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The preceding types of corrosion were discussed with principal reference to boilers in marine steam power plants. Of these, oxygen corrosion and carbon dioxide corrosion are equally important in the condensate and feedwater systems. It was also stated that oxygen and carbon dioxide corrosion are important with regard to both carbon steel and copper system materials.

Another important form of corrosion which can affect the copper tubing in a condenser and in feedwater heaters occurs through the presence of excessive amounts of ammonia and oxygen. It was previously discussed that maintaining an elevated pH or free hydroxyl ions is beneficial in maintaining the passivity of copper and copper alloy surface. It was also stated that there is an optimum range of pH control above which corrosion could occur. In general, the best range of pH control for copper alloys is pH = 8.3-9.0.

Ammonia or other amides are used to maintain the optimum pH conditions in condensate and feedwater systems. This can be done by adding ammonia or other compounds directly or by adding slightly more hydrazine than is needed for oxygen scavenging. You will remember that hydrazine reacts with oxygen as follows:

N2H4 + O2 —> 2H2O+N2

Excess hydrazine that leaves a boiler drum with the steam will decompose at the high temperatures in a superheater by the following reaction.

3N2H4 Heat—> 4NH3+N2

When the Steam condenses in the surface condenser, the ammonia gas that has formed by decomposition becomes dissolved in the condensate.

NH3 + H2O — NH4OH

or

NH3 + H2O — NH4+ + OH-

From the above reaction, it can be seen that ammonia is an alkaline material and that it should have the effect of elevating the pH of a solution because it leads to the formation of free hydroxyl ions. When the ammonia causes the formation of only enough free hydroxyl ions to maintain a pH of from 8.3 to 9.0 there is no problem. However, when both oxygen and excessive amounts of ammonia are present corrosion can occur. Oxygen causes protective cuprous oxides to form cupric oxides. Cupric oxides then react with ammonia, forming copper-ammonium compounds which are extremely soluble. When the protective oxide is removed from the copper surface, the passivity of the surface is destroyed and corrosion can progress very rapidly.
All copper alloys can be corroded by this reaction. The form of corrosion is one of general wastage of pitting under normal circumstances. However, brasses such as admiralty can be caused to crack as a result of corrosion by the combined effects of oxygen and ammonia.

Related Terms

CORROSION INHIBITOR

Additive for protecting lubricated metal surfaces against chemical attack by water or other contaminants. There are several types of corrosion inhibitors. Polar compounds wet the metal surface preferentially, protecting it with a film of oil. Other compounds may absorb water by incorporating it in a water-in-oil emulsion so that only the oil touches the metal surface. Another type of corrosion inhibitor combines chemically with the metal to present a non- reactive surface.

False brinelling

False brinelling of needle roller bearings is actually a fretting corrosion of the surface since the rollers are the I.D. of the bearing. Although its appearance is similar to that of brinelling, false brinelling is characterized by attrition of the steel, and the load on the bearing is less than that required to produce the resulting impression. It is the result of a combination of mechanical and chemical action that is not completely understood, and occurs when a small relative motion or vibration is accompanied by some loading, in the presence of oxygen.

INHIBITOR

A substance that slows or prevents such chemical reactions as corrosion or oxidation.

FRETTING CORROSION

Can take place when two metals are held in contact and subjected to repeated small sliding, relative motions. Other names for this type of corrosion include wear oxidation, friction oxidation, chafing, and brinelling.

SCC

Stress corrosion cracking.

FILIFORM CORROSION

Corrosion that occurs under organic coatings on metals as fine wavy hairlines.

CAVITATION CORROSION

A process involving conjoint corrosion and cavitation.

PIT

Corrosion localized in a small spot.

CHEMICAL CLEANING

Using a solvent solution to remove mill scale and corrosion products.

DEPOSIT CORROSION

Corrosion occurring under or around a discontinuous deposit on a metallic surface (also called poultice corrosion).

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