Resistance protection created by hot-dip galvanizing

- Resistance of galvanizing in different environments: atmosphere, urban environment, sea air, industrial environment. Resistance in liquids, soil, in contact with other ferromagnetic metals (without iron properties) and with organic substances, in reinforced concrete armature.
- Resistance to other influences
- Reddish-brown discoloration
- The spots caused by moisture: white rust

Resistance protection to corrosion in different environments

Hot-dip galvanizing (HDG) is now widely tested in numerous conditions of use. Resistance protection to corrosion is highly dependent on environmental conditions; lifetime is estimated as exceeded 10-30 times than service lifetime for the unprotected steel, located in the same conditions.

The corrosion products

Zinc and zinc alloys have an excellent corrosion resistance in the atmosphere and water. Of course, indicated resistance varies and depends on their components. The property, which gives to zinc a special opportunity of corrosion resistance - its ability to form a protective layer consisting of a mixture of zinc oxide, zinc hydroxide and various salts of basic nature depending on the type of environment.

When this layer is formed and completely covered the surface of metal the reaction rate is greatly reduce.

Zinc is a material with a relatively low galvanic potential and a tendency to rust. However, it has a low rate of corrosion in most environments because of the fact that the coating quickly covered with corrosion products, which protect the underlying part of the coating from the progressive corrosion action.

Corrosion in the atmosphere

When the hot-dip galvanized product “goes out” of the galvanizing bath the surface immediately subject to the aggression of atmospheric oxygen ; a thin film of zinc oxide (ZnO) formed there. A formed membrane quickly transformed under the action of, carbon dioxide and other contaminants in the atmosphere.

In the atmospheric moisture zinc oxide change into zinc hydroxide Zn (OH) 2 which in the carbon dioxide change into basic carbonate of zinc, 2ZnCO3 • Zn (OH) 2. Impermeable membrane is characterized by excellent adhesion. In theory it is insoluble in water and thus provides a good surface protection for zinc coating.

At the beginning disappears a bright covering, metallic sheen, which replaces the mut color of light-gray color.

Influence of pH

The corrosion products of zinc have a volume much greater than that had a zinc which gave a birth to them. So, even a small loss of zinc gives the impression of a large number of corrosion products.

The formation of the protective layer is strongly depends on the pH of the environment. The rate of corrosion varies with pH, the impact on the protective coating becomes much greater at pH values below 6 and above 12.5.

The outer atmosphere contains different concentrations of corrosive substances such as various gases, soot, moisture (fog, dew, rain and snow) and different inert and aggressive dust. Concentration levels can vary greatly between different localities and at different times of the year, with favorable and unfavorable combinations and effects.

The corrosion is usually considered in connection with four types of environment:
1) rural atmosphere
2) marine environment
3) urban atmosphere
4) industrial atmosphere

Urban atmosphere

Air in industrial areas and in urban centers is contaminated by various components of the sulfur which, together with the air moisture transform the impenetrable and corrosion resistant layer in normal conditions: zinc carbonate and zinc oxide into zinc sulphate and zinc sulphite. These zinc components are water-soluble and have a weak grip. Therefore, it is relatively easy to wash them away by rain. Thus, the next new surface get under the influence of oxygen and the corrosive cycle repeats.

In accordance with the atmosphere in which corrosion products formed they are also differ in appearance.

Marine environment

In the marine environment zinc corrosion depends on the salt in the air. However, in the sea air, there are also small levels of magnesium salts, which have a passivating effect. So, the corrosion is not so strong in this environment.
The salt content in the air rapidly decreases from the shore towards the inland.
The corrosion products formed in the marine atmosphere have color lighter than those formed in the rural or urban atmosphere.

Due to the large number of factors that cause the corrosion it is impossible to establish a general formula for determining the speed of process.
It is obvious that the use of zinc coatings to protect steel from corrosion in different conditions has satisfactory results.

Industrial atmosphere

There are many long-term tests. The choice of a particular speed value of shown gamut should be performed for each individual case and making the choice it is necessary to evaluate the corrosion ability of the atmosphere on the basis of the marked conditions. There is often one and the same aggressiveness atmosphere degree as for steel and zinc. Nevertheless, it is not. The steel corrosion increases for example, with the increase in atmospheric levels of sulfur and chlorine and with increasing moisture. At the same time the zinc corrosion is slower.

Resistance protection in liquids

In this environment, as well as in the case of corrosion in atmosphere, forms a protective layer of corrosion products on the surface of the zinc.
Liquids may be acidic or alkaline and may contain various aggressive substances dissolved in the liquid or solid particles.
There are also co-operating factors determined by the velocity and temperature of the stream, which can become an extremely important.
For this reason, the protective layer of corrosion products can dissolve at different speeds or may not be formed.

pH is the determining factor for the formation of a protective layer In liquids, even more strongly than in other surroundings. Electrochemical corrosion, which has a secondary importance in an open atmosphere in liquids, is very important.

In accordance with the electrical liquid’s conductivity electrochemical corrosion and the protective effect of zinc layer appear in a big or small proportions.

Hard water

Types of hard water which contain calcium and magnesium are not highly aggressive. Carbon dioxide and calcium form insoluble carbonates on the zinc surface that protect from the action of corrosion.

Soft water

Soft water is strongly attacks the zinc because it doesn’t contain salt and can’t form a protective layer.

In some cases it may also provide treatment of the polarity between zinc and steel. And therefore the last becomes the electrode (element) with a lower electric potential (anode). In these cases, there is a high risk of pitting corrosion.

This polarity treatment counteracts the carbon dioxide, sulfates and chlorates in water. And it can’t be detected, for example, in sea water.

If water has a flow rate greater than 0.5 m / s the formation of a protective layer on the zinc coating surface is often difficult and there is a rapid corrosion of the coating. In the same way the temperature is important. With water temperatures around 55 ° C above the corrosion products formed on zinc have a structure with large grains and lose contact with the surface of zinc. Polarity inversion (when the steel becomes more electronegative than zinc) may occur at approximately 70 ° C with the following development of pitting corrosion.

The sequence of corrosive reactions in different types of water is complex. And it can be affected by moderate changes in the composition of water. Thus, it is difficult to establish common rules for the consideration of consistency and completeness of corrosion depending on the different types of water.

Corrosion in the soil

In its structure, the soil has a different permeability to air and moisture. Usually, the oxygen concentration is lower than in the air, but the content of carbon dioxide is higher. In addition the soil may contain products that change in atmospheric conditions: free or sequestering salt, acid or alkali, a mixture of organic compost, hydrogen, methane, oxidizing or reducing mold, bacteria, etc. As a consequence, the conditions of corrosion in the soil are very complex and there may be significant differences between different localities even very closely spaced to each other. The soil is not particularly aggressive. The average corrosion rate is approximately 5 mm per year. Soils with significant aggression are rare. A special method to determine corrosion capacity of the soil is a measure the resistivity.

Indeed, the ability of soil corrosion is always proportional to the resistivity (ohm) and is directly proportional to the available moisture content. If parts of galvanized steel or other galvanized metals will be installed in the soil – it need to consult with experts.

The behavior of zinc coatings in contact with nonferrous metals

Zinc is almost always more electronegative than the most widely used metals. This means that in the electrochemical activity the zinc acts as anode,(self-destruction). Therefore there is a good rule to isolate the elements of rubber and plastic in contact with other metals. The risk of galvanic corrosion of zinc coatings in with copper contact is greatly increased. So, it is important as to avoid any contact with the water-rich in Cu ions.

Being in the air galvanized products can remain in contact with stainless steel and aluminum, but they must be isolated when they immersed in water.

Behavior in contact with organic materials and structural materials: mortar, gypsum plaster and wood. Mortar and gypsum plaster immediately after application attack zinc but the aggression ends at a time when they dry.

Raw wood shouldn’t be in a contact with galvanized steel, because some substances that present in the tree in this condition have a corrosive effect on zinc. You also can’t use hot-dip galvanized nails in the water was the wood impregnated or not. In such cases it is better to use stainless steel nails.

Conversely dry or moderately wet wood, with / or without impregnation may be connected with the hot-dip galvanized nails with good results. Other dry materials (such as mineral wool) don’t attack zinc.

Concrete

Unprotected reinforcement can corrode under some environmental conditions, due to moisture that penetrate in the concrete through cracks and crevices. The rust has a volume greater than steel the coating layer on top of rebar can "explode" and crumble. Steel partially coated with mortar or concrete can cause rust spots on the surface of the concrete especially when it poorly protected from rust (for example, in the case of bolts and fenders). Such damage can be avoided if the steel reinforcement will be hot-dip galvanized.

Numerous benefits give the fact that the coating on the reinforcement rods may have thickness thinner reducing the weight of structure. The last may be thinner and more accurate. Hot-dip galvanized crossbar or mesh can be effective in precast facades. For example when you don’t want to stain facade construction with a rust.

During the laying the concrete has a Ph about 13. In solutions with such a high Ph zinc is subjected to aggression and produces hydrogen, which could cause a poor adhesion, but the aggression ceases as soon as concrete grasp.

Almost all types of cement contain modest amounts of chromate. They passivate the surface of zinc which is no longer exposed to the aggression of raw concrete. If the cement contains chromate less then necessary amount in order to give a final mixture concentration of chromate at least 40 ppm. Zinc galvanized rails can be passivated after hot-dip galvanizing in chromate solution or chromate may be added to water when mixed a concrete.

Resistance to other influences

Abrasion resistance

In the hot-dip galvanizing coating has a metallurgical type and completely envelops the metal substrate with a barrier strength of about 80 Vickers for the outermost layers. And the most internal layers of the alloy reach and exceed 200 HV.

Thus, the alloy layer is much more resistant to abrasion than pure zinc.

The experiments showed that the abrasion resistance of the alloy layer is approximately 4-5 times larger than the layer of pure zinc.

The hot-dip galvanized products can be used where the surface is exposed to abrasion. For example, transport wagons, ladders, fenders, doors or floor hatches, handrails and so on.

Zinc coatings at high temperatures

Zinc coatings may be under the constant influence of the temperature up to 200 C. The diffusion reaction is detected in the coating when the temperature is high. Because of that outer layer of zinc is easily peeled and separated from the underlying layer of iron-zinc. This layer of iron-zinc resists the corrosion and depending on thickness it may protect steel against rust for a long period of time.

Reddish-brown discoloration

After a period of time, some galvanized steel can acquire a reddish-brown color, which in the sulphurous atmosphere may even be black.

An iron-zinc reacting with the environment releases an iron and forms compound based on iron with a reddish color. Even small amounts of these compounds can cause significant discoloration. Thus, it would seem that the protection against corrosion has been exhausted (especially if the discoloration is strong) but it happens rarely.

In fact the effect is quite opposite because iron-zinc coating protects steel from rust better (30-40%) than pure zinc.

Such discolored areas can also be painted with a good result. The corrosion rate of zinc surface is not too dependent on the purity of zinc. The zinc content is 98% or 99.9% and it behaves in the same way in many cases. Some of the elements (which present in the alloy) significantly increase the resistance to zinc.

Aluminium is more advantageous in coatings and it’s a major addition to the products with a trade names Galfan and Galvalume (such coatings are used only on a sheet and highly specialized companies).

Spots because of the moisture delay: white rust

Sometimes there is a white porous coating on the galvanized surface, which can crumble. This phenomenon is commonly known as white rust or white corrosion. It is found on new galvanized products with a shiny surface and especially between gaps that exist between too closely packed sheets, bath, etc., when the surfaces contact with condensate or rain water and if moisture can’t dry quickly.
Zinc surface (which already have a normal protective layer of corrosion products) is rarely exposed to this defect.
If the zinc coating has the opportunity to respond normally in the open air there are formed zinc oxide and zinc hydroxide and in the presence of carbon dioxide in the atmosphere these compounds are converted into basic zinc carbonate.

If the air circulation on the surface of the zinc coating is limited carbon dioxide and oxygen are not available in sufficient quantity to form a natural layer of zinc carbonate. Consequently, the received layer of zinc oxide and zinc hydroxide is porous and volume. And it doesn’t abut to the surface of zinc in a compact way.

Thus, the corrosion can continue until the moisture stay on surface.
Once white corrosion discovered all products should be positioned in such way in order to dry surface quickly.
In this way, the aggression stops and we get a normal protective layer of corrosion products with the help of regular blowing the surface.
White corrosion products are washed away step-by-step. And surface coating get a typical sort of hot dipped galvanized product put up in the air.

Note that given corrosion products are very bulky (about 500 times bigger that zinc) aggression may seems serious, but it doesn’t greatly affect the longevity of protection against corrosion. In the case of thin zinc coatings, such as electro-production the massive aggression of this type can, cause damage.

Temporary protection from white rust can be achieved if the products will be chromed or phosphate. Paint after galvanizing also provides an effective protection. Already formed corrosion products of white corrosion can be partially removed by brushing with a stiff bristle brush. They can be completely removed by washing in acetic acid solution with following rinsing with water.

However, these treatments don’t return the original form and the brilliant sparkle of the metal. The recent standards of zinc testing in white corrosion, say the following: "Possible formation of moisture delay points (white corrosion products or dark in color, most often the main zinc oxide formed during storage in a damp environment after galvanizing) are not the reason for not accepting the material."