Electroplating surface treatment methods

Electroplating is a method in which metal is precipitated from the electrolyte by the action of an applied current and deposited on the surface of the object to obtain a metal covering layer.

Galvanized:

Zinc is easily corroded in acids, alkalis, and sulfides. The zinc layer is generally passivated. After passivation in chromate solution, the passivation film formed is not easy to interact with moist air, and the anti-corrosion ability is greatly enhanced. In dry air, zinc is relatively stable and not easy to change color. In water and humid atmosphere, it reacts with oxygen or carbon dioxide to form an oxide or alkaline carbonic acid film, which can prevent zinc from continuing to oxidize and play a protective role.

Applicable materials: steel, iron parts

chrome:

Chromium is very stable in a humid atmosphere, alkali, nitric acid, sulfide, carbonate solutions, and organic acids, and is easily soluble in hydrochloric acid and hot concentrated sulfuric acid. The disadvantage is that it is hard, brittle, and easy to fall off. Direct chromium plating on the surface of steel parts as an anti-corrosion layer is not ideal. Generally, multi-layer electroplating (ie copper plating → nickel → chromium) can achieve the purpose of rust prevention and decoration. At present, it is widely used to improve the wear resistance of parts, repair size, light reflection, and decoration.

Applicable materials: ferrous metal, copper, and copper alloy zero decorative chrome plating, wear-resistant chrome plating

Copper plating:

Copper is not stable in the air, and at the same time, it has a high positive potential and cannot protect other metals from corrosion. However, copper has high electrical conductivity, the copper plating layer is tight and fine, it is firmly combined with the basic metal, and it has good polishing performance. It is generally used to improve the conductivity of other materials, as the bottom layer of another electroplating, as a protective layer to prevent carburization, and to reduce friction or decoration on the bearing.

Applicable materials: black metal, copper, and copper alloy nickel-plated, the chrome-plated bottom layer

Nickel plating:

Nickel has good chemical stability in the atmosphere and lye and is not easy to change color, but it is easily soluble in dilute nitric acid. It is easy to passivate in concentrated nitric acid, and its disadvantage is porosity. To overcome this disadvantage, multi-layer metal plating can be used, and nickel is the intermediate layer. The nickel plating layer has high hardness, is easy to polish, has high light reflectivity and can increase the appearance and resistance, and has good corrosion resistance.

Applicable materials: can be deposited on the surface of various materials, such as steel-nickel-based alloys, zinc-based alloys, aluminum alloys, glass, ceramics, plastics, semiconductors, and other materials

Tin plating:

Tin has high chemical stability. It is not easy to dissolve in dilute solutions of sulfuric acid, nitric acid, and hydrochloric acid. Sulfides have no effect on the tin. Tin is also stable in organic acids, and its compounds are non-toxic. It is widely used in food industry containers and parts of aviation, navigation, and radio equipment. It can be used to prevent copper wires from being affected by sulfur in rubber and as a protective layer for non-nitriding surfaces.

Applicable materials: iron, copper, aluminum, and their respective alloys

Copper tin alloy:

Copper-tin alloy electroplating is to plate copper-tin alloy on the parts without nickel plating, but directly chromium plating. Nickel is a relatively rare and precious metal. At present, copper-tin alloy electroplating is widely used in the electroplating industry to replace nickel plating, which has good anti-corrosion ability.

Applicable materials: steel parts, copper, and copper alloy parts

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.

Hydrochloric acid pickling process control

In the Hydrochloric acid pickling process control, the most important thing is to control the pickling time and life of the pickling tank to ensure the maximum productivity and service life of the pickling tank.

To obtain the best pickling effect, first, control the concentration of hydrochloric acid, and secondly control the content of iron ions (iron salts) in the pickling solution. Because not only the concentration of acid will affect the pickling effect of the workpiece, the content of iron ions (iron salt) will reduce the mass fraction of the pickling solution, which will also affect the pickling effect and speed of the workpiece. It is worth noting that to obtain the best pickling efficiency, the pickling solution must also contain a certain amount of iron ions (iron salts).

Control of the hydrochloric acid washing tank

(1) Pickling time

The pickling time basically depends on the concentration of hydrochloric acid/iron ion (iron salt) and the temperature of the pickling solution.

(2) Best pickling

The best pickling effect of the workpiece should be when the concentration of hydrochloric acid and the concentration of precipitated iron ions (iron salts) reach a relative balance.

(3) Remedy for decreased acid effect

When the pickling solution decreases or loses its pickling effect due to the saturation of iron ions (iron salt), it can be diluted with water to restore the pickling function. Although the concentration of hydrochloric acid is reduced, it can still perform the pickling function, but at a slower rate. If new acid is added to the pickling solution with saturated iron content, the concentration of the new hydrochloric acid solution will fall above the saturation point, and the pickling of the workpiece will still be impossible.

(4) Treatment measures after acid solubility drops

After the pickling solution has been used for a period of time, its concentration decreases and even becomes waste acid. However, the manufacturer cannot recover the acid at this time and still retains a certain use-value. To use the reduced concentration of low acid, at this time, the workpiece that needs to be re-dipped due to partial leakage in the hot-dip galvanizing is generally put into Among them, pickling and reprocessing is also an effective use of waste acid.

Method of replacing old acid with hydrochloric acid pickling solution:

When the iron salt in the old acid exceeds the specified content, it should be replaced with a new acid. The method is that the new acid accounts for 50% and the old acid is added to the new acid after precipitation. The amount of the old acid is ~50%. A part of the old acid is used to maintain an appropriate amount of iron salt in the pickling solution because the hydrochloric acid Iron salts with a content of less than 16% can increase the activity of the pickling solution.

However, with the advancement of hot-dip galvanizing technology, the amount of old acid added must be carried out based on strictly controlling the iron salt content of the old acid, and the iron salt concentration of the newly prepared hydrochloric acid solution must be controlled within the range.

Workpiece steel material and pickling speed

The pickling speed varies with the composition of the steel workpiece to be pickled and the scale produced.

The carbon content in steel has a great influence on the dissolution rate of the steel matrix. When the carbon content increases, the dissolution rate of the steel matrix increases rapidly.

The dissolution rate of the steel workpiece matrix after cold and hot processing will increase, and the dissolution rate of the steel matrix after annealing the steel workpiece will decrease. In the iron oxide scale on the surface of steel workpieces, the dissolution rate of iron monoxide is higher than that of ferroferric oxide and ferric oxide. The rolled iron sheet has more iron monoxide content than the annealed iron sheet. Therefore, its pickling speed is also faster. The thicker the iron oxide layer, the longer the pickling time. If the thickness of the scale is uneven, it is easy to produce local defects of “under-pickling” or “over-pickling”. If you want to know more, please contact us.