The pH value of boiler feed water plays very important role in controlling the corrosion. To understand the importance of pH value for controlling the corrosion it is necessary to know little about electro chemical corrosion.
ELECTROCHEMICAL THEORY OF CORROSION; Each substance even metals dissolve in water to a certain extent like sugar and salt . No doubt the amount of dissolved substance required for saturation varies considerably according to the nature of the substance. A limited amount of any given substance dissolved in water no matter how much the substance is added.
All metals also tend to dissolve in water to a certain extent according to electrochemical theory. Water in contact within iron dissolves very small quantity of iron into solution. The water quickly becomes saturated with iron which stops further dissolving. The dissolved iron remains in different conditions from that in the solid metallic state . Every iron particle going into solution takes on an electric charge and is known as ferrous ion.
Fe(metallic state) = Fe ++ (Ferrous ion when goes into solution)
Water 9in natural condition also ionizes to a very small extent and forms positive H2 ions and negative hydroxyl OH _ as given in the following equation
H2O = H(+ ) + OH( -)
The ferrous ions combine with negatively charged Hydroxyl ions to form ferrous hydroxides.
Fe ++ + 2 OH - = Fe (OH) 2 (ferrous hydroxide)
The dissolved oxygen in water oxidizes formed ferrous hydroxides to ferric hydroxide as given by the following equation
4 Fe(OH) 2 + O2 + 2 H2O = 4 Fe(OH)3 ( insoluble ferric hydroxide)
The formed ferric hydroxides is much less soluble than the ferrous hydroxides and tend to precipitate. This in turn allow s more iron to go into the solution.
The another important role is played by excess hydrogen ions that form on the metal surface to accelerate corrosion. Hydroxil ions form the water unite with the ferrus ions and hydrogen ions are left free in the water as given by the following chemical equation
Fe++ + 2 HOH = Fe(OH)2 + 2H+
These excess hydrogen ions form a protective coating over thye metallic surface and tend to prevent further solution of metallic ions. The dissolved oxygen in the water also tends to combine with free excess hydrtogen ions and forms water
Therefore the dissolved oxygen also removes the protective layer of hydrogen ions and allows more iron to dissolve in the water.
The above explained electrochemi9cal theory of corrosion states that the corrosion is controlled by the free Hydrogen ions present in water. Higher hydrogen content means increased acidity. The acidic or alkaline nature of water is also responsible for the corrosion .
pH and its role in corrosion
The pH is a number denoting the degree of acidity or alkalinity of a substance. It does not indicated the quantity of acid or alkaline as found by titration method. It is derived by measuring the amount of Hydrogen ion (H+) in grams per liter of solution.
For example Pure water ionizes to produce ( 10 -7) grams of H2 ions per liter. Therefore any substance producing 10( -7) grams of Hydrogen ions per liter considered to be neutral solution.
The greater the amount of Hydrogen ions present in solution its acid reaction becomes stronger. Therefore pure water being neutral solution any solution producing more hydrogen ions than pure water will be acidic and degree is governed by the difference.
Other solution producing less hydrogen ions than pure water will be alkaline and the degree is also governed by the difference.
The law of mass action in mass dissociations is OH (-) + H(+) = 10 (- 14)
The pH value of water is the logarithm of the reciprocals of Hydrogen ion concentration. It is numbered from 0 to 14 indicating 7 for neutral water. As the pH is a logarithmic function solution having a pH as 6, 5 or 4 are respectively 10 ,100, 1000 times more acidic than one with pH value as 7
THE ROLE OF pH in CORROSION OF METALS
The role of pH in corrosion of metals is extremely important . The corrosion rate of iron in the absence of oxygen is proportional to pH up to a value of 9.6. At this point hydrogen gas formations and dissolving of iron practically stops . This is the same pH produced by a saturated solution of Ferrus Hydroxide ( Fe(OH)2.
Alkalinity adjustment and film formation are closely related. The pH value of feed water should be maintained greater than 9.6 to reduce the corrosion effects caused by the reason. Film formation occurs when the alkalinity is kept in the desired range The simplest film is composed of the iron hydroxide initially formed on the metal surface . As long as the alkalinity is high the iron hydroxide remains insoluble and acts as a protective layer.
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