The most common way to protect metals from corrosion is galvanizing, which gives the longest lasting protection - 25-30 years. In the air, zinc is covered by a thin oxide or carbonate film, which protects iron from interaction with the environment. It is both barrier and electrochemical protection. That is, when damaged (integrity defects), zinc takes the impact of corrosion on itself and then metal rusts.
Galvanized steel sheet is currently the main material for the manufacture of car body parts. Moreover, the shares of the use of hot-dip galvanized and electrolytically galvanized sheet are comparable.
Corrosion from the outside
Electrolytic (galvanic) galvanizing is a very common way to create zinc anticorrosive coatings. It is much cheaper and simpler than hot-dip galvanizing, but has some limitations on its use, since it does not allow you to create a protective layer thicker than 40 microns. The basis of this technology is an electrochemical process of zinc deposition from an electrolytic solution on the part, connected to the negative terminal of the power supply. The main focus of zinc electroplating is a variety of fasteners, tools, brackets, supports, suspensions, external parts of machines and equipment, and cold-rolled thin sheets. In order to improve anticorrosive properties of galvanized fasteners, tools and small parts of simple shape they are often subjected to additional processing - passivation in solutions of chromic acid or its salts. This process is called chromate passivation, and two types are most widely used: colorless and iridescent (yellow). After passivation, a very thin hard film is formed on the surface of the product, which greatly inhibits the corrosion process.
Electrolytically galvanized steel sheet is used for the manufacture of exterior car body parts, because it has the greatest adhesion with the paint coating. Hot-dip galvanizing is the most traditional method of zinc coating application. Hot-dip galvanizing is the steel coating for corrosion protection by dipping a steel sheet or article into a bath of molten zinc at a temperature of approximately 460°C. When exposed to air oxygen, zinc reacts with air oxygen to form ZnO oxide, which reacts with carbon dioxide CO2 to form zinc carbonate ZnCO3, which prevents corrosion. And, in addition, having a lower reduction potential than Fe, it takes the oxidizing hit in its interaction with the environment. In other words, until all the zinc is used up, corrosion will not approach the iron. The conclusion is: that the thicker the coating, the more durable the product is.
Hot-dip galvanizing is considered one of the most reliable methods of protecting iron and steel from corrosion. The thickness of the galvanized layer is 30 to 100 microns, and usually it is 45-65 microns. Hot-dip galvanizing protects against atmospheric corrosion for 50 to 100 years. Hot-dip galvanizing is applied to both readymade steel structures and rolled metal sheets. Hot-dip galvanized steel sheet is used to manufacture interior parts of car bodies.
Of course, the most reliable as well as the most expensive way to protect against corrosion is full body galvanization. This is done by VAG and BMW and Volvo and other car manufacturers. Not for all models, but mainly for expensive ones. They dip bodies in molten zinc or electrolyte.
Gas-dynamic zinc spraying
A breakthrough in the field of metal repair and restoration was the technology of spraying. The gas dynamic spraying technology involves feeding of finely dispersed zinc particles (0,01-50 microns) through a supersonic (600-700 m/s) nozzle and heating the compressed air to 400-600°C. As a result, a zinc layer is formed on the metal surface, which has a high adhesion (40-100 N/mm2) and low porosity (1-3%). Coatings that are formed in the process of spraying have significant advantages:
- when applying the coatings, the temperature of the substrate does not exceed 150 ° C;
- coatings have high adhesion (40-100 N/mm2) and low porosity (1-3%);
- it is possible to spray layers of almost any thickness;
- light alloys can be sprayed: zinc, aluminum, silver.
