AQUEOUS, ALKALINE ELECTROLYTE FOR THE DEPOSITION OF ZINC-CONTAINING LAYERS ON SURFACES OF METALLIC PIECE GOODS

Description

[0001] The present invention relates to an aqueous, alkaline electrolyte and a method for depositing zinc-containing layers on the surfaces of metallic lump goods. In particular, the present invention relates to an aqueous, alkaline electrolyte and a method for depositing zinc-containing layers on the surfaces of lump goods, in which the lump goods are introduced into the aqueous, alkaline electrolyte. The disclosure further relates to a lump goods provided with a zinc-containing layer and the use of the zinc-containing layer as corrosion protection on metallic lump goods, in particular those made of iron and steel.

[0002] Various methods are available in the state of the art for protecting metallic material surfaces from corrosive environmental influences. One widely used and established method in technology is the application of a metallic coating to the metallic workpiece to be protected. For example, workpieces made of iron and steel are often galvanized to protect them from corrosive environmental influences. The coating metal can behave electrochemically in a more or less noble manner in the corrosive medium than the base metal alone. If the coating metal behaves in a less noble manner, it acts as a sacrificial anode in the corrosive medium, providing cathodic corrosion protection to the base metal. The corrosion protection provided by zinc is based on the fact that it is even less noble than the base metal and therefore initially attracts the corrosive attack exclusively.

[0003] The deposition of zinc-containing coatings on surfaces is widely used in many areas of technology. Zinc coatings are particularly suitable for functional coatings. For example, it is common practice to coat small parts such as screws, nuts, washers, prefabricated structural elements such as angle brackets or connecting plates, and the like in large quantities.

[0004] The zinc layer can be applied using various chemical and physical processes, for example, hot-dip plating, where alloys are common, but especially by electrolytic deposition. Different electrolytes with very specific properties are used for galvanizing, with uniformity and gloss levels being adjustable using organic additives. Typical electrolyte compositions are described in numerous patents; only the most important electrolyte types are listed below: More or less strongly acidic sulfate electrolytes (practically exclusively for continuous pipe and strip galvanizing with very high current densities and high relative speed, usually without any organic additives) Weakly acidic chloride electrolytes (only in exceptional cases for continuous strip galvanizing, as they necessarily require organic additives; chloride electrolytes are almost exclusively used in batch electroplating for relatively fast and sometimes high-gloss galvanizing) Alkaline-cyanide electrolytes, primarily historically for batch electroplating in rack and barrel applications Alkaline, cyanide-free electrolytes, for batch electroplating

[0005] Weakly acid chloride electrolytes are generally characterized by very good coverage and a fast deposition rate with excellent efficiency, but they typically have poor metal distribution, meaning the resulting zinc layer exhibits large variations in layer thickness. In alkaline electrolytes, the zinc is present as an anion, i.e., as a zincate ion, and complexing agents may also be present, historically particularly preferred, cyanide. However, these have largely been replaced by cyanide-free alkaline zinc electrolytes, which, with very good metal distribution, have quite acceptable values for efficiency and thus deposition rate. "Cyanide-free" here means that sodium or potassium cyanide are not intentionally added as a conducting salt, as was previously the case. Naturally present or developing small traces of cyanide can also occur in cyanide-free electrolytes.In addition, specific organic additives are mandatory in both the weakly acidic chloride and the various alkaline electrolytes, such as polymers, surfactants, complexing agents and polar molecules that influence the degree of gloss, so-called brighteners.

[0006] The electrolytically deposited zinc layer, for its part, typically degrades so rapidly in corrosive media such as salt solutions, acids, or alkalis, forming massive, voluminous corrosion products that it is almost always protected from premature degradation by an additional barrier layer, typically a conversion layer (chromating, passivation) and / or a thin layer of paint (sealing, sealer, topcoat). Consequently, the resulting corrosion protection is usually expressed in terms of two types of corrosion: coating corrosion, i.e., the formation of zinc corrosion products, also called "white corrosion," and base metal corrosion, in the case of iron or steel also called "red rust." Common test methods include the neutral salt spray test DIN EN ISO 9227 or ASTM B117, as well as climatic cycling tests such as VDA 233-102.A certain degree of white corrosion is normal for cathodic protection as it is part of the protection mechanism, but company specifications are increasingly demanding high levels of corrosion protection in salt spray tests with no visible change.

[0007] Here, the zinc + passivation system has reached a technical limit that cannot be exceeded any further, but is certainly sufficient for many applications. Since around the 1980s, numerous zinc alloys (co-depositions of zinc with one or more other metals) have been proposed for more stringent requirements. Of these, zinc / cobalt and zinc / iron (both with very low alloying contents of less than 1% Co or Fe), and zinc / nickel (>7% Ni) have achieved widespread practical application. Of these, zinc / nickel with a nickel content of 13-15% has now become almost the only zinc alloy system established. This represents the current optimum in terms of corrosion protection, heat resistance, and the prevention of contact corrosion with aluminum alloys. This coating is widely used, especially in the automotive industry.The other zinc alloys electrolytically deposited in batch electroplating have been completely or largely replaced by zinc / nickel.

[0008] Unfortunately, nickel has the disadvantage of being a strong allergen. Furthermore, zinc / nickel coatings sometimes fail when the nickel content becomes too high, which begins at around 17% nickel. Such a coating is no longer less noble than the base metal, and therefore loses its function as a sacrificial anode in the cathodic corrosion protection system.

[0009] The aim is therefore to develop a zinc-containing layer that, even without nickel, provides approximately the same level of corrosion protection as zinc / nickel layers, but without their disadvantages.

[0010] A large number of nickel-free zinc alloys have been described in the literature. For example, DE 103 06 823 A1 discloses the deposition of zinc-manganese alloys, but the corrosion products here are bright reddish-brown in color and can hardly be distinguished from red rust. Since the 1980s, zinc / iron coatings with higher iron content than the coatings cited above, which contain only around 0.5% iron, have also been described, for example in patent applications JP 58210191 NISSHIN STEEL (1982), DE3428345 OMI (1983), and DE3619385 Elektro-Brite (1987). However, these have not yet been able to gain widespread acceptance, partly because their corrosion protection is not consistently high and there are repeatedly massive outliers with large amounts of white corrosion products.Zinc / iron electrolytes were proposed as more or less strongly acidic sulfate electrolytes for continuous strip and tube coating and as alkaline electrolytes for batch plating in rack and barrel systems.

[0011] EP 2292679 A1 describes a process using various additives in an electrolytic bath intended for the deposition of zinc or zinc alloy coatings. The aim of the process is to obtain "zinc and zinc alloy coatings that are as bubble-free and burn-free as possible, with the most uniform layer thickness distribution possible and high gloss." To achieve this, the process comprises the electrolytic deposition of a layer of zinc or a zinc alloy, wherein the electrolytic bath contains a polymer. It is further described that the bath may contain alloys in the form of 0.1 to 50 grams / liter of metal ions, for example, iron and manganese. Nickel may also be present.

[0012] DE 102005049789 A1 describes an aqueous, alkaline, cyanide-free bath containing I) a zinc ion source and optionally a source for further metal ions, II) at least one cationic polycondensation product.

[0013] DE 3943243 A1 describes an iron-manganese-clad steel sheet comprising a steel sheet, a lower layer of zinc or a zinc alloy applied by electrodeposition to at least one surface of the steel sheet, and an upper layer of at least 0.5 g / m 2 of an iron-manganese alloy having a manganese content of not more than 60 wt.% applied to the lower layer by electrodeposition.

[0014] EP 2290133 A1 describes a steel component with a

[0015] A steel substrate consisting of a steel containing 0.3-3 wt.% manganese and a corrosion protection coating applied to the steel substrate, comprising an electrolytically deposited ZnNi alloy coating consisting of γ-ZnNi and Γ-ZnFe on the steel substrate and having a Mn-containing layer on its free surface, in which Mn is present in metallic or oxidic form. A process for its production is also described.

[0016] EP 0329057 A1 describes a zinc or zinc alloy coated steel sheet comprising: a base steel sheet; one or more layers coated on the steel sheet, at least one of the coated layers containing zinc as a main component; and an oxidized layer formed on the outermost one of the coated layers and containing zinc in an amount of 0.03 to 3.0 g / m 2 , the oxidized layer containing at least one of zinc oxide and zinc hydroxide.

[0017] DE 102012024616 A1 describes a steel sheet, in particular a press-hardening steel sheet, comprising a substrate layer made of steel and a corrosion protection layer galvanically applied to the substrate layer, which contains zinc and manganese in a proportion of at least 5 wt.%.

[0018] The object of the present invention is to provide a zinc-containing layer that offers the highest possible corrosion protection even without nickel, without losing its properties as a sacrificial anode. Furthermore, the object of the present invention is to be heat-resistant in the sense of the component's application and to provide good protection against contact corrosion with aluminum alloys. In particular, the inevitably formed corrosion products should be as inconspicuous as possible, especially not white and voluminous like typical zinc corrosion products.

[0019] Although there is a Japanese patent application from 1987 (JP63176490A) describing a phosphatizable zinc / iron / manganese coating, this process involves a sulfate electrolyte with very high current densities and strip speeds, as is common for electrolytic strip galvanizing. Sulfate electrolytes are unsuitable for batch plating because they are optimized for the high speeds and current densities (approximately 50-100 times higher than those typical for batch plating) and are also very sensitive to varying anode-cathode distances. Furthermore, they are difficult or impossible to adjust with organic additives. In strip galvanizing, the anode-cathode distance is fixed and virtually unchanging. In batch plating, parts are coated that are not simply flat sheet metal, but rather finished formed or even cast parts, sometimes with complex three-dimensional geometries.Therefore, the teaching described in JP63176490A is not suitable for the present task.

[0020] It is therefore necessary to find an alkaline electrolyte suitable for rack and barrel plating with good metal distribution and a uniform alloy composition that can be adjusted with organic additives. Zinc, iron, and manganese should be able to be electrolytically deposited over a wide current density range with sufficient homogeneity, even on heavily shaped components.

[0021] This object is achieved by an aqueous, alkaline electrolyte for the galvanic deposition of a zinc, iron, manganese-containing layer on surfaces of metallic piece goods, in particular piece goods made of iron and / or steel, characterized in that the electrolyte contains: Zinc ions in an amount of 4-60 g / l, preferably 4-45 g / l, more preferably 4-30 g / l, even more preferably 5-20 g / l, in particular 7-10 g / l; iron ions in an amount of 0.5-30 g / l, preferably 0.5-25 g / l, more preferably 0.6-25 g / l, even more preferably 0.7-10 g / l, in particular 1 to 3 g / l; manganese ions in an amount of 0.2-8 g / l, preferably 0.2-5 g / l, in particular 0.3 to 1 g / l.

[0022] Furthermore, preferably include: 1. Sufficient sodium or potassium hydroxide to produce soluble zincate ions, 2. Anions such as acetate, carbonate, chloride, silicate, sulfate, as counterions to the above-mentioned cations and - together with the sodium and potassium ions - as conductive salts, and / or 3. organic additives to stabilize soluble complexes, for uniform deposition, for improved metal distribution and for adjusting the desired degree of gloss.

[0023] Surprisingly, it has been shown that a zinc coating with a higher iron content and a certain manganese content not only avoids the aforementioned disadvantages, but is also capable of surpassing the already excellent corrosion protection values of zinc / nickel. This coating can be passivated in trivalent or chromium-free conversion coatings and can also be coated with organic or inorganic topcoats.

[0024] The electrolyte according to the invention has the following economic and ecological advantages: The electrolyte according to the invention does not contain nickel, which, as a strong allergen, is often avoided for occupational health and safety reasons. However, the corrosion protection produced with this electrolyte is comparable to state-of-the-art zinc / nickel coatings and thus represents a significantly more compatible alternative. Zinc, iron, and manganese are essential for humans and generally well tolerated. The electrolyte according to the invention is alkaline, preferably highly alkaline, with a pH value of more than 13, preferably 13.5-14.5, especially around 14. Beyond that, however, it does not pose any particular hazards. Despite the increase in the number of alloying partners from one to two and the associated complexity, the electrolyte according to the invention can be operated with the same economic efficiency as an alkaline zinc / nickel bath.

[0025] Suitable sources of zinc ions can be soluble zinc compounds such as zinc chloride, zinc sulfate, or organic zinc compounds such as zinc methanesulfonate. Typically, zinc oxide or metallic zinc is dissolved in the highly alkaline electrolyte, generating the necessary zincate ions.

[0026] Suitable sources of iron ions can be soluble iron compounds such as ferrous chloride, ferrous sulfate, ferrous carbonate or organic iron compounds such as ferrous acetate.

[0027] Suitable sources of manganese ions can be soluble manganese compounds such as manganese chloride, manganese sulfate, manganese carbonate, or potassium permanganate. The latter is preferably reduced to a soluble manganese compound with a small amount of methanol during the bath preparation.

[0028] The electrolyte may also contain complexing agents, especially amines, polyalkyleneimines, dicarboxylic acids, tricarboxylic acids, hydroxycarboxylic acids, other chelating ligands such as acetylacetone, urea, urea derivatives, and other complexing ligands in which the complexing functional group contains nitrogen, phosphorus, or sulfur. Other optional components of the electrolyte are additives selected from the group consisting of brighteners, wetting agents, and mixtures thereof. These preferably include benzylpyridinium carboxylate, nicotinic acid, N-methylpyridinium carboxylate, and aldehydes.

[0029] The anode is preferably made of steel, nickel, nickel-plated steel, platinized titanium or another platinized inert metal, or of titanium coated with mixed oxides or another inert metal coated with mixed oxides.

[0030] The metallic workpieces connected as cathodes are attached to the frame or coated in a drum or other system suitable for bulk piece goods.

[0031] According to the invention, a method for the galvanic deposition of zinc-containing layers on surfaces of piece goods is also provided, in which the piece goods are introduced into an electrolyte as described above and zinc-containing layers are galvanically deposited on the piece goods.

[0032] The deposition preferably takes place at a temperature of 20 to 40°C, particularly preferably at a temperature of 25°C. The current density during the deposition is preferably in a range of 0.1 to 20 A / dm <2> , especially from 0.5 to 3 A / dm <2> .

[0033] Another subject of the present disclosure is a zinc-containing layer produced by a method as described above.

[0034] In an unclaimed embodiment of the disclosure, the zinc, iron, and manganese-containing layer contains metallic zinc and iron, as well as metallic and / or oxide manganese. The weight fractions of the elements can be measured using energy-dispersive X-ray spectroscopy (EDX).

[0035] In practical tests, it was found that the weight fraction of the elements in a zinc, iron, manganese-containing layer deposited by the process according to the invention, measured by energy-dispersive X-ray spectroscopy (EDX) at an excitation voltage of 20 kV, is usually in the following ranges: Zinc is usually in the range from 40 wt.% to 96 wt.%, preferably from 65 wt.% to 92 wt.%, more preferably from 77 wt.% to 89 wt.%, in each case based on the total weight of zinc, iron, manganese.

[0036] The weight fraction of iron is usually in the range from 4 wt.% to 50 wt.%, preferably from 8 wt.% to 30 wt.%, more preferably from 10 wt.% to 20 wt.%, in each case based on the total weight of zinc, iron, manganese.

[0037] The weight fraction of manganese is usually in the range from 0.05 wt.% to 10 wt.%, preferably from 0.1 wt.% to 5 wt.%, more preferably from 0.5 wt.% to 3 wt.%, in each case based on the total weight of zinc, iron, manganese.

[0038] The thickness of the zinc-containing layer can vary, for example, depending on the desired corrosion protection properties. For most applications, it has proven advantageous to set the zinc-containing layer to an average thickness of 3 µm to 30 µm, preferably 5 µm to 20 µm, and especially 7 µm to 15 µm. The layer thickness can be determined magnetically inductively, by X-ray fluorescence on copper parts, or by measuring a fracture using a scanning electron microscope.

[0039] According to an unclaimed embodiment of the disclosure, the zinc, iron, manganese-containing layer with adapted passivation, for example SurTec 680 chromiting, imparts to an article corrosion protection in the salt spray test according to ISO 9227 and / or ASTM B 117-73 without or with heat stress, for example of 120 °C for 24 hours, until initial attack according to DIN 50961 Chapter 10 of more than 400 hours, preferably of more than 500 hours and in particular of more than 600 hours.

[0040] Objects or articles containing a zinc, iron, or manganese layer can therefore be permanently and thus particularly advantageously protected against corrosion. Objects or articles containing a zinc-containing layer are also the subject of this disclosure.

[0041] The present disclosure also relates to the use of a zinc, iron, manganese-containing layer produced from an aqueous, alkaline electrolyte according to claim 1 as corrosion protection on metallic piece goods, in particular those made of iron and steel, wherein the weight fraction of the elements in the layer is in the following ranges: 65 to 92 wt.% zinc, 4 wt.% to 30 wt.% iron and 0.1 to 5 wt.% manganese, each based on the total weight of zinc, iron, manganese, wherein the zinc-containing layer has an average layer thickness of 3 µm to 30 µm.

[0042] The invention is explained in more detail below using several non-limiting examples: Two electrolytes according to the invention were prepared as follows: I. Two zinc solutions were prepared as follows: 1. 35 kg of NaOH was dissolved in approximately 50 kg of softened water. 4 kg of zinc oxide was then dissolved in the hot solution while stirring. As soon as it had completely dissolved, the solution was made up to 100 kg with softened water. = SODIUM ZINCATE SOLUTION 1.5 g / l of iron (as sulfate) with 0.66 g / l EDTA and 15 g / l triethanolamine as a complexing agent were added to 500 ml / l of softened water and 225 ml / l of the sodium zincate solution. Then 2 g / l of potassium permanganate was dissolved in it and reduced with 4 ml / l methanol. The resulting solution was made up to just under 1 liter of electrolyte with softened water. 2. 40 kg of KOH was dissolved in approximately 50 kg of softened water. 3 kg of zinc oxide was then dissolved in the hot solution while stirring. Once it had completely dissolved, the solution was topped up to 100 kg with softened water. = POTASSIUM ZINCATE SOLUTION.

[0043] To 500 ml / l of softened water and 225 ml / l of potassium zincate solution, 1.5 g / l of iron (as sulfate) with 0.66 g / l EDTA and 15 g / l triethanolamine as a complexing agent were added. Then, 2 g / l of potassium permanganate was dissolved and reduced with 4 ml / l methanol. The resulting solution was made up to just under 1 liter of electrolyte with softened water.

[0044] Both electrolytes were adjusted to a semi-gloss finish using commercially available base and brightening additives for alkaline galvanizing, such as SurTec 704 I and II. Degreased and pickled steel sheets were immersed in the respective electrolytes and coated at 23 °C with a current density of 2 A / dm².

[0045] The resulting approximately 6 µm thick layer was examined using EDX. The following values were measured: Potassium zincate electrolyte: Iron: 11.8-12.5%, Manganese: 0.2-2.0%, balance zinc Sodium zincate electrolyte: Iron: 11.9-12.5%, Manganese: 0.2-2.0%, balance zinc

[0046] Both sheets were passivated with SurTec 680 Chromiting and dried. The dried sheets were annealed for 24 hours at 120 °C to weaken the corrosion protection according to VDA requirements.

[0047] In the neutral salt spray test, both sheets achieved corrosion protection of > 600 hours without discoloration or black spots. (Comparison: Unalloyed zinc from alkaline electrolytes would exhibit more severe corrosion under the same conditions, and zinc / nickel from alkaline electrolytes would exhibit a more or less noticeable gray discoloration.) II. Another zinc solution according to the invention was prepared as follows: 1. 1.5 g / l iron (as chloride) and 12 g / l gluconic acid as a complexing agent were added to 500 ml / l of softened water and 225 ml / l of the sodium zincate solution from Example I.1. Then 2 g / l potassium permanganate was dissolved therein and reduced with 4 ml / l methanol. The resulting solution was made up to just under 1 liter of electrolyte with softened water. III. Two zinc solutions NOT according to the invention were prepared for comparison as follows: 1. 1.5 g / l iron (as chloride) and 12 g / l gluconic acid as a complexing agent were added to 500 ml / l softened water and 225 ml / l of the sodium zincate solution from Example I.1. The resulting solution was made up to just under 1 liter of electrolyte with softened water. 2. To 500 ml / l of softened water and 225 ml / l of the potassium zincate solution from Example I.2, 1.5 g / l of iron (as sulfate) and 12 g / l of gluconic acid as a complexing agent were added.The resulting solution was topped up with softened water to just under 1 liter of electrolyte.

[0048] For comparison, manganese addition was omitted in examples III.1 and III.2.

[0049] All three electrolytes were adjusted to a semi-gloss finish using commercially available base and brightening additives for alkaline galvanizing, such as SurTec 704 I and II. Degreased and pickled steel sheets were immersed in the respective electrolytes and coated at 23 °C with a current density of 2 A / dm².

[0050] The resulting approximately 6 µm thick layers were examined using EDX. The following values were measured: II.1: Iron: 11.8-12.5%, Manganese: 0.2-2.0%, balance zinc III.1: Iron: 11.9-12.5%, balance zinc III.2: Iron: 11.9-12.5%, balance zinc

[0051] Sheets from all three electrolytes were passivated and dried in SurTec 675 / 551, a cobalt-free, silicate-containing middle layer passivation.

[0052] While the samples from Example II showed neither coating corrosion ("white corrosion") nor red rust in the neutral salt spray test up to 1608 h ( Figure 1 ), the samples from examples III.1 and III.2 performed significantly worse.

[0053] Sample III.1 ( Figure 2 ) showed voluminous coating corrosion and incipient red rust at 1032 h NSS. The corrosion test was discontinued at this point.

[0054] Sample III.2 ( Figure 3 ) showed coating and base metal corrosion already at 384 h NSS, the test was stopped at 768 h neutral salt spray test (NSS) with more base metal corrosion and voluminous coating corrosion.

Claims

1. Aqueous, alkaline electrolyte for the electrodeposition of a zinc-, iron-, manganese-containing layer on the surfaces of metal items, in particular items made of iron and / or steel, characterized in that the electrolyte contains: - zinc ions in a quantity of 4-60 g / l; - iron ions in a quantity of 0.5-30 g / l and: - manganese ions in a quantity of 0.2-8 g / l.

2. Method for the electrodeposition of a zinc-, iron-, manganese-containing layer on surfaces of metal items, in particular items made of iron and / or steel, in which the items are introduced into an aqueous, alkaline electrolyte according to claim 1 and a zinc-iron-manganese alloy is electrodeposited on the items.