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## I. Hot-dip Coatings
1. Process Principle: Immerse steel workpieces in molten metals (zinc, aluminum, tin, lead, etc.) whose melting points are far lower than that of steel. Dissolution, diffusion and chemical reactions occur between the substrate and coating metal to form a metallurgically bonded alloy layer. After being lifted out, the molten metal on the surface cools and solidifies into a thick coating.
2. Core Advantages: Extremely strong bonding force between the coating and substrate, large coating thickness, and far superior corrosion resistance compared with electroplating and electroless plating.
## II. Electroplated Coatings
1. Process Principle: The workpiece serves as the cathode, while coating metals or insoluble materials act as the anode. They are immersed in electrolyte containing coating metal ions. Direct current is applied to deposit metals onto the workpiece surface without altering the fundamental properties of the substrate.
2. Two Major Classifications
- By application: Protective coatings, protective-decorative coatings, functional coatings (wear-resistant, conductive, heat-resistant, repair, etc.).
- By electrochemical relationship: Anodic coatings (provide dual mechanical and electrochemical protection and corrode prior to the substrate); Cathodic coatings (only offer physical isolation, and coating damage will accelerate substrate corrosion).
3. Applications: Widely adopted in numerous industries including machinery, electronics, instruments, military industry, transportation and light industry.
## III. Electroless Coatings
1. Process Principle: No external direct current is required. Reducing agents spontaneously reduce metal ions on catalytic metal surfaces via controllable autocatalytic chemical deposition, enabling unlimited coating thickness growth.
2. Advantages: Applicable to metals, semiconductors and non-metals; uniform coating thickness on complex workpieces; dense coatings with few pores featuring excellent corrosion and wear resistance, capable of producing amorphous alloys; suitable for electroforming.
3. Drawbacks: Poor stability of plating solutions, high operating temperatures and relatively brittle coatings. Common coating metals include nickel, copper, silver, gold, platinum and their alloys.
## IV. Thermal Spray Coatings
1. Process Principle: Special heat sources melt or semi-melt metallic and non-metallic materials. High-speed airflow atomizes particles and sprays them onto workpiece surfaces to form dense overlay coatings that enhance corrosion, wear and heat resistance.
2. Classifications: Flame spraying, arc spraying, plasma spraying, detonation spraying; spray materials cover metals, alloys, ceramics, plastics and glass.
3. Advantages: Portable equipment suitable for large outdoor components such as bridges and mechanical frames; no restrictions on workpiece shapes, allowing partial or full spraying, and capable of preparing protective layers unachievable via other processes.
## V. Diffusion Coatings
1. Core Principle: Bidirectional diffusion of coating metals into steel substrates at high temperatures forms metallurgically bonded layers with ultra-high bonding strength and strong peel resistance, which is distinctly different from electroplating coatings that only form atomic attachments and peel easily.
2. Subdivided Processes
(1) Cementation: Diffuse elements into substrates at high temperatures to modify surface microstructure and composition, improving corrosion resistance and high-temperature oxidation resistance.
(2) Chemical Vapor Deposition (CVD): Gas-phase metal compounds are synthesized at low temperatures and decomposed at high temperatures to deposit metal films, featuring fast deposition, high purity and compatibility with complex parts.
(3) Physical Vapor Deposition (PVD): Evaporate or sputter target materials under vacuum to deposit functional thin films, including vacuum evaporation plating, sputter coating and ion plating.
(4) Ion Implantation: Inject elemental ions into workpiece surfaces under high voltage to boost surface hardness, wear resistance and corrosion resistance.
## VI. Chemical Conversion Films
Stable compound thin films are generated via reactions between metal surfaces and anions in media. Applicable to nearly all metals with both protective and decorative effects, divided into three categories:
1. Chemical Oxide Films (Bluing): Treat steel in high-temperature concentrated alkali to form Fe₃O₄ thin films with good adsorptivity; corrosion resistance improves after oil sealing. The thin film (0.6~1.5μm) delivers limited standalone protective performance.
2. Chromate Films: Immerse metals in chromic acid or chromate solutions to form composite films of trivalent and hexavalent chromium with outstanding corrosion resistance, simple processes and low costs. Primarily used for post-treatment of galvanized layers and protection of aluminum, magnesium and copper alloys.
3. Phosphate Films (Phosphating): Immerse metals in acidic phosphate solutions to generate insoluble phosphate films. Compatible with steel and non-ferrous alloys such as zinc and aluminum, it is the mainstream surface pretreatment process for steel that enhances paint adhesion and decorative performance.
## VII. Non-metallic Coatings
Isolate corrosive media with non-metallic materials, classified into three types:
1. Paint Coatings: Easy construction, low costs and wide application range; disadvantages include weak mechanical properties and susceptibility to damage under impact, high temperature and severe corrosion.
2. Plastic Coatings
- Plastic Film Coatings: Composite plastic films laminated with steel plates, the mainstream organic coated steel sheets.
- Plastic Powder Coatings: Electrostatic coating of synthetic resin powders with excellent protective performance.
3. Temporary Protective Coatings: Temporary rust prevention materials including anti-rust oil, anti-rust paper and vapor phase corrosion inhibitors. Used for raw material storage, inter-process rust prevention and storage & transportation protection, providing only short-term anti-corrosion effects.
Overall Core Comparison Summary
1. Bond Strength Ranking: Diffusion coatings, hot-dip coatings (metallurgical bonding, strongest adhesion) > thermal spray coatings, electroplating, electroless plating (physical/deposition bonding) > conversion films, paint coatings, plastic coatings (surface thin films, weakest adhesion).
2. Distinction of Protection Mechanisms
- Metallic Coatings: Physical isolation plus partial cathodic protection capacity.
- Conversion Films: Improve substrate corrosion resistance and enhance coating adhesion, mostly serving as intermediate pretreatment layers.
- Non-metallic Coatings: Pure physical isolation without electrochemical protection capacity.
- Temporary Coatings: Only provide short-term anti-corrosion for processing and transportation, and must be removed before service.