May I ask about the anti-corrosion measures for the water side of water-cooled heat exchangers?

All three methods are in use, each with its own advantages and disadvantages. The cost of Ni-P is relatively high, and there is a significant variation in the actual service life of the products. It is not always possible to find suppliers that can provide aluminum coating of good quality; I have encountered cases where the aluminum coating peeled off before the product was even used. The high-temperature resistance of the coatings also differs considerably from what is claimed in the promotional materials. For our unit, we use a lot of coatings; it depends on how various users evaluate them, as some of these aren’t technical issues.

Many unit water cooler tube bundles use stainless steel

If all stainless steel tubes are used, the cost will be extremely high. Considering aluminum infiltration and Ni-P plating, it’s not clear which method will have a longer lifespan or result in higher costs. After all, refineries carry out maintenance every 3 years, and as long as this cycle can be maintained, that’s sufficient. The circulating water in our plant is slightly acidic; without adding chemicals

Since the process media on the inner and outer walls of the heat exchange tubes are generally different, the mechanisms of corrosion also vary. It is crucial to determine whether corrosion in the tube side or the shell side is more significant, or which side has higher requirements regarding its media. Rather than considering corrosion on one side unilaterally. Therefore, the material of the heat exchange tubes should be considered comprehensively when making changes. It is determined based on a comprehensive consideration of the process media in the tube and shell sides as well as the operating conditions. In my opinion, if only the corrosion caused by water is considered and no phase change occurs, it is more economical and practical to use coatings on the water side; especially, the effect on the tube side should be good. Our plant’s thermal water station has used bundles with coatings, and after one cycle, the protection at the tube ends (expansion joints) or welds (welded joints) was effective, and there were no perforations in the inner tubes. The application process and inspection of coatings must be strictly controlled; otherwise, if the coating layer is not applied properly, it will lead to the concentration of substances in the gaps, accelerating corrosion. For the tube banks of the initial vacuum distillation column and the initial vacuum distillation column coolers in our plant, coatings were used in the past, while rare earth materials are being used now. It is more durable than paint. Other devices such as deaerators have been put into use one after another; it is advisable to use deoxygenated water, as this results in a much better corrosion resistance. The use of recycled water yields poorer results, mainly due to blockages caused by sludge or microorganisms; high-pressure water cleaning can be carried out regularly whenever the process permits.

The anti-corrosion method chosen varies depending on the corrosion mechanism; aluminum infiltration is generally used in high-temperature areas, while nickel-phosphorus plating is suitable for low-temperature areas. For cost savings, heat curing can be employed

In my opinion, the choice of pipe fittings depends on the water quality; for example, if the water used in boilers causes minimal corrosion, a corrosion margin of 1 mm is sufficient. If the water quality is very poor, then the measures suggested by the original poster need to be taken. Generally speaking, applying paint is less costly. However, if the equipment used in the workshop is not large in size, it is recommended to use stainless steel for the pipes, while the housing can be made thicker as a solution

Applying paint is the most cost-effective option; it usually works fine for one cycle (3 years). There are many types of coatings, such as TH847, TH901, PVB-A, etc., each with different operating temperatures; TH847 requires a slightly lower temperature. But they can all meet the requirements of water coolers. Additionally, after coating protection, care should be taken not to use steam cleaning, as this may cause the coating to peel off. Aluminized steel is commonly used in high-temperature environments and is resistant to sulfur corrosion. If the Ni-P plating is applied properly, its performance in use is also good.

I found everyone’s answers very insightful, but I have a question: does applying paint affect the heat exchange efficiency?

I believe that the type of anti-corrosion measures to be adopted should mainly be chosen based on the quality of the cooling water. It is necessary to address the main factors causing corrosion in order to achieve the desired result. I’ve read some information on cooling water corrosion; it’s copied below in the hope that it will be helpful to the original poster. In circulating cooling water systems, a large number of devices are heat exchangers made of metal. In heat exchangers made of carbon steel in particular, the long-term use of circulating cooling water can lead to corrosion and perforation, and the cause of this corrosion is the result of a combination of various factors. 1. Electrochemical corrosion caused by dissolved oxygen in cooling water: Since water can come into full contact with oxygen in the air, the O2 dissolved in the water can reach a saturated state. When carbon steel comes into contact with cooled water containing O2, many corrosion microcells form on the metal surface, and these microcells cause the metal in the anodic area to be continuously dissolved and corroded. 2. Corrosion by harmful ions: As circulating cooling water becomes more concentrated, the levels of bicarbonates and other salts such as chlorides and sulfates also increase. An increase in the concentrations of Cl－ and SO2－4 ions accelerates the corrosion of carbon steel. Ions such as Cl－ and SO2－4 reduce the protective capacity of the protective film on the metal surface. In particular, Cl－ ions have a small radius and strong penetration ability, allowing them to easily pass through the film layer, displace oxygen atoms to form chlorides, and accelerate the anodic process, thereby speeding up corrosion. Therefore, chloride ions are one of the causes of pitting corrosion. Cl－ is the main cause of stress corrosion in heat exchangers used in stainless steel manufacturing; therefore, high levels of Cl‑ ions in the cooling water can lead to rapid corrosion and damage at the stress concentration points on the equipment, such as the edges of the expanded tubes on the heat exchanger plates. In circulating cooling water systems equipped with stainless steel heat exchangers, the Cl‑ content should be kept at no more than 300 mg/l. 3. Corrosion caused by microorganisms: The growth of microorganisms can also lead to corrosion of metals. The mucus excreted by microorganisms, together with deposits formed by inorganic scale and sedimentary debris, adheres to the metal surface, creating a concentration cell for oxygen that promotes metal corrosion. Furthermore, there is a lack of oxygen between the precipitates on the metal surface, allowing certain anaerobic bacteria (mainly sulfate-reducing bacteria) to thrive; their growth is even faster at temperatures of 25–30°C. It decomposes sulfates in water to produce H2S, causing corrosion of carbon steel. Iron bacteria are the main cause of steel rust nodules; they can oxidize Fe2+ to Fe3+, releasing energy that is needed for the survival of the bacteria. The various factors mentioned above, which cause corrosion in carbon steel, often lead to the corrosion and perforation of the heat exchanger walls, resulting in leaks; or the process medium leaks into the cooling water, causing material loss and water contamination ； Or cooling water seeps into the process medium, affecting product quality. When the number of pipes that have been corroded and punctured is small, temporary plugging of those pipes can be used to allow the heat exchanger to continue operating with a reduced heat transfer surface area. When there are too many perforated tubes, the heat transfer surface of the heat exchanger is significantly reduced, resulting in a loss of cooling capacity; in such cases, production must be stopped for replacement. Therefore, corrosion, just like scale, is a \"major threat\" that endangers safe production in enterprises and causes economic losses. (III) Microbial sludge causes system failure. Microorganisms in cooling water generally refer to bacteria and algae. In fresh water, there are generally fewer bacteria and algae. However, in circulating water, the concentration of nutrients, rising water temperatures, and sunlight create conditions that allow bacteria and algae to reproduce rapidly. The mucus secreted by large numbers of bacteria, along with the sticky substances produced by algae, act like adhesives that cause dust particles, impurities, and chemical precipitates floating in water to stick together, forming sticky deposits. This biofouling that adheres to the walls of heat exchanger tubes not only causes microbial corrosion of the equipment’s pipes but also reduces the cooling efficiency of the heat exchangers; in severe cases, it can even block these pipes, forcing companies to suspend operations temporarily for cleaning. Experts downstairs are also welcome to continue sharing their opinions