[0001] This invention relates to a method for electroplating an uncoated steel strip with a plating layer and an improvement thereof.

[0002] In continuous steel strip plating, a cold-rolled steel strip is provided which is usually annealed after cold-rolling to soften the steel by recrystallisation annealing or recovery annealing. After the annealing and before plating the steel strip is first cleaned for removing oil and other surface contaminants. Mostly, an alkaline cleaner is used for this purpose, wherein steel is electrochemically passive, i.e. the steel strip surface is covered with a stable and protective oxide film and therefore the steel will not dissolve in the alkaline cleaner. The alkaline cleaner is a complex mixture of various ingredients. The main component is caustic soda for providing alkalinity, conductivity, and saponification. Other common components are sodium metasilicate, sodium carbonate, phosphates, borates, and surfactants.

[0003] After the cleaning step, the steel strip is pickled in a sulphuric or hydrochloric acid solution for removing the oxide film. Between different treatment steps the steel strip is always rinsed with deionised water to prevent contamination of the solution used for the next treatment step with solution of the preceding treatment step. Consequently the steel strip is thoroughly rinsed after the pickling step. During rinsing and transport of the steel strip to the plating section a fresh thin oxide layer is formed instantly on the bare steel surface.

[0004] The process used in electroplating is called electrodeposition. The part to be plated (the steel strip) is the cathode of the circuit. The anode of the circuit may be made of the metal to be plated on the part (dissolving anode, such as those used in conventional tinplating) or a dimensionally stable anode (which does not dissolve during plating). Both components are immersed in a solution called an electrolyte. At the cathode, the metal ions in the electrolyte solution are reduced at the interface between the solution and the cathode, such that they deposit onto the cathode.

[0005] In many cases electrolytes are acidic solutions. As a consequence the oxide layer that was formed after the pickling step will dissolve rapidly. Bare steel without any oxide film is prone to corrosion. Corrosion means that iron from the steel substrate is oxidised to Fe ²⁺ , where the liberated electrons are consumed by the reduction of hydrogen ions or oxygen gas that is dissolved in the electrolyte.

2H ⁺ + 2e ^" → H ₂ (g) 0 ₂ (g) + 4H ⁺ + 4e ^" → 2H ₂ 0

The consequence is that the electrolyte becomes enriched in Fe ²⁺ . Depending on the electrolyte these Fe ²⁺ -ions are subsequently reduced in the following electroplating step to Fe and this Fe is deposited onto the substrate along with the metal that is intended to be plated onto the substrate. The codeposited iron adversely affects the properties of the plated layer, particularly the corrosion performance.

[0006] It is an object of the present invention to provide an improved method for electroplating an uncoated steel strip with a plating layer from a trivalent Cr- electrolyte.

[0007] It is also an object of the present invention to provide a steel strip with a plating layer produced by electroplating an uncoated steel strip using a trivalent Cr- electrolyte with improved properties.

[0008] One or more of the objects is reached by a method for electroplating an uncoated steel strip with a plating layer from a trivalent Cr-electrolyte, wherein the uncoated strip is subjected to a cleaning and pickling step prior to the plating process to remove oxides and any other contaminants present on the surface or surfaces of the strip, and wherein the strip is subsequently subjected to a plating process in a plating section comprising of a series of consecutive plating cells, wherein in a first stage of the plating process a current is applied to the strip entering the first plating cell which current is insufficient to deposit a plating layer from the trivalent Cr-electrolyte, but which is sufficient to provide cathodic protection of the strip in the electrolyte, and wherein in a second stage of the plating process a higher current is applied to the strip to deposit a plating layer comprising chromium metal, chromium carbide and chromium oxide from the trivalent Cr-electrolyte according to the invention.

[0009] US3316160 discloses a process for preventing a bluish tint on a chromium plated steel strip from a chromic acid plating solution in a plating operation involving two or more vertical plating tanks. In the process the current density is high in the first downward and upward pass to effect electrolytic chromium plating. The steel strip is then led into a second plating tank and the current density is much lowered in the second downward pass, and any subsequent downward pass, and back to the high level of current density again in the second upward pass. This treatment of low and high current density during the downward and upward pass is repeated in every subsequent tank. The reduction in current density during the upward pass removes the film of complex chromium oxide that is responsible for the bluish tint.

The invention is explained by referring to a specific lay-out of a plating section used in industry, but it should be noted that the invention is not intended to be limited thereto, and is applicable to any plating section comprising a series of consecutive plating cells. In an embodiment of the invention a plating section consists of a series of vertical plating cells for obtaining a sufficient total anode length on a limited floor space. In the method known in the art no current is applied during the first down-pass. In the first down-pass, where the strip enters the plating solution for the first time, the remaining water film sticking to the steel strip surface from the rinsing step is replaced by the electrolyte that is present in the plating cells and also the steel strip is heated to the temperature of the electrolyte. When the steel strip is exposed to the electrolyte the oxide layer that was formed after the pickling step will dissolve rapidly (see figure 1). In the method according to the invention a current is applied to the strip entering the electrolyte for the first time (see figure 2). It is essential that the current is chosen such that no deposition of a plating layer is achieved, but that the potential of the steel in the electrolyte is shifted such that the steel strip is cathodically protected and does not dissolve. In the method according to the invention the electrolyte in the first plating cell is therefore not being enriched in Fe ²⁺ , whereas the electrolyte in the first plating cell in the prior art method is being enriched in Fe ²⁺ . This lack of enrichment of the electrolyte in the first plating cell therefore prevents the drag-out of Fe ²⁺ to subsequent plating cells. In subsequent plating cells the current is increased to deposit a plating layer comprising chromium metal, chromium carbide and chromium oxide from the trivalent Cr-electrolyte. Iron in the Cr(III) electrolyte deposits on the strip together with chromium. It was found that iron in the Cr-CrCx-CrOx coating adversely affects the corrosion performance. Therefore, it is important to keep the iron level in the Cr(III) electrolyte as low as possible. This is achieved by applying a small current at least in the first down-pass, and preferably also in all other passes which are not in use for plating. The method according to the invention can be applied in any inactive plating cell in the series of plating cells through which a strip to be plated is led. With inactive plating cell the plating cell is meant through which the strip is led, but in which no plating action takes place, for instance when one or more plating cells are skipped, but through which the strip has to be led due to the construction of the entire plating facility. In an embodiment of the invention the electrolyte is acidic.

[OOIO] In a study about the deposition mechanism of chromium layers from trivalent chromium electrolytes (J.H.OJ. Wijenberg, M. Steegh, M.P. Aarnts, K.R. Lammers, J.M.C. Mol, Electrodeposition of mixed chromium metal-carbide-oxide coatings from a trivalent chromium-formate electrolyte without a buffering agent, Electrochim. Acta 173 (2015) 819-826.), it was found that the trivalent chromium plating process is very different from regular plating processes, in which the metal ions are directly reduced by an electrical current to metal : Me ⁿ⁺ + ne— > Me. This process is known, for instance, from the tinplating process. In contrast, the Cr(III) plating process is based on a fast, stepwise deprotonation of the water ligands in the Cr(III)-complex ion induced by a surface pH increase due to the hydrogen evolution reaction. This leads to the existence of a so called 'regime Γ, wherein no metal is deposited even though an electrical current is applied (see figure 3). Applying a small current evokes the hydrogen evolution reaction. The removal of H ⁺ -ions ions is accompanied by a surface pH increase, which leads to following acid-base reaction :

[Cr(HCOO)(H ₂ 0) ₅ ] ²⁺ + OH-→ [Cr(HCOO)(OH)(H ₂ 0) ₄ ] ⁺ + H ₂ 0

The existence of regime I is unique for the Cr(III) plating process and is absent in regular plating processes. The inventors arrived at the novel idea to make advantageously use of this special feature of the Cr(III) plating process. By applying a small current in the first down-pass not only a small amount of hydrogen gas is formed, but also the potential of the steel shifts in negative direction, a phenomenon known as cathodic protection. Due to the negative potential the steel strip will not corrode anymore. The steel strip is not only protected against corrosion, but also (part of) the iron oxide film will be reduced to iron metal, thereby reducing the iron pick up in the electrolyte even further. Obviously, the water film will still be replaced by the electrolyte when a current is applied and also the steel strip will be heated. The current that must be applied for protecting the steel strip can be very small. The upper limit is restricted by the onset of regime II (see figure 3).

[Cr(HCOO)(OH)(H ₂ 0)4] ⁺ + OH-→ Cr(HCOO)(OH) ₂ (H ₂ 0)3 + H ₂ 0

Cr(HCOO)(OH) ₂ (H ₂ 0) ₃ forms a deposit on the cathode. A part of the Cr(III) of the deposit is reduced to Cr-metal and formate is broken down leading to the formation of Cr-carbide. If the Cr(III) is not fully reduced to Cr-metal, then Cr- oxide is also present in the deposit. The amount and composition of the deposit depend on the applied current density, mass flux and electrolysis time. The threshold value of the current density for entering regime II increases with increasing line speed, because it is related to the mass flux of H ⁺ as is explained in the article mentioned above. The surface pH increase, which is required to deposit Cr(HCOO)(OH) ₂ (H ₂ 0) ₃ , is thwarted by the faster replenishment of H ⁺ from the bulk of the electrolyte to the electrode surface. Consequently, a higher current density is required with increasing line speed for obtaining the same pH increase at the electrode surface. There is therefore not a fixed threshold value where regime I ends and regime II starts, but it is easy to determine this threshold value by simply monitoring the onset of the deposition of the plating layer as a function of the current density by means of simple experimentation. The regimes I - III are visible when the deposition of chromium is plotted against the current density (cf. for example Figure 4). Regime I is the region where there is a current, but no deposition yet. The surface pH is insufficient for chromium deposition. Regime II is when the deposition starts and the total chromium coating weight increases with the current density until it peaks and drops of in regime III where the deposit starts to dissolve:

Cr(HCOO)(OH) ₂ (H ₂ 0) ₃ + OH ^" → [Cr(HCOO)(OH) ₃ (H ₂ 0) ₂ ] ^" + H ₂ 0

[0011] A high speed continuous plating line is defined as a plating line through which the substrate to be plated, usually in the form of a strip, is moved at a speed of at least 100 m/min. A coil of steel strip is positioned at the entry end of the plating line with its eye extending in a horizontal plane. The leading end of the coiled strip is then uncoiled and welded to the tail end of a strip already being processed. Upon exiting the line the coils are separated again and coiled, or cut to a different length and (usually) coiled. The electrodeposition process can thus continue without interruption, and the use of strip accumulators prevents the need for speeding down during welding. It is preferable to use deposition processes which allow even higher speeds. So the method according to the invention preferably allows producing a coated steel substrate in a continuous high speed plating line, operating at a line speed of at least 200 m/min, more preferably of at least 300 m/min and even more preferably of at least 500 m/min. Although there is no limitation to the maximum speed, it is clear that control of the deposition process, the prevention of drag-out and of the plating parameters and the limitations thereof becomes more difficult the higher the speed. So as a suitable maximum the maximum speed is limited at 900 m/min.

[0012] Although the method according to the invention is applicable to any steel strip, it is preferred to select a strip from :

o cold-rolled full-hard blackplate, single or double reduced;

o cold-rolled and recrystallisation annealed blackplate;

o cold-rolled and recovery annealed blackplate,

o tinplate, as deposited or flow-melted; snijkanten, tin lost niet op o tinplate, diffusion annealed with an iron-tin alloy consisting of at least

80% of FeSn (50 at.% iron and 50 at.% tin);

wherein the resulting coated steel substrate is intended for use in packaging applications.

In case of tinplate dissolution of Fe may occur at the edges of the strip where the strip may have been cut to the correct width. The method according to the invention also ensures that no tin dissolves during the passes through the plating cells when no plating takes place.

[0013] It will be clear that the current density required in regime I to achieve the cathodic protection, but avoid crossing the threshold into regime II is not only dependent on the process conditions like line speed, but also on the nature of the substrate. Also the composition of the electrolyte is relevant, because the kinematic viscosity of the electrolyte influences the threshold value between regime I and regime II (see figure 4 for the difference between a sodium based bath and a potassium based bath).

[0014] The invention is also embodied in an apparatus for performing the method according to the invention. In this apparatus comprising a series of consecutive plating cells, filled with a suitable trivalent Cr-electrolyte for depositing a plating layer comprising chromium metal, chromium carbide and chromium oxide from the trivalent Cr-electrolyte, first means are provided for applying a current to the strip entering the electrolyte in the first plating cell which current is insufficient to deposit a plating layer from the trivalent Cr-electrolyte, but which is sufficient to provide cathodic protection of the strip in the electrolyte. Second means are provided to apply a higher current to the strip downstream of the first plating cell to deposit a plating layer comprising chromium metal, chromium carbide and chromium oxide from the trivalent Cr-electrolyte.

[0015] The invention is also embodied in an apparatus wherein means are also provided for applying a current to the strip residing in or passing through the electrolyte in a subsequent plating cell in which no plating is to take place, which current is insufficient to deposit a plating layer from the trivalent Cr-electrolyte, but which is sufficient to provide cathodic protection of the strip in the electrolyte residing in said plating cell. Subsequent plating cell means any one cell or any combination of cells following the first plating cell.

[0016] The invention will now be described with reference to the following non-limiting examples.

[0017] A double-walled glass vessel connected with a thermostat bath was filled with a freshly prepared trivalent chromium electrolyte. The temperature of the electrolyte was kept constant at 50 ± 1 °C by circulation of hot water through the double-walled glass vessel. The composition of the electrolyte was: 120 g I ¹ basic chromium sulphate, 100 g I ¹ sodium sulphate, and 41.4 g I ¹ sodium formate. The pH was adjusted to 2.8 measured at 25 °C by adding sulphuric acid. The experiments were conducted using a three electrode system (i.e. a working electrode, a counter electrode and a reference electrode) connected to an Autolab PGSTAT303N potentiostat/galvanostat. A galvanostat maintains a controlled constant current as defined by the user between the working electrode and the counter electrode, while the potential of the working electrode is monitored as a function of time vs. the potential of the reference electrode. The working electrode was a mild steel cylinder insert with an outer diameter of 12 mm and a height of 8 mm, thus having an electro active surface area of ca. 3 cm ² , fitted in a special holder from Pine Instruments Company.

[0018] The auxiliary (counter) electrode was a meshed strip of a titanium with a catalytic mixed metal oxide coating of iridium oxide and tantalum oxide. The reference electrode was a Saturated Calomel Electrode (SCE). In the reference experiment the steel cylinder was exposed to the electrolyte for 24 h while no current was applied and only the corrosion potential was recorded every 60 s. The corrosion potential was -0.602 V vs. SCE. The experiment was repeated, but now a small cathodic current of 2 A dm ² was applied . By doing so, the potential shifted about 0.6 V in negative direction to -1.2 V vs. SCE. The steel cylinder was weighed before and after the electrolysis experiment and the Fe content of the electrolyte was analysed by means of Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). When no current is applied, an iron concentration of 147 mg I ¹ is measured, which corresponds very well with the value calculated from the weight loss of the steel cylinder insert. In contrast, only a negligible amount of iron was measured in the electrolyte, in which the steel electrode was protected against corrosion by applying a small current. No weight loss of the steel cylinder insert was measured and no chromium was deposited on the steel electrode, because the experiment was executed in regime I.

[0019] Table 1 - Overview of experiments with analysis results.