Factors Affecting Scale Formation

In addition to inorganic scale, circulating water can also have fouling, which is often caused by the presence of many sands, humus, various suspended solids, and colloidal substances in surface water. It may also be due to the entry of excessive dust into the water, leading to an increase in water turbidity. The primary cause of inorganic scale formation is still water evaporation, which increases the concentration of scaling ions in the water.

Factors Affecting Corrosion

· pH: This primarily depends on the solubility of the metal oxide in water in relation to the pH value. Generally, for materials like nickel, iron, and magnesium, the lower the pH, the faster the corrosion (for iron, at very high pH, iron also dissolves to form iron salts).

· Anions: The different ions in water affect the corrosion rate of metals in the following order: NO₃^- < CH₃COO^- < Cl^- < ClO₄^-, with the concentration of chloride ions being the main focus of monitoring.

· Complexing Agents: We mainly consider NH₃. When certain pipes in chemical plants leak, it can sometimes cause an increase in ammonium ions, accelerating corrosion.

· Hardness: When calcium and magnesium ion concentrations are too high, they react with carbonate, phosphate, or silicate ions in the water to form calcium carbonate, calcium phosphate, and calcium silicate scale, leading to under-deposit corrosion. 

· Metal Ions: Heavy metal ions such as copper and silver can have harmful effects on common metals like steel, aluminum, and magnesium in cooling water. These ions often form micro-cells that cause corrosion of the base metal.

· Dissolved Gases: The corrosion rate of carbon steel, copper, and copper alloys increases as the oxygen content rises. When the pH of water is low, carbon dioxide can cause the dissolution and breakdown of the metal surface film. When oxidants are present in cooling water, ammonia selectively corrodes copper. Hydrogen sulfide accelerates corrosion of copper, steel, and alloy steels, but does not corrode aluminum. Sulfur dioxide lowers pH and increases corrosion. 

· Concentration: Most metals experience increased corrosion as concentration rises in non-oxidizing environments. In oxidizing environments, corrosion rate increases with concentration up to a maximum value, after which a protective film forms and corrosion rate decreases.

· Suspended Solids: When the flow rate of cooling water decreases, suspended solids tend to form loose deposits on the heat exchanger surface, leading to under-deposit corrosion.

· Flow Rate: At low flow rates, the dissolved oxygen in the water increases, which accelerates the corrosion rate of metals. 

· Temperature: In general, the corrosion rate of metals increases as the temperature rises. 

· Chlorine:

·· Carbon Steel: When the residual chlorine concentration in water reaches 0.5 mg/L, the corrosion rate of carbon steel increases rapidly. At 0.7 mg/L, it exceeds the permissible upper limit specified by design standards.

·· Nickel Cast Iron: When the chlorine concentration is less than 2 mg/L, the impact is minimal.

·· Copper Alloys: When the chlorine concentration is less than 2 mg/L, the impact is minimal.

·· Aluminum: Corrosion occurs, but it is not severe.

·· Stainless Steel: The effect depends on the material type.

Factors Affecting the Growth of Microorganisms and Algae

Generally, the ions that promote the growth of microorganisms and algae are primarily inorganic phosphates, which are the main source of growth for these organisms. In addition to inorganic phosphates, poor water quality in circulating water can also introduce a significant amount of microorganisms and algae. Furthermore, when the ammonia nitrogen content is high in water in chemical plants, it can also promote the growth of microorganisms and algae. 

· Chromates: Sodium chromate, an oxidizing corrosion inhibitor, has a critical concentration that needs to be above the threshold. It is typically used in lower doses combined with other inhibitors (such as zinc salts, polyphosphates, and organophosphates). Its advantages include providing excellent protection for steel, copper, zinc, aluminum, and their alloys, with a wide pH range (6-11) and good effectiveness.

· Nitrites: Sodium nitrite, an oxidizing corrosion inhibitor, has a critical concentration that depends on corrosive ions. It is widely used as a passivator after acid cleaning of cooling equipment and as a non-chromate corrosion inhibitor in closed-loop cooling water systems. However, it requires a high concentration and can promote microbial growth, possibly being reduced to ammonia, which causes corrosion and is toxic. 

· Silicates: Water glass with a SiO₂ to Na₂O ratio of 2.5–3.0. The optimal pH is 8.0–9.5, and it is not suitable for water with high pH or high magnesium hardness. When used as a corrosion inhibitor, cooling water must have aerobic metals for effective protection. The concentration ranges from 8–20 mg/L in direct water and 40–60 mg/L in circulating cooling water, with a minimum of 25 mg/L. It is especially useful for dezincification of brass.

· Molybdates: Non-oxidizing corrosion inhibitors, with poor effectiveness and high cost. 

· Zinc Salts: Zinc sulfate, a cathodic corrosion inhibitor, is often used in combination with other inhibitors and is highly effective. It has low cost, forms a film quickly, but requires a pH < 8.

· Phosphates: Anodic corrosion inhibitors that are non-toxic and inexpensive but require use with specialized co-polymers. They have moderate corrosion inhibition and can promote algae growth.

· Polyphosphates: Sodium hexametaphosphate and sodium tripolyphosphate, linear inorganic polymers, not only have corrosion inhibition properties but also have a low-concentration scale inhibition effect on calcium carbonate and calcium sulfate in cooling water. They are often used in combination with chromates, zinc salts, molybdates, and organophosphates. While they have many advantages, they are prone to hydrolysis into orthophosphate, which forms scale with calcium ions and can promote algae growth. They are also corrosive to copper and copper alloys.

· Organic Phosphonates: Common ones include ATMP (Aminotris(methylenephosphonic acid)), HEDP (Hydroxyethylidene diphosphonic acid), DETMP (Diethylenetriamine penta(methylenephosphonic acid)), PBTCA (2-Phosphonobutane-1,2,4-tricarboxylic acid), and HPA (Hydroxyphosphonylacetic acid). Compared to polyphosphates, they are less prone to hydrolyze into orthophosphates. According to Starostina's research, the stability of phosphonate scale inhibitors follows this order: PBTCA > HEDP > ATMP > Sodium Hexametaphosphate. Organic phosphonates have the disadvantage of being highly corrosive to copper and its alloys and are more expensive.

· Mercaptobenzothiazole (MBT): An anodic corrosion inhibitor that is very effective for copper and copper alloy corrosion. It works well at low concentrations but is easily oxidized by chlorine or chloramine.

· Benzotriazole (BTA) and Methyl Benzotriazole (TTA): BTA is a highly effective corrosion inhibitor for steel and copper alloys. The corrosion inhibition rate is highest when BTA and TTA are used in a pH range of 6–10. When free chlorine is present in the cooling water, BTA’s corrosion inhibition ability is compromised, but once the free chlorine is consumed, the inhibition effect is restored. Both BTA and TTA are more resistant to chlorine oxidation but are expensive. 

· Ferrous Sulfate: Widely used in power plant condensers. Currently, power plants worldwide use a combination of ball cleaning and ferrous sulfate film formation treatment to prevent corrosion of copper tubes in condensers by cooling water.