Control of Oxygen Corrosion

in Marine Steam Generating Systems

Dissolved oxygen corrosion is seen as pitting in the pre-boiler, boiler, and after-boiler sections of the steam generation process. The effect of pits on metal surfaces is the same as that of general corrosion, 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 dissolved oxygen. Once oxygenated water enters into the boiler, the oxygen will be liberated in the higher temperatures 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 operating boiler or very close to the point of feedwater entering the boiler. Pitting of metal surfaces below the waterline is virtually nonexistent in operating boilers. Some of the oxygen will pass through the upper portion of the steam space and may lead to pitting on the upper boiler surfaces. Pitting may occur at any point in the steam or condensate systems.

Reducing the dissolved oxygen content in the makeup water through mechanical, thermal or chemical 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. Deaerating heaters as contrasted to closed, non-vented stage heaters serve three purposes in a steam generating 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 water to less than 0.007 ppm (7 ppb). The remaining oxygen can be removed by adding a chemical oxygen scavenger to the storage section of the deaerator.

Thermal Deaeration (Temperature Elevation)

Increasing the temperature of water reduces the solubility of gases such as oxygen. When mechanical deaeration (atomization) is not possible, it is recommended to heat the feedwater to 90°C or as high a temperature as practical followed by the addition of an oxygen scavenger to the feedwater.

Although mechanical and thermal deaeration removes most of the dissolved oxygen in water, chemical oxygen scavengers are used to scavenge remaining oxygen. Chemical oxygen scavengers 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 the differences 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.

SODIUM SULFITE

Reaction with Oxygen

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

2Na2SO3         +      O2       →         2Na2SO4

sodium sulfite           oxygen          sodium sulfate

The stoichiometry (the quantity of one substance to react with another substance) of this reaction reveals 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 Marine’s CATALYZED SULFITE corrosion 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 corrosion protection. 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 the boiler water. The increased level of dissolved solids necessitates the need for additional boiler blowdown. The additional dissolved solids also increase the potential for carryover. Sulfites only provide protection against oxygen from the point of feed, up to and including the boiler. They do not 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 sulfur dioxide. When sulfur dioxide dissolves in the condensate, sulfuric acid forms. The acid condensate can then cause corrosion in the after-boiler section. Therefore, the use especially of sulfite above 600 psig (42 bar) is not the preferred oxygen scavenger.

Na2SO3         +       H2O        +      heat      →         2NaOH       +       SO2

sodium sulfite             water                                  sodium               sulfur dioxide

hydroxide

4Na2SO3         +       2H2O       +        heat      →          3Na2SO4       +       2NaOH       +        H2S

sodium sulfite             water                                 sodium sulfate            sodium           hydrogen

hydroxide             sulfide

Volatility

Sodium sulfite provides protection against oxygen from the point of feed, up to and including the boiler. Sodium sulfite does not protect the steam and after-boiler section from oxygen corrosion due 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-volatile and 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. The additional 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 of Health and Human Services.

Mouse LD50 – Route: Oral; Dose: 820 mg/kg

HYDRAZINE

Reaction with Oxygen

Hydrazine was first introduced as an oxygen scavenger for boiler water treatment in the 1 950s and has been a standard treatment for high pressure systems since the 1 960s. Hydrazine reacts with oxygen to form water and inert nitrogen gas.

N2H4         +       O2        →            2H2O       +       N2

hydrazine           oxygen                 water             nitrogen

The stoichiometry of this reaction (the theoretical requirement of the amount of chemical to react with oxygen) is one part of hydrazine reacts with one part of dissolved oxygen. This reaction shows 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, increased solution pH and the presence of a catalyst as in Drew Marine’s AMERZINEcorrosion inhibitor.

Passivation

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

Hydrazine is an excellent passivator, producing magnetite (Fe3O4), which is a passivated oxide state of iron with a very compact and adherent crystalline structure.

N2H4        +      6Fe2O3        →        4Fe3O4       +       2H2O      +       N2

hydrazine        ferric oxide           ferrous oxide          water           nitrogen

(hematite)               (magnetite)

Decomposition

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

3N2H4     +       heat     →            4NH3       +      N2

hydrazine                               ammonia        nitrogen

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

Volatility

Although hydrazine is volatile and does not add dissolved solids to the boiler water, its volatility is limited. Volatility of an amine is classified by its vapor liquid distribution ratio. The vapor liquid distribution ratio of hydrazine is reported as 0.08. This means that at any point in the system there should be 0.08 ppm of hydrazine in the steam or vapor phase for every 1 ppm of hydrazine in the water phase. The low distribution ratio of hydrazine means that it does not travel far into the 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 that there will be 10 ppm of ammonia in the vapor phase for every 1 ppm of ammonia in the liquid phase. It is known that ammonia travels far into the after-boiler sections and can return to the boiler where it can be recycled.

Dissolved Solids

Hydrazine and the reaction products of hydrazine do not add any 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 of Health and Human Services.

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

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

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

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 overall reaction of DEHA with oxygen can be summarized as:

                         4(C2H5)2NOH       +      9O2→                 8CH3COOH        +      2N2     +       6H2O

DEHA                   oxygen              acetic acid             nitrogen          water

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

The stoichiometry of this reaction (the theoretical requirement of the amount of chemical to react with 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, increased solution pH and the presence of a catalyst as in Drew Marine’s DREW PLEXOX corrosion inhibitor.

Passivation

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

2(C2H5)NOH          +      6Fe2O3       →         4Fe3O4        +        4CH3COOH          +       3H2O

DEHA                  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 are two main dialkylamines; diethylamine and ethylmethylamine. These dialkylamines are volatile and contribute to elevating condensate pH, thereby reducing the requirement for a separate primary neutralizing amine such as morpholine in higher pressure systems. DEHA also decomposes 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. This means that
there will be approximately 1.26 ppm of DEHA in the vapor phase of the system for every 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 be approximately 16 times more DEHA than hydrazine in the vapor phase. This results in a much better 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/thermal deaerator 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 the boiler. In marine systems, the deaerator may be vented to the gland exhaust condenser rather than to the atmosphere. The vent line to the gland exhaust condenser may or may not have an orifice plate. In cases where the vent line to the gland exhaust condenser does have an orifice plate, the size of the orifice may need to be increased to permit effective removal of gases such as oxygen, ammonia and amines.