Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/34—Pretreatment of metallic surfaces to be electroplated
  • C25D5/36—Pretreatment of metallic surfaces to be electroplated of iron or steel
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D17/00—Constructional parts, or assemblies thereof, of cells for electrolytic coating
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/04—Electroplating: Baths therefor from solutions of chromium
  • C25D3/06—Electroplating: Baths therefor from solutions of chromium from solutions of trivalent chromium
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D7/00—Electroplating characterised by the article coated
  • C25D7/06—Wires; Strips; Foils
  • C25D7/0614—Strips or foils
  • C25D7/0628—In vertical cells
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D7/00—Electroplating characterised by the article coated
  • C25D7/06—Wires; Strips; Foils
  • C25D7/0614—Strips or foils
  • C25D7/0642—Anodes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D9/00—Electrolytic coating other than with metals
  • C25D9/04—Electrolytic coating other than with metals with inorganic materials
  • C25D9/08—Electrolytic coating other than with metals with inorganic materials by cathodic processes
  • C25D9/10—Electrolytic coating other than with metals with inorganic materials by cathodic processes on iron or steel

Definitions

• This invention relates to a method for electroplating an uncoated steel strip with a plating layer and an improvement thereof.
• 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.
• 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.
• the steel strip is pickled in a sulphuric or hydrochloric acid solution for removing the oxide film.
• a sulphuric or hydrochloric acid solution for removing the oxide film.
• 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.
• a fresh thin oxide layer is formed instantly on the bare steel surface.
• 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.
• the electrolyte becomes enriched in Fe 2+ .
• 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.
• 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-electroly
• 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.
• 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.
• a plating section consists of a series of vertical plating cells for obtaining a sufficient total anode length on a limited floor space.
• no current is applied during the first down-pass.
• 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.
• the oxide layer that was formed after the pickling step will dissolve rapidly (see figure 1).
• 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.
• 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.
• 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.
• 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.
• the electrolyte is acidic.
• 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.
• 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) 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.
• 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.
• 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.
• 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.
• 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.
• the invention is also embodied in an apparatus for performing the method according to the invention.
• 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.
• 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.
• 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.
• 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).
• SCE Saturated Calomel Electrode
• 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.
• ICP-AES Inductively Coupled Plasma Atomic Emission Spectroscopy

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Electroplating Methods And Accessories (AREA)
• Electroplating And Plating Baths Therefor (AREA)
• Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)

Priority Applications (11)

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• 2017
  • 2017-11-08 BR BR112019009702-3A patent/BR112019009702B1/pt active IP Right Grant
  • 2017-11-08 KR KR1020197014843A patent/KR102387496B1/ko active IP Right Grant
  • 2017-11-08 CA CA3043486A patent/CA3043486C/en active Active
  • 2017-11-08 ES ES17797316T patent/ES2883716T3/es active Active
  • 2017-11-08 JP JP2019524932A patent/JP7066707B2/ja active Active
  • 2017-11-08 WO PCT/EP2017/078582 patent/WO2018087135A1/en active Application Filing
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• 2019
  • 2019-05-15 ZA ZA2019/03049A patent/ZA201903049B/en unknown

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