Classified by Electrochemical Properties
Electroplating can be divided into anodic and cathodic coatings.
- Anodic Coatings
Anodic coatings are those where the potential of the coating metal is lower than that of the base metal. Examples include zinc plating on steel, cadmium plating on iron products in marine environments, and tin plating on iron products in organic acid environments.
The characteristic feature of these coatings is that during the formation of corrosion micro-cells, i.e., when electrochemical corrosion occurs, the anodic coating metal is consumed while the base metal is protected.
Thus, the coating metal (such as zinc or cadmium, relative to steel products) corrodes first, providing electrochemical protection to the steel parts.
The thickness of the anodic coating has a decisive impact on its protective ability.
- Cathodic Coatings
Cathodic coatings are those where the potential of the coating metal is higher than that of the base metal. Examples include copper, nickel, chromium, gold, and silver coatings on steel parts. Cathodic coatings protect the base metal only when they are intact.
Otherwise, once corrosion micro-cells form, the base metal, serving as the anode, corrodes first from the inside out, accelerating the corrosion process instead of protecting the base metal.
Therefore, it is particularly important for cathodic coatings to have a certain thickness and as low a porosity rate as possible.
The electrode potential of metals varies with the medium and working conditions, determining whether a coating is anodic or cathodic based on the medium and environment.
For example, zinc is typically an anodic coating for iron under normal conditions but becomes cathodic in hot water between 70-80°C. Similarly, tin is usually cathodic to iron but turns anodic in organic acid media.
Classified by Purpose
Coatings can be divided into protective coatings, protective-decorative coatings, restorative coatings, and functional coatings.
- Protective Coatings
Protective coatings prevent the base metal from corroding in the atmosphere or other environments. Zinc, cadmium, tin, and zinc alloy coatings (zinc-iron, zinc-cobalt, zinc-nickel) fall into this category. Galvanizing is widely used for protection in general atmospheric conditions, accounting for more than 50% of electroplating production.
Cadmium plating is used in humid and marine atmospheres. Cadmium or cadmium-tin alloys are chosen for fasteners’ protection, such as in chemical production where high-pressure gasket seals are corrosion-resistant.
Tin coatings are highly resistant to organic acids, offering strong anti-rust capabilities and producing harmless corrosion compounds, making them extensively used in the food processing industry.
- Protective-Decorative Coatings
Coatings that prevent base metal corrosion and are aesthetically pleasing are called protective-decorative coatings. Examples include copper/nickel/chromium and nickel-iron/chromium coatings. These coatings must not only resist corrosion but also be decorative.
Such coatings often involve plating a “base layer” followed by a “surface layer” and sometimes an “intermediate layer” because it’s challenging to find a single metal coating that meets both protective and decorative requirements. Some coating metals, though highly corrosion-resistant, cannot maintain their luster under certain conditions.
Others, while less corrosion-resistant, can provide a lasting luster and wear resistance. Therefore, using multi-layer plating combines the strengths of different coatings to compensate for their weaknesses.
For instance, copper/nickel/chromium coatings first plate copper as the base layer, then nickel as the intermediate layer to improve corrosion resistance, and finally, a slightly bluish bright chrome as the outer layer for excellent decorative qualities. This combination is commonly used on the exterior parts of instruments, automobiles, and bicycles.
- Restorative Coatings
Restorative coatings thicken or restore the dimensions of partially worn parts. Major mechanical components such as axles, crankshafts, gears on trains, automobiles, and petrochemical machinery can be electroplated to extend their lifespan.
Deep well pump shafts, for example, can be repaired with hard chrome or iron plating.
- Functional Coatings
Functional coatings leverage the mechanical, physical, and chemical properties of the coating metals to meet the needs of various applications. These can be further classified into several main types:
- Wear-resistant and friction-reducing coatings: These increase the surface hardness of products to enhance their wear resistance. Hard chrome is commonly used industrially for various shafts and crankshaft journals, printing roller surfaces, engine cylinder walls and piston rings, and stamping mold cavities. Lead-tin alloys and lead-tin-copper alloys serve as friction-reducing coatings for sliding contact surfaces, reducing sliding friction.
- High-temperature oxidation-resistant coatings: These protect components likely to oxidize at high temperatures, improving their resistance to high-temperature oxidation. Examples include jet engine rotors and blades made from high-melting-point metals, which may be plated with nickel, chromium, or platinum-germanium alloys. In some cases, composite coatings like nickel-germanium oxide and nickel-aluminum oxide are used to enhance the high-temperature oxidation resistance of metal materials.
- Magnetic coatings: Magnetic materials are used in magnetic cores, drums, disks, and magnetic storage components in electronic computing equipment. Common magnetic alloy coatings include cobalt-nickel, nickel-iron, and nickel-phosphorus-cobalt. Electroplating and chemical plating methods are often employed to prepare magnetic materials, with plating process parameters adjusted to optimize the magnetic properties of the coatings.
- Solderable coatings: Some electronic components assembly requires soldering. To improve their solderability, components are often plated with tin, silver, or tin-lead alloys. For wear resistance, silver-antimony alloys, gold-cobalt alloys, and gold-antimony alloys may be used.
Many other functional coatings are applied in production practice. For example, brass (copper-zinc alloy) plating on steel wire enhances adhesion during hot pressing with rubber; copper or silver plating improves surface conductivity, and when both good conductivity and wear resistance are required, gold or silver alloys are used; lead plating resists sulfuric and chromic acid corrosion; copper plating prevents local carburization, and tin plating prevents local nitriding; chrome, silver, or high-tin bronze plating increases reflectivity; for matte or light-absorbing properties, black nickel or black chrome may be plated.
Classification Based on Coating Structure
The classifications include simple structures, multilayer composite structures, and compound coatings.
- Simple Structure
This involves applying a single coating layer, also known as single metal electroplating. A layer of metal is used to meet the requirements.
- Multilayer Composite Structure
Multiple coating layers can consist of different metals, such as a copper/nickel/chrome three-layer structure, or the same metal, like the highly corrosion-resistant double nickel and triple nickel layers.
- Compound Coatings
These are coatings with a metal base and non-metal or metal particles as the dispersed phase, forming a dispersed structure.
Compound coatings are characterized by high wear and corrosion resistance. Utilizing compound plating technology to improve the surface structure or state of materials, enhance their functionality, expand their applications, and obtain coatings with specific functions represents a trend in the development of electroplating.
Analysis of the Causes of Poor Uniformity in Tin Coating
The primary cause of poor uniformity in tin coatings is an excessively high solution temperature, particularly noticeable when the temperature exceeds 35°C.
The unevenness of the tin coating significantly affects quality since the thinness of the tin layer means that any uneven areas will be even thinner, potentially leading to fogging and poor weldability.
- Control the solution temperature and apply cooling measures when necessary.
- Use a moving cathode. Moving the cathode not only improves the uniformity of the coating but also increases the cathode current density, preventing burnt coatings, pinholes, streaks, dark gray patterns, and enhancing the brightness of the coating.