CN110062819A - Method for using coating layer that uncoated steel band is electroplated - Google Patents

Abstract

The present invention relates to a method for electroplating uncoated steel strip using a coating from a trivalent Cr-electrolyte, wherein the uncoated strip is subjected to cleaning and pickling steps prior to the plating process and then The plating process is subjected to a plating process in the plating section of the series of continuous plating tanks, wherein in the first stage of the plating process a current is applied to the strip entering the first plating tank, the current being insufficient to remove the trivalent Cr-electrolyte A plating layer is deposited, but sufficient to provide cathodic protection of the strip in the electrolyte, and wherein a higher current is applied to the strip in the second stage of the plating process to deposit from the trivalent Cr-electrolyte containing chromium metal, chromium Coatings of carbides and chromium oxides.

Description

Method for electroplating uncoated steel strip with cladding

This invention relates to a method for electroplating uncoated steel strip with a coating and improvements thereof.

In continuous steel strip coating, cold rolled steel strip is provided, which is usually annealed after cold rolling to soften the steel by recrystallization annealing or recovery annealing. The steel strip is first cleaned to remove oil and other surface contaminants after annealing and before plating. Mainly, alkaline cleaners are used for this purpose, in which the steel is electrochemically passivated, ie the steel strip surface is covered with a stable and protective oxide film and thus the steel will not dissolve in alkaline cleaners . Alkaline cleaners are complex mixtures of ingredients. The main component is caustic soda to provide alkalinity, conductivity and saponification. Other common components are sodium metasilicate, sodium carbonate, phosphates, borates and surfactants.

After the cleaning step, the steel strip is pickled in a sulfuric acid or hydrochloric acid solution to remove the oxide film. The steel strip was always rinsed with deionized water between different treatment steps to prevent the solution of the previous treatment step from contaminating the solution for the subsequent treatment step. The steel strip is then completely rinsed after the pickling step. A new thin oxide layer is formed immediately on the bare steel surface during the flushing and transfer of the steel strip to the plated section.

The method used in electroplating is called electrodeposition. The part to be plated (steel strip) is the cathode of the circuit. The anodes of the circuit may be made of the metal to be plated on the part (dissolved anodes, such as those used in conventional tin plating) or dimensionally stable anodes (which do not dissolve during the plating process). Both parts 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, causing them to deposit on the cathode.

In many cases the electrolyte is an acidic solution. Therefore, the oxide layer formed after the pickling step will dissolve rapidly. Bare steel without any oxide film is prone to corrosion. Corrosion means that the iron from the steel substrate is oxidized to Fe ²⁺ where free electrons are consumed by reduction of hydrogen ions or oxygen dissolved in the electrolyte.

2H ⁺ +2e ^- →H ₂ (g)

O ₂ (g)+4H ⁺ +4e ^- →2H ₂ O

The result is that the electrolyte becomes rich in Fe ²⁺ . Depending on the electrolyte, these Fe ²⁺ ions are subsequently reduced to Fe in a subsequent electroplating step and this Fe is deposited onto the substrate together with the metal that is intentionally plated onto the substrate. Co-deposited iron adversely affects the properties of the coating, especially the corrosion properties.

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

It is also an object of the present invention to provide improved properties to a steel strip using a coating produced by electroplating an uncoated steel strip using a trivalent Cr-electrolyte.

One or more objects are achieved by a method for electroplating uncoated steel strip with a coating from a trivalent Cr-electrolyte, wherein the uncoated strip undergoes cleaning and pickling steps prior to the plating process to Removal of oxides and any other contaminants present on one or more surfaces of the strip, and wherein the strip is subsequently subjected to a plating process in a plating section comprising a series of continuous plating baths, wherein the plating process A current is applied to the strip entering the first plating tank in the first stage of A higher current is applied to the strip in the second stage of the plating process to deposit a plating layer comprising chromium metal, chromium carbides and chromium oxides from the trivalent Cr-electrolyte according to the invention.

US3316160 discloses a method for preventing bluish tint from chromic acid plating solutions on chromed steel strip in plating operations comprising two or more vertical plating tanks. In this method, the current density is high in the first down and up passes to achieve electrolytic chrome plating. The steel strip is then directed into a second plating bath and the current density is much lower in the second down pass and any subsequent down passes, and returns to the high current density level in the second up pass again . This low, high current density process during the down and up passes is repeated in each subsequent cell. The reduction in current density during the upward pass removes the film of complex chromium oxide (which is responsible for the bluish tint).

The present invention is explained with reference to specific arrangements of plating sections used in the industry, but it should be noted that the invention is not intended to be so limited and may be applied to any plating section comprising a series of continuous plating tanks. In an embodiment of the invention, the plating section consists of a series of vertical plating tanks to obtain sufficient overall anode length on a limited footprint. In methods known in the art, no current is applied during the first down pass. In the first downward pass where the strip enters the plating solution for the first time, the remaining water film sticking to the surface of the steel strip from the rinsing step is replaced by the electrolyte present in the plating tank, and the strip is also heated to the electrolyte temperature. The oxide layer formed after the pickling step will dissolve rapidly when the steel strip is exposed to the electrolyte (see Figure 1). In the method according to the invention, an electric current is applied to the strip entering the electrolyte for the first time (see Figure 2). It is necessary to select the current so that no deposition of the coating is achieved, but to change the potential of the steel in the electrolyte 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 tank is therefore not enriched in Fe ²⁺ , whereas in the prior art method the electrolyte in the first plating tank is enriched in Fe ²⁺ . This lack of enrichment of electrolyte in the first plating tank thus prevents drag-out of Fe ²⁺ to subsequent plating tanks. The current is increased in a subsequent plating bath to deposit a plating layer comprising chromium metal, chromium carbides and chromium oxides from the trivalent Cr-electrolyte. Iron is deposited on the strip together with chromium in a Cr(III) electrolyte. Iron was found to adversely affect corrosion performance in Cr-CrCx-CrOx coatings. 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 at the first down pass and preferably also in all other passes not used for plating. The method according to the invention can be applied in any inactive plating tank of a series of plating tanks through which the strip to be plated is guided. Using a plating tank that is inactive, meaning that the strip is guided through but no plating action occurs in it, such as when skipping one or more plating tanks, but is due to the configuration of the entire plating facility The strip has been guided through. In embodiments of the present invention the electrolyte is acidic.

In studies on the mechanism of deposition of chromium layers from trivalent chromium electrolytes (JHOJ, Wijenberg, M. Steegh, MP Aarnts, KRLammers, JMCMol, Electrodeposition of mixed chromium metal-carbide-oxide coatings from a trivalent chromium-formate electrolyte without a buffering agent, Electrochim . Acta 173 (2015) 819-826.) found that the trivalent chromium plating process is very different from the conventional plating process, in which the metal ions are directly reduced to the metal by an electric current: Me ⁿ⁺ +ne ⁻ →Me. Such a method is known, for example, from tinning methods. In contrast, the Cr(III) plating process is based on rapid, stepwise deprotonation of water ligands in the Cr(III) complex ions initiated by the increase in surface pH due to the hydrogen evolution reaction. This leads to the existence of the so-called "Mode I", in which no metal is deposited even when an electric current is applied (see Figure 3). Applying a small current causes a hydrogen evolution reaction. Ionic removal of H ⁺ ions is accomplished by surface pH increase, which results in the following acid-base reaction:

[Cr(HCOO)(H ₂ O) ₅ ] ²⁺ +OH ⁻ → [Cr(HCOO)(OH)(H ₂ O) ₄ ] ⁺ +H ₂ O Presence of Mode I for Cr(III) Plating Process is unique and does not exist in conventional plating processes. The inventors have come up with new ideas to take advantage 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 the potential of the steel is shifted negatively (a phenomenon known as cathodic protection). Due to the negative potential, the steel strip will no longer corrode. Not only is the steel strip protected from corrosion, but (part of) the iron oxide film is reduced to iron metal, thereby reducing iron pick up in the electrolyte even further. Obviously, the water film will still be replaced by the electrolyte when the current is applied and will also heat the steel strip. The current that must be applied to protect the steel strip can be very small. The upper limit is limited by the onset of mode (II) (see Figure 3).

[Cr(HCOO)(OH)(H ₂ O) ₄ ] ⁺ +OH ^- →Cr(HCOO)(OH) ₂ (H ₂ O) ₃ +H ₂ O Cr(HCOO)(OH) ₂ (H ₂ O ) ₃ to form deposits on the cathode. Part of the Cr(III) of the deposit is reduced to Cr-metal and the formate decomposes leading to the formation of Cr-carbides. If Cr(III) is not completely reduced to Cr-metal, Cr-oxide is also present in the deposit. The amount and composition of the deposits depend on the applied current, mass flux and electrolysis time. The critical value of the current density for entry mode II increases with increasing linear velocity since this is related to the mass flux of H ⁺ as explained in the above mentioned article. The increase in surface pH required to deposit Cr(HCOO)(OH) ₂ ( _H2O ) ₃ is hindered by the faster replenishment of H ⁺ from the bulk electrolyte to the electrode surface. Therefore, a higher current density is required for obtaining the same pH increase at the electrode surface with increased line speed. There is therefore no fixed threshold at which mode I ends and mode II begins, but this threshold is easily determined by simply monitoring the onset of plating layer deposition as a function of current density by means of a simple experiment. Modes I-III are visible when the deposition of chromium is plotted against current density (see eg Figure 4). Mode I is a region where current is present but not yet deposited. Surface pH is not sufficient for chromium deposition. Mode II is when deposition begins and the total chromium coating weight increases with current density until it peaks and falls with Mode III, where the deposit begins to dissolve:

Cr(HCOO)(OH) ₂ (H ₂ O) ₃ +OH ^- →[Cr(HCOO)(OH) ₃ (H ₂ O) ₂ ] ^- +H ₂ O

A high-speed continuous coating line is defined as a coating line through which the substrate to be coated, usually in strip form, is moved at a speed of at least 100 m/min. The coil of steel strip is placed at the entry end of the coating line, with its eye extending in a horizontal plane. The leading end of the coiled strip is then unwound and welded to the finished strip end. The coils are again split and wound on exiting the line, or cut to different lengths and (usually) wound into rolls. The electrodeposition process can thus continue without interruption, and the use of a strip accumulator avoids the need for a speed drop during welding. It is preferred to use deposition methods that allow even higher speeds. The method according to the invention therefore preferably allows the production of coated steel substrates in a continuous high speed coating line operating at line speeds of at least 200 m/min, more preferably at least 300 m/min and even more preferably at least 500 m/min. While there is no limit to the maximum speed, it is clear that the higher the speed, the more difficult it becomes to control the deposition process, prevent carryover and plating parameters and their limitations. So as a suitable maximum, limit the maximum speed to 900m/min.

Although the method according to the invention can be applied to any steel strip, the strip is preferably selected from the following:

○Single or double thinned cold-rolled full-hard black steel plate;

○Cold rolled and recrystallized annealed black steel plate;

○ Cold rolled and recovery annealed black steel sheet,

○ Tin plate in the as-deposited or melted state; trimmed, insoluble tin (snijkanten, tin lost niet op)

o Diffusion annealed tin plate with an iron-tin alloy consisting of at least 80% FeSn (50 atomic % iron and 50 atomic % tin);

The coated steel substrates produced therein are intended for use in packaging applications.

In the case of tin plates, the dissolution of Fe can occur at the edges of the strip (the strip can be cut to the correct width). The method according to the invention also ensures that no tin is dissolved during the pass through the plating bath when no plating has occurred.

It will be clear that the current density required to achieve cathodic protection in mode I but avoid crossing the critical value to mode II depends not only on process conditions such as line speed, but also on the properties of the substrate. The composition of the electrolyte is also relevant, as the dynamic viscosity of the electrolyte affects the critical value between Mode I and Mode II (see Figure 4 for the difference between sodium- and potassium-based baths).

The invention is also embodied in an apparatus for carrying out the method according to the invention. In this apparatus, the apparatus comprises a series of continuous plating tanks filled with a suitable trivalent Cr-electrolyte for depositing coatings comprising chromium metal, chromium carbides and chromium oxides from the trivalent Cr-electrolyte, A first approach is provided for applying a current to the strip entering the electrolyte in the first plating tank, the current being insufficient to deposit a coating from the trivalent Cr-electrolyte, but sufficient to provide cathodic protection of the strip in the electrolyte. A second route is provided to apply a higher current to the strip downstream of the first plating tank to deposit a plating layer comprising chromium metal, chromium carbides and chromium oxides from a trivalent Cr-electrolyte.

The present invention is also embodied in an apparatus wherein means are also provided for applying an electrical current to the strip present in or through the electrolyte in a subsequent plating bath (where no plating occurs), the electrical current of which is insufficient to remove the trivalent Cr - the electrolyte deposits the plating layer, but which is sufficient to provide cathodic protection of the strip in the electrolyte present in the plating tank. Subsequent plating tanks mean any one tank or any combination of tanks following the first plating tank.

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

A double-walled glass vessel connected to a thermostatic bath was filled with freshly prepared trivalent chromium electrolyte. The temperature of the electrolyte was kept constant at 50±1°C by circulating hot water through the double-walled glass vessel. The composition of the electrolyte was: 120 g l ^-1 basic chromium sulfate, 100 g l ^-1 sodium sulfate and 41.4 g l- ¹ sodium formate. The pH was adjusted to 2.8 measured at 25°C by adding sulfuric acid. Experiments were performed using a three-electrode system (ie, working, counter, and reference electrodes) connected to an Autolab PGSTAT303N potentiostat/galvanostat. A galvanostat maintains a user-defined controlled constant current between the working and counter electrodes, while monitoring the potential of the working electrode over time relative to the potential of the reference electrode. The working electrode was a mild steel cylindrical insert with an outer diameter of 12 mm and a length of 8 mm, mounted in a special holder from Pine Instrument Company, giving an electroactive surface area of approximately 3 cm ² .

The auxiliary (counter) electrode is a mesh ribbon of 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 column was exposed to the electrolyte for 24 h without current application and the corrosion potential was recorded only every 60 s. The corrosion potential was -0.602V relative to SCE. The experiment was repeated, but now a small cathodic current of 2A dm ^-2 was applied. By doing this, the potential is shifted negatively by about 0.6V to -1.2V relative to SCE. The steel column was weighed before and after the electrolysis experiment and the Fe content of the electrolyte was analyzed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). When no current was applied, an iron concentration of 147 mg 1 ⁻¹ was measured, which corresponds very well to the value calculated from the weight loss of the steel cylindrical insert. In contrast, only a negligible amount of iron was measured in the electrolyte, where the steel electrode is protected from corrosion by applying a small current. No weight loss was measured for the steel cylindrical insert and no chromium was deposited on the steel electrodes, since the experiment was performed using Mode I.

Table 1 - Experimental overview and analysis results.

Claims (7)

1. Method for electroplating uncoated steel strip with a coating layer in a coating section comprising a series of continuous coating tanks, characterized in that the coating layer is deposited from a trivalent Cr-electrolyte during the coating process , wherein the uncoated tape is subjected to cleaning and pickling steps prior to the plating process to remove oxides and any other contaminants present on one or more surfaces of the tape, and wherein the tape is subsequently subjected to The section is subjected to a plating process, wherein a current is applied to the strip entering the first plating tank in the first stage of the plating process, the current being insufficient to deposit the plating layer from the trivalent Cr-electrolyte, but sufficient to provide Cathodic protection of the strip in the electrolyte and wherein a higher current is applied to the strip in the second stage of the plating process to deposit a chromium metal, chromium carbide and chromium oxide containing plating from a trivalent Cr-electrolyte cladding. 2. The method of claim 1, wherein a current is applied to the strip in one, more or all subsequent plating baths where no plating occurs, wherein the current is insufficient to remove the electrolyte from the plating bath A plating layer is deposited, but where the current is sufficient to provide cathodic protection of the tape in the electrolyte. 3. The method of claim 1 or 2, wherein the Cr-electrolyte comprises chromium(III) sulfate and one or more of the following: sodium sulfate, sodium formate, potassium sulfate, potassium formate and sulfuric acid. 4. The method of claim 1 or 2, wherein the Cr-electrolyte comprises chromium(III) sulfate, sodium sulfate, sodium formate and sulfuric acid. 5. The method of claim 1 or 2, wherein the Cr-electrolyte comprises chromium(III) sulfate, potassium sulfate, potassium formate and sulfuric acid. 6. The method of claim 1 or 2, wherein the Cr-electrolyte comprises chromium(III) hydroxysulfate ( _CrOHSO4 ), formic acid and optionally sulfuric acid and/or NaOH. 7. The method of any one of claims 1 to 6, wherein the anode in the plating bath comprises a catalytic coating of iridium oxide or mixed metal oxide.