A simple question often asked in the water treatment business is, what is the optimum pH for operating water-containing, inhibited closed-loop heating/cooling/chilled water loops systems? Unfortunately, there is no simple answer. As usual in many water treatment situations, it “all depends”! For example, it depends upon the materials of construction, which are typically mild steels, cast iron, copper, and brasses (together with elastomers and packing and gasket materials). However, aluminum, zinc, nickel, titanium, and stainless alloy – and occasionally glass fiber and plastics, may also be present. Each of these materials is subject to pH and other chemistry limitations – and often flow velocity limitations.
Additionally, it depends upon the nature and purpose of the closed loop and the heat-transfer fluid employed. The system may be water, glycol, or brine based. Alternatively, there may be oils, lithium bromide, or ammonia present. Is the closed loop genuinely 100% closed, or does it contain a save-all or hot-well tank that is open to atmosphere, where oxygen ingress is likely? Is there a venting system? Is there a water phase change, as when condensers or heat recovery systems are employed? Does the system function in a dairy, process industry, or a diesel engine electrical generator?
Even if the system is a simple, chilled water closed loop containing a “standard” inhibitor package, we have to consider the water source and its analysis (potable/non-potable supply, or reused/recycled water, salinity, saturation indices, common ion effects, the presence of copper ions, dissolved iron, etc.).
Also issues of inhibitor biodegradability, calcium tolerance, vapor pressure, dissociation constants, plus flow rates. Finally, is the system a new or a dozen years old? Are internal surfaces clean or are scale and fouling/biofouling deposits present? Is corrosion evident in the form of old pits and tubercules, or is there any evidence of new corrosion/erosion attack?
At one time chromates were particularly popular as corrosion inhibitors; phosphate, silicate, and nitrite chemistries continue to remain popular – supplemented by borates, azoles, and perhaps molybdates. But increasingly, these inorganic-based programs are being partially or totally replaced by organic blends, based on chemistries such as phosphonates, and amines or ketoamines, oxazolidines, and substituted triazines. Older organic chemistries, such as fatty amines, tannins, succinates, alkanolamides, imidazolines, and the various vapor phase corrosion inhibitors (VpCI) continue to be used. Each of these chemistries provides optimum functionality at specific narrow pH ranges – which changes when blends of chemistries are employed.
The types of inhibitors employed include
• Reducing agents, e.g tannins, DEHA
• Direct oxidizing agents, (which react directly with the metal surface) e.g. nitrate, chromate. They are very effective as they do not need the presence of oxygen
• Indirect oxidizing agents (which require some oxygen), e.g. molybdate
• Inorganic precipitating film formers, such as (anodic) ortho-phosphate and silicate
• Organic cathodic film formers, e.g. phosphonates, including Belcor 575 (HPA)
• Organic film formers, e.g. coco/tallow diamines (such as Duomeen T as water-soluble acetates), substituted triazines (Belcor 590), imidazolines, azoles, complex oligomeric succinates (e.g. Bricorr 288), keto-amines (Rhodines) and dibasic organic acids
The commonest chemical inhibitor programs used in the simplest closed-loop systems tend to be based on various formulatory ingredients such as the various phosphates, or sodium nitrite together with azole, borate, and perhaps molybdate or silicate.