Efficient Plant January 2018 - 28
feature | reliability issues
are up against
is crucial in
Neville Sachs, P.E.
Understand It to Fight It
THE PLANT'S ROOF was typical in that
it was almost flat, with well-placed drains.
One section, though, was not draining
properly and vegetation had begun growing
in the shallow pond that resulted from the
situation. After discussions with the site's
engineering and maintenance staff and a
walk-through of the facility, failure-analysis
consultants who had been working on other
projects at the site uncovered the problem:
The troublesome section of the roof was
over an area that was frequently wet, and
the supporting columns had buckled several
inches because of corrosion.
The issue with that plant roof was a close
cousin to that shown in Fig. 1 on p. 29. The
railroad-bridge-support column in this
image is an excellent example of the costly,
potentially dangerous, damage that corrosion can cause in infrastructure and other
There have been many studies about the
cost of corrosion.
NACE International, formerly known
as the National Association of Corrosion
Engineers (nace.org, Houston), conducted research from 1999 to 2001 that found
direct corrosion costs in the United States
amounted to $276 million (about 3.2% of
the country's GDP). In March 2016, NACE
released a study that estimated worldwide
corrosion amounted to $2.5 trillion (about
3.4% of the GDP) and indirect costs doubled that.
In the 2016 study, NACE estimated that
between 15% and 35% of those corrosion-related costs could be eliminated using
current technology. Researchers also offered
that, comparing corrosion costs in 1975
with those in 1999, an intelligent approach
to automobile design and elimination of
corrosion appeared to have reduced the cost
to American consumers by about 52%.
HOW CORROSION OCCURS
Energy is needed to convert mined ores into
useful metals. Corrosion is the natural result
of those metals trying to revert back to their
original states. Consider, for example, that
there's very little difference between the rust
from corroded steel and the iron ores that
were originally refined to make that steel.
The actual corrosion process is an
electrochemical reaction. Depicting a steel
bar in a liquid, Fig. 2 on p. 29, shows how
this reaction takes place. In the diagram,
corrosion is attacking the anode, with iron
ions being released into the solution, while
hydrogen is being generated at the cathode.
Water (H2O), is made up of two hydrogen
ions and one oxygen ion. The iron ions from
the anode (the Fe symbols) will ultimately