Discussion on Anti-corrosion Experience in Circulating Water Systems

+Currently, the main methods for corrosion inhibition in circulating cooling water are: A. Anodic corrosion inhibitors: chromates, phosphates, silicates. The anode of corrosion forms a high-density layer of Fe++ ions; any corrosion inhibitor that can react with these Fe++ ions to form an insoluble coating is known as an \"anodic corrosion inhibitor\". CrO4= + Fe++ → γ‑Fe2O3 •Cr2O3; PO4-3 + Fe++ → Fe2O3•Fe3(PO4)2. B. Cathodic corrosion inhibitors: bicarbonates, zinc salts, polyphosphates. The cathode of corrosion produces OH-, raising the local pH. Any corrosion inhibitor that can form an insoluble film as a result of this increased local pH is known as a \"cathodic corrosion inhibitor\". HCO3- + Ca++ + OH- → CaCO3 + H2O; Zn++ + 2OH- → Zn(OH)2. Furthermore, when using corrosion inhibitors in practice, it is necessary to control factors such as the concentration of the chemicals, the temperature of the metal surface, and pH. Poor control can lead to deposition problems, as well as pitting or scaling issues. Maintaining the cleanliness of the metal interfaces is the best way to prevent oxygen concentration cells (deposition corrosion); therefore, the effective use of dispersants and proper control of microorganisms are also essential factors that cannot be ignored for corrosion prevention.

In typical circulating water systems, the above method can achieve good corrosion inhibition effects. In some special systems, such as those with acid added, the total hardness can reach 800–1000 mg/L, while the alkalinity is only 50–150 mg/L. In such systems, conventional corrosion inhibitors are not very effective, or they only work when used in large quantities, but the results are not ideal. Has anyone discovered better corrosion inhibition methods or water treatment chemicals during operation? Let’s discuss them together to make progress as a team.

While effective corrosion inhibition methods or water treatment chemicals are certainly important, the timing of their addition and the amount added each time are also essential factors. Continuous adjustment is necessary, taking into account environmental influences such as the differences in corrosion inhibition effectiveness between winter and summer, as well as the material of the equipment being cooled.

At present, the concept of a cycle is not being considered; instead, we are only looking for chemicals that provide better corrosion inhibition under the same conditions, especially those that have been used successfully in the aforementioned conditions, so that others can share their experiences.

There are very few types of corrosion inhibitors that are currently in most common use, and the corrosion-inhibiting effect of organophosphates is much weaker compared to their scale-inhibiting chelating effect. The other corrosion inhibitors are zinc salts, polyphosphates, and azole agents specific to copper. Therefore, there are too few options. The former oxide film type, such as molybdate. Chromates and similar substances are also used very little these days. Moreover, the dosage of such chemicals is too high, and their cost is difficult to control. Regarding corrosion inhibitors, it is indeed necessary to introduce some agents with better cost-performance ratios; this is also a great topic for research.

I’m just starting to work with circulating water treatment, still *learning*...

Hardness level is one aspect; the key issue is that the high chloride content in the circulating water causes corrosion.

With the conventional chemicals currently available, corrosion in closed-loop water systems can be fully controlled. There are cases; if there are any difficult ones, we can discuss them.