WO2011036306A2

WO2011036306A2 - Corrosion protection based on a zinc-alloy

Info

Publication number: WO2011036306A2
Application number: PCT/EP2010/064368
Authority: WO
Inventors: Hermann Kronberger; Alexander Tomandl
Original assignee: Voestalpine Stahl Gmbh; Technische Universität Wien
Priority date: 2009-09-28
Filing date: 2010-09-28
Publication date: 2011-03-31
Also published as: WO2011036306A3

Abstract

The invention relates to an anti-corrosion layer containing predominantly zinc, in particular for steel materials. Said anti-corrosion layer is permeable to hydrogen. The invention also relates to a steel component comprising said layer and to a method for producing the steel component.

Description

Corrosion protection based on zinc alloy

The invention relates in particular to electrolytically produced anticorrosion coatings, in particular those based on zinc, for metallic substrates.

In order to improve its resistance to corrosion, steel is electrolytically coated with a layer of zinc or a zinc alloy-based alloy in the art, the less noble zinc acting as a surface sacrificial anode in the event of corrosive attack. In the electrolytic coating process or in other chemical reactions during steel processing, such. As the recrystallization annealing in a hydrogen atmosphere, occurs in addition to the respective desired effect always an unavoidable chemical side reaction, namely the hydrogen formation on the steel surface. In particular, in electrolytic zinc galvanizing can only be reduced by optimizing the process parameters, the hydrogen formation, but never completely ^¬ prevented from a thermodynamic point of view. This atomic water present ^¬ material able to diffuse into the steel work into and partly preferably store therein at defects, grain boundaries and interstitial sites of the steel grid.

To protect steel materials against corrosion and insbesonde ^¬ re to hydrogen embrittlement in steel materials with corrosion To counteract rosionsschutzschichten, various approaches are known from the prior art.

Thus, EP discloses 0566121 Bl a process for producing a with a zinc-chromium alloy electroplated steel sheet having excellent adhesiveness to the protective layer and elekt ^¬ rolytisch prepared Corrosion resistant ^¬ ness, which (using an acidic electrolytic bath, the addition of a nonionic organic additive zinc ions Zn ²⁺ ) and chromium ions (Cr ³⁺ ) in a molar concentration ratio of 0.1 <Cr ³⁺ / (Zn ²⁺ + Cr ³⁺ ) <0.9 in a total amount of at least 0.5 mol / l.

In EP 0285931 Al a corrosion-resistant electromagnetic ^¬ lytic coated steel strip is described which consists of the sub ^¬ strat and comprises at least one basic coating ^¬ layer which is formed on at least one surface side of the steel strip substrate which a simultaneous electrolytic plating on Zinc-chromium-based alloy includes chromium in an amount of 5 wt .-% to 40 wt .-%, the balance being zinc.

EP 0607452 Al discloses a steel sheet coated with a zinc-chromium alloy having different phases ^¬ compositions.

One possibility of the Zn-based coating of high-strength and higher-strength steel materials is known from US 2008/0131721 AI, in which a corrosion protection based on zinc-nickel is proposed. The zinc-nickel layer has a cover ^¬ de and at the same time porous nature, so that on the one hand protects against corrosion and on the other hand, due to the hydrogen ^¬ permeability has a low tendency for water toffversprö- tion of high and high strength steel materials. adversely however, in these ZnNi layers, the composition of the layer composition can not be varied in composition, and the fact that the hydrogen permeability is enabled only by a so-called "columnar structure", that is cracks in the broadest sense. These cracks reduce the layer quality. Another disadvantage of the ZnNi layers is that is composed by the nobler nickel is cathodically reduced shear ^¬ protection. Zinc-nickel-based anti-corrosion layers are further likely to be limited in the future due to carcinogenic properties of Ni-dusts only applications where no dust generating Nachbear ^¬ processing, eg, carried out by grinding.

The hydrogen absorption has a particularly disturbing effect on Zn-based electrolytic plating of high-strength and high-strength steel materials because, unlike conventional steel, it can lead to delayed crack formation there. As a result, there is currently only a very limited applicability of Zn-based corrosion protection coatings on high-strength and higher-strength steel materials. As a guideline for the maximum strength which steels may have for the electrolytic coating, the DIN 50969 specifies 1,000 MPa. In addition, the risk of hydrogen embrittlement is too high.

Steels must be coated electrolytically ^¬ table with higher strength than 1,000 MPa, so 50969 consuming heat treatments are carried out according to DIN. For this purpose, the steel is coated in the first step with a thin Zn layer (<2ym). This thin-layer Zn allowed by their porosity the off ^¬ drive of the hydrogen introduced through a subsequent thermal treatment. Thereafter, the required layer thickness is adjusted with a new electrolytic coating. This process is very complex and practically impractical in the electrolytic coating of strip steel.

Especially versprödungsanfällige high-strength steels are therefore currently galvanized as general cargo with the multi-stage process electrolytically ^¬ table or must be coated with other methods. In many cases, however, hot-dip galvanizing is also excluded in these steels, since the high alloy contents of these steels impede the reaction between iron and zinc necessary for the adhesion of the zinc layer.

It is also known that it may spontaneously paint craters and / or blisters in the paint layer in galvanized steel sheets for painting, on the one hand, optically particular visible areas (outer skin) are the body ^¬ rend very troublefree and on the other hand, long-term potential corrosion- ^¬ onskeimstellen are ,

There are in the prior art various references to the causes of these paint damage that may occur when painting steel materials that have previously been electrolytically coated with anticorrosive coatings based on zinc or zinc alloy or coated by hot dip ("feu ^¬ galvanized").

In "Plating and Surface Finishing 1997, pages 14 -21" The system View in Automotive Finishes - Essential but Overloo- ked "by James H. Lindsay in addition to the Lackierparametern itself also contaminants or surface defects of the sub ^¬ strats, to locally poor adherence Layers lead, as a cause discussed. The object of the present invention is to provide a corrosion protection layer ^¬ to provide zinc-based, which bility improves the paint and lacquer in particular avoids craters and blisters.

The object is achieved with a corrosion protection layer having the feature of claim 1. Advantageous embodiments are specified in the dependent claims.

It is also object of the present invention to provide a low toxic anticorrosive layer Zn-based for high and higher strength steels to provide that prevents ^¬ strength losses caused by hydrogen embrittlement of the coated material.

The object is achieved with a corrosion protection layer having the features of claim 9. Advantageous Ausführungsfor ^¬ men are given in the dependent therefrom dependent claims.

The inventors have found that predominantly hydrogen f effusion from the material during paint firing is responsible for paint craters and bubbles. This long time is not understood phenomenon can be explained by now that diffused in the steel hydrogen present during the Lackeinbrands and thereby the the material enveloping Wasserstoffundurchlässige zinc or zinc alloy coating may break through, since the zinc layer for the water ^¬ material is a diffusion barrier. Since the varnish layer is UNMIT ^¬ telbar deposited on the zinc or zinc alloy layer, and this is mechanically damaged, and finally remain paint craters and blisters in the paint layer.

For the formation of paint craters, therefore, the origin of hydrogen is irrelevant. There are two main ones Sources, the recrystallization annealing of the steel and the electrolytic coating itself. Although a hydrogen absorption in the steel by these two processes can be reduced by complex measures, but never completely prevented.

Since it is also known in the prior art that lack of hydrogen permeability of the anticorrosion layers is responsible for hydrogen embrittlement, the inventors have sought a coating system that, in the same Verarbeitungsei ^¬ properties such as zinc is permeable to hydrogen.

It is known that pure zinc layers from about 2 ym represent a more ^¬ same hydrogen barrier. These layers are present in the hexagonal lattice known for zinc. The ratio of the lattice parameters c / a is 1.86 as known from the JCPDS card PDF 03-065-5973. To hydrogen permeable layers Sustainer ^¬ th, the expert can now search layers having a different structure of the crystal lattice. In the case of Zn alloy layers, this is possible by depositing a cubic gamma phase. However, these phases are very brittle, reduce the adhesion and increase the abrasion of the layer during forming, as known from the prior art (eg EP 0 566 121 B1).

Since zinc layers from 2 μm thick represent a barrier to hydrogen, hydrogen introduced into the steel material can not escape again before painting.

On the other hand, the inventors have found that when using hydrogen permeable corrosion protection layers no Lackkrater- and blistering occur in an overlying lacquer layer. Therefore, in principle, all water Stoffpermeablen corrosion protection layers can be used to prevent subsequent damage to the paint.

Such a hydrogen permeable corrosion protection layer is achieved with the zinc-chromium alloy system. For this purpose, the inventors have found that electrolytically deposited zinc-chromium alloy layers are permeable to hydrogen even at chromium contents of greater than 1%. This is quite überra ^¬ prevalent, since it is known from the prior art that greater than 10%, there is a phase change only with chromium contents. Up to a content of 10%, the electrolytic abge ^¬ different zinc-chromium layer is located as well as a layer of pure zinc present in a hexagonal phase. The phases which are produced via an electrodeposited path differ markedly from the thermodynamically stable phases with the same composition.

Thus, from "Journal of Alloys and Compounds", Vol. 480, Issue 2, July 8, 2008, pages 259 to 264, "Composition of electro-deposited Zn-Cr alloy coatings and phase transformations induced by thermal treatment" by Tz . Boiadjieva, K. Petrov, H. Kronberger, A. Tomandl, G. Avdeev, W. Artner, T. and M. Lavric Monev is known that at the case of zinc-chromium alloys until ei ^¬ nem chromium content of greater than 12 % creates a new cubic Γ (Zn, Cr) phase.

The inventors have also found that the hexagonal zinc lattice can also be altered by alloying chromium so that it is permeable to hydrogen. In this case, the ratio of the lattice parameters c / a <1.75 must be set, which is already the case in the case of an electrolytic coating with chromium content> 1%. It is assumed that the zinc-chromium alloy according to the invention is permeable to hydrogen when the hexagonal lattice is changed such that the ratio of c / a shifts from 1.86 (as in the case of Reinzink) to less than 1.75.

A significant advantage of the invention is that for the first time high-strength and high-strength steel materials can be coated electrolytically ^¬ table with a corrosion protection layer based on zinc-chromium, without risking the risk of hydrogen embrittlement.

The proportion of chromium in the alloy layer changes the lattice parameters of the hexagonal phase, which is called the delta phase. In order to obtain a hydrogen-permeable layer, it is not necessary to deposit a cubic Γ- (Zn, Cr) phase, but it is sufficient to set the chromium content just high enough so that the ratio of the lattice parameters in the hexagonal lattice is c / a <1.75.

Since, in contrast to pure zinc, zinc-chromium alloy layers are also hydrogen permeable without phase change, a broad field of the possible content of chromium in the alloy, optimized for the respective intended use, results. For example, it is known from the prior art (EP 0566121 B1) that adhesion and abrasion during forming of ZnCr layers deteriorate significantly if more than 12% Cr and thus Γ- (Zn, Cr) phase is present.

According to the invention, the coating is carried out by conventional coating methods such as electrolytic coating with a corrosion protection layer which is permeable to hydrogen. This hydrogen permeability ensures that hydrogen diffused into the material can escape through the alloy layer as far as possible before painting, so that a subsequently applied lacquer layer is not damaged by breaking the anti-corrosion layer. In this way, a Lackkrater- or blistering can be effectively prevented. According to the invention the concentra ^¬ onsverhältnis of zinc and chromium ions is adjusted such that respect. Chromium supersaturated alloy phases form, the befin outside of thermodynamic equilibrium are the and the processing properties and the corrosion protection in the optimal ratio for use ^¬.

The invention particularly relates to a corrosion protection ^¬ layer comprising primarily zinc, especially for steel ^¬ materials, said corrosion protective layer has a crystalline structure which is permeable to hydrogen.

The invention particularly relates to a corrosion- ^¬ onsschutzschicht that contains as an alloy component with chromium ei ^¬ nem chromium content> 1%.

Furthermore, the invention relates in particular to a corrosion ^¬ protective layer consisting wholly or partially of a hexagonal phase, the ratio of the lattice parameter c / a <1.75 is.

Furthermore, the invention relates in particular to a corrosion protection ^layer which contains a zinc gamma phase in addition to the hexagonal phase.

The invention further particularly relates to a corrosion ^¬ protective layer is applied electrolytically or by Schmelztauchbe ^¬ layers.

The invention furthermore relates, in particular, to a steel component with a corrosion protection ^{layer according} to the invention. The invention particularly applies to the use ^¬ dung of a steel component according to the invention for outer skin panels of automobiles.

Furthermore, the invention relates in particular to a method for producing a steel component ^{according to} the invention, in particular an outer skin sheet, wherein in a first step on the steel electrolytically or by hot dip coating a corrosion protection layer according to the invention is applied and then in a second step by conventional La ^¬ ckiermethoden a one or multilayer paint system is applied.

Further, the invention particularly relates to a high- and hö ^¬ herfestes steel component with a corrosion protective layer according to the invention ^¬.

The invention further relates to the use of a steel component fiction, contemporary ^¬ for particularly stressed parts of Au ^¬ tomobilen.

The invention further relates, in particular, to a process for producing a high-strength and high-strength steel component according to the invention with an anticorrosive layer of the invention applied electrolytically to the steel.

In the following the invention will be explained in more detail by way of example with reference to the figures. Show it:

Figure 1 is a pure zinc layer (one side) is applied to a steel substrate, wherein the non-coated Be ^¬ te 10 min long loaded with a current density of 42 mA / cm ^2; FIG. 2 shows the zinc-chromium layer according to the invention (one-sided) on a steel substrate, wherein the uncoated side was charged for 25 minutes with a current density of 42 mA / cm ² ;

Figure 3: the zinc-chromium layer of the invention on a

Substrate having a non-loaded part A and a loaded part B, the loaded part being loaded for 20 minutes each with a current density of 42 mA / cm ² ,

Figure 4: the inventive zinc-chromium layer of Figure 3 with an over-galvanized 7 μιη thick layer of pure zinc and then re-loading for 20 minutes the uncoated side with a current density of 42 mA / cm ² .

The invention is explained below by way of example with reference to a zinc-chromium layer.

Investigations were carried out on the hydrogen permeability of zinc-chromium layers. For this purpose, one steel sample is galvanized on one side and a steel sample is coated with the zinc-chromium alloy according to the invention. Subsequently, both samples are loaded in an electrolysis cell on the uncoated side with hydrogen for up to 30 minutes (electrochemical loading in sulfuric acid). The electrolyte for the loading consisted of 35 g / 1 H ₂ SO ₄ conc. + 0.5 g / 1 arsenic oxide.

Figure 1 shows a 10 min loaded with pure zinc layer with egg ^¬ ner current density of 42 mA / cm ² steel sample. In addition to the fine-grained, original origina To see the zinc surface structure 3 the newly added many small bubbles 1.

As a comparison to FIG. 1, FIG. 2 shows a steel sample coated with the zinc-chromium layer according to the invention and then loaded for 25 minutes with a current density of 42 mA / cm ² . It is shown here that even at 15 minutes of charging time longer than that of the pure zinc layer, the zinc-chromium layer has no bubbles, son ^¬ countries retains its coarse surface structure. 4

Figure 3 shows the zinc-chromium layer of the invention on egg ^¬ ner steel sample, not loaded the Part A and Part B is loaded with egg ^¬ ner current density of 42 mA / cm ^2. There are no differences in the surface condition between part A and part B.

In order to obtain evidence for the relevance of the hydrogen permeability, the steel sample with the zinc-chromium layer according to the invention from FIG. 3 is provided with a 7 μm thick and thus hydrogen-impermeable zinc layer and then again with a current density of 42 mA for 20 minutes. cm ² loaded. As can be seen in FIG. 4, a considerable number of lacquer craters 2, which only occur in part B '(loaded part), can be seen. It can be concluded indirectly that he ^¬-making contemporary zinc-chromium layer for hydrogen must and it is not damaged for this reason be transparent. The damage only occurs in the case of the water-permeable zinc layer. Bezugszeichenl

A non-loaded part A of the zinc-chromium layer according to the invention on a substrate

A 'galvanized and loaded part A

1 bubble

2 paint craters

3 original zinc surface structure

Claims

claims

1. corrosion protection layer containing predominantly zinc, in particular ^¬ special for steel materials, wherein the corrosion protection layer ^{¬ has} a crystalline structure which is permeable to water ^¬ substance, so that the corrosion protection layer for hydrogen is permeable.

2. corrosion protection layer according to claim 1, characterized in that it contains as alloying ingredient chromium with ei ^¬ nem chromium content> 1 mass%.

3. Corrosion protection layer according to claim 1 or 2, characterized ge ^¬ indicates that it consists wholly or partly of a hexagonal phase whose ratio of the lattice parameter c / a <1.75.

4. corrosion protection layer according to any one of the preceding claims, characterized in that the corrosion protection ^¬ layer in addition to the hexagonal phase contains a zinc gamma phase.

5. Corrosion protection layer according to one or more of claims 1 to 4, characterized in that the corrosion protection ^¬ onsschicht is applied electrolytically or by Schmelztauchbe ^¬ layers.

6. steel component with a corrosion protection layer according to one or more of claims 1 to 5.

7. Steel component according to claim 6, characterized in that the steel component is formed of a high and higher strength steel.

8. Use of the steel component according to claim 6 for outer skin ^¬ plates of automobiles.

9. Use of the steel component according to claim 7 for particularly stressed parts of automobiles.

10. A method for producing a steel component, in particular a skin panel, according to claim 6, characterized in that in a first step on the steel elekt ^¬ rolytically or by hot dip coating, a corrosion protection layer according to one of claims 1 to 5 is applied ^¬ and then in a second step by conventional Lackiermethoden a single or multi-layer paint system is applied.

11. A method for producing a high and high strength steel component, in particular according to claim 7, characterized ge ^¬ indicates that on the steel electrolytically a Korro ^¬ sion protective layer according to one or more of claims 1 to 5 is applied.