Metal Surface Treatment 101: Basics You Should Know

1. What is surface treatment process

The surface treatment process is a process that uses the technology of modern physics, chemistry, metalology, heat treatment and other disciplines to change the condition and properties of the part surface and optimize the combination with the core material to meet the predetermined performance requirements.

Function of surface treatment:

  • Improve the surface corrosion resistance and wear resistance, slow down, eliminate and repair the change and damage of the material surface;
  • Enable ordinary materials to obtain surfaces with special functions;
  • Save energy, reduce costs and improve the environment.

2. Classification of metal surface treatment process

It can be divided into four categories: surface modification technology, surface alloying technology, surface conversion film technology and surface coating technology.

1. Surface modification technology

1. Surface quenching

Surface quenching refers to the heat treatment method of austenitizing the surface layer by rapid heating and quenching to strengthen the surface of parts without changing the chemical composition and core structure of steel.

The main methods of surface quenching are flame quenching and induction heating.

The common heat sources are oxyacetylene or oxypropane flame.

2. Laser surface strengthening

Laser surface strengthening is to use the focused laser beam to shoot at the workpiece surface, heat the extremely thin material on the workpiece surface to the temperature above the phase transformation temperature or melting point in a very short time, and then cool it in a very short time to harden and strengthen the workpiece surface.

Laser surface strengthening can be divided into laser phase transformation strengthening treatment, laser surface alloying treatment and laser cladding treatment.

Laser surface strengthening has small heat affected zone, small deformation and convenient operation.

It is mainly used for locally strengthened parts, such as blanking die, crankshaft, cam, camshaft, spline shaft, precision instrument guide rail, high-speed steel cutter, gear and cylinder liner of internal combustion engine.

3. Shot peening

Shot peening strengthening is a technology to spray a large number of high-speed moving bullets onto the surface of parts, just like countless small hammers hammering the metal surface, so that the surface and sub surface of parts undergo certain plastic deformation to realize strengthening.


  • Improve the mechanical strength, wear resistance, fatigue resistance and corrosion resistance of parts;
  • Used for surface extinction and descaling;
  • Eliminate the residual stress of casting, forging and welding parts.

4. Rolling

Rolling is a surface treatment process in which hard rollers or rollers are pressed on the rotating workpiece surface at room temperature and move along the bus direction to plastically deform and harden the workpiece surface, so as to obtain an accurate, smooth and strengthened surface or specific pattern.

Application: cylindrical surface, conical surface, plane and other parts with relatively simple shape.

5. Wire drawing

Wire drawing refers to the surface treatment method that makes the metal forcibly pass through the die under the action of external force, the metal cross-sectional area is compressed, and the required cross-sectional area shape and size are obtained, which is called metal wire drawing process.

Wire drawing can be made into straight lines, random lines, ripples and spiral lines according to the needs of decoration.

6. Polishing

Polishing is a finishing method to modify the surface of parts.

Generally, it can only get a smooth surface and can not improve or even maintain the original machining accuracy.

With different pre-processing conditions, the Ra value after polishing can reach 1.6 ~ 0.008 μ m.

It is generally divided into mechanical polishing and chemical polishing.

2. Surface alloying technology

Chemical surface heat treatment

The typical process of surface alloying technology is chemical surface heat treatment.

It is a heat treatment process that places the workpiece in a specific medium for heating and insulation, so that the active atoms in the medium penetrate into the surface of the workpiece, so as to change the chemical composition and organization of the surface of the workpiece, and then change its performance.

Compared with surface quenching, chemical surface heat treatment not only changes the surface structure of steel, but also changes its chemical composition.

According to different elements, carburizing and nitriding can be divided into multiple elements and other elements.

Chemical heat treatment process includes three basic processes: decomposition, absorption and diffusion.

The two main ways of chemical surface heat treatment are carburizing and nitriding.

ObjectiveImprove the surface hardness, wear resistance and fatigue strength of the workpiece, while maintaining good toughness of the heart.Improve the surface hardness, wear resistance and fatigue strength of the workpiece, and improve the corrosion resistance.
MaterialLow carbon steel containing 0.1 ~ 0.25% C. High carbon reduces the toughness of the core.It is medium carbon steel containing Cr, Mo, Al, Ti and V.
Common methodGas carburizing, solid carburizing and vacuum carburizingGas nitriding method, ion nitriding method
Surface thicknessGenerally 0.5 ~ 2mmNot more than 0.6 ~ 0.7mm
PurposeIt is widely used in mechanical parts such as aircraft, automobile and tractor; Such as gear shaft, camshaft, etc.It is used for parts with high wear resistance and precision requirements and heat-resistant, wear-resistant and corrosion-resistant parts. Such as small shaft of instrument, light load gear and important crankshaft, etc.

3. Surface conversion film technology

1. Blackening and phosphating


The process of heating steel or steel parts to an appropriate temperature in air steam or chemicals to form a blue or black oxide film on their surface. Also become blue.


The process in which the workpiece (steel or aluminum or zinc) is immersed in phosphating solution (some acid phosphate based solution) and deposited on the surface to form a crystalline phosphate conversion film insoluble in water is called phosphating.

2. Anodic oxidation

It mainly refers to the anodizing of aluminum and aluminum alloys.

Anodizing is to immerse aluminum or aluminum alloy parts in acidic electrolyte and act as anode under the action of external current to form an anti-corrosion and oxidation film firmly combined with the substrate on the surface of the parts.

This oxide film has special characteristics such as protection, decoration, insulation and wear resistance.

Before anodizing, polishing, degreasing, cleaning and other pretreatment shall be carried out, followed by washing, coloring and sealing.

Application: it is commonly used in the protective treatment of some special parts of automobiles and aircraft, as well as the decorative treatment of handicrafts and daily hardware products.

4. Surface coating technology

1. Thermal spraying

Thermal spraying is to heat and melt metal or non-metallic materials and continuously blow them onto the surface of the workpiece by compressed gas to form a coating firmly combined with the substrate, so as to obtain the required physical and chemical properties from the surface of the workpiece.

Thermal spraying technology can improve the wear resistance, corrosion resistance, heat resistance and insulation of materials.

Applications: aerospace, atomic energy, electronics and other cutting-edge technologies in almost all fields.

2. Vacuum plating

Vacuum plating is the surface treatment process of depositing various metal and non-metallic films on the metal surface by distillation or sputtering under vacuum conditions.

Very thin surface coating can be obtained by vacuum plating.

At the same time, it has the advantages of high speed, good adhesion and less pollutants.

Principle of vacuum sputter plating

According to different processes, vacuum plating can be divided into vacuum evaporation, vacuum sputtering and vacuum ion plating.

3. Electroplating

Electroplating is an electrochemical and redox process.

Take nickel plating as an example: immerse the metal parts in the solution of metal salt (NiSO4) as the cathode and the metal nickel plate as the anode.

After connecting the DC power supply, the metal nickel coating will be deposited on the parts.

Electroplating methods are divided into ordinary electroplating and special electroplating.

4. Vapor deposition

Vapor deposition technology is a new coating technology that deposits the vapor phase substances containing deposition elements on the surface of materials by physical or chemical methods to form thin films.

According to different principles of deposition process, vapor deposition technology can be divided into physical vapor deposition (PVD) and chemical vapor deposition (CVD).

Physical vapor deposition (PVD)

Physical vapor deposition refers to the technology of vaporizing materials into atoms, molecules or ions by physical methods under vacuum, and depositing a thin film on the surface of materials through vapor process.

Physical deposition technology mainly includes three basic methods: vacuum evaporation, sputtering and ion plating.

Chemical vapor deposition (CVD)

Chemical vapor deposition is a method of forming metal or compound films on the substrate surface by the interaction between mixed gas and substrate surface at a certain temperature.

Due to its good wear resistance, corrosion resistance, heat resistance, electricity, optics and other special properties, chemical vapor deposition film has been widely used in mechanical manufacturing, aerospace, transportation, coal chemical industry and other industrial fields.

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1 thought on “Metal Surface Treatment 101: Basics You Should Know”

  1. Hi .enjoyed reading your post. I am working on a rusted out vehicle and would like to know your thoughts on getting sheet metal ready for paint. I am currently cleaning as much rust off as I can but what would be the next step to halt further rusting before applying a top coat? Thanks

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