Water Treatment In Closed Systems

Closed systems are commonly classed by their function — either heating (hot water) or cooling (chilled water). They typically rely on water or water-based solutions as their heat transfer fluid.

While various ways exist to classify water treatment product technologies, one of the simplest is whether the primary corrosion inhibitors are reducing agents, oxidizing agents, or film formers. In addition to the primary corrosion inhibitors, factors such as fluid pH and copper ions also affect corrosion. Supplemental agents are used to buffer/control pH and minimize yellow metal corrosion.

The primary corrosion inhibitors, which focus on ferrous alloys, and the supplemental inhibitors comprise the total treatment package.

Reducing Agents

Reducing agents are not commonly used due to inherent limitations with available chemicals. Reducing agents work by removing oxygen from solution so it is not available to corrode metals. Although rarely seen nowadays, tannins have been used both to remove oxygen and form an iron-tannin film on steel surfaces. Tannins are low cost, easy to formulate, testable, and readily available.

The drawbacks for tannins include their tendency to form organic deposits on heat exchange surfaces. These deposits may eventually require chemical cleaning to remove. More significantly, while they do scavenge oxygen, the rate of reaction is not rapid. It is common to have oxygen corrosion in spite of maintaining reasonable tannin residual. Sulfite is another type of reducing agent treatment that still shows up from time to time. The reaction of sulfite with oxygen has been well studied. It is generally recognized that the reaction proceeds via a free radical mechanism. The overall equation is:

½ O₂ + Na₂SO₃ --------> Na₂SO₄

With this technology, sufficient sulfite must be present at all times. Otherwise free oxygen will exist and the boiler metal will corrode. The normal approach involves maintaining an excess of sulfite (30 – 50 mg/L Na2SO3) in the water.

The sulfite residual acts as a “sponge” to react with oxygen that enters the system. Without regular testing, the risk of corrosion is quite high, since even temporary losses of the sulfite residual can lead to corrosion.

A secondary drawback is that as sulfite is fed to the system (to cope with the ongoing ingress of oxygen), the concentration of sulfate builds. Increasing sulfate increases the conductivity of the water and its corrosively as well as the potential for SRB (sulfate reducing bacteria) growth. Organic reducing agents such as hydrazine (N2H4) and DEHA (diethyl hydroxylamine) have been used.

However, decomposition (catalyzed by copper) and health concerns for hydrazine have largely eliminated their use. In the case of hydrazine, its breakdown to ammonia has resulted in failures related to intensive copper corrosion by the following mechanism.

3 N₂H₄ + 6 OH^¯ ---------> 2 NH₃ + 2 N₂ + 6 H₂O

NH₃ + Cu+² ---------> Cu (NH₃)²⁺

Both hot and cold loops have all the conditions for these reactions to take place, including the oxygen needed to oxidize the copper metal and ammonia that dissolves copper oxide. The dissolved copper is free to plate on to steel surfaces in the system where it can cause galvanic corrosion. When the copper plates out, it releases the ammonia, which is free again to repeat the cycle. As a result, ammonia will rapidly corrode copper (and its alloys).