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While the
external signs can sometimes be subtle, learning of a corrosion problem often
requires little more than a quick look into the cooling tower pans. A rust
build-up at the tower, fouled drift eliminators, pipe tubercles, and flakes of
scale and rust caught in the condenser water tubes are obvious indications of a
corrosion problem. Likewise, encrustations at the threaded joints, buckets of
rust at start-up, a 10 MPY corrosion coupon result, lost heat transfer, and
failed equipment should prompt the need to investigate further.
Under real world conditions, some degree
of corrosion must be expected to exist within any piping system. However,
identifying the type and severity of corrosion responsible requires far more
effort - with the ultimate control of any problem generally determined by the
amount of money expended.
The monitoring,
testing, diagnosis, and repair of piping related corrosion problems is a
multi-billion dollar business in itself. Yet, corrosion caused piping
replacement in the United States still exceeds $75 billion per year. Volumes of
information exist on the subject - with many corrosion mechanisms still
remaining unknown and/or unexplained.
While hundreds of
different corrosion scenarios exist, there are actually only six main forms
that affect most HVAC related piping systems. Of the many different piping
systems and services available, condenser water traditionally exists under the
greatest threat and suffers the most severe losses.
See Technical Bulletin
# C-10 regarding some common corrosion trends. The six most common
forms of corrosion are:
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Generalized corrosion is the well distributed and low level
attack against the entire metal surface with little or no localized
penetration. It is the least damaging of all forms of corrosion. Generalized
corrosion usually occurs in environments in which the corrosion rate is
inherently low or well controlled - such as for chemically treated closed
circulating systems, and in some open water systems.
It is the only form
of corrosion whereby weight loss or metal loss data from corrosion coupons or
ultrasonic testing can be used to accurately and reliably estimate corrosion
rates and future pipe life expectancy.
View a photo gallery
of various corrosion types.
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Often termed "under deposit corrosion," this is a localized,
deep penetration of the metal surface with little general corrosion in the
surrounding area. Due to surface deposits, electrical imbalance or some other
initiating mechanism, all existing corrosion potential attacks a select number
of individual sites.
In most cases, pitting is extended throughout the
entire metal surface, creating it an irregular or very rough surface profile.
In other instances, pits are concentrated in specific areas, leaving the
majority of the metal surface in like new condition.
Pitting is the
most common form of corrosion found where there are incomplete chemical
protective films, and insulating or barrier deposits of dirt, iron oxide,
organic, and other foreign substances at the pipe surface. It is very prevalent
at galvanized steel pipe, where any failure of the galvanizing invokes a
pitting condition. Pitting corrosion may include: crevice corrosion, water-line
attack, under deposit attack, impingement or erosion corrosion attack, and
concentration-cell corrosion.
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This is an aggressive and localized
form of corrosion due to the electrochemical reaction often found between two
or more dissimilar metals in an electrically conductive environment. It occurs
because the more electronegative material (the anode) is attacked by the more
electropositive material (the cathode).
The most common example of such
corrosion activity, widely found throughout HVAC and process plant operations,
is the direct connection of brass valves to carbon steel pipe, or between
copper tubing and steel pipe - where the steel serves as the anode. Carbon
steel pipe, without the protection of a galvanic insulator, will show the
highest rate of corrosion under such conditions - usually developing over many
years.
The severity of pipe loss due to galvanic activity is often
found relative to the general corrosion activity of the piping system itself -
with little or no galvanic activity found where extremely low general corrosion
rates exist. Under conditions of high corrosion rate activity, galvanic losses
often become aggressive - making an existing pipe corrosion problem
significantly worse at the threads.
While galvanic corrosion is
generally assumed to involve only dissimilar metals, millivolt potentials can
actually be measured between similar metals under certain conditions. New steel
pipe installed during a repair or renovation is often more electronegative than
older existing pipe, and therefore may suffer from some degree of galvanic
attack. View a photo gallery
of galvanic corrosion examples.
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Microbiologically Influenced Corrosion
(MIC) is, by far, the most severe and threatening form of corrosion to HVAC
piping systems. It is caused by the presence of various microbiological agents
under specific environmental conditions - in some cases resulting in advanced
and widespread failure of entire piping systems within a few years.
An
MIC presence usually signals a very severe threat to the entire system -
requiring extensive cleaning and sterilization at great expense. For many
affected systems, MIC cannot be eliminated, and an elevated corrosion and
pitting condition will exist for the remainder of system life.
MIC
produces large and deep pits due to the microorganism's utilization of iron as
an energy source (often as an alternative to oxygen), and through the
production of strongly corrosive metabolic by-products such as sulfuric acid -
which further assists the microorganism in dissolving pipe metal. MIC exists to
varying degrees of severity, and is not exclusive to carbon steel piping
systems or open condenser water systems.
MIC is less commonly found in
closed chill water piping, in hot water heating and domestic water systems, and
has been documented to destroy copper, brass, and stainless steel pipe.
See Technical Bulletin
# C-5 for more about the threat of MIC.
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This is the gradual and selective
deterioration of a metal surface due to mechanical wear and abrasion. It is
attributed to entrained air bubbles, suspended matter and particulates under a
flow rate of sufficient velocity.
Erosion is similar to impingement
attack, and is primarily found at elbows and tees, or in those area where the
water sharply changes direction. Softer metals such as copper and brass are
inherently more susceptible to erosion corrosion than steel.
High
pressure steam will often contribute to the erosion of carbon steel. Though
typically not a problem at the water velocities encountered within most HVAC
piping systems, high corrosion rates and the entrainment of high volumes iron
oxide particulates can produce an erosion condition under certain conditions.
Erosion at the base of elbows or after multiple sharp turns of the pipe has
been documented to occur.
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Known as Corrosion Under Insulation,
CUI is a threat to any piping system or holding tank which operates at lower
temperatures in humid environments. In the absence of an effective moisture
barrier and a protective metal coating, any available moisture will penetrate
commonly used fiberglass or foam insulation to condense at the cold metal
surface.
Often, moisture can accumulate sufficiently to waterlog the
insulation and cause its total deterioration. This effectively creates an
untreated water condition at the outer pipe surface, and produces a corrosion
problem on two fronts.
In outdoor environments, moisture, rain, snow,
and ice can also penetrate the insulation due to physical damage, wear, or by
the failure to install sealants at the overlap of the hard metal outer shell.
CUI is commonly found at cold water domestic piping, free cooling
condenser water systems, and especially at chill water piping - being most
severe at the colder supply side piping. The degree of CUI type corrosion
depends upon a combination of pipe temperature and humidity. In extreme
examples of high humidity, CUI corrosion will even occur on typically warm
condenser water piping. Conversely, the extremely cold temperatures of a brine
or ammonia refrigeration plant can create substantial exterior pitting even
from a relatively dry atmosphere.
CUI corrosion usually remains hidden
until severe damage has occurred to the pipe, producing telltale discoloration
at the insulation itself, or failure. In many cases, CUI corrosion can exceed
the degree of physical damage caused by internal corrosion of poorly treated
open condenser or process cooling water piping.
View a photo gallery
of under insulation corrosion examples. |
While controlled
generalized corrosion may take many decades to present even minor operating
problems, aggressive and localized corrosion can accelerate the need for pipe
replacement to as little as a few years. Typically, a visual diagnosis is all
that is necessary for problems such as CUI or galvanic corrosion. Pitting
corrosion and erosion usually require a reasonable degree of nondestructive
testing or metallurgical analysis, coupled with the ability to investigate the
most problem prone areas.
MIC is
unquestionably the most difficult to test and diagnose, as well as the most
difficult to control. Combined with the potential to destroy piping at rates
exceeding 50 mils per year, MIC is clearly the greatest threat to any piping
system. For that reason, continuous, reliable and effective monitoring for any
corrosion problem is required, and quick attention to correct it
essential.
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Copyright
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