Waterways WW-1 Re Control of Oxygen Corrosion

7/31/2019 Waterways WW-1 Re Control of Oxygen Corrosion

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Control of Oxygen Corrosionin Marine Steam Generating Systems

Dissolved oxygen corrosion is seen as pitting in the pre-boiler, boiler, and after-boiler sections ofthe steam generation process. The effect of pits on metal surfaces is the same as that of generalcorrosion, but at a localized and more rapid rate of metal loss.

Pitting may occur in the pre-boiler section due to the transport of water containing dissolvedoxygen. Once oxygenated water enters into the boiler, the oxygen will be liberated in the highertemperatures and pose potential problems in the boiler and in the after-boiler sections.Generally, oxygen pitting will occur near or above the waterline of the steam drum in an operatingboiler or very close to the point of feedwater entering the boiler. Pitting of metal surfaces belowthe waterline is virtually nonexistent in operating boilers. Some of the oxygen will pass throughthe upper portion of the steam space and may lead to pitting on the upper boiler surfaces. Pittingmay occur at any point in the steam or condensate systems.

Reducing the dissolved oxygen content in the makeup water through mechanical, thermal orchemical means controls corrosion caused by oxygen.

Mechanical Deaeration (Atomization of the makeup into a steam or elevated temperature area)

Efficient operation of vented deaerating heaters is essential to good boiler operation. Deaeratingheaters as contrasted to closed, non-vented stage heaters serve three purposes in a steamgenerating system. They are:

Removal of non-condensable gases such as oxygen and ammonia Increase feedwater temperature Storage of boiler feedwater

A properly functioning vented deaerator normally removes dissolved oxygen in the makeup waterto less than 0.007 ppm (7 ppb). The remaining oxygen can be removed by adding a chemicaloxygen scavenger to the storage section of the deaerator.

Drew Marine Division


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Thermal Deaeration (Temperature Elevation)Increasing the temperature of water reduces the solubility of gases such as oxygen as seen inFigure 1. When mechanical deaeration (atomization) is not possible, it is recommended to heatthe feedwater to 90 C or as high a temperature as practical followed by the addition of an oxygenscavenger to the feedwater.

Figure 112

8ml/L

Oxygen 4Content

0-1 21 43 66 88 110

Degrees Centigrade

Chemical Deaeration (Scavenging)Although mechanical and thermal deaeration removes most of the dissolved oxygen in water,chemical oxygen scavengers are used to scavenge remaining oxygen. Chemical oxygenscavengers differ in their ability to remove oxygen, the amount required for the chemical reaction,under various conditions of temperature and pH, to passivate metal surfaces, their volatility(distribution ratio), thermal stability and toxicity. The remainder of this paper will discuss thedifferences between the primary oxygen scavengers used in marine steam generating systems.The discussion of each oxygen scavenger includes six categories; reaction with oxygen,passivation, decomposition, volatility or distribution ratio, dissolved solids and health hazard data.An Oxygen Scavenger Summary Chart is attached and offered as a quick reference guide.


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SODIUM SULFITEReaction with Oxygen Sodium sulfite was the first widely used oxygen scavenger in marine boiler water treatment. Itreacts readily with oxygen to form sodium sulfate by the following reaction:

2Na 2SO 3 + O 2 2Na 2SO 4

sodium sulfite oxygen sodium sulfate

The stoichiometry (the quantity of one substance to react with another substance) of this reactionreveals that 8 parts of sodium sulfite are required to react with one part of dissolved oxygen.

The rate of reaction of sodium sulfite with oxygen increases with increased temperature,increased solution pH and the presence of a catalyst as in Drew Marines CATALYZED SULFITEcorrosion inhibitor.

Passivation While sodium sulfite is an excellent oxygen scavenger, it does not have any passivation property.Therefore, it does not produce a barrier on the metal surface to provide additional corrosionprotection. Sodium sulfite and the sodium sulfate formed in the reaction with oxygen are non-volatile and will remain in the water phase, thereby adding to the dissolved solids content of theboiler water. The increased level of dissolved solids necessitates the need for additional boilerblowdown. The additional dissolved solids also increase the potential for carryover. Sulfites onlyprovide protection against oxygen from the point of feed, up to and including the boiler. They donot protect the after-boiler section due to the fact that they are non-volatile.

Decomposition At high pressures (above 600 psig or 42 bar) sulfite can decompose into sulfur (gas) and sulfurdioxide. When sulfur dioxide dissolves in the condensate, sulfuric acid forms. The acidcondensate can then cause corrosion in the after-boiler section. Therefore, the use especially ofsulfite above 600 psig (42 bar) is not the preferred oxygen scavenger.

Na 2SO 3 + H 2O + heat 2NaOH + SO 2sodium sulfite water sodium sulfur dioxide

hydroxide

4Na 2SO 3 + 2H 2O + heat 3Na 2SO 4 + 2NaOH + H 2Ssodium sulfite water sodium sulfate sodium hydrogen

hydroxide sulfideVolatility Sodium sulfite provides protection against oxygen from the point of feed, up to and including theboiler. Sodium sulfite does not protect the steam and after-boiler section from oxygen corrosiondue to the fact that it is non-volatile.

Dissolved Solids Since sodium sulfite and the sodium sulfate formed in the reaction with oxygen are non-volatileand will remain in the water phase, they add to the dissolved solids content of the boiler water.The increased level of dissolved solids necessitates the need for additional boiler blowdown. Theadditional dissolved solids, unless controlled, increases the potential for carryover.

Health Hazard Data The following toxicity data is from the RTECS (Registry of Toxic Effects of Chemical Substances)compiled by the National Institute for Occupational Safety and Health of the U.S. Department ofHealth and Human Services.Mouse LD50 Route: Oral; Dose: 820 mg/kg


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HYDRAZINEReaction with Oxygen Hydrazine was first introduced as an oxygen scavenger for boiler water treatment in the 1950sand has been a standard treatment for high pressure systems since the 1960s. Hydrazine reactswith oxygen to form water and inert nitrogen gas.

N2H4 + O 2 2H 2O + N 2

hydrazine oxygen water nitrogenThe stoichiometry of this reaction (the theoretical requirement of the amount of chemical to reactwith oxygen) is one part of hydrazine reacts with one part of dissolved oxygen. This reactionshows that much less hydrazine is required to react with oxygen than sodium sulfite.

The rate of reaction of hydrazine with oxygen increases with increased temperature, increasedsolution pH and the presence of a catalyst as in Drew Marines AMERZINE corrosion inhibitor.

Passivation In boiler systems passivation is attributed to the formation of a thin adherent oxide film on metalsurfaces. The oxide film provides a barrier between the metal and the water, thereby minimizingcorrosion attack.

Hydrazine is an excellent passivator, producing magnetite (Fe 3O4), which is a passivated oxidestate of iron with a very compact and adherent crystalline structure.

N2H4 + 6Fe 2O3 4Fe 3O4 + 2H 2O + N 2hydrazine ferric oxide ferrous oxide water nitrogen

(hematite) (magnetite)

Decomposition Hydrazine begins to decompose at 235 psig (16 bar) to form ammonia and nitrogen.

3N 2H4 + heat 4NH 3 + N 2hydrazine ammonia nitrogen

A significant amount of ammonia can be formed due to the decomposition of hydrazine. Theammonia can, in the presence of oxygen, attack copper and copper alloys in the condensate andfeedwater systems. Keeping the excess hydrazine level low and the systems tight with minimalair inleakage can avoid this.

Volatility Although hydrazine is volatile and does not add dissolved solids to the boiler water, its volatility islimited. Volatility of an amine is classified by its vapor liquid distribution ratio. The vapor liquiddistribution ratio of hydrazine is reported as 0.08. This means that at any point in the systemthere should be 0.08 ppm of hydrazine in the steam or vapor phase for every 1 ppm of hydrazinein the water phase. The low distribution ratio of hydrazine means that it does not travel far intothe after-boiler section and the majority of any unreacted hydrazine stays in the boiler.

As a comparison, the distribution ratio for ammonia is noted as approximately 10.0 meaning thatthere will be 10 ppm of ammonia in the vapor phase for every 1 ppm of ammonia in the liquidphase. It is known that ammonia travels far into the after-boiler sections and can return to theboiler where it can be recycled.


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Dissolved Solids Hydrazine and the reaction products of hydrazine do not add any dissolved solids to the boilerwater.

Health Hazard Data The following toxicity data is from the RTECS (Registry of Toxic Effects of Chemical Substances)compiled by the National Institute for Occupational Safety and Health of the U.S. Department ofHealth and Human Services.

Rat LD50 Route: Oral; Dose: 60 mg/kg

According to the NTP (National Toxicology Program), hydrazine has been categorized a Group 2agent, a substance which may reasonably be anticipated to be a carcinogen. There is sufficientevidence of carcinogenicity in experimental animals, but inadequate evidence in humans.

According to the IARC (International Agency for Research on Cancer, hydrazine is categorized asa Group 2B agent, a substance that is possibly carcinogenic to humans. The exposurecircumstance entails exposures that are possibly carcinogenic to humans. It was found thatevidence in humans was inadequate and evidence in animals was sufficient.


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DEHA (Diethylhydroxylamine)Reaction with Oxygen DEHA was introduced commercially in 1981 as an oxygen scavenger for treating boiler systems.The reaction of DEHA with oxygen is a complex process involving several reactions. The overallreaction of DEHA with oxygen can be summarized as:

4(C 2H5)2 NOH + 9O 2

8CH 3COOH + 2N 2 + 6H 2ODEHA oxygen acetic acid nitrogen water

When sodium hydroxide is present in the boiler water, the acetic acid is neutralized and removedby blowdown as sodium acetate.

The stoichiometry of this reaction (the theoretical requirement of the amount of chemical to reactwith oxygen) is 1.24 parts of DEHA reacts with one part of dissolved oxygen.

The rate of reaction of DEHA with oxygen increases with increased temperature, increasedsolution pH and the presence of a catalyst as in Drew Marines DREWPLEX OX corrosioninhibitor.

Passivation DEHA has been shown to be a good metal passivator by forming magnetite oxide film on metalsurfaces.

2(C 2H5)NOH + 6Fe 2O3 4Fe 3O4 + 4CH 3COOH + 3H 2ODEHA ferric oxide ferrous oxide acetic acid water

(hematite) (magnetite)

Decomposition DEHA is thermally stable up to 300 psig (21 bar). The major thermal decomposition products aretwo main dialkylamines; diethylamine and ethylmethylamine. These dialkylamines are volatileand contribute to elevating condensate pH, thereby reducing the requirement for a separateprimary neutralizing amine such as morpholine in higher pressure systems. DEHA alsodecomposes to ammonia but to a lesser extent than hydrazine.

Volatility Like many other amines, DEHA is very volatile. Its vapor liquid distribution ratio is 1.26. Thismeans that there will be approximately 1.26 ppm of DEHA in the vapor phase of the system forevery 1 ppm of DEHA in the liquid phase. Therefore, DEHA will be distributed through the after-boiler section of the system providing protection against dissolved oxygen corrosion.

With a vapor liquid distribution ratio of 1.26 versus that of hydrazine at 0.08, there will beapproximately 16 times more DEHA than hydrazine in the vapor phase. This results in a muchbetter passivation of the after-boiler section with the use of DEHA than hydrazine.

Because of the high volatility of DEHA it is important to sufficiently vent the mechanical/thermaldeaerator to remove any non-condensable gases that may have formed. Venting of the

deaerator directly to the atmosphere is an important practice that allows the escape of non-condensable gases such as oxygen, ammonia and amines that otherwise would return to theboiler. In marine systems the deaerator may be vented to the gland exhaust condenser ratherthan to the atmosphere. The vent line to the gland exhaust condenser may or may not have anorifice plate. In cases where the vent line to the gland exhaust condenser does have an orificeplate, the size of the orifice may need to be increased to permit effective removal of gases suchas oxygen, ammonia and amines.


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Below are suggestions for evaluating deaerator venting process efficiency. It is essential tocheck the accuracy of the gauges used for measuring temperature and pressure before makingan evaluation.

1. Difference in temperature between the water space and the steam space in thedeaerator should be a maximum of 1 C or 2 F.

2. A convenient rule of thumb is that the saturation temperature increases by 3F (or1.6 C) per psig between 0 and 10 psig (0 and 0.7 bar). Therefore a deaerator

operating at 3 psig (0.2 bar) should maintain a temperature of 221 F (105 C).3. An alternate method is to check the saturated steam tables to obtain the

corresponding temperature and pressure figures.

Dissolved Solids DEHA and its reaction products do not add dissolved solids to the boiler water.

Health Hazard Data The following toxicity data is from the RTECS (Registry of Toxic Effects of Chemical Substances)compiled by the National Institute for Occupational Safety and Health of the U.S. Department ofHealth and Human Services.

Mouse LD80 Route: Oral; Dose: 2150 mg/kg


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Oxygen Scavenger Summary Chart

Chemical Reaction with Oxygen

Stoichiometry

Metal Passivation Volatility

Distribution Ratio

Thermal

Decomposition

Sodium Sulfite 8:1

Reacts with oxygen to form

sodium sulfate.

Increases dissolved solids level

in the boiler.

None N/A Above 600 psig (42

bar) can decompose

to sulfur gas and

sulfur dioxide

Hydrazine 1:1


water and nitrogen.

Does not add dissolved solids to

the boiler water.

Good passivation in

pre-boiler and boiler

sections.

0.08 Begins at 235 psig

(16 bar) to form

ammonia and

nitrogen.

DEHA 1.24


acetic acid, nitrogen and water.

Acetic acid is neutralized by

hydrate alkalinity in the boiler

water forming sodium acetate

that is then removed by boiler

blowdown.

DEHA does not add dissolved

solids to the boiler water.

Good passivation

throughout the boiler

system.

1.26

Good distribution

through the after-

boiler section.

Over 300 psig (21

bar) to form volatile

diethylamine and

ethylmethylamine

which contribute to

condensate pH

neutralization.

All statements, information and data presented herein are believed to be accurate and reliable but are not to be taken as a guarantee,express warranty or implied warranty of merchantability or fitness for a particular purpose, or representation, express or implied, for whichseller assumes legal responsibility, and they are offered solely for your consideration, investigation and verification. Statements orsuggestions concerning possible use of this product are made without representation or warranty that any such use is free of patentinfringement and are not recommendations to infringe on any patent.

2002 Ashland, Inc. Registered trademark, Trademark of Ashland Inc. Printed in U.S.A.*Responsible Care and the Responsible Care logo are registered servicemarks of the American Chemistry Council in the U.S., of the CanadianChemical Producers Association in Canada and of different entities in other countries.

WW-1 (2/02)

Waterways WW-1 Re Control of Oxygen Corrosion

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