Surface treatment is a process of artificially forming a layer on the surface of matrix material with different mechanical, physical and chemical properties. The purpose of surface treatment is to meet the requirements of corrosion resistance, wear resistance, decoration or other special functions of products.
For metal castings, our common surface treatment methods are mechanical grinding, chemical treatment, surface heat treatment, spraying surface, surface treatment is to clean, clean, deburr, oil stain, oxide skin and so on.
In the process of processing, transportation, storage and so on, the surface of workpiece often contains oxide scale, residual sand, welding slag, dust, oil and other dirt. In order to firmly adhere to the workpiece surface, the workpiece surface must be cleaned before painting. Otherwise, it will not only affect the adhesion and corrosion resistance of the coating to the metal, but also make the base metal continue to corrode even with the protection of the coating, which will make the coating peel off and affect the mechanical properties and service life of the workpiece. Therefore, the surface treatment before painting is an important guarantee and measure to obtain high quality protective coating and prolong the service life of products.
In order to provide a good workpiece surface, the following points should be paid attention to in surface treatment:
1. No Oil Pollution and Water
2. Rust-free and oxide-free
3. Non-Adhesive Impurities
4. Residues without acid or alkali
5. Surface roughness of workpiece
Surface treatment:
Manual processing:
Such as scrapers, wire brushes or grinding wheels. Rust marks and oxide scales on the surface of workpiece can be removed by hand, but manual treatment has the advantages of high labor intensity, low production efficiency, poor quality and incomplete cleaning.
Chemical treatment:
The main purpose is to use acid-alkaline or alkaline solution to react with oxides and oil stains on the surface of workpiece and dissolve them in acidic or alkaline solution so as to remove the oxide scale and oil stains on the surface of workpiece. Then the brush roll made of nylon or 304# stainless steel wire (steel wire brush roll made of acid-alkali resistant solution) can be cleaned.
Mechanical treatment:
It mainly includes steel wire brush roller polishing method, shot blasting method and shot blasting method. The polishing method is that the brush roll is driven by the motor, and the brush roll rotates at high speed on the upper and lower surfaces of the strip in the direction opposite to the movement of the rolled piece. Scrubbed iron oxide sheet is washed out by closed circulating cooling water flushing system. Shot blasting cleaning is a method that uses centrifugal force to accelerate the projectile and eject it to the workpiece for rust removal.
Shot peening is divided into shot peening and sand blasting.
Surface treatment with shot peening has great impact and obvious cleaning effect. However, shot peening can easily deform the thin plate workpiece, and steel shot peening can strike the workpiece surface (whether shot peening or shot peening) to deform the metal base material. Because Fe304 and FE203 are not plastic, they are broken and peeled off, and the oil film is deformed with the workpiece, so shot peening and shot peening can not completely remove the oil contamination on the workpiece with oil contamination.
According to the different methods used, surface treatment technology can be divided into the following categories.
I. ELECTROCHEMICAL METHODS
This method is to use the electrode reaction to form a coating on the surface of the workpiece. The main methods are:
(1) Electroplating
In electrolyte solution, the workpiece is a cathode, under the action of external current, the process of forming coating on its surface is called electroplating. The coating can be metal, alloy, semiconductor or containing various solid particles, such as copper plating, nickel plating, etc.
(2) Oxidation
In electrolyte solution, the workpiece is an anode. Under the action of external current, the process of forming oxide film on its surface is called anodic oxidation, such as the anodic oxidation of aluminium alloy. Iron and steel can be oxidized by chemical or electrochemical methods. The chemical method is to put the workpiece into the oxidation solution and form oxide film on the workpiece surface by chemical action, such as bluing treatment of steel.
II. Chemical Methods
This method is to form a coating on the surface of the workpiece by the interaction of chemical substances without current action. The main methods are:
(1) Chemical conversion membrane treatment
In electrolyte solution, metal workpiece has no external current action, and the process of forming coating on its surface by the interaction of chemical substances and workpiece in solution is called chemical conversion film treatment. Such as metal surface bluing, phosphating, passivation, chromium salt treatment, etc.
(2) Electroless plating
In electrolyte solution, the workpiece surface is catalyzed without external current effect. In solution, due to the reduction of chemical substances, some substances are deposited on the workpiece surface to form a coating, which is called electroless plating, such as electroless nickel plating, electroless copper plating and so on.
THREE. HOT WORKING METHODS
This method is to melt or diffuse the material at high temperature to form a coating on the surface of the workpiece. Its main methods are:
(1) Hot dip plating
The process by which metal parts are put into molten metals to form coatings on their surfaces is called hot dipping, such as hot galvanizing and hot aluminizing.
(2) Thermal spraying
The process of atomizing molten metal and spraying it on the surface of workpiece to form coating is called thermal spraying, such as thermal spraying zinc, thermal spraying aluminium, etc.
(3) Hot stamping
The process of covering metal foil with temperature and pressure on the surface of workpiece to form coating layer is called hot stamping, such as hot stamping of aluminium foil.
(4) Chemical heat treatment
The process of contacting and heating a workpiece with a chemical substance and ordering an element to enter the workpiece surface at high temperature is called chemical transformation.
