Surface Finish Definition
Surface treatment involves methods that create a coating on the workpiece to enhance its appearance and provide corrosion resistance.
Surface treatment methods are attributed to the following commonly used methods:
- electroplating,
- hot dip galvanizing,
- mechanical plating,
- chemical plating,
- dip plating, anodizing,
- chemical conversion layer,
- chromate treatment,
- metal dyeing,
- painting,
- hot dip plating,
- hot dip tin plating,
- cathodic sputtering vacuum plating,
- ion plating,
- surface hardening,
- and so on.

Common Surface Treatment
1.0 Electroplating: workpiece as cathode
The part to be plated is immersed in a metal compound solution, and an electric current is applied to deposit the metal onto the part. General plating includes galvanizing, copper, nickel, chromium, copper-nickel alloys, etc., sometimes including blackening (bluing) and phosphating.
1.1 Hardware plating pre-treatment and plating: grinding to remove oxidized skin, polishing, oil degreasing, washing, pickling, descaling, alkaline cleaning and neutralization, washing, checking, racking and plating, drying, checking, packing and shipping.
1.2 The quality of plating is primarily measured by its corrosion resistance and secondarily by its appearance.
Corrosion resistance was used to imitate the working environment of the product, set up for the test conditions, and a corrosion test.
The quality of plating products from the following aspects to be controlled:
① Appearance:
the product surface is not allowed to have local no plating, burning, roughness, gray, peeling orange peel condition, and obvious stripes, are not allowed to have pinholes and pimples, black slag, passivation film loss, cracking, peeling and serious traces of passivation.
② Plating thickness:
◆ Fasteners in the corrosive atmosphere of the operating life and its plating thickness is directly proportional. The generally recommended economic galvanized plating thickness is 0.00015 in~0.0005 in(4~12um).
◆ Hot dip galvanizing: the standard average thickness of 54um (call diameter ≤ 3/8 for 43um), the minimum thickness of 43um (diameter ≤ 3/8 for 37um).
③ Plating distribution:
◆ Using different deposition methods, plating on the surface of the fastener gathered in different ways. During plating, the plated metal is not uniformly deposited on the peripheral edges, and thicker plating is obtained at the corners. In the threaded part of the fastener, the thickest plating is located at the top of the threaded teeth, along the threaded side of the thinning, the thinnest deposition is at the bottom of the teeth, and hot dip galvanizing is just the opposite of thicker plating deposited in the corners and the bottom of the threads, mechanical plating to deposition of plated metal and hot dip plating the same, but much smoother and more even thickness on the entire surface.
④ Hydrogen embrittlement:
◆ Fasteners in the processing and treatment process, especially in the plating before the pickling and alkaline cleaning and subsequent plating process, the surface absorbs hydrogen atoms, the deposited metal coating, and then captured hydrogen. When the fastener is tightened, hydrogen is transferred toward the most concentrated part of the stress, causing the pressure to increase beyond the strength of the base metal and producing a small surface rupture. Hydrogen is particularly active and quickly penetrates the newly formed cracks. This cycle of stress-rupture-infiltration continued until the fastener broke. This usually occurs within a few hours of the first stress application.
- To eliminate the threat of hydrogen embrittlement, fasteners are heated and baked as quickly as possible after plating to allow hydrogen to leach out of the coating, with baking usually occurring at 375-4000F (176-190°C) for 3-24 hours.
- Since mechanical galvanizing is non-electrolytic, this virtually eliminates the threat of hydrogen embrittlement.
2.0 Hot Dip Galvanizing:
This is accomplished by submerging carbon steel parts in a bath of dissolved zinc at a temperature of approximately 510 degrees Celsius. The result is a gradual transformation of the iron-zinc alloy on the surface of the steel part into passivated zinc on the exterior surface of the product. Hot-dip aluminum plating is a similar process.
2.1 General screws are mostly plated, but hexagonal screws used in electric power, highways, and other outdoor uses undergo hot-dip galvanizing; the cost of plating is generally $0.08-0.1 per kilogram, and hot-dip galvanizing is generally $0.2-0.28 per kilogram, which is more costly.
3.0 Mechanical Plating:
Impacts the surface of the product by particles of the plated metal and cold welds the coating to the product’s surface.
4.0 Chemical Plating:
Autocatalytic plating, generally known as “chemical plating”, “electroless plating” and other autocatalytic nickel-phosphorus plating in industrial applications.
5.0 Impregnation plating:
using copper sulfate, and stannous sulfate as the main salt of the replacement reaction impregnation plating method on the surface of the steel parts to produce a gold-colored copper-tin alloy layer. Through the addition of fluorine anion and carboxyl anion complexing agent, control of copper, and tin ions replacement reaction rate, to obtain 15% ~ 39% copper content of the alloy coating, the appearance of 18 ~ 22K gold color, good film bonding, excellent corrosion resistance. 5.1 There are two types of chemical plating imitation gold, one is the use of a reducing agent chemical reduction type formed by the coating is generally russet or bad color gold, and the adhesion of its coating is poor.
Another type: Dip plating, i.e., replacement plating or contact plating, is a replacement reaction driven by the electric potential generated by the potential difference between two metals without the need of an external current or reducing agent. Dip plating requires less equipment, high efficiency and low cost.
6.0 Anodizing:
The electrochemical oxidation of metals or alloys close to the surface plating treatment. A metal or alloy part is used as an anode to form an oxide film on its surface by electrolysis. The metal oxide film changes the surface state and performance, such as surface coloring, improving corrosion resistance, enhancing wear resistance and hardness, and protecting the metal surface. For example, aluminum anodic oxidation, aluminum and its alloys in the corresponding electrolyte (such as sulfuric acid, chromic acid, oxalic acid, etc.) as an anode, oxalic acid, etc.) as anode, under specific conditions and applied current, electrolysis is carried out.
①
Anode of aluminum or its alloy oxidation, the formation of a thin layer of aluminum oxide on the surface, the thickness of 5 ~ 20 microns, hard anodic oxide film up to 60 ~ 200 microns. Anodic oxidation of aluminum or its alloys, improves its hardness and wear resistance, up to 250500 kg / mm2, good heat resistance, hard anodic oxidation film melting point of up to 2320K, excellent insulation, breakdown voltage of up to 2000V, enhanced corrosion resistance, in the ω = 0.03NaC1 salt spray by thousands of hours without corrosion. Oxide film thin layer with a large number of microporous, can adsorb a variety of lubricants, suitable for the manufacture of engine cylinders or other wear-resistant parts; film microporous adsorption capacity can be colored into a variety of beautiful and colorful colors.
②
Non-ferrous metals or their alloys (such as aluminum, iron and its alloys, etc.) can be anodic oxidation treatment, this method is widely used in machinery parts, aircraft and automobile parts, precision instruments and radio equipment, daily necessities and architectural decoration and so on.
Supplementary:
①In addition to metal, other substances in the anode are caused by oxidation, also known as “anodic oxidation”. ②In life, anodic oxidation of aluminum alloy is more and more, which can be applied in daily life, and it is thought that the characteristics of this process make the surface of aluminum parts produce a hard protective layer, which can be used in the production of kitchenware and other daily necessities. However the casting aluminum anodic oxidation effect is not good, the surface is not good, but also can only be black. Aluminum profiles are better.
7.0 Chemical Conversion Coating:
electrochemical or chemical method using metal ions that are suspended in a solution of insolubility, so that the metal or its corrosion products and the environment of the constituent components of the chemical reaction, to form the inorganic material on the surface of the metal cover layer. a. Steel blue (Blackening), commonly known as “pot black or black”. b. Steel phosphating (Phosphating). c. Composite plating (diffusion plating).
8.0 Chromating
9.0 Metal Colouring
10.0 Paint Finishing:
Includes a variety of coatings such as hand painting, electrostatic painting, electrophoretic painting, etc.
11.0 Hot Dip Plating:
is a method of plating metal materials by immersing them in other liquid metals or alloys with lower melting points. The method of plating. The basic characteristic of this method is the formation of an alloy layer between the base metal and the plated metal.
11.1 Hot-dip galvanizing, commonly known as “lead plating”.
11.2 Hot-dip Tinning
12.0 cathodic sputtering:
the collision of charged or neutral particles with sufficient energy to transfer energy to the atoms on the surface of an object. As long as the energy obtained by the surface atoms is greater than their own ionization energy, they can get rid of the surrounding atoms and leave the surface of the object, a phenomenon known as sputtering.
13.0 Vacuum Plating:
Vacuum plating mainly includes vacuum evaporation plating, sputtering plating, and ion plating several types. They are used in vacuum conditions through the distillation or sputtering and other ways in the plastic surface deposition of various metals and non-metallic films, through this way you can get a very thin surface plating, which at the same time has the outstanding advantages of fast adhesion. Because of its high price, it is generally used for higher-grade products. It is generally used for functional plating of high-grade products due to its high price.
13.1 Ion Plating:
Ion plating under vacuum conditions, the use of gas discharge to make the gas or evaporated substances partially ionized, in the gas ions or evaporated ions under the bombardment, the evaporated substances or their reactants are deposited on the substrate method. These include magnetron sputtering ion plating, reactive ion plating, hollow cathode discharge ion plating (hollow cathode evaporation method), and multiple arc ion plating (cathode arc ion plating).
14.0 Surface Hardening:
Surface hardening methods treat the surface layer of a part to increase hardness. The core remains strong and ductile. This process enhances wear resistance and fatigue resistance. The core maintains good toughness and strength, providing resistance to impact loads. Commonly used surface hardening treatment methods include carburizing, nitriding, hard anodizing, chromium plating, surface hardening and metalizing.
14.1 Carburizing:
Carburizing involves heating low-carbon steel parts in a carburizing medium. This allows carbon atoms to penetrate the surface. The process achieves a specific surface carbon content. After quenching, the surface has high hardness due to carbon. Meanwhile, the low-carbon core maintains good toughness.
- The purpose is to make the parts obtain high surface hardness, wear resistance and high contact fatigue strength and bending fatigue strength.
- It is mainly used for parts subjected to wear, alternating contact stress bending stress and impact load, such as shafts, gears, camshafts, etc. These parts require high surface hardness and sufficient strength and toughness in the heart.
- The carburizing method is divided into three kinds: solid carburizing method, liquid carburizing method and gas carburizing method. There is also a vacuum carburizing method.
14.2 Nitriding:
Nitriding refers to the alloy steel (generally containing Al, C, Mo) in anhydrous ammonia (NH3) flow at 500 ℃ 570 ℃ or so long time heating so that the steel surface forms a layer of high hardness and corrosion-resistant nitride (mainly Fe2N, Fe3N, Fe4N).
①
Generally, there are gas nitriding, liquid nitriding and glow ion nitriding. Ammoniation of alloy steel carbon content is generally 0.2%-0.5%, mainly by the mechanical properties of the heart of the parts to determine the carbon content, high carbon content to hinder the diffusion, thereby reducing the thickness of the nitrided layer, and less carbon content, the parts cross-section of the hardness gradient will be large, resulting in the nitrided layer is easy to peel off.
②
Al increases the nitriding surface hardness of steel. Cr enhances the thickness of the nitrided layer. Mo prevents nitrogen oxides at 500℃. Mo also prevents tempering brittleness from prolonged heating at 500℃-570℃. Both Cr and Mo improve material properties. The longest history of the internationally commonly used nitriding steel is 38CrMoAl.
- The general process of nitrided parts is forging, annealing, rough machining, tempering, finishing and nitriding.
- The parts that do not need nitriding can be plated with Ni or Sn or Pb20%+Sn80%.
- The parts are tempered before nitriding treatment so that they have good plasticity and toughness. After nitriding treatment, quenching and tempering are no longer implemented, and the temperature of nitriding treatment itself is relatively low, so the deformation of workpiece is small.
- The hardness of the workpiece surface after nitriding treatment can reach HRC65-72, which is higher and more wear-resistant than the hardness after carburizing treatment, and the corrosion resistance is better.
- It is used for the parts with high requirements of wear resistance and precision, and heat-resistant, wear-resistant and corrosion-resistant parts. For example, small shafts of instruments, light load gears and important crankshafts.
14.3 Hard anodizing:
①
- Anodic oxidation of aluminum is to use aluminum or aluminum alloy as anode and lead plate as cathode in the electrolyte to generate oxide film layer on its surface. After anodic oxidation, the surface of aluminum can generate a thickness of several to several hundred microns of oxide film. The surface of this oxide film is porous and honeycomb, compared with the natural oxide film of aluminum alloy, its corrosion resistance, wear resistance and decorative properties are significantly improved and enhanced. Different properties of anodic oxide film can be obtained by using different electrolytes and process conditions.
②
- Process flow: degreasing and degreasing-alkali corrosion-polishing-anodic oxidation-dyeing and pore treatment.
- The purpose of alkali corrosion is to remove the dense but uneven oxide film on the surface of aluminum alloy. For high silicon aluminum alloy, the mixed solution of HNO3 and HF is used, while other aluminum alloys use alkaline tank solution mainly with NaOH solution.
- The purpose of polishing is to get a bright substrate surface, which is divided into mechanical polishing, electrolytic polishing and chemical polishing. Mechanical polishing is the use of mechanical methods to grind a bright surface. Electrolytic polishing uses electric current to cause an electrochemical reaction in aluminum alloy. This dissolves the surface’s uneven areas, creating a smooth mirror effect. Chemical polishing immerses parts in a chemical solution to achieve a bright surface. This process can be divided into acid polishing and alkaline polishing.
- Dyeing the porous structure and strong adsorption of anodic oxide film. Anodized aluminum parts are soaked in dye solutions, allowing the oxidized film to absorb color.
- The oxide film on aluminum and its alloys has high porosity, making it easily contaminated. This is especially true in corrosive environments.
- The oxide film should be closed after dyeing to increase the color of sunlight resistance and corrosion resistance. In industrial production, anodized aluminum films require a sealing treatment, regardless of dyeing. Common methods include hot water and steam sealing. These methods cause a hydration reaction between the oxide film and aluminum trioxide. This generates hydrated alumina, increasing the film’s volume by 33–100% and narrowing the pores. The expansion of the oxide film volume significantly reduces the membrane hole, thus achieving the purpose of sealing the hole.
- Various chemical solutions can also be used for sealing, such as potassium dichromate solution. This treatment produces a light yellow anodic oxidation film with high corrosion resistance.
③
- The electrolytes for anodic oxidation mainly include sulfuric, oxalic, and chromic acids. Sulfuric acid anodization is the most widely used due to its strong passivation properties. If there is no special indication, it generally refers to sulfuric acid anodizing.
14.4 Chrome plating:
- Chromium is the hardest metal with a hardness of 800-1000 HV. The chromium layer passivates quickly in the atmosphere, remaining stable and retaining its luster for a long time. It resists alkali, nitric acid, and organic acids but dissolves in hydrohalic acids, like hydrochloric and concentrated sulfuric acid.
- The advantages of chromium plating are high hardness, good abrasion resistance and strong light reflection. It also has good heat resistance, does not change color below 480°C, begins to oxidize only after 500°C, and its hardness decreases at 700°C. The disadvantage of chromium plating is that it is too hard, brittle and easy to fall off. Chromium plating is porous, making it ineffective for steel corrosion protection. Copper plating is applied first, followed by nickel plating, and finally chromium plating for effective corrosion prevention and decoration.
- Hard chrome plating is thicker than standard chrome plating, requiring an extended processing time, typically lasting more than ten hours.
14.5 Surface quenching:
- Surface quenching rapidly heats the workpiece surface to quenching temperature and then cools it to achieve hardness.
- This process forms a martensitic structure at the surface while preserving the core’s original state, ensuring durability. It is mainly used for medium carbon tempered steel and ductile iron machine parts.
The primary methods for surface quenching include induction heating, flame quenching, and laser or electron beam heating.
14.5.1 Induction heating surface hardening:
- Definition: Induction heating is a metal heat treatment method that rapidly heats the workpiece surface and then quenches it.
- Induction heating principle: The workpiece is placed in an inductor that passes medium or high-frequency alternating current (500-300 kHz) through a hollow copper tube. This generates an alternating magnetic field, producing an induced current within the workpiece. The induced current distribution is non-uniform: stronger at the surface and weaker internally, approaching zero at the core. This phenomenon, known as the skin effect, allows the surface temperature to rise to 800-1000°C within seconds, while the core temperature rises minimally.
- Induction heating surface hardening can be divided into three categories based on current frequency:
- High-frequency induction heating quenching (200-300 kHz) achieves a hardened layer depth of 0.5-2.0 mm, suitable for small gears and shafts.
- Medium frequency induction heating quenching (2500-800 Hz) provides a hardened layer depth of 2-10 mm, ideal for larger shafts and gears.
- Low-frequency induction heating quenching (50 Hz) does not require frequency conversion and reaches a hardened layer depth of 10-15 mm, suitable for larger parts like rolls and train wheels.
Induction heating surface quenching offers advantages like good surface quality, low brittleness, reduced oxidation and decarburization, and minimal deformation.
Thus, induction heating equipment is widely used in metal surface heat treatment.
14.5.2 Flame heating quenching:
This quenching method employs a flame to directly heat the surface of the workpiece, followed by immediate water cooling, thereby achieving surface hardening. The flame reaches about 3000 ℃, allowing rapid heating to quenching the temperature. By adjusting the burner position and movement speed, different hardened layer thicknesses can be obtained. Surface quenching hardened layer depth is generally counted to half martensite zone.
For metal castings, the more commonly used surface treatment methods are:
- Mechanical grinding.
- Chemical treatment.
- Surface heat treatment
- Sprayed surfaces