Beton çeliklerin oksidasyonu ve oksit tabakasının karakterizasyonu
Başlık çevirisi mevcut değil.
- Tez No: 55645
- Danışmanlar: DOÇ.DR. M. KEMALİ ŞEŞEN
- Tez Türü: Yüksek Lisans
- Konular: Metalurji Mühendisliği, Metallurgical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1996
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 64
Özet
ÖZET Bu çalışmanın amacı farklı kimyasal bileşimdeki düşük alaşımlı beton çeliklerinin oksidasyon davranışlarını incelemek ve seçilen bazı beton çeliklerinin oksidasyon direçleri açısından karşılaştırmasını yapmaktır. Bu sebeple öncelikle oksidasyon davranışı incelenecek olan beton çeliklerinin üretim yöntemleri kısaca bahsedilmiştir. Daha sonra yüzeydeki oksit filmin oluşum mekanizması ve spesifik alaşım elementlerinin oksidasyon üzerindeki etkileri anlatılmıştır. Teorik bölümde son olarak, düşük alaşımlı ticari çeliklerin havada ve oksijenli ortamdaki davranışları ve beton çeliklerinin korozyonu anlatılmıştır. Beton çeliklerinin oksidasyon davranışlarını incelemek amacıyla GR40, GR60 ve Vanadyum içeren GR40 çelikleri seçilerek muhtelif ortamlarda oksidasyon deneyleri, metallografik çalışmalar, x-ışmlan ve optik mikroskop çalışmaları yapılmış ve aşağıdaki sonuçlar elde edilmiştir: i-Tempcore Prosesi ile üretilen çelikler üretimleri sonunda hızlı soğutulduğundan yüzeylerinde koruyucu oksit film oluşmamakta ve geleneksel yöntemlerle üretilen çeliklere kıyasla, atmosferik ortamda yüzeyleri daha çabuk bozulmaktadır. ii-Tempcore Prosesi ile üretilen çeliklerin yüzeyleri martenzitik yapıda, geleneksel yöntemlerle üretilen çeliklerin yüzeyleri ferritik+perhtik yapıdadır. Martenzitik yapı daha aktif olduğu için Tempcore Prosesi ile üretilen çelikler oksidasyona daha duyarlıdır. iii- Beton çeliklerinin bünyesindeki C, Mn,V gibi alaşım elementleri miktarları arttıkça, yüksek sıcaklıktaki oksidasyon direnci de artmaktadır. iv-Farkh kimyasal bileşime sahip beton çeüklerinin oksidasyon aktivasyon enerjileri 26.8-30.1 kcal/moPK arasında olup, oksidasyon aktivasyon enerji sıralaması şöyledir: GR40
Özet (Çeviri)
OXIDATION OF CONCRETE REINFORCING BARS AND CARACTERIZATION OF OXIDE LAYER SUMMARY Iron is one of the most important metals that is widely used in daily life. Iron has maintained its importance for centuries and a period in the history of man has been named after it. Continuing its technological development incessantly, iron and steel has become one of the basic raw materials of the industry today. In the 20th century, steel is produced from iron ore and scrap. The steel production from iron ore consists of several metallurgical steps. Firstly raw iron obtained from blast furnace is converted to raw steel in alkaline oxygen converter, then raw steel is processed to the desired steel composition by secondary metallurgical methods. On the other hand, steel production from scrap consist of electrical arc furnace and secondary metallurgical steps. Recently the steel casting is making continuously by special equipment's. This method has got more advantages to produce steel billets, blooms and slabs. In this study, various defects of continuous casting has been introduce as surface defects and internal defects. After continuous casting, the billets which are produced have been rolled in rolling sections. Then the final product is reinforcing steel bars. High yield Tempcore concrete reinforcing bars are produced by a new process which constitutes a real advance when compared with conventional techniques. Hot rolling is immediately followed by a heat treatment the principle of which is described here under. On leaving the last the concrete reinforcing bars pass trough a short cooling installation where they undergo surface hardening. This is immediately followed by a further stage in which the surface layer is tempered by use of the residual heat in the core of the bars. Finally they are subjected to normal cooling in an ambient temperature. All these specifications characterized by excellent weldability and sperior ductility. The oxide films which formed on the surface of the steel,is protected the steel from corrosion. Therefore its important to explain the mechanism and growth of oxide films and oxidation. Oxidation or“dry”corrosion implies the reaction between a metal and any oxidizing gas, e.g. carbon dioxide, sulfur, oxygen or a mixture of oxidizing gases at ambient and elevated temperatures. However, the most common form of metal oxidation is brought about the oxygen. VIOxide films shows different growth rates in different conditios. Growth ratesis described as a relation between film thickness and time. In practise film thickness can be measured as weight loss or gain per unit area. Logarithmic law applies to thin films formed at low temperatures. y = Ki.log [ t / K2+l] and dy/dt a 1/ 1 Parabolic law is obeyed by many metals at higher temperatures and, therefore, relates to thicker films than the logarithmic law. y 2= K3. t + Kt and dy/dt a 1/y Parabolic and logarithmic growth afford protection to the metal after a time. The linear law ; y = K5. t + Kg and dy/dt = constant applies to the initial stages of oxidation before the thin films is thick enough to be protective.At low temperatures and for thin oxide films inverse logarithmic and asymptotic laws have been identified. l/y = K7+K8.logt and y = K9 [ 1- exp (-K10. t) ] A cubic law, y = Kn.t + Kn intermediate between the logarithmic and parabolic laws also exists for some metals. (Kı,....,Kı2 are constants) It is important to note that a metal may obey different rate laws over different ranges of temperatures and in different atmospheres. Oxides, like metals, contain defects. These defects may be ion vacancies or interstitials and the concentration of each is not usually equal. This unequality gives rise to non-stoichiometry. However, even though these defects are present the agreement of ions and defects in the oxide lattice must maintain electrical neutrality. In fact the presence of defects results in metal oxides having semiconductor properties. So, far an oxide on the surface of a pure metal has been considered to be one of composition. For the growth of a thin oxide films, electrons needed to chemisorb oxygen and create O2" ions at an oxide-gas interface are supplied from a narrow region at the surface which therefore suffers electron depletion. Thus a negative surface charge is associated with a layer containing positive 'space charges'. Electrical neutrality does not exist within this layer. The effect of the electric field created tents to counteract the chemisorbtion process which therefore becomes slower. Ions move across this layer under the action of the potential gradient rather than by diffusion under a concentration gradient. Similarly a negative space charge layer may be considered at a metal-oxide interface. Between the layers an electrically neutral zone exists across which diffusion of defects can occur. Specific alloying elements also effects oxidation resistance of steel. vnThe addition of carbon to iron principally affects the subsequent oxidation response via the oxidation of the carbon in the steel to form CO and C02. Carbon together with alloying elements which forms carbides, reduces the oxidation rate of steel. At 850 °C and 1.2% C the oxidation rate is in the order Fe-Cr-C > Fe-C>Fe-Ni-C>Fe-Ti-C>Fe-Ta-C>Fe-Nb-C>Fe-V-C>Fe-W-C, which is nearly sequence of carbide stability. On oxidation aluminium forms highly refractory and protective AI2O3. Formation of iron oxide nodules is suppressed the aluminium content exceed approximately 7 wt%. Total aluminium + crom content to suppressed nodule formation is in the range of 7-8% with 7% Al required at 0% Cr, but only 3% Al required at 5% Cr. The addition of silicon to iron is confersignificant corrosion protection. Fe-Si oxides are generally immiscible. Corrosion protection arises from the formation of a SİO2 healing layer, beneath magnetite, which acts as a barrier to outward transport of metal ions. The addition of manganese to steels has little effect on the over all scaling rate in air or oxygen since Mn both soluble in iron oxide and mobile to the same extent as Fe. The presence of small quantities of sulphur in steels has little effect on the initial scaling rate in air, but may be detrimental to long-term scale adhesion. Phosphorus plays a role in suppressing oxidation in carbon steels in CO/C02 environments. The addition of nickel to iron reduces the oxidation rate due to the virtual insolubility of NiO in FeO. Of all of the alloying elements added to steels, Cr has been the most used for improving the corrosion properties. Bare iron and steel are liable to rust in most environments but the extent of the corrosion depends upon a number of factors, the most important of which are the composition and surface condition of the metals, the corrosive medium itself and the local conditions. With regard to the effect of composition, ferrous metals fall in three broad categories; 1- The ordinary cast irons, wrought irons and steel, to which no-alloying elements are added, and which are vulnerable to corrosion. 2- Low-alloy steels, which contain about 2-3 % of alloying elements, commonly copper, chromium and nickel. These steels still rust, but under certain conditions in the atmosphere, the rust formed becomes adherent and protective so that the corrosion rate becomes several times less rapid than with the ordinary steels mentioned above. These steels are often termed weathering steels. VIII3- Stainless steels, which contain high percentages of alloying elements, e.g. 18% chromium, 8% nickel and 3% molybdenum. Steel of this type are particularly non-corrodible in appropriate circumstances. Moreover, since the corrosion of cast iron is discussed elsewhere, and since little wrought iron is produced nowadays, the subject matter will virtually resolve itself into the corrosion of ordinary carbon steels, as used in mass for general purposes. The treatment will begin with a brief consideration of the mechanism of rusting and of the influence of variations in the steel itselves. It will be completed by short surveys of present knowledge of rusting of ordinary mild steel in the three natural media : air, water and soil. A great number of observations on the concrete cracking caused by the corrosion of the reinforcements was attributed to the unsatisfactory quality of the materials used for the preparation of concrete ( moer, sand, water ) and their mixing proportions, to the unsatisfactory quality of steel and its history, to the unsatisfactory construction technique and to stray currents. For the protection of reinforcements in marine atmospheres, there are four ways to protect the reinforcement from atmospheric corrosion ; 1- Cathodic protection of reinforcements : This method is rarely used for constructions in the atmosphere. A very promising new method of needle-diodes is under going long term field tests before being recommended for general application. 2- Hot dip galvanizing of reinforcement : This method is commonly used. 3- Epoxy coatings on the concrete : This method is used also in certain cases in Greece. 4- Additives in the concrete : Besides the known commercial additives, an additive of volcanic origin, SİO2, shows satisfactory anti-corrosive properties. As the aim of the study to investigate of oxidation behavior of low alloyed reinforcement steels and compare some of them in terms of oxidation resistance, oxidation, corrosion, experiments and metallographic, optical microscope and X-Rays studies were made by using GR40,GR60,GR40 alloyed with V,and GR40-Tempcore steels. IXAccording to results of experiments; 1) In atmospheric environment, surfaces of steels which produced by using Tempcore Process can be destroyed faster than the steels produced by using common process. 2) Steels produced by using Tempcore Process has martensite surface ; produced by using common process has ferrite+perlite surface. As martensite is more active than ferrite+perlite.Tempcore steels are sensitive to oxidation. 3) It is observed that oxidation resistance arises with increasing carbon and manganes. Alloying elements such as V improves oxidation resistance together with mechanical properties. 4) Over 500°C temperatures, scale which forms in high temperatures on surface of reinforcement steels cracs and fresh oxide films forms in this cracks. 5) It is observed from x-rays difractions of scale that forms in high temperatures, Fe304 and Fe203 are together in scale structure and Fe304 transforms to Fe203 with increasing temperatures. X
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