1. CORROSIVE DAMAGE IN MATERIALS & ITS PREVENTION
Dr. T. K. G. NAMBOODHIRI
Professor of Metallurgy (Retired)

2. INTRODUCTION
Definition: Corrosion is the degeneration of materials by reaction with environment. Examples: Rusting of automobiles, buildings and bridges, Fogging of silverware, Patina formation on copper.

3. UNIVERSALITY OF CORROSION
Not only metals, but non-metals like plastics, rubber, ceramics are also subject to environmental degradation
Even living tissues in the human body are prone to environmental damage by free radicals-Oxidative stress- leading to degenerative diseases like cancer, cardio-vascular disease and diabetes.

4. CORROSION DAMAGE Disfiguration or loss of appearance Loss of material
Maintenance cost
Extractive metallurgy in reverse- Loss of precious minerals, power, water and man-power
Loss in reliability & safety
Plant shutdown, contamination of product etc

5. COST OF CORROSION
Annual loss due to corrosion is estimated to be 3 to 5 % of GNP, about Rs crores
Direct & Indirect losses
Direct loss: Material cost, maintenance cost, over-design, use of costly material
Indirect losses: Plant shutdown & loss of production, contamination of products, loss of valuable products due to leakage etc, liability in accidents

6. WHY DO METALS CORRODE?
Any spontaneous reaction in the universe is associated with a lowering in the free energy of the system. i.e. a negative free energy change
All metals except the noble metals have free energies greater than their compounds. So they tend to become their compounds through the process of corrosion

7. ELECTROCHEMICAL NATURE
All metallic corrosion are electrochemical reactions i.e. metal is converted to its compound with a transfer of electrons
The overall reaction may be split into oxidation (anodic) and reduction (cathodic) partial reactions
Next slide shows the electrochemical reactions in the corrosion of Zn in hydrochloric acid

8. ELECTROCHEMICAL REACTIONS IN CORROSION

9. ELECTROCHEMICAL THEORY
The anodic & cathodic reactions occur simultaneously at different parts of the metal.
The electrode potentials of the two reactions converge to the corrosion potential by polarization

10. PASSIVATION
Many metals like Cr, Ti, Al, Ni and Fe exhibit a reduction in their corrosion rate above certain critical potential. Formation of a protective, thin oxide film.
Passivation is the reason for the excellent corrosion resistance of Al and S.S.

11. FORMS OF CORROSION Corrosion may be classified in different ways
Wet / Aqueous corrosion & Dry Corrosion
Room Temperature/ High Temperature Corrosion

12. WET & DRY CORROSION
Wet / aqueous corrosion is the major form of corrosion which occurs at or near room temperature and in the presence of water
Dry / gaseous corrosion is significant mainly at high temperatures

13. WET / AQUEOUS CORROSION
Based on the appearance of the corroded metal, wet corrosion may be classified as
Uniform or General
Galvanic or Two-metal
Crevice
Pitting
Dealloying
Intergranular
Velocity-assisted
Environment-assisted cracking

14. UNIFORM CORROSION
Corrosion over the entire exposed surface at a uniform rate. e.g.. Atmospheric corrosion.
Maximum metal loss by this form.
Not dangerous, rate can be measured in the laboratory.

15. GALVANIC CORROSION
When two dissimilar metals are joined together and exposed, the more active of the two metals corrode faster and the nobler metal is protected. This excess corrosion is due to the galvanic current generated at the junction
Fig. Al sheets covering underground Cu cables

16. CREVICE CORROSION
Intensive localized corrosion within crevices & shielded areas on metal surfaces
Small volumes of stagnant corrosive caused by holes, gaskets, surface deposits, lap joints

17. PITTING
A form of extremely localized attack causing holes in the metal
Most destructive form
Autocatalytic nature
Difficult to detect and measure
Mechanism

18. DEALLOYING
Alloys exposed to corrosives experience selective leaching out of the more active constituent. e.g. Dezincification of brass.
Loss of structural stability and mechanical strength

19. INTERGRANULAR CORROSION
The grain boundaries in metals are more active than the grains because of segregation of impurities and depletion of protective elements. So preferential attack along grain boundaries occurs. e.g. weld decay in stainless steels

20. VELOCITY ASSISTED CORROSION
Fast moving corrosives cause
a) Erosion-Corrosion,
b) Impingement attack , and
c) Cavitation damage in metals

21. CAVITATION DAMAGE Cavitation is a special case of Erosion-corrosion.
In high velocity systems, local pressure reductions create water vapour bubbles which get attached to the metal surface and burst at increased pressure, causing metal damage

22. ENVIRONMENT ASSISTED CRACKING
When a metal is subjected to a tensile stress and a corrosive medium, it may experience Environment Assisted Cracking. Four types:
Stress Corrosion Cracking
Hydrogen Embrittlement
Liquid Metal Embrittlement
Corrosion Fatigue

23. STRESS CORROSION CRACKING
Static tensile stress and specific environments produce cracking
Examples:
1) Stainless steels in hot chloride
2) Ti alloys in nitrogen tetroxide
3) Brass in ammonia

24. HYDROGEN EMBRITTLEMENT
High strength materials stressed in presence of hydrogen crack at reduced stress levels.
Hydrogen may be dissolved in the metal or present as a gas outside.
Only ppm levels of H needed

25. LIQUID METAL EMBRITTLEMENT
Certain metals like Al and stainless steels undergo brittle failure when stressed in contact with liquid metals like Hg, Zn, Sn, Pb Cd etc.
Molten metal atoms penetrate the grain boundaries and fracture the metal
Fig. Shows brittle IG fracture in Al alloy by Pb

26. CORROSION FATIGUE S-N DIAGRAM
Synergistic action of corrosion & cyclic stress. Both crack nucleation and propagation are accelerated by corrodent and the S-N diagram is shifted to the left

27. CORROSION FATIGUE, CRACK PROPAGATION
Crack propagation rate is increased by the corrosive action

28. PREVENTION OF CORROSION
The huge annual loss due to corrosion is a national waste and should be minimized
Materials already exist which, if properly used, can eliminate 80 % of corrosion loss
Proper understanding of the basics of corrosion and incorporation in the initial design of metallic structures is essential

29. METHODS Material selection Improvements in material
Design of structures
Alteration of environment
Cathodic & Anodic protection
Coatings

30. MATERIAL SELECTION
Most important method – select the appropriate metal or alloy .
“Natural” metal-corrosive combinations like
S. S.- Nitric acid, Ni & Ni alloys- Caustic
Monel- HF, Hastelloys- Hot HCl
Pb- Dil. Sulphuric acid, Sn- Distilled water
Al- Atmosphere, Ti- hot oxidizers
Ta- Ultimate resistance

31. IMPROVEMENTS OF MATERIALS
Purification of metals- Al , Zr
Alloying with metals for:
Making more noble, e.g. Pt in Ti
Passivating, e.g. Cr in steel
Inhibiting, e.g. As & Sb in brass
Scavenging, e.g. Ti & Nb in S.S
Improving other properties

32. DESIGN OF STRUCTURES Avoid sharp corners Complete draining of vessels
No water retention
Avoid sudden changes in section
Avoid contact between dissimilar metals
Weld rather than rivet
Easy replacement of vulnerable parts
Avoid excessive mechanical stress

33. ALTERATION OF ENVIRONMENT
Lower temperature and velocity
Remove oxygen/oxidizers
Change concentration
Add Inhibitors
Adsorption type, e.g. Organic amines, azoles
H evolution poisons, e.g. As & Sb
Scavengers, e.g. Sodium sulfite & hydrazine
Oxidizers, e.g. Chromates, nitrates, ferric salts

34. CATHODIC & ANODIC PROTECTION
Cathodic protection: Make the structure more cathodic by
Use of sacrificial anodes
Impressed currents
Used extensively to protect marine structures, underground pipelines, water heaters and reinforcement bars in concrete
Anodic protection: Make passivating metal structures more anodic by impressed potential. e.g. 316 s.s. pipe in sulfuric acid plants

35. COATINGS Most popular method of corrosion protection
Coatings are of various types:
Metallic
Inorganic like glass, porcelain and concrete
Organic, paints, varnishes and lacquers
Many methods of coating:
Electrodeposition
Flame spraying
Cladding
Hot dipping
Diffusion
Vapour deposition
Ion implantation
Laser glazing

36. CONCLUSION
Corrosion is a natural degenerative process affecting metals, nonmetals and even biological systems like the human body
Corrosion of engineering materials lead to significant losses
An understanding of the basic principles of corrosion and their application in the design and maintenance of engineering systems result in reducing losses considerably