CONTROLLED METHOD FOR DEPOSITING A LAYER OF CHROMIUM OR CHROMIUM ALLOY ON AT LEAST ONE SUBSTRATE

Abstract

The present invention relates to a controlled method for depositing a layer of chromium or chromium alloy onto at least one substrate. The method comprises the steps of (a) providing an aqueous deposition bath, wherein the bath comprises trivalent chromium ions, bromide ions, and alkali metal cations in a total amount of 0 mol / L to 1 mol / L, based on the total volume of the deposition bath, and the bath has a target pH within the range of 4.1 to 7.0, (b) providing at least one substrate and at least one anode, (c) immersing at least one substrate in the aqueous deposition bath and applying a direct electric current such that the layer of chromium or chromium alloy is deposited onto the substrate, the substrate being the cathode, wherein during or after step (c) the pH of the deposition bath is lower than the target pH, (d) adding NH4OH and / or NH3 during or after step (c) to the deposition bath such that the target pH of the bath the bowel movement is restored.

Description

The present invention relates to a controlled method for depositing a layer of chromium or a chromium alloy and an aqueous deposition bath. In particular, the present invention relates to functional chromium layers, also called hard chromium layers. BACKGROUND OF THE INVENTION Functional chrome plating usually has a much greater average layer thickness (from at least 1 pm up to several hundred micrometers) compared to decorative chrome plating (typically less than 1 pm) and is characterized by excellent hardness and wear resistance. Functional chromium layers obtained from a deposition bath containing hexavalent chromium are known in the prior art and are a well-established standard. After chromium deposition, these chromium surfaces are subsequently treated, typically in finishing or superfinishing steps. In these steps, the chromium layer is further ground and polished to obtain a very smooth surface, typically exhibiting an average surface roughness (Ra) of 0.2 µm or less. In recent decades, chromium deposition methods that rely on hexavalent chromium have been increasingly replaced by deposition methods that rely on trivalent chromium. These trivalent chromium-based methods are much healthier and more environmentally friendly. WO 2015 / 1 10627 A1 refers to a galvanizing bath for depositing chromium and a method for depositing chromium onto a substrate using the galvanizing bath. US patent 2,748,069 relates to a chromium electroplating solution that allows for the rapid application of a chromium coating with excellent physical and mechanical properties. The chromium electroplating solution can be applied using special electrolysis methods, such as spot or trace electroplating. In these special methods, the substrate is typically not immersed in the electroplating solution. However, it has been observed that trivalent chromium-based methods frequently lead to a chromium or chromium alloy layer with a significantly high average surface roughness (Ra) (even up to 1.5 pm, based on an average layer thickness of at least 20 pm) compared to chromium layers obtained from hexavalent chromium-based methods (typically in the range of 0.2 to 0.4 pm, based on an average layer thickness of at least 20 pm). Furthermore, it has been observed in trivalent chromium-based methods that the average surface roughness of chromium or chromium alloy coatings increases continuously with long-term use of an aqueous deposition bath. Our experiments have shown that a freshly prepared deposition bath primarily results in a desirable low average surface roughness. However, with intensive use of the deposition bath, the average surface roughness increases rapidly, such that the surface quality of the treated substrate continuously decreases. Finally, substrates with a chromium or chromium alloy coating from a long-term deposition bath very frequently exhibit lower surface quality (i.e., they have a comparatively high average surface roughness p / oznn / eznz / E / YiAi) compared to substrates treated in a freshly prepared deposition bath.Of course, the goal is to obtain substrates with consistent surface quality. Significant changes in the average surface roughness of the substrate cause an undesirable disadvantage because well-established finishing and / or superfinishing steps, such as grinding and polishing, are often developed and specifically adapted for chrome plating obtained using hexavalent chromium methods—that is, for substrates with an average surface roughness usually less than 0.5 µm. These processes cannot be easily adapted to plating with a significantly higher average surface roughness or require at least sophisticated modifications, which typically increase costs and maintenance. Furthermore, if the substrates have variable surface quality, the surface quality of the respective substrates must be individually and carefully determined before the finishing steps to adapt the equipment used. This additional effort is even more undesirable. Objectives of the Invention: Therefore, a primary objective of the present invention was to provide a deposition method, based on trivalent chromium ions, for obtaining a substrate with a functional chromium or functional chromium alloy layer having an average surface roughness (Ra) very similar to the average surface roughness of layers obtained using hexavalent chromium-based methods. Furthermore, this method must ensure that these substrates exhibit this good and acceptable average surface roughness during extended use of the deposition bath, preferably throughout the entire lifetime of the deposition bath. Thus, the surface quality obtained needs to be as consistent as possible. In addition, the method should be easy to control and preferably more environmentally acceptable. The second objective was to provide an aqueous deposition bath containing trivalent chromium ions, which is more environmentally acceptable and enables (i) good and acceptable average surface roughness and (ii) a functional chromium or functional chromium alloy layer with excellent hardness and wear resistance. This aqueous deposition bath will thus be applicable to the desired method described above. BRIEF DESCRIPTION OF THE INVENTION The first objective is achieved by a controlled method for depositing a layer of chromium or chromium alloy onto at least one substrate, the method comprising the steps of (a) providing an aqueous deposition bath, where the bath comprises - trivalent chromium ions, - bromide ions, - alkali metal cations in a total amount of 0 mol / L to 1 mol / L, based on the total volume of the deposition bath, and the bath has - a target pH within the range of 4.1 to 7.0, (b) providing at least one substrate and at least one anode, (c) immersing at least one substrate in the aqueous deposition bath and applying a direct electric current so that the chromium or chromium alloy layer is deposited onto the substrate, the substrate being p / oznn / eznz / E / YiAi the cathode, wherein during or after step (c) the pH of the deposition bath is less than the target pH, (d) adding NHUOH and / or NH3 during or after step (c) to the deposition bath so that the target pH of the deposition bath is recovered. The second objective solved by an aqueous deposition bath for depositing a layer of chromium or chromium alloy, the bath comprising (i) trivalent chromium ions in a total amount in the range of 17 g / L to 30 g / L, on the basis of the total volume of the deposition bath, (ii) at least one organic complexing compound, (iii) ammonium ions, (iv) at least one halide ion species, wherein at least one species is bromide, (v) alkali metal cations in a total amount of 0 mol / L to 1 mol / L, on the basis of the total volume of the deposition bath, where - the trivalent chromium ions are from a source containing soluble trivalent chromium ion, the source used being in a total weight of less than 100 g per liter of aqueous deposition bath and the source comprising alkali metal cations in a total amount of 1% by weight or less, on the basis of the total weight of the source used, - the pH of the bath is in the range of 4.1 to 7.0, - the bath does not contain compounds containing sulfur with a sulfur atom having an oxidation number lower than +6, - the bathroom does not contain compounds containing boron. BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows a graphical representation of Example 1 consisting of a graph, which describes the average surface roughness on the y-axis. Figure 2 shows a graphical representation of Example 2 consisting of a graph, which describes the average surface roughness on the y-axis. DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a graphical representation of Example 1, consisting of a graph where the y-axis plots the average surface roughness (Ra) in pm and the x-axis plots the total amount of alkali metal cations (represented as the total amount of sodium cations in g / L), based on the total volume of the respective deposition bath samples. Each bar ((i) to (v)) represents a specimen. Further details are given in the “Examples” section below. Figure 2 shows a graphical representation of Example 2, consisting of a graph plotting the average surface roughness (Ra) in pm on the y-axis and the aqueous deposition bath usage in Ah / L on the x-axis. The graph is divided into two sections, A and B. Section A represents a method according to the present invention (addition of NH4OH), while Section B represents a method not according to the present invention (addition of NaOH). Sections A and B are interrupted by an interval from approximately 180 Ah / L to 230 Ah / L. During this interval, the hydroxide replenishment was changed from NH4OH to NaOH, and a simulated coating was carried out for a certain period. For further details, see Example 2 in the "Example" section later in the text. In the context of the present invention, the term “at least one” denotes (and is interchangeable with) “one, two, three, or more than three.” Furthermore, “trivalent chromium” refers to chromium with the oxidation number +3. The term “trivalent chromium ions” refers to Cr3+ ions in free or complexed form. Similarly, “hexavalent chromium” refers to chromium with the oxidation number +6 and also to related compounds that include ions containing hexavalent chromium. The method of the present invention includes steps (a) and (b), where the order is (a) followed by (b) or vice versa. Step (c) is carried out after both steps (a) and (b) have been completed. The present invention is based on the discovery (i) to carry out the method of the present invention with an aqueous deposition bath having a comparatively low total amount of alkali metal cations (for example, with an aqueous deposition bath according to the present invention as described later in the text or at least with an aqueous deposition bath containing a total amount of alkali metal cations of no more than 1 mol / L, on the basis of the total volume of the deposition bath) and (ii) maintaining this comparatively low total amount of alkali metal cations during the use of the deposition bath, preferably for the entire lifetime of the deposition bath. During the lifetime of an aqueous plating bath, a large quantity of chemical compounds is usually added, which can potentially contaminate it. A primary contributor to alkali metal cation contamination in functional chromium coating methods is the hydroxide source. During operation of such an aqueous plating bath, the pH typically decreases and is frequently restored by adding a hydroxide, which often contains alkali metal cations. In addition, trivalent chromium ions are consumed during deposition and must be replenished. This is a second major contributor to alkali metal cation contamination because in many cases these sources comprise significant amounts of alkali metal cations. Therefore, it is crucial to select the chosen hydroxide sources and soluble sources containing trivalent chromium ion that contain a low amount or even no alkali metal cations to maintain a total amount of alkali metal cations in the aqueous deposition bath in the range of 0 mol / L up to a maximum of 1 mol / L, based on the total volume of the deposition bath. It is generally assumed that an increase in the total amount of alkali metal cations in the bath results in a corresponding increase in the average surface roughness of the deposited chromium or chromium alloy layer (see experiments below). It appears that the total amount of alkali metal cations in the deposition bath crucially affects the average surface roughness of the chromium or chromium alloy layer deposited in step (c). Our experiments indicate that the maximum tolerable total amount of alkali metal cations in the deposition bath is 1 mol / L, based on the total volume of the deposition bath. A preferred method of the present invention states that the total amount of alkali metal cations in the deposition bath is in the range of 0 mol / L to 0.8 mol / L, based on the total volume of the deposition bath, preferably in the range of 0 mol / L to 0.6 mol / L, more preferably in the range of 0 mol / L to 0.4 mol / L, and even more preferably in the range of 0 mol / L to 0.2 mol / L. Most preferably, the deposition bath contains alkali metal cations in a total amount of 0 mol / L to 0.08 mol / L, or, in the best case, contains no alkali metal cations at all.According to our own experiments, the lower the total amount of alkali metal cations in the deposition bath, the more reliably constant the average surface roughness is during prolonged use of a respective deposition bath. In the context of the present invention, “constant” does not necessarily denote that all substrates have an identical average surface roughness. Instead, it denotes that the average surface roughness remains within a reasonable range that is suitable and desirable for common finishing steps, for example, within a range of 0.2 µm to 0.6 µm (see Example 2 and compare with Figure 2). The term “total amount of alkali metal cations” refers to the sum of the maximum individual amounts of lithium, sodium, potassium, rubidium, cesium, and francium metal cations. Typically, rubidium, francium, and cesium ions are not used in an aqueous deposition bath. Thus, in most cases, the total amount of alkali metal cations includes lithium, sodium, and potassium metal cations, primarily sodium and potassium. Our experiments have shown that the method of the present invention results in very smooth chromium and chromium alloy coatings relative to the initial average surface roughness of the substrate before step (c) of the method of the present invention. In other words, the method of the present invention does not increase the average surface roughness of a substrate to an undesirable degree. Furthermore, in the method of the present invention, this effect is not only achieved by a few substrates at the beginning of the method (or after a freshly prepared aqueous deposition bath) but also through prolonged use of the deposition bath. This can be easily achieved if the total amount of alkali metal cations is carefully controlled so that it does not exceed 1 mol / L or, preferably, is well below 1 mol / L, and if the target pH is restored by adding NH4OH and / or NH3.This controlled method allows for continuous operation with consistent surface quality. Thus, a method of the present invention is preferred, wherein the method is a continuous method. This means that A: steps (a) (d) are repeated continuously, and / or B: Step (c) is repeated at least once with another substrate before step (d) is carried out. Scenario B preferably includes repeating step (c) several times with other substrates before step (d) is carried out. After step (d) is completed, the deposition bath obtained after step (d) is provided in step (a), and the method proceeds. This also includes ensuring that the aqueous deposition bath obtained after step (d) does not exceed the maximum tolerable total amount of alkali metal cations of 1 mol / L or, preferably, the upper limits defined above as initially preferred. A method of the present invention is preferred, wherein the aqueous deposition bath provided in step (a) is used repeatedly in the method of the present invention, preferably for a usage of at least 100 Ah per liter of aqueous deposition bath, preferably at least 150 Ah per liter, more preferably at least 200 Ah per liter, and more preferably at least 300 Ah per liter. If the total amount of alkali metal cations significantly exceeds 1 mol / L in many cases, the average surface roughness reaches an undesirable degree relative to the initial surface roughness before step (c) and the surface quality continually decreases during prolonged use of the respective deposition bath. The method of the present invention is specifically designed for an aqueous deposition bath having a pH within the range of 4.1 to 7.0 (at 20°C). The method is not compatible with an identical deposition bath, provided in step (a), with the sole exception that it has a pH significantly lower than 4.1 because undesirable precipitation occurs if the pH is significantly lower than 4.1. Furthermore, if the pH is significantly lower than 4.1 or significantly higher than 7, a functional chromium layer or chromium alloy layer with sufficient wear resistance and hardness is not obtained. A method of the present invention is preferred, wherein the target pH is within the range of 4.5 to 6.5, preferably within the range of 5.0 to 6.0, and more preferably within the range of 5.3 to 5.9. Optimal results in terms of functional chromium and chromium alloy coatings were obtained at a target pH within the range of 5.0 to 6.0; better results were obtained at a target pH within the range of 5.3 to 5.9. The functional chromium and chromium alloy coatings obtained from an aqueous deposition bath at this target pH exhibit good or even excellent wear resistance and hardness. The pH ranges and values ​​mentioned above and below are also referred to a temperature of 20°C. According to our own experiments, at least one substrate obtained after step (c) exhibits a Vickers hardness of at least 700 HV (0.05) (determined with a 50 g load).The wear resistance is comparatively good with the wear resistance obtained from hexavalent chromium-based deposition methods. In step (d) of the method of the present invention, the target pH is restored by adding NH4OH and / or NH3 because, during or after step (c), the pH of the deposition bath is typically lower than that of the preceding step (c). Although after each step (c) the pH of the deposition bath may be slightly lower than before step (c), it is not necessarily required to restore the target pH in step (d) during or after each step (c). Those skilled in the art know that the target pH must be restored if the pH of the deposition bath shifts during or after step (c) outside the predefined tolerance range. In the method of the present invention, during or after step (c), the pH of the deposition bath does not fall below pH 4.1 or exceed pH 7.0. Preferably, during or after step (c), the pH does not fall below or exceed the aforementioned preferred pH ranges, if applicable. Preferably, the target pH in step (a) of the method of the present invention comprises a tolerance range of ± (plus / minus) 0.3 pH units (i.e., the tolerance range is from 0.3 to +0.3 pH units around the target pH and includes all values ​​in between), preferably a tolerance range of ± (plus / minus) 0.2 pH units. This most preferably applies to the aforementioned preferred ranges of 5.0 to 6.0 and 5.3 to 5.9. The tolerance range does not result in a target pH falling below or above the defined maximum pH range of 4.1 to 7.0 or, if applicable, the other aforementioned preferred pH ranges. If that target pH is recovered in step (d), it is preferable to recover a pH value within the tolerance range. For example: in step (a) the target pH is 6.4 and comprises a tolerance range of ± 0.2 pH units, leading to a target pH range of 6.2 to 6.6. In step (d), the target pH is recovered by obtaining any pH within the tolerance range, for example, a pH of 6.3 or 6.43, etc. A preferred method of the present invention is that the target pH in step (a) comprises a tolerance range of ±0.3 pH units, preferably ±0.2 pH units, and in step (d) the target pH is recovered by obtaining a pH value within the tolerance range. More preferably, a single target pH (preferably comprising the aforementioned tolerance range) is defined and thus recoverable throughout the lifetime of the aqueous deposition bath.In other words, a single target pH (preferably comprising the tolerance range mentioned above) is defined for all steps (a) and recovered in a number of steps (d) if the method of the present invention is carried out as a continuous method. In other applications it is preferred that the target pH in step (a) (preferably within one of the tolerance ranges mentioned above) be within the preferred pH range of 5.0 to 6.0 (preferably within the pH range of 5.3 to 5.9), and in step (d) the target pH is recovered to a pH within that pH range of 5.0 to 6.0 (preferably within that pH range of 5.3 to 5.9). In step (d), NhUOH and / or NH3 are added. Preferably, no other hydroxides are added. In the method of the present invention, NH4OH and NH3 are the only compounds used in step (d). In the method of the present invention, the trivalent chromium ions in the aqueous deposition bath are from a soluble source containing trivalent chromium ions, typically a water-soluble salt comprising the trivalent chromium ions. Preferably, the soluble source containing trivalent chromium ions comprises alkali metal cations in a total amount of 1% by weight or less, based on the total weight of the source. More preferably, this source is used to replenish the trivalent chromium ions if the method is operated continuously. A preferred water-soluble salt comprising the trivalent chromium ions is alkali metal-free trivalent chromium sulfate or alkali metal-free trivalent chromium chloride.Thus, in some cases it is preferred that the aqueous deposition bath used in the method of the present invention contains sulfate ions, preferably in a total amount in the range of 50 g / L to 250 g / L, based on the total volume of the deposition bath. A method of the present invention is preferred, wherein the soluble source containing trivalent chromium ions is used in the aqueous deposition bath in a total weight of less than 100 g per liter of aqueous deposition bath, particularly if the aqueous deposition bath is freshly prepared. Typically, if the trivalent chromium ions are replenished during the use of the aqueous deposition bath, significantly less than 100 g per liter of aqueous deposition bath is preferably used. A method of the present invention is preferred, wherein the total amount of trivalent chromium ions in the deposition bath is in the range of 10 g / L to 30 g / L, based on the total volume of the deposition bath, preferably in the range of 17 g / L to 24 g / L. If the total amount is significantly less than 10 g / L, insufficient deposition is often observed, and the deposited chromium or chromium alloy layer is usually of poor quality. If the total amount is significantly greater than 30 g / L, the deposition bath is no longer stable, which includes the formation of undesirable precipitates. In the method of the present invention, the aqueous deposition bath comprises bromide ions, preferably in a total amount of at least 0.06 mol / L, based on the total volume of the deposition bath, preferably at least 0.1 mol / L, more preferably at least 0.15 mol / L. The bromide ions effectively suppress the formation of anodically formed hexavalent chromium. In the method of the present invention, the aqueous deposition bath preferably contains at least one additional compound selected from the group consisting of at least one organic complexing compound and ammonium ions. The preferred organic complexing compounds are organic carboxylic acids and salts thereof, preferably aliphatic monocarboxylic organic acids and salts thereof. More preferably, the aforementioned organic complexing compounds (and their preferred variants) have from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, and even more preferably from 1 to 3 carbon atoms. The complexing compounds preferably form complexes with trivalent chromium ions in the aqueous deposition bath to increase the bath's stability. Preferably, the molar ratio of trivalent chromium ions to organic complexing compounds is in the range of 1:0.5 to 1:10.The ammonium ions are provided either solely by means of NH4OH and the NH3 added in step (d) of the method of the present invention or are added additionally, preferably in step (d). A method of the present invention is preferred, wherein the aqueous deposition bath does not contain sulfur-containing compounds with a sulfur atom having an oxidation number lower than +6 and boron-containing compounds. The term “does not contain” denotes that, for example, sulfur-containing compounds and boron-containing compounds are not intentionally added to the aqueous deposition bath. In other words, the aqueous deposition bath is substantially free of these compounds. This does not preclude these compounds from being carried over as impurities from other chemical compounds (preferably a total amount of less than 10 mg / L of sulfur-containing compounds and a total amount of less than 10 mg / L of boron-containing compounds, each based on the total volume of the deposition bath). However, typically the total amount of these compounds is below the detection range and is therefore not critical during step (c) of the method of the present invention. It is assumed that the absence of sulfur-containing compounds results in an amorphous chromium layer and chromium alloy layer, respectively. Therefore, a method of the present invention is preferred, wherein the layer deposited in step (c) is amorphous, as determined by x-ray diffraction. This applies to the chromium or chromium alloy layer obtained during step (c) of the method of the present invention and before any further surface treatment after deposition that affects the atomic structure of the deposited layer, modifying it from amorphous to crystalline or partially crystalline. It is further assumed that these sulfur-containing compounds negatively affect the hardness of the functional chromium or functional chromium alloy layer deposited in step (c). In the aqueous deposition bath used in the method of the present invention, boron-containing compounds are undesirable because they are environmentally problematic. The presence of boron-containing compounds makes wastewater treatment expensive and time-consuming. Furthermore, boric acid, which is known to also function as a buffer, typically exhibits poor solubility and therefore tends to form precipitates. Although these precipitates can be solubilized upon heating, a corresponding aqueous deposition bath cannot be used during this time. There is a significant risk that these precipitates will contribute to undesirable surface roughness. Thus, the aqueous deposition bath used in the method of the present invention preferably does not contain boron-containing compounds.Surprisingly, the aqueous deposition bath used in the method of the present invention works very well without boron-containing compounds, particularly in the preferred pH ranges mentioned above. In the method of the present invention, hexavalent chromium is not intentionally added to the aqueous deposition bath. Thus, the aqueous deposition bath contains no hexavalent chromium except for very small amounts that may form anodically. The aqueous deposition bath used in the method of the present invention is sensitive to a number of undesirable metal cations. Accordingly, a method of the present invention is preferred wherein the aqueous deposition bath contains copper ions, zinc ions, nickel ions, and iron ions, each independently in a total amount of 0 mg / L to 40 mg / L, based on the total volume of the deposition bath, preferably each independently in a total amount of 0 mg / L to 20 mg / L, and more preferably each independently in a total amount of 0 mg / L to 10 mg / L. This preferably also includes compounds comprising the metal cations. More preferably, none of the aforementioned metal cations are present at all, i.e., each is present independently in a total amount of zero mg / L.However, the results of our own experiments have shown that a small amount of these metal cations can be tolerated. If such small amounts are present, they are insufficient to serve as alloying metal to form a chromium alloy layer on at least one substrate. If the total amount mentioned above is significantly exceeded, the chromium layer and the chromium alloy layer deposited in step (c) of the method of the present invention exhibit undesirable discoloration. Even more preferably, in the aqueous deposition bath used in the method of the present invention, chromium is the only side group element. Furthermore, a method of the present invention is preferred, wherein the aqueous deposition bath does not comprise glycine, aluminum ions, and tin ions. This ensures a functional chromium or chromium alloy layer, respectively, with the desired attributes as set forth throughout the text. Our own experiments have shown that in a number of cases, aluminum and tin ions, particularly aluminum ions, significantly perturb and even inhibit deposition in step (c). In step (b) of the method of the present invention, at least one substrate and at least one anode are provided, wherein at least one substrate is the cathode. Preferably, more than one substrate is used simultaneously in the method of the present invention. p / oznn / eznz / E / YiAi A method of the present invention is preferred, wherein at least one substrate provided in step (b) is a metal or metal alloy substrate, preferably a metal or metal alloy substrate comprising one or more of a metal selected from the group consisting of copper, iron, nickel, and aluminum, more preferably a metal or metal alloy substrate comprising iron. Most preferably, at least one substrate is a steel substrate, which is a substrate of a metal alloy comprising iron. In many technical applications, a steel substrate with a smooth, wear-resistant, functional chromium or chromium alloy layer is required. This can be achieved particularly by the method of the present invention. In some cases, a method of the present invention is preferred, wherein at least one substrate, preferably the metal substrate, and more preferably the steel substrate, does not exhibit a pre-ground and / or pre-polished surface. In other words, the substrate surface exhibits an initial average surface roughness (Rade) of 0.2 µm or more before step (c) of the method of the present invention, for example, an average surface roughness of 0.25 µm or more, or even 0.3 µm or more. In such cases, the method of the present invention advantageously (i) does not increase the average surface roughness of the substrate to an undesirable degree after step (c) is completed and (ii) ensures a consistently low average surface roughness during prolonged use of the aqueous deposition bath. However, in other cases, it is preferable that the surface already be pre-ground and / or pre-polished. In that case, the initial average surface roughness Ra is preferably less than 0.2 µm before step (c) of the method of the present invention, for example, 0.17 µm or less. In such cases, the method of the present invention also does not increase the average surface roughness of the substrate to an undesirable degree after step (c) is completed and (i) ensures a consistently low average surface roughness during prolonged use of the aqueous deposition bath. In some cases, at least one substrate is preferably a coated substrate, more preferably a coated metal substrate (for preferred metal substrates, see the preceding text). The coating is preferably a layer of metal or metal alloy, preferably a layer of nickel or nickel alloy, and more preferably a semi-bright nickel layer. In particular, a steel substrate coated with a layer of nickel or nickel alloy is preferred. However, other coatings are preferably present alternatively or additionally. In many cases, a coating significantly increases corrosion resistance compared to a metal substrate without that coating. However, in some cases, substrates are not susceptible to corrosion due to a corrosion-inert environment (e.g., in an oil bath).In that case, a coating is not necessarily necessary, preferably a layer of nickel or nickel alloy. Therefore, a method of the present invention is preferred, wherein in step (c) the chromium or chromium alloy layer is deposited directly onto at least one substrate, or at least one substrate defined in step (b) additionally comprises a nickel or nickel alloy layer and in step (c) the chromium or chromium alloy layer is deposited onto it. A method of the present invention is preferred, wherein at least one anode is independently selected from the group consisting of graphite anodes and metal oxide anodes (MMO), preferably independently selected from the group consisting of graphite anodes and mixed metal oxide anodes on titanium. These anodes have been shown to be sufficiently resistant in the deposition bath of the present invention. Preferably, at least one anode does not contain lead or chromium. In step (c) of the method of the present invention, either a chromium layer or a chromium alloy layer is deposited. In most cases, a method of the present invention is preferred, wherein the layer deposited in step (c) of the method of the present invention is a chromium alloy layer. The preferred alloying elements are carbon and oxygen. Carbon is typically present due to the organic compounds usually present in the aqueous deposition bath. Preferably, the chromium alloy layer does not comprise one, more than one, or all of the elements selected from the group consisting of sulfur, nickel, copper, aluminum, tin, and iron. More preferably, the only alloying elements are carbon and / or oxygen, most preferably carbon and oxygen.Preferably, the chromium alloy layer contains 90 percent by weight of chromium or more, on the basis of the total weight of the alloy layer, more preferably 95 percent by weight or more. A method of the present invention is preferred, wherein the cathodic current density of the direct electric current is in the range of 5 A / dm2 to 100 A / dm2, preferably in the range of 10 A / dm2 to 70 A / dm2, more preferably in the range of 20 A / dm2 to 60 A / dm2. The electric current is a direct current (DC), preferably a direct current without interruption during the passage (c). The direct current is preferably non-pulsed (non-pulsed DC). Furthermore, the direct current preferably does not include reverse pulses. As mentioned previously, in the method of the present invention, the layer obtained in step (c) is preferably a functional chromium or functional chromium alloy layer (also frequently referred to as a hard chromium or hard chromium alloy layer) and not a decorative chromium or chromium alloy layer. Accordingly, a method of the present invention is preferred wherein the average layer thickness of the chromium or chromium alloy layer deposited in step (c) is 1.0 µm or more, preferably 2 µm or more, more preferably 4 µm or more, and even more preferably 5 µm or more. Most preferably, the average layer thickness is in the range of 5 µm to 200 µm, and preferably 5 µm to 150 µm. These are typical average layer thicknesses for functional chromium or chromium alloy layers. These thicknesses are necessary to provide the required wear resistance, which is typically demanded.In some cases the lower limit preferably and specifically includes 10 pm, 3 pm or 8 pm. A method of the present invention is preferred, wherein the aqueous deposition bath in step (c) has a temperature in the range of 20°C to 90°C, preferably in the range of 30°C to 70°C, more preferably in the range of 40°C to 60°C, and more preferably in the range of 45°C to 60°C. If the temperature significantly exceeds 90°C, undesirable evaporation occurs, which adversely affects the concentration of the bath components (even to the point of risk of precipitation). Furthermore, the undesirable anodic formation of hexavalent chromium is significantly less suppressed. If the temperature is significantly below 20°C, the deposition is insufficient. Temperatures significantly lower than 40°C are generally acceptable, but in a few cases the quality of p / oznn / eznz / E / YiAi deposition and the degree of deposition are not sufficient, particularly between 20°C and 35°C.In a number of cases, the chromium and chromium alloy layer becomes undesirably dull, layer adhesion and deposition rate are low, and reproducibility is sometimes difficult. However, optimal and improved results are obtained at a temperature of at least 40°C, preferably in the range of 40°C to 90°C, more preferably in the range of 40°C to 70°C, and even more preferably in the range of 40°C to 60°C. The most preferred temperature is at least 45°C, preferably in the range of 45°C to 90°C, more preferably in the range of 45°C to 70°C, and even more preferably in the range of 45°C to 60°C. During step (c), the aqueous deposition bath is preferably continuously agitated, preferably by shaking. A method of the present invention is preferred, wherein the layer deposited in step (c) has an average surface roughness Rade of 0.6 µm or less, based on an average layer thickness of at least 20 µm, preferably 0.5 µm or less, and more preferably 0.4 µm or less. This applies most preferably to steel substrates (preferably steel substrates as described herein). A substrate with a chromium or chromium alloy layer exhibiting that average layer thickness is much less desirable and can be readily subjected to common finishing and / or superfinishing steps. In many cases, a method of the present invention is preferred, wherein the substrate obtained after step (c) is subjected to heat treatment at a temperature of 250°C or less. This heating is typically applied to harden the chromium or functional chromium alloy layer. However, a method of the present invention is preferred that does not include, after step (c), a heat treatment step at a temperature of 500°C or more, preferably 400°C or more, even more preferably 300°C or more, and most preferably 260°C or more. In the method of the present invention, it is preferred that at least one substrate and at least one anode be present in the aqueous deposition bath so that the trivalent chromium ions are in contact with at least one anode. In this preferred method, a membrane or diaphragm for separating the trivalent chromium ions from the anode can be completely avoided (i.e., no additional compartments are formed). In other words, in the method of the present invention, no separation means are used to separate the trivalent chromium ions in the deposition bath from the anode. This reduces maintenance costs and allows for simplified operation of the method of the present invention. As mentioned above, the present invention also relates to an aqueous deposition bath. With respect to this deposition bath, the features mentioned above regarding the method of the present invention (including features specifically related to the aqueous deposition bath used in that method) preferably apply equally to the aqueous deposition bath of the present invention and vice versa. More preferably, the aqueous deposition bath according to the present invention is used in the method of the present invention discussed above. For further details regarding the amount of trivalent chromium ions in the p / oznn / eznz / E / YiAi deposition bath and the molar ratios with complexing compounds, see the text above regarding the method of the present invention. An aqueous deposition bath according to the present invention is preferred, wherein at least one organic complexing compound is selected from the group consisting of organic carboxylic acids and salts thereof, preferably selected from the group of aliphatic monocarboxylic organic acids and salts thereof. More preferably, the aforementioned organic complexing compounds (and preferred variants thereof) have from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, and even more preferably from 1 to 3 carbon atoms. An aqueous deposition bath according to the present invention is preferred, wherein the sum of the total weight of trivalent chromium ions and the total weight of ammonium ions corresponds to 90% by weight or more of the total weight of all cations in the aqueous deposition bath, preferably 95% by weight or more, and more preferably 98% by weight or more. In this way, essentially the entire quantity of cations in the deposition bath consists of trivalent chromium ions and ammonium ions. In the aqueous deposition bath according to the present invention, at least one halide ion species is bromide. Preferably, the total amount of bromide ions in the deposition bath is at least 0.06 mol / L, based on the total volume of the deposition bath, preferably at least 0.1 mol / L, and more preferably at least 0.15 mol / L. An aqueous deposition bath according to the present invention is preferred, wherein the total amount of alkali metal cations in the aqueous deposition bath is in the range of 0 mol / L to 0.5 mol / L, preferably in the range of 0 mol / L to 0.3 mol / L, more preferably in the range of 0 mol / L to 0.1 mol / L, and more preferably in the range of 0 mol / L to 0.08 mol / L. An aqueous deposition bath according to the present invention is preferred, which does not contain iron ions, nitrogen-containing compounds other than NH4+ and NH3, and reducing agents to reduce trivalent chromium ions. An aqueous deposition bath according to the present invention is preferred, wherein the soluble source containing trivalent chromium ions comprises or is chromium sulfate, preferably acid chromium sulfate, more preferably chromium sulfate with the general formula Cr₂(SO₄)₃ and a molecular weight of 392 g / mol. This chromium sulfate may preferably be used in a total weight of significantly less than 100 g per liter of aqueous deposition bath and usually further comprises alkali metal cations in a total amount of 1% by weight or less, based on the total weight of the chromium sulfate used. It is desirable to keep the total weight of the soluble source containing trivalent chromium ions used in the aqueous deposition bath according to the present invention below 100 grams per liter.This reduces the risk of contamination, not only from alkali metal cations, but also from other undesirable cations (see above) and chromium counterions. It should be noted that the method of the present invention and the aqueous deposition bath of the present invention are specifically designed for industrial application and long-term use (i.e., use for weeks and months). During such prolonged use, the trivalent chromium ions in the aqueous deposition bath are replenished several times, so it is assumed that it also contains small amounts of contaminants in the soluble source containing trivalent chromium ions, which accumulate significantly over time. This effect can be minimized by optimizing the amount of the soluble source containing trivalent chromium ions used. Preferably, the soluble source containing trivalent chromium ion is used in the aqueous deposition bath of the present invention in its dissolved form as an aqueous solution. Thus, the soluble source containing trivalent chromium ions does not contain crystallized water (if present in a solid form). Therefore, it is preferable that the soluble source containing trivalent chromium be used in a non-solid form (i.e., not in its solid form when used for replenishment). If the source is solid, it can, in some cases, adversely affect the average surface roughness. Thus, an aqueous deposition bath according to the present invention is preferred, wherein the source is used in a total weight in the range of 70 ga 99.9 g per liter of aqueous deposition bath, preferably in the range of 70 ga 99 g, more preferably in the range of 70 ga 95 g, more preferably in the range of 70 ga 90 g. An aqueous deposition bath according to the present invention is preferred, wherein the pH of the bath is in the range of 4.5 to 6.5, preferably in the range of 5.0 to 6.0, more preferably in the range of 5.3 to 5.9. For further details regarding pH, see the text above concerning the method of the present invention. The present invention is described in greater detail by the following non-limiting examples. Examples Example 1 In the first step, five deposition bath samples ((I), (II), (III), (IV), and (V), each approximately 1 L in volume, were prepared. Each sample contained an identical amount of trivalent chromium ions, typically 10 g / L to 30 g / L, sulfate ions, 50 g / L to 250 g / L, at least one organic complexing compound (an aliphatic organic monocarboxylic acid), ammonium ions, and bromide ions. No boron-containing compounds were used. In each deposition bath sample, a soluble source containing dissolved trivalent chromium ion (Cr₂(S₄)₃; molecular weight: 392 g / mol) was used. The total amount of source in each case was approximately 75 g per liter of aqueous deposition bath sample, which is significantly less than 100 g per liter of deposition bath sample. Furthermore, the source was substantially free of alkali metal cations. The pH of each sample ranged from 5.3 to 5.9 (at 20°C) and was adjusted using alkali metal cation-free compounds. However, each deposition bath sample differed in the total amount of alkali metal cations, represented by sodium ions (molar mass of 23 g / mol), as shown in Table 1. The amounts of sodium ions were intentionally added to the corresponding deposition bath samples. The total amount was based on the total volume of the corresponding deposition bath sample. Only in deposition bath sample (I) were sodium ions not intentionally added. p / oznn / eznz / E / YiAi Table 1 Na+ deposition bath sample [g / L] (i) 0 (II) 20 (IN) 40 (iv) 60 (V) 80 ρ / οζηη / ρζηζ / Ε / γίΛΐ The deposition bath samples (I) and (II) are in accordance with the present invention, wherein samples (III), (IV) and (V) are comparative examples. In a second step, five specimens (10 mm diameter rolled steel rods with pre-ground and polished surfaces; initial Ra < 0.2 pm) were pre-coated with a semi-bright nickel layer (standard nickel deposition bath, Atotech, Mark 1900; cathode current density 4 A / dm² for 10 minutes) to obtain nickel-coated specimens. After nickel deposition, the average surface roughness Ra of the nickel-coated specimens was still < 0.2 pm. In a third step, nickel-coated specimens were subjected to chromium deposition in the aforementioned deposition bath samples under respective deposition scenarios, and a chromium alloy layer comprising minimal amounts of carbon was deposited. In each scenario, deposition was carried out for 45 minutes at a cathodic current density of 40 A / dm² with graphite anodes and at a temperature of 50°C. After the third step, chromium-deposited specimens ((i), (ii), (iii), (iv), and (v)) were obtained with an average layer thickness in the range of 25 µm to 30 µm, exhibiting an average surface roughness (Ra) as summarized in Table 2 (see also Figure 1). Table 2 specimen sample deposition bath Ra [pm] () (I) 0.17 (i) (H) 0.20 (i) (III) 0.25 (iv) (IV) 0.26 (v) (V) 0.34 Specimens (i) and (II) were treated in a deposition bath containing a total sodium ion concentration below 1 mol / L, thus representing the advantage of the method of the present invention. Specimen (i) represents the most preferred case of complete absence of contamination with alkali metal cations (zero g / L sodium ions). As a result, specimen (i) exhibits the lowest average surface roughness, below 0.20 µm, which is one of the most desirable outcomes. However, specimen (II) also exhibits an excellent average surface roughness of approximately 0.2 µm. It can be concluded that the chromium deposition in the third step did not significantly increase the initial average surface roughness of the specimens. Specimens (iii), (iv), and (v) were treated in deposition bath samples comprising more than 1 mol / L of alkali metal cations, thus representing a disadvantage of many trivalent chromium deposition methods that use compounds containing alkali metal cations. This disadvantage is a significantly increased average surface roughness. The average surface roughness for each specimen was determined on the middle part of the rod at four different positions. As strongly indicated by Example 1, the average surface roughness increases with an increase in the total amount of alkali metal cations and thus clearly shows the relationship between the total amount of alkali metal cations in an aqueous deposition bath and the average surface roughness of the respective substrates. This relationship can be advantageously used in the method of the present invention to control the average surface roughness of substrates by carefully using alkali metal-free hydroxides. Example 2 In the first step, a 25 L aqueous deposition bath was provided with a target pH within the range of 5.3 to 5.9 (at 20°C) and encompassing a tolerance range of ±0.3 pH units. The pH was adjusted using alkali metal cation-free compounds. The bath also contained a typical amount of 10 g / L to 30 g / L of trivalent chromium ions, 50 g / L to 250 g / L of sulfate ions, at least one organic complexing compound (an aliphatic monocarboxylic organic acid), ammonium ions, and bromide ions (boron-containing compounds were not used). The total amount of alkali metal cations was almost zero, and thus within the highly preferred range of 0 mol / L to 0.2 mol / L.To prepare the aqueous deposition bath, a soluble source containing trivalent chromium ion, substantially free of alkali metal cation, was used (which is typically a source comprising alkali metal cations in a total amount of 1% by weight or less, on the basis of the total weight of the source) in a total amount significantly less than 100 g per liter of deposition bath (for details see Example 1). In a second step, a plurality of specimens (nickel-coated mild steel rods with a diameter of 10 mm; not pre-ground and not polished; Initial Rain > 0.2 pm) were provided and subsequently subjected to nickel deposition as described in Example 1. The anodes used during Example 2 were graphite anodes. In a third step, some of the specimens were sequentially immersed in the aqueous deposition bath, and a direct current (DC) of 40 A / dm² cathodic current density was applied for 45 minutes. The deposition bath temperature was 50°C. As a result, a layer of chromium alloy (containing chromium and minimal amounts of carbon) was deposited on these first specimens. After some specimens had been treated (i.e., step (c) of the method of the present invention was repeated several times), the target pH of the deposition bath was lower than the target pH. Therefore, it was necessary to add p / oznn / eznz / E / YiAi hydroxide to restore a pH value within the aforementioned tolerance range. For this purpose, NHUOH was used. This procedure continued until a deposition bath utilization of approximately 180 Ah / L was achieved.Until 180 Ah / L was reached, twelve specimens were treated according to the method of the present invention. In Figure 2, this process is described and graphically designated as section A. Figure 2 also shows the average surface roughness (Ra) for each of the twelve specimens. Most specimens exceeded an average surface roughness of 0.4 pm; an average surface roughness of 0.6 pm was never exceeded. It can be concluded that during section A, the average surface roughness was comparatively low and constant for a functional chromium alloy layer obtained from an aqueous deposition bath as defined above. After 180 Ah / L (i.e., after the 12d° specimen was treated), the target pH was recovered by adding NaOH instead of NH4OH (not in accordance with the method of the present invention). A number of specimens were used for simulated coating during the 180 Ah / L to 230 Ah / L range to allow the deposition bath to adjust to this difference. Figure 2, section B (from 230 Ah / L to 390 Ah / L) represents a comparative example, showing the effect on average surface roughness if NaOH is used. As described in section B of Figure 2, the average surface roughness increases rapidly to 1 µm or even more. Example 2 was determined at 390 Ah / L throughout the entire use (approximately 4 months) of the aqueous deposition bath (or, in other words, after another 160 Ah / L starting at 230 Ah / L), and 9 additional specimens were treated during the comparative example. The last specimen obtained after 390 Ah / L exhibited a maximum average surface roughness of more than 1.6 µm. Figure 2 strongly indicates that even beyond 390 Ah / L, the average surface roughness would most likely increase even further. The average layer thickness of the deposited chromium alloy layer was in the range of 25 µm to 30 µm for each specimen (sections A and B). The average surface roughness was determined as described in Example 1. The comparative example shows that the deposition during section B is no longer controlled in the sense of the present invention, i.e., in terms of the average surface roughness of the treated substrates. This example further indicates that during prolonged use of an aqueous deposition bath, the average surface roughness continuously increases to undesirable values, such as beyond 0.8 µm. This increase can be successfully avoided by the method of the present invention. Based on section A, it can be reasonably concluded that the average surface roughness remains comparatively constant during at least prolonged use of the deposition bath, even throughout the bath's lifetime. Furthermore, functional chromium alloy layers, characterized by excellent hardness and wear resistance as mentioned above, were obtained in section A.

Claims

1. A steel substrate comprising a chromium alloy layer deposited thereon, characterized in that the chromium alloy layer is deposited directly onto the steel substrate, or the steel substrate further comprises a nickel or nickel alloy layer and the chromium alloy layer is deposited thereon, comprises carbon and oxygen, does not comprise alkali metal cations, has an average layer thickness in the range of 5 pm to 200 pm, and has an average surface roughness Ra of 0.6 pm or less, based on an average layer thickness of at least 20 pm.

2. The steel substrate comprising a chromium alloy layer deposited thereon according to claim 1, further characterized in that it has a Vickers hardness of at least 700 HV(0.05).

3. The steel substrate comprising a chromium alloy layer deposited thereon according to claim 1 or 2, further characterized in that the chromium alloy layer is amorphous, as determined by x-ray diffraction.

4. The steel substrate comprising a chromium alloy layer deposited thereon in accordance with any of claims 1 to 3, further characterized in that the chromium alloy layer has an average layer thickness in the range of 5 pm to 150 pm.

5. The steel substrate comprising a chromium alloy layer deposited thereon in accordance with any of claims 1 to 4, further characterized in that the chromium alloy layer has a lower average layer thickness limit of 10 pm, 15 pm, or 20 pm.

6. The steel substrate comprising a chromium alloy layer deposited thereon in accordance with any of claims 1 to 5, further characterized in that the nickel or nickel alloy layer is a semi-bright nickel layer.

7. The steel substrate comprising a chromium alloy layer deposited thereon according to any of claims 1 to 6, further characterized in that the chromium alloy layer contains 90% by weight or more of chromium, based on the total weight of the chromium alloy layer, more preferably 95% by weight or more.

8. The steel substrate comprising a chromium alloy layer deposited thereon in accordance with any of claims 1 to 7, further characterized in that the chromium alloy layer has an average surface roughness Ra of 0.5 pm or less, preferably 0.4 pm or less.

9. The steel substrate comprising a chromium alloy layer deposited thereon in accordance with any of claims 1 to 8, further characterized in that the chromium alloy layer is a hard chromium alloy layer.

10. The steel substrate comprising a chromium alloy layer deposited thereon according to any of claims 1 to 9, further characterized in that the chromium alloy layer is deposited from trivalent chromium ions. p / oznn / eznz / E / YiAi 11. The steel substrate comprising a chromium alloy layer deposited thereon according to any one of claims 1 to 10, further characterized in that the steel substrate has an initial average surface roughness Ra of less than 0.2 pm, preferably 0.17 pm or less.

12. The steel substrate comprising a chromium alloy layer deposited thereon according to any one of claims 1 to 11, further characterized in that the chromium alloy layer does not comprise sulfur.