REACTOR CHEMISTRY AND CORROSION
Section 3: The Eight Forms of Corrosion
D.H. Lister & W.G. Cook
Department of Chemical Engineering
University of New Brunswick
The Eight Forms of Corrosion
1. Uniform attack (general corrosion);
2. Galvanic corrosion;
3. Crevice corrosion;
5. Intergraular attack (“IGA”);
6. Selective leaching;
7. Flow-Accelerated Corrosion;
8. Stress corrosion cracking (“SCC”)
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.
• Uniform attack minimized by:
• specifying proper materials;
• correctly applying coatings;
• using corrosion inhibition;
• protecting cathodically.
1800-year-old Roman nail shows
how iron and steel can withstand
Note: Environment is crucial!
• Usually “uniform”.
• Dry, damp or wet conditions have profound effect on
• 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-
• “passivity” of metals like SS, Ti, Cr depends on protective oxide
films (but such passivity extends to other environments, e.g.,
• 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.
• 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
• 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.
• Corroded weathering steel
• Note how corrosion has
thinned the bottom of the
vertical web where
corrosion products have
fallen and formed a moist
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.
Corroded weathering steel gutter.
Rusting of iron and steel, formation of patina on copper,
examples of damp wet corrosion.
Corroded regions of a painted highway bridge.
Corroded weathering steel highway bridge girder.
• Wet atmospheric corrosion is often governed by level of
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.
• SO2 selectively adsorbs on metals – under humid conditions
metal oxide corrosion products catalyze oxidation to SO 3:
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.
• Nitrogen compounds promote atmospheric corrosion -
from fuel burning (NOx as well as SOx), as well as by
N2 + x O2 → 2 NOx;
nitrogen-based fertilizers (from NH3) increase nitrogen
pollutants in atmosphere.
• 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.
SRB + SO42- → H2S
• 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).
• 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
• Winter exposure generally more severe (more combustion
products in atmosphere, temperature inversions, etc.),
though summer gives higher surface temperatures.
• Relative humidity very important: for clean Fe, critical
• 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
• 1.5 nm - diameter pore (capillary) condenses
water at 50% RH;
• 36 nm - diameter pore at 98% RH.
• Note: dust, particles, etc. on surfaces create
crevices that can condense moisture at various
• Salt or soluble corrosion products will form
electrolytes in condensed moisture - lower critical
RH, also increase corrosion.
“ General Corrosion”
Damp and wet corrosion are described in terms of
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)
BOTH REACTIONS OCCUR SIMULTANEOUSLY
AND AT THE SAME RATE.
occurring during corrosion
of zinc in air-free
It follows, that during metallic corrosion …
THE RATE OF OXIDATION EQUALS THE RATE OF
• Implies that a corroding surface has anodic and cathodic areas for
• These must be distributed evenly over the surface and in fact must
• Some anodic “half reactions” for corrosion:
Zn Zn2+ + 2 e-
Na Na++ e-
Fe Fe2+ + 2 e-
Cu Cu2+ + 2 e- … etc.
• Anodic “half” reaction must be balanced by cathodic “half”
• Primary cathodic “half reactions” include:
hydrogen evolution 2 H + + 2 e- H 2
oxygen reduction O 2 + 4 H + + 4 e- 2 H 2 O
oxygen reduction O2 + 2 H2O + 4 e- 4 OH-
(neutral or basic solution)
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 Faraday’s Law
M wt I t
• 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
• Say the imeasured = 10-5 amps/cm2
• Using Faraday’s Law the corrosion rate is calculated as:
mol 105 A cm2
2.9 109 g cm2s
2 96485C mol
• Or in a more useable unit … (divide by the density –
7.86 g/cm3 for iron; convert cm to mm and seconds to
CR 116 mm / yr 0.12 mm / yr