Transcript corrosive) Environment

FUNDAMENTALS OF
CORROSION CONTROL &
CATHODIC PROTECTION
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Outline
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Fundamentals of Corrosion
Corrosion Cell:
 Conditions required for corrosion
 Corrosion Rate
 Corrosion Types
 Commonly Affected Structures
Corrosion Control
 Materials Selection
 Inhibitors/Altering the environment
 Coatings
 Cathodic Protection
 Measuring potentials
 Criteria
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Concerns
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Structural failure
Contamination
Property damage
Costs to repair/replace
 Safety
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Introduction to Corrosion
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Corrosion can be defined as the deterioration of metal
due to its interaction with the environment. Corrosion
is a natural phenomenon, which should not surprise
one, but rather should be expected to occur. Metals are
high energy materials, which exist because heat energy
was added to natural iron ores during the smelting
process. Nature, by environmental contact, constantly
attacks these high energy materials and breaks them
down to the natural elements from which they were
derived.
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Conditions Required for Corrosion
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Corrosion is an electrochemical process
occurring at the interface between metal and
environment. Three conditions must be present
for this to occur.
1. Two areas on a structure or two structures
must differ in electrical potential.
2. Those areas, called anodes and cathodes, must
be electrically interconnected.
3. Those areas must be exposed to a common
electrolyte (soil or water).
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Two areas or two structures that differ in electrical
potential (different amounts of stored energy)
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The two areas, called the “Anode” and “Cathode”,
must be electrically connected (conductive path)
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Those areas must be exposed to a common
electrolyte
CURRENT
FLOW
WIRE
CATHODE
(COPPER)
ANODE
(IRON)
CURRENTFLOW
IONIZEDELECTROLYTE
CURRENTFLOW
(- TO+INELECTROLYTE)
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Corrosion Cell – Dissimilar Metals
i.e. steel pipe connected to copper ground rod
CURRENT
FLOW
WIRE
CATHODE
(COPPER)
ANODE
(IRON)
CURRENTFLOW
IONIZEDELECTROLYTE
CURRENTFLOW
(- TO+INELECTROLYTE)
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Corrosion Cell – Within a Metal
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Corrosion Rate
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As a result of this process, electric current flows
through the interconnection between cathode
and anode. The cathodic area is protected from
corrosion damage at the expense of the metal,
which is consumed at the anode. The amount
of metal lost is directly proportional to the
current flow. Mild steel is lost at approximately
20 pounds for each ampere flowing for a year.
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Factors Affecting Corrosion Rate
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Potential Difference Between Anode and
Cathode (Galvanic Series)
Circuit resistance – Resistivity of the Electrolyte
Chemical Activity
Stray Currents
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Potential Difference
Interconnecting two dissimilar metals in an
electrolyte will create a corrosion cell. The
strength of this cell increases as the distance
within the galvanic series increases.
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Galvanic Series
Example: connecting magnesium to copper will produce a corrosion cell with a
potential of about 1.5 volts.
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METAL
VOLTS (CSE)
Commercially Pure Magnesium -1.75
Magnesium Alloy
-1.60
Zinc
-1.10
Aluminum Alloy
-1.05
Commercially Pure Aluminum -0.80
Mild Steel (clean & shiny)
-0.50 to -0.80
Mild Steel (rusted)
-0.20 to -0.50
Cast Iron (not graphitized)
-0.50
Lead
-0.50
Mild Steel in Concrete
-0.20
Copper, Brass, Bronze
-0.20
High Silicon Cast Iron
-0.20
Carbon, Graphite, Coke
+0.30
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Circuit Resistance
Circuit resistance includes the following:
 Resistance of the anode
 Resistance of the cathode
 Resistance of the electrolyte
 Resistance of the metallic path
Increasing the resistance will reduce the corrosion rate.
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Resistance of Common Electrolytes
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Soils – High resistivity water reduces the
corrosion rate, while low resistivity water
increases the corrosion rate.
CLASSIFICATION
ELECTROLYTE
RESISTIVITY
(ohm-cm)
ANTICIPATED
CORROSIVITY
Low Resistance
0 to 2,000
Severe
Medium
2,000 to 10,000
Moderate
High
10,000 to 30,000
Mild
Very High
Above 30,000
Increasingly Less
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Resistance of Common Electrolytes
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Water – Approximate resistivity values
Water resistivity
open sea
seawater (coastal)
river water
tap water
rain water
distilled water
pure water
Ohms-cm
20-25
30-40
500-10,000
1,000-10,000
20,000
500,000
20,000,000
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Chemical Activity
Passive (less corrosive) Environment
 High pH (neutral or basic)
 Low Moisture Content
 Lack of Salts
 High Resistivity
 Low Temperature
 Homogenous Environment
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Chemical Activity
Active (more corrosive) Environment
 Low pH (acidic)
 High Moisture Content
 Salts
 Low Resistivity
 Moderate to High Temperature
 Heterogeneous Environment
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Stray current corrosion
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Boat
Boat
Boat
Ground
wire
Corrosion Type
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Uniform or near uniform - Corrosion attacks all
areas of the metal at the same or similar rate.
Localized - Some areas of the metal corrode at
different rates due to heterogeneities in the
metal or environment. This type of attack can
approach pitting.
Pitting - Very highly localized attack resulting in
small pits that may penetrate to perforation.
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Commonly Affected Structures
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Buried Piping
Steel Piles
Storage Tanks - Above Ground / Underground
Reinforcing Steel in Concrete
 Ships/Boats
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Ships/Boats
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Methods to Control Corrosion
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Use of Corrosion resistant materials (plastic, stainless
alloys, fiberglass).
Use of the same or similar metals per the galvanic
series.
Altering the environment (utilizing homogeneous high
resistivity backfill or inhibitors).
Utilize coatings and linings that electrically insulate the
structure from the electrolyte (paints, plastic films, etc).
Use of Cathodic Protection.
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Fundamentals of Cathodic
Protection
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Cathodic Protection (CP) Defined - Minimize
corrosion by utilizing an external source of
electrical current which forces the entire
structure to become a cathode.
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1824: Earliest practical use of
cathodic protection
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Sir Humphrey Davy’s work on protecting the
copper sheathing on wooden hulls in the British
Navy by sacrificial zinc or iron anodes is
generally considered to be the earliest example
of practical cathodic protection.
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Galvanic CP
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When metals such as magnesium or zinc are
placed in the environment in contact with a
more noble metal such as steel, a current flows
from the more active anode to the noble
cathode (corrosion cell).
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Anodes Used for Galvanic CP
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Magnesium – Magnesium is often used in soil to
protect small electrically isolated structures, such
as underground storage tanks, and well coated
pipelines.
Zinc – Zinc is often used in marine
environments. They are commonly found on
boats.
Aluminum – Aluminum can be used for a variety
of marine applications.
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Keys to obtaining enough
cathodic protection
Determine amount of current required
Theoretical calculations based on coating quality
and environment
Or, perform current requirement testing
 Calculate output expected from anode and
determine number of anodes required.
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Galvanic CP
Pros of Galvanic CP
 Inexpensive
 Little Maintenance cost
 No external Power source
 A variety of install methods can be used. For
example, many UST’s are shipped with anodes
attached.
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Galvanic CP
Cons of Galvanic CP
 Typically work best with electrically isolated structures.
 No external power source, limited driving potential
(driving potential based on the galvanic series)
 Limited output makes it ineffective when trying to
protect large uncoated surfaces.
 Require a low resistivity electrolyte to function well.
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Impressed Current CP
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Utilize an external power source to develop a
high potential difference between the surface to
be protected and an anode.
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Impressed Current CP
IM P R E S S E D
An o d e
C U R
G ro u n d b e d
P o s i ti v e
C a b l
R e c ti fi e r
AC
P o w e N
r S
u a
p t
pi v
l ye
e g
P ip e lin e
C a b
Impressed Current CP
Pros of Impressed Current CP
 Unlimited driving potential.
 Capable of protecting large steel structures
when designed properly.
 Requires less anodes then a galvanic system.
 Output can be controlled using a permanent
reference electrode, desirable when the
electrolyte resistivity is known to change.
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Impressed Current CP
Cons of Impressed Current CP
 Initial costs can be more expensive.
 Requires an external DC power source along
with an AC supply.
 System requires routine maintenance and
monitoring.
 Anode wires can be susceptible to damage.
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Impressed Current CP Rectifier
A rectifier converts available AC power to low
voltage DC power. Most cathodic protection
rectifiers are provided with a means to vary the
DC output voltage in small increments, or in
some cases offer complete control from zero to
100% of rated DC output.
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CP System Testing
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A CP system can be evaluated by obtaining a
reference cell potential. This potential
determines the amount of cathodic polarization
the structure is receiving.
Adequate protection is indicated by obtaining a
value more negative then the established criteria.
This criteria is specific to the reference cell used
to obtain potentials.
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Questions?
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