Introduction to Common Surface Treatment Processes
I. Dacromet
Dacromet is the transliteration and abbreviation of DACROMET, also referred to as Dacromet, Dacromet coating, or Dicron. In China it is designated as zinc-chromium coating, a new type of anti-corrosion coating with zinc powder, aluminum powder, chromic acid, and deionized water as its main components.
Dacromet was first developed in the late 1950s. American scientist Mike Martin developed a highly dispersed water-soluble coating based on metallic zinc flakes with the addition of aluminum flakes, chromic acid, and deionized water as solvent. The coating adheres to the metal substrate and, through a fully closed-loop coating and baking process, forms a thin coating. The Dacromet coating successfully resists chloride ion attack, taking anti-corrosion technology to a new level and overcoming the shortcoming of short anti-corrosion life associated with traditional processes.
China formally introduced Dacromet technology from Japan in 1994, initially for use only in the defense industry and domestically produced automotive parts, and later expanded to power, construction, marine engineering, household appliances, small hardware and standard fasteners, railways, bridges, tunnels, highway guardrails, petrochemicals, bioengineering, medical devices, powder metallurgy, and many other industries.
Dacromet process flow: organic solvent degreasing — mechanical polishing — spraying — baking — second spraying — baking — drying.
Main applications of Dacromet:
1. Anti-corrosion of various high-strength chassis small and medium brackets, connectors, exposed parts, and fasteners on heavy-duty trucks, such as various profiled steel brackets, bolts (including U-bolts, wheel bolts, etc.), nuts, etc.
2. Anti-corrosion of metal parts in high-temperature environments around engines, such as heat shields, exhaust pipes, radiators, engine cylinder heads, and other components.
3. Anti-corrosion of various elastic parts on heavy-duty trucks, such as rings, semicircular clamps, various springs, leaf springs, etc.
4. Anti-corrosion of pipe-type and cavity-type components of various complex shapes on heavy-duty trucks, such as engine exhaust pipes, mufflers, etc.
II. PVD Treatment
PVD is the abbreviation of Physical Vapor Deposition. It refers to the process, under vacuum conditions, of using low-voltage, high-current arc discharge technology, utilizing gas discharge to evaporate the target material and ionize both the evaporated material and the gas, and using the acceleration effect of an electric field to deposit the evaporated material and its reaction products onto the workpiece.
PVD technology emerged in the late 1970s. The films prepared have advantages such as high hardness, low friction coefficient, good wear resistance, and chemical stability. Its initial successful application in the field of high-speed steel cutting tools attracted great attention from manufacturing industries worldwide. While developing high-performance, high-reliability coating equipment, researchers have also conducted more in-depth coating application research on cemented carbide and ceramic cutting tools. Compared with the CVD process, the PVD process has a low treatment temperature and has no effect on the bending strength of tool materials below 600°C; the internal stress state of the film is compressive stress, making it more suitable for coating precision and complex cemented carbide tools; the PVD process is simple, pollution-free to the environment, and consumes few consumables, in line with the development direction of modern green manufacturing.
PVD films are uniform and dense, with strong adhesion to the substrate. This technology is widely used in aerospace, electronics, optics, machinery, construction, light industry, metallurgy, materials, and other fields, and can prepare film layers with properties such as wear resistance, corrosion resistance, decorative, conductive, insulating, light-guiding, piezoelectric, magnetic, lubricating, and superconducting.
III. Polishing
Polishing refers to a processing method that uses mechanical, chemical, or electrochemical action to reduce the surface roughness of a workpiece in order to obtain a bright, smooth surface. It is a finishing process that uses polishing tools and abrasive particles or other polishing media to modify the surface. Polishing cannot improve the dimensional accuracy or geometric shape accuracy of a workpiece; its purpose is to obtain a smooth surface or mirror gloss, and sometimes it is also used to eliminate gloss (matting).
Mechanical polishing usually uses a polishing wheel as the polishing tool. The polishing wheel is generally made by stacking multiple layers of canvas, felt, or leather, clamped on both sides with metal discs, and coated on its rim with a polishing agent uniformly mixed from fine abrasive powder and grease. During polishing, the high-speed rotating polishing wheel (peripheral speed above 20 m/s) is pressed against the workpiece, causing the abrasive to produce rolling and micro-cutting on the workpiece surface, thereby obtaining a bright machined surface, with surface roughness generally reaching Ra 0.63–0.01 μm; when a non-greasy matting polishing agent is used, the bright surface can be matted to improve appearance.
For different polishing processes—rough polishing (basic polishing process), medium polishing (finishing process), and fine polishing (glossing process)—selecting the appropriate polishing wheel can achieve the best polishing effect while improving polishing efficiency.
IV. Sandblasting
Sandblasting is the process of cleaning and roughening the substrate surface using the impact of a high-speed abrasive stream. Compressed air is used as power to form a high-speed jet that projects abrasive material (copper ore sand, quartz sand, emery, iron sand, Hainan sand) at high speed onto the surface of the workpiece to be treated, changing the appearance or shape of the workpiece surface. Due to the impact and cutting action of the abrasive on the workpiece surface, the surface obtains a certain degree of cleanliness and different roughness, and the mechanical properties of the workpiece surface are improved, thereby increasing the fatigue resistance of the workpiece, enhancing its adhesion to coatings, extending the durability of the coating film, and also facilitating the leveling and decoration of the paint.
V. Brushing
Brushing is a surface treatment method that forms lines on the workpiece surface through abrasive products to achieve a decorative effect. According to the different patterns after brushing, it can be divided into: straight-line brushing, random-line brushing, wave pattern, and spin pattern.
Surface brushing treatment is a surface treatment method that forms lines on the workpiece surface through abrasive products to achieve a decorative effect. Because surface brushing treatment can express the texture of metal materials, it has gained increasing favor among users and increasingly wide application.
VI. Anodizing
Anodizing is an electrolytic oxidation process in which the surface of aluminum and aluminum alloys is typically converted into an oxide film that is protective, decorative, and has other functional properties. Based on this definition, aluminum anodizing includes only the process portion that generates the anodic oxide film. The metal or alloy workpiece is used as the anode, and electrolysis is used to form an oxide film on its surface. The metal oxide film changes the surface state and properties, such as surface coloring, improved corrosion resistance, enhanced wear resistance and hardness, and protection of the metal surface. For example, in aluminum anodizing, aluminum and its alloys are placed in a corresponding electrolyte (such as sulfuric acid, chromic acid, oxalic acid, etc.) as the anode and electrolyzed under specific conditions and applied current.
In anodized aluminum or its alloys, a thin aluminum oxide layer forms on the surface, with a thickness of 5–30 μm; hard anodic oxide films can reach 25–150 μm. After anodizing, aluminum or its alloys have improved hardness and wear resistance (up to 250–500 kg/mm²), good heat resistance (the melting point of hard anodic oxide film is as high as 2320K), excellent insulation (breakdown voltage up to 2000V), and enhanced corrosion resistance (no corrosion after several thousand hours in ω=0.03 NaCl salt spray).
The thin oxide film contains a large number of micropores that can adsorb various lubricants, making it suitable for manufacturing engine cylinders or other wear-resistant parts; the strong adsorption capacity of the film micropores allows coloring into various beautiful and vivid colors. Non-ferrous metals or their alloys (such as aluminum, magnesium, and their alloys) can all undergo anodizing treatment. This method is widely used in mechanical parts, aircraft and automobile components, precision instruments and radio equipment, daily necessities, and architectural decoration.
Generally speaking, aluminum or aluminum alloy is used as the anode, and a lead plate is selected as the cathode. The aluminum and lead plate are placed together in an aqueous solution containing sulfuric acid, oxalic acid, chromic acid, etc., for electrolysis, causing an oxide film to form on the surfaces of the aluminum and lead plate. Among these acids, sulfuric acid anodizing is the most widely used.
Process flow: Single color, gradient color: polishing/sandblasting/brushing → degreasing → anodizing → neutralization → dyeing → sealing → drying Two-color: ① polishing/sandblasting/brushing → degreasing → masking → anodizing 1 → anodizing 2 → sealing → drying ② polishing/sandblasting/brushing → degreasing → anodizing 1 → laser engraving → anodizing 2 → sealing → drying
Technical features: 1. Improved strength. 2. Achieves any color except white. 3. Achieves nickel-free sealing, meeting the nickel-free requirements of Europe, the United States, and other countries.
VII. Electrophoresis (EP)
Electrophoresis refers to the phenomenon in which charged particles move toward the electrode of opposite polarity under the action of an electric field. The technology that uses the different migration speeds of charged particles in an electric field to achieve separation is called electrophoresis technology. Electrophoresis processes are divided into anodic electrophoresis and cathodic electrophoresis. If the coating particles are negatively charged and the workpiece is the anode, the coating particles deposit and form a film on the workpiece under the action of the electric field force, which is called anodic electrophoresis; conversely, if the coating particles are positively charged and the workpiece is the cathode, the coating particles deposit and form a film on the workpiece, which is called cathodic electrophoresis.
The electrophoresis process was founded in the 1960s and was first applied by Ford Motor Company to automotive primer. Due to its excellent anti-corrosion and anti-rust functions, it was quickly widely used in the military industry. Only in recent years has it been applied to the surface treatment of daily hardware. Due to its excellent quality and high environmental friendliness, it is gradually replacing traditional paint spraying.
The general process flow of anodic electrophoresis is: workpiece pretreatment (degreasing → hot water rinse → derusting → cold water rinse → phosphating → hot water rinse → passivation) → anodic electrophoresis → workpiece post-treatment (clean water rinse → drying).
1. Degreasing. The solution is generally a hot alkaline chemical degreasing solution, at a temperature of 60°C (steam heating), for about 20 min.
2. Hot water rinse. Temperature 60°C (steam heating), time 2 min.
3. Derusting. Use H2SO4 or HCl, for example, with a hydrochloric acid derusting solution, total HCl acidity ≥43 points; free acidity >41 points; add 1.5% cleaning agent; wash at room temperature for 10–20 min.
4. Cold water rinse. Rinse in running cold water for 1 min.
5. Phosphating. Use medium-temperature phosphating (phosphating at 60°C for 10 min); a commercially available phosphating solution can be used.
The above steps can also be replaced by sandblasting → water rinse.
6. Passivation. Use a chemical matched with the phosphating solution (provided by the phosphating solution supplier), at room temperature for 1–2 min.
7. Anodic electrophoresis electrolyte composition: H08-1 black electrophoretic paint, solid content mass fraction 9%–12%, distilled water mass fraction 88%–91%. Voltage: (70±10) V; time: 2–2.5 min; paint liquid temperature: 15–35°C; paint liquid pH: 8–8.5. Note that power must be cut off when the workpiece enters and exits the tank. During electrophoresis, the current gradually decreases as the paint film thickens.
8. Clean water rinse. Rinse in running cold water.
9. Drying. Dry in an oven at (165±5)°C for 40–60 min.
VIII. Electroplating
Electroplating is the process of plating a thin layer of another metal or alloy on the surface of certain metals using the principle of electrolysis. It is a process that uses electrolysis to attach a metal film to the surface of metal or other material workpieces, thereby preventing metal oxidation (such as rust), improving wear resistance, solderability, conductivity, reflectivity, corrosion resistance, and enhancing appearance.
Schematic diagram of electroplating (rack plating) principle
Different electroplating requirements also serve different purposes. Examples are as follows:
1. Copper plating: used as a base layer to improve the adhesion and corrosion resistance of the electroplated layer. (Copper oxidizes easily; after oxidation, copper green is no longer conductive, so copper-plated products must have copper protection.)
2. Nickel plating: used as a base layer or for appearance, improving corrosion resistance and wear resistance (among these, electroless nickel in modern processes has wear resistance exceeding that of chromium plating). (Note that many electronic products, such as DIN connectors and N connectors, no longer use nickel as a base layer, mainly because nickel is magnetic and can affect passive intermodulation in electrical performance.)
3. Gold plating: improves conductive contact resistance and enhances signal transmission. (Gold is the most stable and also the most expensive.)
4. Palladium-nickel plating: improves conductive contact resistance, enhances signal transmission, and has higher wear resistance than gold.
5. Tin plating (bright tin and matte tin): improves solderability.
6. Silver plating: improves conductive contact resistance and enhances signal transmission. (Silver has the best performance but oxidizes easily; it remains conductive after oxidation.)
Comparison between zinc plating and zinc-nickel alloy plating:
1. Characteristics of zinc-nickel alloy coating Among zinc-based alloys, zinc-nickel alloy coating is a new type of excellent protective coating suitable for use in harsh industrial atmospheres and severe marine environments. The corrosion resistance of zinc-nickel alloy with a nickel content of 7–9% is more than 3 times that of zinc coating; the corrosion resistance of zinc-nickel alloy coating with a nickel content of about 13% is more than 5 times that of zinc coating, and it has the best corrosion resistance.
Because zinc-nickel alloy has excellent characteristics such as high corrosion resistance, low hydrogen embrittlement, weldability, and machinability, it has long attracted great attention, and its application range is becoming wider and wider. Zinc-nickel alloy coating has a high melting point and is suitable for electroplating automotive engine components; it has low hydrogen embrittlement and is suitable for electroplating on high-strength steel; it can serve as a cadmium substitute coating and is mostly used for military products.
Zinc-nickel alloy plating solutions are mainly divided into two types: one is a weakly acidic system, which has simple composition, high cathodic current efficiency (generally above 95%), a stable plating solution, and is easy to operate. The other is an alkaline zincate plating solution, whose main advantages are: good throwing power of the plating solution, relatively uniform alloy composition of the coating over a wide current density range, uniform coating thickness, low corrosion to equipment and workpieces, easy process operation, stable process, and relatively low cost.
2. Comparison with zinc coating 1) Corrosion resistance. Zinc plating and zinc-nickel alloy are used as functional coatings, and zinc-nickel alloy is far superior to zinc plating in corrosion resistance, which is also the main reason researchers have spent a great deal of time and effort developing zinc-nickel alloy. After color passivation treatment, zinc-nickel alloy coating easily passes 1000 hours in neutral salt spray without white rust, while zinc coating after color passivation rarely passes 120 hours. After white passivation treatment, zinc-nickel alloy coating can pass 400 hours in neutral salt spray without white rust, while zinc coating after blue-white passivation rarely passes 96 hours.
2) Appearance. Under normal circumstances, the color-passivated layer of zinc-nickel alloy is not as vivid as that of zinc plating, especially when the nickel content in the alloy coating is relatively high; the white passivation color of zinc-nickel alloy is not as white and clean as that of zinc plating. It should be noted that people's understanding of things is often preconceived, and the same is true for product color. As zinc-nickel alloy products have increased in the market, people have been able to accept the color difference between zinc-nickel alloy passivation layers and zinc passivation layers.
3) Production cost. Because zinc-nickel alloy coating contains about 13% metallic nickel, and the price of nickel is far higher than that of zinc (nickel: 191,000 yuan/ton, zinc: 18,000 yuan/ton), the production cost of zinc-nickel alloy electroplating is much higher than that of zinc electroplating.
4) Process maintenance. Alkaline zincate zinc plating is now widely accepted by the market, while zinc-nickel alloy process, as a "new" electroplating process, has not yet been widely popularized and is known to a limited number of people. In fact, zinc-nickel alloy has been used in the market for more than twenty years and has developed rapidly. The process is now quite mature, with stability even exceeding that of zinc plating.
Electroplating methods are mainly divided into rack plating, barrel plating, continuous plating, and brush plating, mainly related to the size and batch of the parts to be plated. Rack plating is suitable for products of general size, such as automobile bumpers and bicycle handlebars. Barrel plating is suitable for small parts, such as fasteners, washers, and pins. Continuous plating is suitable for mass-produced wire and strip, and continuous plating includes full immersion plating and localized selective plating (brush plating, spot plating, line plating, etc.). Brush plating is suitable for localized plating or repair. Plating solutions include acidic, alkaline, and acidic and neutral solutions with complexing agents. Regardless of the plating method used, the plating tank, hanging fixtures, etc. that contact the parts to be plated and the plating solution should have a certain degree of versatility.
IX. Chemical Etching
Etching as generally referred to is also called photochemical etching. It refers to removing the protective film in the area to be etched after exposure, plate making, and development, and then contacting a chemical solution during etching (commonly ferric chloride or copper chloride solution) to achieve dissolution and corrosion, forming a raised, recessed, or hollowed-out shape.
Process flow: 1. Exposure method: engineering opens material preparation dimensions according to the pattern - material preparation - material cleaning - drying → film lamination or coating → drying → exposure → development → drying - etching → film removal → OK 2. Screen printing method: cutting → cleaning the plate (stainless steel or other metal materials) → screen printing → etching → film removal → OK
X. Spray Coating
Spray coating is a coating method in which paint is dispersed into uniform and fine droplets by means of pressure or centrifugal force through a spray gun or disc atomizer and applied to the surface of the object to be coated. It can be divided into air spraying, airless spraying, electrostatic spraying, and various derived methods of the above basic spraying forms, such as high-flow low-pressure atomizing spraying, thermal spraying, automatic spraying, and multi-component spraying.
Spray coating operations have high production efficiency and are suitable for manual work and industrial automated production. The main application areas include hardware, plastics, furniture, military, marine, and other fields, and it is the most commonly used coating method today. Spray coating operations require a cleanroom environment ranging from Class 1,000,000 to Class 100. Spray coating equipment includes spray guns, spray booths, paint supply rooms, and curing