Corrosion in Chemical Industry Lecture Notes 2024

YILDIZ TECHNICAL UNIVERSITY CHEMICAL ENGINEERING DEPARTMENT CORROSION SECTION 5 PROF. DR. EMEK MÖRÖYDOR DERUN RES. ASSIST. ENİS MUHAMMET GÜL 1 Graphitic corrosion  Graphitic corrosion has occurred primarily in buried cast iron pipe and becomes evident only after decades of service.  As a result of...

YILDIZ TECHNICAL UNIVERSITY CHEMICAL ENGINEERING DEPARTMENT CORROSION SECTION 5 PROF. DR. EMEK MÖRÖYDOR DERUN RES. ASSIST. ENİS MUHAMMET GÜL 1 Graphitic corrosion  Graphitic corrosion has occurred primarily in buried cast iron pipe and becomes evident only after decades of service.  As a result of this corrosion, pipes which weaken and can not bear the operating pressure can explode and cause direct or indirect losses. 2 Graphitic corrosion To prevent; ........................................... ........................................... ........................................... 3 Stress Corrosion Cracking (SCC)  It occurs in case of combine action of tensile stress, aggressive (corrosive) environment to sensitive materials.  The materials subjected to the stress corrosion cracking are damaged by the formation of cracks. The directions of forms cracks are perpendicular to the tensile stress. The stress that produces stress corrosion cracking can be externally applied or residual. The residual stress may be a result of phase changes due to heat treatment, cold-work or welding.  4 To prevent Stress Corrosion Cracking (SCC).......................................................................................................................................................................................................................................................................................................................................................................... 5 Fretting Corrosion  Fretting Corrosion defines as the abrasion caused by vibration and friction between two metal surfaces which are contacted and under load.  The prongs that are amalgamate under high tension will be scraped during the sliding movement and oxidize by oxygen of that air that enters the interface.  The formed oxide particles are hard and play a scraper role because they can not be removed from the interface. 6 Fretting Corrosion To prevent; ................................................. ................................................. ................................................. ...................................................................................................................................................................................................................................................... 7 CORROSION IN VARIOUS ENVIRONMENTS Corrosion rate (mdd) Steel Zinc Copper - 0,17 0,14 Sea atmosphere 2,9 0,31 0,32 Industrial 1,5 1,0 0,29 Sea water 25 10 8 Soil 5 3 0,7 Rural atmosphere atmosphere 8 Atmospheric corrosion In dry regions relative humidity is very low and the thickness of water layer is not enough to create a continuous electrolytic layer on material surface. In moist regions the relative humidity of air is suitable for formation a particular water layer on the material surface and in such regions corrosion rate is high. In IIIth zone the water layer on the surface become thicker and visible. The reason for decrease in corrosion rate is weakness of diffusion of O2 through the thick water layer. In IVth zone, the water layer is very thick so material behaves like it is in water. 9 Underground corrosion   ....................... of soil........................of soil As the thickness of water layer on the material surface increase, the diffusion of O2 will be decrease so the corrosion rate will be smaller. However, before this event the fall in resistance makes corrosion rate higher. 10 Underground corrosion  The penetration of air into the soil; 11 pH and Salt Content of Soil    The salts, alkalines, which are dissolved in soil, react with water and win conductivity properties. The corrosion rate of material in soil, increase with increasing salt and water content. The corrosion rate in dry soil is........................... The pH of soil is between 4 and 8. But when bacteria change these values and make soil more acidic. Thus, more aggressive, corrosive soil forms. 12 Temperature of soil  When temperature of soil rises moisture evaporates so the soil becomes drier and the corrosion rate decreases. But temperature change is not much effective on soil corrosion. Temperature of soil is............................................. 13 Effects of bacteria There are many types of microorganism in the soil.  Aerobic: Metabolism activities are maintained in presence of O2. This type of microorganism reduces sulphur and sulphuric compounds to H2SO4. They are seen in oil areas and places where sewage flows into the sea. 2S-2 + 3O2 + 2H2O → 2H2SO4  Anaerobic: Metabolism activities are maintained in the environments that not content O2 or in low O2 concentrations. This type of microorganism reduces sulphates to sulphur. SO4-2 + 4H2 → S-2 + 4H2O 14 OXIDATION DRY CORROSION O2, CO2, S mixture.  Oxidation is the reaction of any oxidizing gas with metals.  Oxidizing gas can be O2, CO2, S or their mixture. Reaction can be occurring at room temperature or at high temperatures.  The common form of metal oxidation happens in presence of O2.  15 OXIDATION DRY CORROSION  The oxide film over the metal surface can be named in different ways according to its thickness. The film which’s thickness is less than 200 nm is called as....................................... 16 Pilling-Bedworth Ratio Rate of oxidation and the tendency of the film to protect the metal from further oxidation are related to the relative volumes of the oxide and metal. The ratio of these volumes, termed the Pilling–Bedworth ratio,........................................................................................... moxide= weight of oxide doxide = density of oxide mmetal= weight of metal dmetal= density of metal 17 Pilling-Bedworth Ratio nM + mO2 → MnO2m PB < 1 Oxide covers smaller volume than metal. The formed porous oxide film over Mg is not protective. PB = 1 The volume of oxide film equals to metal’s volume. The oxide film adhered to the metal surface and it is protective. PB >1 the volume of oxide film is bigger than metal and gradually thickens. The oxide film breaks and leaves the surface. 1