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Note for Design of Steel Structure - DSS by Iliyas Khan

  • Design of Steel Structure - DSS
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  • Civil Engineering
  • B.Tech
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Design of Steel Structures Prof. S.R.Satish Kumar and Prof. A.R.Santha Kumar 1.5 Corrosion Corrosion, fire protection and fatigue failure of steel structures are some of the main concerns of an engineer involved in the design and construction of structural steel work and these aspects do warrant extra attention. A review of international literature and the state-of-the-art in steel construction would reassure the designer that many aspects of corrosion, fire and fatigue behaviour of structural steel work are no longer the major issues. For example, the steel construction industry has developed excellent protective coatings that would retain service life even after 20 years without any attention! Similarly, the emergence of ‘fire engineering of steel structures’ as a specialised discipline has addressed many of the concerns regarding the safety of structural steel work under fire. In India ‘Fire Resistant Steels (FRS)’ are available which are quite effective in steelwork subjected to elevated temperatures. They are also cost effective compared to mild steel! Similarly, fatigue behaviour of steel structural systems has been researched extensively in the past few decades and has been covered excellently in the published literature. The revised Indian code IS 800 has introduced separate sections on each of these aspects pertaining to steel structures. Corrosion is an electro-chemical process involving an anode, a cathode and an electrolyte. In the case of steel, when favourable condition for corrosion occurs, the ferrous ions go into solution from anodic areas. Electrons are then released from the anode and move through the cathode where they combine with water and oxygen to form hydroxyl ions. These react with the ferrous ions from anode to produce hydrated ferrous oxide, which further gets oxidised into ferric oxide, which is known as the ‘red rust’. From the above discussion, it is clear, that the main interest of the structural designers is to prevent the formation of these “corrosion batteries”. For example, if we can wipe out the ‘drop of water’ shown in Fig.1.11, the corrosion will not take place! Hence using the “eliminate the electrolyte” principle, wherever possible we need to device detailing and protection to surfaces of structural steel work to ensure that the Indian Institute of Technology Madras

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Design of Steel Structures Prof. S.R.Satish Kumar and Prof. A.R.Santha Kumar combination of oxygen and water are avoided and hence the corrosion batteries are avoided. On the other hand, steel is anodic in the presence of stainless steel or brass and cathodic in the presence of zinc or aluminium and the second property can be used to protect it from corrosion. The types of corrosion encountered in structural steel elements are: Pitting corrosion: As shown in Fig.1.11, The anodic areas form a corrosion pit. This can occur with mild steel immersed in water or soil. This common type of corrosion is essentially due to the presence of moisture aided by improper detailing or constant exposure to alternate wetting and drying. This form of corrosion could easily be tackled by encouraging rapid drainage by proper detailing and allowing free flow of air, which would dry out the surface . Fig 1.11. Mechanism of corrosion in steel Crevice corrosion: The principle of crevice corrosion is shown in Fig.1.12. The oxygen content of water trapped in a crevice is less than that of water which is exposed to air. Because of this the crevice becomes anodic with respect to surrounding metal and hence the corrosion starts inside the crevice. Bimetallic corrosion: When two dissimilar metals (for e.g. Iron and Aluminium) are joined together in an electrolyte, an electrical current passes between them and the corrosion occurs. This is because, metals in general could be arranged, depending on their electric potential, into a table called the ‘galvanic series’. The farther the metals in the galvanic series, the greater the potential differences between them causing the Indian Institute of Technology Madras

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Design of Steel Structures Prof. S.R.Satish Kumar and Prof. A.R.Santha Kumar anodic metal to corrode. A common example is the use of steel screws in stainless steel members and also using steel bolts in aluminium members. Obviously such a contact between dissimilar metals should be avoided in detailing. Fig 1.12. Mechanism of crevice corrosion Stress corrosion: Fig 1.13. Mechanism of stress corrosion This occurs under the simultaneous influence of a static tensile stress and a specific corrosive environment. Stress makes some spots in a body more anodic (especially the stress concentration zones) compared with the rest as shown in Fig.1.13. The crack tip in Fig.1.13 is the anodic part and it corrodes to make the crack wider. This corrosion is not common with ferrous metals though some stainless steels are susceptible to this. Fretting corrosion: If two oxide coated films or rusted surfaces are rubbed together, the oxide film can be mechanically removed from high spots between the contacting surfaces as shown in Fig. 1.14. These exposed points become active anodes compared with the rest of the surfaces and initiate corrosion. This type corrosion is common in mechanical components. Indian Institute of Technology Madras

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Design of Steel Structures Prof. S.R.Satish Kumar and Prof. A.R.Santha Kumar Bacterial corrosion: This can occur in soils and water as a result of microbiological activity. Bacterial corrosion is most common in pipelines, buried structures and offshore structures. Hydrogen embrittlement: This occurs mostly in fasteners and bolts. The atomic hydrogen may get absorbed into the surface of the fasteners. When tension is applied to these fasteners, hydrogen will tend to migrate to points of stress concentration. The pressure created by the hydrogen creates and/or extends a crack. The crack grows in subsequent stress cycles. Although hydrogen embrittlement is usually included in the discussion about corrosion, actually it is not really a corrosion phenomenon. Fig 1.14. The mechanism of fretting corrosion 1.5.1 Corrosion protection to steel structure elements Corrosion Protection methods – The methods of corrosion protection are governed by actual environmental conditions as per IS: 9077 and IS: 9172. The main corrosion protection methods are given below (Cl. 15.2.3): a) Controlling the Electrode Potential b) Inhibitors c) Inorganic/Metal Coatings or Organic/Paint systems Taking care of the following points can provide satisfactory corrosion protection to most structural steel elements: The design, fabrication and erection details of exposed structure should be such that good drainage of water is ensured. Standing pool of water, moisture accumulation and rundown of water for extended duration are to be avoided. The details of connections should ensure that Indian Institute of Technology Madras

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