1. Basic Concepts of Plating and Coating

Plating and coating are surface-engineering technologies that form a new material layer on a substrate to provide properties that cannot be sufficiently obtained from the base material alone.

The main purposes include:

• Corrosion protection

• Improved wear resistance

• Increased hardness

• Control of the coefficient of friction

• Improved lubricity

• Electrical conductivity

• Electrical insulation

• Improved solderability and joining performance

• Improved decorative appearance

• Thermal reflection and thermal insulation

• Improved chemical resistance

• Anti-fouling and anti-adhesion performance

• Dimensional restoration

• Optical functionality

• Biocompatibility or food-contact compliance

The performance of plating and coating is not determined solely by the name of the coating material.

The following factors must be considered together:

• Type of substrate

• Condition of surface pretreatment

• Chemical composition of the coating

• Coating thickness

• Porosity

• Grain size

• Internal stress

• Interfacial adhesion

• Post-treatment

• Service temperature

• Corrosive environment

• Counter-material

• Load and friction conditions

2. Difference Between Plating and Coating

2.1 Plating

Plating is a process that deposits a metallic, alloy, or metal-based composite layer onto the surface of a substrate.

Representative processes:

• Electroplating

• Electroless plating

• Hot-dip plating

• Immersion plating

• Mechanical plating

• Diffusion coating

Representative deposits:

• Zinc

• Nickel

• Chromium

• Copper

• Tin

• Silver

• Gold

• Zinc-nickel

• Nickel-phosphorus

• Nickel-boron

2.2 Coating

Coating is a broader term that includes the formation of metallic, polymeric, ceramic, oxide, carbide, and composite films.

Representative processes:

• Liquid painting

• Powder coating

• Electrodeposition coating

• Thermal spraying

• PVD

• CVD

• DLC coating

• Ceramic coating

• Fluoropolymer coating

• Rubber lining

• Glass lining

• Sol-gel coating

2.3 Conversion Coating

Conversion coating forms a film by chemically or electrochemically reacting the substrate surface rather than simply depositing a separate external layer.

Representative processes:

• Anodizing

• Phosphate coating

• Chromate conversion coating

• Black oxide treatment

• Passivation

• Zirconium-based conversion coating

• Titanium-based conversion coating

3. Basic Structure of a Surface-Coating System

A general surface-treatment system may consist of:

1. Substrate

2. Pretreatment layer

3. Interfacial layer

4. Main functional layer

5. Sealing layer or sealer

6. Topcoat or lubricating layer

A typical zinc-plated steel structure may consist of:

• Steel substrate

• Zinc-plated layer

• Chromate or chromium-free passivation layer

• Sealer

• Optional paint layer

A typical paint system may consist of:

• Substrate

• Surface-treatment layer

• Primer

• Intermediate coat

• Topcoat

4. General Composition of Coating Materials

4.1 Metallic Plating Deposits

Metallic plating deposits consist of a principal metal, alloying elements, co-deposited substances, and trace impurities.

4.2 Organic Coatings

Paints and resin coatings generally consist of the following components.

The solids content is the proportion of the applied coating material that remains as a dry film after solvent or water evaporation.

4.3 Thermal-Spray and Thin-Film Coatings

Thermal-spray, PVD, and CVD coatings may consist of:

• Single metals

• Metallic alloys

• Oxide ceramics

• Carbides

• Nitrides

• Carbon-based films

• Metal-ceramic composites

• Multilayer structures

• Functionally graded layers

Representative compositions:

• WC-Co

• WC-CoCr

• Cr₃C₂-NiCr

• Al₂O₃

• Al₂O₃-TiO₂

• ZrO₂-Y₂O₃

• TiN

• CrN

• TiAlN

• AlCrN

• DLC

• SiC

• Si₃N₄

5. Corrosion-Protection Mechanisms

5.1 Barrier Protection

The coating prevents water, oxygen, salts, and chemicals from reaching the substrate.

Representative coatings:

• Nickel

• Chromium

• Paint

• Epoxy

• Fluoropolymer

• Ceramic coating

If a barrier coating contains pinholes or cracks, localized corrosion may become concentrated at the defects.

5.2 Sacrificial Protection

The coating corrodes preferentially and protects the substrate.

Representative coatings:

• Zinc

• Zinc-aluminum

• Certain aluminum thermal-spray coatings

Zinc on steel can electrochemically protect exposed steel within a limited area even when the coating is locally damaged.

5.3 Passivation

A stable oxide or compound layer is formed on the surface to suppress corrosion reactions.

Representative examples:

• Passivation of stainless steel

• Anodizing of aluminum

• Chromate conversion coating

• Titanium oxide films

5.4 Inhibitive Pigments

Corrosion-inhibiting pigments within a paint film suppress corrosion reactions.

Representative pigments:

• Zinc phosphate

• Zinc dust

• Calcium-based corrosion inhibitors

• Molybdate-based pigments

6. Surface Pretreatment

Coating failure is often caused more by inadequate pretreatment than by the coating material itself.

A general pretreatment sequence is:

1. Degreasing

2. Rinsing

3. Removal of rust and oxide scale

4. Surface activation

5. Conversion treatment or primer application

6. Rinsing

7. Drying

8. Plating or coating

6.1 Degreasing

Contaminants to be removed:

• Cutting oil

• Rust-preventive oil

• Grease

• Fingerprints

• Polishing compound

• Mold-release agents

Representative methods:

• Alkaline cleaning

• Solvent cleaning

• Electrolytic cleaning

• Ultrasonic cleaning

• Vapor degreasing

Incomplete degreasing may cause:

• Unplated areas

• Pinholes

• Blistering

• Poor adhesion

• Craters

• Localized corrosion

6.2 Pickling

Pickling removes rust, scale, and oxide films using an acidic solution.

Representative acids:

• Hydrochloric acid

• Sulfuric acid

• Phosphoric acid

• Nitric acid

• Hydrofluoric-acid-based mixtures

High-strength steels may absorb hydrogen during pickling and electroplating, creating a risk of hydrogen embrittlement. Process conditions and post-baking must therefore be controlled.

6.3 Blasting

Representative blast media:

• Steel shot

• Steel grit

• Alumina

• Glass beads

• Ceramic beads

• Plastic media

Purposes:

• Rust removal

• Scale removal

• Formation of surface roughness

• Increase in effective bonding area

• Removal of existing coatings

If the roughness is too low, adhesion may be insufficient. If it is too high, the coating may fail to cover surface peaks completely.

7. Electroplating

Electroplating uses an external direct-current power source to reduce metal ions from an electrolyte and deposit them on the workpiece, which acts as the cathode.

Basic elements:

• Workpiece: cathode

• Soluble metal anode or insoluble anode

• Metal-salt electrolyte

• Power supply

• Additives

• Temperature-control and agitation equipment

Requirements for electroplated zinc coatings include the substrate, service condition, coating thickness, post-treatment, and heat-treatment requirements, all of which should be clearly specified by the purchaser.

7.1 Thickness Distribution in Electroplating

Areas with high current density:

• Protrusions

• Edges

• Outer surfaces

• Areas close to the electrode

Areas with low current density:

• Deep grooves

• Internal diameters

• Inside holes

• Shielded regions

Electroplating therefore produces thickness variation on complex geometries.

Methods for improvement:

• Auxiliary anodes

• Shields

• Optimized rack design

• Adjustment of electrode distance

• Current-waveform control

• Optimization of workpiece orientation

8. Zinc Plating

8.1 Basic Composition

The main component of a zinc-plated deposit is Zn.

A typical electroplated zinc system consists of:

• Zn: principal component

• Fe and other impurities: trace amounts

• Residual organic brighteners: trace amounts

• Passivation and sealer: separately applied top layers

8.2 Representative Thickness

Typical application ranges:

• Indoor small parts: approximately 3–8 μm

• General industrial parts: approximately 8–15 μm

• Higher corrosion resistance: approximately 15–25 μm or more

The final required thickness is determined by the corrosion environment, post-treatment, and applicable standard.

8.3 Properties

Advantages:

• Sacrificial protection of steel

• Low cost

• Suitable for mass production

• Good appearance

• Compatible with various passivation treatments

• Widely used for bolts and fasteners

Limitations:

• Reduced corrosion resistance at elevated temperature

• Limited resistance to acids and alkalis

• White-rust formation

• Thickness variation

• Hydrogen embrittlement risk in high-strength steel

• Possible galvanic corrosion depending on the contact environment

8.4 Main Applications

• Bolts

• Nuts

• Brackets

• Sheet-metal components

• Electrical components

• Automotive components

• Machine-frame accessories

9. Zinc-Nickel Alloy Plating

9.1 Basic Composition

Representative composition:

• Zn: approximately 84–92 wt.%

• Ni: approximately 8–16 wt.%

Commercial high-corrosion-resistance zinc-nickel deposits commonly contain approximately 12–15 wt.% Ni.

9.2 Properties

• Higher corrosion resistance than conventional zinc plating

• High protection with a relatively thin deposit

• Better retention of corrosion resistance after elevated-temperature exposure

• Suitable for automotive and high-corrosion-resistance fasteners

• More difficult process control

• Higher cost than conventional zinc plating

• Performance depends strongly on alloy-composition consistency

9.3 Representative Thickness

• Approximately 5–15 μm

• Passivation and sealer are generally added for high-corrosion-resistance specifications

10. Nickel Plating

10.1 Electrolytic Nickel Plating

Representative plating baths:

• Watts bath

• Sulfamate bath

• Sulfate bath

• Chloride bath

A typical Watts-bath system contains:

• Nickel sulfate: primary source of nickel ions

• Nickel chloride: improves anode dissolution and conductivity

• Boric acid: buffering agent

• Brighteners and wetting agents: trace additions

10.2 Types of Nickel Deposits

Dull Nickel

• Low additive content

• Relatively ductile

• May have low internal stress

• Suitable as an underlayer and for functional applications

Semi-Bright Nickel

• Low sulfur content

• Improves corrosion resistance in multilayer decorative systems

• Used in copper-nickel-chromium systems

Bright Nickel

• High gloss

• Excellent leveling

• Sulfur may co-deposit from organic additives

• Widely used for decorative purposes

Sulfamate Nickel

• Low internal stress can be achieved

• Suitable for thick electroforming and dimensional restoration

• Used for precision components

10.3 Representative Thickness

• Decorative undercoat: approximately 5–25 μm

• Functional coating: approximately 10–100 μm

• Electroforming and dimensional restoration: several hundred micrometers or more

10.4 Properties

• Good corrosion resistance

• High hardness

• Decorative appearance

• Good solderability and joining performance

• Functions as an undercoat

• Magnetic

• Nickel-allergy considerations may be required

• Pinholes may cause localized corrosion on steel substrates

11. Electroless Nickel-Phosphorus Plating

Electroless nickel plating is an autocatalytic process that deposits a nickel alloy through a chemical-reduction reaction without an external electrical power source.

11.1 Basic Coating Composition

• Ni: principal component

• P: approximately 2–13 wt.%

• Impurities: trace amounts

11.2 Classification by Phosphorus Content

Low-Phosphorus Type

• P: approximately 2–5 wt.%

• High hardness

• More crystalline structure

• Some grades perform well in alkaline environments

• Corrosion resistance may be lower than that of high-phosphorus grades

Medium-Phosphorus Type

• P: approximately 6–9 wt.%

• Balanced hardness, wear resistance, and corrosion resistance

• Most common industrial grade

High-Phosphorus Type

• P: approximately 10–13 wt.%

• Amorphous structure may be formed

• High corrosion resistance

• Nonmagnetic behavior

• As-deposited hardness may be lower than that of medium-phosphorus deposits

11.3 Representative Bath Components

• Nickel salt

• Hypophosphite reducing agent

• Complexing agent

• Buffer

• Stabilizer

• Wetting agent

• pH adjuster

11.4 Properties

Advantages:

• Uniform thickness on complex shapes

• Relatively uniform deposition inside bores and grooves

• High corrosion resistance

• Good wear resistance

• Hardness can be increased by heat treatment

• Suitable for precision components

Limitations:

• Higher cost than electroplating

• Bath life and composition require careful control

• Risk of localized overheating and pitting

• Thick deposits may develop internal stress and delamination

• Aluminum requires special pretreatment

11.5 Representative Thickness

• General corrosion protection: approximately 5–15 μm

• Wear-resistant and precision components: approximately 15–50 μm

• Dimensional restoration and severe environments: approximately 50–100 μm or more

11.6 Effect of Heat Treatment

Heat treatment can increase hardness through precipitation of phases such as Ni₃P.

However, heat treatment may also affect:

• Corrosion resistance

• Dimensions

• Substrate hardness

• Distortion

• Interfacial diffusion

• Coating cracking

Heat-treatment conditions must therefore be specified according to the substrate and intended application.

12. Chromium Plating

12.1 Decorative Chromium

Typical structure:

• Substrate

• Copper plating

• Semi-bright nickel

• Bright nickel

• Thin chromium layer

The chromium layer provides appearance and tarnish resistance.

Representative thickness:

• Chromium layer: approximately 0.1–1 μm

• Underlying nickel: several micrometers to several tens of micrometers

12.2 Hard Chromium

Representative thickness:

• Approximately 10–100 μm

• Several hundred micrometers may be applied for dimensional restoration

Properties:

• High hardness

• High wear resistance

• Low coefficient of friction

• Good heat resistance

• Suitable for dimensional restoration

• Microcracked structure

• Possible tensile residual stress

• Thickness buildup at edges

• Hydrogen-embrittlement control required

12.3 Main Applications

• Hydraulic-cylinder rods

• Molds

• Rollers

• Shafts

• Piston rings

• Valve components

• Measuring tools

12.4 Environmental and Safety Considerations

Hexavalent chromium compounds present significant health and environmental hazards.

Alternative technologies may include:

• Trivalent chromium plating

• Chromium-free post-treatment

• HVOF coatings

• Other hard-coating systems

Regulatory compliance must be checked according to the latest requirements in the applicable country and product sector.

13. Copper Plating

13.1 Main Functions

• High electrical conductivity

• High thermal conductivity

• Undercoat for nickel and chromium plating

• Surface leveling

• Electronic circuit conductors

• Electroforming

• Masking against carburization

13.2 Types of Bath

• Acid copper sulfate bath

• Copper pyrophosphate bath

• Copper cyanide bath

• Electroless copper bath

Cyanide-based processes require strict safety and environmental control. Non-cyanide alternatives should be considered where practical.

13.3 Representative Thickness

• Undercoat: approximately 5–30 μm

• Electronic circuits: several micrometers to several tens of micrometers

• Electroforming: several hundred micrometers or more

14. Tin Plating

14.1 Properties

• Excellent solderability

• Relatively low toxicity

• Used for electrical contacts

• Long history of use in food-container materials

• Soft deposit

• Susceptible to wear

• Tin-whisker formation may occur

14.2 Representative Composition

Pure Tin

• Sn: generally 99% or higher

Tin Alloys

• Sn-Bi

• Sn-Cu

• Sn-Ag

• Sn-Ni

Alloying elements control:

• Hardness

• Solderability

• Whisker formation

• Melting point

• Electrical-contact behavior

14.3 Representative Thickness

• Electronic terminals: approximately 1–10 μm

• General corrosion protection and solderability: approximately 5–30 μm

15. Silver and Gold Plating

15.1 Silver Plating

Properties:

• Very high electrical conductivity

• High thermal conductivity

• Good solderability

• Low contact resistance

• Susceptible to sulfide tarnishing

• Possible ion migration

• Relatively soft deposit

Main applications:

• Electrical contacts

• High-current components

• RF components

• Bearings and sliding components

• Limited specialized uses in food equipment

15.2 Gold Plating

Properties:

• High corrosion resistance

• Low and stable contact resistance

• Excellent solderability and wire-bonding performance

• High cost

• Pore control required in thin deposits

Representative types:

Soft Gold

• High purity

• Used for wire bonding and electronic components

• Low hardness

Hard Gold

• Contains small additions of Co, Ni, Fe, or similar elements

• Higher hardness and wear resistance

• Used for connector contacts

Representative thickness:

• Gold flash: approximately 0.05–0.2 μm

• Electronic contacts: approximately 0.5–2 μm

• Special functional applications: several micrometers or more

16. Hot-Dip Galvanizing

Hot-dip galvanizing immerses fabricated steel products in molten zinc to form zinc-iron alloy layers and an outer zinc layer.

16.1 Basic Coating Structure

From substrate to surface:

• Steel substrate

• Fe-Zn alloy layer

• Intermediate alloy layers

• Outer zinc layer

16.2 Representative Bath Composition

• Zn: principal component

• Small additions of Al, Ni, Bi, or Sn may be used to control processing and appearance

• Non-zinc metals are generally limited depending on the applicable specification

16.3 Representative Thickness

Depending on the steel thickness and material:

• Generally approximately 45–100 μm or more

Actual thickness varies with steel composition, immersion time, surface condition, and the applicable standard.

16.4 Properties

Advantages:

• Thick corrosion-protection coating

• Sacrificial protection

• Can cover edges and internal surfaces

• Suitable for outdoor structures

• Long corrosion-protection life

Limitations:

• Dimensional change

• Rough surface

• Possible distortion

• Thread-clearance allowances required

• Vent and drain holes required

• Appearance and thickness affected by welds and silicon content

• Limited suitability for precision components

17. Hot-Dip Aluminum and Zinc-Aluminum Coatings

17.1 Hot-Dip Aluminum Coating

Properties:

• High-temperature oxidation resistance

• Thermal reflectivity

• Resistance to sulfur-containing environments

• Used in exhaust systems and heating components

• Formability and weldability must be reviewed

17.2 Zinc-Aluminum Coatings

Representative systems:

• Zn-Al

• Zn-Al-Mg

• Al-Zn

Functions of alloying elements:

• Al: improves barrier protection and heat resistance

• Mg: stabilizes corrosion products at cut edges and damaged areas

• Zn: provides sacrificial protection

18. Anodizing

Anodizing is an electrochemical process in which a metal, especially aluminum, is used as the anode to convert its natural oxide film into a thicker and controlled oxide layer.

18.1 Coating Composition

Anodized aluminum coating:

• Main component: Al₂O₃

• Hydrated oxides may form during sealing

• Dyes or inorganic coloring substances may be included

• Oxides of alloying elements may be present depending on the substrate alloy

18.2 Conventional Anodizing

Representative thickness:

• Approximately 5–25 μm

Main properties:

• Corrosion resistance

• Decorative appearance

• Colorability

• Electrical insulation

• Adhesive pretreatment

• Dimensional change

18.3 Hard Anodizing

Representative thickness:

• Approximately 25–100 μm

• The range of approximately 30–60 μm is widely used

Properties:

• High hardness

• High wear resistance

• High electrical insulation

• Friction depends on the counter-material and lubrication

• Possible cracking and brittleness

• Edge-burning risk

• Part of the oxide grows into the substrate and part grows outward

18.4 Sealing

Anodized films are porous and are therefore commonly sealed.

Representative methods:

• Hot-water sealing

• Nickel-salt sealing

• Low-temperature sealing

• Organic sealing

• PTFE impregnation

Sealing improves corrosion and stain resistance but may affect hardness and adhesion.

19. Conversion Coatings

19.1 Phosphate Coatings

Representative types:

• Iron phosphate

• Zinc phosphate

• Manganese phosphate

Coating composition:

• Metal-phosphate crystals

• Oxides

• Absorbed oil or sealer

Applications:

• Paint pretreatment

• Retention of rust-preventive oil

• Initial running-in of friction components

• Lubrication for cold forming

19.2 Zinc Phosphate

Properties:

• Improved paint adhesion

• Suppression of underfilm corrosion

• Crystalline coating

• Used as a pretreatment for automotive and industrial painting

Representative coating mass:

• Approximately 1–5 g/m² for paint pretreatment

• Heavier coatings may be used for other functions

19.3 Manganese Phosphate

Properties:

• Relatively thick crystalline coating

• Good oil retention

• Reduces initial wear of friction components

• Applied to gears, cams, pistons, and similar parts

19.4 Chromate and Chromium-Free Passivation

Chromate conversion coatings form thin conversion films on zinc, aluminum, and related metals.

Main functions:

• Suppression of white rust

• Improved paint adhesion

• Limited self-healing of damaged areas

• Coloring

For environmental and occupational-safety reasons, trivalent-chromium, zirconium-based, and titanium-based chromium-free treatments are increasingly used.

19.5 Black Oxide

Black oxide treatment forms a magnetite-based black oxide film on steel.

Main component:

• Fe₃O₄

Properties:

• Very thin film

• Minimal dimensional change

• Black appearance

• Usually used with oil or wax

• Limited corrosion resistance when used alone

• Suitable for tools and precision parts

20. Passivation of Stainless Steel

Passivation of stainless steel removes free iron and contaminants and promotes the formation of a stable chromium-oxide-rich surface layer.

Representative treatment chemistries:

• Nitric-acid-based

• Citric-acid-based

Effects:

• Removal of free iron

• Restoration of corrosion resistance

• Stabilization after welding and machining

• Improved cleanliness

Considerations:

• Passivation is different from pickling used to remove scale.

• Heavy rust and heat tint may require separate pickling.

• Chloride contamination and cleaning residues must be controlled.

21. Liquid Painting

Liquid painting applies a coating containing resin, pigment, solvent or water, and additives in liquid form, followed by drying or curing.

Representative application methods:

• Air spray

• Airless spray

• Roller coating

• Brush application

• Dip coating

• Flow coating

• Electrodeposition coating

22. Epoxy Coating

22.1 Basic Composition

Representative two-component epoxy coating:

22.2 Properties

Advantages:

• High adhesion

• Excellent corrosion protection

• High chemical resistance

• High film strength

• Thick-film capability

• Suitable for primers and tank linings

Limitations:

• UV-induced chalking

• Discoloration

• Limited low-temperature curing

• Mixing-ratio and pot-life control required

• Solvent entrapment may occur in thick films

22.3 Main Applications

• Machine frames

• Tanks

• Piping

• Floors

• Marine structures

• Chemical equipment

• Heavy-duty anticorrosive primers

23. Polyurethane Coating

23.1 Basic Composition

Representative two-component system:

• Polyol resin

• Polyisocyanate curing agent

• Pigment

• Solvent

• UV stabilizer

• Leveling agent

23.2 Properties

• High-quality appearance

• Excellent gloss retention

• Excellent weather resistance

• Good wear resistance

• Adjustable elasticity

• Sensitive to moisture and curing conditions

• Isocyanate handling requires control

Main applications:

• Machine exteriors

• Automotive parts

• Outdoor equipment

• Topcoats for epoxy heavy-duty coating systems

24. Acrylic, Alkyd, and Silicone Coatings

24.1 Acrylic Coating

• Excellent weather resistance and color retention

• Fast drying

• Transparent grades available

• Chemical and thermal resistance vary by grade

24.2 Alkyd Coating

• Economical

• Good application properties

• Cures through air oxidation

• Used for general industrial painting

• Limited in severe chemical and immersion environments

24.3 Silicone-Resin Coating

• High heat resistance

• Good thermal-cycle performance

• Used for exhaust systems and heating equipment

• Mechanical strength and solvent resistance depend on composition

25. Fluoropolymer Coating

Representative materials:

• PTFE

• PFA

• FEP

• ETFE

• PVDF

Representative coating composition:

• Fluoropolymer: principal component

• Primer resin and adhesion promoters

• Pigments

• Fillers

• Functional particles

25.1 Properties

Advantages:

• Low coefficient of friction

• Non-stick behavior

• High chemical resistance

• Good heat resistance

• Easy cleaning

Limitations:

• Difficult adhesion because of low surface energy

• Limited scratch and impact resistance

• Pinholes must be controlled

• Thick coatings affect dimensions

• Some grades require high-temperature baking

25.2 Main Applications

• Food-processing equipment

• Molds

• Hoppers

• Guides

• Chemical tanks

• Valves

• Heat-exchanger components

• Semiconductor wet-process components

26. Powder Coating

Powder coating electrostatically deposits a powdered coating material and then heats it to melt and cure the film.

Representative resins:

• Epoxy

• Polyester

• Epoxy-polyester hybrid

• Acrylic

• Nylon

• Fluoropolymer

26.1 General Composition

26.2 Representative Thickness

• Generally approximately 50–120 μm

• Greater thicknesses may be used for functional or heavy-duty anticorrosive systems

26.3 Properties

Advantages:

• High solids content

• Little or no solvent

• Thick and relatively uniform film

• Overspray can often be recovered

• High productivity

• Good appearance

Limitations:

• Heat curing required

• Large components require large ovens

• Thickness variation at edges and deep recesses

• Losses during color changes

• Limited very-thin-film capability

• Dust-explosion prevention required

27. Electrodeposition Coating

Electrodeposition coating uses an electric field to deposit waterborne paint particles onto a conductive workpiece.

Representative types:

• Cathodic electrodeposition

• Anodic electrodeposition

Properties:

• Relatively uniform film on complex geometries and internal surfaces

• High automation potential

• Excellent corrosion protection

• Suitable for automotive bodies and high-volume components

• Requires an electrically conductive substrate

• Pretreatment and rinsing must be carefully controlled

Representative thickness:

• Approximately 15–35 μm

28. Heavy-Duty Anticorrosive Paint Systems

Heavy-duty corrosion protection for steel structures is generally designed as a multilayer system rather than as a single coating.

Representative systems:

Zinc-Rich Primer + Epoxy Intermediate Coat + Polyurethane Topcoat

• Sacrificial protection

• Barrier protection

• Weather resistance

• Suitable for outdoor steel structures

Epoxy Primer + High-Build Epoxy

• Tanks

• Immersion environments

• Chemical equipment

Hot-Dip Galvanizing + Paint

• Duplex system

• Protects the zinc coating

• Long-term corrosion resistance

• Provides color and appearance

29. Thermal-Spray Coating

Thermal spraying heats and accelerates powder or wire material and impacts molten or semi-molten particles onto the substrate to form a coating.

29.1 Thermal-Spray Processes

• Flame spraying

• Arc spraying

• Plasma spraying

• HVOF

• HVAF

• Cold spraying

• Detonation spraying

29.2 Structure of Thermal-Spray Coatings

Thermal-spray coatings generally contain:

• Flattened splats

• Interparticle oxides

• Pores

• Unmelted particles

• Interfaces

• Sealers

29.3 Representative Thermal-Spray Materials

Metals

• Zn

• Al

• Ni

• NiCr

• Mo

• Cu

Ceramics

• Al₂O₃

• Cr₂O₃

• ZrO₂

• TiO₂

Cermets

• WC-Co

• WC-CoCr

• Cr₃C₂-NiCr

29.4 Representative Compositions

WC-Co

• WC: approximately 80–94 wt.%

• Co: approximately 6–20 wt.%

WC-CoCr

• WC: approximately 80–90 wt.%

• Co: approximately 5–12 wt.%

• Cr: approximately 3–10 wt.%

Cr₃C₂-NiCr

• Cr₃C₂: approximately 70–80 wt.%

• NiCr: approximately 20–30 wt.%

Alumina-Titania

• Al₂O₃: approximately 60–97 wt.%

• TiO₂: approximately 3–40 wt.%

29.5 Representative Thickness

• Metallic corrosion-protection spray: approximately 100–500 μm

• Wear-resistant HVOF coating: approximately 50–500 μm

• Plasma-sprayed ceramic: approximately 100 μm to several millimeters

30. PVD Coating

Physical vapor deposition transfers material from a solid source in vacuum by evaporation, sputtering, or arc processes and condenses it as a thin film on the substrate.

PVD processes are often performed in high vacuum at approximately 150–500°C, although the actual temperature depends on the process and substrate.

30.1 Representative Processes

• Vacuum evaporation

• Sputtering

• Arc ion plating

• Ion-beam deposition

• Reactive PVD

30.2 Representative Coatings

• TiN

• CrN

• TiCN

• TiAlN

• AlTiN

• AlCrN

• ZrN

• MoN

• Metallic thin films

30.3 Representative Coating Composition

TiN

• Compound of Ti and N

• Properties vary with stoichiometry and impurities

TiAlN and AlTiN

• Ti, Al, and N

• Aluminum content affects oxidation resistance and crystal structure

CrN

• Cr and N

• Balanced corrosion and wear resistance

30.4 Representative Thickness

• Cutting tools: approximately 1–5 μm

• Molds and machine parts: approximately 2–10 μm

• Decorative thin films: approximately 0.1–1 μm

30.5 Properties

Advantages:

• High hardness

• Thin coating

• Low dimensional change

• Excellent wear resistance

• Various colors available

• Low-friction behavior can be achieved

• Improved high-temperature oxidation resistance

Limitations:

• Vacuum equipment required

• Limited coverage in deep recesses

• Strong influence of substrate surface roughness

• Too thin for large dimensional restoration

• Coating may fail if substrate hardness and support are insufficient

31. CVD Coating

Chemical vapor deposition forms a solid coating through chemical reactions of gaseous precursors on a heated substrate.

Representative types:

• Thermal CVD

• Plasma-enhanced CVD

• Low-pressure CVD

• Metal-organic CVD

• Atomic-layer deposition

Representative coatings:

• TiC

• TiN

• Al₂O₃

• SiC

• Si₃N₄

• Diamond

• DLC-related films

31.1 Properties

Advantages:

• Relatively good coverage of complex geometries

• Dense coating

• High adhesion

• High-purity films can be formed

• Multilayer structures are possible

Limitations:

• High process temperature

• Possible substrate distortion and heat-treatment effects

• Reactive-gas handling required

• Long process time

• Post-treatment may be required

32. DLC Coating

DLC is an amorphous carbon-based coating containing both diamond-like and graphite-like bonding characteristics.

Representative constituents:

• Carbon

• Hydrogen

• Dopants such as Si, W, or Cr

• sp² bonds

• sp³ bonds

Representative types:

• a-C

• a-C:H

• ta-C

• Si-DLC

• W-DLC

32.1 Properties

• High hardness

• Low coefficient of friction

• High wear resistance

• Chemical stability

• Reduced adhesive wear

• Internal stress may be high

• Coating thickness is limited

• Friction behavior varies with temperature and humidity

• Counter-material and lubricant have strong effects

Representative thickness:

• Approximately 0.5–5 μm

• Special multilayer systems may be thicker

32.2 Main Applications

• Automotive engine components

• Fuel-injection components

• Molds

• Precision shafts

• Valves

• Medical components

• Sliding components

33. Ceramic Coating

Representative materials:

• Al₂O₃

• ZrO₂

• Cr₂O₃

• TiO₂

• SiO₂

• SiC

• TiN

• CrN

Main functions:

• Wear resistance

• Thermal insulation

• Electrical insulation

• Corrosion resistance

• Non-stick behavior

• High-temperature oxidation protection

33.1 Thermal-Barrier Coating

Representative structure:

• Metallic substrate

• Bond coat

• Thermally grown oxide

• Zirconia-based topcoat

Representative topcoat:

• Y₂O₃-stabilized ZrO₂

Properties:

• Low thermal conductivity

• Reduces substrate temperature in hot-section components

• Risk of thermal-cycle delamination

• Growth of oxide layer

• Thermal-expansion mismatch must be controlled

34. Rubber and Lining Coatings

Representative materials:

• Natural rubber

• Butyl rubber

• EPDM

• Neoprene

• Hard rubber

• Polyurethane

• PVC

• PE

• PP

Main applications:

• Acid and alkali tanks

• Piping

• Slurry equipment

• Vibration and impact protection

• Wear-resistant lining

Considerations:

• Adhesive-layer degradation

• Blistering

• Pinholes

• Thermal expansion

• Permeation

• Compatibility with solvents and chemicals

35. Mechanical Properties of Plating and Coating

35.1 Hardness

Coating hardness is influenced by:

• Chemical composition

• Grain size

• Heat treatment

• Porosity

• Internal stress

• Co-deposited elements

• Test load

• Coating thickness

• Substrate hardness

When measuring the hardness of a thin coating, the influence of the substrate should be minimized.

35.2 Adhesion

Adhesion mechanisms include:

• Mechanical interlocking

• Chemical bonding

• Diffusion bonding

• Metallic bonding

• Electrostatic interaction

Causes of poor adhesion:

• Oil

• Oxide films

• Moisture

• Insufficient surface roughness

• Excessive internal stress

• Thermal-expansion mismatch

• Contamination

• Improper curing

35.3 Residual Stress

Coatings may contain tensile or compressive residual stress.

Excessive tensile stress may cause:

• Cracking

• Delamination

• Reduced fatigue life

Excessive compressive stress may cause:

• Blistering

• Buckling delamination

• Distortion

35.4 Friction and Wear

Friction performance is not determined by coating hardness alone.

Items to review:

• Counter-material

• Lubricant

• Contact pressure

• Speed

• Temperature

• Surface roughness

• Wear particles

• Coating thickness

• Substrate hardness

36. Representative Plating and Coating Defects

36.1 Pinholes

Causes:

• Gas generation

• Surface contamination

• Entrapped resin

• Incomplete powder melting

• Porosity

Effects:

• Localized corrosion

• Leakage

• Dielectric breakdown

• Contaminant penetration

36.2 Delamination

Causes:

• Poor pretreatment

• Interfacial contamination

• Excessive internal stress

• Thermal-expansion mismatch

• Impact

• Moisture penetration

36.3 Blistering

Causes:

• Interfacial corrosion

• Osmosis of moisture or solvent

• Hydrogen or gas

• Incomplete curing

• Residual surface salts

36.4 Cracking

Causes:

• Excessive coating thickness

• Brittleness

• Rapid temperature change

• Bending deformation

• Residual stress

• Substrate deformation

36.5 Edge Buildup and Burning

In electroplating and powder coating, electric-field concentration may produce excessive thickness at edges.

Possible problems:

• Assembly interference

• Edge chipping

• Poor appearance

• Dimensional variation

36.6 Orange Peel and Sagging

Orange peel is an uneven surface resembling citrus peel, while sagging occurs when the coating flows downward.

Causes:

• Viscosity

• Spray distance

• Solvent evaporation rate

• Film thickness

• Curing temperature

• Poor leveling

37. Hydrogen Embrittlement

High-strength steel may absorb atomic hydrogen during pickling, electrolytic cleaning, and electroplating.

Possible results:

• Delayed fracture

• Reduced ductility

• Cracking

• Failure of fasteners

High-risk components:

• High-strength bolts

• Springs

• Gears

• Aerospace components

• High-hardness tool steels

Countermeasures:

• Minimize acid pickling.

• Use mechanical pretreatment where possible.

• Apply appropriate post-baking.

• Use low-hydrogen processes.

• Control current efficiency.

• Begin baking promptly after plating.

• Validate the process.

Baking conditions must be specified according to material strength, part geometry, coating type, and applicable standard.

38. Dimensional and Tolerance Design

Plating and coating change component dimensions.

Change in shaft outside diameter:

• If the coating thickness on one side is (t), the outside diameter increases by approximately (2t).

Change in hole inside diameter:

• If the coating thickness on one side is (t), the inside diameter decreases by approximately (2t).

For threaded parts, review:

• Change in pitch diameter

• Plating allowance for male and female threads

• Timing of gauge inspection

• Masking

• Post-plating machining

• Tightening torque and coefficient of friction

39. Geometric Design for Plating and Coating

39.1 Drain and Vent Holes

Closed sections used in hot-dip galvanizing, dip coating, or chemical treatment require adequate vent and drain holes.

Reasons:

• Explosion prevention

• Prevention of trapped solution

• Improved rinsing

• Uniform coating

• Improved drying

39.2 Sharp Edges

Liquid paint may become thin on sharp edges because of surface tension.

By contrast, electroplating and powder coating may become excessively thick at edges because of electric-field concentration.

Countermeasures:

• Appropriate chamfers

• Fillets

• Rounded edges

• Stripe coating

• Shielding design

39.3 Overlaps and Crevices

Overlapping joints and narrow gaps may cause:

• Retained solution

• Capillary corrosion

• Inadequate rinsing

• Gas generation

• Incomplete coating penetration

• Localized corrosion

Where possible, the joint should either be seal-welded or designed as a sufficiently open structure.

39.4 Welded Areas

Items to inspect at welds:

• Slag

• Spatter

• Undercut

• Porosity

• Heat tint

• Silicon contamination

• Oxide scale

Welded areas should be smoothed and cleaned before plating or painting.

40. Selection of Coating Thickness

A thicker coating is not always better.

Advantages of increased thickness:

• Longer corrosion-protection life

• Longer wear life

• Reduced porosity

• Dimensional restoration

Disadvantages of increased thickness:

• Increased internal stress

• Cracking and delamination

• Dimensional interference

• Higher cost

• Increased brittleness

• Incomplete curing

• Thermal-expansion stress

Thickness should be determined based on:

• Service environment

• Target life

• Expected wear

• Allowable tolerance

• Coating material

• Post-treatment

• Substrate strength

• Manufacturing process

41. Inspection of Plating and Coating

41.1 Thickness Measurement

Representative methods:

• Magnetic method

• Eddy-current method

• X-ray fluorescence

• Microscopic cross-section measurement

• Gravimetric method

• Coulometric method

• Ultrasonic method

The method should be selected according to the combination of coating and substrate.

41.2 Adhesion Testing

Representative tests:

• Cross-cut test

• Pull-off test

• Bend test

• Impact test

• Thermal-cycle test

• Tape test

• Scratch test

41.3 Corrosion Testing

Representative tests:

• Neutral salt-spray test

• Cyclic corrosion test

• Humidity test

• Immersion test

• Condensation-water test

• Outdoor exposure

• Electrochemical testing

Salt-spray hours should not be directly converted into actual service life.

These tests are mainly tools for process comparison and quality control. Correlation with the real service environment must be evaluated separately.

41.4 Porosity and Defect Inspection

• Ferroxy test

• Dye-penetrant inspection

• Electrical porosity test

• High-voltage holiday test

• Microscopic inspection

• Cross-sectional analysis

• SEM and EDS

• X-ray inspection

41.5 Surface Roughness and Appearance

Inspection items:

• Gloss

• Color difference

• Roughness

• Waviness

• Staining

• Burning

• Sagging

• Orange peel

• Particles

• Craters

42. Plating and Coating in Food Machinery

Food machinery requires evaluation of both corrosion resistance and the risk of coating delamination or contamination.

Main candidates:

• Electroless nickel

• Hard chromium

• Tin

• Stainless-steel passivation

• PTFE and PFA coatings

• Ceramic coatings

• Food-contact epoxy

• Electropolishing

• DLC

42.1 Main Requirements

• Food-contact compliance

• Resistance to cleaning chemicals

• Resistance to hot water and steam

• Low extractables

• Non-absorbent surface

• Low surface roughness

• No pinholes or cracks

• Suppression of flaking and wear particles

• Easy cleaning and inspection

42.2 Considerations

• Substrate corrosion after coating damage

• Nickel allergy and metal release

• Cracking of chromium deposits

• Scratching of fluoropolymer coatings

• Contamination by coating flakes

• Detectability by metal detectors

• Thermal shock during cleaning

• Insufficient coating thickness at machined edges

Food-contact compliance must be evaluated for the complete coating system, including pretreatment, pigments, curing agents, sealers, and the finished product, not only for the principal coating material.

43. Coatings for Semiconductor and Precision Equipment

Representative applications:

• Electroless nickel

• Hard anodizing

• PVD

• DLC

• Ceramic coatings

• Fluoropolymers

• Black surface treatments

• Al₂O₃ insulating coatings

Main requirements:

• Low outgassing

• Low particle generation

• Plasma resistance

• Chemical resistance

• Electrical insulation or grounding

• Low friction

• High dimensional accuracy

• Uniform thickness

• Compatibility with clean processing

44. Comparison of Representative Surface Treatments

The thicknesses shown are general design ranges. Actual values vary with process, standard, geometry, and supplier grade.

45. Selection by Component

45.1 General Steel Fasteners

Recommended:

• Electroplated zinc

• Zn-Ni

• Zinc-flake coating

• Phosphate coating with oil

Selection criteria:

• General indoor use: electroplated zinc

• High corrosion resistance: Zn-Ni

• High-strength bolts sensitive to hydrogen embrittlement: consider non-electrolytic zinc-flake systems

• Controlled assembly friction: specify sealer and lubricant topcoat

45.2 Hydraulic-Cylinder Rods

Recommended:

• Hard chromium

• HVOF WC-CoCr

• Electroless Ni-P

• DLC

Selection criteria:

• General industrial use: hard chromium

• Severe wear and corrosion: HVOF

• Complex geometry and uniform thickness: electroless nickel

• Low-friction precision use: DLC

45.3 Aluminum Housings

Recommended:

• Conventional anodizing

• Hard anodizing

• Conversion coating with paint

• Electroless nickel

Selection criteria:

• Appearance and general corrosion protection: conventional anodizing

• Wear resistance and insulation: hard anodizing

• Conductive grounding areas: selective masking or conductive conversion coating

• Precision wear resistance: electroless nickel

45.4 Cutting Tools and Molds

Recommended:

• TiN

• TiAlN

• AlCrN

• CrN

• DLC

• Hard chromium

45.5 Chemical Tanks and Piping

Recommended:

• Epoxy lining

• Vinyl-ester lining

• Fluoropolymer

• Rubber lining

• Glass lining

• Ceramic lining

Selection must be based on the exact chemical, concentration, temperature, and immersion time.

45.6 Outdoor Steel Structures

Recommended:

• Hot-dip galvanizing

• Zinc or aluminum thermal spray

• Heavy-duty paint system

• Hot-dip galvanizing with paint

46. Designation on Drawings and Specifications

It is not sufficient to specify only “zinc plating,” “nickel plating,” or “paint.”

The following should also be specified.

46.1 Substrate

Examples:

• SPCC

• S45C

• SCM435

• SUS304

• A6061-T6

• ADC12

46.2 Coating Type and Composition

Examples:

• Zn electroplating

• Zn-Ni, Ni 12–15 wt.%

• Electroless Ni-P, P 10–13 wt.%

• Hard anodizing

• TiAlN PVD

• Epoxy/polyurethane system

46.3 Coating Thickness

Examples:

• Zn, 8 μm minimum

• Electroless Ni-P, 25 ±5 μm

• Hard anodizing, 40 ±10 μm

• Dry-film thickness, 250 μm minimum

It should be clearly stated whether the requirement is minimum, average, or local thickness.

46.4 Post-Treatment

• Trivalent passivation

• Sealer

• Hot-water sealing

• PTFE impregnation

• Baking

• Polishing

• Lubricant topcoat

46.5 Color and Appearance

• Black

• Clear

• Yellow

• Natural

• Gloss

• Semi-gloss

• Matte

• Standard color code

46.6 Functional Requirements

• Salt-spray requirement

• Adhesion

• Hardness

• Coefficient of friction

• Electrical resistance

• Dielectric strength

• Surface roughness

• Chemical resistance

• Heat resistance

• Food-contact compliance

46.7 Masking Areas

Typical masking areas:

• Threads

• Fitting surfaces

• Grounding surfaces

• Welding surfaces

• Bearing seats

• Sealing surfaces

• Electrical contacts

• Precision-tolerance areas

The masking boundary and allowable coating overlap should be clearly indicated on the drawing.

47. Plating and Coating Selection Procedure

47.1 Confirm the Primary Purpose

• Corrosion protection?

• Wear resistance?

• Low friction?

• Electrical insulation?

• Electrical conductivity?

• Decoration?

• Thermal insulation?

• Chemical resistance?

• Dimensional restoration?

47.2 Confirm the Substrate

• Carbon steel

• High-strength steel

• Stainless steel

• Aluminum

• Copper

• Titanium

• Cast iron

• Plastic

• Ceramic

47.3 Confirm the Environment

• Indoor

• Outdoor

• Marine

• High temperature

• Immersion

• Acid

• Alkali

• Solvent

• Food

• Vacuum

• Plasma

47.4 Confirm Mechanical Conditions

• Contact pressure

• Reciprocating motion

• Rotary motion

• Impact

• Bending

• Fatigue

• Abrasive particles

• Lubrication condition

• Counter-material

47.5 Confirm Dimensions and Geometry

• Tolerances

• Threads

• Internal diameters

• Deep grooves

• Edges

• Closed cavities

• Drain holes

• Masking

• Post-coating grinding

47.6 Confirm Manufacturing and Quality Conditions

• Production volume

• Racking

• Maximum process size

• Process temperature

• Distortion

• Inspection method

• Reworkability

• Wastewater and environmental management

• Supplier process capability

48. Core Principles of Plating and Coating Selection

Plating and coating can compensate for deficiencies in the substrate but cannot completely eliminate the structural limitations of the base material.

Even if a very hard thin film is applied to a soft substrate, the coating may crack or delaminate if the substrate deforms under high contact load.

Similarly, a corrosion-resistant coating cannot achieve its expected service life if surface pretreatment, edge coverage, pinholes, or damaged areas are inadequately controlled.

The following must therefore be designed as one integrated system:

• Substrate

• Pretreatment

• Interface

• Coating material

• Coating thickness

• Post-treatment

• Tolerance

• Friction conditions

• Corrosion environment

• Inspection criteria

49. Final Summary

Plating primarily refers to the deposition of metallic or alloy layers, while coating is a broader concept that includes metallic, polymeric, ceramic, and composite films.

Electroplated zinc is widely used for general steel components because of its low cost and sacrificial corrosion protection.

Zinc-nickel plating provides higher corrosion resistance and better thermal stability than conventional zinc plating and is suitable for automotive and high-corrosion-resistance fasteners.

Nickel plating provides corrosion resistance, decorative appearance, hardness, and undercoat functionality.

Electroless nickel-phosphorus plating provides uniform thickness on complex geometries, together with good corrosion and wear resistance, making it suitable for precision mechanical components.

Hard chromium provides high hardness, wear resistance, and dimensional-restoration capability, but coating cracks, hydrogen embrittlement, and environmental impact must be considered.

Hot-dip galvanizing forms a thick zinc-iron alloy coating and provides long-term corrosion protection for outdoor steel structures.

Anodizing converts the aluminum surface into an aluminum-oxide film and improves corrosion resistance, wear resistance, electrical insulation, and decorative appearance.

Conversion coatings such as phosphate, passivation, and black oxide are used for paint adhesion, corrosion resistance, oil retention, and appearance control.

Epoxy coatings provide high adhesion, corrosion protection, and chemical resistance, while polyurethane coatings provide better weather resistance and appearance retention.

Powder coating offers thick and relatively uniform films, high productivity, and low solvent usage.

Thermal spraying can apply relatively thick metallic, ceramic, and cermet coatings for corrosion protection, wear resistance, thermal protection, and dimensional restoration.

PVD and CVD form high-hardness functional thin films, generally several micrometers thick, for tools, molds, precision components, and semiconductor equipment.

DLC provides both high hardness and a low coefficient of friction, but internal stress, process temperature, counter-material, and lubrication must be reviewed.

Fluoropolymer coatings provide low friction, non-stick behavior, and high chemical resistance and are widely used in food and chemical equipment.

However, plating and coating must not be selected solely by the generic name of the treatment.

The following should be specified together:

• Substrate material and heat-treatment condition

• Surface pretreatment

• Coating type and chemical composition

• Minimum and average coating thickness

• Post-treatment and sealing

• Color and appearance

• Masking areas

• Dimensional tolerances

• Hydrogen-embrittlement controls

• Adhesion

• Hardness and coefficient of friction

• Corrosion and chemical-resistance requirements

• Inspection methods

• Applicable standards

• Supplier process grade

Selecting plating or coating is not merely the act of applying a protective layer to a surface.

It is a surface-engineering design process that treats the substrate, interface, coating, geometry, load, environment, and manufacturing process as one integrated system in order to achieve the required function and service life.