“Surface pretreatment” refers to the mechanical, chemical or electrochemical treatment of materials and their products before they are subjected to surface processing. This process is done to purify, roughen or passivate the surface, leaving it ready for further treatment or surface adjustment.
Pretreatment of metal surfaces includes the following methods:
- Surface leveling, which covers both mechanical leveling and mechanical polishing.
- Engraving, which can be done through chemical attack or electrochemical attack.
- Surface degreasing, which can be achieved through organic solvent degreasing, chemical degreasing or electrochemical degreasing.
Surface leveling
Surface leveling encompasses a variety of methods, including: mechanical polishing, chemical polishing, electrolytic polishing, rolling, brushing, sandblasting and others.
The specific surface treatment process used depends on the condition of the parts and the technical requirements of the work.
1. Mechanical polishing
The main purpose of polishing is to make the rough and uneven surface of metal parts smooth and flat. In addition, it can also remove burrs, scale, rust, sand holes, grooves, bubbles and other surface imperfections on metal parts.
Polishing is carried out using an elastic grinding wheel attached to a grinder. The working surface of the grinding wheel is covered with abrasive particles, which act as small cutting edges. When the grinding wheel rotates at high speed, the surface of the metal workpiece is gently pressed against the working surface of the grinding wheel, causing the raised parts of the surface to be cut and become smooth and flat.
Polishing can be used on all metallic materials and its effectiveness depends on the characteristics of the abrasive, the stiffness of the grinding wheel and the rotation speed of the grinding wheel. Abrasives commonly used for polishing include artificial corundum and emery. Artificial corundum, which is composed of 90-95% alumina and has a certain level of toughness, is widely used due to its lower fragility and the greater number of edges and corners of its particles.
Figure 1 polishing machine
Figure 2Al 2 Ó 3 abrasive (400X)
Abrasives can be classified into different grades based on particle size. The particle size of abrasives is generally determined by the number of holes per unit area (square centimeter) on a sieve. The greater the number of sieves, the smaller the holes. Particle size is represented by the amount of abrasives that can pass through the sieve. The more abrasives passed through, the finer the particle size, and the fewer abrasives, the coarser the particle size.
Table 1 highlights the characteristics and uses of common abrasives. Table 2 lists the most suitable grinding wheel speeds for polishing different metallic materials.
Table 1 Characteristics and uses of common abrasives
| Abrasive name | Artificial emery (SiC) | Artificial corundum (A2O3) | Natural emery | Silica sand (SiO 2 ) |
| Mineral hardness/Mohs hardness | 9.2 | 9 | 7~8 | 7 |
| Toughness | Fragile | Relatively difficult | Tenacity | Tenacity |
| form | Sharp | Rounder | cylinder | Rounder |
| Particle size/mm (mesh) | 0.045 ~ 0.800 (24 ~ 320) | 0.053 ~ 0.800 (24 ~ 280) | 0.063 ~ 0.800 (24 ~ 240) | 0.045 ~ 0.800 (24 ~ 320) |
| Appearance | Purple black shiny crystal | White to black gray grain | Grayish red to black sand | White to yellow sand |
| Purpose | It is mainly used for polishing low-strength metals (such as brass, bronze, aluminum, etc.) and hard and brittle metals (such as cast iron, carbon tool steel, high-strength steel) | It is mainly used to polish high-strength metals with certain toughness (such as hardened steel, malleable cast iron and manganese green steel) | Used to polish metals in general | General purpose grinding and polishing materials, also used for blasting and rolling |
Table 2 Ideal grinding wheel speed for polishing different metallic materials
| Material type | Steel, nickel, chrome | Copper and copper, silver and zinc alloys | Aluminum and aluminum alloy, lead and tin | ||
| Abrasive linear speed / (M/s) | 18~30 | 14~18 | 10~14 | ||
| Suitable speed/(R/min) | Grinding wheel diameter / mm | 200 | 2850 | 2400 | 1900 |
| 250 | 2300 | 1900 | 1530 | ||
| 300 | 1880 | 1500 | 1530 | ||
| 350 | 1620 | 1530 | 1090 | ||
| 400 | 1440 | 1190 | 960 | ||
2. Polishing
2.1M mechanical polishing
Polishing is a type of grinding process. Some believe it works by “tearing” atoms from the surface layer of the workpiece, causing the bottom layer to instantly become smooth due to the action of surface tension before it solidifies. Others consider polishing to be a result of surface tension. During the polishing process, the heat generated by friction can soften or even melt the surface, making it more than a simple mechanical polishing process.
During polishing, the surface layer of the metal is melted, but it quickly solidifies into an amorphous state due to the high thermal conductivity of the substrate metal. Before solidifying, the surface becomes smooth as a result of the combined effects of surface tension and friction from the polishing agent.
Parts that require a high degree of finishing must undergo fine polishing after the initial polishing process.
Mechanical polishing is performed by using a polishing agent on the polishing wheel of a polishing machine. The polishing agent can be a polishing paste or a polishing fluid. The first is a mixture of polishing abrasive and an adhesive, such as stearic acid or paraffin. The latter is a mixture of abrasive and an oil or water emulsion.
As the polishing wheel rotates at high speed, it removes small irregularities from the workpiece and gives it a mirror-like shine. Mechanical polishing is used not only for surface pretreatment before coating, but also for finishing coating after coating to improve surface finish.
It is important to note that mechanical polishing is different from buffing. During polishing, obvious metal shavings are cut away, while mechanical polishing does not result in significant metal loss. The high temperature generated by friction between the high-speed rotating polishing disc and the part causes plastic deformation of the metal surface, filling in slight irregularities.
Furthermore, repeated removal of the extremely thin oxide film or other composite film formed on the metal surface under the influence of the surrounding atmosphere results in a flat, shiny surface.
2.2 C hemic polishing
Chemical polishing is a type of controlled chemical etching. It is a metal processing method that involves etching the metal in a specific polishing solution, making the surface flat and shiny by selectively dissolving the metal.
Compared with other polishing techniques, chemical polishing has the advantages of simple equipment, low cost, easy operation, high efficiency and not being affected by the shape and structure of parts. Furthermore, compared to electrolytic polishing, chemical polishing does not require a power source and can be used to process parts with complex shapes. However, the production efficiency is higher, but the surface processing quality is lower than that of electropolishing.
Chemical polishing is a type of electrochemical process similar to electrolytic polishing. The chemical polishing reaction belongs to the attack of a micro battery in the electrochemical process. Therefore, the principle of chemical polishing is similar to that of electrolytic polishing.
During the chemical dissolution process, an oxide film is generated on the metal surface, which regulates the diffusion speed in the continuous dissolution process. The convex parts of the surface dissolve more quickly due to the thin thickness of the oxide film, while the concave parts dissolve more slowly.
A passive oxide film and an oxide film are constantly formed on the surface of steel parts, the former being stronger than the latter. Due to the micro-irregularity of the surface, the micro-convex parts dissolve first, at a faster rate than the concave parts. Film dissolution and film formation occur simultaneously, but at different rates.
As a result, the surface roughness of the steel part is evened out, resulting in a smooth and shiny surface. Chemical polishing can effectively remove the surface damage layer caused by mechanical polishing, as it has a strong dissolving effect on the surface.
2.3 And electrolytic polishing
Electropolishing involves placing the part as an anode and conducting electrolysis in a specific solution. During the process, the microprotruding parts on the surface of the part have high current density and dissolve quickly, while the current density in the microrecesses is low, causing dissolution to be slow. This results in a flat, shiny surface.
Electropolishing is commonly used for decorative finishing of parts such as carbon steel, stainless steel, aluminum and copper, as well as surface finishing of some tools or for creating highly reflective surfaces and metallographic samples.
Phosphoric acid chromic anhydride polishing solution is widely used for iron and steel materials and contains components such as phosphoric acid, sulfuric acid, chromic anhydride and additives such as corrosion inhibitors, brighteners and thickeners. The cathode is typically made of lead and the power supply voltage can be 12V.
In recent years, with the increasing demand for stainless steel products, the demand for electrolytic polishing solutions has also increased. To prevent environmental pollution caused by electropolishing solutions containing phosphoric acid and chromic anhydride, China has made efforts to develop environmentally friendly stainless steel electrolytic polishing solutions and has achieved significant progress.
Table 3 presents the solution composition and processing conditions of several new stainless steel electrolytic polishing solutions. Formulas 1 and 2 in the table do not use chromic anhydride, which solves the problem of wastewater disposal and provides a pollution-free and environmentally friendly electrochemical polishing agent.
Table 3 Solution composition and process conditions of environmentally friendly stainless steel electropolishing solution
| Solution composition and process conditions | Recipe 1 | Recipe 2 | Recipe 3 |
| Phosphoric acid (H 3 PO 4 .85%) /% Sulfuric acid (H 2 SO 4 .98%) / % Nitric acid (HNO 3 ) /% perchloric acid /% Glacial acetic water (H 2 O) additive | 40~50 15~20 allowance Adequate dextrin |
20~30 20~30 allowance Adequate amount of glycerol |
10~15 8~10 allowance Small amount of additives |
| Temperature / ℃ Current density / (A / dm 2 )Time/minute | 60~70 20~30 3~5 |
65~70 15~30 3~8 |
High temperature 10~30 3~5 |
Compared to mechanical polishing, electropolishing flattens the polished surface through electrochemical dissolution, leaving no deformation layer on the surface and preventing the mixing of foreign substances. Furthermore, the electrolysis process results in the precipitation of oxygen, forming an oxide film on the polished surface, which improves its corrosion resistance.
Electropolishing is also useful for parts with complex shapes, wires, thin plates and small parts, which are difficult to polish mechanically. In addition to leveling, electropolishing can also remove surface inclusions and reveal defects such as cracks, sand holes and inclusions on the surface of parts.
3. Rolling
Lamination is a common method used for surface preparation before coating or surface modification after coating for large quantities of small parts. Roll polishing is a process in which parts and abrasives are placed together in a drum or bell machine for rolling grinding to remove burrs, roughness and rust from the surface of parts and obtain a smooth surface.
In addition to abrasives, chemical reagents such as acids or alkalis are often added during the rolling process. Thus, the rolling process serves to remove burrs, roughness and rust as the parts and abrasives roll together, as well as the role of chemical reagents.
Fig. 3 is the schematic diagram of the calender.
Fig.3 Schematic diagram of the calendar
Roller polishing can eliminate oil stains and oxide scale on the surface of parts and produce a shiny surface. It can partially or completely replace polishing and polishing, but it is only suitable for large quantities of parts with low surface roughness requirements.
Lamination can be divided into dry method and wet method. The dry method uses abrasives such as sand, emery, broken glass and leather, while the wet method uses steel balls, crushed stones, sawdust, caustic soda, tea powder, etc.
The rotational speed during rolling depends on the characteristics of the parts and the drum structure, typically ranging from 15 to 50 RPM. If the speed is too high, centrifugal force will prevent the pieces from rubbing against each other in the drum, reducing the effectiveness of the lamination. On the other hand, if the speed is too low, the efficiency will be low.
When there is a large amount of oil stains or rust on the surface of parts during rolling, degreasing and etching should be carried out first. If there is a small amount of oil stain, a small amount of alkaline or emulsifying substances such as soda ash, soap or washing powder can be added to curl. Dilute sulfuric acid or hydrochloric acid can be added to parts with rusted surfaces. After the parts are rolled in an acidic medium, the acidic solution must be rinsed off immediately.
4. Brushing
Brushing is a surface processing technique that uses a brushing wheel made from materials such as metal wire, animal hair, or natural or synthetic fibers. This method is mainly used to remove surface contaminants such as oxidation, rust, welding slag, old paint and other debris. Additionally, brushing is also used to remove burrs left on the edges of a part after machining.
The most commonly used brush wheels are made of steel wire or brass wire. If the workpiece material is hard, a high rigidity steel brush wheel should be used in conjunction with a high speed. On the other hand, for softer materials, a brass wire brush wheel is recommended.
Brushing can be done mechanically or manually. Both methods typically employ the use of a wet technique, with water being the most commonly used brushing solution. In some cases, a 3% to 5% (by mass) solution of soda ash or sodium phosphate may also be used when brushing steel materials.
5. Sandblasting
Sand blasting is a process that uses compressed air to blow dry sand, such as quartz sand, steel sand, or alumina, onto the surface of metal parts to remove surface defects such as burrs, scale, rust, carbon deposits, slag. welding, molding sand residues, salt residues, old paint films and dirt.
This method is commonly used to clean the surface of workpieces, such as removing residual sand and high carbon layers on castings and eliminating rust and scale on welds.
Sand blasting and acid washing are techniques used to remove rust. However, although acid washing can cause hydrogen to penetrate the interior of steel parts, increasing internal stress and reducing plasticity, sandblasting does not result in hydrogen embrittlement.
After sandblasting, parts made of high-carbon steel, high-strength steel, or materials such as brass, stainless steel, and aluminum may have better adhesion of coatings or oxide layers. Hard chrome-plated and coated workpieces are typically sandblasted. Machine tool accessories and measuring tools are often sandblasted before milky white chrome plating.
Sand blasting is an effective method for surface pretreatment. It can completely remove impurities such as oxide scale, rust, old paint films and oil stains from metal surfaces, resulting in uniform metallic color and uniform surface roughness. This roughness can improve the bonding strength between the anti-corrosion coating and the base metal and increase the corrosion resistance of the metal.
Sand blasting is commonly used in thermal spray coatings and plastic roughening treatments. Other surface roughening techniques include threading, knurling, electric spark roughening, and more.
There are two types of sand blasting: dry blasting and wet blasting. Wet blasting uses abrasives mixed with water to form a grout, and a corrosion inhibitor is typically added to the water to prevent the metal from rusting. Dry blasting is efficient, but it results in a rough surface, generates a large amount of dust and makes the abrasive break down more easily. On the other hand, wet blasting has minimal environmental impact, can have a decorative and protective effect on the surface, and is often used for more precise processing.
Engraving
Etching is a process used to remove rust, oxide scale (formed during casting, forging, rolling, and heat treating), and other corrosion products from the surface of a workpiece. This is achieved through the use of acidic solutions, which have a strong ability to dissolve metal oxides. As a result, etching is also called pickling.
For some non-ferrous metals, alkali etching can be used. Removing a large amount of oxides and poor surface structure is known as hard etching, while removing a thin film of oxide on the surface of the part to prepare it for electroplating is called weak etching.
Inorganic acids such as sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid and hydrofluoric acid are typically used for pickling steel. Organic acids such as acetic acid, fatty acid and citric acid can also be used. The action of organic acids is mild and the residual acid does not have significant side effects. Furthermore, the surface of the workpiece is clean after treatment and is less likely to rust again.
Organic acids, although they have the advantage of not causing significant consequences, have a high cost and low efficiency in removing rust, which is why they are mainly used to clean rust incrustations inside electrical equipment containers and other components with special requirements.
Inorganic acids, on the other hand, feature high rust removal efficiency, high speed, wide variety of raw material sources, and low cost. However, if the concentration of inorganic acids is not properly controlled, the metal can become “over-corrosive” and the residual acid is highly corrosive. If the acid solution is not completely cleaned, it will affect the coating effect.
To retard corrosion and hydrogen embrittlement of metals, an adequate amount of buffers such as rutin, urotropin and thiourea should be added to the rust removal solution.
1. Pickling of steel products
(1) Pickling principle
The purpose of the acid in pickling is to dissolve and mechanically remove oxides from the surface of the part. Using sulfuric acid as an example, sulfuric acid reacts with iron oxides (FeO, Fe3O4) to form ferrous sulfate and ferric sulfate.
Sulfuric acid reacts with iron in the matrix through the gaps in the oxide scale, causing the iron to dissolve and hydrogen to be released. The reaction between sulfuric acid and iron in the matrix accelerates the rate of chemical dissolution, reducing low-solubility iron sulfate to high-solubility ferrous sulfate. The hydrogen produced under the oxide scale also creates mechanical cracking and stripping effects on the oxide scale, improving pickling efficiency.
However, the reaction between sulfuric acid and matrix iron can result in excessive matrix corrosion and changes in part size. These are the disadvantages of using sulfuric acid in the pickling process.
Hydrogen evolution during the pickling process can also result in hydrogen permeation into the part, causing hydrogen embrittlement.
Hydrochloric acid mainly dissolves oxides. It reacts with iron oxide to form ferrous chloride and ferric chloride, both with high solubility. As a result, the mechanical removal effect of hydrochloric acid etching is less pronounced than that of sulfuric acid.
For loose oxide scale, hydrochloric acid etching is rapid and there is less matrix corrosion and hydrogen permeation. However, for compact oxide scale, a large amount of acid is consumed when using only hydrochloric acid. A mixed acid solution of hydrochloric acid and sulfuric acid is often used to achieve the effect of mechanical hydrogen removal.
Nitric acid is mainly used for the treatment of high-alloy steel and is often mixed with hydrochloric acid for the treatment of non-ferrous metals. Nitric acid has a strong ability to dissolve iron oxides, and the solubility of ferrous nitrate and ferric nitrate is high, with minimal hydrogen evolution reaction.
When used on stainless steel, nitric acid does not cause matrix corrosion due to its passivating properties. However, when used on carbon steel, the issue of matrix corrosion must be addressed.
Hydrofluoric acid is primarily used to remove silicon-containing substances such as alloying elements in certain types of stainless steel and alloy steels, mixed welding slag in welds, and residual molding sand in castings.
The combination of hydrofluoric and nitric acids is often used to treat stainless steel, but hydrofluoric acid is extremely corrosive and must be handled with care.
Nitric acid releases toxic nitrides and can be difficult to dispose of, so extra care must be taken to avoid harm to the human body.
Phosphoric acid has good solubility for iron oxide and is less harmful to metal because it forms a water-insoluble phosphate layer (phosphating film) on the metal surface, which helps prevent corrosion.
Plus, it's an excellent base coat before painting. It is commonly used to remove rust from precision parts, but the cost of phosphoric acid is relatively high.
When phosphoric acid is used for rust removal, the main function is to transform oxide scale and rust into water-soluble Fe (H 2 PO 4 ) 3 and water-insoluble FeHPO 4 and Fe 3 (PO 4 ) 2 .
Hydrogen diffusion is a weak process.
When using phosphoric acid for pickling, the amount of hydrogen produced is approximately 1/10 to 1/5 of that produced by pickling with hydrochloric or sulfuric acid. Furthermore, the diffusion and penetration rate of hydrogen is half that of the latter two acids.
Stainless steel and alloy steel have a complex composition and dense structure for their oxide scale, making it difficult to remove in common carbon steel rust removal solution. A mixture of acids is typically used for this purpose.
When pickling steel alloys containing titanium, it is necessary to add hydrofluoric acid.
The thick, dense oxide scale formed from heat treatment can be “loosened” in a hot, concentrated alkaline solution containing a strong oxidant and then attacked using a mixture of hydrochloric and nitric acids, or sulfuric and nitric acids.
(2) Pickling additive
It is crucial to use a corrosion inhibitor in the pickling solution. The general belief is that a corrosion inhibitor can form an adsorption film or an insoluble protective film on the surface of the base metal in an acidic solution.
The formation of this film occurs through an electrochemical reaction when metallic iron comes into contact with acid, which charges the metallic surface. The corrosion inhibitor, being a polar molecule, is attracted to the metal surface and forms a protective film, thus preventing the continuous action of the acid on the iron and achieving the objective of inhibiting corrosion.
From an electrochemical point of view, the protective film formed not only significantly blocks the anodic polarization process, but also promotes cathodic polarization, inhibits hydrogen production and slows down the corrosion process.
Oxide scale and rust do not adsorb the polar corrosion inhibitor molecules to form a film because they interact with the acid through ordinary chemical action and do not have any charge on their surfaces.
Therefore, adding a specific amount of corrosion inhibitor to the rust removal solution does not affect its rust removal efficiency.
To evaluate the effectiveness of various corrosion inhibitors, it is crucial to determine their corrosion inhibition efficiency.
The corrosion inhibition efficiency can be determined by comparing the weight loss (g/(m2·h)) of a sample with and without the corrosion inhibitor in the same medium and under the same conditions.
The specified amount of different corrosion inhibitors used in various acidic solutions varies.
As the temperature of the acid washing solution increases, the inhibition efficiency of the corrosion inhibitor will decrease or even fail completely.
Therefore, each corrosion inhibitor has a specific allowable operating temperature.
The wetting agents used in pickling solutions are mainly non-ionic and anionic surfactants, with cationic surfactants being rarely used. This is because nonionic surfactants are stable in strong acidic media, and the only acceptable anionic surfactant is the sulfonic acid type.
The use of surfactants with wetting, penetrating, emulsifying, dispersing, solubilizing and decontaminating properties can greatly improve the pickling process and reduce pickling time.
To minimize matrix corrosion loss, reduce the impact of hydrogen permeation, decrease acid fog and improve the working environment, it is advisable to add an efficient corrosion and fog inhibitor to the pickling solution.
However, it is important to note that the corrosion inhibitor can form a film on the surface of the part, which must be thoroughly cleaned. Furthermore, the corrosion inhibitor can reduce the mechanical removal effect of the hydrogen evolution reaction.
(3) Selection of acid type, concentration and temperature for pickling
The method of cleaning the surface of a workpiece depends on the material of the workpiece, the presence of rust and oxide scale, and the desired level of surface cleaning quality.
For steel parts, sulfuric acid, hydrochloric acid, or a combination of the two is commonly used.
To dissolve silicon-containing compounds on the surface of castings, hydrofluoric acid is added to sulfuric acid or hydrochloric acid.
The concentration of sulfuric acid is normally around 20%. At this concentration, the rate of attack of the oxide scale is rapid and damage to the underlying material is minimal.
The concentration of hydrochloric acid is usually less than 15%, as it produces vapors when the concentration exceeds 20%.
As the concentration of hydrochloric acid increases, the pickling speed accelerates and the pickling time decreases.
Table 4 illustrates the relationship between pickling time and acid concentration for steel parts with the same degree of corrosion in hydrochloric acid and sulfuric acid.
Table 4 Relationship between hydrochloric acid concentration and sulfuric acid pickling time of iron and steel
| Hydrochloric acid content /% | two | 5 | 10 | 15 |
| Pickling time/min | 90 | 55 | 18 | 15 |
| Sulfuric acid content /% | two | 5 | 10 | 15 |
| Pickling time/min | 135 | 135 | 120 | 95 |
| Hydrochloric acid content /% | 20 | 25 | 30 | 40 |
| Pickling time/min | 10 | 9 | / | / |
| Sulfuric acid content /% | 20 | 25 | 30 | 40 |
| Pickling time/min | 80 | 65 | 75 | 95 |
As the temperature increases, the pickling speed also increases and the time required is reduced.
Table 5 shows the relationship between pickling time and temperature for steel parts with the same level of corrosion in hydrochloric acid and sulfuric acid.
Table 5 Relationship between pickling time and temperature
| Acid content/% | Sulfuric acid pickling time/min | Hydrochloric acid pickling time/min | ||||
| 18℃ | 40°C | 60°C | 18℃ | 40°C | 60°C | |
| 5 | 135 | 45 | 13 | 55 | 15 | 5 |
| 10 | 120 | 32 | 8 | 18 | 6 | two |
(4) Pickling process of iron and steel parts
Pickling and rust removal methods include dip pickling, spray pickling, and acid paste rust removal.
After undergoing degreasing treatment, the impregnated and pickled metal is placed in an acid tank.
Once the oxide scale and rust have been removed, the metal is rinsed with water and neutralized with an alkali to produce a surface suitable for painting.
Table 6 provides information on the etching process parameters for steel parts.
Table 6 parameters of the strong engraving process of steel parts
| Project | Forged and stamped parts | Steel parts in general | Foundry | ||
| 1 | two | 1 | two | ||
| Concentrated sulfuric acid / (g/L.) hydrochloric acid / (g/L) hydrofluoric acid / (g/L) Rodin / (g / L) Urotropin / (g/L) |
200~250 2~3 |
150~200 1~3 |
150~200 1~3 |
80~150 | 100 10~20 |
| Temperature / ℃ Time/minute |
40~60 until everything is divided | 30~40 until everything is divided | 1.5 | Up to 40~50 is divided | 30~40 until everything is divided |
2. Electrochemical etching
Electrochemical etching involves using electrolysis to remove the surface of a workpiece, which serves as the anode or cathode, in an acidic or alkaline solution. The process can also be accelerated by stirring the solution, which generates hydrogen at the cathode and renews the etching solution on the surface of the part.
Electrochemical rust removal can be classified into anodic attack and cathodic attack, depending on the polarity of the part.
During anodic etching, oxide scale is removed through a combination of chemical and electrochemical dissolution of the workpiece metal and mechanical removal of oxygen.
In cathodic etching, oxide scale is removed mainly through the mechanical effect of the large amount of hydrogen generated and the reduction effect of primary atomic hydrogen on the oxide.
Anodic etching results in large, few oxygen bubbles with limited mechanical stripping effect, but if it takes too long, it can cause excessive corrosion of the underlying metal.
On the other hand, cathodic attack minimizes metal corrosion, preserving the size of the part, but can lead to hydrogen permeation and ash residue.
Anodic corrosion is slow and corrosive to the base metal, making it only suitable for workpieces with a thin oxide layer. However, it does not cause hydrogen embrittlement.
On the other hand, cathodic attack is rapid and does not result in excessive corrosion of the part, making it suitable for parts with thick oxide films. However, it has the disadvantage of hydrogen permeation.
Currently, most of the methods used in China are anodic recording or a combination of cathodic and anodic recording. Electrochemical etching is used for strong and weak etching.
Compared to chemical etching, electrochemical etching is more effective in quickly removing oxide scale firmly attached to the metal surface. It is also less affected by changes in acid concentration and has little impact on the underlying material.
This method is easy to operate and manage, but requires specialized equipment and requires more suspension operations. There is also a risk of uneven dissolution of the oxide scale.
The advantages of electrochemical etching include fast etching speed, low acid consumption and little influence of the iron ion content in the solution on the etching ability.
However, this method requires power equipment and consumes energy.
Workpieces with complex shapes are difficult to engrave due to poor dispersion ability.
When the oxide scale is thick and dense, it must be pretreated with chemical etching with sulfuric acid to loosen the oxide scale before being subjected to electrochemical attack.
Surface degreasing
1. Organic solvent degreaser
Organic solvent degreasing is a common method for removing grease from metal materials. It works by using the physical dissolving properties of organic solvents in both types of oils.
Gasoline and kerosene are commonly used solvents, but chlorobenzene and kerosene are more affordable and less toxic alternatives.
Organic solvent degreasing is characterized by its heatless process, fast degreasing speed and absence of corrosion on the metal surface. It is particularly suitable for removing mineral oils with high viscosity and high melting points, which are difficult to remove with alkaline solutions.
Therefore, it is a suitable pretreatment for almost all surface treatment technologies, especially for parts with severe oil pollution or metal parts that are susceptible to corrosion by alkaline degreasing solutions.
However, this method is not comprehensive and chemical and electrochemical methods may be required to complement the degreasing process. Furthermore, most organic solvents are flammable and toxic, and the cost can be high.
It is important to prioritize safety, take precautions and maintain good ventilation during operation.
2. Alkaline solution chemical degreasing
Currently, chemical degreasing with alkaline solution is widely used in production.
Although the oil removal time for this method is longer than that of organic solvents, it has the advantages of being non-toxic, non-flammable, requiring simple equipment, and being inexpensive and easy to operate, making it a reasonable choice for oil removal. of Oil. .
The core of this method is to remove oil through saponification and emulsification. The first removes animal and vegetable oils, while the second removes mineral oils.
With proper process selection, removing both types of grease is not difficult.
However, when there are high requirements for the bond strength of the coating, relying solely on an alkaline solution for chemical removal of oil from coated parts may not be sufficient.
This is particularly true when the oil stain is mainly mineral oil, as it takes a long time to remove and may not be completely removed due to the limited emulsification effect of the alkaline oil removing solution.
In these cases, it is necessary to use electrochemical (electrolytic) oil removal with stronger emulsification to obtain satisfactory results.
3. Electrochemical oil removal
Electrochemical oil removal, also known as electrolytic oil removal, is an oil removal process by placing metal parts in an oil removal liquid and using the parts as an anode or cathode while connected to a direct current.
The composition of the electrochemical degreasing solution is similar to that of chemical degreasing solutions.
A nickel plate or nickel-plated iron plate is commonly used as a counter electrode, which only serves as a conductor.
Production experience has shown that electrochemical oil removal is several times faster than chemical oil removal and effectively removes oil pollution. This is due to the electrochemical oil removal mechanism.
New surface pretreatment technology
1. Ultrasonic strengthening
Ultrasonic cleaning uses a high-frequency oscillation signal that is converted into high-frequency mechanical oscillation by a transducer.
The ultrasonic wave can propagate effectively in different media, including gas, liquid, solid, solid solution, and can transmit strong energy. The ultrasonic wave is transmitted to the tank cleaning liquid through the tank wall and causes the microbubbles in the liquid to vibrate due to reflection, interference and resonance.
Ultrasonic waves create strong impacts and cavitation at the interface, which is the basis of ultrasonic cleaning. The effectiveness of ultrasonic cleaning depends on several factors, including the type of cleaning fluid, cleaning method, cleaning temperature and time, ultrasonic frequency, power density, and complexity of the parts to be cleaned.
Common liquids used for ultrasonic cleaning include organic solvents, alkaline solutions, and water-based cleaning solutions.
The most commonly used ultrasonic cleaning and degreasing device consists of an ultrasonic transducer, cleaning tank and generator. It may also include additional components for circulation cleaning, filtration, heating and fluid transport.
Ultrasonic cleaning is a popular method due to its simplicity, fast cleaning speed and good results.
2. Low temperature oil removal and high efficiency cleaning agent
Using a low-temperature, high-efficiency cleaning agent to remove oil stains on metal surfaces is not only highly effective, but also energy-efficient due to its low cleaning temperature.
3. Vacuum degreasing cleaning
Vacuum degreasing cleaning is a new and environmentally friendly cleaning technology. It uses hydrogen carbide cleaning agent, which has minimal impact on human health, is less irritating and has no odor.
This technology provides the same level of cleaning as triethanolamine and is even more effective than alkaline liquor. Furthermore, the cleaning agent can be recovered and regenerated.
The vacuum degreasing cleaning device is a closed system, free from pollution, has a high safety factor, is highly productive and allows automatic loading and unloading of materials, making it easy to operate.
In the future, vacuum degreasing technology, with or without liquid cleaning, is expected to be widely used.
4. Spray plastic shot to remove paint (coating layer)
When performing non-destructive surface testing on large, important components, such as aircraft, to detect fatigue cracks and hard damage, the surface coating (paint) must be removed first.
Traditional coating removal methods include chemical stripping or manual grinding with a grinding wheel, but both methods have disadvantages. Chemical pickling can corrode and damage the metal matrix, while grinding with a grinding wheel can easily damage the substrate and has low efficiency.
Recently, a new paint removal process using plastic spraying was developed and has shown good results. This process involves spraying granular plastic onto the surface of the part at high speed using a spray gun powered by compressed air.
The paint layer is removed by the sharp edges and corners of the plastic grit, cutting and impacting the surface. This provides an efficient way to remove paint.
Removing paint from plastic grit has several advantages, such as not damaging the substrate or coating due to the plastic grit having a hardness that is greater than the paint layer, but less than the substrate or coating and the anodized surface layer. This also provides a clean surface for the new coat of paint, improving its adhesion. Furthermore, plastic pellets can be recycled and easily separated from the peeling paint layer.
5. Supersonic sand blasting with air flame and blasting
Ultrasonic sandblasting is a process of thickening the surface of a substrate by using compressed air to spray hard sand particles at high speed onto the surface, resulting in a mechanical cleaning effect. The speed of ultrasonic sand blasting is 300 to 600 meters per second and is more efficient than traditional sand blasting, with a blasting efficiency three to five times higher.
It is commonly used in surface pretreatment of large structural parts, such as surface cleaning before applying surface coating to bridges, ships, boilers and pipelines. In addition, it is often used to thicken surfaces before spraying large parts or equipment with high requirements for spraying effects, and cleaning equipment surfaces with heavy natural pollution such as paint, cement and organic or inorganic scale.
The thickening treatment increases the “anchor hook” effect between the coating and the substrate, reducing the shrinkage stress of the coating and improving the bond strength between the coating and the substrate.
The sand used for sandblasting must have high hardness, density, crush resistance and low dust content. The particle size must be determined based on the required surface roughness. Commonly used sand grains include corundum sand (alumina), silica sand, silicon carbide and emery.
Supersonic surface shot peening is a process in which supersonic projectiles are sprayed onto the surface of the part, causing plastic deformation on the surface and forming a reinforcement layer of a certain thickness.
