Zinc and zinc alloy plating
Upcoming SlideShare
Loading in...5
×
 

Like this? Share it with your network

Share

Zinc and zinc alloy plating

on

  • 1,236 views

An overview of zinc and zinc alloy plating technology, mainly for automotive industry. A comparison between their properties is made.

An overview of zinc and zinc alloy plating technology, mainly for automotive industry. A comparison between their properties is made.

Statistics

Views

Total Views
1,236
Views on SlideShare
1,235
Embed Views
1

Actions

Likes
0
Downloads
48
Comments
0

1 Embed 1

https://www.linkedin.com 1

Accessibility

Categories

Upload Details

Uploaded via as Adobe PDF

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Processing…
Post Comment
Edit your comment

Zinc and zinc alloy plating Presentation Transcript

  • 1. Zinc and zinc alloy plating Erik Galdames Bach. of Eng. E-46800 Xativa (Spain)
  • 2. Content • • • • • • • • • • • • • Electroplating fundamentals Zinc plating Zinc plating coating properties Zinc plating hardware Zinc plating flow-chart Zinc plating process parameters Zinc plating process control Zinc alloy plating Zinc-nickel plating flow-chart Zinc-nickel process parameters Zinc-nickel process control Zinc and zinc-nickel plating performance Zinc and zinc alloy performance
  • 3. Electroplating. Fundamentals • • Faraday’s law: m = electrodeposited mass (g): I = Current applied (A), t = time (s), M = Atomic weight (g) n = interchanged electrons, Faraday’s law for thickness e = electrodeposited thickness (µm) (g), j = current density (A/dm2), t = time (s), n = interchanged electrons, d = element density (g/cm3). • • • . Theoretical performance. It is the expected performance according to Faraday’s law expressed in percentage. Theoretical performance 100% Actual performance. It is the real performance taking into account physical constraints (conductivity of the solution, heat losses, etc.) Actual performance < 100% Electrodeposition rate (µm/min.): Thickness of electrodeposited layer per unit of time
  • 4. Electroplating. Fundamentals
  • 5. Electroplating. Fundamentals FACTORS WITH AN INFLUENCE IN ELECTROPLATING • Part geometry • Position of anodes • Position of cathodes • Ratio cathode/anode • Nature and surface condition of part • Current density • Agitation of electrolyte (bath) • Temperature • Concentration of metallic ions • Concentration of hydrogen ions (pH) • Additives (brighteners, surfactants, etc.)
  • 6. Electroplating. Fundamentals  Current density – – –  Temperature – – – –  The higher the current density, the higher the electrodeposited layer thickness Size of crystal formed is reduced, provided that current density is within limits If current density increases beyond practical limits, a more rugous film will be electrodeposited Temperature generally favours electroplating processes. Less hydrogen is diffused on the cathode (the part to be coated) If temperature and current density both increase, brighter deposits will be obtained If temperature increases and current density is not changed, the size of metallic crystals will increase and consequently, rugosity of film will increase The higher the temperature, the higher the consumption of brighteners and other additives in the bath Metallic concentration – – If the metal concentration is low, brighter deposits with fine crystals can be obtained. If concentration is too low, layer thickness will be minimum If the metal concentration is too high, layers of higher roughness will be obtained
  • 7. Electroplating. Fundamentals     Electroplated finishes are coatings of pure or allied metals by means of DC It is the reversed process produced in a battery. Electricity is generated by means of ionic dissolution of metals in a battery In the case of electroplating processes metals are deposited from a ionic solution by means of an electric current Electroplating processes follow Faraday’s law – – – –  The quantity of an electrodeposited coating depends of the metal to be electrodeposited (chemical equivalent) The higher the current, the higher the quantity of electrodeposited metal The larger the surface area to plate, the lower the quantity of electrodeposited metal The longer the electroplating process, the higher the quantity of electrodeposited metal Other factors – – – – The higher the conductivity of the bath, the higher the quantity of electrodeposited metal. Type of electrolyte (acidic, alkaline) Temperature influences in the rate of metal electrodeposition Cleanliness of surface of the metal to be plated Content of impurities on the surface of the metal to be plated
  • 8. Zinc plating         Electroplated zinc is widely used for coating steel parts for corrosion protection Zinc offers cathodic protection to steel due to a redox reaction. Zinc is rusted to protect steel. As far as zinc is present, there will be cathodic protection against corrosion If the zinc layer has cracks, steel will still be protected against corrosion This phenomenon does not occur with paints or with nickel and copper plating where surface cracks or discontinuities allow corrosion to penetrate below the surface and extend the corrosion under the coated surface and corrodes the base mteal Zinc plating offers temporary corrosion protection Zinc also can be deposited in form of alloy with other metals such as nickel, iron, cobalt and tin Zinc layers can be treated with chromic passivations and sealers, which increase corrosion protection substantially (white and red rust). Zinc layers also allow further lubrication, which changes the friction behaviour of metallic fasteners Nowadays, most of the passivation products do not contain Cr6+
  • 9. Zinc plating   Zinc layers can be treated with chromic passivations and sealers, which increase corrosion protection substantially (corrosion resistance to white and red rust). Lubrication with liquid or solid lubricants is also possible. Lubrication changes the friction behaviour of the electrodeposited surface completely, thus the properties of the coated part, especially in a metallic threaded fastener Some years ago, it was common to see chromic formulations with Cr6+ for passivation processes. Cr6+ is carcinogenic and has been prohibited for use in many industries. These formulas were known as chromating or bichromating. Typical colours seen were: – – –   Cr6+ was widely used by its corrosion protective properties, since it had self-healing effect, i.e. sBadl scratches or superficial damage at microscopically level were self-repaired and corrosion protection was maintained. Due to restrictions of use of Cr6+, the majority of passivation products are made of formulas without Cr6+ (Cr6-free processes). They contain Cr3+ mainly. Typical colours: – – – –  Yellow to bronze. Iridescent yellow Olive green Black Transparent or clear, silver colour or bluish. Normally thin layer passivation Transparent iridescent with reddish, yellowish or bluish nuances. Known as thick layer passivation Yellow to bronze, iridescent yellow (not present in Europe) Black (not widely used) Cr3+ provides no self-healing effect and needs sealers or top-coats to increase corrosion protection
  • 10. Zinc plating Zn + transparent passivation Zn + transparent iridescent passivation
  • 11. Zinc plating  Surface preparation before coating mainly is: – – – – – – – –  Zinc plating has the following types of electrolyte – – –  Chemical cleaning Rinse Acid pickling Rinse Anodic electrocleaning Rinse Acid activation (not used for acid zinc plating) Rinse Acid zinc plating Alkaline zinc plating Alkaline cyanide * Post-treatments – – – – – Rinse Baking (for high tensile bolts > 10.9) Passivation (optional) Rinse Top-coats, sealers or lubrication (optional) – *Not used anymore
  • 12. Zinc plating  Acid zinc plating  Widely used technology  High plating efficiency > 90%  Less quantity of H2 generated  Faster electrodeposition  Excellent covering power  Poor throwing power  Bright deposit, special for decoration purposes  Less permeable to hydrogen effusion  Poor thickness distribution  Poor ductility
  • 13. Zinc plating  Alkaline zinc plating  Widely used technology  Good plating efficiency > 60%  More quantity of H2 generated  Medium electrodepositing rate  Less brighter deposit  Good covering power  Medium throwing power  More permeable to hydrogen effusion  Good thickness distribution  Good ductility
  • 14. Zinc plating coating properties  Layer thickness distribution 5 µm 20 µm 5 µm 8 µm 15 µm Long part The longer the part, the higher the differences in layer thickness between high current /low current density areas Small part Recess areas are low current density areas. Layer thickness is lower Uneven coating layer thickness in low current density areas wiith acid zinc plating. Recess areas with poor layer thickness Alkaline zinc plating Acid zinc plating 10 µm
  • 15. Zinc plating technology  Barrel zinc plating        For small parts Bulk process Widely used and automated technology Good productivity Low labour cost Reproducible results Rack zinc plating        For large parts Parts fixed on racks Widely used and almost totally automated technology Good productivity Higher labour cost if loading not automated Excellent appearance of coated parts Reproducible results, but effect of low/high current density areas must be taken into consideration
  • 16. Zinc plating hardware Zinc plating line on barrel
  • 17. Zinc plating hardware Zinc plating barrel
  • 18. Zinc plating hardware Zinc plating line on rack
  • 19. HYDROGEN EMBRITLEMENT RELIEF OVEN Lay-out of a zinc plating line ZINC PLATING ZINC PLATING ZINC PLATING ZINC PLATING ZINC PLATING HOT AIR DRYING ZINC PLATING SPINNING ZINC PLATING TOP-COAT ZINC PLATING COLD DRYING RINSE RINSE RINSE RINSE ACID ACTIVATION PASSIVATION RINSE RINSE RINSE RINSE ELECTROCLEANING ACID ACTIVATION RINSE RINSE RINSE RINSE ACID PICKLING RINSE RINSE RINSE CLEANING Zinc plating hardware Typical lay-out of a conventional zinc plating line on barrel. Design may vary and may include other additional baths or plating stations to increase production (up to 20 plating stations) Hydrogen embrittlement relief is done just after plating. Passivation is applied after hydrogen embrittlement relief
  • 20. Zinc plating hardware Plating station Ancillary tank for zinc anode dissolution Reservoir Rectifier Every plating station has its own rectifier. An ancillary tank is used for all stations to provide the necessary amount of Zn ions. A filter is used to filtrate the plating solution Lay-out of a zinc plating line
  • 21. Zinc plating flow-chart CLEANING 2-STEP RINSE YES RINSE ACID PICKLING Hydrogen embrittlement relief? NO 2-STEP RINSE DRYING BAKING ACID ACTIVATION COOL DOWN ELECTROCLEANING PASSIVATION 2-STEP RINSE 2-STEP RINSE ACID ACTIVATION COLD DRYING RINSE TOP-COAT ZINC PLATING DRYING
  • 22. Zinc plating process parameters Agent Temperature Process parameters Description Cleaning Alkaline soak cleaner 50 to 60ºC Removal of soils (oil, grease, machining residues) Acid cleaning HCl or H2SO4 with acid inhibitor RT or 35ºC HCl = 10-25% H2SO4 = 10 – 15% Removal of rust Electrocleaning Alkaline soak cleaner applied under current 60ºC to 70ºC NaOH: 50 to 100 g/L Current dens..: 5 to 10 A/dm2 Cleaning l of soils not removed by previous steps (e.g. fingerprints, Acid activation HCl RT 5% Activation of the surface prior to coating Zinc plating (alkaline noncyanide) Zn NaOH Carriers Brightener 20º to 28ºC Zn: 7 to 10 g/L NaOH: 110 to 120 g/L Current density: 1 to 2 A/dm2 Voltage: 5 to 10 V t = 50 to 60 min. Carriers and brightener acc. to formulator Plating Baking -- 200ºC to 220ºC > 4 h (1 h max. within electroplating and baking) Removal of hydrogen introduced by acid pickling and electroplating itself. Applicable to highstrength fasteners P.C. ≥ 10.9 and 1000 N/mm 2 Acid activation HNO3 RT 1% Previous step to passivation Passivation Cr containing acidic solutions RT or 35ºC to 60ºC Conc. depending on formulator pH 1,3 to 2,5 Increase of corrosion protection (white rust prevention), colour (silver, silver iridescent, black, yellow Top-coat Organic and mineral based alkaline solution Mainly RT, up to 50ºC Depending on formulator pH slightly alkaline Enhancement of corrosion protection of metallic coatings. Integrated solid lubrication (optional) Typical process parameters of zinc plating on barrel. They can be different according to the needs of the plater
  • 23. Zinc plating process control   Characteristics to be checked Concentration of cleaner, temperature, filtration and oil skimming  Concentration of acid pickling bath, pollutants such as Zn and Fe. Inhibitor performance > 90%  Anodic electrocleaning must be used for fasteners. Temperature  Concentration of Zn, NaOH of zinc plating bath  Temperature of zinc plating bath  Current density. Current and voltage of rectifiers  Plating time  Hull cell to control additives (carriers, brighteners)  Automated dosing of chemicals  Filtration of the zinc plating bath  Ratio cathode/anode  Alkaline vs. acid electrolyte for hydrogen embrittlement avoidance  Hydrogen embrittlement relief for high-strength fasteners mandatory (P.C. >10.9 bolts and parts with Rm ≥ 1000 N/mm2  Concentration of passivation, pH, pollutants (mainly Zn, Fe), temperature  Efficiency of rinsing operations. Concentration of pollutants. Use of flow-meters
  • 24. Zinc alloy plating  Zinc allows its electrodeposition as an alloy with other metals such as cobalt, nickel, iron and tin  The most widely used are zinc-iron, zinc-nickel and also tinzinc.  Zinc-iron. It consists of an alloy of zinc and iron with a content of Fe of 0,3 to 0,8% approximately. It allows Cr3-free black passivation and also transparent passivation, though black passivation is widely used. Medium corrosion protection. This finish is not longer preferred  Zinc-nickel. It consists of an alloy of zinc and nickel with a 12% to 15% Ni content. Excellent corrosion protection even on high temperature conditions (vehicle compartment applications). It allow Cr3-free transparent and black passivations. Good base for e-coat paints. High temperature resistance. High ductility after bending and crimping operations  Tin-zinc. It consists of an alloy of zinc and tin (Sn 70% Zn 30%). Excellent corrosion protection. Very good weldability and electrical conductivity
  • 25. Zinc-Nickel plating flow-chart CLEANING 2-STEP RINSE YES RINSE ACID PICKLING Hydrogen embrittlement relief? NO DRYING BAKING 2-STEP RINSE COOL DOWN ELECTROCLEANING PASSIVATION 2-STEP RINSE 2-STEP RINSE ACID ACTIVATION COLD DRYING RINSE TOP-COAT ZINC-NICKEL PLATING DRYING
  • 26. Zinc-Nickel plating process parameters Agent Temperature Process parameters Description Cleaning Alkaline soak cleaner 50 to 60ºC Removal of soils (oil, grease, machining residues) Acid cleaning HCl or H2SO4 with acid inhibitor RT or 35ºC HCl = 10-25% H2SO4 = 10 – 15% Removal of rust Electrocleaning Alkaline soak cleaner applied under current 60ºC to 70ºC NaOH: 50 to 100 g/L c.d.: 5 to 10 A/dm2 Cleaning l of soils not removed by previous steps (e.g. fingerprints, Acid activation HCl RT 5% Activation of the surface prior to coating Zinc-Nickel plating (alkaline) Zn NaOH Carriers Brightener 20º to 28ºC Zn: 8 to 11 g/L Ni: 1,9 to 2,3 NaOH: 110 to 130 g/L c.d.: 1 to 2 A/dm2 t = 120 to 160 min. Plating Baking -- 200ºC to 220ºC > 4 h (1 h max. within electroplating and baking) Removal of hydrogen introduced by acid pickling and electroplating itself. Applicable to high-strength fasteners P.C. ≥ 10.9 and 1000 N/mm2 Passivation Cr containing acidic solutions RT or 35ºC to 60ºC Conc. depending on formulator pH 1,3 to 2,5, depending on type of passivation Increase of corrosion protection (white rust prevention), colour (silver, silver iridescent, black, yellow Top-coat Organic and mineral based alkaline solution Mainly RT, up to 50ºC Depending on formulator pH slightly alkaline Enhancement of corrosion protection of metallic coatings. Integrated solid lubrication (optional) Typical process parameters of zinc-nickel plating on barrel. They can be different according to the needs of the plater
  • 27. Zinc-nickel plating process control                   Characteristics to be checked Concentration of cleaner, temperature, filtration and oil skimming Concentration of acid pickling bath, pollutants such as Zn and Fe. Inhibitor performance > 90% Anodic electrocleaning must be used for fasteners. Temperature Concentration of Zn, Ni, NaOH of zinc-nickel plating bath. Use of AAS (Atomic Absorption Sprectroscopy) for analysis of metal content Temperature of zinc-nickel plating bath(use of heat exchangers) Current density. Current and voltage of rectifiers Plating time Hull cell to control additives (carriers, brighteners) Automated dosing of chemicals Filtration of the plating bath Ratio cathode/anode Turnover (recycling of bath to maintain homogeneous concentration) Removal of carbonates (chiller or membrane technology) Alkaline vs. acid electrolyte for hydrogen embrittlement avoidance. Acid electrolyte for parts made of cast iron and high-carbon steel Hydrogen embrittlement relief for high-strength fasteners mandatory (P.C. >10.9 bolts and parts with Rm ≥ 1000 N/mm2 Concentration of passivation, pH, pollutants (mainly Zn, Fe), temperature Efficiency of rinsing operations. Concentration of pollutants. Use of flow-meters
  • 28. Zinc and zinc-nickel plating performance Zn Zn Zn ZnNi ZnNi ZnNi Zn+transp. passivation Zn+passiv. transp.+sealer Zn+thick layer passivation ZnNi+transp. passivation ZnNi+transp. passivation ZnNi+black passivation 8 µm 8 µm 8 µm 8 µm 8 µm 8 µm NSS WR 24 120 h 72 h 96 h 120 h 96 h NSS RR 144 h 240 h 144 h 720 h 720 h 720 h Colour Silver Silver Iridescent yellow Silver Silver Black Cr6+ No No No No No No Hydrogen embrittlement risk Yes Yes Yes Fair Fair Fair Temp. Resistance 80ºC 100ºC 80ºC 150ºC 150ºC 150ºC Multi-tightening NO Conditional No No Conditional No Sealing No Yes No No Yes Yes + ++ + ++++ ++++ ++++ Coating Layer thickness Cost
  • 29. Zinc and zinc alloy properties Properties of zinc and zinc alloy coatings Properties Appearance Weldability Wear resistance Ductility Corrosion protection (as plated) Alkaline zinc Alkaline zinc nickel Alkaline zinc-iron Good Fair Fair Fair Good Fair Good Fair Good Fair Fair Fair White rust Red rust Fair Fair Excellent Excellent Excellent Good Corrosion resistance (after thermal conditioning) White rust Red rust Low Low Good Good Low Low Corrosion resistance (after bending) White rust Red rust Fair Fair Good Good Fair Fair Characteristics of electrolyte Distrib. (min./max) Plating efficiency Covering power Bath control Good Fair Fair Good Excellent Low Fair Fair Good Fair Fair Fair Analysis and measurement techniques X-ray fluorescence Magnetic induction Coulometric Good Good Good Good Bad Good Good Fair Good -- Good -- Alloy content % (X-ray fluorescence)
  • 30. Summary          Electroplated zinc provides cathodic protection to steel parts. It provide medium term corrosion protection Zinc can also be electrodeposited as an alloy to provide better corrosion protection Zinc-nickel and tin-zinc can provide excellent corrosion protection compared to pure zinc Zinc-nickel is widely used in the automotive industry Tin-zinc is used in welding applications and where conductivity is important. It is used in aerospace industry Zn, ZnNi, SnZn provide good electrical conductivity Good (pure and tin-zinc) to fair weldability (zinc-nickel) Zn, ZnNi provide uniform coating distribution except on large parts (high-current density areas) Risk of hydrogen embrittlement on high-strengh fasteners (≥ 10.9 bolts or parts with tensile strength > 1000 N/mm2). Necessary to apply hydrogen embrittlement relief processes