Stainless steel surface treatment process

 Stainless steel has unique strength and superior anti-corrosion properties. At the same time, it has high wear resistance and is not easy to rust. Therefore, it is widely used in the chemical industry, food machinery, electromechanical industry, and household appliance industry. The application and development prospects of stainless steel will become wider and wider, but the application and development of stainless steel largely determine the degree of development of its surface treatment technology.

Introduction of stainless steel varieties

Stainless steel generally contains high-quality metal elements such as chromium (CR), nickel (NI), molybdenum (MO), and titanium (TI). Common stainless steels include chromium stainless steel and nickel-chromium stainless steel. Classification from the metallographic structure of stainless steel: austenitic stainless steel, martensitic stainless steel

Stainless steel surface treatment varieties

• Whitening treatment of the natural color of the surface
• Surface mirror stone bright treatment
• Surface coloring treatment

1. The surface natural color whitening treatment

In the process of processing, the stainless steel undergoes coiling, edge binding, welding, or the handler’s warm surface fire treatment, resulting in a black oxide scale. This kind of hard gray-black oxide scale was usually removed by a strong corrosion method with hydrofluoric acid and nitric acid in the past. However, this method is costly, pollutes the environment, and is harmful to the human body. The corrosion is relatively large, and it is gradually eliminated.

At present, there are two kinds of treatment methods for this kind of oxide scale: (1) Using shot peening/sandblasting method. (2) Use chemical methods.

2. Mirror bright treatment method of stainless steel surface

According to the complexity of stainless steel products and the different requirements of users, mechanical polishing, chemical polishing, electrochemical polishing, and other methods can be used to achieve mirror stone luster.

Mechanical polishing

Good leveling performance, bright labor intensity, serious pollution, difficult to process complex parts, reduced gloss, high investment, and cost. Simple workpieces, medium and small products, and complex parts cannot be processed. The gloss of the entire product is not consistent, and the gloss retention time is not long.

Chemical polishing

Low investment, complex parts can be polished, high efficiency, high speed, insufficient brightness, polishing liquid to be humidified, gas overflows, and wind-friendly equipment is required.

Electrochemical polishing

Mirror gloss, long-term maintenance, stable process, less pollution, low cost, and good anti-pollution performance. The one-time investment is large, and the complicated parts must be equipped with tooling and auxiliary electrodes.

3. Surface coloring treatment

Stainless steel coloring not only imparts various colors to stainless steel products, increases the variety of products, but also improves the wear resistance of products.

There are several coloring methods for stainless steel:

(1) Ion deposition of oxides or oxides

(2) High-temperature oxidation method

(3) Chemical oxidation method, electrochemical oxidation method

(4) Gas-phase cracking method

Briefly describe the overview of various methods:

(1) The ion deposition oxide or oxide method is to place the stainless steel workpiece in a vacuum coating machine for vacuum evaporation plating; this method is suitable for mass product processing. Because of the large investment and high cost, small-batch products are not economical.

(2) The high-temperature oxidation method is to immerse the workpiece in a specific molten salt to maintain a certain process range, so that the workpiece forms a certain thickness of oxide film, and presents a variety of different colors.

(3) The chemical method is to form the color of the film through chemical oxidation in a specific solution. However, if the color of a batch of products is consistent, it must be controlled by a reference telegram.

(4) The electrochemical method is to form the color of the film through electrochemical oxidation in a specific solution.

(5) The gas-phase cracking method is more complicated and less used in industry.

The difference between pickling, phosphating, and passivation

In an effort to prevent corrosion or simply improve appearance, engineers will often specify an additional finishing treatment process for a portion, or all, of a metal part’s exterior surface after it has been machined or fabricated.

No matter what the reason, each of these metal surface treatment finishing processes plays an important role in the manufacturing process:

Pickling

The definition of pickling

Acid is used to remove oxide scale chemically according to a certain concentration, temperature, and speed, which is called pickling.

Pickling classification

• According to the type of acid: sulfuric acid pickling, hydrochloric acid pickling, nitric acid pickling, hydrofluoric acid pickling. Pickling must choose different media according to the material of the steel, such as pickling carbon steel with sulfuric acid and hydrochloric acid, and pickling stainless steel with mixed acid of nitric acid and hydrofluoric acid.
• According to the shape of steel: wire pickling, forging pickling, steel plate pickling, strip steel pickling, etc.
• According to the type of pickling equipment: tank pickling, semi-continuous pickling, full continuous pickling, tower pickling.

The principle of pickling

Pickling is the process of chemically removing the oxide scale on the metal surface, so it is also called chemical pickling. The iron oxide scale formed on the surface of the steel pipe is an alkaline oxide that is insoluble in water. When they are immersed in an acid solution or sprayed on the surface of the acid solution, these alkaline oxides can undergo a series of chemical changes with the acid.

Because the iron oxide scale on the surface of carbon structural steel or low-alloy steel is loose, porous and cracked, and the iron oxide scale undergoes repeated straightening, stretching, and conveying along with the strip steel on the pickling line, these pores and cracks further increase and expand, so while the acid solution chemically reacts with the iron oxide scale, it also reacts with the base iron of the steel through the cracks and pores. That is to say, at the beginning of pickling, three kinds of chemical reactions between iron oxide scale and metallic iron and acid solution are carried out at the same time.

1. The iron oxide scale reacts chemically with acid and is dissolved (dissolution).
2. Metal iron reacts with an acid to generate hydrogen, which mechanically peels off the iron oxide scale (mechanical peeling action).
3. The generated atomic hydrogen reduces iron oxides to ferrous oxides that are easy to react with acid, and then react with acid to be removed (reduction).

Passivation

The Principle of passivation

The passivation mechanism can be explained by film theory. That is to say, passivation is due to the action of metal and oxidizing substances, which produces a thin and dense passivation film on the metal surface with good covering performance, and can also be firmly adsorbed on the metal surface. Passivation film firmly adsorbed on the metal surface. This film exists as a separate phase, usually a compound of oxidized metal. It plays the role of completely separating the metal from the corrosive medium, preventing the metal from contacting the corrosive medium, so that the metal basically stops dissolving and forming a passive state to achieve the role of anti-corrosion.

Advantages of passivation

1. Compared with the traditional physical sealing method, the passivation process has the characteristics of absolutely no increase in the thickness of the workpiece and no change in color, which improves the precision and added value of the product, and makes the operation more convenient; 　　
2. Since the passivation process is carried out in a non-reactive state, the passivation agent can be added and used repeatedly, so the life is longer and the cost is more economical.　　
3. Passivation promotes the formation of the oxygen molecular structure passivation film on the metal surface, the film layer is dense, the performance is stable, and it has a self-repairing effect in the air at the same time, so compared with the traditional method of applying anti-rust oil, the passivation formed The passivation film is more stable and corrosion-resistant.　　

Most of the charge effects in the oxide layer are directly or indirectly related to the thermal oxidation process. In the temperature range of 800-1250℃, the thermal oxidation process with dry oxygen, wet oxygen, or water vapor has three continuous stages. First, the oxygen in the ambient atmosphere enters the formed oxide layer, and then the oxygen diffuses into the inside through the silicon dioxide. When it reaches the Si02-Si interface, it reacts with silicon to form new silicon dioxide. In this way, the oxygen entry-diffusion-reaction process continues to occur, so that the silicon near the interface is continuously converted into silicon dioxide, and the oxide layer grows into the silicon wafer at a certain rate.

Phosphating

Phosphating treatment is a kind of chemical reaction to form a layer of film (phosphating film) on the surface of a surface treatment process.

The phosphating process is mainly used on the metal surface. The purpose is to provide a protective film for the metal surface to isolate the metal from the air and prevent it from being corroded. It is also used for the primer before the painting of some products. With this layer of phosphorus, The chemical film can improve the adhesion and anti-corrosion ability of the paint layer, improve the decoration and make the metal surface look more beautiful, and can also play a lubricating role in the cold working of some metals.

After phosphating treatment, the workpiece will not be oxidized and rusted for a long time. Therefore, the application of phosphating treatment is very wide, and it is also a commonly used metal surface treatment process, which is more and more used in automobiles, ships, machinery manufacturing, and other industries.

Classification and application of phosphating

Normally, a kind of surface treatment will show one color, but phosphating treatment can show different colors by using different phosphating agents according to actual needs. This is why we often see phosphating The processing is gray, colored, or black.

Iron-based phosphating: After phosphating, the surface will appear iridescent and blue, so it is also called color phosphor. The phosphating solution is mainly made of molybdate, which will form an iridescent phosphate film on the surface of steel materials. It is also mainly used for coating the bottom layer to achieve the anti-corrosion ability of the workpiece and improve the bonding force of the surface coating.

Phosphating wastewater treatment process

 Phosphating wastewater is the pretreatment of metal surface treatment, generally including degreasing and rust removal, surface conditioning, and phosphating passivation.

Phosphating wastewater treatment process description:

1. Chemical removal of zinc

Chemical methods for removing zinc include the sulfide method and alkali method coagulation precipitation.

2. Phosphorus removal by chemical precipitation by the lime method

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Adding lime milk clarified liquid and adjusting the pH value to 10~11, most of the phosphate in the wastewater can be precipitated and removed. The lime milk also neutralizes while removing phosphorus.

3. Phosphorus removal and oil removal by coagulation air flotation

The addition of flocculant makes the residual phosphorus in the wastewater and other pollutants undergo a coagulation reaction, and the flocs produced are attached to the air floatation method to produce a large number of fine bubbles, and the bubble buoyancy is used to bring them out of the water. At the same time as dephosphorization, it can also absorb and remove oil droplets. The bubble rises fast, and the operation is safe and reliable. It is an economical and practical deoiling technology.

4. Activated carbon adsorption phosphorus removal

Activated carbon is the most commonly used adsorbent in water treatment. It has good adsorption performance and chemical stability is not easy to break and has low airflow resistance. The commonly used ones are powder and granular. The process utilizes the huge specific surface area of activated carbon to fully adsorb trace residual phosphorus and other pollutants in wastewater.

5. Stirring and mixing device

Both the primary and secondary reactors are equipped with stirring devices. The stirring speed in the tank is gradually reduced by using a pulley or a reducer to meet the needs of stirring and mixing at different stages in the entire reaction process, making the reaction more thorough and more conducive to the subsequent precipitation Process. The pipeline mixer in the process makes the neutralization reaction easier when adding H2SO4, and can make the effluent pH value fully meet the standard.

6. Discharge mud by gravity or water pressure

The sludge or scum produced in the primary and secondary reactors and air flotation tanks in the system are regularly discharged into the sludge thickening tank by gravity or hydraulic pressure to be concentrated and then transported to landfill after natural drying. The clear liquid is returned to the conditioning tank for physical and chemical treatment.

7. Backwashing and desorption

Both the silica sand filter and the activated carbon adsorber in the system need to be backwashed regularly to prevent clogging, and the wastewater produced by them is returned to the regulating tank. In addition, in order to regenerate the activated carbon, a high-temperature heating regeneration method is used to activate, and water vapor is used to desorb the granular carbon in the activated carbon filter to ensure the normal adsorption effect.

Pickling passivation of stainless steel

The corrosion resistance of stainless steel mainly depends on the surface passivation film. If the film is incomplete or defective, the stainless steel will still be corroded. In engineering, pickling and passivation treatment is usually carried out, so that the corrosion resistance of stainless steel can be exerted even more. In engineering, pickling and passivation treatment is usually carried out, so that the corrosion resistance of stainless steel can be exerted even more. Stainless steel surface cleaning, pickling, and passivation not only maximize corrosion resistance but also prevent product contamination and obtain a beautiful appearance.

The corrosion resistance of stainless steel is mainly because the surface is covered with a very thin and dense passivation film, which is the basic barrier for stainless steel protection. Stainless steel passivation has dynamic characteristics and should not be regarded as a complete stop of corrosion but as a barrier to diffusion, which greatly reduces the anode reaction speed. Generally, it tends to damage the membrane in the presence of a reducing agent, while maintaining or repairing the membrane in the presence of an oxidizing agent.

The stainless steel workpiece will form an oxide film when placed in the air, but the protection of this film is not perfect. Usually, thorough cleaning, including alkali cleaning and acid cleaning, and then passivation with an oxidizing agent are required to ensure the integrity and stability of the passivation film. One of the purposes of pickling is to create favorable conditions for passivation treatment to ensure the formation of high-quality passivation films. Because the surface of the stainless steel is corroded by an average of 10μm thick by pickling, the chemical activity of the acid makes the dissolution rate of the defect parts higher than other parts on the surface. Therefore, pickling can make the entire surface tend to be evenly balanced, and some hidden dangers that are easy to cause corrosion are removed. But more importantly, through pickling passivation, the iron and iron oxides are dissolved preferentially than the chromium and chromium oxides, and the chromium-poor layer is removed, resulting in the enrichment of chromium on the surface of the stainless steel. This chromium-rich passivation film The potential is close to that of precious metals, which improves the stability of corrosion resistance. Different passivation treatments will also affect the composition and structure of the film, thereby affecting the rust resistance. For example, the electrochemical modification treatment can make the passivation film have a multi-layer structure, so that the stainless steel can exert maximum corrosion resistance.

Comparison of stainless steel pickling passivation methods

Method	Scope of application	Pros and cons
Dipping method	Used for parts that can be put into the pickling tank or passivation tank, but not suitable for large equipment	Pickling liquid can be used for a long time, with high production efficiency and low cost; large-volume equipment is filled with pickling liquid and immersion consumes too much liquid
Brushing method	Suitable for internal surface and local treatment of large equipment	Manual operation, poor working conditions, acid can not be recovered
Ointment method	For installation or maintenance site, especially for welding part treatment	Manual operation, poor working conditions, high production cost
Spray method	For the installation site, the inner wall of the large container	Low liquid consumption, low cost, fast speed, but need to be equipped with spray gun and skewer ring system
Round robin	Used for large-scale equipment, such as heat exchanger, tube and shell treatment	Convenient construction, acid liquid can be reused, but pipes and pumps are required to connect the circulation system
Electrochemical method	Not only can be used for parts but also can be used to brush the surface treatment of field equipment	The technology is more complicated and requires a DC power supply or a potentiostat