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Cooling Water Chemistry
Scale and Corrosion
Scale and Deposit Problems
A Basic Discussion of Mechanisms and
Inhibitors
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Problems Caused by Scale
Loss of heat transfer
Reduced flow
Under deposit corrosion
Tower fill collapse
Why Cycling Up is Good
Blowdown -vs- Cycles of Concentration
35
30
25
Blowdown in gpm
•
•
•
•
Save Water
and Chemical
7.9 million gal/yr
20
15 gpm
15
2.6 mm gal/yr
10 gpm
10
1.26 mm gal/yr
7.5 gpm
0.79 mm gal/yr
6 gpm
5 gpm
5
0
2
3
4
5
Cycles
6
7
8
9
Example using 1000 ton load
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Effect of CaCO3 Scale on Efficiency
50
45
40
% Energy Increase
35
30
25
% Energy Increase
20
15
10
5
0
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
Scale Thickness (inches)
The Cost of Deposits
• A calcium carbonate scale of just 0.01” can
reduce efficiency by 10%.
• Running 500 tons of AC, 24 hours, 365 days @
0.6Kw/ton and a cost of $0.07/Kw will cost
about $184,000.
• Reducing efficiency by 10% costs $18,400.
• A good water treatment program costs $6 12,000.
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Deposits
•
•
•
•
Calcium carbonate most common.
Calcium phosphate.
Magnesium Silicate.
Corrosion by-products.
– Fe2O3
• Most deposits heterogeneous mixtures.
– Contain Ca, P, Fe, Si, Al, Mg, and Mn
• Pure silica deposits not as common.
– Does not exhibit inverse solubility.
How Scale Forms
•
•
•
•
Concentration (by evaporation)
Supersaturation
Nucleation
Crystal Formation
Concentration by
evaporation and
supersaturation
Nucleation
Crystal Growth
Scale
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Deposit Formation
Na2CO3
Ca ++
NaHCO3
Ca ++
Na2CO3
SS
Ca ++
-CO3
SS
Na2CO3
Na2CO3
Ca ++
SS
Na2CO3
-CO3
Ca ++
SS
Ca
++
NaHCO3
Ca ++
NaHCO3
NaHCO3
-CO3
SS
SS
SS
Corrosion sites may provide nucleation site.
HEAT
Biofilm can accumulate calcium and provide nucleation sites
Nucleation and growth at surface irregularity, existing deposit, or high heat
flux surface.
Influencing Factors
• Concentration
• Competing ion pairs (i.e. sulfate/chloride)
• Temperature (Most HVAC condensers are NOT hot
enough to promote spontaneous nucleation).
• pH
• Velocity
• Surface characteristics
• Biofilm
How Scaling is the Water?
• Water treater’s use solubility indices to determine scaling
potential.
• Other software such as Watercycle can be used to
determine solubility of salts.
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Controlling Scale
•
•
•
•
•
Control pH
Soften the water
Remove the alkalinity
Increase blowdown
Add chemical inhibitors
Deposit Inhibitors
• Main mechanisms:
– Threshold inhibition
– Crystal modification
– Dispersion
• Dispersion of suspended solids
• Dispersion of biofilm
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Threshold Inhibition
• Keeping a large amount of scale forming species in
solution by adding select chemicals at very low dose.
• Dose is usually < 5 ppm.
• Certain chemicals interact with forming crystal nuclei
preventing their development keeping them in solution.
Concentration by
evaporation and
supersaturation
Scale
Nucleation
Crystal Growth
Crystal Modification
• Crystal modification is the process by which
agents adsorb onto forming crystals past the
nucleation stage and modify them in such a way
to limit directional development .
C
O
O-
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Concentration by
evaporation and
supersaturation
Nucleation
Crystal Growth
Scale
• Calcium carbonate crystals
•
Modified calcium carbonate crystals
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Dispersion
• A process by which charged particles are prevented from
agglomerating.
• Most cooling water particulates have a net negative
charge. Addition of dispersant increases the charge
inhibiting agglomeration.
• Essentially the opposite of coagulation.
Dispersion
Commonly Used Crystal Modifiers and
Threshold Inhibitors
• Phosphonates
– HEDP
– PBTC
– Others
• Polymers
–
–
–
–
AA
AA/AMPS
MA
PCA
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Phosphonates
•
•
•
•
•
Organic molecules with phosphonic acid groups attached
Very strong threshold inhibitors and crystal modifiers.
Performance characteristics vary.
Typically maintained at 3-7 ppm active in system.
HEDP and PBTC most commonly used.
HEDP
•
•
•
•
•
•
Good performance
Provides good chlorine stability
Less stable in bromine
Overfeed can precipitate Ca complexes
Fair corrosion inhibitor
Typically maintained at 3-7 ppm active in system
• MW = 206
• %PO4 = 92.2
HEDP
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PBTC
• Good performance especially under high stress
conditions.
• Excellent chlorine and bromine stability.
• Typically maintained at 3-7ppm active in system
• Higher cost.
PBTC
• MW = 270
• %PO4 = 35%
Dispersants/Stabilizers
• Polymers based on acrylic acid disperse suspended
colloidal solids.
• Acrylate (AA) polymers are also threshold inhibitors.
• Polymaleates very good crystal modifiers but do not
always prevent precipitation.
• Certain sulfonated (i.e. AA/AMPS) polymers stabilize
precipitants such as Ca3(PO4)2, Fe, and Zn salts.
• Typically maintained at 5-10 ppm active in system.
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Dispersants/Stabilizers
•
•
•
•
•
•
•
Polyacrylates
AMPS/Acrylate
AMPS/Acrylate terpolymers
Sulfonated styrene
Maleic Acid Homopolymer
Maleic acid co and terpolymers
Phosphinocarboxylates
Polymer Structure
AA
AA/AMPs
Scale Control Hints
•
•
•
•
•
•
•
•
•
Control biofilm.
Test total phosphate in make up water.
Run indecis based on estimated skin temps not bulk water.
Running Ca vs cycles indicates gross precipitation only!
Do not overfeed inhibitors.
Maintain feed and control equipment.
Reduce cycles if acid feed is lost.
Side stream filtration.
Others?
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Review
Corrosion
A discussion of mechanisms and
inhibition
Corrosion
• Deterioration of a metal due to the interaction
with the environment.
• All waters are corrosive to some degree.
• Metals like to return to their natural low energy
state.
• Corrosion is initiated by potential differences on
metal surfaces.
• Ryznar and LSI are NOT indices of corrosion.
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The Essential Elements of a Corrosion
Cell
• Anode: The sight of higher negative potential
from which electrons flow and metal is lost.
• Cathode: Where electrons are consumed.
• Electrolyte: a solution capable of conducting
electric current.
• Metallic path : The anode and cathode must be
in electrical contact for corrosion to occur.
Basic Corrosion Model
A potential difference initiates formation of an anode and a cathode. The electrons from
the anode are accepted at the cathode by the reduction of oxygen. Hydroxyls formed at
the cathode will combine with ferrous ions at the anode and begin to form a deposit, a
protective layer, or a tubercle.
Anodic reaction Fe0  Fe++ + 2e–
o2
o2
Cathodic reaction 1/2O2 + H2O  2OHOH-
Fe++
Fe0
Fe++
Fe++
Fe++OHOH-
e-
OH-
OH-
e-
Basic Corrosion Model
Acid pH
Alkaline pH
H+ + H+ + 2e → H2↑
1/2O2 + H2O + 2e → 2OH-
OH - OH
H2
Fe 0
2e-
Fe
++
-
2e-
H+ may take the place of O2 in acidic environments with the
formation of molecular hydrogen at the cathode. At low pH soluble
iron complexes are made such as FeCl2, FeSO4, and Fe(HCO3)2.
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Corrosion ModelO 2cont’d.
O
2OH-
Fe +
→ Fe(OH)2 ↓
O
2Fe(OH)2+1/2O2 + H2O → 2Fe(OH)3↓
2 O
2
2
O
O
2
2
O
O
2
2
++
Fe
Fe
++
++
Fe
Fe
OH
++
++
Fe
e-
At high pH insoluble
complexes (rust) are
made.
ee- e- eee-
OH
e-
OH
OH
OH
OH
1/2O2 + H2O → 2OH-
e-
Occluded Corrosion Cell
Cathode
OHCl-
Fe(OH)2 + ½ H2O + 1/2O2  Fe(OH)3
SO4- -
2e- + H2O +1/2O2 → 2OH-
HCO3-
OHOH-
OH-
OH-
OH-
Fe++
Fe++
Fe++ + 2Cl- + 2H2O→ Fe(OH)2 + 2HCl
2HCl + Fe++ →FeCl2 + 2H+
Fe++
e
H+
-
H+
H+
H+
Fe++
Fe++
H+
e
-
Anode
o
Fe  Fe++ + 2e-
General Corrosion
General corrosion due to water chemistry.




Metal loss across entire surface.
Anodes and cathodes shifting.
Unpassivated metal will corrode more rapidly than
metal with an established oxide film.
Factors such as pH, alkalinity, calcium, chlorides,
sulfates, conductance, and temperature will
determine the rate.
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Air Handlers
• Don’t forget about the air handlers.
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Localized Corrosion
• Significant loss of metal
from one area.
• Anode fixed.
• Types: Dissimilar
metals, Crevice
corrosion, Under
deposit corrosion (O2
concentration), pitting,
Erosion or flow assisted
corrosion.
Types of Corrosion
Chemical Attack
• Overfeed of acid
• Overfeed of inhibitor
• Overfeed of chlorine
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Chemical attack due to overfeed of
acid and low pH when control
equipment failed.
Concentration Cell Corrosion
• Occurs where a potential difference is created by a
higher concentration of ions or oxygen in contact
with one area of the surface compared to another.
• Crevice and Under Deposit Corrosion are examples.
• Pitting in condenser water systems usually the result
of some under deposit mechanism or microbiological
corrosion.
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Corrosion formed under debris likely
differential oxygen concentration cells.
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Corrosion formed under deposits likely
differential oxygen concentration cells.
Seam defect on pipe. Likely low bid.
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Corrosion along seam defect likely started as
crevice corrosion and formation of corrosion
products led to further under-deposit corrosion.
Dissimilar Metals
• Often referred to as galvanic corrosion, occurs as the
result of placing a more active metal such as steel or
aluminum in direct contact with a more noble metal such
as copper.
• The galvanic series.
Simplified Galvanic Series
Standard Electrode Potentials
More Active
More Active
Eo (volts)
Na ↔ Na+
-2.71
Zinc
Mg ↔ Mg++
-2.38
Aluminum
Al ↔ Al+++
-1.66
Steel
Zn ↔ Zn ++
-0.763
Fe ↔ Fe++
-0.409
Lead
Nickel
Brass
Copper
Ni ↔ Ni++
-0.25
Pb ↔ Pb++
-0.126
H ↔ H+
Bronze
More Noble
Reaction
Magnesium
0
Stainless 304
Cu ↔ Cu++
0.34
Stainless 316
Fe++ ↔ Fe+++
0.771
Titanium
Ag ↔ Ag+
0.799
Au ↔ Au+++
1.498
Gold
More Noble
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Galvanic Cell
• In order to have a galvanic cell the ΔG° must be
negative.
• The standard electrode potential (E°) must be positive,
where:
• E°cell = E°cathode − E°anode
• E° Cu = +0.34, E° Zn -0.76
• 0.34-(-0.76) = + 1.10V
Corrosion due to dissimilar metals (galvanic). This is a
materials selection error likely from trying to keep cost
low.
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Erosion Corrosion
• Cavitation occurs when gas bubbles form on
lower pressure impeller surfaces and implode
when pressure increases.
• Impingement occurs as the result of turbulence
created by high velocities and directional
changes. Low alloy copper metallurgies are
especially prone.
– The corrosion can be made worse by suspended
solids.
Corrosion due to cavitation.
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Erosion type or flow assisted corrosion due
to impingement. High flow velocities will
erode protective oxide film on soft metals
such as copper.
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Stress Cracking
• Stress cracking occurs to certain metals when placed
under stress. Microscopic fissures occur along stressed
areas and attack occurs within. Over time the metal is
weakened and failure occurs.
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White Rust
• White rust is the corrosion that often occurs on galvanized
surfaces.
• When zinc carbonate forms at high pH it takes on a
voluminous grey or waxy type deposit rather than a
invisible film.
• Often just a cosmetic problem.
• Galvanize metal is not compatible with highly alkaline
cooling water.
• It is important that new galvanize towers be “seasoned”.
• Please see the AWT Paper for
detailed information!
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‘White rust’ on galvanized steel due to changes in
galvanizing process and incompatibility with high pH
cooling water.
Why you don’t use galvanize pipe in
hot water systems.
• Potential reverses about 135°F.
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Inhibiting Corrosion
•
•
•
•
•
Changing the water chemistry.
Using corrosion resistant metals.
Applying protective coatings.
Installing sacrificial anodes or cathodic protection.
Chemical inhibitors.
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Corrosion Inhibition
• Inhibiting corrosion relies on the chemistry of the water,
the environment, and the additives you use to maintain
a protective oxide film on the metal.
Corrosion Inhibitors
•
•
•
•
•
•
Calcium
Orthophosphate
Zinc
Phosphonates
Molybdate
Amines
•
•
•
•
•
Nitrite
Azoles
Silicate
Polyphosphate
pH and alkalinity
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Calcium Carbonate
• Forms precipitant at cathode.
• Prevalent corrosion inhibitor in alkaline programs.
• Usually not thought of as a significant inhibitor but it is.
Corrosion Inhibition by Ca
1/2O2 + H2O + 2e → 2OH-
H2
Fe
0
Fe
OH - OH
++
-
2e CaHCO3 + OH → CaCO3 + H2O
pH Buffers
• Maintain pH of electrolyte.
• High pH in electrolyte prevents cathodic area from
rapidly depolarizing.
• Neutralize acids formed in glycol systems.
• Buffers include borax, amines and carbonates.
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Effect of pH on Corrosion
60
Corrosion Rate
50
40
30
20
10
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
pH
Carbon Steel
Aluminum
Copper
Galvanized Steel
Poly and Ortho-phosphate
• Form complexes with Ca at cathode
• Need to formulate stabilizing polymer with package
• Reacts at anode to form iron phosphate complex holding
iron in place for further oxidation to protective film.
• Levels of 2-20ppm typically used depending on program
Corrosion Inhibition by PO4
H2
1/2O2 + H2O + 2e → 2OH-
Fe
0
3Fe++ + 2PO4 → Fe3(PO4)2
Fe
3Ca + 2PO4 → Ca3 (PO4)2
OH - OH
++
-
2e -
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Phosphonates
•
•
•
•
•
•
Including HPA, HEDP, and others.
Form calcium complexes at cathode.
Calcium reliant mechanism.
Some reaction at anode with Fe.
Need 5-10 ppm to be effective.
Effectiveness varies.
Zinc
• Forms zinc hydroxide and zinc carbonate complexes at
cathode.
• Good for soft water.
• Above pH 8 will begin to precipitate in bulk water.
• Zn stabilizing polymer recommended.
• Levels from 0.25-3.0 used.
Nitrite
• Active inhibitor forces rapid oxidation of metal surface to
a protective oxide state.
• Need high levels of 200-1000 ppm.
• Possible activity loss due to microbial oxidation or
reduction.
• Provides needed oxidizing action for molybdate in closed
systems.
• Can accelerate localized corrosion at low levels in some
cases.
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Molybdate
Functions somewhat like orthophosphate.
Forms insoluble complex with ferrous ion at anode.
Has some tubercle penetrating ability.
Need high levels in closed systems.
Typical use in open systems 3-5ppm as Mo.
•
•
•
•
•
Silicate
•
•
•
•
•
Adsorbtive inhibitor forming iron-silicate complex.
Reduces porosity of oxide film.
Has some benefit in previously corroded systems.
Can be used in potable water.
Thermal storage systems.
Azoles
• Tolyltriazole and Benzotriazole most common.
• Form complexes with cuprous oxide film helping to
strengthen it.
• Typical use is 1-4ppm in open and 20-100ppm in closed
systems.
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Corrosion Monitoring
•
•
•
•
Coupons
Linear polarization (Corrator® or CorrTrans® )
Test Spools
Visual Inspection
Corrosion Coupons
• Corrosion coupons measure the corrosive
tendencies of the system on fresh unpassivated
metal surfaces.
• Coupons may not reflect localized corrosion in the
system due to deposition, microorganisms, or
other factors.
• Maintain proper velocity in the corrosion rack. 3 –
5 feet per second or 8 – 13 gpm in 1” pipe.
• Passivated or non-passivated?
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Coupon Racks
• Coupon racks can be
made out of pvc, carbon
steel, or stainless.
• Do not plumb in copper
pipe!
• Coupon order in
direction of flow is least
to most noble.
8 – 12 gpm
in 1” pipe
3 – 5 ft/sec
Velocity
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AWT Corrosion Rates in mpy
High TDS
Moderate1 H2O
High TDS
Moderate1 H2O
Carbon Steel
Carbon Steel
Copper
Copper
Excellent
< 1.0
<0.5
<0.1
<0.1
Very Good
1-3
0.5 – 1.0
0.1 – 0.25
0.1 – 0.2
Good
3-5
1-2
0.25 – 0.35
0.2 – 0.3
Fair
5-8
2-3
0.35 – 0.5
0.3 – 0.5
Poor
8 – 10
3-5
0.5 – 1.0
0.5 – 1.0
>10
>5
>1.0
> 1.0
Description
Metal
Severe
1. HVAC for institutional and commercial facilities.
How is Corrosion Continuously
Monitored?
• The most common method is with a LPR (linear
polarization resistance) device such as a Rohrback
Cosasco Systems Corrator® or Fuchs CorrTrans.
• Probes are inserted and either report directly to a self
contained data log or transmit a 4 – 20 mA signal which
can be data logged remotely.
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Continuous Monitoring
• Continuous monitoring offers significant advantages over
corrosion coupons.
– You don’t have to wait 90 days to get results.
– You can be immediately alerted to changes in water chemistry which
may negatively impact your system.
– With new control and data reporting systems you can trend corrosion
against other system parameters.
– Affordable.
Important to install the probes
correctly. Be sure to read manual
for installation recommendations.
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Visual Inspection
Visual Inspection
• Visual inspection of vital equipment provides us with the
actual results.
• Techniques such as using a video scope and eddy
current testing can provide evidence of corrosion related
damage.
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Review
48