CHEMICAL FACTORS

WATER COMPOSITION

The nature of the water phase, including dissolved and suspended matter, is integral to the subject of corrosion control. These aspects are discussed briefly to establish general guidelines. The exact constituents of the water phase are unique to any one water supply or application. 

Therefore, the specific problems encountered in any plant' operations are, in a sense, similarly unique. Proper corrosion control requires that each case be considered separately. Often, the treatment program that works well for one plant may fail at a neighboring installation.

pH

Between pH 4.3 and 10.0 in natural, aerated waters at normal temperatures, changes in pH will affect the degree to which steel is corroded. In the case of ferrous-based materials, the solution just outside the anode is mildly alkaline since it is generally saturated with ferrous hydroxide. 

However, below pH 4.3, where there is free mineral acidity in the water, corrosion progresses rapidly. In the pH range, overvoltage plays a more significant role in the corrosion process. Variations in the macrostructure of the metal or in the water interface become more important.

The actual effect of pH on a particular metal is determined by the behavior of its oxide. If the oxide is soluble in acidic media, the metal will corrode rapidly in this environment. If the oxide will readily dissolve in alkaline media, there will be extensive corrosion in that pH range. Most metals fall into the first category.

Occasionally, a metal oxide will dissolve in both acid and alkaline solutions; these metals are referred to as "amphoteric". The metal will have its greatest stability, from a corrosion standpoint, at some intermediate pH range. Aluminum and zinc are amphoteric. Their corrosion rates are minimal at pH 6.5 and 11.5, respectively. Some metal oxides are insoluble at any pH. Their corrosion rate will, therefore, be independent of pH. "Noble" metals, those at the top of the galvanic series, will behave in this way. The figure illustrates these points further.

A number of odd trends are evident in the behavior of iron. As the pH increases to 4.0, the behavior is similar to that of an acid-soluble metal. Between pH 4.3 and 10.0, the corrosion rate is less influenced by pH, because oxygen depolarization is the principal factor determining corrosion. Any further pH increase reduces the corrosion rate, until a minimum value is attained at about pH 12. At this point, iron behaves amphoterically; the corrosion rate again begins to rise with increasing pH. Interestingly enough, this results from hydrogen evolution; iron reacts with strong caustic solutions to liberate hydrogen and form the ferroate ion.

Fe° + 2NaOH ———> Na₂FeO₂ + H₂

Thus, we see that hydrogen evolution controls corrosion of ferrous-based materials at both extremes of the pH scale.

DISSOLVED SALTS

The corrosion rate for metals associated with natural, low-solids water, at normal temperature, will accelerate as the concentration of the dissolved salts in the water increases. The corrosion rate may, thereafter, decrease in more heavily concentrated solutions, as a result of precipitation of dissolved salts as their solubility products are reached. In a dilute solution, increased electrical conductivity causes the corrosion rate to increase. For a concentrated solution, precipitation could quite possibly result in a barrier film, which would retard corrosion.

The Ionic constituents in normal waters have various effects on the corrosion rate. For instance, the chloride ion, and to a lesser extent, the sulfate ion are capable of penetrating passive films and setting up highly active, local anodic sites. Conversely, hardness ions and alkalinity have an inhibiting effect on corrosion; the precipitated products of various hardness salts often inhibit corrosion.

DISSOLVED GASES

A number of gases are normally found dissolved in water; these include carbon dioxide and oxygen. Other gases may also be present as a result of contamination or other control programs. Three typical examples of these are ammonia, hydrogen sulfide and chlorine.

Carbon Dioxide

The gradual solution of carbon dioxide into water will decrease pH by the formation of carbonic acid, which adds acidity to the water and thereby promotes hydrogen evolution.

CO₂ + H₂O ———> H₂CO₃

Oxygen

The amount of dissolved oxygen in water is directly related to its temperature, pressure, and surface area. Dissolved oxygen in the water acts as a cathodic depolarizer, promoting corrosion.

A special case of oxygen corrosion is mentioned briefly here and discussed in greater detail later in this section. When water has unequal oxygen concentrations, a differential aeration cell is set up. A very common manifestation of this is under-deposit corrosion. 

Any porous deposit on a metal surface, whether from precipitated salts, suspended matter, or biological growth, will almost always have an oxygen-deficient environment underneath it. This leads to the formation of an active anodic site under the deposit and severe localized corrosion.

Ammonia

Ammonia is generally introduced into water as a result of process contamination. It will selectively corrode copper in the presence of oxidizing agents.

NH₃ + H₂0 ————> NH₄OH NH₄OH + Cu⁺² ————> Cu(NH₃)⁺² + H₂O 

This soluble copper-ammonium complex is very corrosive to copper.

Hydrogen Sulfide

Hydrogen sulfide is one of the most harmful gases that can enter a cooling water system. It normally results from process contamination, especially in refineries and petrochemical plants, or is produced by the reduction of sulfate ions by sulfate-reducing bacteria. 

The gas promotes active corrosion in two ways. Because it is acidic, it causes low pH attack. Secondly, it is responsible for the formation of iron sulfide, which is cathodic to iron and leads to galvanic corrosion.

Chlorine

Chlorine gas is the most commonly used toxicant for the control of microorganisms in cooling water systems. Upon entering the water, it hydrolyzes to form hypochlorous and hydrochloric acid. This action reduces the pH of the recirculating water and causes increased corrosion. On many metals it also retards formation of certain protective corrosion inhibitor films.

SUSPENDED MATTER

Mud, sand, silt, clay, dirt and other particles may enter a cooling water system either as airborne contamination or as part of the system's makeup water supply. In areas of the system where sedimentation of these materials take place, porous deposits are easily formed and differential aeration cells are quickly established, which can cause more corrosive damage than precipitated salts.

MICROORGANISMS

Microbiological growth often presents very special problems. Hydrogen is metabolized by many species, causing depolarization of the corrosion cell, similar to the action caused by dissolved oxygen. Anaerobic bacteria form differential aeration cells and accelerate local attack. Some species produce acidic compounds.

Desulfovibrio desulfuricans, a type of sulfate-reducing bacteria, produce hydrogen sulfide by reduction of the sulfate ions found in almost all water supplies. Such sulfate oxidizers as the Thiobacillus oxidize the sulfate ion to sulfuric acid and cause low pH attack.