Classifications

• • 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
  • C25D17/00—Constructional parts, or assemblies thereof, of cells for electrolytic coating
  • C25D17/10—Electrodes, e.g. composition, counter electrode
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D21/00—Processes for servicing or operating cells for electrolytic coating
  • C25D21/12—Process control or regulation
• • 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
  • C25D3/10—Electroplating: Baths therefor from solutions of chromium characterised by the organic bath constituents used
• • 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
  • C25D5/18—Electroplating using modulated, pulsed or reversing current
• • 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

Definitions

• This invention relates to a method for electrodepositing a functional or decorative chromium layer from a trivalent chromium electrolyte onto a metallic substrate.
• Hexavalent chromium electrodeposition has been used for many years to provide decorative, durable coatings with excellent wear and corrosion resistance properties.
• hexavalent chromium baths have come under increasing scrutiny due to the toxic nature of the bath, effects on the environment, and workers' health.
• chromium coating must be applied from a Cr(III) electrolyte.
• Cr(III) electrolytes for applying decorative chromium coatings are available in the market. Typical applications are automotive parts (interior and exterior), sanitary and plumbing fixtures, furniture, and hand tools.
• a diffusion barrier layer prevents diffusion of iron or other detrimental elements from the steel substrate to the solar cells that are deposited at a temperature up to 600 °C.
• One of the barrier layer combinations is a chromium layer on a nickel-plated steel substrate.
• the term "detrimental" element means an element that adversely affects the efficiency of the solar cell.
• Decorative chromium coatings are usually applied on a duplex nickel base coating.
• the nickel layer provides corrosion resistance and levelling of the substrate surface.
• the principle is that by applying two nickel layers, the first being semi-bright with a columnar structure (13 - 30 mhi), the second being bright with a laminar structure (5 - 20 mhi), exceptional corrosion resistance is obtained, because the bright nickel offers cathodic protection to the semi-bright nickel.
• the bright nickel acts as the anode and sacrificia lly protects the semi-bright nickel. This results in corrosion spreading laterally rather than penetrating the substrate.
• Decorative chromium coatings have numerous micro-cracks and micro-pores. Since these micro-defects are uniformly spread over the chromium surface, corrosion is not localised and therefore proceeds slowly.
• Chloride based electrolytes risk of chlorine evolution at the anodes, a depolariser (bromide is often used for this purpose) is required to suppress Cr(VI) formation at the anode, and moderate chromium deposition rates (0.2 pm min 1 );
• Sulphate based electrolytes low chromium deposition rates (0.05 pm min- v) Commercially available trivalent chromium baths often result in cracked coatings after a heat treatment.
• a method for the electrodeposition of a functional or decorative chromium layer onto a metallic substrate in a batch or a continuous electrodeposition process from a halide-ion free and boric acid free aqueous electrolyte solution comprising: i) a trivalent chromium compound provided by a water-soluble chromium(III) salt wherein the electrolyte solution contains at least 50 mM and at most 1000 mM Cr 3+ -ions; ii) a total amount of from 25 to 2800 mM of sodium sulphate or potassium sulphate; iii) a formate salt as a complexing agent at a molar ratio of at least 1 : 1 and at most 4.0: 1.0 (or 4: 1); iv) optionally sulphuric acid or sodium hydroxide or potassium hydroxide to adjust the pH to the desired value; v) optionally a surfactant to facilitate the release of hydrogen gas bubbles from the substrate, where
• iridium oxide or ii). a mixed metal oxide comprising iridium oxide and tantalum oxide for reducing or eliminating the oxidation of Cr 3+ -ions to Cr 6+ -ions, and wherein the electrodeposition is performed by means of pulsed electrodeposition comprising two or more current pulses at a selected current density for a selected pulse duration, wherein each current pulse (the "on-time”) is followed by an interpulse period (the "off-time") wherein the current density is set to 0.
• a practical minimum off-time is 0. 1 s. A shorter time will not result in the required relaxation of concentration gradients including the pH in the diffusion boundary layer near the cathode and the establishment of new chemical equilibria of Cr(III) complexes during the time period wherein the current is switched off.
• the use of a trivalent chromium compound renders the method REACH compliant as the use of hexavalent chromium in the electrolyte is avoided.
• the absence of halide- ions in the electrolyte prevents the formation of toxic gases such as chlorine and bromine at the anode.
• a buffering agent such as the often-used boric acid (H3BO3), is not present in the electrolyte to prevent hexavalent chromium formation at the anode during electrodeposition. Even without boric acid chromium metal will be deposited under the conditions of the method according to the invention.
• the electrolyte does not contain a depolarizer, such as potassium bromide. The absence of this compound prevents the risk of bromine formation at the anode.
• the electrodeposition process may be a batch electrodeposition process or a continuous electrodeposition process.
• the preferred metallic substrate is an unalloyed or low-alloyed steel substrate.
• the steel substrate can be of varying thickness, preferably from 25 pm to 3 mm.
• the lower thickness forms a flexible solar module, whereas thicknesses of over 0.3 mm can take a rigid form or even be directly integrated to building elements, in which case an electrically insulating layer is applied.
• the unalloyed or low-alloyed steels comprise unalloyed or low-alloy steel mild steel, low carbon (LC), extra low-carbon (ELC) or ultra-low carbon (ULC) could be used, such as the steels DC01 to DC07 as defined by EN 10130:2006, e.g. in table 2.
• the surface condition of the steels is bright (Ra £ 0.4 pm, EN 10130:2006) or mirror-like (R a £ 0.10 pm, more preferably R a £ 0.08 pm) to minimise the possible negative effects of surface roughness.
• the unalloyed or low-alloyed steels also include cold-rolled structural steels or a high strength low alloy (HSLA) steel could also be chosen. These may be used if a higher strength of the steel substrate is needed.
• HSLA high strength low alloy
• the unalloyed or low-alloyed steel substrate steel types referred to above specifically exclude stainless steels.
• Stainless steel is an alloyed steel with a minimum of 10.5 wt.% Cr. Chromium is expensive.
• the method according to the invention allows to use a less expensive steel substrate than stainless steels and still provide the required corrosion properties and protection against poisoning of a photovoltaic device provided on top of the steel.
• the metallic substrate that has been provided with a functional or decorative chromium layer according to the invention can also be used for other applications where the functional and/or decorative properties of a chromium layer are required.
• Pulsed electrodeposition in the context of this invention comprises or consists of a plurality of current pulses (i.e. two or more) at a selected current density for a selected pulse duration each current pulse followed by an interpulse period wherein the current density is set to 0.
• the current density of 0 in the interpulse period encompasses a very low current density, cathodic or anodic, which has no material effect on the electrodeposition in the interpulse period and has the same technical effect as a current density of exactly 0.
• the term “comprises” means that at least the components as recited are present in the amounts or within the ranges as recited.
• the term “consists of” and variations thereof have a limiting meaning where these terms appear in the description and claims.
• the term “consists of” means that the components as recited are present in the amounts or within the ranges as recited and that no other components are present at all, unless indicated as an optional element, in which case it may be present in the given amounts or not at all, or at least not in an amount that materially affects the working of the claimed invention. This also means that inevitable impurities or ineffective additions of components are deemed to not materially affect the way the invention works.
• halide-ion and boric acid free electrolyte means that the aqueous electrolyte contains no halide-ions and no boric acid in an amount that materially affects the way the invention works.
• the claimed buffering action in prior art electrolytes of boric acid is not necessary and not even desired in the electrolyte and method according to the invention.
• the aqueous electrolyte solution consists of, and preferably consists only of: i) the trivalent chromium compound provided by a water-soluble chromium(III) salt wherein the electrolyte solution contains at least 50 mM and at most 1000 mM Cr 3+ -ions; ii) a total amount of from 25 to 2800 mM of sodium sulphate or potassium sulphate; iii) a formate salt as a complexing agent at a molar ratio of at least 1 : 1 and at most 4.0: 1; iv) optionally sulphuric acid or sodium hydroxide or potassium hydroxide to adjust the pH to the desired value; v) optionally a surfactant to facilitate the release of hydrogen gas bubbles from the substrate; vi) remainder inevitable impurities.
• the pulse duration in a batch electrodeposition process is between 0.1 and 2.5 seconds and preferably between 0.5 and 2.5 seconds, and the interpulse period is between 0.1 and
• the pulse duration in a continuous electrodeposition process is between 0.1 and 2.5 seconds and preferably between 0.5 and 2.5 seconds
• the interpulse time is between 0.1 and 5 seconds and preferably between 0.5 and 5 seconds.
• the pulse duration in the continuous electrodeposition process is between 0.1 and 2 seconds and preferably between 0.5 and 2 seconds
• the interpulse time is between 0.1 and 2 seconds and preferably between 0.5 and 2 seconds.
• the aqueous electrolyte solution (also referred to as “the electrolyte” in this description) consists of the compounds in the ranges as mentioned hereinabove in the range, and more preferably consists only of the compounds in the ranges as mentioned hereinabove in the range.
• the temperature of the electrolyte during electrodeposition is at most 55°C, more preferably at most 50°C.
• a suitable minimum temperature of the electrolyte during electrodeposition is 35°C.
• the electrolyte has a pH of 2.00 or more and or 3.00 or less. All references to pH relate to the pH value as measured at 25 °C. More preferably the pH is between 2.25 and 2.75.
• the optional sulphuric acid or sodium hydroxide or potassium hydroxide only needs to be added if the pH has to be adjusted to the desired value. If the pH is already at the desired value, then no such addition will be needed.
• the complexing agent is a formate salt, preferably a sodium or potassium formate.
• the ratio of molar complexing agent to Cr 3+ is 2.0: 1.
• the optional surfactant can be added if required and is added to promote the release of hydrogen gas bubbles, formed during the electrodeposition, from the substrate.
• TriChrome Regulator LR in an amount of between 2-4 ml/l in accordance with the recommendations of the technical datasheet as provided by the supplier.
• Other surfactants are available and the skilled person will have no problem selecting a suitable one and amounts to be added in accordance with the relevant the technical datasheet.
• a surfactant is generally not needed in a continuous process where the inherent relative movement between the electrolyte and the substrate already removes any bubbles from the substrate, particularly if the substrate is a strip and the continuous process is performed in a strip electrodeposition line.
• the chromium coating thickness should be between 10 and 1000 nm. If the chromium coating is defect free, it may serve as a barrier layer on its own. However, in many cases a nickel layer between the substrate and the chromium layer is preferably used, wherein the chromium and nickel layer together form the barrier layer. The nickel layer smoothens out the steel substrate and offers a degree of insurance in case the chromium layer, despite all due care, contains some defect or pinhole.
• the underlayer is not particularly restrictive as long as the underlayer provides a smooth and defect free layer between the steel substrate and the chromium layer on top. Copper layers of between 50 and 300 nm have also shown to be useful and effective as underlayers. Preferably the Cu-layer is between 50 and 150 nm (e.g. about 100 nm) and the subsequent Cr-layer is between 450 and 550 nm (e.g. about 500 nm).
• the nickel layer thickness is between 0.25 and 5.5 pm, and the chromium layer thickness is between 0.01 pm (10 nm) and 1.0 pm (1000 nm).
• the Ni and Cr layer can be thinner than without a dielectric layer.
• a suitable minimum chromium layer thickness is 15 nm.
• a suitable maximum chromium layer thickness is at most 800 nm, preferably at most 700 nm.
• a suitable minimum nickel layer thickness is 0.4 pm.
• a suitable maximum nickel layer thickness is at most 3.5 pm, preferably at most 2.5 pm.
• the nickel layer thickness is between 1.75 and 2.5 pm and/or the chromium layer thickness is between 0.450 and 0.550 pm.
• These layer thicknesses are particularly suitable for production of PV-modules requiring a high process temperature (such as in CIGS solar technology).
• a high process temperature such as in CIGS solar technology.
• the nickel and chromium layers can be thinner, and the dielectric layers also prevents the movement of detrimental elements, such as iron and manganese, to the CIGS layer, depending on the nature of the coating.
• the chromium coating must be defect free and crack free to prevent the steel substrate interfering with the functioning of the PV-application.
• Commercially available trivalent chromium baths resulted in cracked coatings, either already before or after the annealing, depending on the thickness.
• the cold rolled steel substrate needs to be recrystallisation annealed or recovery annealed, then this has to be done before applying the optional nickel layer or the optional copper layer and the chromium layer, because otherwise detrimental elements may diffuse into the nickel, copper or chromium layer during the recrystallisation annealing or recovery annealing and diffuse through the molybdenum back contact layer during the growing of the CIGS absorber layer and finally potentially end up in the CIGS absorber layer.
• the nickel, copper and chromium layers are defect free, they should reduce diffusion of elements (Fe, Mn, etc) from the substrate to (ideally) ⁇ 10 ppm, because these detrimental elements negatively impact the efficiency of the PV-application.
• the chromium coatings deposited according to the invention provide good protection against the diffusion of elements from the steel substrate up to the maximum temperature of 650 °C.
• the method according to the invention is suitable for use in a batch type process, for instance for rack electrodeposition or piecewise electrodeposition and in a continuous process, for instance for electrodeposition of strip material.
• the line speed of the electrodeposition line in the continuous electrodeposition process is at least 50 m/min, preferably at least 100 m/min.
• HILAN ® nickel plated steel coil Two variants of HILAN ® nickel plated steel coil were used as substrate: a variant with a high surface roughness and a dull surface appearance (Ra. min 0.6 and max. 2.5 pm) and a bright finish variant with a low surface roughness and a shiny appearance (Ra ⁇ 0,2 pm).
• Tata Steel's HILAN ® is a cold-rolled steel strip product electroplated with bright nickel. Bright nickel creates an extra hard and extra bright surface and is suitable for stamping and deep-drawing operations. It is produced by electrodepositing a bright nickel layer of between 0.5 and 3.0 pm on a cold-rolled steel strip which offers low contact resistance and high corrosion resistance.
• the material was activated in a 50 g/l sulphuric acid solution by dipping it for 10 seconds in the solution at room temperature. After activation a Woods nickel strike layer was applied in an electrolyte at 30 °C at a cathodic current density of 10 A/dm 2 with nickel anodes.
• the aqueous electrolyte comprises 240 g/l nickel(II)chloride hexahydrate and 125 ml/l of hydrochloric acid 37 %
• the aqueous electrolyte solution for electrodeposition of the chromium coating is prepared as follows: Table 1: 30 g/l Cr electrolyte with a formate/Cr ratio of 2.0 (surfactant (V) is optional).
• the electrolyte was treated to remove sulphite as disclosed in EP3428321-A1 and the electrolyte temperature was 43 °C.
• the chromium coating weight was measured with Inductively Coupled Plasma - Mass Spectrometry (ICP-MS), a benchtop spectrometer (SPECTRO XEPOS) or with a Byk handheld XRF-spectrometer (type 4443).
• ICP-MS Inductively Coupled Plasma - Mass Spectrometry
• SPECTRO XEPOS benchtop spectrometer
• SPECTRO XRF-spectrometer type 4443
• the inventors also found that shiny coatings can be obtained at longer aggregated electrodeposition times when the current is interrupted.
• the hydrogen that also evolves during electrodeposition forms bubbles on the surface and these bubbles are being stimulated to come off the metallic substrate which is being plated, e.g. by means of agitation, a shaking action or a mechanical action.
• the next electrodeposition step can then be performed on a surface free from hydrogen bubbles each time.
• the inventors consider this removal of the hydrogen bubbles as very important in the production of a bright chromium plated surface.
• the intermittant removal of hydrogen and intermittant electrodeposition results in a very shiny surface and also in much thicker chromium layers (figure 2). There appears to be no limit to the thickness of the chromium layer when applied in this way and layers up to 2 pm could be applied.
• colour is one of the most important coating properties. It is desired that the colour from Cr(III) electrolytes is close to the colour from Cr(VI) electrolytes, because this allows different parts plated from Cr(III) and Cr(VI) electrolytes to be combined without perceivable colour differences.
• the interrupted electrodeposition process causes a relaxation of concentration gradients including the pH in the diffusion boundary layer near the cathode and the establishment of new chemical equilibria of Cr(III) complexes during the time period wherein the current is switched off.
• the interruption allows the hydrogen that has evolved during the electrodeposition to dissipate, move away from the cathode surface, or be actively removed from the cathode surface. This results in preventing the formation of chromium-oxide during the electrodeposition.
• Evidence is provided by XPS results performed on two samples (corresponding SEM images are provided in figure 4).
• the dull sample contains a massive amount of chromium oxide, and the shiny sample does not.
• the electrolyte composition, temperature, pH and current density in both examples in table 3 are identical.
• Table 3 Composition of shiny and dull substrate.
• Known trivalent chromium electrolytes for electrodeposition decorative chromium layers contain boric acid as a buffer. This ensures that the pH in the diffusion boundary layer is maintained at a set value. In this known technology this is a prerequisite for depositing Cr-metal, because the prior art states that without these buffers mainly or only chromium oxide is deposited.
• the process according to the invention shows that it is possible to deposit decorative chromium layers without this boric acid buffer, thereby simplifying the electrolyte.
• the inventors also found that the total process time of the interrupted electrodeposition process can be limited by adding a surfactant to the electrolyte.
• This surfactant facilitates the removal of the hydrogen that has evolved during the electrodeposition.
• the interpulse time can be reduced to below 2 seconds. If the interpulse time becomes too short, then the hydrogen cannot be removed sufficiently effectively, and the establishment of new chemical equilibria of Cr(III) complexes during the time period wherein the current density is 0 is not obtained. This leads to a dull surface of the chromium layer.
• the preferred interpulse time lies between 0.1 and 2 seconds.
• FIG 2 the relation is shown between the number of pulses and the amount of chromium deposited for a current density of 26 A/dm 2 , an "on-time" of 1 s and an off-time of 10 s.
• the chromium coating weight is directly proportional to the number of current pulses. For different values of current densities and combination of on- and off-time similar proportional relationships were found.
• a comparison of the deposition rate of the electrolyte according to the invention with commercially available electrolytes shows that the deposition rate obtainable with the inventive method is much higher.
• the inventors obtained deposition rates of up to 0.40 pm/min.
• Experiments with the commercially available sulphate based Trylite ® Flash SF by MacDermid Enthone show that a deposition rate of 0.05 pm/min can be obtained under optimum conditions (deposition temperature 60 °C, cathodic current density 10 A/dm 2 , anodic current density 3 A/dm 2 and a pH of 3.7.).
• This electrolyte contains boric acid and Trylite specific compounds.
• Figure 1 Single pulse electrodeposition process with pulse duration of 1 second as a function of current density.
• Left hand side Chromium coating weight in mg/m 2
• right hand side gloss expressed in GU (Gloss Units).
• Figure 2 Relation between the number of pulses and the chromium coating weight deposited for a current density of 26 A/dm 2 , pulse durations of 1 s and an interpulse time of 10 s.
• the top line presents the ICP-MS measurements
• the middle line presents the benchtop XRF measurements
• the lower line presents the handheld measurements.
• Figure 3 Loss in process efficiency with increasing interpulse time. S means that the underlying nickel layer of the Hilan was shiny (see table 3) and D that the underlying nickel layer was dull. 8 and 20 mean the number of Is pulses of 26 A/dm 2 used to deposit the chromium layer.
• Figure 4 SEM images of chromium surface obtained by single-pulse process vs multi-pulse process. Both images have been made at the same magnification.
• the measurement bar represents 1 pm.
• the I Probe was 150 pA, and the WD is 4.6 mm. Pixel size 9,766 nm).

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