CE Module 2.pptx all topics covered from phontena

Corrosion engineering module 2 topics from phontena..

1. CORROSION ENGINEERING Module 2 : ENVIRONMENTAL FACTORS & CORROSION - HEMANGI ADHIKARI

2. 01. CORROSION IN WATER AND AQUEOUS SOLUTIONS 04. CORROSION IN ACIDIC AND ALKALINE PROCESS STREAMS 02. CORROSION IN SULPHUR BEARING SOLUTIONS 03. MICROBIOLOGICALLY INDUCED CORROSION CONTENTS

3. 01. Corrosion in water and aqueous solutions

4. SEA WATER

5. Contains 3.4% salt . It Is a good electrolyte and can cause galvanic corrosion & crevise corrosion. Corrosion is affected by: oxygen content velocity temperature biological systems Corrosion are less severe in rivers & bays than in sea water but the guidelines for MoC are almost the same.

6. The greatest corrosion attack occurs in splash zone because of alternate wetting and drying and aeration. Corrosion rates may vary depending on local conditions but values are relatively accurate. Corrosion by seawater at greaterdepths is usually decreased because of lower temperature.(40^o F) Brakish water is contaminated with chlorides of tidal action in rivers and bays near ocean.

7. FRESH WATER

8. Corrosion in fresh water depends on: o2 content hardness Chlorides present sulfur content other factors Fresh water can be hard or soft depending on the mineral dissolved. In hard water, carbonates oftem deposite on the metal surface and protect it from corrosion, but pitting may occur if the coating is incomplete. Soft water are usually more corrosive because protective deposites do not form. Low alloy ss offers resistance to corrosion but in water applications it doesnot offer any advantage over ordinary steel. Eg : most boiler tubes and boiler water systems are made from low carbon steel. Cast iron, tseel and galvanised steel are most widely used materials for handling freah water.

9. HIGH PURITY WATER

10. We know that when we neeed to use water as heat transfer medium (when very less corrosion can be tolerated) then we use high purity water. Eg : Atomic power plants, conventional high pressure power units. Corrosion decreases when purity increases, because of less solids and gases present and increasing electrical resistance. Ordinary DW exhibits resistance of 200000 ohm-cm. At high temp(600-700 oF ) about 10 ppm o2 and h2 are formed. (radiolytic decomposition) In high purity water containing oxygen, intergranular attack and cracking of solution quenched ss and alloys have been observed. Corrosion products cause contamination “crud” deposits on heat transfer surfaces and plugging of thin annular spaces & control mechanisms. Zirconium, zircalloys, ss, Inconel, incoloy , are used in these services.

11. HYDROGEN PEROXIDE

12. Eg : Molybdenum in SS alloys and other than high purity aluminium alloys can cause decomposition. Doesnot corrode metals but it causes decomposition

13. LIQUID METALS

14. Just like aqueous solutions, even liquid metals causes corrosive attack. Liquid metal corrosion has a physical effect, it is the result of direct solution or solid state interactions. Types: Solution of structural metal Diffusion of a liquid metal into a solid meta Mass transfer Mass transfer by thermal gradient is very prevent in heat exchange systems because of thermal gradients. The most dangerous form of attack is the formation of intermetallic compounds on or under the surface of structural material. Eg : There is a direct relationship between corrosion rate and solubility of structural components in liquid mercury, it is necessary to have small amounts of titanium or magnesium present in the solute to prevent excessive attack. 18Cr-8Ni and higher alloy ss gives good resistance to both liquid metal and high temperature oxidation. Nickel an Ni-based alloys like monel also shows favorable resistance. Austenitic ss are attacked by magnesium at its meeting point.

15. In recent years, 5% chromium steel & Si-Cr—Mo steels have replaced Carbon steel because of their superior corrosion resistance and high rupture strength. Gallium has the widest liquidus range of any known metal and it is one of the most corrosive liquid metals. Only refractory metals are capable of resisting attack by liquid gallium.

16. FUSED SALTS

17. We can observe: Mass transport due to thermal gradient. Electrochemical interactions with formation of corrosion product in fused salts systems. Fused salts will act as a conducting electrolyte and thus sustain electrochemical reactions. Nickel and Nickel – base alloys have been observed to be the most resistant to this fused salts. Sodium Hydroxide is one of the most common fused salts used for heat transfer application.

18. 02. CORROSION IN SULPHUR BEARING COMPOUNDS

19. It is a significant concern in various industrial applicatons particularly in oil & gas industry where sulphur containing compounds are abundant. Eg : Hydrogen Sulphide, Sulphuric Acid, Etc. Presence of Sulphur introduces corrosive elements that when combines with water to form corrosive environment. Hydrogen Sulphide Corrosion: It can react with water to form corrosive species, including hydrogen sulfide ions and sulfide ions. They promote localised corrosion like pitting and stress corrosion. Sulfuric acid corrosion: Sulfuric acid forms when sulphur compounds oxidize. This acid can lead to general corrosion and attack various metals particularly when concentration are high.

20. 03. MICROBIOLOGICALLY INDUCED CORROSION

21. 04. CORROSION IN ACIDIC AND ALKALINE PROCESS STREAMS

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