JP7066707B2 - How to electroplat an uncoated steel strip with a plating layer - Google Patents

Description

The present invention relates to a method of electroplating an uncoated steel strip with a plating layer and an improvement thereof.

In continuous steel strip plating, a cold rolled steel strip is prepared, which is usually allered after cold rolling to soften the steel by recrystallization or recovery allering. After allering and before plating, the steel strips are first cleaned to remove oil and other surface contaminants. In most cases, alkaline cleaners are used for this purpose, in which case the steel is electrochemically passivated, i.e. the steel strip surface is covered with a stable protective oxide film. Therefore, the steel does not dissolve in the alkaline cleaner. Alkaline cleaners are complex mixtures of various components. The main component is caustic soda for imparting alkalinity, conductivity and saponification. Other common ingredients are sodium metasilicate, sodium carbonate, phosphates, borates and surfactants.

After the cleaning step, the steel strips are pickled in sulfuric acid or hydrochloric acid to remove the oxide film. During different treatment steps, the steel strips are always rinsed with deionized water to prevent the solution used in the next treatment step from being contaminated with the solution in the previous treatment step. As a result, the steel strips are thoroughly rinsed after the pickling step. During transport of the steel strip to the rinse and plating sections, a fresh thin oxide layer is instantly formed on the bare steel surface.

The process used in electroforming is called electrodeposition. The part to be plated (steel strip) is the cathode of the circuit. The anode of the circuit is the metal to be plated on this component (dissolving anode, for example the dissolving anode used in conventional tin plating) or the dimensionally stable anode (which does not dissolve during plating). ) Can be formed. Both components are immersed in a solution called an electrolyte. At the cathode, metal ions in the electrolyte solution are reduced at the interface between the solution and the cathode, and they deposit on the cathode.

Often the electrolyte is an acidic solution. As a result, the oxide layer formed after the pickling step dissolves rapidly. Bare steel without an oxide film is easily corroded. Corrosion means that iron derived from the steel substrate is oxidized to Fe ²⁺ , where the liberated electrons are consumed by the reduction of hydrogen ions or oxygen gas dissolved in the electrolyte.
2H ⁺ + ^2e- → H ₂ (g)
O ₂ (g) + 4H ⁺ + ^4e- → 2H ₂ O
As a result, the electrolyte becomes rich in Fe ²⁺ . Depending on the electrolyte, these Fe ²⁺ ions are subsequently reduced to Fe in the next electroplating step, which Fe is based on, along with the metal intended to be plated on the substrate. Precipitated on the material. The co-precipitated iron adversely affects the properties of the plating layer, especially the corrosive performance.

One object of the present invention is to provide an improved method for electroplating uncoated steel strips with a plating layer from a trivalent Cr electrolyte.

Also, one object of the present invention is to provide a steel strip having a plated layer, which is produced by electroplating an uncoated steel strip using a trivalent Cr electrolyte having improved properties. That is.

One or more objectives are methods of electroplating uncoated steel strips with a plating layer from a trivalent Cr electrolyte.
The uncoated steel strip is subjected to a cleaning and pickling step to remove oxides and other contaminants present on the surface of the strip prior to the plating process.
The strip is then subjected to a plating process in a plating section containing a series of continuous plating cells.
In the first step of the plating process, a current sufficient to precipitate the plating layer from the trivalent Cr electrolyte but sufficient to cathodic protect the strip in the electrolyte is first. Applied to the strip entering the plating cell,
In the second step of the plating process, a higher current is applied to the strip to precipitate a plating layer containing chromium metal, chromium carbide and chromium oxide from the trivalent Cr electrolyte, achieved by the method. Ru.

FIG. 1 is a diagram showing the layout of the plating section. FIG. 2 is a diagram showing the layout of the plating section. FIG. 3 is a diagram showing the relationship between the current density (A / dm ² ) and Cr (mg / m ² ). FIG. 4 is a diagram showing the relationship between i (A / dm ² ) and Cr (mg / m ² ).

US Pat. No. 3,316,160 discloses a method of preventing a bluish tint on a chrome-plated steel strip from a chromic acid plating solution in a plating operation involving two or more vertical plating tanks. In this process, the first downward pass and upward pass have high current densities and electrolytic chrome plating is performed. The steel strip is then led to a second plating tank, and at the second downward path and any subsequent downward path, the current density is significantly reduced, and at the second upward path again a high level of current density. Return to. Processing at low and high current densities in the downward and upward paths is repeated in all subsequent tanks. The decrease in current density in the upward path removes the film of complex chromium oxide that causes the bluish tint.

The present invention is described by reference to a particular layout of the plating section used in the industry, but the invention is not intended to be limited thereto and includes a series of continuous plating cells. Applicable to any plating section. In one embodiment of the invention, the plating section consists of a series of vertical plating cells to obtain sufficient total anode length in a limited floor area. In the methods known in the art, no current is applied during the first down-pass. In the first downward path when the strip first enters the plating solution, the residual water film adhering to the surface of the steel strip from the rinsing process is replaced by the electrolyte present in the plating cell and the steel strip is the temperature of the electrolyte. Is heated to. When the steel strip is exposed to the electrolyte, the oxide layer formed after the pickling step dissolves rapidly (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 FIG. 2). The current is selected so that precipitation of the plating layer is not achieved, but it is important that the potential of the steel in the electrolyte is shifted so that the steel strip is cathodically protected but not melted. Therefore, in the method according to the present invention, the electrolyte in the first plating cell is not rich in Fe ²⁺ , but the electrolyte in the first plating cell in the prior art method is rich in Fe ²⁺ . Therefore, the lack of electrolyte enrichment in the first plating cell prevents the subsequent drag-out of Fe ²⁺ into the plating cell. In subsequent plating cells, the current is increased to precipitate a plating layer containing chromium metal, chromium carbide and chromium oxide from the trivalent Cr electrolyte. Iron in the Cr (III) electrolyte precipitates on the strip along with chromium. It has been found that iron in the Cr-CrCx-CrOx coating adversely affects corrosive performance. Therefore, it is important to keep the iron level in the Cr (III) electrolyte as low as possible. This is achieved by passing a small current, at least in the first downward pass, preferably in all other passes not used for plating. The method according to the invention can be applied to any inert plating cell in a series of plating cells from which the strip to be plated is guided. An inert plating cell is one in which the strip is guided but no plating action occurs, for example, when one or more plating cells are skipped, no plating action occurs, while the strip is guided due to the structure of the entire plating facility. Means a plating cell that must be. In one embodiment of the invention, the electrolyte is acidic.

In the study on the precipitation mechanism of the chromium layer from the trivalent chromium electrolyte (JHO Wijenberg, M. Steeg, MPArants, KR Rammers, JMC Mol, buffering. Electrodeposition of mixed chromium metal-carbohydrate-oxide coating from a trivalent chromium formate electrolyte free of agents, in Electrochim.Acta 173 (2015) 819-826), the trivalent chromium plating process is carried out by metal ions. It was found that it is very different from the usual plating process, which is directly reduced by the current to the metal (Men ⁺ + ne- → Me ⁾ . This method is known, for example, in the tin plating process. In contrast, the Cr (III) plating process involves rapid, stepwise deprotonation of water ligands in Cr (III) complex ions caused by an increase in surface pH due to a hydrogen evolution reaction. Is based. As a result, there is a so-called "regime I" in which the metal does not precipitate even when an electric current is applied (see FIG. 3). A hydrogen generation reaction occurs when a small current is passed. Removal of H ⁺ ions involves an increase in surface pH, 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

The presence of Regime I is unique to the Cr (III) plating process and is not present in the normal plating process. We have arrived at a novel idea to take advantage of this special feature of the Cr (III) plating process. Applying a small amount of current in the first downward path not only produces a small amount of hydrogen gas, but also shifts the potential of the steel in the negative direction. This is a phenomenon known as cathode protection. Due to the negative potential, the steel strips 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 further reducing the pick-up of iron in the electrolyte. Obviously, when an electric current is applied, the water film is still replaced by the electrolyte and the steel strip is also heated. The current that must be applied to protect the steel strip may be very small. The upper limit is limited by the initiation of Regime 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 forms a precipitate on the cathode. A part of Cr (III) in the precipitate is reduced to Cr metal, and the formate is decomposed to form Cr carbide. If Cr (III) is not completely reduced to the Cr metal, Cr oxide is also present in the precipitate. The amount and composition of the precipitate depends on the applied current density, mass flux and electrolysis time. The current density threshold to enter Regime II increases with increasing line velocity as it is related to the mass flux of H ⁺ , as explained in the paper above. The increase in surface pH required to precipitate Cr (HCOO) (OH) ₂ (H ₂ O) ₃ is hampered by the faster replenishment of H ⁺ from the bulk of the electrolyte to the electrode surface. As a result, higher current densities are required with increasing line speeds to obtain the same pH increase on the electrode surface. Therefore, there is no fixed threshold at which regime I ends and regime II begins, but determining this threshold by simply monitoring the initiation of plating layer precipitation as a function of current density by simple experimentation. Is easy. Regimes I-III are visualized when chromium precipitation is plotted against current density (see, eg, FIG. 4). Regime I is a region where current is present but precipitation is not yet present. The surface pH is insufficient for chromium precipitation. Regime II is when the total chromium coating weight increases with current density until precipitation begins and peaks and drops in regime III where precipitation 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 plating line is defined as a plating line in which the substrate to be plated, usually in the form of strips, moves at a rate of at least 100 m / min. The coil of the steel strip is placed at the inlet end of the plating line with its eye extending in a horizontal plane. The tip of the coiled strip is then rewound and the end of the strip that has already been processed is welded. Upon exiting the line, the coil is again separated and coiled, or cut to a different length and (usually) coiled. The electrodeposition process can thus continue uninterrupted and the use of strip accumulators prevents the need for speed down during welding. It is preferable to use an electrodeposition process that allows for even higher velocities. Therefore, the method according to the invention produces a steel substrate coated with a continuous high speed plating line, preferably operating at a line speed of preferably at least 200 m / min, more preferably at least 300 m / min, and even more preferably at least 500 m / min. Allows you to do. There is no limit to the maximum speed, but it is clear that the higher the speed, the more difficult it is to control the electrodeposition process, prevent dragging and limit the plating parameters, and limit them. Therefore, the maximum speed is limited to 900 m / min as an appropriate maximum speed.

The method according to the invention is applicable to any steel strip, but the strips are as follows:
-Cold-rolled full-hard blackplate (single or double reduced)
-Cold-rolled and recrystallization annealed blackplate
-Cold-rolled and recovery annealed blackplate
-Tinplate, eg, thin-film or flow-melted; snijkanten, tin lost niet op
A tin plate (tinplate) diffusely allylated with an iron-tin alloy consisting of at least 80% FeSn (50 atomic% iron and 50 atomic% tin).
The resulting coated steel substrate is intended for use in packaging applications.

In the case of tin plates (tinplates), dissolution of Fe can occur at the ends of the strip where the strip may have been cut to the correct width. The method according to the invention also ensures that tin does not dissolve while passing through the plating cell when plating is not performed.

The current density required in region I to achieve cathode protection but avoid crossing the threshold to region II depends not only on process conditions such as line velocity, but also on the nature of the substrate. Will be clear. The composition of the electrolyte is also important because the kinematic viscosity of the electrolyte affects the threshold between Regime I and Regime II (see Figure 4 for the difference between a sodium-based bath and a potassium-based bath).

The present invention is also embodied in a device for carrying out the method according to the present invention. In this device, which includes a series of continuous plating cells filled with a suitable trivalent Cr electrolyte for precipitating a plating layer containing chromium metal, chromium carbide and chromium oxide from the trivalent Cr electrolyte, the trivalent Cr electrolyte to the plating layer A first means is provided in which a current is applied to the strip that enters the electrolyte in the first plating cell, but is insufficient to precipitate the .. A second means is provided to apply a higher current to the strip downstream of the first plating cell in order to precipitate a plating layer containing chromium metal, chromium carbide and chromium oxide from the trivalent Cr electrolyte.

The present invention also provides means for applying an electric current to a strip that is present in or passes through an electrolytic solution in a subsequent plating cell that is not plated, and the current is trivalent Cr. Although insufficient to precipitate the plating layer from the electrolyte, it is sufficient to provide cathode corrosion protection for the strips in the electrolyte present in the plating cell. Subsequent plating cell means any one cell following the first plating cell or any combination of cells.

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

A double-walled glass container connected to a thermostat bath was filled with a newly prepared trivalent chromium electrolyte.

By circulating hot water through a double-walled glass container, the temperature of the electrolytic solution was kept constant at 50 ± 1 ° C. The composition of the electrolyte was 120 g / L basic chromium sulfate, 100 g / L sodium sulfate and 41.4 g / L sodium formate. The pH measured at 25 ° C. was adjusted to 2.8 by adding sulfuric acid. The experiment was performed using a three-electrode system (ie, working electrode, counter electrode and reference electrode) connected to the Autolab PGSTAT303N potentiostat / galvanostat. I was. The galvanostat maintains a constant current controlled by the user between the working electrode and the counter electrode, while the working electrode potential is monitored as a function of the time vs. reference electrode potential. 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 electrically active surface area of about 3 cm ² , and was attached to a special holder from the Pine Instruments Company.

The auxiliary (pair) electrode was a titanium mesh strip 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 hours without current and only the corrosion potential was recorded every 60 seconds. The corrosion potential was -0.602V vs. SCE. The experiment was repeated, but this time a small cathode current of 2 A / dm ² was applied. By doing so, the potential was shifted in the negative direction by about 0.6V to -1.2V vs. SCE. Steel cylinders were weighed before and after the electrolysis experiment and the Fe content of the electrolyte was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES). When no current is applied, an iron concentration of 147 mg / L is measured, which is in very good agreement with the value calculated from the weight loss of the steel cylinder insert. In contrast, only a very small amount of iron was measured in the electrolyte and the steel electrodes were protected from corrosion by applying a small current. Since the experiment was performed in Regime I, no weight loss of the steel cylinder insert was measured and no chromium was deposited on the steel electrode.

Claims (7)

A method of electroplating an uncoated steel strip with a plating layer in a plating section containing a series of continuous plating cells.
The plating layer is precipitated from the trivalent Cr electrolyte in the plating process.
The uncoated steel strip is subjected to a cleaning and pickling step to remove oxides and other contaminants present on the surface of the strip prior to the plating process.
The strip is then subjected to the plating process in the plating section.
In the first stage of the plating process, a current that does not precipitate a plating layer from the trivalent Cr electrolyte but cathodes the strip in the electrolyte is applied to the strip that enters the first plating cell. ,
The method, wherein in the second step of the plating process, a higher current is applied to the strip to precipitate a plating layer containing chromium metal, chromium carbide and chromium oxide from the trivalent Cr electrolyte. In one, two or more or all subsequent plating cells where plating is not performed, an electric current is applied to the strip.
The method of claim 1, wherein the current does not precipitate a plating layer from the electrolyte in the plating cell, but cathodic protects the strip in the electrolyte. The method according to claim 1 or 2, wherein the Cr electrolyte comprises chromium (III) sulfate and one or more of sodium sulfate, sodium formate, potassium formate, potassium formate and sulfuric acid. The method according to claim 1 or 2, wherein the Cr electrolyte comprises chromium (III) sulfate, sodium sulfate, sodium formate and sulfuric acid. The method according to claim 1 or 2, wherein the Cr electrolyte comprises chromium (III) sulfate, potassium sulfate, potassium formate and sulfuric acid. The method of claim 1 or 2, wherein the Cr electrolyte comprises chromium (III) hydroxysulfuric acid (CrOHSO ₄ ) and formic acid . The method according to any one of claims 1 to 6, wherein the anode in the plating cell comprises a catalytic coating of iridium oxide or a mixed metal oxide.

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