QUESTIONS

AND ANSWERS

Close-up of “pit” formation — the characteristic oxygen attack mechanism.

Oxygen Scavengers

SM

Figure 2 — In the basic corrosion cell,iron dissolves at the anode while oxygenreacts to form hydroxide ions at thecathode

Can an oxygen scavenger be usedif the plant currently does notdeaerate the feedwater?

Yes. Oxygen scavengers areeffective at high oxygen levels, butmechanical deaeration is moreefficient and cost-effective in thelong run. Oxygen corrosion cannotbe completely inhibited by chemicalsalone. There is no real substitutefor good mechanical deaeration.Chemicals are most effective whenused only to polish a properlyoperating system.

Should oxygen scavengers be fedwith other products?

No. Oxygen scavengers should notbe mixed with other chemicalsbecause this may result in acomplete loss of product activity.

The addition of water to a liquidoxygen scavenger may also resultin a loss of product activity.Dilution water contains oxygen,which is consumed by the scaven-ger and reduces the overall effec-tiveness of the product. All liquidoxygen scavengers should be fedneat (undiluted).

What Nalco chemicals can beused for oxygen scavenging?

• Sodium sulfite based:NALCO® 19P NALCO® 2549NALCO® 1720 NALCO® 7408NALCO® 1722

• Proprietary formulations:NALCO® 1700 NALCO® 1721NALCO® 1710 ELIMIN-OX®

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Oxygen Corrosion

Is oxygen corrosive?

Yes. Oxygen is a highly reactivegaseous element. In the presenceof steel, the corrosion rate ofoxygen doubles for each 30°F risein temperature. For example, in aboiler system operating at 400 psigand 444°F, the corrosion rate foroxygen is 256 times more reactivethan at room temperature.

How does oxygen attack metalsurfaces?

Oxygen forms localized corrosionareas referred to as “pits.” Thisdistinctive formation is readilydistinguishable from acid attack,caustic gouging or chelate corro-sion. (See Figure 1.) Oxygen pitscan rapidly “drill” through metalsurfaces, leading to metal fatigueand failure.

by oxygen at the cathode. (SeeFigure 2.) Pitting is the result ofthis localized mechanism.

Anode: Fe0 → Fe+2 + 2e–

Cathode: 2e– + H2O + 1/2O2 → 2OH–

Where can oxygen corrosionoccur in a boiler system?

Because oxygen is a gas, it will“flash” into the condensate sys-tem, turbines, and other steamoperated equipment. Every metalsurface of the boiler system isvulnerable to oxygen attack.

Why is oxygen corrosion aserious problem?

As oxygen corrodes the boilermetal, it dissolves the iron surface.This weakens the metal site, butmore importantly, sends dissolvediron into the boiler. This dissolvediron can deposit onto boiler tubes,causing overheating and tubefailure.

Oxygen Scavenging

How can oxygen corrosion beminimized?

Oxygen control is generally both amechanical and chemical process.The majority of oxygen in theboiler feedwater is typicallyreduced to less than 20 parts perbillion (ppb) by heating the waterto reduce its solubility and releas-ing it out of the system via venting(deaeration). Since even very lowlevels of oxygen will cause corro-sion, a chemical scavenger is usedto supplement mechanical deaera-tion to reduce the level to zero.For more information on mechani-cal deaeration, see NalcoTECHNIFAX® 148.

Figure 1 — Oxygen attack occurs as adistinctive pit formation

What is the oxygen corrosionmechanism?

Oxygen corrosion is an electro-chemical process similar to asimple battery. Iron dissolves atthe anode and releases electronswhich are subsequently consumed

O2 Scavengers O 2 Scavengers & Passivators

19P 1720 1722 2549 7408 1710 1721 1700 ELIMIN-OX

Form Dry Liquid Liquid Dry Liquid Liquid Dry Liquid Liquid

Max. pressure 900 900 900 900 900 1500 900 2500 Unlimited

Catalyzed Yes Yes Yes Yes No Yes Yes No No

Passivates No No No No No No Yes Yes Yes

Adds solids Yes Yes Yes Yes Yes Trace Yes Trace No

Spray attemperation No No No No No Yes No Yes Yes

Superheater lay-up No No No No No No No No Yes

Boiler lay-up Yes Yes Yes Yes Yes No Yes No Yes

FDA approved Yes Yes Yes Yes Yes No Yes Yes No*

USDA approved No No Yes Yes No No No Yes No

*Can be used in boilers which are FDA regulated for pulp and paper mills only

Table 1 — Oxygen scavenger characteristics

Each of these chemicals is a strongreducing agent which quicklyremoves oxygen from boilerfeedwater. (See Table 1.) Todetermine the maximum recom-mended boiler pressure for eachproduct, see Figure 3.

0 1000 2000 3000 4000

content of water from the satura-tion point to 70% with sodiumsulfite at room temperature. Thistype of product is recommendedonly when the application cannottolerate the presence of a cobaltcatalyst (catalyst beds, foodapplications, etc.).

Catalyzed sulfite — The catalyst,usually a cobalt salt, speeds up thereaction rate of sulfite at lowtemperatures (<240°F). A fullysaturated oxygen solution can becompletely deoxygenated in aslittle as 30 seconds. A potentialdrawback is that the catalyst canbe deactivated or precipitated byimproper feed application, such aswater dilution or mixing withcaustic.

Decharacterized sulfite — This is aterm used by the USDA. It refersto specific chemicals which mustbe removed from sodium sulfitefor the product to be USDA andFDA approved. Sulfite which isnot decharacterized has the abilityto give meat a red or “fresh”appearance. Nalco's two dechar-acterized products are bothcatalyzed.

What are the disadvantages ofusing sodium sulfite?

Several costly disadvantages areinherent with using sodium sulfiteas an oxygen scavenger. Theseproblems occur regardless ofwhether the sulfite is catalyzed oruncatalyzed.

• Adds solids to the boiler — Addi-tional solids to the boiler con-tributes to an increased blow-down requirement. Blowdownis literally “money down thedrain.” It is a direct loss ofenergy and increases watertreatment costs.

• Cannot be used for sprayattemperation — The solidscontribution of sodium sulfitewould cause superheater andturbine deposits. Also, sincechemical feed must occur afterthe attemperation water take-offpoint, all equipment upstream ofthat point would be subject tooxygen corrosion and costlyrepairs.

Pressure, psi

Figure 3 — Oxygen scavengerpressure limits

What types of sodium sulfite areavailable?

Sodium sulfite is a highly effectiveoxygen scavenger and is availablein several forms:

Uncatalyzed sulfite — The reac-tion of sulfite with oxygen is veryslow at temperatures below 200°F.Without a catalyst, it takes up toten minutes to reduce the oxygen

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Sulfite

NALCO 1710

NALCO 1700

Hydrazine

ELIMIN-OX

• Cannot be used at boiler pressuresabove 900 psig. Sodium sulfitebegins to decompose at approxi-mately 600 psig and is completeat 900 psig. The decompositionproducts are H2S and SO2, bothhighly corrosive gases, whichcan cause catalyst poisoning andsevere corrosion on steamoperating equipment.

What is NALCO 1710?

NALCO 1710 is a nonvolatile,organic oxygen scavenger whichcan be used as a direct replacementfor sulfite without the associateddissolved solids contribution.

What are the advantages of usingNALCO 1710?

• Minimal solids added to the boiler— Less boiler solids meanslower conductivity levels andlower blowdown rates, therebysaving energy dollars.

• No poisoning of catalyst beds —Unlike sulfite, NALCO 1710 willnot poison hydrogen, ammoniaor methanol catalysts with sulfurdioxide or hydrogen sulfide.

• Can be used for spray attemperation— Because it breaks down tovolatile components, the uniquechemistry of NALCO 1710permits it to be added upstreamof the spray attemperation take-off point which provides moreequipment protection fromoxygen attack.

• Can be used at boiler pressures upto 1500 psig — Unlike sulfite,which has a pressure limit of 900psig, NALCO 1710 can be usedup to pressures of 1500 psig. Thedecomposition products ofNALCO 1710 are completelyvolatile and have been provennot to adversely affect steamoperated equipment.

• Reduced sodium level variation inthe boiler feedwater — Lowervariation will reduce the prob-lems in maintaining the properNa:PO4 ratio in phosphate-basedprograms. This enhances boilercorrosion protection and foulingcontrol by eliminating thecontrol conflict between theoxygen scavenger and internaltreatment programs.

Can NALCO 1710 actually savemoney over sulfite?

Yes. Since NALCO 1710 does notcontribute dissolved solids to theboiler, this product may pay foritself within the first year of treat-ment. For example, in a typical400 psig boiler using 500,000 lb/hrof feedwater which is conductivitylimited, the savings in energy andwater cost can be as high as $200/day or $70,000 per year! The returnon investment (ROI) for using aNALCO 1710 program can be over500% while only slightly raisingthe cost of chemical treatment. Foran estimate of your potentialblowdown savings, contact yourlocal Nalco representative.

How is a NALCO 1710 programmonitored?

Iron, copper, and oxygen testingare recommended to monitor theoxygen consumption of theproduct. Oxygen residual testingprovides an extra measure ofinsurance, especially duringupsets. Your Nalco representativecan provide the necessary productmonitoring guidelines.

How does the performance ofNalco's oxygen scavengerscompare?

At typical boiler feedwater tem-peratures (>210°F), sulfite,NALCO 1710 and NALCO 1700provide the fastest oxygen scav-

0 20 40 60 80 100Oxygen removal, %

Figure 4 — Oxygen scavengerperformance (240°F)

enging properties, whereas,NALCO 1721, hydrazine, andELIMIN-OX all provide superiorprotection from oxygen by alsoproviding metal passivation. (SeeFigure 4.)

Metal Passivation

What is metal passivation?

Metal passivation has traditionallybeen considered to be the reduc-tion of hematite to magnetite iniron-based boiler tubes. Actually,it is a process by which bare metalsurfaces form a protective oxidefilm. The passive film is very thinand dense. It is distinguishablefrom the base metal by the colora-tion. In the case of carbon steel,this protective film is magnetite(Fe3O4) and is black in color.

Can oxygen scavengers enhancemetal passivation?

Yes. Hydrazine has traditionallyshown passivation properties andNalco has developed severalproprietary products which effec-tively enhance metal passivation.

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Sulfite

NALCO 1710

NALCO 1721

Hydrazine

NALCO 1700

ELIMIN-OX

What is NALCO 1700?

NALCO 1700 is a highly effectiveoxygen scavenger that also en-hances metal passivation. NALCO1700 is nonvolatile and the ingre-dients are listed as GenerallyRecognized As Safe (GRAS) by theFDA for use as boiler water addi-tives. NALCO 1700 is also ap-proved for use in USDA regulatedmeat and poultry applications.NALCO 1700 cannot be used indairy applications.

What are the advantages ofNALCO 1700?

• Minimal solids added to the boiler— Less boiler solids meanslower conductivity levels andlower blowdown rates, therebysaving energy and water treat-ment dollars.

• Can be used for spray attemperation— Because it breaks down tovolatile components, the uniquechemistry of NALCO 1700permits it to be added upstreamof the spray attemperation take-off point which provides moreequipment protection fromoxygen attack.

• Provides metal passivation —Passivation is the extra measureof protection that helps improvesystem reliability. Less corrosionmeans less downtime and fewerrepairs.

• Can be used at boiler pressures upto 2500 psig — NALCO 1700begins to decompose at tempera-tures associated with pressuresof approximately 1800 psig. Thedecomposition products ofNALCO 1700 are completelyvolatile and have been provennot to adversely affect steamoperated equipment.

• Fast reaction rate — At typicalfeedwater temperatures(>210°F), NALCO 1700 is nearly100% faster than hydrazine.

Will NALCO 1700 carry over intothe steam?

No. NALCO 1700, under boilerconditions, is a nonvolatile sub-stance that will not carry over. It isacceptable for turbines, evapora-tors, and other steam operatedequipment.

Will NALCO 1700 adversely affectcondensate polisher resin?

Unlike some oxygen scavengerswhich can carry over into thesteam, NALCO 1700 cannot foulor adversely affect condensatepolisher resin.

What is NALCO 1721?

NALCO 1721 has been speciallyformulated to provide the scav-enging power of catalyzed sulfiteand metal passivation propertiesof NALCO 1700. This productoffers sulfite users the additionalbenefits of metal passivation andprotection against oxygen attack.

What is ELIMIN-OX?

ELIMIN-OX is an all-volatileoxygen scavenger which can beused in all types of boilers, exceptwhere FDA or USDA complianceis required.

Is ELIMIN-OX just a catalyzed formof hydrazine?

No. ELIMIN-OX oxygen scaven-ger contains carbohydrazidewhich reacts directly with oxygen.At temperatures above 300°F, itbegins to break down to hydra-zine. ELIMIN-OX is unique in thatboth the product and its by-products are oxygen scavengersand metal passivation enhancers.

What Nalco oxygen scavengerscan be used for metalpassivation?

• NALCO 1700• NALCO 1721• ELIMIN-OX

Each of these chemicals is anexcellent metal passivator whichquickly forms a protective oxidelayer on the boiler metal surface.(See Table 1.)

What are the disadvantagesassociated with hydrazine?

• Generates ammonia — One of thedecomposition products ofhydrazine is ammonia (NH3). Atconcentrations above 0.5 ppm,ammonia will attack copper inthe presence of oxygen.

• Suspected carcinogen — Hydra-zine is a suspect carcinogen,therefore, handling is a specialconcern. Precautions, records,and special equipment may berequired to avoid inhalation orfrequent exposure to hydrazine.

• Slow reaction rate — At low tem-peratures, hydrazine is slow toreact. Slow reaction time meansthere is always the possibility ofoxygen attack, even with hydra-zine in the system, unless veryhigh levels are maintained.

Can the low temperature reactionof hydrazine be catalyzed tospeed it up?

Yes. Although a catalyst helps atambient temperatures, it is of littlevalue at boiler operating tempera-tures. Even catalyzed hydrazinereacts more slowly than theuncatalyzed form of sulfite at lowtemperatures.

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NALCO CHEMICAL COMPANY ONE NALCO CENTER NAPERVILLE, ILLINOIS 60563-1198SUBSIDIARIES AND AFFILIATES IN PRINCIPAL LOCATIONS AROUND THE WORLD

Registered Trademarks of Nalco Chemical Company ©1983, 2000 Nalco Chemical CompanyAll Rights Reserved Printed in U.S.A. 12-00 Bulletin 116

Poor Good Excellent

Figure 5 — Metal passivationperformance (240°F)

Poor Good Excellent

Figure 6 — Metal passivationperformance (>300°F)

Sulfite

NALCO 1710

NALCO 1721

Hydrazine

NALCO 1700

ELIMIN-OX

Sulfite

NALCO 1710

NALCO 1721

Hydrazine

NALCO 1700

ELIMIN-OX

What are the advantages of usingELIMIN-OX?

• Wet lay-up of boilers and superheaters— Oxygen protection is asimportant when a boiler comesdown as while it is on line. Whenproperly used, ELIMIN-OXhelps prevent oxygen damageand continues to passivate metalsurfaces during wet lay-up.

• Once-through boilers — Thesuperior metal passivationproperties of ELIMIN-OX makeit the product of choice for once-through systems.

• Fast reaction rate — At typicalfeedwater temperatures(>210°F), ELIMIN-OX is nearly15% faster than hydrazine.

• No handling concerns — Unlikehydrazine, ELIMIN-OX poses nospecial handling problems.

• No solids added to the boiler —Zero boiler solids means lowerconductivity levels and lowerblowdown rates, thereby savingenergy and water makeupdollars.

• Can be used for spray attemperation— Because it breaks down tovolatile components, the uniquechemistry of ELIMIN-OX permits itto be added upstream of the sprayattemperation take-off pointwhich provides more equipmentprotection from oxygen attack.

• Provides metal passivation —Passivation is the extra measureof protection that helps improvesystem reliability. Less corrosionmeans less downtime and fewerrepairs.

How does the performance ofNalco's metal passivatorscompare?

Temperature has an effect on theperformance of certain metalpassivators. (See Figures 5 and 6.)At typical feedwater temperatures(>210°F), hydrazine has onlymoderate passivation properties,whereas, NALCO 1700, NALCO1721 and ELIMIN-OX provideexcellent metal passivation. Atboiler operating conditions(>360°F), all products provide fullmetal passivation protection.ELIMIN-OX has also been shownto be significantly more effectivethan DEHA and MEKO as avolatile oxygen scavenger in bothlab and field studies.

How will I know which product isbest for my plant?

Nalco can provide technical service,consulting and chemicals for totalboiler control of your plant. Pro-ductivity improvement throughdecreased energy losses, reduceddowntime, and superior corrosionprotection are goals that Nalcoshares with your plant. Your localNalco representative will be happy toconduct a survey of your particu-lar application and recommend acost-effective program for your use.

• Unlimited boiler pressures —ELIMIN-OX can be used for anyboiler operating pressure. Thedecomposition products ofELIMIN-OX are completelyvolatile and have been provennot to adversely affect steamoperated equipment.

How are NALCO 1700, NALCO1721 and ELIMIN-OX programsmonitored?

Testing procedures have beendeveloped to monitor residualNALCO 1700, NALCO 1721 andELIMIN-OX products present inthe system.

Additional iron, copper, andoxygen testing are recommendedto monitor the passivation proper-ties and oxygen consumption ofthe products. Oxygen residualtesting provides an extra measureof insurance, especially duringupsets. Your Nalco representativecan provide the necessary productmonitoring guidelines.