Galvanizing, among other procedures involving the use of zinc for iron and steel protection, continues to gain traction in industry as an effective and low-cost method.
by Inpra Latina
The properties of steel allow this metal to be used in the manufacture of different types of products, from vehicles and lightweight tubes, to solid tools made for heavy work. However, corrosion is a threat that puts more than one manufacturer in trouble.
Fortunately, technology has managed to standardize and make more accessible some physical processes that protect steel from corrosion and even improve its appearance and finishes. This makes it easy to get protection with good finishes for a low-carbon steel used in a vehicle, or a high-carbon steel used in heavy-duty tools.
Zinc has long been recognized as one of the most versatile elements for the protection against atmospheric corrosion of sheets and parts made of steel in its different alloys. The function of this metal is to act as a physical barrier that isolates the metal substrate from the medium and corrosive agents, as if it were
of a painting. In some other cases, it is used as a sacrificial anode in the process of cathodic protection.
Zinc-based protection
According to the Colombian Manuel Arroyo, of the Colombian company Corpacero, depending on the characteristics that the corrosion protection must present, different zinc-based protection techniques are applied. Among them are the following:
1. Hot dip galvanized: The parts to be treated are immersed, once the surface has been cleaned, in a molten zinc bath that is usually at a temperature of 445°-460°C. Zinc reacts with iron, or steel, forming alloy layers on the surface. The outermost layer is usually unalloysed ductile zinc. Zinc binds metallurgically to the base metal to form a protective coating that possesses excellent corrosion resistance.
2. Electrolytic galvanizing or zinc plating: This technique consists of depositing on the piece a layer of zinc by direct current from a saline solution containing zinc. The process is used to protect smaller parts, when they require a more uniform finish provided by galvanizing.
3. Sherardization: Prepared parts of iron or steel are heated with a mixture of zinc powder and sand in rotary drums at a temperature below the melting point of zinc (380° C – 400°C), until it forms a closed coating on the surface of the piece. The coating is very uniform. The process, which results in a gray matte coating, is mainly used for small parts due to the difficulty of heating large parts evenly.
4. Coatings with zinc powder: In this kind of coating a very fine zinc powder is used that is suspended in an organic or inorganic binder. The application techniques of this type of coatings are similar to those used for the application of lacquer (projection, immersion). Zinc powder layers have limited conductivity, since zinc is not found on the entire surface in contact with the base material and does not form zinc-iron alloys in the bordering areas.
5. Cathodic corrosion protection: A metal in a state of corrosion dissolves anodically. In cathodic corrosion protection, corrosion is prevented by making the metal protect a cathode. This is achieved by arranging a short circuit element composed of the material of the piece to be protected and a less noble metal alloy. Both materials have a metal conductive connection. By immersing the pair of materials in an electrolyte like seawater, the noble minor metal dissolves, dissociating into ions and electrons. The ions pass to the electrolyte, while the electrons pass through the metal connection to the surface of the noblest metal. Cathodic corrosion protection is used, for example, as external protection of the submerged part of ships, walkways, docks, breakwaters, stakes, lock doors, buoys and underwater equipment for, for example, the extraction of oil and natural gas.
Table 1 presents a comparison made by Arroyo, which details characteristic aspects of each of the procedures used for zinc protection.
Zinc coating
The most common procedure for zinc protection is galvanizing, consisting of immersing the part in a solution of molten zinc and, in some cases, alloyed with other metals, for a short period of time. Although there is also a procedure called electrodeposition, which allows to protect with zinc without the need for immersion, the low costs and the possibility of having a mass production make galvanizing the most common system in our environment.
According to Manuel Arroyo, of the Colombian company Corpacero, the newly obtained galvanized coatings are characterized by having a very characteristic shiny metallic appearance, but that is lost over time until it acquires a matt metallic gray tone. "This change is due to the reaction between zinc and air, resulting in the formation of a thin layer that constitutes a protective barrier that isolates the surface of zinc from the environment," he explains.
It adds that the zinc used is of a high purity and complies with the ASTM B 6 high special grade standard and that, on some occasions, it is advisable to make an addition of aluminum to the zinc bath. This process controls and inhibits the iron-zinc reaction, prevents excessive coating thickness, provides uniformity in appearance and increases coating adhesion and formability.
Special conditions
One of the big questions that have arisen in the industry revolve around the possibility of applying paints to improve the appearance of galvanized surfaces, mainly for advertising purposes.
Arroyo assures that, in fact, everything galvanized can be painted. However, it is necessary to make a proper primer of the surface; if conventional phosphates are used, it is recommended not to stack the pieces during drying, as this will affect the adhesion of the paint. Similarly, galvanized surfaces can be coated long ago, as long as the proper primer process is done.
In the event that the process of applying the paint is done after galvanizing and before the installation of the part, the light abrasive jet, or sweep blasting, can be used, since the degree of contamination is much lower.
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