zn-rich inter-branched phase mixture has been depleted, the Al-rich alpha phase is also Will continue to provide proper level of sacrificial protection and barrier protection for the steel substrate.

However, there are many cases where the barrier protection and sacrificial protection levels imparted by the Al-rich alpha vine are not sufficient, and the properties of the coated steel ribbon may be impaired. Three such areas are as follows.

1. In “acid rain” or “contaminated” environments containing high concentrations of nitrogen oxides and sulfur oxides.

2. Under the paint film in the marine environment.

3. At areas where the cutting edge or other metallic coating has been damaged to expose the steel substrate to the marine environment.

As an example, Applicants have discovered that Al/Zn based alloy coatings on steel strips are particularly thin (i.e., coatings have a total coating mass of coated per square meter). Less than 200 grams, typically less than 150 grams; which is equal to less than 100 grams per square meter of coating on each surface of the steel strip when there are equal coating thicknesses on both surfaces, typically less than 75 g), the microstructure tends to be more columnar or bamboo extending from the steel strip to the surface of the coating when the coating is formed at a standard cooling rate typically from 11 ° C/s to 100 ° C/s. structure. The microstructure comprises (a) Al-rich alpha-phase vines and (b) a Zn-rich eutectic phase mixture formed as a series of discrete columnar channels extending directly from the steel ribbon to the surface of the coating.

Applicants have also discovered that steel ribbons having such thin Al/Zn based alloy coatings with columnar microstructures are exposed to low pH environments commonly described as "acid rain" environments, or exposed The Zn-rich inter-branched eutectic phase mixture is rapidly eroded and extends directly from the steel strip to the environment of high concentrations of sulfur dioxide and nitrogen oxides, which are generally described as "contaminated" environments. The columnar passage of this phase mixture of the surface of the coating acts as a direct corrosion path to the steel strip. Where there is such a direct corrosion path from the surface of the coating to the steel strip, the steel strip may be corroded and the corrosion products (iron oxides) will migrate freely to the surface of the coating and develop One is known as the "red rust stain" appearance. Red rust stains degrade the aesthetic appearance of coated steel products and reduce product performance. For example, red rust stains reduce the thermal efficiency of coated steel products used as roofing materials.

Applicants have also discovered that in areas where thin Al/Zn-based coatings are scratched, cracked or otherwise damaged to reveal steel strips and exposed to "acid rain" or "pollution" environments, red Rust stains may even appear in the absence of columnar or bamboo structures.

It is also known that in an "acid rain" environment or a "contaminated" environment, the alpha phase rich in Al cannot be protected from sacrificial steel strips.

An "acid rain" environment is understood herein to mean that rain and/or agglomerates formed on the coated steel ribbon have an environment having a pH of less than 5.6. As an example, a “contaminated environment” would be typically, but not exclusively, defined as a P2 or P3 category in ISO 9223.

Also by way of example, in a marine environment where Al-rich alpha-phase vines are conventionally considered to provide good sacrificial protection against steel substrates, this ability is applied to the metallic coated steel ribbon. The change in the microenvironment under the paint film is weakened.

The above description is not intended to be recognized as a general general knowledge of Australia or elsewhere.

Summary of invention

Applicants have discovered that red rust stains in "acid rain" or "contaminated" environments of steel strips coated with Al/Zn based alloys can be formed into Al-Zn-Si- by coating The Mg alloy coating ensures and minimizes the OT:SDAS ratio of the coating to a value greater than 0.5:1, where OT is the coating thickness on the surface of the ribbon and SDAS is rich in Al in the coating A measure of the spacing of the secondary branches of the alpha vines.

The term "coverage thickness" is understood herein to mean the total thickness of the coating on the ribbon minus the thickness of the coating intermetallic alloy layer, wherein the intermetallic alloy layer is applied to the ribbon as a coating. The Al-Fe-Si-Zn quaternary metal phase layer immediately adjacent to the steel substrate is formed by the reaction between the molten coating and the steel substrate.

According to the present invention, there is provided a corrosion-resistant Al-Zn-Si-Mg alloy for forming an "acid rain" or "contaminated" environment suitable for use as an exemplary metal strip of steel. A method of coating comprising:

(a) passing the metal strip through a molten bath of Al-Zn-Si-Mg alloy and forming an alloyed coating on one or both surfaces of the strip,

(b) curing the coating on the ribbon to form a cured coating having a microstructure comprising a dendritic body having an alpha phase rich in Al and a eutectic rich in Zn An inter-branched channel of the phase mixture extending from the metal ribbon and having Mg ₂ Si phase particles in the inter-branched inter-body channels, and the method comprising controlling steps (a) and (b) and forming OT:SDAS A cured coating having a ratio greater than 0.5:1, wherein the OT system is the cover thickness, and the SDAS is the secondary dendritic arm spacing of the Al-rich alpha-branched body of the coating.

The term "Zn-rich eutectic phase mixture" is understood herein to mean a mixture of eutectic reaction products, and the mixture contains a Zn-rich β phase and a Mg:Zn compound phase, for example, MgZn ₂ .

According to the present invention, there is also provided a metal ribbon having Al-Zn-Si-Mg alloy on one or both surfaces of which is suitable for use as an exemplary "acid rain" or "contaminated" environment. a coating comprising a microstructure comprising a dendritic body having an alpha phase rich in Al and an inter-branched intervening channel having a mixture of Zn-rich eutectic phases extending from the metal ribbon and having Mg ₂ Si phase particles are in the inter-branched inter-body channels, and the coating has an OT:SDAS ratio of more than 0.5:1, wherein the OT system is a cover thickness, and the SDAS system is a coating-rich Al-rich alpha-branched body The secondary branches are separated by arms.

It is noted that where the coating is on both surfaces of the ribbon, the thickness of the cover on each surface may vary or be the same as desired by the coated ribbon. In any event, the present invention requires that the OT:SDAS ratio of the coating on each of the two surfaces is greater than 0.5:1.

The OT:SDAS ratio can be greater than 1:1.

The OT:SDAS ratio can be greater than 2:1.

The coating can be a thin coating.

In this context, a "thin" coating on a metal, such as steel, ribbon is understood herein to mean having a total coating mass on both surfaces of the ribbon that is coated per square meter. A coating of less than 200 grams is equal to less than 100 grams per square meter of coating on one surface of the steel strip, although instances may not always be the case.

The thickness of the coating can be greater than 3 μm.

The coating may have a cover thickness of less than 20 μm.

The coating may have a cover thickness of less than 30 μm.

The coating may have a cover thickness of 5-20 μm.

The Al-Zn-Si-Mg alloy may contain 20-95% Al, up to 5% Si, up to 10% Mg, and the remaining Zn and a small amount of other elements, typically less than 0.5% for each of the other elements. .

The Al-Zn-Si-Mg alloy may contain 40-65% of Al.

The Al-Zn-Si-Mg alloy may contain 45-60% of Al.

The Al-Zn-Si-Mg alloy may contain 35-50% Zn.

The Al-Zn-Si-Mg alloy may contain 39-48% of Zn.

The Al-Zn-Si-Mg alloy may contain 1-3% Si.

The Al-Zn-Si-Mg alloy may contain 1.3 to 2.5% of Si.

The Al-Zn-Si-Mg alloy may contain less than 5% of Mg.

The Al-Zn-Si-Mg alloy may contain less than 3% of Mg.

The Al-Zn-Si-Mg alloy may contain not only 1% of Mg.

The Al-Zn-Si-Mg alloy may contain 1.2 to 2.8% of Mg.

The Al-Zn-Si-Mg alloy may contain 1.5 to 2.5% of Mg.

The Al-Zn-Si-Mg alloy may contain 1.7-2.3% of Mg.

The metal strip can be a steel strip.

In addition or in the above-mentioned OT:SDAS ratio is not maintained and the coating has an OT:SDAS ratio of less than 0.5:1, the applicant has also found red rust stains in the "acid rain" or "contaminated" environment and Corrosion at the cutting edge in a marine environment can be thin Al-Zn-Si on the steel strip by the choice of coating alloy composition (primarily Mg and Si) and the control of the microstructure of the coating -Mg alloy coatings are prevented or minimized.

The above composition selection and microstructure control system are particularly useful for thin coatings and/or coatings having an OT:SDAS ratio of less than 0.5:1, but are not limited to these coatings, but are also suitable for thick coating. A coating of coating and/or OT: SDAS ratio greater than 0.5:1.

Applicants have also found that corrosion in the edge of coated steel strips in the marine environment and red rust stains in "acid rain" or "contaminated" environments can be found in sensitive Al/Zn-based coatings. Eliminated or minimized by:

1. Block corrosion along the Zn-rich inter-branched channel to the steel strip, and/or

2. The Al-rich alpha phase is active in these environments so that it can be sacrificed to protect the steel ribbon.

In general, in both instances, in accordance with the present invention, a metal strip is provided having one or both surfaces having an Al that is suitable for use as an exemplary "acid rain" or "contaminated" environment. a coating of a Zn-Si-Mg alloy, the coating comprising a microstructure comprising a dendritic body having an alpha phase rich in Al and an inter-body channel extending from a mixture of Zn-rich eutectic phases a metal ribbon with Mg ₂ Si phase particles in the inter-branched channels.

The term "particle" as used herein in the context of the Mg ₂ Si phase is understood to mean the physical form of the phase precipitate in the microstructure. It is to be understood herein that the "particles" are formed from the solution via precipitation during the curing of the coating and are not specifically added to the composition.

Block

According to the present invention, there is provided a corrosion-resistant Al-Zn-Si-Mg alloy for forming an "acid rain" or "contaminated" environment suitable for use as an exemplary metal strip of steel. A method of coating comprising:

(a) passing the metal strip through a molten bath of Al-Zn-Si-Mg alloy and forming an alloyed coating on one or both surfaces of the strip,

(b) curing the coating on the ribbon to form a cured coating having a microstructure comprising a dendritic body having an alpha phase rich in Al and a eutectic rich in Zn An inter-branched channel of the phase mixture extending from the metal ribbon and having a Mg ₂ Si phase in the inter-branched channel, and the method comprises selecting a concentration of Mg and Si and controlling a cooling rate in step (b) Mg ₂ Si phase particles that block corrosion along the inter-branched channels are formed in the inter-branched channels in the cured coating.

As an explanation, in the Al/Zn-based coating with a dendritic structure, Si is present in the form of flaky particles, and although it is not corroded, it is not filled with inter-plant channels. And blocking the inter-branched inter-body channel from the inter-corrosion of the branches of the steel strip. Applicants have discovered that Mg added to the Al/Zn-based coating containing Si can be combined with Si to form the appropriate size and morphology in the inter-branched inter-channel between the Al-rich alpha-branched arms. The Mg ₂ Si phase particles, which may otherwise be a direct corrosion path to the steel strip, are blocked and help to isolate the underlying steel substrate cathode. Particles of the appropriate size and morphology are formed by controlling the cure of the coating, i.e., the rate of cooling.

In particular, Applicants have discovered that the cooling rate CR during curing of the coating should be maintained at less than 170-4.5 CT, where CR is the cooling rate in ° C/sec and CT is the ribbon in microns. The thickness of the coating on the surface.

The morphology of properly sized Mg ₂ Si phase particles can be described as "Chinese character" when viewed in a planar image, and as a petal form when viewed in a 3D image. The morphology as an example is shown in Figures 12 and 13, and is further discussed below.

The lobes of the Mg ₂ Si particles may have a thickness of less than 8 μm.

The lobes of the Mg ₂ Si phase particles may have a thickness of less than 5 μm.

The lobes of the Mg ₂ Si phase particles may have a thickness in the range of 0.5 to 2.5 μm.

The Mg concentration can be selected to be greater than 0.5%. Below this concentration, the Mg ₂ Si phase particles will not be sufficient to fill and block the intervening channels.

The Mg concentration can be selected to be less than 3%. Above this concentration, large Mg ₂ Si particles having a square shape which is ineffective in blocking inter-canal corrosion are formed.

In particular, the Al-Zn-Si-Mg alloy may contain not only 1% of Mg.

In the case of a coating having a Si concentration of from 0.5 to 2%, the volume fraction of the Mg ₂ Si phase between the branches may be greater than 50% compared to the other Si-containing phases.

The volume fraction of the Mg ₂ Si phase between the branches can be greater than 80% compared to other Si-containing phases.

The ratio of the Mg ₂ Si phase between the branches of the coating in the lower two-thirds of the thickness of the coating can be greater than 70% of the total integral of the Mg ₂ Si phase in the coating in order to provide a good barrier between the inter-body channels. .

The proportion of inter-plant channels that are "blocked" by the Mg ₂ Si phase can be greater than 60% of the total number of channels, typically greater than 70%.

Applicants have also discovered that the application of the possible improved protection of the present invention spans a range of microstructures ranging from OT:SDAS ratio of 0.5:1 dendritic structure to OT:SDAS ratio of 6:1 twig structure .

Corrosion along these pathways and red rust stains via these pathways, particularly in "acid rain" or "contaminated" environments, are therefore retarded.

In Al/Zn alloy coatings, corrosion along the inter-branched channels can also be limited by reducing the channel size. Reducing the channel size is the result of increasing the cooling rate during curing and reducing the SDAS of the coating, such as US. Patent 3,782,909. However, while this may slow the surface corrosion of the coating (as often measured by mass loss testing), it limits the availability of the zinc-rich phase mixture to provide sacrificial protection to the steel substrate. Thus, corrosion of the steel substrate is more likely to occur.

2. Activation of alpha phase

According to the present invention, there is provided a corrosion-resistant Al-Zn-Si-Mg alloy for forming an "acid rain" or "contaminated" environment suitable for use as an exemplary metal strip of steel. A method of coating comprising:

(a) passing the metal strip through a molten bath of Al-Zn-Si-Mg alloy and forming an alloyed coating on one or both surfaces of the strip,

(b) curing the coating on the ribbon to form a cured coating having a microstructure comprising a dendritic body having an alpha phase rich in Al and a eutectic rich in Zn An inter-branched channel of the phase mixture extending from the metal ribbon and having a Mg ₂ Si phase in the inter-branched channel, and the method comprises selecting a concentration of Mg and Si and controlling a cooling rate in step (b) Mg ₂ Si phase particles having a size range, morphology, and spatial distribution that activates the alpha phase rich in Al to provide sacrificial protection are formed in the interbranched channels in the cured coating.

In particular, Applicants have discovered that the Mg ₂ Si phase itself is reactive and can be susceptible to corrosion. However, Applicants have also discovered conditions for passivating the Mg ₂ Si phase, contributing to channel blockage, and promoting and enhancing the activation of the Al-rich alpha phase in the sacrificial protection of the steel ribbon.

In particular, Applicants have discovered that the addition of a suitable Mg and Si concentration to an Al/Zn based alloy coating composition and the selection of a cooling rate to cure the coating with an alloy composition on the steel strip results in The formation of the Mg ₂ Si phase is suitably dispersed and positioned in the inter-branched inter-body channels to activate the Al-rich alpha phase to provide sacrificial protection for certain marine and "acid rain" and "contaminated" environments.

The activation of the alpha phase rich in Al contributes to the application of the finer vine structure without the loss of sacrificial protection at the cutting edge or other areas where the steel substrate has been exposed.

The choice of Mg and Si concentration and cooling rate is consistent with the description of these parameters under the "Blocking" heading.

In particular, in terms of cooling rate, Applicants have discovered that the cooling rate CR during curing of the coating should be maintained at less than 170-4.5 CT, where CR is the cooling rate in ° C/sec, while the CT system is The thickness of the coating on the surface of the ribbon expressed in microns.

As far as the composition is concerned, as an example, in an "acid rain" or "contaminated" environment and an "acidic" microenvironment, the Mg concentration may be greater than 0.5% for the formation of Mg ₂ Si.

The Mg concentration can be greater than 1% to ensure efficient activation of the alpha phase.

The Mg concentration can be less than 3%. At higher concentrations, a coarse, widely dispersed primary Mg ₂ Si phase will form, but will not provide a uniformly activated Al-rich alpha phase.

In particular, the Al-Zn-Si-Mg alloy may contain not only 1% of Mg.

Applicants have also discovered that the application of the possible improved sacrificial protection of the present invention spans a range of microstructures ranging from OT:SDAS ratio of 0.5:1 dendritic structure to OT:SDAS ratio of 6:1 twig. structure.

Applicants have also discovered that ribbons coated with Al-Zn-Si-Mg alloys made in accordance with the present invention and subsequently painted exhibit a development of a narrower and more uniform corrosion front as Al-rich alpha phase activation and oceans. The result of a marginal overcut in the environment.

Samples made in accordance with the present invention exhibited a reduced rate of "edge creep" or "overcut" from the cut edge as compared to conventional Al/Zn coatings in the experimental work performed by the Applicant.

The improved properties have been shown to be suitable for a range of coating structures and a range of paint films.

Simple illustration

The present invention is further described with reference to the accompanying drawings, wherein: Figure 1 is an edge overcut of an example of an Al-Zn-Si-Mg alloy coating according to the present invention on a test sample in a marine environment. Graph of Mg concentration; Figures 2 to 4 are test plate photographs and corrosion front images, which verify the improved version of the Al-Zn-Si-Mg alloy coating according to the present invention in the marine environment. Performance; Figure 5 is an experimentally accelerated test plate photograph showing improved surface weathering and improved sacrificial protection of the metallic coated steel strip according to the present invention; Figures 6 through 11 are Photographs of test panels, which verify the improved performance of an example of an Al-Zn-Si-Mg alloy coating on a steel strip in accordance with the present invention in an "acid rain" or "contaminated"environment; 12 is a plan view of a scanning electron microscope image of an Al-Zn-Si-Mg alloy coating according to the present invention, which illustrates the morphology of Mg ₂ Si phase particles in the microstructures appearing in the image; and FIG. Network 3D image of the morphology of Mg ₂ Si phase particles in the Al-Zn-Si-Mg alloy coating of Fig. 12 .

The improved corrosion performance of the example of the Al-Zn-Si-Mg alloy coated steel strip according to the present invention has been verified by the applicant for exposure to a range of actual "acid rain", "contaminated" and marine environments. On the test sample of the site.

The test samples include test panels developed by the applicant to provide coating corrosion information.

Figures 1 to 5 and Tables 1 and 2 verify the improved performance of an example of an Al-Zn-Si-Mg alloy coating produced on a steel strip produced in accordance with the present invention in a marine environment.

Performance in the marine environment is assessed by the outdoor exposure test at the address of the ISO rating of C2 to C5 in Appendix B of AS/NZS 1580.457.1.1996 and by the experimental cyclic corrosion test (CCT).

The data presented in Table 1 reveals an example of an Al-Zn-Si-Mg coated steel test panel for a range of metallic coating qualities in accordance with the present invention. The impact in harsh marine environments is exposed to the edge of the paint. Improved performance over the cut level (unit: mm). The table also includes comparative data for traditional Al/Zn based alloy coated test panels.

It is apparent from Table 1 that the Al-Zn-Si-Mg coated steel test panel according to the present invention has significantly less edge overcut than the conventional Al/Zn based alloy coated test panel. .

Further information presented in Table 2 reveals an example of an Al-Zn-Si-Mg coated painted steel test panel for a range of paint types in accordance with the present invention for exposure to overcutting in harsh marine environments. Improved performance at the level (unit: mm). The table also includes comparative data for traditional Al/Zn based alloy coated test panels.

Obviously from Table 2, the Al-Zn-Si-Mg coated painted steel test panel according to the present invention is significantly more than the conventional Al/Zn based alloy coated paint test panel. Less edge overcutting.

The test plate prints and corrosion front image in Figures 2 through 4 further illustrate the improved performance of an example of an Al-Zn-Si-Mg coating in accordance with the present invention in a marine environment. Figure 2 shows the improved corrosion performance of the fluorocarbon-coated Al-Zn-Si-Mg coating according to the present invention for unimpacted exposure in harsh marine environments. Figure 3 is an example of a conventional Al/Zn coating having a large corrosion front under paint in a marine environment. Figure 4 is an example of a narrower and more uniform corrosion front of an Al-Zn-Si-Mg coating according to the present invention when painted in a marine environment.

The test panel photograph in Figure 5 demonstrates the improved corrosion performance of an example of an Al-Zn-Si-Mg coating according to the present invention under accelerated test conditions. In particular, Figure 5 shows that the Al-Zn-Si-Mg coating according to the present invention has a coarse or fine structure in salt spray cycle corrosion and improved compared to conventional Al/Zn coatings. Surface weathering and improved sacrificial protection.

Figures 6 through 11 verify the improved performance of the Al-Zn-Si-Mg coated steel test panels in an "acid rain" or "contaminated" environment when produced in accordance with the present invention. The photographs show red rust stains on conventional Al/Zn based alloy coated steel test panels, while there is no red rust on Al-Zn-Si-Mg coated steel test panels made in accordance with the present invention. Stains. A comparison of Figure 9 with Figure 7 shows that interest is retained over time. In particular, Figure 6 shows a conventional Al/Zn-based coated steel strip exposed to a harsh "acid rain" environment for 6 months (total coating mass per square meter of coating system) 100 grams) with red rust stains. Figure 7 shows an Al-Zn-Si-Mg coating according to the invention exposed to a harsh "acid rain" environment for 6 months (total coating mass is 100 grams per square meter of coating system) There are no red rust stains on it. Figure 8 shows a conventional Al/Zn-based coated steel strip exposed to a harsh "acid rain" environment for 18 months (total coating mass per 100 m of coating system) There are red rust stains on it. Figure 9 shows an Al-Zn-Si-Mg coating according to the invention exposed to a harsh "acid rain" environment for 18 months (total coating mass is 100 grams per square meter of coating system) There are no red rust stains on it. Figure 10 shows a conventional Al/Zn-coated steel ribbon with a columnar structure exposed to a harsh "acid rain" environment for 4 months (total coating mass per square meter) There are red rust stains on the system 50 grams). Figure 11 shows an Al-Zn-Si-Mg coating according to the invention having a columnar structure exposed to a harsh "acid rain" environment for 4 months (the total coating mass is coated per square meter) There were no red rust stains on the system 50 grams).

Finally, Applicants have found in the microstructure analysis of an example of an Al-Zn-Si-Mg coating according to the present invention that the microstructure comprises a Mg ₂ Si phase particle having a special morphology in an alpha phase rich in Al. The inter-branched channels between the vines with a mixture of Zn-rich eutectic phases, and this morphology is important in improving the corrosion resistance of the coating, as discussed above. Applicants have discovered that the size and distribution of the Mg ₂ Si phase particles are also important factors in promoting the improved corrosion performance of the Al-Zn-Si-Mg coatings according to the present invention. Applicants have also discovered that a satisfactory morphology, size and distribution of Mg ₂ Si phase particles is possible by selecting the coating composition and controlling the cooling rate during curing of the coating.

Examples 12 and 13 illustrate one example of the morphology of the Mg ₂ Si phase particles discussed above.

In the plane image of Fig. 12, the darker region is Al-rich phalanx, and the bright region is the inter-body channel with a mixture of Zn-rich eutectic phases, and some of the channels filled with such channels. Word pattern "Mg ₂ Si phase particles.

In the 3D image of Fig. 13, the Mg ₂ Si "valve" is shown in red color, while the other phases include: Si (green), MgZn ₂ (blue), and Al-rich alpha phase (dark substrate).

Many modifications may be made to the invention described above without departing from the spirit and scope of the invention.

Figure 1 is a reduction in exposure to edge overcut levels in a severe marine environment in accordance with the coated metallic coated steel strip of the present invention.

Figure 2 is an improved corrosion performance of a fluorocarbon coated metallic coated steel strip in accordance with the present invention for impact exposure in severe marine environments.

Figure 3 is an example of a conventional Al/Zn coating having a large corrosion front under paint in a marine environment.

Figure 4 is an illustration of a narrow and uniform corrosion front of a metallic coated steel strip in accordance with the present invention in a marine environment.

Figure 5 is an Al/Zn coating with a very fine dendritic structure. Compared to the conventional structure (B vs A), there is improved surface weathering in the salt spray test but the level of protection is reduced. The metal coated steel strip according to the present invention has improved surface weathering and improved type in the salt spray test compared to the Al/Zn coating (C and D vs A and B) having coarse or fine structure. Admitted to protection.

Figure 6 is a conventional Al/Zn-coated steel strip (total coating mass per 100 m of coated system 100 g) exposed to a harsh "acid rain" environment for 6 months. There are red rust stains.

Figure 7 is an Al/Zn metallic coated steel strip according to the present invention exposed to a harsh "acid rain" environment for 6 months (total coating quality is 100 grams per square meter of coating system) There are no red rust stains on it.

Figure 8 shows a conventional Al/Zn-coated steel ribbon (total coating quality of 100 grams per square meter of coating) exposed to a harsh "acid rain" environment for 18 months. Red rust stains.

Figure 9 is an Al/Zn metallic coated steel strip according to the present invention exposed to a harsh "acid rain" environment for 18 months (total coating quality is 100 grams per square meter of coating system) There are no red rust stains on it.

Figure 10 is a conventional Al/Zn-coated steel ribbon with a columnar structure exposed to a harsh "acid rain" environment for 4 months (total coating quality per square meter of coating system) 50 grams) with red rust stains.

Figure 11 is an Al/Zn metallic coated steel strip according to the present invention having a columnar structure exposed to a harsh "acid rain" environment for 4 months (total coating quality per square meter) There were no red rust stains on the system 50 grams).

Figure 12 is a SEM plan view.

Figure 13 is a network 3D configuration of Mg ₂ Si particles (in red) patterned by successive EPMA.

Claims (30)

A method for forming a coating of a corrosion-resistant Al-Zn-Si-Mg alloy on a metal strip, comprising: (a) passing a metal ribbon through an Al-Zn-Si-Mg alloy a molten bath and forming an alloy coating on one or both surfaces of the ribbon, wherein the alloy contains 40-65% Al, 35-50% Zn, 1-3% Si, greater than 0 And less than 3% of Mg, and a small amount of other elements, the amount of each other element is less than 0.5%, wherein all percentages are percentage by weight, (b) curing the coating on the ribbon to form a cured coating having a microstructure comprising a dendritic body having an alpha phase rich in Al and an intervening channel having a mixture of Zn-rich eutectic phases extending from the metal ribbon And having Mg ₂ Si phase particles in the inter-branched inter-body channels, and the method comprises controlling steps (a) and (b) and forming a cured coating having an OT:SDAS ratio greater than 0.5:1, wherein OT The thickness is the cover thickness, and the SDAS is the secondary branching arm spacing of the Al-rich alpha-branched body of the coating. The method of claim 1, wherein the OT: SDAS ratio is greater than 1:1. A metal ribbon having a coating of an Al-Zn-Si-Mg alloy on one or both surfaces of the ribbon, wherein the alloy contains 40-65% Al, 35-50% Zn, 1-3% of Si, greater than 0 and less than 3% of Mg, and a small amount of other elements, each of which is less than 0.5% by weight, wherein all percentages are by weight, and the coating comprises one a microstructure comprising a dendritic body having an alpha phase rich in Al and a branching intervening channel having a Zn-rich eutectic phase extending from the metal ribbon, and having Mg ₂ Si phase particles between the dendrites In the channel, and the coating has an OT:SDAS ratio of greater than 0.5:1, wherein the OT is the cover thickness and the SDAS is the secondary dendritic arm spacing of the Al-rich alpha-branched body of the coating. A coated metal ribbon as claimed in claim 3, wherein the OT:SDAS ratio is greater than 1:1. A coated metal ribbon according to claim 3 or 4, wherein the coating has a total coating mass on both surfaces of the ribbon of less than a coating per square meter At 200 grams, which is equal to less than 100 per square meter of coating on one surface of the steel strip when the strip is coated only on one surface and the thickness of the coating is the same on both surfaces. Gram. The coated metal ribbon of claim 3, wherein the coating has a cover thickness greater than 3 μm. The coated metal ribbon of claim 3, wherein the Al-containing alpha-branched SDAS of the coating is greater than 3 μm but less than 20 μm. The coated metal ribbon of claim 3, wherein the metal ribbon is a steel ribbon. A method for forming a coating of a corrosion-resistant Al-Zn-Si-Mg alloy on a metal strip comprising: (a) passing a metal ribbon through an Al-Zn-Si-Mg a molten bath of alloy and forming an alloy coating on one or both surfaces of the ribbon, wherein the alloy contains 40-65% Al, 35-50% Zn, 1-3% Si, greater than 0 and less than 3% of Mg, and a small amount of other elements, the amount of each other element is less than 0.5%, wherein all percentages are by weight, and (b) curing the coating on the ribbon Forming a cured coating having a microstructure comprising a dendritic body having an Al-rich alpha phase and an inter-branched intervening channel having a Zn-rich eutectic phase mixture extending from the metallic ribbon And having a Mg ₂ Si phase in the inter-branched channel in the cured coating, and the method comprises selecting a concentration of Mg and Si and controlling a cooling rate in step (b) to channel between the branches Mg ₂ Si phase particles are formed in the middle. The method of claim 9, wherein the method comprises selecting a Mg concentration of greater than 0.5%. A method of claim 9 or 10, which comprises selecting a Mg concentration of greater than 1%. The method of claim 9 or 10, wherein the steps of selecting the concentration of Mg and Si and the cooling rate in the controlling step (b) are formed in the channels of the branches to have an appropriate size and shape to block the edge. The Mg ₂ Si phase particles corroded by the inter-branched channels. The method of claim 12, wherein the form of the Mg ₂ Si phase particles in the inter-branched channel is in the form of a "Chinese character" when viewed in a planar image, and in a 3-dimensional image view The time is in the form of a petal. The method of claim 13, wherein the petals have a small thickness At 5 μm. The method of claim 13, wherein the lobes have a thickness in the range of 0.5-2.5 μm. The method of claim 9, wherein the step of selecting the Mg and Si concentrations and controlling the cooling rate in the step (b) to form the Mg ₂ Si phase particles in the inter-branched channels is in the cured coating. Mg ₂ Si phase particles having a size range and a spatial distribution that activates the Al-rich alpha phase to provide sacrificial protection are formed in the inter-branched inter-body channels. The method of claim 9, wherein the cooling rate CR during curing of the coating is less than 170-4.5 CT, wherein CR is a cooling rate expressed in ° C/sec, and CT is a band expressed in microns. The thickness of the coating on the surface of the object. A metal ribbon having a coating having an Al-Zn-Si-Mg alloy on one or both surfaces of the ribbon, wherein the alloy contains 40-65% Al, 35-50% Zn , 1-3% of Si, greater than 0 and less than 3% of Mg, and a small amount of other elements, each of which is less than 0.5% by weight, wherein all percentages are by weight, and the coating comprises a microstructure comprising a dendritic body having an Al-rich alpha phase and an inter-branched intervening channel having a Zn-rich eutectic phase mixture extending from the metal ribbon and having Mg ₂ Si phase particles in the dendrites In the interchannel. A coated metal ribbon as claimed in claim 18, wherein the Mg concentration is greater than 0.5%. A coated metal ribbon as claimed in claim 18, wherein the Mg concentration is greater than 1%. The coated metal ribbon according to any one of claims 19 to 20, wherein the coating having a Si concentration of from 0.5 to 2% is compared with other Si-containing phases The volume fraction of the interbody Mg ₂ Si phase is greater than 50%. The coated metal ribbon of any one of claims 18, wherein the volume fraction of the Mg ₂ Si phase between the branches is greater than 80% compared to the other Si-containing phases. The coated metal ribbon of any one of claims 18, wherein greater than 70% of the total integral ratio of the Mg ₂ Si phase in the coating is two thirds below the coating coverage thickness. in. The coated metal ribbon of any one of claims 18, wherein greater than 60% of the inter-branched inter-channel channels are "blocked" by the Mg ₂ Si phase particles. A metal ribbon as described or claimed in claim 1, 3, 9 or 18 wherein the Al concentration is 45-60%. A metal ribbon as described or claimed in claim 1, 3, 9 or 18 wherein the Zn concentration is 39-48%. A metal ribbon as described or claimed in claim 1, 3, 9 or 18 wherein the Si concentration is from 1.3 to 2.5%. A metal ribbon as described or claimed in claim 1, 3, 9 or 18 wherein the Si concentration is from 1.3 to 2.5%. The coating has a cover thickness of less than 30 μm as described in the claims 1, 3, 9 or 18 of the patent. Metal strip as described or claimed in claim 1, 3, 9 or The coating has a cover thickness of 5-20 μm.