Nanoceramic Coating an Introduction

Dr E. Ramanathan PhD

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Advantages of Nanoceramic Coating over Zinc phosphate coating – A Summary

• Zinc Phosphate Coating has been widely used in metal pretreatment for more than five decades.
• Limitations of traditional zinc phosphate coating:
  • High energy and power consumption.
  • Excessive sludge formation, posing disposal issues.
  • Prolonged processing time, especially for room-temperature phosphate coatings.
• Modern Metal Pretreatment Systems:
  • Use of sol-gel methods involving hexafluorozirconic acid and hexafluorotitanic acid.
  • Incorporation of adhesion promoters like polyacrylic acid, polyvinyl alcohol, and tannic acid to enhance surface treatment for painting.
• Advantages of Nano-Ceramic Coatings:
  • These coatings use zirconium oxide instead of fluoride on metal surfaces, resulting in reduced sludge.
  • Zirconia, titania, ceria, and zinc oxide coatings have been researched for superior corrosion resistance.
• Coating Methods and Materials:
  • Nano-zirconia coatings have been developed for various metals (mild steel, aluminum, and galvanized iron).
  • Utilizes dispersants like polyacrylic acid and ammonium polymethacrylate for effective nanocoating.
• Characterization of Nano-Zirconia Coatings:
  • Studied using AFM (Atomic Force Microscopy), SEM (Scanning Electron Microscopy), EIS (Electrochemical Impedance Spectroscopy), and other techniques.
  • These nano-coatings show improved adhesion between the metal and organic top coat.
• Commercial Applications and Testing:
  • Coatings based on hexafluoro zirconic acid (FZ) and commercial Bonderite NT-1 have been investigated.
  • Corrosion resistance is tested against various environmental and chemical conditions.
• Environmentally Safe Solutions:
  • Nano-coatings do not form insoluble sludge, making them eco-friendly alternatives.
  • No heavy metals such as chromium, nickel, or manganese are involved.
• Applications in Marine and Industrial Sectors:
  • Nano-ceramic coatings help in preventing microbial colonization and fouling in marine environments and in improving durability in industrial applications.

This summary highlights the environmental benefits, technological advancements, and broad application potential of nano-ceramic coatings in modern industrial metal treatment processes.

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Surface Preparation and Conversion Coatings

• Quality of Paint vs Substrate:
  • High-quality paint on a poorly prepared substrate performs worse than average paint on a clean, well-prepared substrate.
• Importance of Surface Preparation:
  • Critical for enhancing adhesion of the top coat.
  • Achieved via three methods:
    1. Surface cleaning without altering the metal’s nature.
    2. Physical or chemical cleaning of the surface.
    3. Chemical conversion of the metal.
• Primary Cleaning Methods:
  • Alkali and solvent cleaning: Removes oil or grease without affecting metal properties.
  • Mechanical polishing: Physically removes tool marks or rust.
  • Acidic de-rusting: Chemically removes oxide layers from the metal.
• Inorganic Chemical Conversion Coatings:
  • Common coatings include:
    • Zinc phosphate coating.
    • Iron phosphate coating.
    • Manganese phosphate coating.
  • Applications:
    • Protects metal surfaces for organic top coat application.
    • Enhances lubrication (e.g., oiling or soaping for tube/wire drawing or cold forming).
    • Improves adhesion for rubber coatings.
    • Strengthens concrete structures (e.g., zinc phosphate coating on thermo-mechanically treated rebars).
• Application on Various Metals and Alloys:
  • Suitable for metals like:
    • Iron, cold-rolled mild steel, hot-rolled steel.
    • Galvanized steel, electro-galvanized steel.
    • Aluminum and its alloys, copper, manganese, zinc, titanium.
• Key Factor for Quality Coatings:
  • Effective cleaning of the base metal is essential to ensure high-quality conversion coatings.

The Need for Metal Surface Treatment

• Pre-requisites for Surface Treatment:
  • Metal surface must be free from:
    • Corrosion types: crevice, pitting, galvanic, creeping, and cosmetic.
    • Defects and contaminants: scales, tool marks, oil, grease, and dirt.
• Types of Corrosion:
  • Crevice corrosion:
    • Caused by stagnant electrolytes in joints or surface deposits.
  • Pitting corrosion:
    • Results from prolonged contact with corrosive acids on localized areas.
    • Leads to a rough surface.
  • Galvanic corrosion:
    • Occurs when two dissimilar metals are in contact.
  • Cosmetic corrosion:
    • Visible on scratches of paint layer on sheet metal.
    • Can lead to creeping corrosion beneath paint, weakening the substrate.
• Mechanical Cleaning Benefits:
  • Processes:
    • Sanding with emery sheets.
    • Polishing.
  • Outcomes:
    • Increases active sites on bare metal.
    • Facilitates electrochemical reactions for corrosion resistance.
• Role of Phosphate Coating:
  • Acidic phosphate coating:
    • Increases active sites for nucleation of phosphate crystals.
    • Produces smaller grains, reducing porosity and enhancing corrosion resistance.
  • Rough metal surfaces:
    • Offer greater surface area, enabling thicker phosphate conversion coatings.
• Key Characteristics of Phosphate Coatings:
  • Smaller grain size leads to better corrosion resistance.
  • Thicker coatings with more porosity are ideal for lubrication applications.
    • Porous layers absorb more oil, improving lubrication properties.
• Surface Profile Impact:
  • Differences in metal roughness affect coating weight and corrosion resistance capabilities.

Different Types of Metal Pre-Treatment

• MIL Specification Classification:
  • M-type: Manganese-based phosphate coatings.
  • Z-type: Zinc-based phosphate coatings.
• Varieties of Chemical Conversion Coatings:
  • Traditional coatings:
    • Zinc phosphate.
    • Manganese phosphate.
    • Amorphous iron phosphate.
  • Modified coatings:
    • Calcium-modified zinc phosphate.
    • Low-zinc manganese phosphate.
    • Zinc phosphate combined with nickel and manganese.
  • Nano-enhanced coatings:
    • Nanoparticle-incorporated zinc phosphate.
  • Metal oxides-based coatings:
    • Zirconium oxide.
    • Cerium oxide.
    • Chromium oxide.
    • Yttrium oxide.
• Processing Techniques for Conversion Coatings:
  • Spraying.
  • Hot immersion.
  • Cold immersion.
• Coating Measurement:
  • Phosphate coatings are typically reported in terms of weight per unit surface area.

Coating Weight and Thickness – Bullet Points:

• Measurement of Coating Weight:
  • Reported as weight per unit surface area (g/m²).
  • Procedure:
    1. Weigh coated specimen to nearest mg (w₁).
    2. Immerse specimen in 50 g/L chromic acid stripping solution at 165°C for 15 minutes to remove coating.
    3. Clean in running water, dry, and reweigh (w₂).
    4. Repeat until constant weight is obtained.
  • Formula:

• Coating Weight Ranges:
  • Amorphous iron phosphate layer: 30–100 mg/ft².
  • Spray-type phosphate coating: 100–400 mg/ft².
  • Medium hot immersion zinc phosphate coating: 450–1000 mg/ft².
  • Heavy zinc phosphate and manganese phosphate coatings: 1000–3000 mg/ft².
• Porosity and Applications:
  • Heavy zinc phosphate and manganese phosphate coatings:
    • Feature high porosity and rough surfaces.
    • Not suitable for painting.
    • Ideal for applications requiring oiling, waxing, and lubrication.
• Adhesion Improvement:
  • Cathodic and anodic phosphate coatings:
    • Enhance adhesion of paints on mild steel components.

Selection of Suitable Chemical Conversion Coating – Bullet Points:

• Factors for Selection:
  • Based on the nature of the base metal and type of topcoat.
  • Consider key parameters: coating weight, porosity, and thickness.
• Coatings for Specific Metals:
  • Aluminum:
    • Chromate conversion.
    • Fluoride-activated phosphate coating.
    • Silane coating and in situ phosphating.
    • Organosilane post-treatment as an eco-friendly alternative to hexavalent chroming.
  • Mild steel:
    • Amorphous iron phosphating.
    • Crystalline zinc phosphating.
    • Manganese phosphating.
    • Calcium-modified zinc phosphating.
    • Trication phosphating and low-zinc manganese phosphating.
• Barrier Coatings:
  • Low-porosity phosphate layers act as efficient barriers against corrosion.
• Adhesion Factors:
  • Primary adhesion: Between conversion coating and bare metal.
  • Secondary adhesion: Between base coat and top coat.
  • Thin, tightly packed zinc phosphate coatings improve corrosion and humidity resistance.
• Pre-Treatment Techniques for Enhanced Coatings:
  • Electrochemical treatment.
  • Galvanic coupling with other metals.
  • Pre-dip in titanium phosphate suspension and post-dip in chromic solution or demineralized (DM) water.
• Surface Preparation:
  • Essential for mild steel to ensure good performance of:
    • Phosphate layers.
    • Subsequent paint films, rubber coats, lubricant oiling, or concrete applications.
• Phosphating Mechanism:
  • Steps include:
    1. Electrochemical attack.
    2. Amorphous phosphate precipitation.
    3. Phosphate crystallization.
    4. Reorganization of phosphate crystals.
• Key Benefits of Fine Grain Coatings:
  • Smaller grains reduce porosity and enhance corrosion resistance.
• Material Contributions to Corrosion Resistance:
  • Nature of base material, component design, manufacturing processes, and post-coating steps.
  • Zn-Ni alloy modifications improve corrosion protection of epoxy coatings.

Phosphate Coatings

• Use in Automotive Industry:
  • Zinc phosphating is used for metal pretreatment to improve paint adhesion and corrosion resistance.
  • Tightly packed zinc phosphate crystals enhance resistance to creeping corrosion.
• Salt Spray Test:
  • Scratched paint film is tested to study corrosion extent over untreated and treated panels.

• Types of Zinc Phosphate Crystals:
  • Hopeite: Zn₃(PO₄)₂·4H₂O.
  • Parahopeite: Triclinic dimorph of hopeite.
  • Phosphophyllite: Zn₂Fe(PO₄)₂·4H₂O.
• Performance Insights:
  • Zinc phosphate provides better insulation but is less porous than manganese phosphate.
  • Aging bath solutions can cause sludge formation, reducing coating quality.
• Environmental and Energy Considerations:
  • Room-temperature phosphating saves energy but requires more chemicals and time.
  • Galvanic coupling (e.g., mild steel with copper or brass) enhances corrosion resistance.
• Coating Improvement Techniques:
  • Incorporation of:
    • Nickel ions (Ni²⁺): Slows zinc phosphating, forms thin, corrosion-resistant layers.
    • Calcium ions: Reduces crystal grain size and increases corrosion resistance.
    • Titanium salts: Refines zinc phosphate crystal lattice.
• Mechanical and Analytical Testing:
  • Methods include:
    • Visual inspection for physical characteristics.
    • Thickness and porosity testing.
    • Hygroscopicity and thermal stability evaluation.
    • SEM (Scanning Electron Microscopy) and AFM (Atomic Force Microscopy) for surface morphology.
    • XRD (X-Ray Diffraction) to analyze crystal forms and measure “P ratio”: P ratio=PP+HP \text{ ratio} = \frac{P}{P + H} where PP is intensity of phosphophyllite and HH is intensity of hopeite.
• Operational Steps in Phosphating:
  • Includes alkaline degreasing, water rinsing, acid de-rusting, zinc phosphating, chrome passivation, and hot air blow.
• Factors Affecting Coating Quality:
  • Temperature, agitation, and bath maintenance (acid levels, accelerators).
• Additional Benefits of Phosphate Coatings:
  • High porosity for lubrication applications.
  • Improved adhesion of subsequent paint films.

Nano-Ceramic Coatings

• Zinc Phosphate Coatings:
  • Have been widely used in metal pretreatment for over five decades.
  • Limitations:
    • High temperature requirements.
    • Excessive power and water consumption.
    • Formation and disposal of excessive sludge.
    • Slow processing time, especially for cold or room-temperature applications.
• Modern Metal Pretreatment Systems:
  • Utilize sol-gel methods with advanced formulations.
  • Key components:
    • Hexafluorozirconic acid and hexafluorotitanic acid.
    • Adhesion promoters: polyacrylic acid, polyvinyl alcohol, and tannic acid.
  • Fluoro zirconic acid forms zirconium oxide on iron surfaces, avoiding heavy sludge formation.
• Nano-Material Coatings:
  • Materials used include zirconia, titania, ceria, and zinc oxide.
  • Dispersants like polyacrylic acid and ammonium polymethacrylate enhance coating performance.
• Synthesis and Characterization:
  • Nano-zirconia coatings have been applied on metals like:
    • Mild steel, aluminum, galvanized iron, and their alloys.
  • Techniques for characterization:
    • AFM (Atomic Force Microscopy), SEM (Scanning Electron Microscopy), EDAX (Energy Dispersive X-ray Analysis).
    • DC polarization and EIS (Electrochemical Impedance Spectroscopy).
• Adhesion and Corrosion Resistance:
  • Studies reveal failure of adhesion and corrosion resistance in zinc phosphate-treated mild steel with epoxy primer.
  • Nano-zirconia coatings improve adhesion and corrosion resistance, but further research is needed to optimize inter-coat adhesion with organic top coats.
• Organic Promoters:
  • Succinic acid creates hydroxyl sites on metal surfaces, enhancing nano-zirconia coating adhesion.
• Benefits of Nano-Ceramic Coatings:
  • Minimized sludge formation.
  • Better corrosion resistance.
  • Compatibility with a variety of top coats and applications.

Synthesis of Hexafluoro Zirconic Acid and Its Application in Nano-Ceramic Coating

• Modern Metal Pretreatment Systems:
  • Utilize sol-gel methods with hexafluorozirconic acid (H₂ZrF₆) and hexafluorotitanic acid.
  • Incorporate adhesion promoters like polyacrylic acid, polyvinyl alcohol, and tannic acid.
  • H₂ZrF₆ forms zirconium oxide on metal surfaces, reducing sludge formation compared to conventional phosphate coatings.
• Nanocoatings with Zirconia:
  • Materials like zirconia, titania, ceria, and zinc oxide are applied using dispersants (e.g., polyacrylic acid, ammonium polymethacrylate).
  • Coatings improve corrosion resistance and paint adhesion properties.
• Processes for Zirconia Conversion Coatings:
  • Sol Formation: Mixing zirconium and cerium sols, followed by calcination.
  • Metal Acetate Precursors: Mixed with non-aqueous solvents and heated.
  • Zirconium Hydroxide: Treated with cerium sol and nitric acid for re-precipitation, followed by calcination and pulverization.
• Applications of Nano-Zirconia Coatings:
  • Can be applied to mild steel (MS), galvanized iron (GI), and aluminum alloys.
  • Achieved using pre-treatment baths containing 50% H₂ZrF₆ and hydrated copper nitrate.
  • Flocculating agents like sodium and potassium salts of acrylic acid enhance corrosion resistance.
• Characterization Techniques:
  • Microscopic analysis: AFM (Atomic Force Microscopy), SEM (Scanning Electron Microscopy), EDX (Energy Dispersive X-ray Spectroscopy).
  • Corrosion testing: DC polarization, EIS (Electrochemical Impedance Spectroscopy).
• Key Advantages:
  • Minimal sludge formation at ambient temperature and pH.
  • No use of heavy metals like nickel, manganese, or chromium.
  • Eliminates the need for additional anti-corrosive treatments.
• Commercial Applications and Research:
  • Hexafluorozirconic acid-based coatings:
    • FZ and MFZ: Developed as replacements for phosphating (e.g., Henkel’s TecTalis and Bonderite NT-1).
    • Tested on cold-rolled steel, hot-dip galvanized steel, and Galfan-coated steel.
  • Studies show effectiveness of H₂ZrF₆ with tannic acid for better adhesion.
• Epoxy Resin Compatibility:
  • Epoxy resins provide superior hardness and corrosion resistance.
  • Adhesion failures often result from incorrect pre-treatment selection for specific metals and resin types.
• Challenges in Heat Treatment:
  • High temperatures are required for thermal decomposition during coating processes, increasing production costs.

Conversion Coating on Copper

• Key Properties and Applications of Copper:
  • Copper exhibits excellent electrical and thermal conductivity, strength, malleability, ductility, and fungicidal/bactericidal activities.
  • Widely used in:
    • Architecture, automotive, electrical components, tubing, fuel gas systems, industrial applications, sea water environments, telecommunications, and machined products.
• Need for Protection:
  • Copper tends to oxidize when unprotected.
  • Protective coatings:
    • Tough, abrasion-resistant coatings are required to prevent daily handling damage and oxide layer formation.
    • Clear coatings preserve the natural beauty of decorative copper items.
• Challenges in Pretreatment:
  • Limited pretreatment options for improving adhesion of organic coatings on polished copper surfaces.
• Nanocoatings for Copper:
  • Nanocoatings of zirconia and titania have been investigated.
  • γ-aminopropyltriethoxy silane coatings on copper improve adhesion and durability at copper-epoxy interfaces.
• Organic Coatings on Copper:
  • Protective coatings with imidazole derivatives act as inhibitors to corrosion.
  • Thermal degradation of polymer films may lead to formation of copper (I) oxide, compromising protection.
• PNZ Coating (Hexafluorozirconic Acid-Based):
  • Developed as a nano-zirconia film for copper panels and components.
  • Provides corrosion resistance and improves dry film adhesion for organic top coats.
  • Forms covalent bonds with metal substrates (M–O–O–Zr or M–O–Zr–O).
• Advantages of PNZ Coating:
  • Chromate-free sol-gel process with significant anti-corrosion properties.
  • Minimal equipment required:
    • Low number of tanks.
    • Ambient temperature processing.
  • Eliminates sludge formation:
    • Does not produce insoluble materials in the processing bath.
    • Operates effectively under normal temperature and pH conditions.
• Eco-Friendly Alternative:
  • Unlike traditional chromate coatings, PNZ coatings avoid altering the natural color of copper.

Anti-Fouling Coatings

• Properties of Zirconium and Related Materials:
  • Zirconium (Zr), Titanium (Ti), and Ti-Zr alloys:
    • Exhibit excellent corrosion resistance and microbial growth prevention.
    • Used in medical applications due to biocompatibility and bone-implant compatibility.
  • Nano-structured composite materials:
    • Embed bioactive compounds within inorganic nanostructured matrices for antibacterial and anti-infective applications.
• Applications of Antibacterial Biomaterials:
  • Used in medical devices and implants to prevent bacterial colonization and treat infections.
  • Designed for specific clinical applications considering biocompatibility and tissue interactions.
  • Anti-fouling, bactericidal, and anti-biofilm coatings developed using:
    • Intrinsic antibacterial coatings.
    • Nano-materials and biofilm-interfering molecules.
• Factors Influencing Bacterial Adhesion:
  • Morphological factors:
    • Surface porosity and roughness at micro or nano levels.
  • Physico-chemical factors:
    • Surface energy, hydration degree, hydrophilicity or hydrophobicity, and functional groups.
  • Environmental conditions:
    • Electrolyte, pH, temperature, fluid flow rate, and host proteins.
  • Pathogen characteristics, such as gram-positive/negative species, strain type, and surface energy.
• Fouling in Marine Environments:
  • Prolonged seawater exposure leads to colonization by bacteria, algae, fungi, and barnacles.
  • Fouling increases hull weight, damages surfaces, reduces speed, and raises fuel consumption.
  • Occurs on underwater surfaces made of wood, concrete, or metal.
• Conventional Antifouling Coatings:
  • Previously used biocides (tributyl tin oxide, lead, arsenic, copper, tar, wax) are harmful to aquatic life.
  • Alternatives:
    • PTFE membranes, zwitter ionic coatings, pulsed electrical fields.
    • Triple-action coatings with:
      • Chlorhexidine (antibacterial antiseptic).
      • Zinc peroxide (reacts with seawater to create hydrogen peroxide).
      • Tween 85 (non-ionic surfactant disrupting colonization).
• Anti-Fouling Process Stages:
  1. Initial conditioning of the base metal.
  2. Accumulation of adsorbed organic molecules (proteins, polysaccharides, glycoproteins).
  3. Formation of biofilm matrix by planktonic bacteria:
     • Initial reversible adhesion via weak forces (e.g., Van der Waals, electrostatic).
     • Irreversible bacterial anchoring leads to macroscopic biofilm development.
  4. Growth of micro-fouling species (algae, fungi).
  5. Colonization of macro-fouling species (macro-algae, sponges, barnacles, mollusks).
• Protective Coating in Current Study:
  • Nano-copper composite coatings applied to:
    • Polyurethane paint: For PNZ-treated mild steel (MS) and galvanized iron (GI).
    • Polyurethane clear coat: For wood.
    • Acrylic emulsion paint: For cement panels.
  • Coated specimens were tested for anti-biofilm activity (bacteria) and antifouling performance (algae).