Effects of Various Dissolved Species

Demineralized water is relatively non-corrosive toward steel because of its high electrical resistance (ohmic control) and low hydrogen ion concentration. However, when demineralized water is in contact with the atmosphere, it will absorb carbon dioxide and form carbonic acid, which wiU decrease its resistance so that significant corrosion of steel will occur, the cathodic reaction being primarily reduction of dissolved oxygen rather than reduction of hydrogen ions. 

In this case, minimal concentrations of inhibitors such as sodium chromate, sodium nitrite, polyphosphates, sodium benzoate, or borax are effective. Steel is easily passivated in demineralized or distilled water because the pH is neutral and there are no dissolved ions to interfere with formation of the passive layer. 

Industrial and domestic waters contain dissolved substances that affect their aggressiveness and corrosion inhihitor requirements in various ways, depending on the nature of the substances. The most common dissolved substances and their effects on corrosion inhibition are as follows. 

like many other metals, is more difficult to passivate in the presence of the chloride ion, therefore, a higher concentration of passivating inhibitor is required. Non-passivating inhibitors must also be used in higher concentrations because chloride ions are strongly absorbed by steel. 

The effects of sulfate on passivity are similar to those of chloride, but to a lesser degree. Sulfates or chlorides must not be allowed to build up in a system by evaporation because depassivation may 

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