Zinc Chloride And Orthophosphate In Cooling Water System

Zinc is a very effective corrosion inhibitor, forming a protective film on metal surfaces, especially in steel mill cooling water systems with high heat fluxes. Zinc chloride and even zinc hydroxide can be used as soluble zinc salts.

Work of zinc salts

In circulating cooling water systems, zinc ions can rapidly form a precipitated film of zinc hydroxide in the high pH region at the cathode site. The anions of zinc salts generally do not affect their corrosion inhibition properties. Commonly used zinc salts are ZnSO₄ ·H₂O, ZnSO₄ ·7H₂O or zinc chloride (ZnCl₂).

Zinc salts form films very quickly, so they are often used as a pre-film agent when the system is first applied. However, zinc films do not last long, so they cannot be used alone, but must be used in combination with other corrosion inhibitors. Zinc is unique in that it can be used in combination with many corrosion and scale inhibitors and can have a good synergistic effect.

When the calcium content in the water is low, zinc can be used in combination with polyphosphates to improve corrosion inhibition. Applications are widespread.

One of the disadvantages of zinc is that it has a tendency to precipitate when the pH is high. It is generally accepted that the pH should be controlled at ≤ 8.3. When low doses of zinc ions are used in combination with a good scale inhibitor, operation at pH > 8.3 is also permitted.

Another disadvantage of zinc is that it is toxic to aquatic organisms and is limited in terms of discharge. The concentration of zinc ions in the circulating water is appropriate at low doses and should be lower than the discharge concentration specified by environmental regulations, which also reduces the tendency of zinc scale precipitation...

The work of phosphate

The main role of orthophosphate is the anodic corrosion inhibitor, the mechanism is orthophosphate and dissolved divalent iron ion Fe²⁺ to generate a small solubility of iron phosphate compounds, deposited in the anodic area, in the presence of oxygen, it prompted divalent iron oxide into γ iron oxide - a strong passivation film, this passivation film plays a role in anodic protection.

According to the application of phosphates over the decades, orthophosphates, phosphonates, zinc salts and extremely well-dispersed calcium phosphates, suspensions, zinc deposits and iron oxide polymers are generally used in concert. The advantages are a wide range of applicable water quality, easy to adjust, low cost, good treatment effect, in line with the environmental requirements of most countries.

Phosphate tends to promote the growth of algae in water, so the environment also has restrictions on it. When using phosphate compounding agents, as matched with high-efficiency scale inhibitors, the general treatment effect is very good, not only can operate under alkaline conditions, but also allows a high orthophosphate content, but taking into account the requirements of the environment, it is still appropriate to use low orthophosphate content. Domestic examples generally use phosphate, zinc salt and multifunctional scale inhibitor compound formulations.

Synergistic effect

When more than two agents are used to form corrosion inhibitors, often better than the corrosion inhibition of these agents alone and less dosage, this phenomenon is called the synergistic effect of corrosion inhibitors or synergistic effect. Due to the synergistic effect of the compound corrosion inhibitor, but also to save the agent, so the current single corrosion inhibitor is rarely used. Molybdenum or tungsten formulations are also due to the compounding of other corrosion inhibitors, so that the amount of molybdate or tungstate is greatly reduced, the cost is reduced, which is conducive to the promotion.

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