CORROSION GUIDE 181108_new table content format ... - Reichhold

Common Types of Metal Corrosion
Fiber reinforced composites do not match the
characteristically high elastic modulus and ductility of
steel and other metals, yet they display lower density,
this often translates to favorable strength/ weight ratio
which, in turn, leads to favor in transportation and
various industrial and architectural applications.
Composites can present other advantages over
steel, such as low thermal conductivity and good
dielectric or electrical insulating properties. However,
an overwhelming advantage to composites rests with
corrosion resistance.
When the cost and benefi ts of FRP and special resins
are considered for particular environments, it is useful to
understand the common mechanisms by which metals
are oxidized or corroded. FRP is immune or otherwise
quite resistive to many of these infl uences, at least
within the range of practical limits of temperature and
stress.
Oxygen Cell-Galvanic Corrosion
The most commonly observed instances of corrosion
to carbon steel involve oxidation-reduction galvanic
couplings in the presence of molecular oxygen and
hydrogen ion associated with acids.
Oxidation (anode)
Fe – 2e - → Fe 2+
Reduction (cathode)
O 2 + 2H 2 O + 4e - → 4OH -
2H + + 2e - → H 2
Most forms of steel corrosion relate to some variation
of these mechanisms, as hereby the steel effectively
functions as an anode and becomes oxidized. Dissolved
salts and ionic components can accelerate this type
of corrosion by increasing electrical conductivity. It
can also occur in the presence of stray leaks of direct
current, such as in the vicinity of mass transit systems.
Galvanic corrosion of steel is accelerated in the vicinity
of metals such as copper which are cathodic to steel.
Due to impurities, as well as various metallurgical or
geometric factors, steel substrates are not always
uniform. There can be numerous microscopic anodecathode
couplings along the surface or cross-sectional
gradients of the steel, and each can effectively function
as a galvanic oxidation cell.
Apart from paints and other protective or dielectric
coatings, various forms of cathodic protection are often
employed with steel. For small structures, sacrifi cial
anodes may be located near to the steel, so that
these anodes corrode selectively, or preferentially,
to the steel. Sacrifi cial anodes employ metals which
are more electronegative than iron within the galvanic
series. Examples include zinc, magnesium, or various
aluminum alloys. For larger structures, such as tanks,
impressed current methods are frequently used. This
involves use of separate anodes and DC current to
reverse or alter polarity, allowing the steel to function as
a cathode rather than as an anode, which is where the
oxidation occurs.
Galvanic corrosion is exceptionally severe in wet
acidic environments where free oxygen is present.
Flue gas desulfurization is a good example of where
the conditions strongly favor this type of corrosion. This
is due to the presence of sulfuric acid in combination
with oxygen associated with the excess air ordinarily
employed in coal combustion. Polyesters and vinyl
esters display excellent acid resistance and common
galvanic corrosion mechanisms do not infl uence
properly designed FRP.
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