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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

0

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2 3 4 5 6 7 8 9

Cycles

Blowdow

n in g

pm

Save Water and Chemical

Example using 1000 ton load

15 gpm

10 gpm 7.5 gpm

6 gpm 5 gpm

7.9 million gal/yr

2.6 mm gal/yr

1.26 mm gal/yr

0.79 mm gal/yr

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Effect of CaCO3 Scale on Efficiency

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0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05Scale Thickness (inches)

% E

nerg

y In

cre

ase

% Energy Increase

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

Crystal Growth

Nucleation

Scale

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Deposit Formation

Ca ++

Ca ++

Ca ++

Ca ++ Ca ++

Ca ++

Ca ++

CO 3 --

CO 3 --

CO 3 --

Nucleation and growth at surface irregularity, existing deposit, or high heat flux surface.

HEAT

Na2CO3

SS

SS SS

Biofilm can accumulate calcium and provide nucleation sites

SS SS

SS

SS

NaHCO3

Na2CO3

Na2CO3

Na2CO3

Na2CO3

Corrosion sites may provide nucleation site.

NaHCO3

NaHCO3

NaHCO3

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

Nucleation

Crystal Growth Scale

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

HEDP • MW = 206

• %PO4 = 92.2

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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

Anodic reaction Fe0 Fe++ + 2e–

Cathodic reaction 1/2O2 + H2O 2OH-

o2

OH-

OH- OH-

o2

Fe++ Fe++

Fe++

Fe0

e- e-

OH-

Fe++ OH-

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.

-

Basic Corrosion Model

1/2O2 + H2O + 2e → 2OH-

2e-

Fe Fe

OH - OH -

++ 0

Alkaline pH Acid pH

H+ + H+ + 2e → H2↑

2e-

H2

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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2Fe(OH)2+1/2O2 + H2O → 2Fe(OH)3↓

Fe ++

OH

OH

OH

OH OH

OH Fe

++

Fe ++

Fe ++ Fe

++

e - e -

e - e -

e -

e - e - e -

e -

O 2

O 2 O 2

O 2

O 2

O 2

O 2

O 2

Corrosion Model cont’d.

At high pH insoluble complexes (rust) are made.

1/2O2 + H2O → 2OH-

Fe + 2OH- → Fe(OH)2 ↓

H+

Feo

Fe++ + 2e-

2e- + H2O +1/2O2 → 2OH-

Cathode

Anode

OH-

OH-

OH- OH-

OH-

e-

e-

Fe++

Fe++

Fe++ Fe++

Fe++ + 2Cl- + 2H2O→ Fe(OH)2 + 2HCl H+ H+

H+ H+

OH-

Cl-

SO4- -

HCO3-

Fe++

Fe(OH)2 + ½ H2O + 1/2O2 Fe(OH)3

Occluded Corrosion Cell

2HCl + Fe++ →FeCl2 + 2H+

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

More Active Magnesium

Zinc

Aluminum

Steel

Lead

Nickel

Brass

Copper

Bronze

Stainless 304

Stainless 316

Titanium

More Noble Gold

Standard Electrode Potentials

More Active Reaction Eo (volts)

Na ↔ Na+ -2.71

Mg ↔ Mg++ -2.38

Al ↔ Al+++ -1.66

Zn ↔ Zn ++ -0.763

Fe ↔ Fe++ -0.409

Ni ↔ Ni++ -0.25

Pb ↔ Pb++ -0.126

H ↔ H+ 0

Cu ↔ Cu++ 0.34

Fe++ ↔ Fe+++ 0.771

Ag ↔ Ag+ 0.799

More Noble Au ↔ Au+++ 1.498

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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-

2e -

Fe Fe

OH - OH -

++ 0

H2

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

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

3Fe++ + 2PO4 → Fe3(PO4)2

3Ca + 2PO4 → Ca3 (PO4)2

1/2O2 + H2O + 2e → 2OH-

2e -

Fe Fe

OH - OH -

++ 0

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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

Description

High TDS Moderate1 H2O High TDS Moderate1 H2O

Metal 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

Severe >10 >5 >1.0 > 1.0

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