CORROSIVE DAMAGE IN MATERIALS

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

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COST OF CORROSION

• Annual loss due to corrosion is estimated to be 3
  to 5 of GNP, about Rs.700000 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

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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

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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

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ELECTROCHEMICAL REACTIONS IN CORROSION
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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

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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.

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FORMS OF CORROSION

• Corrosion may be classified in different ways
• Wet / Aqueous corrosion Dry Corrosion
• Room Temperature/ High Temperature Corrosion

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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

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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

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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.

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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

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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

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PITTING

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

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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

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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

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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

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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

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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

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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

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CORROSION FATIGUE, CRACK
PROPAGATION

• Crack propagation rate is increased by the
  corrosive action

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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

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METHODS

• Material selection
• Improvements in material
• Design of structures
• Alteration of environment
• Cathodic Anodic protection
• Coatings

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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

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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

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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

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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

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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

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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

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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