Dissolved oxygen cooling water

The production of hydroxide ions creates a localized high pH at the cathode, approximately 1—2 pH units above bulk water pH. Dissolved oxygen reaches the surface by diffusion, as indicated by the wavy lines in Figure 8. The oxygen reduction reaction controls the rate of corrosion in cooling systems the rate of oxygen diffusion is usually the limiting factor. 

Foulants enter a cooling system with makeup water, airborne contamination, process leaks, and corrosion. Most potential foulants enter with makeup water as particulate matter, such as clay, sdt, and iron oxides. Insoluble aluminum and iron hydroxides enter a system from makeup water pretreatment operations. Some well waters contain high levels of soluble ferrous iron that is later oxidized to ferric iron by dissolved oxygen in the recirculating cooling water. Because it is insoluble, the ferric iron precipitates. The steel corrosion process is also a source of ferrous iron and, consequendy, contributes to fouling. 

This represents a special case of high-level turbulence at a surface by the formation of steam and the possibility of the concentration of ions as water evaporates into the steam bubbles . For those metals and alloys in a particular environment that allow diffusion-controlled corrosion processes, rates will be very high except in the case where dissolved gases such as oxygen are the main cathodic reactant. Under these circumstances gases will be expelled into the steam and are not available for reaction. However, under conditions of sub-cooled forced circulation, when cool solution is continually approaching the hot metal surface, the dissolved oxygen... 

Preparation of Potassium Nitrite. Melt 1-2 g of potassium nitrate in a test tube and heat it above its melting point until oxygen bubbles stop evolving. Dissolve the cooled substance in water and add 1-2 ml of a dilute sulphuric acid solution. What happens ... 

Iron(n) Hydroxide. Prepare 200 ml of water containing no dissolved oxygen. To do this, boil distilled water for 5-10 minutes while passing carbon dioxide through it, and then cool the water in a stream of carbon dioxide to room temperature. Close the mouth of the flask with a stopper. 

In a cooling system where the water is saturated with oxygen, the primary rate-determining step is usually the rate of diffusion of dissolved oxygen at the cathode. Thus deaeration and the use of inhibitor treatments are ways of accomplishing cathodic polarization. Anodic polarization can also occur by the formation of a thin impervious oxide film, chemisorbed at the anode, such as takes place on the surface of stainless steels. However, for most metals used in cooling systems, the chemisorption process must be aided by anodic corrosion inhibitors. 

Mechanical and Chemical Deaeration. The corrosive quahties of water can he reduced by deaeration. Vacuum deaeration has been used successftdly in once-through cooling systems. When all oxygen is not removed, catalyzed sodium sulfite can be used to remove the remaining oxygen. The sulfite reaction with dissolved oxygen is... 

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