Corrosion Basics: Inhibitors for Waters of Low to Moderate Salt Concentrations

Waters of low to moderate salt concentrations are often encountered in marine and offshore activities.

Waters of low to moderate salt concentrations are encountered in municipal water systems, cooling waters, marine and offshore activities, and oilfield water injection systems. Because metals adsorb ions of dissolved salts in water, an inhibitor has more difficulty reaching the metal surface and displacing adsorbed ions than it has in demineralized water; hence, a higher concentration of inhibitor is required. Furthermore, chloride ions have a depassivating effect; that is, they make it more difficult for passivating inhibitors to control corrosion. Therefore, it is important to maintain inhibitor concentrations at a safe level in waters containing dissolved salts, particularly if these include chlorides.

Potable water cannot be treated with most inhibitors because of their toxicity. Fortunately, treatment with lime to raise the pH usually affords sufficient protection to steel or cast iron water pipes. If the water is high in chlorides or sulfates, then polyphosphates may be used for added inhibition. Silicates may also be used in municipal waters, but they have the disadvantage of forming precipitates with iron and calcium, forming scale inside pipes and on heat transfer surfaces.

Cooling water systems may be of either recirculating or once-through types. In closed-loop recirculating systems, oxygen can be excluded, and corrosion often can be controlled by adjusting the pH to an alkaline value. Recirculating systems are more easily controlled by inhibitors because higher concentrations can be applied and maintained with limited replenishments.

Glycol/water mixtures, such as those used to cool engines and to transfer solar heat, are usually inhibited with a mixture of borax, for maintaining an alkaline pH, and mercaptobenzothiazole, which inhibits the corrosion of brass and copper. Borax alone is satisfactory for steel in contact with glycol/water mixtures, but borax and glycol attack zinc (including galvanizing) rapidly, and selectively attack the zinc in brass because of the formation of complex zinc compounds at low temperatures.

The addition of mercaptobenzothiazole is necessary in many alloy-based or mixed metal cooling systems. A soluble oil also is often added to increase protection and to lubricate moving parts in the cooling system. In some mixed metal systems, silicates and nitrates are now used. Amine phosphates have also long been used in such systems.

In once-through cooling systems, corrosion is typically more severe, and good inhibition is imperative. Cooling systems that are open to the atmosphere use evaporation as part of the heat-transfer process, providing a ready source of oxygen from the atmosphere. Once-through open systems require inexpensive corrosion inhibitors that can be completely replaced in each pass. The situation is similar to that in municipal water supplies, so comparable remedial measures are often used, namely, addition of lime or polyphosphates.

Waters that may contain appreciable quantities of organic matter, such as seawater and oilfield injection brines, usually are not inhibited with oxidizing inhibitors because oxidation of the organics requires high consumption of the inhibitor. Non-oxidizing inorganic inhibitors, such as sodium silicate (Na2SiO3), must also be used in high concentrations to address the high chloride content of brines. Generally, organic inhibitors offer the best means for protection in organic-contaminated brines. Inhibitor concentrations as low as 10 to 20 ppm in fatty amines often effectively control corrosion in oilfield brines.

This article is adapted from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 538-539.

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