Immersion method, this means that the preparation of the surface of structural components, as well as zinc coating, is carried out by immersion in baths that contain baths of the appropriate chemical composition. This technology ensures that every nook and cranny can be reached, cleaned and protected from corrosion. The final stage of the galvanizing process is the application of a zinc coating (Zn-Al. alloy with other alloying additives) at a temperature of 445 °C to 455 °C to clean steel parts, which are immersed in molten zinc. There is a reaction between zinc and iron, which leads to the formation of a zinc coating on the surface of steel.
Its properties (structure) depend on the chemical composition of the steel substrate, as well as on its design and thickness.
The conditions for the grade of steel must be met, it is about the content of phosphorus and silicon, the result is to obtain an anti-corrosion coating free of porosity, resistant to mechanical damage, erosion, abrasion, impact, showing excellent adhesion to the substrate.
Diffusion phenomenon in hot-dip galvanizing
This technology uses the phenomenon of diffusion, that is, when “bathing” in molten zinc, the zinc atoms “penetrate” the outer layer of the steel. This creates an iron-zinc alloy on the surface of the steel component, forming a protective coating, which has a layered structure on the inside, while a layer of pure zinc is formed on the outer surface of the alloy. In a nutshell, the resulting coating consists of alloyed, iron-zinc phases and a layer of pure zinc. Alloy layers make the zinc coating inseparable from the steel substrate. This feature is very important if we take into account the fact that steel components are subjected to surface loads during transportation, installation, and subsequent use. Coatings bound to the substrate by the physical effect of adhesion, do not withstand this.
Hot-dip galvanizing – anti-corrosive properties
The anti-corrosive properties of coatings are formed when they come into contact with air and water. Forming extremely resistant and difficult to dissolve opaque coatings, they are responsible for proper corrosion protection and consist mainly of basic zinc carbonate. With the passage of time (years) and the influence of atmospheric factors, they are worn away in small amounts, however, due to the zinc underneath them, they are formed anew.
Zinc fire coating is a physical barrier shielding the steel, this provides not only passive protection, but also electrochemical protection, because zinc being a more active metal itself is oxidized, thus protecting the steel from this.
Zinc coating protects the steel for many years and does not require maintenance.
It is possible to prolong its durability and give the products the desired aesthetic qualities by coating the galvanized surface with additional varnish or paint.
Stages of hot-dip galvanizing
Hot-dip galvanizing is the direct immersion of a steel product (component) in liquid zinc (stage 7), precedes it, several preliminary stages (stages 1-6), as a result of these activities, a coating is formed to protect the material from corrosion, mechanically durable and impermeable.
Stages:
1. treatment.
The process of removing rust, sand, dirt, etc. In a closed circuit using abrasive materials, all contaminants are removed. The effect achieved is: hardening, smoothing, heating, removal of deformations, or strengthening of the surface structure.
2. degreasing
Products before immersion in a zinc bath should be free of grease (waxy, oily), all substances that if they are on the surface can prevent the element from reacting with zinc. The purpose of the procedure is to obtain a chemically clean surface, for this purpose inorganic solutions of acidic, alkaline and neutral reaction are used.
Degreasing of steel surfaces causes that:
– the duration of the next phase of the process is shortened (pickling),
– the pickling bath is less contaminated.
– labor and costs are reduced,
– consumption of hydrochloric acid, zinc, flux, is significantly reduced.
3. rinsing.
Prevents excessive transfer of process baths to subsequent chemical processing steps (inter-process). Two-stage rinsing is recommended, in a cascade with added wetting agents.
4. surface etching
The purpose of this process is to effectively remove non-metallic substances (i.e., scale, rust, and other corrosion products) formed during annealing of structural components and rolling. For this purpose, a bath in hydrochloric acid (the most popular) is used, if digestion in hydrochloric acid did not have the desired effect, a solution of hydrofluoric acid, or a mixture of hydrofluoric and hydrochloric acids is used.
5. fluxing.
Another phase that the acid-etched steel undergoes is fluxing, in an aqueous solution of zinc chloride and ammonium chloride (ZnCl2/NH4Cl), steel components are immersed, in order to ensure that the chemical reactions occurring during galvanizing take place properly. In the wet method, ammonium chloride is used in a ratio of 1:3, and in the dry method the salts make up 15% of the solution. The process is designed to clean the steel surfaces of trace amounts of oxides to minimize oxidation of the steel before it enters the liquid zinc bath.
6. drying.
A very important process of drying the flux on the surface of the components, for this purpose they are subjected to drying at 120°C – 150°C. A stage that requires great care and special precision. The temperatures of the system should be constantly monitored. Drying should be carried out efficiently, as well as quickly, so that adverse processes do not occur.
7. zinc plating.
Application of zinc coating in molten zinc (Zn-Al. alloy with other alloying additives) at a temperature of 445 °C to 455 °C.
Additional processes – extend the life of the zinc coating.
Paint coating.
Is made by applying a special paint coating to the fire zinc coating, this additional protection against corrosion, also makes it possible to achieve the desired color effects. The paint coating protects the zinc from oxidation and prolongs its life.
Passivation.
The process by which a chemically active substance in a given environment produces a passive coating on its surface, formed from the products of the chemical reaction of that substance with the environment.
The passive layer consists of zinc oxide and hydroxide. White rust is the result of the high reactivity of zinc.
The effect of white corrosion is :
– loss of gloss,
– loss of elastic properties of the coating and general deterioration of the value of the galvanized structure.
There are two types of protective agents – organic and inorganic. Both of these products can be used immediately after galvanizing in cooling water.
