CORROSION & ITS CONTROL -Content. Introduction Classification Galvanic series Factors affecting Protection methods Summary

Transcription

1 CORROSION & ITS CONTROL -Content Introduction Classification Galvanic series Factors affecting Protection methods Summary 3

2 Learning Objectives At the end of the chapter the learner will be able to; 1. Understand the terms: Corrosion, Galvanic Series, Inhibition 2. Describe the mechanism of corrosion by electrochemical theory 3. Understand how consequences of corrosion leads to destruction & loss in the domestic & industrial sectors 4. Describe different forms of corrosion- galvanic, pitting, stress, & inter-granular 5. Explain the importance of the factors affecting corrosion 6. Know about the different techniques to control corrosion 7. Distinguish between galvanizing & tinning, electrochemical series & galvanic series 8. Unfold the practical aspects of metal corrosion with real life examples 2

3 Corrosion Metal State of higher energy Corrosion (Oxidation) Metallurgy (Reduction) Metallic compound State of lower energy + Energy Examples in real world: Zinc/Iron and copper couple Steel screws in a brass marine hardware Lead antimony solder around copper wire Importance Steel propeller shaft in bronze bearing 3

4 Corrosion Classification Dry or Chemical Corrosion Wet or Electrochemical Corrosion 5

5 Dry Corrosion Types 6 Oxidation corrosion Corrosion by gases Liquid metal corrosion

6 Electrochemical Theory of Corrosion 7

7 9 Comparison Between Dry & Wet Corrosion Dry Corrosion Involves direct attack of atmospheric gases No need of a corroding medium Types include oxidation corrosion, liquid metal corrosion & corrosion by gases Less common Wet Corrosion Existence of separate anodic & cathodic areas Need of a corrosive medium Types include galvanic, pitting, stress & intergranular More common

8 Galvanic Series Mg Mg alloys Zn Al Cd Mild Steel Cast Iron Stainless steel.. Nickel Ag Ti Graphite Au Pt Active (more anodic) Increasingly Active Passive (more cathodic) Increasingly Inert 10

9 Comparison between ES & GS Electrochemical series Galvanic series EPs are measured by dipping pure metals in their salt solution of 1M conc. without any oxide film. Eps are measured w.r.t. standard hydrogen electrode. Position of a given metal in is fixed. Gives no information regarding the position of alloys. Metals & non- metals are included. Series developed by studying corrosion of metals & alloys in unpolluted sea water, without their oxide films. EPs are measured relative to a SCE. The position of a given metal may shift as it takes active & passive states. Alloys are included based on their corrosion behavior Similarities: In both, base metals are placed higher & noble metals lower in the series 11

10 Galvanic Corrosion 12

11 Pitting Corrosion Corrosion product-fe(oh) 2 Cathodic region, OH - Pit Anodic region, Fe 2+ Iron surface Important Reasons: Surface roughness or non- uniform finish. Scratches or cut edges. Local straining of metal due to non- uniform stress. Deposition of extraneous matter such as sand, scale, water drop & dust Small anodic & large cathodic areas

12 Inter-granular Corrosion 14

13 Stress corrosion Stress corrosion or cracking refers to cracking caused by the combined effect of a tensile stress & specific corrosive environment Examples: *Season cracking of brass *Caustic embrittlement of steel 15

14 17. Primary factors-associated mainly with the metal Factors affecting corrosion Secondary factorsassociated mainly with the environment

15 Material selection principles: 19 Pure metals show greater resistance to corrosion than impure ones. Selecting a larger anodic area & smaller cathodic material of smaller area Avoid residual stress in fabricated articles by proper heat treatment The metals chosen should be very close in the galvanic series. Proper insulator fitting may be used in between to avoid direct contact of metals.

16 Anti-corrosion design Important design selection principles. Water collects Danger zone Electrical Box Poor design Leg Good design Water storage tank Free circulation of air, proper drainage & easy washing

17 Further examples of design Weld Recess Weld Weld Sharp corner Poor design Poor design Smooth bend Good design

18 Cathodic protection Sacrificial anode method Advantages: No external power supply is necessary Can be used in remote & difficult to reach areas Low installation cost Minimum maintenance cost Replacement is much cheaper than replacing the tank/oil pipeline Underground iron pipeline- cathode Ground level Mg block - anode More active metal Mg 2+ Insulated copper wire

19 22 Impressed Current Method: Cathodic protection Anode- Inert electrode (graphite/pt) Battery External power source Cathode-Steel storage tank Advantages: Applicable to large structures & long term operations. Uncoated parts can be protected. Disadvantages: Larger installation cost. Higher maintenance cost. High energy consumption Localized corrosion may occur if the impressed current is not uniform on the entire surface.

20 Anodic protection method. Passivated metal to be protectedanode Platinum electrodecathode Calomel electrode 23

21 Anodic protection method Advantages: Disadvantages: Reduced equipment cost-i.e. it is possible to protect two or more vessels with one controller Cost of anodic protection is generally independent of tank capacity Applicable in extremely corrosive environments Low current demand Cannot be applied in case of corrosive medium containing aggressive anions such as chloride Needs continuous supply of electric current Applicable only to those metals that show active passive behavior Initial installation cost is high It cannot reduce the corrosion rate to zero 24

22 Corrosion Inhibitors Chemical substance which when added to a soln. increases the corrosion resistance. Classification: Anodic Inhibitors Cathodic Inhibitors 25

23 Surface Coating Metallic Coating: Coating of a metal on a substrate metal requiring protection against corrosion. Types: Anodic metal coating coating the substrate metal with a more anodic metal eg. Galvanizing Cathodic metal coating - coating the substrate metal with a more cathodic metal eg. Tinning 26

24 Galvanization 27

25 Tinning 28

26 Comparison of Galvanizing and tinning A process of coating Zn over iron/ steel to prevent it from rusting Galvanized containers cannot be used to store acidic foodstuffs because Zn reacts with food acids forming highly toxic Zn-compounds. Zn protects the underlying iron by sacrificial action Zn continues to protect iron by sacrificial action in the event of puncture/break in coating A process of coating Sn over iron to prevent it from corrosion Sn coated containers can be used to store foodstuffs because Sn is nontoxic & does not react to cause any food poisoning Sn protects the base metal iron due to its noble nature. Sn protects the iron till the coating is perfect & any break causes rapid corrosion of iron 29

27 Summary 30 Metallic corrosion is one of the major problems faced by industrial societies. Several engg. systems depend on corrosion control for their reliability, performance & safety. The different forms of corrosion & the factors affecting are discussed. Corrosion control techniques include selection of materials & proper design, cathodic & anodic protection methods, surface coating & the use of corrosion inhibitors.

28 Review questions 1. What is corrosion? Explain the pitting corrosion with an example. 2. Justify the following; i) Iron in contact with copper corrodes faster than that in contact with tin. ii) Corrosion is reverse of extractive metallurgy 3. What is cathodic protection? Explain the sacrificial anode method of corrosion protection. 4. Explain how the following factors affect the rate of corrosion. (i) Hydrogen overvoltage (ii) Nature of the product formed 5. Discuss the importance of design & material selection in controlling corrosion 6. Explain the method of pre-treatment of the electrode surface for plating 7. Sate the distinguishing features of dry & wet corrosion 8. Describe briefly the process of galvanizing and tinning 9. Explain the type of corrosion that occurs when i) Screw & washer are made of different metals ii) Presence of NaOH in mild steel boiler under stress 10. Describe how electrode potential values affect the corrosion. 31