Paint-Anodized-Galvanic (P-A-G) Protection
Although structural steel elements have a high strength and robust structure for the construction sector, deterioration may occur over time, like other structural elements.
The most important of the deteriorations occurring in its structure is “corrosion” damage. The fact that corrosion damage is both a physical and chemical change emphasizes the importance of the damage to occur. Corrosion causes mass loss in metals, resulting in a decrease in strength and service life, and can change the properties of the material with chemical reactions.
Corrosion protection types can be examined under 3 main headings. These:
In order to prevent corrosion, when different metals are used during the design phase, insulations should be made to prevent the contacts of these metals. It should be designed to prevent water accumulation. In this regard, care should be taken not to accumulate water in the most welded areas. In addition, since the rough surface will cause water accumulation, care should be taken not to create a rough surface during the paint application. Precautions should be taken against the formation of cracks or holes.
When choosing materials, economy should be considered along with safety factors. In this context, some corrosion may be allowed depending on the life of the material. In addition, by adding different elements into our material, its resistance to corrosion can be increased. For example, mixing 0.20% copper into the steel material or adding chromium, nickel and molybdenum elements in appropriate proportions can significantly increase the resistance of the steel against corrosion.
Steel gains resistance to electrochemical effects with galvanic coating and paint, and physical effects with barrier coating. There are coatings that do both. A choice is made between coating or paint according to environmental conditions, economic life, and the need for repetition.
Metallic coating by spray method
First, the surface of the steel material is cleaned. Then, molten zinc or aluminum is sprayed hot onto the steel material. During spraying, the metal is cold and its surface is not damaged. One of the most important points here is that the surface should be cleaned by sandblasting before the spray coating process.
Metallic coating with electrolyte method
For detailed information, see the electro galvanizing method!
Metallic coating by hot dip method
For detailed information, see the hot-dip galvanizing method!
Inorganic coatings (Anodized coating)
For detailed information, see the anodized coating method!
Organic coatings (Paints)
Paints are mentioned in detail in the TS EN ISO 12944 standard. Paint is a protective material that everyone knows very well but does not know its function exactly. Paints are generally thought to be used for aesthetic beauty, but the first purpose of paints is to protect, and the second purpose is to provide aesthetic beauty.
Paints can be applied to the surface of almost any metal. It is the most widely used protective method due to its ease of application and cost. Paints insulate metal surfaces from the external environment and prevent corrosion. If the metal surface is directly exposed to the atmosphere, the paint is applied in two layers. While the inner layer prevents corrosion of the metal surface, the outer layer prevents the passage of water and oxygen. When choosing paint, the corrosive properties of the environment to be applied and the life of the facility where the paint will be applied should be taken into consideration. Paint application has various application methods such as brush application, electrostatic method, dipping and spraying.
Paint coatings are used more than the sum of all other coatings in steel structures against corrosion. The main reasons for this are:
Ease of application
Paint is cheap
Application equipment is cheaper
Applicable to damaged structures
Being applicable everywhere (High structures, underwater structures, etc.)
Can be applied even on bad surfaces
For all these reasons, paint is the most advantageous coating method.
For powder coating, see powder coating application method!
Aluminum profiles can be produced in a wide variety of shapes, as well as in a wide variety of surface appearances. Aluminum surface treatments not only give the product the desired aesthetic appearance, but also help to increase the resistance of the profiles against corrosion and wear. In general, we can talk about 3 different surface treatments.
Anodizing, Anodic Oxidation, Anodization (Eloxal, Anodizing, Anodic Oxidation)
Electrostatic Powder Coating (Powder Coating)
Wood Look, Wood Pattern (Wood Effect)
The type of electrochemical surface treatment, which can give aluminum brown and black tones, even copper and gold colors, without spoiling its metallic appearance, is called “anodizing”. The pretreatments applied before the anodizing process determine the texture of the metallic appearance and whether it is matte or glossy.
From the production side, first of all, it should be sought that the anodized aluminum profiles are produced by manufacturers with Qualanod quality certificate.
Anodizing pretreatments are mechanical and chemical pretreatments, respectively, and vary according to the desired surface appearance. If a matte appearance is desired, satination mechanical pre-treatment is applied. According to this method, extrusion lines are eliminated with stainless steel brushes and a unique satin surface texture formed by the brushes dominates the profile. If a bright appearance is desired, polishing pretreatment is applied. Accordingly, the profiles are polished with a liquid or solid polish.
In DIN 17611, standard nomenclatures were used for anodizing pretreatments and color selections. Accordingly, when we say E6EV1 to indicate the color selection in E0, E1, E2, E3, E4, E5 and E6 preprocesses EV1, EV2, EV3 and EV6, it means that EV1 (Natural Color) is desired after E6 (Chemical Matting) preprocessing. C0, C31, C32, C33, C34 and C35 are used in the naming of EURAS (European Association of Anodizing Companies).
E0 degreasing and oxidation
E3 mechanical polishing
E4 sanding + brushing
E5 sanding + polishing
E6 chemical matting
EV1 neutral color
EV3 golden yellow
EURAS color notations
C0 natural anodized color
C31 very light bronze color
C32 light bronze
C34 dark bronze
Accordingly, anodized surface treatment E0 EV1 can be used as a basis to protect aluminum from corrosion. Apart from the decorative appearance, it may be requested to be anodized on the aluminum surfaces where the glass will be transferred to the aluminum on the curtain walls.
The anodizing process is an electrochemical process. Accordingly, the aluminum profile is immersed in an acidic electrolyte as the anode. Direct current is passed between the cathodes in the bath and the profiles with anode behavior, and with this method, an oxide layer of 5-30 microns, based on aluminum oxide, is formed on the surface of the aluminum profile.
How many microns should the anodized layer be?
The chart below can be used in this regard. It is seen that the 25 µm (micron, micrometer) thick anodized coating is eroded by 0.07 to 0.50 µm/year depending on the weather conditions.
The amount of wear of the anodized layer (micro meters – years)
In summary, the durability of the 25 µm thick anodized layer is 40 years, and 30 years in bad weather conditions.
According to DIN 17611, 20 micrometer anodized layer is required for anodized profiles used for architectural purposes. According to the British standard (BS), it is required to be 25 micrometers.
The surface of aluminum profiles can be coated with powder paint and the desired color appearance can be obtained. Standard RAL colours, color mixes, special colours, plus velvet, rough etc. surface effects and even surface properties such as self-cleaning can be achieved with powder coating. Having so many varieties is an advantage when compared to anodized.
From the production side, we must first look for the condition that powder coated aluminum profiles must be produced by manufacturers with Qualicoat quality certificate. There are over 500 products and over 400 licensed paint coating facilities.
What is powder coating?
It is also called electrostatic powder coating, powder coating, powder coating.
Powder coating is a type of coating applied as a freely sprayed dry powder. The coating is applied electrostatically and then cured under heat to allow it to flow to form a thin layer.
There are two main categories of powder coatings: Thermoset and thermoplastic polymers. The most common polymers used are polyester, polyurethane, polyester-epoxy, plain epoxy and acrylics. The components of the powder coating process may vary slightly from manufacturer to manufacturer.
Powder coating is used to create a hard, continuous coating on the aluminum surface that will protect the metal from corrosion and provide an attractive appearance that lasts when exposed to various weather conditions. Powder coating is mainly used for household appliances, aluminum extrusion products (window profiles, other architectural profiles, etc.), automotive and industrial applications.
Powder Coating Properties
Because powder coating does not have a liquid carrier, it can produce thicker coatings than conventional liquid coatings. The coating process emits little volatile organic compounds (VOCs: Volatile Organic Compounds). Several powder colors can be cured together so that the colors can be combined in a single layer.
mixing and special effects are allowed. Thick coatings are relatively easy to apply, allowing for smooth, non-woven coatings, thin coatings are not.
One of the most important advantages of powder coating is that the excess powder that cannot be attached to the product during the powder coating process can be recycled.
Many noticeable advantages (hardness, gloss, etc.) of powder coating over liquid coating (except PVDF) are actually a characteristic of polymers.
Powder coating process
Preparation process and pretreatment
Cleaning of aluminum before powder coating is done by various chemical and mechanical methods. The choice of method depends on the material and size of the part to be powder coated, as well as the performance requirements of the finished product.
The pretreatment process both cleans the metal and increases the adhesion of the powder to the metal. Chemical pretreatments usually take place in multiple stages and consist of steps such as degreasing, etching, de-casting, various rinsing and chromating.
Recently, processes have been developed that prevent the use of chromates. Titanium and zirconium chemicals and silanes also show similar performance against corrosion and increasing dust adhesion.
Another method of preparing the surface before coating is known as abrasive blasting or blasting.
Powder coating application process
The most common way to apply powder coating to metal objects is to spray the powder using an electrostatic gun. The gun imparts a positive electrical charge to the powder. The powder is sprayed by accelerating towards the part to be painted, which is then grounded by mechanical or compressed air. Then, the object covered with powder is heated and the powder is melted and spread on the painted part in the form of a uniform film layer.
When a thermoset powder is exposed to a high temperature, it melts, flows and then chemically reacts to form a higher molecular weight polymer in a network structure.
This curing process can take place at a certain temperature for a certain period of time. Normally powders cure at 200°C (390°F) for 10 minutes.
Applications for powder coated aluminum
Powder coated aluminum is a building material that can find applications in a number of construction projects. Powder coating on aluminum helps to increase the corrosion resistance ability of the metal. This makes the material ideal for outdoor use.
The architectural powder coating offers a wide range of appearances, starting with white, in numerous RAL colors and other special colors, with desired effects.
Industrial uses is one of the largest markets for powder coatings. For example, the automotive industry is experiencing a dynamic growth in this regard.
The choice of powder coating depends on the application and is not simply a personal choice. Powder coating does not contain solvents and the coating process does not create hazardous waste. The recyclability of excess powders that do not adhere to metal during the process makes powder coating an environmentally friendly method.
In order for the aluminum profiles to have the desired surface color and properties, one of the processes of anodizing (anodization) and electrostatic powder painting is applied. As a result of the anodized surface treatment, the metallic surface of the aluminum can still be noticed, while the surface gains a more plastic appearance as a result of powder coating.
When we compare these 2 surface treatment options;
Advantages of Anodizing:
It is easy to maintain, it can be wiped with water and regained its old appearance.
It does not peel or flake.
Since it is a transparent coloring, the natural metallic surface of aluminum is noticeable.
It is resistant to the sun, does not fade. However, organic coatings are affected by ultraviolet rays.
You’ll see consistent color no matter which angle they’re viewed from.
There is no problem of peeling off like paint in silicone facade applications where we glue the glass to aluminum with silicone.
There is no such thing as paint peeling, no peeling. Because anodized coating is part of the surface.
It is more resistant to physical contact and abrasive cleaning materials. In this respect, it is easier to use in places where it can be accessed and where contact may occur.
Disadvantages of Anodized:
It is weak against acidic pollutants in city life.
Color types are very few compared to powder painting.
It is more difficult to keep the same color, especially when intermediate dark colors are desired.
Only matte and glossy surfaces can be achieved
Since only aluminum can be anodized, it is difficult to match with other materials on the facade.
It is difficult to remove scratches etc. that occur afterwards.
Electrostatic powder coating advantages:
Color retention is not a problem, regardless of whether they are produced in the same batch.
Color variations are many.
Resistant to acidic and alkaline chemical cleaners.
The process does not pollute the air.
Powder coatings emit zero or almost zero volatile organic compounds.
Electrostatic powder coating disadvantages:
If the pretreatments are not done correctly, the effect we call “Filiform Corrosion” is seen.
Orangeness may be seen.
Chalking can be seen.
Silicone is not suitable for attaching glass to the facade with silicone.
Galvanizing, whose name comes from its inventor, Luigi Galvani, is roughly the name given to the process of coating steel by dipping it into molten zinc at 450 – 455 degrees. Zinc forms a three-phase layer by forming strong bonds with iron, and it is beneficial in many ways by preventing the contact of the product with oxygen as well as adding durability to the durability of this product. In addition to its contribution to durability, this process is carried out in order to prolong the life of the metal, especially against corrosion. In addition, it should be known that if the galvanized metal is coated with chromate, the life of the metal will increase even more.
In particular, this anti-corrosion process is carried out to prolong the life of metals that will operate outdoors in all weather conditions. If the galvanized metal is coated with chromate, its life will increase. After the galvanization process, attention should be paid to the micron value of the galvanization, its adherence to the material, the burrs in the holes, the galvanized deposits at the ends, the materials stuck together, the dross adhesion, the uncoated areas and the ash adhesion and white rust. Iron material should be designed in accordance with galvanization at the manufacturing stage. Zinc inlet holes and pressure outlet holes are especially important in hollow materials. The galvanizing process is as follows. It consists of degreasing, rinsing, etching, rinsing, fluxing, drying and dipping. There are also some small parts that can be galvanized. Many materials such as chain galvanized, anchor galvanized, profile galvanized, sheet galvanized, stud galvanized can be galvanized. Galvanized grounding stakes and galvanized grounding strips are also used in grounding lines.
The things to be considered after the galvanization process are the micron value of the galvanization, its adherence to the material, the burrs in the holes, the materials stuck together, the ash adhesion and the white rust. At the same time, the iron material must be designed to be suitable for galvanization. We can state that zinc inlet holes and pressure outlet holes are very important especially in hollow materials.
Processing of Galvanized
Galvanized coating, which is a fine work, is essentially done in two ways according to the methods. The first of these is electro galvanized coating, while the second is the coating made with hot dip galvanization. The second method, the hot-dip galvanizing method, prevents oxidation on the metal covered with zinc coatings and allows the life of metals to be extended.
Preventing oxygen, which causes rust, destabilization of the metal and deterioration of its appearance in case of contact with the metal, provides great benefits to the metal both technically and visually. Therefore, the best way to protect against rust is galvanization. Thanks to the fact that the coated zinc decreases and disappears at a certain rate every year, the life of the metal can be easily determined after this method.
Electro Galvanized Coating Method:
This method is accomplished by immersing two anode and canoe electrodes in an electrode-filled vessel. While zinc is connected to the anode electrode, the material to be coated is attached to the canode electrode. At this point, electricity is supplied with the help of various machines and tools, and the electron detached from the anode is connected to the material in the cathode. Following this, the passivation process ensures that the new coating is permanent. It should be noted that the electro galvanized closure method does not change the properties of the material. While there is a risk of removal of the coating in the hot-dip galvanizing process, there is absolutely no such risk in this method. In electro galvanizing, the process is carried out with pure zinc.
Hot Dip Galvanizing Method:
In this process, it is a more economical, longer-lasting and high-quality coating method on iron and steel products. It is known that the life of materials coated with galvanization in the hot-dip method increases by an average of 30 years. It should also be noted that the most used method in surface coating methods compared to other methods is the hot dip galvanizing method and it does not require any maintenance afterwards. One of the biggest handicaps of the hot-dip galvanized coating method is that it cannot be welded. The parts must be galvanized one by one and subjected to this process in disassembled form.
Galvanized coatings are coatings formed by a chemical process, consisting of a series of anti-corrosion, highly wear-resistant zinc/iron alloy layers coated with a pure zinc layer, to which steel and zinc are metallurgically bonded. For most hot rolled steels, the zinc-iron alloy portion of the coating provides the balance, with the free zinc outer layer accounting for 50-70% of the total coating thickness.
The chemistry of the steel affects the appearance of the galvanized coating. Steel compositions vary in strength and service requirements. Trace elements (silicon, phosphorus) in steel affect the galvanizing process as well as the structure and appearance of the galvanized coating. Outside the recommended ranges, steels with these elements are known in the galvanizing industry as highly reactive steel and can produce a coating consisting entirely or almost entirely of zinc-iron alloy layers.
Typical coatings made from reactive steels show coating properties that differ from a typical galvanized coating such as:
Appearance: Atypical galvanized coating may have a dull gray appearance and/or a rougher surface due to the absence of a free zinc layer. The free zinc layer found in typical coatings gives the galvanized coating a brighter surface.
Adhesion: The zinc-iron alloy coating tends to be thicker than a typical galvanized coating. In rare cases where the coating is extremely thick, there is a reduced probability of adhesion under external stress (thermal gradients, sharp impact).
Reactive steels are still normally galvanized and it is important that differences in appearance have no effect on the corrosion protection provided by the galvanized coating. The performance of the coating is based on the zinc thickness. This is why the matte (and thicker) coatings usually produced by reactive steels will last longer. Furthermore, all galvanized coatings that are weathered over time will develop a uniform matte gray appearance.
It is difficult to provide a complete guide to steel selection without considering all the steel grades available on the market. However, these discussed guides will assist you in selecting steels that provide good galvanized coatings.
Carbon levels less than 0.25%
Phosphorus less than 0.04%
Manganese less than 1.35%
Silicon levels less than 0.04% or 0.15 – 0.22%
Since silicon is used in the steel deoxidation process and is constantly present in cast steel, it can generally be present in most galvanized steels, although silicon is not part of the controlled composition of the steel. Both silicon and phosphorus act as catalysts during the galvanizing process, resulting in rapid growth of zinc-iron alloy layers.
Even when both elements are kept separately to the desired limits, the combined effect between them can still form an atypical coating of all or mostly zinc-iron alloy layers. Where possible, galvanizer steel grade to determine the need for special galvanizing techniques
should be informed in advance.
Hydrogen embrittlement is important for steels with a final tensile strength of 150 ksi (1100 MPa). During the chemical reaction between steel, rust and chemicals used in the pickling stages of the galvanizing process, hydrogen is produced and absorbed by the steel. The very small grain structure of the high-strength steel prevents the hydrogen from being ejected during immersion in the molten zinc. When under stress during use, steel can become brittle and brittle.
Of course, galvanized steel can be painted. This process is known as “Duplex Coating”. Here, the synergistic effect of zinc and paint provides more protection against corrosion than just galvanized material. The paint on the outer surface prevents and delays the aggressive attacks of the atmosphere, while the galvanized film layer on the bottom protects the metal as well as prevents the paint from cracking, prolonging its life. Paint on galvanized steel also extends the life of the galvanized layer. In other words, when both applications are together, a high synergistic effect occurs. However, duplex coatings facilitate repainting, security stamping and color coding. It also paves the way for different usage areas where it is desired to be noticed in color (colorful city furniture, playgrounds, stops, etc.).