Efficient Plant January 2018 - 29

feature | reliability issues

H2
H-

H2
Liquid Electrolyte

Fe++
H-

Fig. 1. This railroad-bridge support column
exemplifies the costly (potentially dangerous)
damage that corrosion can cause in infrastructure and other plant assets.

unite with oxygen in the water, whereupon
several different types of rust can form.
At the cathode site of the piece, atomic
hydrogen is being released. Most of those
hydrogen ions then mate with another
hydrogen ion and form molecular hydrogen, the readily flammable gas we're used to
thinking about. But some of the ions remain
solitary and they are the cause of the many
forms of hydrogen damage including hydrogen embrittlement, cracking, and blisters.
For wet corrosion, a liquid must be
present to provide the complete circuit
required by the electromechanical reaction. Electrons that flow from the cathode
to the anode have to eventually return to
the cathode, and they do so by traveling
through the liquid.
Referring back to the railroadbridge-support column in Fig. 1 (above),
note the presence of a fair amount of silt.
This has two substantial effects on the
corrosion rate:
The silt holds moisture that allows
corrosion to attack the column for a longer
period of time than if the steel were dry.
Chemicals such as road salt are in the silt.
As the moisture in it evaporates, the chemical concentration increases. The chemicals,
in turn, make the water more electrically

JANUARY 2018

Fe++

H- H- H- H- H- H- HCathode Site

Fe++
Fe++

Fe++
Anode Site

Steel Bar

Fig. 2. Depicting a steel bar in liquid, this diagram shows how corrosion occurs. The liquid contains water.
When iron ions (Fe) off the bar unite with oxygen in the water, different kinds of rust can form.

conductive and significantly increase the
rate of corrosion.
Temperature is a third important factor in
corrosion. Below freezing, ice can't conduct
corrosion currents. But, as the temperature
increases, the corrosion rate increases. A
good example is the rapid attack on hot
piping with moist insulation. The exact
solution chemistry has a major effect, but
up to about 175 F (80 C), the corrosion rate

usually rapidly increases, then drops off and
ceases when the liquid vaporizes.

TYPES OF CORROSION
While more than 99% of corrosion losses
are from the wet variety, dry corrosion also
occurs, but only at greatly elevated temperatures. A common example would be the formation of oxide scales on a barbecue grill.
In North America, the recognition and

FORGET ABOUT 'THE OTHER GUYS.'
WHAT ABOUT YOUR OPERATIONS?

There's a tendency by some plant personnel to "look at the other guys,"
rather than effectively confront problems in their own operations. With
regard to corrosion, that might mean focusing on the idea that other
sites' costs must be higher, or believing that paper mills, with their hot
and humid environments, must surely have more potential for corrosion
than, say, air-conditioned manufacturing facilities. But two of the most
memorable failure analyses I ever encountered actually involved airconditioned plants.
The first was a manufacturing operation where a buried water line erupted twice in eight years. Each of those two events shut the site's machining lines down for several days.
The second involved a pharmaceutical plant where many millions of dollars of product had to be scrapped because of corrosion in a cooling system's sensor.
Remember this: Practically anywhere the relative humidity exceeds
60%, there's a potential for corrosion. No plant can escape it. -N.S.

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