Metal surface treatment is a process of artificially forming a surface layer on the surface of a metal base material that is different from the mechanical, physical and chemical properties of the base. The purpose of surface treatment is to meet the product's corrosion resistance, wear resistance, decoration or other special functional requirements. For metal castings, our more commonly used surface treatment methods are: mechanical polishing, chemical treatment, surface heat treatment, and sprayed surface. The surface pretreatment of metal castings is to clean, sweep, deburr, degrease, and deoxidize the surface of the workpiece.
There are two explanations for surface treatment. One is the generalized surface treatment, which includes many physical and chemical methods including pretreatment, electroplating, painting, chemical oxidation, thermal spraying, etc.; the other is the narrowly defined surface treatment. That is, only processing including sandblasting, polishing, etc., which is what we often call pre-treatment.
| Surface Treatment | Applications |
| Zinc Plating | Alloy steel castings, carbon steel castings, parts made of powder metallurgy |
| Electroless Zinc Coating | Electroless Zinc rich coating on the steel parts |
| Electroless Nickel Plating | Electroless nickel plating on steel, stainless steel, aluminum, and copper parts |
| Tin-Zinc Plating | Tin-zinc plating on the steel parts |
| Chromium Plating | Alloy steel castings, copper based alloy castings |
| Nickel Plating | Electroless nickel plating on steel, stainless steel, and aluminum parts |
| Chrome-Nickel Plating | Brass parts, bronze parts |
| Zinc Nickel Plating | Steel castings, brass castings, bronze casting parts |
| Copper-Nickel-Chromium Plating | Copper-Nickel-Chromium Plating on steel, stainless steel, aluminum parts |
| Copper Plating | Plating on the steel parts |
| Anodizing | Anodizing and hard anodizing on aluminum profile, machining and die cast aluminum parts |
| Painting | Painting and dry film on iron, aluminum, stainless steel, and steel parts |
| Acid Cleaning | Acid cleaning for stainless steel castings, heat treated parts, super alloy, aluminum alloy, and titanium alloy parts |
| Passivation | Passivation of all kinds of stainless steel |
| Phosphating | Zinc and manganese phosphating of normal castings and machining parts |
| Electrophoresis | Electrophoresis on the steel parts |
| Electrolytic Polishing | Electrolytic polishing on the stainless steel parts |
| Wire Drawing | Stainless steel parts by casting, welding and forging |
1. Surface Pretreatment
In the process of processing, transportation, storage, etc., the surface of metal workpieces often has oxide scale, rust molding sand, welding slag, dust, oil and other dirt. For the coating to be firmly attached to the surface of the workpiece, the surface of the workpiece must be cleaned before painting. Otherwise, it will not only affect the bonding force and corrosion resistance of the coating to the metal, but also make the base metal even if it is coated. It can continue to corrode under the protection of the layer, causing the coating to peel off, affecting the mechanical properties and service life of the workpiece. It can be seen that the purpose of surface pretreatment of metal workpieces is to provide a good substrate suitable for coating requirements, obtain a protective layer of good quality, and prolong the service life of the product.
2. Mechanical Treatment
Mainly include wire brush roller polishing, shot blastingand sand peening.
The brush polishing is that the brush roller is driven by the motor, and the brush roller rotates at a high speed on the upper and lower surfaces of the strip in the direction opposite to the movement of the rolling piece to remove the oxide scale. The brushed iron oxide scale is washed away with a closed circulating cooling water washing system.
Shot blasting is a method of using centrifugal force to accelerate the projectile and project it to the workpiece for rust removal and cleaning. However, shot blasting has poor flexibility and is limited by the site. It is a little blind when cleaning the workpiece, and it is easy to produce dead corners on the inner surface of the workpiece that cannot be cleaned. The structure of the equipment is complex, there are many wearing parts, especially the blades and other parts wear quickly, the maintenance man-hours are many, the cost is high, and the one-time investment is large. Using shot blasting for surface treatment, the impact force is large, and the cleaning effect is obvious.
However, the treatment of thin plate workpieces by shot peening can easily deform the workpiece, and the steel shot hits the surface of the workpiece (no matter shot blasting or shot peening) to deform the metal substrate. Because the ferroferric oxide and ferroferric oxide have no plasticity, they will be broken. After peeling off, the oil film deforms together with the material, so the shot blasting and shot blasting can not completely remove the oil stains on the work piece with oil stains. Among the existing surface treatment methods for workpieces, the best cleaning effect is sandblasting. Sandblasting is suitable for cleaning the workpiece surface with higher requirements.
3. Plasma Treatment
Plasma is a collection of positively charged positive particles and negative particles (including positive ions, negative ions, electrons, free radicals and various active groups, etc.). The positive and negative charges are equal. Therefore, it is called plasma, which is the fourth state in which matter exists in addition to solid, liquid, and gaseous states-plasma state. The plasma surface processor is composed of a plasma generator, a gas delivery pipeline and a plasma nozzle. The plasma generator generates high-pressure and high-frequency energy in the nozzle steel pipe to be activated and controlled to generate low-temperature plasma in the glow discharge, with the help of compressed air The plasma is sprayed to the surface of the workpiece.
When the plasma and the surface of the processed object meet, the object changes and chemical reactions occur. The surface has been cleaned, and hydrocarbon contaminants such as grease and auxiliary additives have been removed, or etched and roughened, or formed a dense cross-linked layer, or introduced oxygen-containing polar groups (hydroxyl, carboxyl), these The group has the effect of promoting the adhesion of various coating materials, and has been optimized in adhesion and paint applications. Under the same effect, the application of plasma treatment surface can get a very thin high-tensile coating surface, which is beneficial to bonding, coating and printing. There is no need for other machines, chemical treatments and other strong components to increase adhesion.
4. Electrochemical Method
Electrochemical surface treatment uses the electrode reaction to form a coating on the surface of the workpiece, which mainly includes electroplating and anodic oxidation.
When the workpiece is the cathode in the electrolyte solution. The process of forming a coating on the surface under the action of an external current is called electroplating. The plating layer can be metal, alloy, semiconductor or containing various solid particles, such as copper plating, nickel plating, etc.
While in the electrolyte solution, the workpiece is the anode. The process of forming an oxide film on the surface under the action of external current is called anodization, such as the anodization of aluminum alloy. The oxidation treatment of steel can be done by chemical or electrochemical methods. The chemical method is to put the workpiece in an oxidizing solution, and rely on chemical action to form an oxide film on the surface of the workpiece, such as bluing of steel.
5. Chemical Methods
The surface treatment of chemical method has no current effect, and uses the interaction of chemical substances to form a plating layer on the surface of the workpiece. The main methods are chemical conversion coating treatment and electroless plating.
In the electrolyte solution, the metal workpiece has no external current action, and the chemical substance in the solution interacts with the workpiece to form a coating on its surface, which is called chemical conversion film treatment. Such as bluing, phosphating, passivation, and chromium salt treatment on the metal surface. In the electrolyte solution, the surface of the workpiece is catalytically treated without the effect of external current. In the solution, due to the reduction of chemical substances, the process of depositing certain substances on the surface of the workpiece to form a coating is called electroless plating, such as electroless nickel, Electroless copper plating, etc.
6. Hot Processing Method
The hot processing method is to melt or thermally diffuse the material under high temperature conditions to form a coating on the surface of the workpiece. The main methods are as follows:
1) Hot Dip Plating
The process of putting a metal workpiece into molten metal to form a coating on its surface is called hot-dip plating, such as hot-dip galvanizing and hot-dip aluminum.
2) Thermal Spraying
The process of atomizing the molten metal and spraying it on the surface of the workpiece to form a coating is called thermal spraying, such as thermal spraying zinc and thermal spraying aluminum.
3) Hot Stamping
The process of heating and pressing the metal foil on the surface of the workpiece to form a coating layer is called hot stamping, such as hot stamping aluminum foil.
4) Chemical Heat Treatment
The process in which the workpiece is in contact with chemical substances and heated, and a certain element enters the surface of the workpiece at a high temperature is called chemical heat treatment, such as nitriding and carburizing.
7. Electrophoresis
As an electrode, the workpiece is put into the conductive water-soluble or water-emulsified paint, and forms a circuit with the other electrode in the paint. Under the action of the electric field, the coating solution has been dissociated into charged resin ions, the cations move to the cathode, and the anions move to the anode. These charged resin ions, together with the adsorbed pigment particles, are electrophoresed to the surface of the workpiece to form a coating. This process is called electrophoresis.
8. Electrostatic Spraying
Under the action of a DC high-voltage electric field, the atomized negatively charged paint particles are directed to fly onto the positively-charged workpiece to obtain a paint film, which is called static spraying.
Post time: Sep-12-2021