CN110062819B

CN110062819B - Method for electroplating uncoated steel strip with a coating

Info

Publication number: CN110062819B
Application number: CN201780076997.0A
Authority: CN
Inventors: J·H·O·J·温伯格; A·J·怀特布鲁德
Original assignee: Tata Steel Ijmuiden BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 2016-11-14
Filing date: 2017-11-08
Publication date: 2021-07-23
Anticipated expiration: 2037-11-08
Also published as: ZA201903049B; EP3538688A1; JP2019533768A; WO2018087135A1; RS62127B1; CN110062819A; EP3538688B1; RU2743357C2; CA3043486A1; MX2019005540A; KR20190077437A; RU2019118305A; ES2883716T3; BR112019009702B1; JP7066707B2; BR112019009702A2; KR102387496B1; CA3043486C; RU2019118305A3

Abstract

The invention relates to a method for electroplating an uncoated steel strip using a plating layer from a trivalent Cr-electrolyte, wherein the uncoated strip is subjected to a cleaning and pickling step prior to a plating process and then to a plating process in a plating section comprising a series of successive plating cells, wherein in a first stage of the plating process an electric current is applied to the strip entering the first plating cell, which electric current is 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 in a second stage of the plating process a higher electric current is applied to the strip to deposit a plating layer comprising chromium metal, chromium carbides and chromium oxides from the trivalent Cr-electrolyte.

Description

Method for electroplating uncoated steel strip with a coating

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

In continuous steel strip plating, a cold rolled steel strip is provided, which is typically annealed after cold rolling to soften the steel by recrystallization annealing or recovery annealing. The steel strip is first cleaned for removal of 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, 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. Alkaline cleaners are complex mixtures of various 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 solution or a hydrochloric acid solution for removing the oxide film. The steel strip is always rinsed with deionized water between the different treatment steps to prevent the solution of the previous treatment step from contaminating the solution used in the next 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 rinsing and transfer of the steel strip to the plating section.

The process used in electroplating is known as electrodeposition. The part to be plated (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 (dissolved anodes such as those used in conventional tin plating) or a dimensionally stable anode (which does not dissolve during the plating process). Both parts are immersed in a solution called 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 onto the cathode.

In many cases the electrolyte is an acidic solution. Thus, the oxide layer formed after the pickling step will dissolve rapidly. Bare steel without any oxide film is susceptible to corrosion. Corrosion means that iron from the steel substrate is oxidized to Fe²⁺Wherein free electrons are consumed by reduction of hydrogen ions or oxygen dissolved in the electrolyte.

2H⁺+2e^-→H₂(g)

O₂(g)+4H⁺+4e^-→2H₂O

As a result, the electrolyte becomes rich in Fe²⁺. Depending on the electrolyte, these Fe²⁺The ions are subsequently reduced to Fe in the next electroplating step and this Fe is deposited onto the substrate along with the metal intended to be plated onto the substrate. Co-depositedIron adversely affects the properties of the plating, particularly the corrosion performance.

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

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

One or more of the objects are achieved by a method for electroplating 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 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 successive plating cells, wherein in a first stage of the plating process an electric current is applied to the strip entering the first plating tank, which electric 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.

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

The invention is explained by reference to the specific arrangement of plated sections used in the industry, but it should be noted that the invention is not intended to be so limitedLimited thereto and may be applied to any plating section comprising a series of successive plating baths. In an embodiment of the invention, the plating section consists of a series of vertical plating tanks for obtaining sufficient total anode length on a limited footprint. In the methods known in the art, no current is applied during the first downward pass. In the first downward pass of the strip into 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 bath and the steel strip is also heated to the temperature of the electrolyte. When the steel strip is exposed to the electrolyte, the oxide layer formed after the pickling step will dissolve rapidly (see fig. 1). In the method according to the invention, an electric current is applied to the strip which first enters the electrolyte (see fig. 2). It is essential that the current is chosen such that deposition of the plating is not achieved, but that the potential of the steel in the electrolyte is changed 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 Fe²⁺However, in the prior art method the electrolyte in the first plating tank is rich in Fe²⁺. This lack of enrichment of the electrolyte in the first plating tank thus prevents Fe²⁺To the subsequent plating tank. The current is increased in the subsequent plating tank 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 cr (iii) electrolytes. It was found that iron adversely affects 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 downward pass and preferably also in all other passes not used for plating. The method according to the invention may be applied in any inactive coating cell of a series of coating cells through which the strip to be coated is guided. With an inactive plating tank is meant that the strip is guided through but no plating action takes place therein, e.g. when one or more plating tanks are skipped, but the strip has been guided through due to the construction of the entire plating facility. In an embodiment of the invention the electrolyte is acidic.

In a mechanistic study on the deposition of chromium layers from trivalent chromium electrolytes (j.h.o.j.wijenberg, m.stegh, m.p.aarnts, k.r.lammers, j.m.c.mol, electro-deposition of mixed chromium metal-carbide-oxides coatings from a three-valued chromium-formed electrolyte with a diffusion agent, electro-deposition.acta 173 (2015819) 826.) it was found that trivalent chromium plating processes are very different from conventional plating processes, where metal ions are directly reduced to metal by current: meⁿ⁺+ne^-→ Me. Such a method is known, for example, from the tin plating method. In contrast, the cr (iii) plating process is based on a rapid, stepwise deprotonation of the water ligands in cr (iii) complex ions initiated by an increase in surface pH due to hydrogen evolution reactions. This results in the presence of a so-called "mode I" in which no metal is deposited even if a current is applied (see fig. 3). Application of a small current causes a hydrogen evolution reaction. Completion of H by surface pH increase⁺Ionic removal of ions, which leads to the following acid-base reactions:

[Cr(HCOO)(H₂O)₅]²⁺+OH^-→[Cr(HCOO)(OH)(H₂O)₄]⁺+H₂the presence of O mode I is unique to cr (iii) plating processes and is not present in conventional plating processes. The inventors have arrived at a new idea to advantageously exploit this particular feature of the cr (iii) plating process. Not only a small amount of hydrogen gas is formed by applying a small current in the first downward pass, but also the potential of the steel is negatively shifted (a phenomenon known as cathodic protection). Due to the negative potential, the steel strip will no longer corrode. Not only the steel strip is protected from corrosion, but also (part of) the iron oxide film is reduced to iron metal, thereby reducing the iron pick-up (pick up) in the electrolyte even further. It is obvious that the water film will still be replaced by electrolyte when the current is applied and will also heat the steel strip. The current that has to be applied for protecting the steel strip can be very small. The upper limit is limited by the start of mode (II) (see fig. 3).

[Cr(HCOO)(OH)(H₂O)₄]⁺+OH^-→Cr(HCOO)(OH)₂(H₂O)₃+H₂OCr(HCOO)(OH)₂(H₂O)₃Forming a deposit on the cathode. A part of the Cr (iii) of the deposit is reduced to Cr-metal and the formate decomposes resulting in the formation of Cr-carbides. If Cr (III) is not completely reduced to Cr-metal, Cr-oxides are also present in the deposit. The amount and composition of the deposit depends on the applied current, mass flux and electrolysis time. The critical value of the current density for entering regime II increases with increasing linear velocity, since this is associated with H as explained in the article mentioned above⁺Is concerned with the mass flux. By H from bulk electrolyte to electrode surface⁺Prevents the deposition of Cr (HCOO) (OH)₂(H₂O)₃The desired surface pH increases. Therefore, with increased linear velocity a higher current density is required for obtaining the same pH increase at the electrode surface. There is therefore no fixed threshold at which mode I ends and mode II begins, but this threshold is readily determined by simply monitoring the onset of deposition of the plating layer as a function of current density by simple experimental means. The modes I-III are visible when the deposition of chromium is plotted against the current density (see e.g. fig. 4). Regime I is the region where current is present but not yet deposited. The surface pH was 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 decreases with mode III, where the deposit begins to dissolve:

Cr(HCOO)(OH)₂(H₂O)₃+OH^-→[Cr(HCOO)(OH)₃(H₂O)₂]^-+H₂O

a high speed continuous plating line is defined as a plating line through which a substrate to be plated, usually in the form of a strip, is moved at a speed of at least 100 m/min. The coil of steel strip is placed at the inlet end of the coating line with its inlet (eye) extending in a horizontal plane. The leading end of the coiled strip is then unwound and welded to the trailing end of the strip which has been processed. Upon leaving the production line the web is again separated and wound into rolls or cut to different lengths and (usually) wound into rolls. The electrodeposition process can thus be continued without interruption, and the use of a strip accumulator avoids the need for a speed reduction during the welding process. Deposition methods allowing even higher speeds are preferably used. The method according to the invention preferably allows the production of coated steel substrates in a continuous high speed plating line operating at a line speed of at least 200m/min, more preferably at least 300m/min and even more preferably at least 500 m/min. Although 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. Therefore, the maximum speed is limited to 900m/min as a suitable maximum value.

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

single or double thinning cold-rolled full-hard black steel plate;

cold-rolled and recrystallization annealed black steel sheet;

a black steel sheet cold rolled and recovery annealed,

a tin plate in the deposited or soft-melt state; trimmed, insoluble tin (snijkanten, tin lost niet op)

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

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

In the case of tin plates, Fe dissolution can occur at the strip edges (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 tank when no plating takes place.

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

The invention is also embodied in an apparatus for carrying out the method according to the invention. In such an apparatus comprising a series of continuous 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, a first route is provided for applying an electric current to the strip entering the electrolyte in the first plating cell, which electric current is insufficient to deposit the plating layer from the trivalent Cr-electrolyte, but which is 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 bath to deposit a plating layer comprising chromium metal, chromium carbide and chromium oxide from a trivalent Cr-electrolyte.

The invention is also embodied in an apparatus in which means are also provided for applying an electric current to the strip present in the electrolyte or passing through the electrolyte in a subsequent plating tank in which no plating takes 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 present in said plating tank. By subsequent plating tank is meant any one tank or any combination of tanks after 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 constant temperature bath was filled with a freshly prepared trivalent chromium electrolyte. The temperature of the electrolyte was kept constant at 50 ± 1 ℃ by circulating hot water through the double-walled glass vessel. The electrolyte composition was: 120g l^-1Basic chromium sulfate, 100g l^-1Sodium sulfate and 41.4gl^-1Sodium formate. The pH was adjusted to 2.8 measured at 25 ℃ by adding sulfuric acid. Experiments were performed using a three-electrode system (i.e., working, counter and reference electrodes) connected to an Autolab PGSTAT303N potentiostat/galvanostat. The galvanostat maintains a controlled constant current defined by the user between the working electrode and the counter electrode, while monitoring the potential of the working electrode as a function of time relative to the potential of the reference electrode. The working electrode was a low carbon steel cylindrical insert mounted in a special holder from the Pine Instrument Company, having an outer diameter of 12mm and a length of 8mm, and thus having a length of about 3cm²The electroactive surface area of (a).

The auxiliary (counter) electrode is a mesh strip of titanium with a catalytic mixed metal oxide coating of iridium oxide and tantalum oxide. The reference electrode isSaturated Calomel Electrode (SCE). In the reference experiment the steel column was exposed to electrolyte for 24h without applying current and the corrosion potential was only recorded every 60 s. The corrosion potential was-0.602V vs SCE. The experiment was repeated, but now 2A dm was applied^-2Small cathode current. By doing so, the potential is shifted in the negative direction by about 0.6V to-1.2V relative to SCE. The steel columns were weighed before and after the electrolysis experiment and the electrolyte was analyzed for Fe content by inductively coupled plasma atomic emission spectroscopy (ICP-AES). When no current was applied, 147mg l was measured^-1Which 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, in which the steel electrodes were protected against corrosion by applying a small current. No weight loss of the steel cylindrical insert was measured and no chromium was deposited on the steel electrode as the experiment was performed in mode I.

Table 1-experimental overview and analytical results.

Claims

1. Method for electroplating an uncoated steel strip using a plating layer in a plating section of a high speed continuous plating line comprising a series of continuous plating tanks, characterised in that the plating layer is deposited from a trivalent Cr-electrolyte in a plating process, 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 one or more surfaces of the strip, and wherein the strip is subsequently subjected to the plating process in the plating section, wherein in a first stage of the plating process an electric current is applied to the strip entering the first plating tank which is insufficient to deposit the 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 electric current is applied to the strip to deposit from the trivalent Cr-electrolyte a plating layer comprising chromium metal, Chromium carbide and chromium oxide.

2. The method of claim 1, wherein an electric current is applied to the strip in one, more or all subsequent plating baths where no plating is occurring, wherein the electric current is insufficient to deposit a plating layer from the electrolyte in the plating bath, but wherein the electric current is sufficient to provide cathodic protection of the strip in the electrolyte.

3. The method according to 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 according to claim 1 or 2, wherein the Cr-electrolyte comprises chromium (III) sulfate, sodium formate and sulfuric acid.

5. The method according to claim 1 or 2, wherein the Cr-electrolyte comprises chromium (III) sulfate, potassium formate and sulfuric acid.

6. The method according to claim 1 or 2, wherein the Cr-electrolyte comprises chromium (III) hydroxy sulphate (CrOHSO)₄) Formic acid and optionally sulphuric acid and/or NaOH.

7. A method according to claim 1 or 2, wherein the anode in the plating tank comprises a catalytic coating of iridium oxide or mixed metal oxide.