Sıcak daldırma yöntemi ile sürekli çinko kaplama
Galvanizing process
- Tez No: 83071
- Danışmanlar: PROF. DR. MEHMET ÇAPA
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Galvanizleme, Kaplama, Sıcak daldırma yöntemi, Çinko, Çinko kaplama, Galvanization, Coating, Hot dip method, Zinc, Zinc coating
- Yıl: 1999
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
SICAK DALDIRMA YÖNTEMİ İLE GALVANIZLEME ÖZET Belirli korozif ortamlara karşı dayanıksız olan yada az dayanım gösteren malzemelerin bu ortamlarda kullanılabilmesi için özel kaplama malzemeleri kullanılmaktadır. Bunlardan çinko kaplama, özellikle çelik yapıların korozyondan korunması yöntemleri içinde en yaygın kullanılan metodlardan biridir. Çinko kaplamalar, genellikel « Sıcak Daldırma Yöntemi ile Galvanizleme » metodu adı verilen, çelik yapıların ergiyik çinko banyosuna daldırılması ile uygulanır. Sicak daldırma ile üretilmiş çinko kaplamalar, geniş bir kullanım alam olan çelik ürünleri için başarılı bir korozyon direnci sağlamaktadır. Çinko kaplamanın etkinliği yüksek korozyon direncinin yanısıra kaplamanın sıyrıldığı kısımlarda açığa çıkan çelik yüzeyini katotik olarak korumasından kaynaklanır. Sıcak daldırma yöntemiyle çinko kaplamanın ilk aşaması kaplanacak metalin yüzeyinin temizlenmesidir. Yüzey temizlemeyi izleyen ikinci aşama flakslama işlemidir. Flakslamada amaç ona metal yüzeyinin galvaniz banyosuna gidinceye kadar oksitlenmesini engellemektir. Yüzeyi temizlenmiş, flakslanmış çelik 430- 470°C de ergimiş çinko banyosuna daldırılır. Belli bir süre bekletilip alındıktan sonra genellikle soğuk havada kurutulur. Oluşan kaplama bir dizi tabakalardan ibarettir. Çeliğin tabanından başlıyarak, her tabaka dış yüzeye kadar yükselen oranlarda çinko içerir. Bu yüzden demir ve çinko arasında belirli bir sınır yoktur, fakat ana metal ile kaplama arasında güçlü bir bağ sağlıyan demir-çinko alaşım dizilerine doğru kademeli bir geçiş vardır. Sıcak daldırma yoluyla galvanizleme yöntemi ticari olarak senelerdir başarıyla uygulanmaktadır. Bu metodun tercih edilmesinin sebebi, yanlız çinkonun çelik üzerinde sabit ve kuvvetli bir şekilde yapışan bir kaplama meydana getirmesinden değil, aynı zamanda sürekli daldırma yöntemlerinin uygulanmasıyla geniş ölçüde bir üretim yapılmasına imkan vermesinden ve dolayısıyla maliyetlerin düşürülmesinden kaynaklanır. Bu avantajları nedeniyle sıcak daldırma yöntemi ile sürekli galvanizleme metodu geliştirilerek önümüzdeki senelerde de sıkça kullanılacaktır. vıı
Özet (Çeviri)
GALVANIZING PROCESS SUMMARY Galvanizing is the practice of immersing clean, oxide-free iron or steel into molten zinc in order to form a zinc coating that is metallurgically bonded to the iron or steel's surface. The zinc coating protects the surface against corrosion by providing protection to the iron or steel in two ways. l.It shields the base metal from the atmosphere 2.Because it is more electronegative than iron or steel, the zinc gives cathodic or sacrificial protection. Even if the surface becomes scratched and the base metal is exposed, the zinc is slowly consumed while the iron or steel remains protected from corrosion [6]. The hot dip galvanizing process is adaptable to coating neraly all types of fabricated and non-fabricated products such as wire, tanks, sheets, strip, pipes and tubes, fittings, hardware, wire cloth, hollow-ware, and structural assemblies. All galvanizing consists of four fundamental steps :. Surface preparation. Prefluxing. Galvanizing. Finishing t“, <\ ',7* <* -?\ ”ivAv**f v^^^v"^- *^_-*.jii5 Figure 1. Cross Section of a hot dip Galvanized Coating vuiThe preparation steps consist of cleaning and pickling operations that free the surface of dirt, grease, rust and scale. The preflux step serves to dissolve any oxide that may have formed on the iron or steel surface after pickling and prevents further rust from forming. Clean, oxide-free work is galvanized by immersion into molton zinc. Finishing operations include quenching, removing excess zinc and inspection. The above photomicrograph and accompanying figure represent a typical cross section of a hot dip galvanized coating. The outermost (Eta) layer of the coating is formed as a covering of pure zinc that is carried out on the material as it is withdrawn from the molten zinc bath. The underlying series of zinc/iron allyos in the coating are the result of a metallurgical reaction between molten zinc and the steel or iron material being galvanized. As the zinc/iron alloys from, they will grow perpendicularly to the steel surface. The effect this has on corners and edges of material is that the coating there is generally thicker than the surrounding coating, as seen in the micrograph below. This is in sharp contrast to other types of protective coatings that tend to thin out at the edges and corners of material. Figure 2. The coating at the edge and Corners of material The galvanized coating itself is considered to be a self inspecting system. This is because the reaction between the molten zinc and the steel will not occur unless the steel surface is chemically clean. In effect, a galvanized coating that appears sound and continuous is in fact sound an continuous. If a coating should become damaged however, the zinc will continue to provide cathodic protection to the exposed steel. Even if areas as much as lA inch in length and/or width become exposed, the surrounding zinc will provide cathodic protection to this area as long as the coating lasts. As for mechanical protection, the galvanized zinc coating literally becomes part of the steel substrate thus, an adhesion bond on the order of several thousand psi exists between the two. Also, since the zinc/iron alloys are harder than the underlying steel, they will provide excellent abrasion resistance to the galvanized material by providing good impact resistance. The combination of all these layers to produce the galvanized cooting provides toughness and resistance to mechanical damage in transport, erection and service. The thickness and appearance of the galvanized coating are affected by several factors. These include : - The chemical composition of the steel IX- Bath immersion time - Bath withdrawal rate - Steel cooling rate 1. The chemical composition of the steel is very important to the quality of the galvanized coating since a metallurgical reaction between the zinc and the iron is occuring. As an example, silicon contents in the steel between 0.05 % -0.1 1 % and above 0.25 % will cause the zinc/iron alloy layers to grow abnormally fast. In fact, the silicon causes the galvanizing reaction to continue unabated and the outermost pure zinc layer is eventually consumed in the process. This not only causes the coating to be thicker than normal, it will also most likely consist entirely of alloy layers. The incerased presence of these alloy layers in the coating causes it to become brittle and a dark gray, matte finish will also result. Steel compositions that affect the coating formation and properties in this manner are referred to as «reactive steels». RECOMMENDED ASTS STEEL GRADES FOR HOT DIP GALVANIZING [Structural Shapes and Plates; A3 6 A441 A529 A242 type 2 A500 A572 A283 A501 A588 ISteel for Sheet Metal: A569 A570 tSteel for Pipe or Tubing: ~ A53 A120 A595GrAorB Steel for Fasteners: Bolt Material Nut Material Carbon Steel High Strength A303 Gr A or ] A325 Type 1 BA563GrA A563GrDH Tower Bolts A394 A563GrA Quenched & Tempered|A449 Carbon Steel Bolts A563 Gr C Quenched & Tempered A354 Gr BC Alloy Steel Bolts A563GrDH GENERAL CONSIDERATIONS The generic composition of a steel to be galvanized that will produce an acceptable coating will be carbon less than 0.25 %, phosphorous less than 0.05 % and manganese less than 1.3 %. Also, as discussed above, in order to avoid the adverse effects of silicon on the galvanized coating, silicon content in the steel should be in the range of 0-0.04 % or 0.15-0.25 %. Steel with these recommended levels should develop a normal galvanized coating. However, should the composition of thesedevelop a normal galvanized coating. However, should the composition of these elements fall outside any of the previously stated ranges, the steel will most likely still able to be galvanized provided that the Galvanizer is notified in advance of any deviations. 2. In general, rough steel surfaces will cause the galvanizing reaction to produce thicker coatings due to the increased surface area. These thicker coatings, however, will be rough and have a generally poor appearance. 3. Galvanizing is a diffusion process. As with all diffusion processes, the reaction between the molten zinc and the steel or iron will proceed quickly at first but will slow down as the alloy layers grow and become thicker. Thus, continued immersion or dipping material more than once will not produce a significantly thicker coating expect in the case of reactive steels. 4. The withdrawal rate of material from the galvanizing bath has the greatest effect on the outermost layer of pure zinc. A rapid withdrawal will produce the thickest coating because the largest amount of zinc will be carried out on the material and will subsequently solidify and become the pure zinc layer. 5. Slower withdrawal allows the zinc to effectively drain back into the kettle producing smoother, thinner, and more uniform coatings. Figure 3. Life of coating XISlower cooling rates, such as those experienced with air cooling or even with thicker sections that have been water quenched, allow the zinc/iron alloying reaction to continue. The inner alloy layers will use the outer. Eta Layer for their supply of zinc causing the coating to become dull, matte gray. Quickly reducing the temperature to less than 300 degrees. Fahrenheit will halt the galvanizing reaction and minimize the formation of dull or matte surfaces. The graph below illustrates the expected service of a galvanized coating as a function of coating thickness in varying atmospheric conditions, the life of a coating is linearly related to its thickness. Very simply, the thicker the coating, the longer it will last. For galvanized sheet steel with better coating adhesion and good coating forming properties, the thickness of the intermetallic layers should be controlled below 20 % of the total coating thickness. Control of intermetallic layer thickness can be achieved by adding 0.1 to 0.3 % Al into the zinc bath. This addition retards the growth of the intermetallic layer so that the coating is mostly pure zinc. Other alloying elements, in addition to aluminium, can influence appearanc,e mechanical properties and durability of coating. Two commercial processes used in the continuous hot dip galvanizing of sheet steels are the COOK - NORTEMAN process and the SENDZİMİR process [1]. In the Cook - Norteman process, the steel sheet in chemically cleaned. Typical cleaning steps may include alkaline degreasing and acid pickling. After cleaning, the sheet is immersed in a liquid flux (zinc - ammonium chloride), dried and preheated to less than 260 C before entering the galvanizing bath. In the Sendzimir process, the steel sheet is heated in a high temperature furnace consisting of an oxidising atmosphere to remove organic oils and surface contaminants followed by heating in a reducing furnace with a hydrogen - rich atmosphere to reduce the surface oxide layer and to anneal the steel substrate. The discharge end of the reducing furnace, is below the surface of the zinc bath this allows the continuous sheet to enter the bath without passing through a contaminating atmosphere. Galvanized sheets are used in various industries, such as white goods, automotive industry, HVAC (Heating Ventilating and Air Conritioning), electrical transmission system, production of bail and cable strip, the dockyards, profile industries and solar system collectors. In this thesis, I tried to explain the production method of hot dip galvanizing and made search in Galvanizing industry. xu
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