3 Types of Metal Surface Pretreatment Technology: The Complete Guide

Surface pretreatment refers to the mechanical, chemical or electrochemical treatment of materials and their products before surface processing, so that the surface is purified, roughened or passivated for subsequent surface treatment, also known as surface preparation or surface adjustment.

Metal surface pretreatment includes the following types:

① Surface leveling, including mechanical leveling and mechanical polishing;

② Etching, including chemical etching and electrochemical etching;

③ Surface degreasing, including organic solvent degreasing, chemical degreasing and electrochemical degreasing.

Surface leveling

Surface leveling mainly includes: mechanical polishing, polishing (mechanical polishing, chemical polishing, electrolytic polishing), rolling, brushing, sand blasting, etc.

Different surface treatment processes are adopted according to the surface conditions of parts and the specific technical requirements of parts.

1. Mechanical polishing

The main purpose of polishing is to make the rough and uneven surface of metal parts flat and smooth;

Secondly, it can also remove burrs, scale, rust, sand holes, grooves, bubbles, etc. on the surface of metal parts.

The polishing is completed by an elastic grinding wheel installed on the grinder (Fig. 1).

The working surface of the grinding wheel is covered with adhesive abrasive, and the abrasive particles are like many small cutting edges.

When the grinding wheel rotates at high speed, the surface of the machined part will be gently pressed against the working surface of the grinding wheel, so that the convex part of the metal part surface will be cut and become flat and smooth.

Polishing is applicable to all metal materials, and its effect mainly depends on the characteristics of the abrasive, the rigidity of the grinding wheel and the rotation speed of the grinding wheel.

The abrasives used for polishing are usually artificial corundum (see Fig. 2, containing 90% ~ 95% alumina) and emery.

Artificial corundum has the certain toughness, less brittleness and more edges and corners of particles, so it is widely used.

Fig. 1 polishing machine

Fig. 2 Al2O3 abrasive (400X)

The abrasive can be divided into several grades according to particle size.

Abrasive particle size is usually divided according to the number of sieves.

The number of sieves is expressed by the number of holes per unit area (square centimeter).

The larger the number of sieves, the smaller the hole.

The particle size of the abrasive is represented by the number of abrasives that can pass through the sieve.

The larger the number of abrasives, the finer the particle, and the smaller the number, the coarser the particle.

Table 1 shows the characteristics and uses of common abrasives.

The most suitable grinding wheel speed for polishing different metal materials is shown in Table 2.

Table 1 Characteristics and uses of common abrasives

Abrasive nameArtificial emery (SiC)Artificial corundum (A2O3)Natural emerySilica sand (SiO2)
Mineral hardness / Mohs hardness9.297~87
ToughnessFragileRelatively toughTenacityTenacity
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)
AppearancePurple black shiny crystalWhite to gray black grainGrayish red to black sandWhite to yellow sand
PurposeIt is mainly used to polish 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 quenched steel, malleable cast iron and manganese green steel)Used for polishing general metalsGeneral purpose grinding and polishing materials, also used for sandblasting and rolling

Table 2 optimum grinding wheel speed for polishing different metal materials

Material type

Steel, nickel, chromium

Copper and copper alloys, silver and zinc

Aluminum and aluminum alloy, lead and tin

Abrasive linear velocity / (M/s)




Suitable speed / (R / min)

Grinding wheel diameter / mm





















2. Polishing

2.1 Mechanical polishing

Polishing is a grinding function.

The polishing agent “tears” (grinds) the atoms from the surface layer of the workpiece, and the lower layer maintains its fluidity in an instant, and becomes smooth due to the action of surface tension before solidification.

Others believe that polishing is a surface tension effect.

In the polishing process, the heat generated by friction can soften or melt the surface, so it is not a simple mechanical polishing.

During polishing, the metal surface layer is melted, but because the substrate metal has high thermal conductivity, the surface layer solidifies rapidly into amorphous state.

Before solidification, it becomes smooth due to the action of surface tension and friction of polishing agent.

The workpieces with high requirements for finish shall be polished after fine polishing.

Mechanical polishing is carried out by using polishing agent on the polishing wheel of the polishing machine.

The polishing agent includes polishing paste and polishing fluid.

The former is the mixture of polishing abrasive and adhesive (stearic acid, paraffin, etc.);

The latter is a mixture of abrasive and oil or water emulsion.

The polishing wheel rotates at high speed to remove the slight unevenness on the workpiece in contact with it and make it have mirror-like luster.

Polishing is used not only for pretreatment before plating, but also for finishing the coating after plating to improve the surface finish.

The polishing process is different from polishing.

Obvious metal chips are cut down during polishing, but not during polishing, so polishing does not cause significant metal loss.

On the one hand, the high temperature generated by the friction between the high-speed rotating polishing wheel and the workpiece causes the plastic deformation of the metal surface, so as to fill the slight unevenness of the metal surface of the workpiece;

On the other hand, the extremely thin oxide film or other compound film formed instantaneously on the metal surface under the oxidation of the surrounding atmosphere is removed repeatedly, so as to obtain a flat and glossy surface.

2.2 Chemical polishing

Chemical polishing is a kind of chemical corrosion under controllable conditions.

It is a metal processing process that carries out chemical etching in a specific polishing solution and makes its surface flat and bright by controlling the selective dissolution of metal.

Compared with other polishing technologies, it has the advantages of simple equipment, low cost, simple operation, high efficiency and not being affected by the shape and structure of parts.

Compared with electrolytic polishing, chemical polishing does not need power supply and can process workpieces with complex shapes.

The production efficiency is higher, but the surface processing quality is lower than that of electrolytic polishing.

Chemical polishing reaction belongs to the electrochemical process of etching micro battery.

Therefore, the principle of chemical polishing is similar to that of electrolytic polishing.

In the process of chemical dissolution, a layer of the oxide film is generated on the metal surface, which controls the diffusion speed in the process of continuous dissolution.

In the convex part of the surface, due to the thin thickness of mucous membrane, the dissolution speed is faster than that of the concave part.

Passive oxide film and oxide film are continuously formed on the surface of steel parts, and the former is stronger than the latter.

Due to the micro inconsistency of the part surface, the micro convex part of the surface is dissolved first, and the dissolution rate is greater than that of the concave part.

Moreover, the dissolution of the film and the formation of the film are always carried out at the same time, but the rate is different.

As a result, the surface roughness of the steel part can be leveled, so as to obtain a smooth and bright surface.

Chemical polishing can effectively remove the surface damage layer caused by mechanical polishing because it has a great dissolving effect on the surface.

2.3 Electrolytic polishing

Electropolishing is placing the workpiece on the anode and conducting electrolysis in a specific solution.

The micro protruding part of the workpiece surface has high current density and rapid dissolution;

The current density at the micro recess is low and the dissolution is slow, so as to achieve the purpose of flatness and brightness.

Electropolishing is often used for decorative finishing of parts such as carbon steel, stainless steel, aluminum and copper or coatings such as copper and nickel and surface finishing of some tools, or for making highly reflective surfaces and metallographic samples.

Phosphoric acid chromic anhydride polishing solution is widely used for iron and steel materials. The main components are phosphoric acid, sulfuric acid and chromic anhydride, in which corrosion inhibitor, brightener, thickener and other additives are also added; The cathode is made of lead, and the power supply voltage can be 12V.

In recent years, with the increasing application scope and output of stainless steel products, the demand for electrolytic polishing solutions is also increasing.

In order to prevent environmental pollution caused by the use of electrolytic polishing solutions containing phosphoric acid and chromic anhydride, China has vigorously developed environmentally friendly stainless steel electrolytic polishing solutions and achieved remarkable results.

Table 3 lists the solution composition and process conditions of several new stainless steel electrolytic polishing solutions.

Formula 1 and Formula 2 in the table do not use chromic anhydride, which solves the problem of wastewater discharge.

It is a new pollution-free and environment-friendly electrochemical polishing agent.

Table 3 solution composition and process conditions of environmentally friendly stainless steel electropolishing solution

Solution composition and process conditionsRecipe 1Recipe 2Recipe 3
Phosphoric acid (H3PO4,85%) /%Sulfuric acid (H2SO4,98%) /%Nitric acid (HNO3) /% perchloric acid /%Glacial acetic acid water (H2O)additive40~50

Proper dextrin

Appropriate amount of glycerol

Small amount of additives
Temperature / ℃Current density / (A / dm2)Time / min60~70
High temperature 

Compared with mechanical polishing, electropolishing flattens the polished surface through electrochemical dissolution, so there is no deformation layer on the surface and foreign substances will not be mixed;

At the same time, due to the oxygen precipitation in the electrolysis process, a layer of oxide film will be formed on the polished surface, which is conducive to improving its corrosion resistance.

In addition, for parts with complex shapes, wires, thin plates and small parts, mechanical polishing is difficult, so electrolytic polishing can be used.

In addition to leveling, electropolishing can also remove surface inclusions, showing defects such as cracks, sand holes and inclusions on the surface of parts.

3. Rolling

Rolling is often used for surface preparation before plating or surface modification after plating of large quantities of small parts.

Rolling polishing is a kind of processing process in which parts and abrasives are put together in a drum machine or bell machine for rolling grinding to remove burrs, roughness and rust products on the surface of parts and make the surface smooth.

In addition to adding abrasives, some chemical reagents such as acid or alkali are often added during rolling.

Therefore, the essence of the rolling process is to remove burrs, roughness and rust when parts and abrasives roll over together, as well as the role of chemical reagents.

Fig. 3 is the schematic diagram of the calender.

Fig. 3 Schematic diagram of calender

Rolling polishing can remove the oil stain and oxide scale on the surface of parts and make the surface of parts shiny.

It can replace polishing and polishing in whole or in part, but it is only applicable to large quantities of parts with low surface roughness requirements.

Rolling light can be divided into the dry method and the wet method.

The dry process uses sand, emery, broken glass and leather as abrasives.

The wet method uses steel balls, crushed stones, sawdust, lye, tea powder, etc. as abrasives.

The rotating speed during rolling depends on the characteristics of the parts and the structure of the drum, which is generally 15 ~ 50R / min.

When the rotating speed is too high, due to the large centrifugal force, the parts can not rub with each other when rotating with the drum, so it can not play the role of rolling light;

When the speed is too low, the efficiency is low.

If there is a large amount of oil stain and rust on the surface of parts during rolling, degreasing and etching shall be carried out first.

When there is less oil stain, a small amount of alkaline substances or emulsifiers such as sodium carbonate, soap and soap pod powder can be added for rolling;

Dilute sulfuric acid or hydrochloric acid can be added when the surface of parts is rusty.

After the parts are rolled in acid medium, the acid solution shall be washed immediately.

4. Brushing

Brushing is a method of processing the workpiece surface with a brushing wheel made of metal wire, animal hair, and natural or man-made fiber.

It is mainly used to remove the oxide scale, rust, welding slag, old paint and other dirt on the workpiece surface;

It is also used to remove the burrs left on the surface edges after machining.

The commonly used brush wheel is generally made of steel wire, brass wire and other materials.

If the material of the parts is hard, the steel wire brush wheel with high rigidity shall be adopted, and a large speed shall be adopted at the same time;

On the contrary, use brass wire brush wheel.

Brushing can be divided into mechanical brushing and manual brushing.

Both of them mostly use wet method.

Generally, water is used as brushing solution, and 3% ~ 5% (mass fraction) sodium carbonate or sodium phosphate solution is also used for brushing of steel materials.

5. Sand blasting

Sand blasting refers to the method of using purified compressed air to strongly spray dry sand (such as quartz sand, steel sand, alumina, etc.) onto the surface of metal workpiece, and grind off the surface defects such as burr, scale, rust, carbon deposit, welding slag, molding sand, residual salt, old paint film, dirt and so on.

Sand blasting is often used for workpiece surface cleaning, such as removing residual sand and high carbon layer on the surface of castings, as well as welds of weldments to eliminate rust and scale.

Sand blasting method and acid washing method are generally used for rust removal.

The latter is easy to make hydrogen penetrate into the interior of steel parts, increase internal stress and reduce plasticity, while sand blasting does not produce hydrogen embrittlement.

Whether high carbon steel, high-strength steel, or elastic parts, brass parts, stainless steel parts and aluminum parts enter the next process after sandblasting, the adhesion of coating or oxide layer can be improved.

Hard chromium plated and coated workpieces are usually cleaned by sand blasting.

Machine tool accessories and measuring tools are usually polished by sand blasting before milky white chromium plating.

Sand blasting is one of the best surface pretreatment methods.

It can not only completely remove the oxide scale, rust, old paint film, oil stain and other impurities on the metal surface, so that the metal surface shows a uniform metal color, but also make the metal surface obtain a certain roughness to obtain a surface with uniform roughness, and change the machining stress into compressive stress, so as to improve the bonding force between the anti-corrosion coating and the base metal and the corrosion resistance of the metal itself.

It is commonly used in coating Thermal spraying and plastic roughening treatment.

In addition to sand blasting, surface roughening treatment methods include threading method, knurling method, electric spark roughening method, etc.

Sand blasting is divided into dry spraying and wet spraying. Wet spraying uses abrasive mixed with water to form mortar.

In order to prevent metal rust, corrosion inhibitor needs to be added to the water.

The dry spraying efficiency is high, but the processing surface is rough, the dust is large, and the abrasive is broken more;

Wet spraying has little environmental pollution and has a certain effect of decoration and protection on the surface.

It is often used in more precise processing.


The purpose of etching is to remove the rust layer, oxide scale (formed during casting, forging, rolling and heat treatment) and other corrosion products on the workpiece surface.

Acid solutions are usually used because they have a strong ability to dissolve metal oxides, so etching is also called pickling.

Some non-ferrous metals adopt alkali etching.

The process of removing a large number of oxides and poor surface structure is called strong etching, while the process of removing thin oxide film on the workpiece surface to obtain an activated surface before electroplating is called weak etching.

The acid used for steel pickling is inorganic acid and organic acid, such as sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, hydrofluoric acid, etc;

Organic acids such as acetic acid, fatty acid, citric acid, etc.

The action of organic acid is mild, the residual acid has no serious aftereffect, it is not easy to re-rust, and the surface of the workpiece is clean after treatment;

However, the cost of organic acid is high and the rust removal efficiency is low, so it is mostly used to clean the rust scale inside the power equipment container and other components with special requirements.

Inorganic acid has the advantages of high rust removal efficiency, fast speed, wide source of raw materials and low price, but the disadvantage is that if the concentration is not controlled properly, the metal will be “over corroded”, and the residual acid is highly corrosive.

The acid solution is not cleaned thoroughly, which will affect the coating effect.

In order to slow down the corrosion and hydrogen embrittlement of metals, an appropriate amount of buffer should be added to the derusting solution.

Such as rutin, urotropine, thiourea, etc.

1. Pickling of steel products

(1) Pickling principle

The role of acid in pickling includes chemical dissolution and mechanical stripping of oxides on the workpiece surface.

Taking sulfuric acid as an example, sulfuric acid reacts with iron oxides (FeO, Fe3O4) to produce ferrous sulfate and ferric sulfate.

Sulfuric acid reacts with matrix iron through the gap of oxide scale, resulting in the dissolution of iron and the precipitation of hydrogen.

The advantage of the reaction between sulfuric acid and matrix iron is that the new atomic hydrogen can reduce the iron sulfate with low solubility to ferrous sulfate with high solubility, so as to speed up the chemical dissolution rate;

The hydrogen generated under the oxide scale can produce mechanical top cracking and stripping effects on the oxide scale, which can improve the pickling efficiency.

The disadvantage is that the reaction between sulfuric acid and matrix iron may cause excessive corrosion of the matrix and change the size of the workpiece;

Hydrogen evolution may also cause hydrogen permeation of the workpiece, resulting in hydrogen embrittlement.

Hydrochloric acid mainly dissolves oxides.

Hydrochloric acid reacts with iron oxide to produce ferrous chloride and ferric chloride, both of which have great solubility, so the mechanical stripping effect of hydrochloric acid etching is smaller than that of sulfuric acid.

For loose oxide scale, hydrochloric acid etching speed is fast, matrix corrosion and hydrogen permeation are less;

However, for the relatively tight oxide scale, the acid consumption is large when pickling with hydrochloric acid alone.

It is best to use the mixed acid-washing solution of hydrochloric acid and sulfuric acid to play the role of mechanical stripping of hydrogen.

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, the solubility of ferrous nitrate and ferric nitrate is also large, and the hydrogen evolution reaction is small.

Nitric acid used in stainless steel will not cause matrix corrosion due to its passivation, but when used in carbon steel, the problem of matrix corrosion must be solved.

Hydrofluoric acid is mainly used to remove compounds containing Si, such as alloy elements in some stainless steel and alloy steel, mixed welding slag in welds, and residual molding sand on the surface of castings.

The mixture of hydrofluoric acid and nitric acid is mostly used to treat stainless steel, but hydrofluoric acid is highly corrosive.

Nitric acid will release toxic nitrides and is difficult to treat. Therefore, special attention should be paid to prevent damage to human body.

Phosphoric acid has good solubility to iron oxide and less corrosion to metal, because it can produce a water-insoluble phosphate layer (phosphating film) on the metal surface, which can prevent corrosion.

At the same time, it is also a good bottom layer during painting. It is generally used for rust removal of precision parts, but the price of phosphoric acid is high.

When phosphoric acid is used for rust removal, the main function is to turn the oxide scale and rust into water-soluble Fe (H2PO4) 3 and water-insoluble FeHPO4 and Fe3 (PO4) 2.

The diffusion phenomenon of hydrogen is weak.

The hydrogen produced during phosphoric acid pickling is 1 / 10 ~ 1 / 5 of that of hydrochloric acid or sulfuric acid pickling, and the hydrogen diffusion and penetration rate is 1 / 2 of that of the latter.

For stainless steel and alloy steel, the composition of oxide scale is very complex and the structure is often dense.

It is difficult to remove it in the derusting solution of ordinary carbon steel. Mixed acid is used in production.

When pickling alloy steel containing titanium, hydrofluoric acid should be added.

The thick and dense oxide scale produced by heat treatment shall be “loosened” in the hot concentrated alkali solution containing strong oxidant, and then etched in the mixed acid of hydrochloric acid and nitric acid or sulfuric acid and nitric acid.

(2) Pickling additive

Corrosion inhibitor must be used in pickling solution. It is generally believed that corrosion inhibitor can form an adsorption film or insoluble protective film on the surface of base metal in acid solution.

The formation of the film is that the electrochemical reaction occurs when the metal iron contacts with the acid to charge the metal surface, and the corrosion inhibitor is a polar molecule, which is attracted to the metal surface to form a protective film, so as to prevent the continuous action of acid and iron and achieve the purpose of corrosion inhibition.

From the electrochemical point of view, the formed protective film can not only greatly block the anodic polarization process, but also promote the cathodic polarization, inhibit the production of hydrogen, and significantly slow down the corrosion process.

Oxide scale and rust will not adsorb polar molecules of corrosion inhibitor to form a film, because they interact with acid to dissolve rust through ordinary chemical action, and there is no charge on its surface.

Therefore, adding a certain amount of corrosion inhibitor to the derusting solution does not affect the derusting efficiency.

To evaluate the effect of various corrosion inhibitors, the most important thing is to determine the corrosion inhibition efficiency.

The corrosion inhibition efficiency can be obtained by comparing the weight loss [g / (m2 · h)] of the sample with and without corrosion inhibitor in the same medium and under the same conditions.

The additional amount of different corrosion inhibitors in various acid solutions has a specified value.

With the increase of acid washing solution temperature, the inhibition efficiency of corrosion inhibitor will also decrease, or even completely fail.

Therefore, each corrosion inhibitor has a certain allowable service temperature.

The wetting agents used in pickling solutions are mostly non-ionic and anionic surfactants, and cationic surfactants are usually not used.

This is because the non-ionic surfactant is stable in strong acid medium, and the anionic surfactant can only be sulfonic acid type.

Using the functions of wetting, penetration, emulsification, dispersion, solubilization and decontamination of surfactant can greatly improve the pickling process and shorten the pickling time.

In order to reduce the corrosion loss of the matrix and the influence of hydrogen permeation, reduce acid mist and improve the operating environment, efficient corrosion and fog inhibitor should also be added to the pickling solution.

However, it should be noted that the corrosion inhibitor may form a film on the surface of the workpiece, which needs to be carefully cleaned, and the corrosion inhibitor can reduce the mechanical stripping effect of hydrogen evolution reaction.

(3) Selection of acid type, concentration and temperature for pickling

It shall be determined according to the workpiece material, surface rust layer and oxide scale, as well as the requirements for surface cleaning quality.

For steel workpieces, sulfuric acid, hydrochloric acid and their mixed acids are commonly used.

In order to dissolve silicon-containing compounds on the surface of castings, hydrofluoric acid needs to be added to sulfuric acid or hydrochloric acid.

The concentration of sulfuric acid is generally about 20%.

At this concentration, the etching speed of the oxide scale is fast and the loss of the matrix is small.

The concentration of hydrochloric acid is generally below 15%, because it will smoke when it is greater than 20%.

With the increase of hydrochloric acid concentration, the pickling speed is accelerated and the pickling time is shortened.

Table 4 shows the relationship between pickling time and acid concentration of steel workpieces 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 /%251015
Pickling time/min90551815
Sulfuric acid content /%251015
Pickling time/min13513512095
Hydrochloric acid content /%20253040
Pickling time/min109//
Sulfuric acid content /%20253040
Pickling time / min80657595

With the increase of temperature, the pickling speed is also accelerated and the time is shortened.

Table 5 shows the relationship between pickling time and temperature of steel workpieces with the same degree 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





















(4) Pickling process of iron and steel workpiece

Pickling and derusting methods include immersion pickling, spray pickling and acid paste derusting.

After degreasing treatment, the impregnated and pickled metal is placed in the acid tank.

After the oxide scale and rust are etched off, it is washed with water, and then neutralized with alkali to obtain a surface suitable for painting.

See Table 6 for the strong etching process parameters of steel workpieces.

Table 6 strong etching process parameters of steel workpieces


Forgings and stamping parts

General steel parts






Concentrated sulfuric acid / (g / L.)
hydrochloric acid / (g / L)
hydrofluoric acid / (g / L)
Rodin / (g / L)
Urotropine / (g / L)










Temperature / ℃

40 ~ 60 until all is divided

30 ~ 40 until all is divided


Until 40 ~ 50 is divided

30 ~ 40 until all is divided

2. Electrochemical etching

Electrochemical etching refers to the method of electrolytic stripping with the workpiece as anode or cathode in acid or alkali solution, or stirring the solution due to hydrogen evolution from the cathode and constantly updating the etching solution on the workpiece surface to accelerate the removal of surface rust layer.

According to the different polarity of the workpiece, electrochemical rust removal can be divided into anode etching and cathode etching.

During anodic etching, the oxide scale is removed by chemical dissolution and electrochemical dissolution of workpiece metal and mechanical stripping of oxygen.

During cathodic etching, the oxide scale is removed by means of the mechanical stripping effect of a large amount of hydrogen evolution on the oxide scale and the reduction effect of primary atomic hydrogen on the oxide.

The oxygen bubbles precipitated during anodic etching are large and few, and the mechanical stripping effect is small.

If it takes a long time, it is easy to cause excessive corrosion of the base metal.

During cathodic etching, the metal matrix will hardly corrode and the size of the workpiece will not change, but it may lead to hydrogen permeation and ash hanging.

Although anodic etching will not cause hydrogen embrittlement, it is slow and corrosive to the base metal. It is only suitable for thin oxide scale workpieces.

Cathodic etching will not cause over-corrosion of the workpiece, and the speed is fast.

It can also be applied to the workpiece with thick oxide skin, but it has the disadvantage of hydrogen permeation.

At present, most of the methods used in China are anode etching or cathode anode combined etching. Electrochemical etching is used for both strong etching and weak etching.

Compared with the chemical etching method, electrochemical etching is easier to quickly remove the oxide scale firmly bonded on the metal surface.

Even if the acid concentration changes, it will not significantly affect the etching effect, and has little corrosion to the matrix.

It is easy to operate and manage. However, this method requires special equipment, requires more hanging operation, and there is the phenomenon of uneven dissolution of oxide scale.

The advantages of electrochemical etching are fast etching speed, less acid consumption and little influence of iron ion content in solution on etching ability.

However, power supply equipment and power consumption are required.

Due to the poor dispersion ability, the workpieces with complex shapes are not easy to remove.

When the oxide scale is thick and dense, it should be chemically etched with sulfuric acid to loosen the oxide scale before electrochemical etching.

Surface degreasing

1. Organic solvent degreasing

Organic solvent degreasing is a common degreasing method for metal materials.

It degreases by using the physical dissolution of organic solvents on both types of oils.

Gasoline and kerosene are widely used, but chlorobenzene and kerosene are cheap solvents, and chlorobenzene and kerosene are commonly used, while chlorobenzene and kerosene are less toxic solvents.

Organic solvent degreasing is characterized by no heating, fast degreasing speed and no corrosion to the metal surface.

It is especially suitable for those mineral oils with high viscosity and high melting point that are difficult to remove with alkaline solution.

Therefore, it is suitable for the pretreatment of almost all surface treatment technologies, especially the preliminary degreasing of parts with serious oil pollution or metal parts that are easy to be corroded by alkaline degreasing solution.

However, this degreasing is not complete, and chemical and electrochemical methods need to be used to supplement degreasing;

Moreover, most organic solvents are flammable and toxic, and the cost is high.

Therefore, pay attention to safety, strengthen protection and maintain good ventilation during operation.

2. Chemical degreasing of alkaline solution

At present, chemical degreasing in alkaline solution is widely used in production.

Although the oil removal time of this method is longer than that of organic solution, the medium is non-toxic, will not burn, the required equipment is simple, the operation is simple and the price is cheap, so it is reasonable to adopt this oil removal method.

The essence of this method is to remove oil by saponification and emulsification.

The former can remove animal and vegetable oil and the latter can remove mineral oil.

As long as the process is properly selected, it is not difficult to remove the two types of grease.

However, when the requirements for the binding force of the coating are high, it is not enough to use only alkaline solution for chemical oil removal of the plated parts.

Especially when the oil stain is mainly mineral oil, it not only takes a long time to remove the oil, but also it is not easy to completely remove it.

This is because the emulsification effect of alkaline oil removal solution is limited, so it is necessary to use electrochemical (electrolytic) oil removal with stronger emulsification to obtain satisfactory results.

3. Electrochemical oil removal

The method of removing oil by putting the metal parts to be removed into the oil removal liquid, using the parts as anode or cathode and connected with direct current is called electrochemical oil removal or electrolytic oil removal.

The composition of the electrochemical degreasing solution is roughly the same as that of the chemical degreasing solution.

Nickel plate or nickel-plated iron plate is usually used as the counter electrode, which only plays a conductive role.

Production practice has proved that the speed of electrochemical oil removal is several times higher than that of chemical oil removal, and the oil pollution is cleaned up, which is inseparable from the mechanism of electrochemical oil removal.

New surface pretreatment technology

1. Ultrasonic strengthening

The high-frequency oscillation signal sent by the ultrasonic power supply is converted into high-frequency mechanical oscillation through the transducer.

Based on the principle that the ultrasonic wave can effectively propagate in gas, liquid, solid, solid solution and other media and can transmit strong energy, the ultrasonic wave is radiated to the cleaning liquid in the tank through the cleaning tank wall, and the microbubbles in the liquid in the tank vibrate under the action of the sound wave, that is, reflection, interference.

When superposition and resonance phenomena and ultrasonic waves propagate in liquid medium, they can produce strong impact and cavitation on the interface.

The effect of ultrasonic cleaning depends on the type of cleaning fluid, cleaning method, cleaning temperature and time, ultrasonic frequency, power density, number and complexity of cleaning parts, etc.

The liquids used for ultrasonic cleaning include organic solvent, alkaline solution, water cleaning solution, etc.

The most commonly used ultrasonic cleaning and degreasing device is mainly composed of the ultrasonic transducer, cleaning tank and generator.

In addition, there are cleaning fluid circulation, filtration, heating and transportation devices.

Ultrasonic cleaning is a new cleaning method, which is widely used because of its simple operation, fast cleaning speed and good quality.

2. Oil removal with low temperature and high-efficiency cleaning agent

Removing the oil stain on the metal surface with low-temperature and high-efficiency cleaning agent not only has high oil removal efficiency, but also has low oil removal temperature, so as to save energy.

3. Vacuum degreasing cleaning

Vacuum degreasing cleaning is a new cleaning technology with less pollution and no pollution. The cleaning agent used is hydrogen carbide cleaning agent, which has little impact on human body, low irritation and no odor.

The cleaning effect reaches the same cleaning degree as triethanolamine, which is better than alkali liquor, and the cleaning agent can be recovered and regenerated.

The vacuum degreasing cleaning device is pollution-free, closed system, high safety factor, high productivity, automatic loading and unloading of materials and convenient operation.

Vacuum degreasing technology, whether without cleaning or with liquid cleaning, will be more widely used in the future.

4. Spray plastic shot to remove paint (coating layer)

For the coating (plating) of important large components such as aircraft, when conducting surface non-destructive testing to find fatigue cracks and hard damage, the surface coating treatment (paint removal) must be carried out first.

The traditional methods are chemical stripping or manual grinding with grinding wheel, but both methods have disadvantages, such as chemical stripping has corrosion and damage to the metal matrix;

Grinding with the grinding wheel is easy to damage the substrate, and the efficiency of coating removal is very low.

Recently, a new paint stripping process of plastic shot spraying has been developed, and the effect is good.

Spraying plastic shot for paint removal is to spray granular plastic to the surface of the workpiece at high speed through the spray gun under the action of compressed air.

Under the dual action of sharp edges and corners cutting and impact of the plastic shot, the paint layer surface will be cut and stripped, so as to achieve the purpose of efficient paint removal.

The main advantages of plastic shot paint removal are: because the hardness of plastic shot is higher than that of the paint layer and lower than that of substrate or coating and anodized surface layer, plastic shot paint removal will not damage the substrate or coating, but also provide a clean surface for the new paint layer, which is conducive to improving the adhesion of paint layer.

The plastic pellets can be recycled and easily separated from the peeled paint layer.

5. Air flame supersonic sand blasting and shot blasting

Ultrasonic sand blasting coarsening is to use compressed air as power to spray hard sand particles to the substrate surface at high speed, and coarsen the surface through the mechanical scouring effect of sand particles on the surface.

The speed of ultrasonic sand blasting is 300 ~ 600m / s, and the efficiency of sand blasting is more than 3 ~ 5 times that of ordinary sand blasting.

Therefore, it is widely used in the surface pretreatment of large structural parts, such as surface cleaning before the surface coating of bridges, ships, boilers, output pipelines and so on.

In addition, due to the fast sand blasting speed and good surface coarsening effect, it is often used for surface coarsening before spraying of parts or large equipment with high requirements for spraying effect, and surface cleaning of equipment surface with heavy natural pollution (such as paint, cement, organic and inorganic scale).

Coarsening treatment can increase the “anchor hook” effect between the coating and the substrate and reduce the shrinkage stress of the coating, so as to improve the bonding strength between the coating and the substrate.

The sand used for sand blasting shall have high hardness, high density, good crushing resistance and low dust content.

The particle size shall be determined according to the required surface roughness. Commonly used sand grains include corundum sand (alumina), silica sand, silicon carbide, emery, etc.

Supersonic surface shot peening is a process in which a large number of supersonic moving projectiles are sprayed onto the workpiece surface to produce a certain plastic deformation on the surface, so as to obtain a certain thickness of strengthening layer.

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