Metal products, for all their strength, have one significant drawback – they easily rust. In production conditions, steel products are protected against corrosion by hot-dip galvanizing. This is a rather complicated and expensive process; moreover, it is not designed to protect large structures. In everyday life, to protect the roof, fence, canopy, car and other structures from rust, it is best to use cold galvanizing of the metal, which is carried out using special zinc-containing compounds. The coating process is as simple as possible, and the cold galvanizing method itself can be called universal for a number of reasons. Let’s figure out the technology, formulations and find out how the zinc “armor” works.
# 1. General principles of cold galvanizing
Hot-dip galvanizing helps to protect the metal from corrosion for the next 40-50 years, and no additional measures are required. A technologically complex and costly process in everyday life has been replaced by cold galvanizing, which allows you to obtain a high-quality protective coating with minimal effort, but it will have to be periodically updated.
The cold galvanizing process resembles simple painting of metal structures, only zinc-rich paints (ZNA) are used instead of conventional paints. They contain at least 94% zinc, the rest is binding additives. ZNK should not be confused with zinc-containing paints: the proportion of zinc in them is lower, and the density of the composition is always less than 2.2 kg / l.
Due to its simplicity, cold galvanizing of metal is carried out not only in industrial conditions, but also at home to protect structures of any size and geometry.
# 2. The nature of cold galvanizing
To better understand the features and advantages of the method, it is necessary to recall the school chemistry course. The process of protecting iron-containing alloys with zinc is possible due to the unique physicochemical properties of this element. Zinc very easily enters into chemical reactions with various types of substances, and when interacting with water forms an almost insoluble hydroxide, which covers the metal surface and prevents further interaction of zinc and water. By the way, aluminum behaves in a similar way.
If you cover a steel structure with a layer of zinc, then the zinc in the air will begin to oxidize over time. The reaction product is zinc oxide, which does not react with water and forms a strong film on the surface of the structure. That’s all, on this further passage of the reaction is impossible, since zinc oxide and zinc bicarbonate (formed in a smaller amount) are inert with respect to water. This is a barrier-based defense. It should be noted that iron also reacts with water to form oxides, which we call rust, but these compounds do not form a continuous dense film, passing moisture deep into the metal and provoking the development of corrosion.
In addition to barrier protection, zinc also provides electrochemical protection. We recall the electrochemical series of metal voltages, in which zinc is earlier than iron. This means that zinc is chemically more reactive and will react first in the zinc / iron pair. In the presence of atmospheric moisture, an electrochemical reaction can take place to form zinc carbonate. This compound is also insoluble in water and stops the further development of the corrosion process.
The principle of “work” of zinc coating remains the same, despite the method of its application:
- hot-dip galvanizing;
- electrolytic zinc plating;
- diffuse zinc plating;
- gas-dynamic galvanizing;
- cold galvanizing.
Immediately after the protection of the metal structure by the method of cold galvanizing, mainly electrochemical protection works: while the coating has not yet reached its maximum strength, moisture particles can penetrate through it and reach the steel. At this point, an electrochemical zinc / iron pair is formed. In the future, the protection is built according to the barrier type, but if the integrity of the paint is violated and moisture penetrates the structure, then the electrochemical protection is activated again.
No. 3. Advantages and Disadvantages of Cold Galvanizing
Cold galvanizing of metal is a simple, reliable and one of the most popular ways to protect it. In everyday life, the technology has become widespread, since it has many advantages:
- the compositions used are distinguished by good adhesion both to the base and to decorative paints and varnishes, so the structure can be easily painted in the required color;
- the coating can be applied to a product of any size and any geometry, and if it is already installed and operated, then it is unnecessary to dismantle it – all work can be done on site;
- surface preparation before cold galvanizing is relatively simple;
- metal parts protected by this method are easily welded together;
- ease of coating, no special skills or tools are required. For painting, spray guns, paint rollers and ordinary brushes are used;
- work can be performed in almost any weather (permissible air temperature -20 … + 400C);
- low financial and time costs.
The main disadvantage of cold galvanizing is the low resistance of the coating to mechanical damage. Simply put, the paint can simply be scratched, exposing the steel structure. On the other hand, it is not that difficult and costly to renew the coverage, so the minus cannot be called very significant.
No. 4. How is cold galvanizing carried out?
The cold galvanizing method is the application of zinc-containing compounds to the metal surface. The technology and procedure for application depends on the type of composition, but most often materials are used whose characteristics are prescribed in GOST 9.305–84. The same document permits the use of compounds for cold galvanizing on any structures, with the exception of high strength steels and magnesium alloys.
The process of applying the composition (we will talk about the most popular below) is preceded by a thorough preparation of the metal surface:
- removal of any kind of dirt, salts, coking;
- abrasive surface treatment (abrasive blasting or hydrodynamic method) to impart the required roughness (provides better adhesion) and remove old rust;
- drying the structure;
- manual cleaning of splashes, burrs and sharp corners;
- dedusting with a jet of air.
If there are grease stains on the metal, the surface must be degreased. The standards imply a thorough check of the level of dust, degreasing and roughness, which requires special instruments. When it comes to protecting a responsible structure, it is better not to neglect this.
This completes the preparation and begins directly cold galvanizing. It is produced at a surface temperature above the dew point by three degrees or more, and the air temperature recommended by the manufacturer. It is advised to apply paint using special equipment (pneumatic, paint chambers, etc.) in several layers, each new layer is applied after the previous one has dried. You can use a brush and roller for some areas. The coating is allowed to dry, then leveled. Subsequently, you can apply regular paint on it. It is advisable to check the quality of the coating after applying ZNK and drying it, using special devices that allow you to measure the thickness of the protective film.
To determine the dew point, use a hygrometer, thermometer, psychrometer or devices that measure both temperature and humidity at the same time. The readings of the devices are compared with the table and it is determined whether the procedure of cold galvanizing of the metal can be carried out.
No. 5. Where can cold galvanizing be used?
Cold galvanizing is suitable for all steel except high strength and magnesium alloy. There are no requirements for the size, weight, shape, thickness of products. The method is suitable for already manufactured and installed products (including large metal structures). You do not need to dismantle them.
Galvanizing can be used both in production and in the maintenance and repair of infrastructure elements (for example, pipelines, tanks and towers), and in everyday life for structures such as:
- fences and gates, gates;
garages made of metal;
- bottom and other body parts of cars;
- various containers and tanks;
- carts, agricultural and construction tools.
No. 6. Compositions for cold galvanizing
According to the standards, ZNK must contain at least 94% zinc with a particle size of 12-15 microns or 88% zinc with particles of 3-5 microns. The higher the zinc content, the better the anti-corrosion properties, and the finer the particles, the better the adhesion. There are many cold galvanizing compounds on the market, but we will focus on the most popular ones:
“Galvanol” is a composition based on pure electrolytic zinc and with a low content of binding components, a domestic development. The composition provides excellent adhesion and can be applied even to rusty structures if the rust adheres well to them. It is applied quickly and easily, can be used at temperatures from -30 to + 500C, as well as at high air humidity. The composition is resistant to salt solutions and alcohol, and the finished coating is resistant to abrasion and impact, has good adhesion to paints. It is produced ready to use, can be applied with a brush, roller, spray, aerosol can. The scope of application is the broadest;
“Tsinotan” is used as an independent agent against corrosion and in combination with other compounds. It can be used in all climatic conditions, it is often used in industry to protect structures in salt water or polluted atmospheres. The composition covers fences, containers for oil products, power transmission line supports, pipes, automobile structures, etc. It is used by the giants of the domestic industry;
“Zincconol” is a polyurethane primer with a high zinc content, very flexible, resistant to high temperatures, salt water, oils, oil and alkalis;
“Tsinol” – a composition including zinc particles of different fractions ranging in size from 4 to 20 microns and more, applied with a thickness of 100-120 microns, dries in 1 hour, has a high resistance to water;
CFES contains zinc powder and ethyl silicate, well suited for high-alloyed and conventional steels;
Master AK-100 is excellent for handling vehicle bodies;
UR-100 “Liquid zinc” is produced on the basis of zinc powder with a particle size of 3-5 microns, it is inexpensive;
Zinga is a cold-galvanizing compound from the Belgian company Zinga Metall. The manufacturer uses 3-5 micron zinc powder and claims the unique properties of its product, which explains the high cost, which is almost equivalent to hot-dip galvanizing.
It is important that the processed product is not subjected to constant mechanical stress – in this case, the coating will not last very long.
Cold galvanizing is suitable for all products that need to be protected against corrosion. Despite the fact that such a coating can be damaged like an ordinary paint, it works effectively and eliminates the appearance of rust spots, and also simplifies painting: the paint on the zinc layer adheres better than on ordinary metal.
Learn more about the method of cold galvanizing at http://spektrlkm.ru/jidkiy_cink