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Title: UN1001: REACTOR CHEMISTRY AND CORROSION Section 3: The Eight Forms of Corrosion


1
UN1001REACTOR CHEMISTRY AND CORROSION Section
3 The Eight Forms of Corrosion
  • By
  • D.H. Lister W.G. Cook
  • Department of Chemical Engineering
  • University of New Brunswick

2
The Eight Forms of Corrosion
  • Uniform attack (general corrosion)
  • Galvanic corrosion
  • Crevice corrosion
  • Pitting
  • Intergraular attack (IGA)
  • Selective leaching
  • Flow-Accelerated Corrosion
  • Stress corrosion cracking (SCC)

3
UNIFORM ATTACK or GENERAL CORROSION
  • This is the most common form of corrosion.
  • A chemical reaction (or electrochemical reaction)
    occurs over entire exposed surface (or large
    areas) more or less uniformly.
  • Metal thins fails.
  • Not usually serious and is typically predictable
    from simple tests (e.g., coupon or specimen
    immersion)
  • Can be designed around by specifying an
    adequate CORROSION ALLOWANCE for the expected
    lifetime of the component.

4
  • Uniform attack minimized by
  • specifying proper materials
  • correctly applying coatings
  • using corrosion inhibition
  • protecting cathodically.

5
  • 1800-year-old Roman nail shows how iron and steel
    can withstand burial underground.
  • Note Environment is crucial!

6
ATMOSPHERIC CORROSION
  • Usually uniform.
  • Dry, damp or wet conditions have profound effect
    on corrosion.
  • Dry atmospheres
  • at ambient temperatures, most metals corrode very
    slowly
  • atmospheric oxygen promotes a protective oxide
    film ... such films are defect-free (sort of!),
    non-porous (more or less!) and self-healing
  • passivity of metals like SS, Ti, Cr depends on
    protective oxide films (but such passivity
    extends to other environments, e.g., aqueous).

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8
  • EXAMPLE
  • Ag Cu tarnish in dry air with traces of H2S
    (undesirable - aesthetically, technically -
    affects electrical contacts, etc.).
  • The S2- incorporation in the normally-protective
    oxides creates lattice defects which destroy
    protective nature of films tarnishing.
  • Moisture not required for tarnishing, it can
    actually retard tarnishing of Cu in presence of
    traces of H2S.

9
  • Damp atmospheres
  • corrosion increases with moisture content
  • at critical moisture level ( 70 RH), an
    invisible, thin film of moisture forms on (metal)
    surface, provides electrolyte for current
    (critical RH depends on surface condition
    cleanliness, presence of oxide or scale, presence
    of salts or other contaminants that may be
    hygroscopic).

10
  • Wet atmospheres
  • promote puddles, pockets, visible water layers
    (from dew, sea spray, rain, etc.)
  • crevices, condensation traps, etc., create water
    pools, and lead to wet atmospheric corrosion
    even when rest of surface dry
  • Corroded weathering steel I-beam. Note how
    corrosion has thinned the bottom of the vertical
    web where corrosion products have fallen and
    formed a moist corrosive deposit. soluble
    corrosion products increase wet corrosion
    (dissolved ions increase conductivity, sustain
    higher electrical currents)
  • insoluble corrosion products may retain moisture
    during alternate wet and dry conditions, lead to
    continuous wet corrosion.

11
  • Corroded weathering steel I-beam.
  • Note how corrosion has thinned the bottom of the
    vertical web where corrosion products have fallen
    and formed a moist corrosive deposit.

12
  • Corroded steel framework on the ceiling of a
    parking garage. The seams in this corrugated
    structure act as condensation traps and lead to
    wet atmospheric corrosion.

13
Corroded weathering steel gutter.
14
  • Rusting of iron and steel, formation of patina on
    copper, examples of damp ?
    wet corrosion.
  • Corroded regions of a painted highway bridge.

15
  • Corroded weathering steel highway bridge
    girder.

16
  • ATMOSPHERIC CONTAMINANTS
  • Wet atmospheric corrosion is often governed by
    level of contaminants.
  • e.g., marine salts vary drastically with
    distance from the sea
  • steel at 25 m from the sea will corrode 12x
    faster than same steel 250 m away.
  • Industrial atmospheres are generally more
    corrosive than rural, mainly because of sulfur
    compounds produced by burning fuels.

17
  • SO2 selectively adsorbs on metals under humid
    conditions metal oxide corrosion products
    catalyze oxidation to SO3
  • SO2 1/2 O2 ? SO3 (with a catalyst)
  • H2O SO3 ? H2SO4
  • Small additions ( 0.2) of Cu, Ni or Cr
    increase resistance of steel to sulfur pollution
    by enhancing the formation of a tighter, more
    protective rust film.
  • NOTE longevity of ancient Fe probably due to
    SO2 - free environments rather than high degree
    of corrosion resistance.

18
  • Nitrogen compounds promote atmospheric corrosion
    - from fuel burning (NOx as well as SOx), as well
    as by thunderstorms.
  • N2 x O2 ? 2 NOx
  • nitrogen-based fertilizers (from NH3) increase
    nitrogen pollutants in atmosphere.

19
  • H2S promotes atmospheric corrosion (e.g., Ag, Cu
    tarnishing)
  • from industry (oil gas, pulp and paper , etc.)
  • from decomposition of organic S compounds
  • from sulfate-reducing bacteria (SRB) in polluted
    rivers etc.
  • H2O
  • SRB SO42- ? H2S

20
  • Dust particles detrimental (stick to metal
    surfaces, absorb water, H2SO4 etc., may contain
    Cl- WHICH IS BAD since it breaks down
    protective oxide films).
  • CO2 dissolution in water can give pH 5.6 (in
    equilibrium with normal atmosphere containing
    CO2) BUT CO2 is not significant in
    atmospheric corrosion, in fact sometimes can
    inhibit it (if SO2 is present).

21
ATMOSPHERIC VARIABLES
  • Surface temperature very important - as T rises,
    corrosion rate rises - though damp and wet
    corrosion stop when moisture driven off
  • Metal surfaces that retain moisture generally
    corrode faster than rain-washed surfaces rain
    flushes impurities off surfaces, removes
    particles, etc. that promote differential
    aeration, etc.
  • Winter exposure generally more severe (more
    combustion products in atmosphere, temperature
    inversions, etc.), though summer gives higher
    surface temperatures.

22
ATMOSPHERIC VARIABLES
  • Relative humidity very important for clean Fe,
    critical RH ? 60
  • above this, rust begins to form slowly from
    deposited water film. At 75 - 80 RH, corrosion
    rate increases rapidly (probably because of
    capillary condensation within the rust layer).
  • if corrosion product rust is microporous,
    moisture will condense at different RHs depending
    on pore size
  • 1.5 nm - diameter pore (capillary) condenses
    water at 50 RH
  • 36 nm - diameter pore at 98 RH.

23
ATMOSPHERIC VARIABLES
  • Note dust, particles, etc. on surfaces create
    crevices that can condense moisture at various
    RHs.
  • Salt or soluble corrosion products will form
    electrolytes in condensed moisture - lower
    critical RH, also increase corrosion.

24
  • General Corrosion

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28
  • Damp and wet corrosion are described in terms of
    ELECTROCHEMISTRY.
  • We have seen how a metal dissolution, such as
  • Zn 2 HCl ? ZnCl2 H2
  • can be regarded as two reactions
  • Zn ? Zn2 2 e- (oxidation - an ANODIC
    process)
  • 2H 2e ? H2 (reduction - a CATHODIC process)

29
  • BOTH REACTIONS OCCUR SIMULTANEOUSLY AND AT THE
    SAME RATE.
  • It follows, that during metallic corrosion
  • THE RATE OF OXIDATION EQUALS THE RATE OF
    REDUCTION.

30
  • Implies that a corroding surface has anodic and
    cathodic areas for UNIFORM CORROSION
  • These must be distributed evenly over the surface
    and in fact must move around.
  • Some anodic half reactions for corrosion
  • Zn ? Zn2 2 e-
  • Na ? Na e-
  • Fe ? Fe2 2 e-
  • Cu ? Cu2 2 e- etc.

31
  • Anodic half reaction must be balanced by
    cathodic half reactions
  • Primary cathodic half reactions include
  • hydrogen evolution 2 H 2 e- ?
    H2
  • oxygen reduction O2 4 H 4 e- ? 2
    H2O
  • (acid solution)
  • oxygen reduction O2 2 H2O 4 e- ? 4
    OH-
  • (neutral or basic solution)

32
  • More cathodic half reactions
  • metal ion reduction M3 e- ? M2
  • (e.g. Fe3 e- ? Fe2)
  • metal deposition (e.g. Cu e- ?
    Cu)
  • Note that the flow of charge (i.e., electrons) is
    a measure of the reaction rate (metal dissolution
    or corrosion rate).
  • Thus, if the corrosion current can be measured,
    the corrosion rate is directly evaluated through
    Faradays Law

33
Faradays Law
  • m mass deposited/released (g)
  • Mwt atomic or molecular weight (g/mol)
  • I current passed (Amps)
  • t time current/potential applied (seconds)
  • n electrons transferred in the half-cell
    reaction
  • F Faraday constant (96485 C/mol).
  • this is the number of charges that must be passed
    to reduce or oxidise one mole of a compound

34
  • To illustrate
  • Current flowing in B electrochemically equivalent
    to rate of weight loss in (lower) anodic portion
    in A.

35
  • Say the imeasured 10-5 amps/cm2
  • Using Faradays Law the corrosion rate is
    calculated as
  • Or in a more useable unit (divide by the
    density 7.86 g/cm3 for iron convert cm to
    mm and seconds to years)
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