Optimum corrosion inhibiting pH ranges for common metals of construction vary. In commercial buildings, it is common to use hot water heating systems with cast iron boilers, but usually, the manufacturer of the boiler will specify water treatment that excludes caustic, as it can damage the gasket materials employed. The newer types of gas-fired, high efficiency (93-95%) condensing boilers tend to use cast aluminum alloy heat exchangers, rather than cast iron. These heat exchangers use an amphoteric metal alloy that will not tolerate fully soft water or hard water over 125 ppm TH. Also, they will not tolerate oxygen being present and need a neutral pH and dielectric insulators to prevent eddy current corrosion with copper connecting pipes. So water treatment specifications usually require a good propylene glycol antifreeze mix at 30 to 50% with water and a good corrosion inhibitor pack. Silicate works well as an aluminum inhibitor, but the formulation has to be buffered down to around pH 8.0 to 8.5.
In large diesel, closed loop cooling systems (as well as automotive cooling systems) metals of construction include a combination of aluminum, cast iron, steel, brass, copper, braising materials, Pb-based solders, and zinc. Modern inhibitor programs are now based on organic acid technology (OAT) and inhibitors include carboxylates such as ethanolamine phosphate, 2-ethyl hexanoic acid (2-EHA), and sebacates. Also, hybrid OAT programs are used, which may include, for example, phosphate or silicate.
European formulations tend to produce additives lacking in phosphates, but include borates and low silicates because their water is harder and it reacts with phosphates to create aluminum phosphate or calcium and magnesium deposits. However, Japanese formulations specify mixtures high in phosphates and low in borates and silicates because they fear lack of maintenance will cause borate corrosion. Most waters in North America also contain calcium, but typically it isn’t as hard as European waters so phosphates are considered okay to use here.
Where amphoteric metals such as zinc and aluminum are widely used in closed loop system, the pH has to be reduced, so caustic is eliminated and replaced with silicate (occasionally a silicate phosphate mix). This means that azoles are not used, as they tend to be of very limited solubility in close to neutral pH. Thus, OAT’s based on chemistries such as polyaminophosphonate and 2- EHA are used, as they protect cast iron, steel, copper, and also aluminum. Nevertheless, silicate content has to be limited as high silicate concentration can cause pump seal failure.
So, the answer to the original question of what is the optimum pH for operating water-containing inhibited closed-loop heating/cooling/chilled water loops systems are, it “all depends”!
Optimum corrosion inhibiting pH ranges for common metals of construction vary. However, if pushed for a more definitive, but simple, answer, our starting point would be as follows:
• Closed loop systems constructed primarily of mild steel = pH 8.5 to11.0
• Closed loop systems containing significant copper and brasses = pH 8.0 to 9.2
• Closed loop systems with significant aluminum = pH 7.5 to 8.5
• Closed loop systems with significant zinc = pH 7.0 to 8.0
Where the system pH requires alkaline buffering do not use caustic, as this will simply precipitate any dissolved iron as a gelatinous hydroxide and cause fouling. Rather use soda ash (sodium carbonate) or an amine, such as morpholine (lower temperatures) or alkanolamines (higher temperatures). Do not add ammonia.
Where the system requires acid buffering, use citric, sulfamic, or phosphonic acids. Do not use mineral acids.