Corrosion in Chemical Industry PDF

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Yıldız Technical University

Prof. Dr. Emek Moröydor Derun, Res. Assist. Enis Muhammet Gül

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corrosion chemical engineering Evans diagrams materials science

Summary

These lecture notes cover various aspects of corrosion in chemical industries, focusing on different types of corrosion, such as pitting corrosion, crevice corrosion, and intergranular corrosion. Key concepts like Evans diagrams and passivation are discussed. The document is from YILDIZ Technical University.

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YILDIZ TECHNICAL UNIVERSITY CHEMICAL ENGINEERING DEPARTMENT CORROSION IN CHEMICAL INDUSTRY SECTION 4 PROF. DR. EMEK MÖRÖYDOR DERUN RES. ASSIST. ENİS MUHAMMET GÜL 1 EVANS DIAGRAMS In Evans diagrams, both the cathodic and anodic partial reactions constituting the overall corrosion reaction are present...

YILDIZ TECHNICAL UNIVERSITY CHEMICAL ENGINEERING DEPARTMENT CORROSION IN CHEMICAL INDUSTRY SECTION 4 PROF. DR. EMEK MÖRÖYDOR DERUN RES. ASSIST. ENİS MUHAMMET GÜL 1 EVANS DIAGRAMS In Evans diagrams, both the cathodic and anodic partial reactions constituting the overall corrosion reaction are presented as linear E-I curves that converge and intersect at a point, which defines the corrosion potential Ecor and the corrosion current Icor. 2 EVANS DIAGRAMS When polarization occurs mostly at the anodes, the corrosion reaction is controlled ……………. and if polarization occurs mostly at the cathode, the corrosion rate is said to be ………………. controlled. 3 EVANS DIAGRAMS  These diagrams show us ……………………… ……………………… ……………………… ……………………… ……………………  If anode area is bigger than cathode area, the corrosion rate at anode will ……………………. 4 PASSIVATION  Passivation is described as the big decrease on the corrosion rate of a metal or metal alloy which is resulted by formation protective corrosion products (thin oxide films) on the metal surface.  The protective film behaves like a barrier to a further corrosion. The passivation process occurs in 2 ways; 1) …………………………… 2) …………………………… 5 CORROSION TYPES             …………………Corrosion …………………Corrosion …………………Corrosion …………………Corrosion …………………Corrosion Dealloying Hydrogen Damage …………………Corrosion Stress Corrosion and Cracking Fretting Corrosion …………………Corrosion …………………………………………. 6 Galvanic Corrosion  Galvanic corrosion occurs because of contact of different metals. When two dissimilar alloys are coupled in the presence of a corrosive electrolyte, one of them is preferentially corroded while the other is protected from corrosion. For example, Fe-Cu couple. 7 Standard Electrode Electrode Reaction Potential, V 0 (V) (25°C) Na → Na+ + e- -2,71 Mg →Mg++ + 2e- -2,37 Al →Al+3 + 3e- -1,66 Zn → Zn+2 + 2e- -0,763 Cr → Cr+3 + 3e- -0,74 Fe → Fe+2 + 2e- -0,44 Ni → Ni+2 + 2e- -0,25 Sn → Sn+2 + 2e- -0,136 Pb → Pb+2 + 2e- -0,126 H2 →2H+ + 2e- 0 Cu → Cu+2 + 2e- 0,337 OH → ½ H2O + ¼ O2 + e- 0,40 Ag → Ag+ + e- 0,8 Pt → Pt+2 + 2e- 1,2 Au →Au+3 + 3e- 1,5 8 Pitting Corrosion The weight loss of material by pitting corrosion is small but it is a dangerous type of corrosion. For example, exposing of stainless steel in seawater. It occurs mostly in ………………………………………………………… ….. 9 Pitting Corrosion Pitting corrosion is a dangerous …………………………….which occurs in forms of …………………………………as a result of corrosion focuses on narrow regions. The formed hollows on the metal surface give a pitting appearance. Meo → Me+2 + 2eMeCl2 + H2O → Me(OH)2 + H+ +Cl10 Pitting Corrosion   The radius, depth and frequency of hollows change depending on the ………………………………………….. Total metal loss is smaller than homogeneous corrosion, but materials are punctured in a short time and become unusable. This corrosion generally occurs in the Cl- and Br- containing environments. Seawater which is rich of NaCl and O2 causes pitting corrosion. 11 Pitting Corrosion To slow down the dissolution process ……………………………. …………………………….. …………………………………… …………………………………………. Increasing the flow rate of solution Decreasing the temperature 12 Crevice Corrosion  Mo → M+ + e½ O2 + H2O + 2e- → 2(OH)-  Crevice corrosion is a ……………………………………………. concentrated in crevices in which the gap is sufficiently wide for liquid to penetrate into and yet narrow for the liquid in the crevice to be stagnant. It occurs under the heads of gaskets, rivets and bolts which cannot be seen easily. The required oxygen for the cathode reaction consumes in a short time because the solution in the crevices is stagnant. Thus, concentration differences arise between the solutions in the crevice and out the crevice. 13 Crevice Corrosion The concentration difference causes a potential difference, so corrosion occurs rapidly. Crevice corrosion is autocatalytic as pitting corrosion. Crevice corrosion occurs not only for passive metals; but the effects of crevice corrosion …………………………………….. 14 Crevice Corrosion To minimize or/and prevent from crevice corrosion;       Rather than ……………………………….. using soldering or welding. Vessels should be designed to provide a ……………………. and should be avoided sharp corners and stagnant areas The solid particles precipitated at the bottom of the fluid containers ………………………………. Non-absorptive ……………………. should be chosen. The ………………………. to the crevice corrosion should be preferred. If solution deposits cause corrosion it can be effective to ……………………. or ……………………. for cleaning. 15 Intergranular Corrosion      Intergranular corrosion is ……………………………………………………………………………… ……………………………………………………………………………… Its most prominent feature is the very high corrosion rate in the grain boundary region despite a small reduction in weight. Corrosion moves through the whole cross-section along the grain boundary. Even though the appearance of alloys does not change, they lose their mechanical resistance. Intergranular corrosion can be observed in high strength Al alloys and Cu alloys. Intergranular corrosion of Al-Mg alloy 16 Intergranular Corrosion    The common example of intergranular corrosion is observed in austenitic steels. When stainless steels which contain 18 Cr-18 Ni is heated and cooled between the range of 400-800 °C, Cr23C6 forms at grain boundaries. Because of high amount of Cr in Cr23C6 structure the Cr amount of neighbour area’s drops under the corrosion resistance limit’s. In this case, the alloy structure is called as sensitized. If the sensitized stainless steel is exposed any aggressive environment it will corrode through the decreased Cr regions. 17 Intergranular Corrosion To minimize or/and prevent intergranular corrosion ;    After welding steel is ……………………. ……………..at 1050°C. Then, with rapid quenching Cr is provided inside of the structure. The stainless steel which contains 0.05–0.02 % C can be chosen. Low-carbon steel (304 L) shows resistance to intergranular corrosion. Addition strong carbide maker’s such as Titanium or Niobium to the steel. Ti, Nb containing steels are called as stabilized steel. They react with carbon and prevent C-Cr reaction. 18 Exfoliation Corrosion    A special form of corrosion ……………………………… is exfoliation corrosion. Exfoliation corrosion takes place when Al alloys are exposed to different industrial and sea atmosphere. It is observed in the swage products of Al alloys. Layer separations in material structure occur as a result of localization of corrosion at the grain boundaries of elongated grains in the rolling direction. The corrosion product can have a bigger volume than the original material so it can dislocate the other grains. It is observed mostly at the alloys of heat-treat 2xxx, 7xxx series and cold worked 5xxx series. 19 Hydrogen Damage  ………………………………………………………. ………………………………. The damage because of H2 can appear in different forms such as leaching C from steel.  Hydrogen has only a proton in its structure and is small, so it easily diffuse into the crystal structure of many metals and alloys.  The diffused H2 into the steel structure during the annealing reacts with C and forms decarburize structures and methane gas. This case may cause an increase in pressure. The formed pores make the material structure brittle and decrease the mechanical properties. The pores in the surface can cause surface cracks. 20 Hydrogen Damage Hydrogen damages are more common in oil exploration and refining plants which have many sulfur compounds.  H2 damage creates stable effects on the metallurgical and mechanical properties of materials. To prevent,   Using ……………………. Using ……………………………………………….  Applying tempering process up to a maximum of 200°C to ensure the exit of H2 from the structure   Addition ……………………. to the environment 21 Fatigue Corrosion Fatigue corrosion is ………………………………………………………… …………………………………………………………  It can be observed at all materials. The difference between fatigue and corrosion fatigue is determined by examining the fracture surface.  The fatigue cracks are typically intragranular. Near the major crack a few cracks can be observed.  22 Fatigue Corrosion To prevent;  ……………………. by decreasing or remove the stress in the material  Addition …………………….  Increase the fatigue life by coating, painting or cladding  Applying the operations that form compressive stress on the surface. (e.g. Shot peening to surface)  …………………….. 23 Erosion Corrosion  touch flowing liquid All types of equipment which ……………………. may undergo erosion corrosion. Erosion corrosion is a problem, especially for piping systems.  Erosion corrosion occurs in elbows, T-pipes, valves, pumps, mixers, heat exchanger frequently. The surface of the corroded material is corrugated.  Rapid movement of the corrosive fluid causes an increase in corrosion rate. Also solid particles (e.g. sand) in the environment can play an abrasive role for the surface, they hit and make it easier to remove the protective film and increase the corrosion rate. 24 Erosion Corrosion To prevent;  The most important precaution for erosion corrosion can be taken at the design stage. Reduce turbulent by removing the irregularities on the material surface. Decrease the flow rate by increasing pipe diameter. Using large scale elbows.  Using high-strength materials  Cleaning environment from O2, solid particles and inhibitor addition.   ……………………. ……………………. 25 Cavitation Corrosion    Cavitation corrosion occurs at the material surfaces where ………………………………………………………………………… ……………………………..are present. For example, hydraulic turbines, ship propellers. Vapour bubbles formed in low-pressure zones and gradually grow and explode. When this happens close to the metal surface, the bubble collapse causes a concentrated and intense impact against the metal, with the induction of high local stress and possibly local plastic deformation of the material. The appearance of the surface is rough and porous. In severe corrosion conditions, honeycomb appearance is dominant. 26 Cavitation Corrosion To prevent;  …………………….  Hard and corrosion resistant surface coatings and linings.  Eliminate the pressure drop and turbulent that cause bubble formation by an appropriate design  ……………………. 27  THANK YOU FOR LISTENING… 28

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