Buffers/pH Control

Although corrosion inhibitors are frequently separated from the buffering/pH control agents, in most cases they are intertwined. Inhibitors generally function only in certain pH ranges. For example, nitrite is typically used under alkaline conditions. Others, such as some of the dibasic acid mixtures, show best performance at neutral pH. Regardless of the inhibitor type, acid pH conditions are never acceptable.

Given the variety of metals and elastomers in closed loops, the pH range is a compromise. The primary metallurgies used in North America are mild steel and copper alloys. These metals require a pH range of 8.5 to 10.5. Aluminum, which is uncommon in North America but is seen in Europe, prefers a neutral pH range for optimum protection. At one time, caustic (sodium hydroxide) was commonly used to raise pH. Since caustic is not a buffer, controlling pH is difficult, and excursions are not unusual. Except for cost, there is little to recommend caustic, and the risk related to its use is high. Also rarely used, carbonate buffers are sometimes encountered where other more suitable buffering systems cannot be used. Carbonate and hydroxide may have application in nuclear plants where borate cannot be accepted. Otherwise, they have little to recommend them.

The most widely used pH control agents are all buffers:

• Ortho phosphate,
• Borate salts, and
• Organic amines.

Ortho phosphate, typically dipotassium phosphate (or adjusted mixtures of di- and tripotassium phosphate) can be used to control the pH between 9 to 10.5. At the high levels used for corrosion control, ortho phosphate provides excellent buffering capacity. This buffering function is particularly useful in glycol containing loops, where the ortho phosphates can neutralize glycol breakdown products for extended periods of time.

Another inorganic buffering system is borate (or metaborate). Borates provide good buffering capacity and allow a slightly higher pH than phosphates. This means that less can be used to obtain the desired pH. Unlike phosphate, borate functions in the sole role of a buffer. It has effectively no impact on corrosion rates beyond that seen from pH.

Organic amines provide a number of benefits compared to inorganic buffers. Depending on the pH range desired, one can select from the commonly available amines to tailor the pH of the fluid to the exact value required. In addition, morpholine has been shown to be a passivity agent in its own right. Another benefit of the amines is that the amount needed in the fluid is low (200 to 300 mg/L). The low concentration coupled with their low intrinsic conductivity means amines contribute less to conductivity than inorganic buffers. The lower conductivity means amines are useful in conductivity sensitive systems like cooling water loops for welders.