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Why should not the salt spray test be used when comparing different types of coatings?
After galvanizing, the steel cools down to about 450 ° C ambient temperature. If it is cooled with water, this process will be fast, but not every galvanizer can be a cooling bath and water is not recommended in some designs due to the distortion potential of the steel. When the new galvanized steel is left to cool off (from the natural way) it can take several hours to reach the steel ambient temperature. Thin steel is rapidly absorbing, while thicker ones take time to cool. Sometimes a single piece can have different cooling rates.
Example: The outer side of a square plate cools faster than the inner side. As the interior remains warm for longer, it is possible to continue the galvanizing reaction by using the free zinc (eta) layer remaining in the outer part, especially at temperatures of 550 F and above. As the interior continues to react, the cooling exterior section does not sustain galvanization, so an irregular color appearance occurs, but this is not a condition that affects corrosion protection performance. For this reason it can be accepted according to ASTM standards.
How does galvanizing protect steel from corrosion?
The galvanizing process creates a leakproof barrier between the steel surface and corrosive agents in the atmosphere. This barrier does not penetrate chloride and sulphites, which are moisture and corrosive effects that may adversely affect the steel surface. The galvanized surface is even more anodic to the steel. This means that the galvanized surface is protected completely from corrosion by steel corrosion before the steel is completely consumed.
What are the basic steps of the galvanizing process?
The process consists of 4 steps.
1. Preliminary inspection: Determine where to use the produced structural steel first. Where necessary, the specific structures on the sample, the pouring gaps and all the general design features should be examined. It is of great benefit to make a quality galvanizing process.
2.Cleaning: The steel specimen is immersed in a burning solution to remove organic materials such as dirt and grease. Subsequently it is immersed in an acid bath (hydrochloric or sulfuric) to remove rust and scale layer. Finally, the steel is reduced to flux (flux). This allows the steel and zinc to enter the reaction easily and delay the steel oxidation. (If the steel is not cleaned perfectly, it will not react with zinc.)
3. Galvanizing: The cleaned steel is immersed in molten zinc at a temperature of 850F. The steel and zinc react metallurgically to form the iron-zinc intermetallic layer (form 3) and the pure zinc layer.
4.Son review: The galvanized steel is visually inspected to check whether galvanization is good or not. Then the coating thickness is measured with a magnetic thickness gauge.
Can galvanized steel paint work be done? What is the benefit?
Of course, galvanized steel painting is done. This process is known as “DUBLEX COATING”. Here, the synergic effect of zinc and paint provides more protection against corrosion only from galvanized material. The paint on the outer surface prevents aggressive attacks of the atmosphere and delays it, while the galvanized film layer on the bottom protects the metal as well as prevents the paint from cracking and prolongs its life. The paint made on the galvanized steel also extends the life of the galvanized layer. That is, when both applications coexist, a high synergistic effect occurs. Duplex coatings, however, facilitate repainting, security stamping and color coding. It also opens up different areas of use where you want to be noticed in color (colorful city furnitures, children’s parks, stops, etc.).
Why does the appearance of galvanized steel vary according to the type of project or material?
The chemical structure of the steel is the main factor affecting the coating thickness and appearance in galvanizing. The chemistry of steel products, which are continuously cast by steel companies, varies on a wide scale. Especially the silicon content in the steel structure is the most important factor affecting the external view (Sandelin Effect). This leads to a variety of coating appearances. Galvanizers can change the coating appearance by putting some additives into the zinc boilers (glossy, matte gray, scaly). This change of appearance does not make any difference in protection from galvanized corrosion. You can check our Technical Information section on our website (www.galder.org.tr) for the galvanized material criteria.
Why are we having trouble keeping the coating on the edges of some sheet materials?
The edges of sheet / plate type materials are prepared by passing through many different production stages. Most edges are cut by large machines if they do not have critical dimensions or if the steel is low strength. The edges of the other plates can be cut on computer controlled laser cutting tables. Another method of cutting the edges of sheet metal is to cut the oxygen with flame. The manufacturer cuts the plate precisely for steel assemblies using one of these methods or other techniques.
Some of these cutting techniques form a clean edge, but in the case where the process produces a significant amount of energy in the steel during cutting, it can change the regional properties of the high energy steel and increase the hardness of the steel at the edge of the steel plate. The increase in hardness can change the diffusion properties of the steel edge and make it difficult to form the galvanized steel on this edge.
This result becomes more complicated due to the fact that the plate has silicon and / or phosphorus content which is highly reactive in the galvanizing pot. Reactive steel forms a thick coating that can be scratched internally and externally, depending on the narrowing of the metal during the cooling from galvanizing temperature to ambient temperature. The thick coatings on the steel sheet and the high stress coating tend to flake, which is the silicon and / or phosphorus content of the underlying steel sheet.
If the galvanized coating is thick and the edge does not form a well-formed coating, it may show edge flaking marks. This can be spread by the forklift transport of the steel plates and the contact with the floor at the place where it is stored in the galvanizing plant. This material will apply pressure to the already weak bond of the coating stages and cause the pouring to start from the edges. Scratch marks on the edges can be examined and spilled parts can be repaired in accordance with ASTM A780.
The communication between the galvaniser, the producer and the steel supplier can bring about this potential problem after the layer has been coated with hot dip galvanized before reaching the examination stage.
I got the feeling that the steel that had been cut for the joint at the ends was cracked when hot dip galvanized it. Are these concerns still valid? What can I do to prevent cracking?
First of all, it is very important to understand how the joint cutting process in columns and beams is affected by the hot dip galvanizing process. According to the American Institute of Steel Construction (US Structural Steel Institute) Structural Steel Handbook, minimum ½ inch clearance should be left when assembling structural parts. In order to provide this clearance, it is useful to remove the material from the flange of the web and / or structural beams. Cutting parts of the material, cope cutting – is a practical method of removing this material. Galvanizers may demand steels with welding process that require extra attention from manufacturers before galvanizing.
Dip galvanizers have witnessed cracks around galvanized cutting sites. The cuts tend to expand radially from the radius of the joining section and appear on both sides; cracks spread throughout the material. It is known that cracks can develop with different methods in various steel sections. Cope cuts can be fairly rough and can contain considerable residual strain. This leads to a lot of concerns about fragility. Liquid metal-assisted cracking, which is a consequence of the reaction between solid steel and liquid zinc, is also a potentially increasing factor in cracking. The cracks can be repaired after galvanizing and the coating can be repaired according to ASTM A780, but it is economically advantageous to take necessary precautions and to avoid cracks.
It is important that manufacturers and galvanizers are aware that it is possible to reduce cracking as cracking occurs only in a certain number of cuts. Application of heat to the interrupted zone can significantly reduce the probability of cracking. A weld bead can be applied directly and is necessary to reduce the cracking potential of thermal application against 1 inch of cut from the cut as highlighted in the frame above.
What kind of problems do the wet storage spots cause and how are they prevented?
Zinc in a new galvanized steel has a highly reactive structure and wants to turn into corrosive structures like zinc-oxide and zinc-hydroxide. Eventually it becomes stable zinc carbonate. When galvanized steel is stacked tightly or stored in damp containers, it is cut off from free air. In such cases, zinc becomes a large number of zinc-hydroxide strata. Even the strongest wet storage stains can easily be cleaned with a cleaner or a nylon brush. To prevent wet storage deposits, galvanized steel should be kept indoors or protected from free flowing air in large quantities.