Comparison of protection systems

Impact on costs
In order to understand what this chapter about we need to remind some characteristics of parts for zinc which are significantly influence on the protective layer thickness. And consequently they cause the increase of processing cost. It also concerns the surface, chemical composition and thickness of steel.

Like a weight, the surface of steel changes in proportion to its thickness. In fact, a sheet surface has a much greater risk of corrosion compared with the profile. The compound of the steel surface also determines the differences in the amount of zinc, which will become a part of the protective coating.  So, it can be much thicker than provided for present standards.

1. The presence of the steel such elements as carbon, silicon, phosphorus, sulfur, aluminum, chrome, copper, molybdenum, nickel, and titanium produces differences in the amount of zinc which is included in the reaction with the steel for coating formation. They also identify some differences in the aesthetic appearance of the galvanized product.

2. The thickness of zinc coating (that is formed during the reaction) is proportional to the thickness of steel.

Comparative savings of different forms of protection.

The organic protective layers (that were received by means of dyeing) sometimes may have a low initial cost, but they can require a continuous attention and service. As the result they will be extremely expensive.

First expenses

The cost of protection by means of hot-dip galvanizing at the first time often overbalance in comparison with the organic protection.  But then this type protection becomes the most advantageous form in all respects during the medium and long-term period.

Hot dip galvanizing is generally cheaper then the coating system with three layers of paint. In order to reduce costs and improve the final product in the future, the designer (projector) in the beginning of process should consult with the manufacturer regarding steel components reaction.

There is frequent mistake in the process of protecting against corrosion. The person takes into account only the initial cost ignoring the countless advantages that provide different protection systems.

Indeed  besides the initial price, you should consider many other factors:
- The total cost attributable to the whole period of the product existence
- Ease of application
- Time
- Easy to control
- Easy to fix
- Strength
- Reliability: the three ways protection
First of all let’s see the initial cost, compare them with the cost of other types of coverage. And then we consider other factors that directly or indirectly have an influence on the cost of galvanized steel.

The cost of hot-dip galvanizing is lower than with coatings of three layers of paint in many applications.

The reason is simple – alternative systems (especially paint) require more manpower than galvanizing and term performance generally longer.The cost of galvanizing increases with increasing steel thickness because of increased time and necessary costs to heat the material to a temperature 450-460 ° C. The thickness doesn’t important when you apply zinc coating with the help of spray.
We must also consider that the presence of some elements in the alloy of steel determines the thickness increase.
Some examples are:

Carbon: the protective layer of zinc is the similar for steel and iron. The only difference is the discontinuity of gamma-phase (region of alloy closer to base). Usually it is favorable for adhesion.
The presence of carbon in the form of graphite doesn’t affect on the rate of growth layers of alloy. On the contrary it is very important a form in which carbon is present in a solid solution, such as perlite, sorbitol, martensite, etc.

Silicon: the urgency  of steel galvanizing question  with the help of deoxidized silicon with following  complications:
- acceleration of the reaction leads to a large coating thickness increasing the slag during the process and use a large number of zinc;
- a large thickness of the coating often makes it more fragile, and the surface may look dark;
- aesthetic appearance may look heterogeneously with a dark patches on a bright background;
- there can be seen premature brown spots on the cover, although the coating remains fully effective.

Phosphorus: Phosphorus is often added to steel to improve its mechanical properties.
In the metallurgical process (especially in conjunction with silicon) phosphorus can cause an increase in the rate of reaction with the following development of abnormal coating. In some cases, the phosphorus concentration can improve the coating adhesion.

Total cost for the whole period of the product

Total cost of galvanizing for the whole period of the product is even more favorable (compare with the cost of painting) because first service for the hot-dip galvanized products is necessary just after a long period of time.
Usually, the hot dip galvanizing is changed outside slowly.  It is typically less than 10 microns per year even in the urban atmosphere or in the presence of sulfur trioxide or the coastal environment in the presence of chlorides.

Dual system

Afterwards to a dual system (Duplex) the final cost is reduced. It consists in painting after hot-dip galvanizing and can have two objectives:
1. Aesthetic, camouflage or signal;
2. The increase of the lifetime protection (especially in the presence of specific aggressive environments). It is supposed that possible intervention in the service may be difficult in the future or when zinc layer is very thin (sheet).
3. Protection from galvanic corrosion; if the hot-dip galvanized steel should be in contact with another metal with high electrical potential (copper).
Double treatment allows increase the protection time from 150 to 225% on the amount of time both types of protection, taken separately: just zinc or just color.

The action of both protection forms is a joint. For example: zinc which is under the paint doesn’t form the rust at the points of penetration through the paint which are caused by moisture, chemical agents or mechanical action. At the same time paint protects the zinc coating against the weather and ensures the safety. It means that the protection by means of a dual system can be active for the whole period of structure’s lifetime.

Painting of  after hot-dipped galvanizing

Numerous tests have shown that the system DUPLEX (dual protection system) is the most effective way to protect the steel and make the brightness of the first layer of paint.
Although the hot-dip galvanizing usually gives a full and satisfactory protection against the corrosion it is necessary sometimes to use an additional protection in aggressive environments.

The Dutch experiments showed that the lifetime of the system can be calculated using the following formula:

LT = K (LZn + Lf), where

LT = lifetime of a dual system in years.

K = depends on environmental factors, varying between 1.5 and 2.3, low figure applies to the sea water  environment.

LZn = estimated service for zinc layer in years (in the medium)

Lf = the estimated service layer of paint in years (in the real world), if it is applied on steel.

Hot-dip galvanized painting of surfaces sometimes requires more effort than any other painting of surfaces. In order to get a good and long lasting grip with paint in practice a hot-dip galvanized surface should be carefully cleaned and you should use the correct type of paint.

The recommendation “to stay hot-dip galvanized surface open to external influences before painting at least 1-2 years” was correct up to 1950.  At that time the atmosphere was relatively clean and it means that corrosive particles occurred on the zinc surface were almost formed of zinccarbonate.
Thus, after a period of time in the open condition, the paint is not applied to the surface of zinc but to an inert layer of carbonates. And the result was mainly positive, even with the colors which now we can name inappropriate.
Today this method is questionable, since the atmosphere is heavily polluted with sulfur oxides and corrosion zinc products often contain zinc salts which are soluble in water. In such cases it is possible blister forming or detachments, regardless of the staining system.
Shiny surface of the fresh galvanization

Hot-dip galvanized surface (with its shiny appearance) often considered to be sufficiently pure for subsequent staining. However in many cases it doesn’t correspond to the facts and give unsatisfactory results.

When you want to paint hot-dip galvanized materials - the best results will be achieved when:
- The subject is cooled on air to avoid contamination from the water used for cooling;
- The object is stored in a safe place and not become contaminated with dust;
- Transportation and warehousing are carefully controlled, the subject is contained in a dry and clean to avoid spots of dampness.

Just hot dip galvanized surface is rarely actually clean as it may seem because of its shining. Thin films of oil due to contact with gloves, shoes and the appliances for movement may actually reinforce the impression of shiny and clean surface. In fact, the contamination occurs in thin, transparent and difficult to find layers.
A galvanized sheet is often chrome or coat with oil to protect from damp spots during storage. Chrome-plated layer to a certain extent is soluble in water and painting on oil film rarely achieves the desired result. So these layers should be removed before painting.
Matt surface

When zinc coating is open its surface is covered with rust and corrosion products. Basic zinc carbonate which is formed in contact with air can be absorbed by the dye. Therefore there is an advice to paint galvanized materials after a year being in the clear.
But now it is much more difficult to determine the composition of corrosion products. Hygroscopic or water-soluble salts contained in them form the layers that are not suitable for staining.  Therefore, it’s necessary to clean the surface before painting.
On the contrary, paint layers are more or less moisture-permeable. If water-soluble products are not removed from the galvanized surface there can be formed large bubbles on the inside layer of paint. They can cause a progressive delamination of paint film between the paint and zinc corrosion products.
Cleaning and surface preparation

The experience has shown that sandblasting is the best preparation for adhesion between the paint and zinc. Its mild mechanical action removes all corrosion products from a surface and other contaminants as well as water-soluble products.
Many surfaces after the hot dip galvanizing are sufficiently smooth and shiny.  For them sandblasting improves the staining, because it determines the best mechanical bond coat of paint.
Inadequate cleaning can prevent the paint adhesion whereas excessive mechanical forces can destroy the layer of zinc and provoke a disruption. And subsequently it can lead to peeling.
About 10 microns of zinc remove when sandblasting done correctly. In the case if it can’t be used we recommend the following procedure:
1. alkaline degreasing
2. washing
3. phosphatizing
4. washing
5. drying
6. coloration
Chrome-plated products require an alkaline degreasing and sometimes cleaning with a brush or a nylon fabric impregnated with an abrasive composition based on aluminum oxide powder.
A phosphate layer should be as thin as possible but should give a uniform 4.2 g/m2 coating. The thickness less than 1 micron is generally considered optimal.
In the case of hand-painting a removal of grease with softening materials is recommended. It is better to combine use brush or nylon with water rinse and under a high pressure (if possible).
Some degreasers contain additives that can dissolve the oxides. This may be an advantage when the composition of corrosion products is not known.
The choice of paint
There is still a certain lack of information of composition and colors. Especially with respect to their use on galvanized steel.
Recommended types of paints below can be applied directly to galvanized steel after a single thorough cleaning. For other colors we recommend applying a one soil (primer) layer (preferably epoxy) on the surface of zinc.
When you choose paint, remember that it can contain up to 10-15 different components. Each production has its own formula for a certain type of paint. Suppliers of raw materials also have different formulas. All this means that it’s possible an infinite number of combinations.
So paints of the same type but from different manufacturers have different properties.
For paint sprayer at high pressure are preferred epoxide resins, polyurethane or based chlorinated rubber.
Epoxy paint should be a two-component polyamide-type: the content of nonvolatile components should not be too high. Volume content of the pigment should be 28-33%.
It is normal to use an epoxy type primer when for the final coating use epoxy, polyurethane materials or based on resin. Always check with the paint manufacturer primers compatibility with the paint to cover. Also never use a primer and paint for the final coating from different manufacturers.
One of the difficult application of the epoxy paints may be their tendency to separation.
Chlorinated rubber and two-component polyurethanes are more sensitive (when you apply it to zinc) to the material of the composition and colors than epoxy resin. For chlorinated rubber is preferable to use the pigment content in the proportion of 26-30%. The best of them contain zinc phosphate as an inhibitor and aluminum iron micaceous oxide to use their barrier effect.
Polyurethane paints shall have the pigment content of 30-35% and preferably pigmented with the iron micaceous oxide or aluminum.
For manual color suitable two-component resin, urethane-modified acrylic or alkyd-modified acrylic resin. Also there are suitable latexes based on acrylic resins or polyvinyl acetate. Although they reach its hardness and adhesion characteristics after 10-14 days. Some types of paints based on mixtures of rubber and rubber-modified asphalt seem reliable and easy to use.
Solutions of asphalt (aluminum pigmented) can be used on structures submerged in water. But  keep in mind that they have low mechanical strength. This type of stains shouldn’t make at temperatures below 10.8 C.
The above paint all relate to the type of drying in the open air. A forced drying is carried out with great care.
It is necessary to achieve the total thickness of 75 microns and not less than 50 microns locally.
You should always consult with the supplier, if you are going to apply powder paints or paints with laminar layers.