Industrial Corrosion and Protection -- 05 Corrosion of industrial waters such as circulating water
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The corrosion of metal equipment caused by industrial water is a common phenomenon. It not only leads to the waste of resources, energy, and materials, but also often threatens the normal and safe production of large enterprises, affects product quality, and even endangers personal safety. Therefore, the issues related to the corrosion and protection against industrial water have attracted widespread attention. Industrial water can be classified by its use into cooling water, boiler water, and other industrial waters. Industrial water can come from various sources, including groundwater, surface water (river water, lake water), and seawater. Therefore, the composition of industrial water varies not only depending on the source of water but also depending on the water treatment methods used. I. Corrosion in circulating cooling water: Both fresh water and seawater can be used as cooling water. Cooling water systems are prone to a range of problems such as corrosion, scaling, and microbial growth, and these issues affect each other – corrosion products can exacerbate scaling, scaling in turn promotes under-scale corrosion, and microorganisms often contribute to both corrosion and the development of fouling. Therefore, the above factors must be considered comprehensively. Corrosion in circulating cooling water is a type of electrochemical corrosion. Taking the dissolution of iron as an example, the anodic and cathodic reactions vary depending on the conditions of the medium: in acidic aqueous solutions, it is the reduction of hydrogen ions ； In aerated acidic solutions, it is the reduction of oxygen ； In natural water, it is the reduction of water ； In naturally aerated water, it is the reduction of dissolved oxygen in the water ； Under acidic and turbulent conditions, it is the reduction of iron ions ； In the presence of sulfate-reducing bacteria, it is the reduction of sulfate ions. The products of the anodic reaction can also lead to a side reaction involving ferrous hydroxide; this substance has very low solubility and deposits rapidly on the metal surface as white flocculent masses. It is then quickly oxidized to Fe(OH)3. The dehydration of these products results in the formation of red rust and other corrosion products consisting of hydrated iron oxides. Corrosion products deposit on the metal surface, forming a thin layer of corrosion products that contains deposited salts, sediment, microbial sludge, and other substances. If the thin layer of corrosion products is porous, metal ions can pass through it to reach the interface with the solution, allowing corrosion to continue. If a dense, adhering layer is formed, it can prevent ion diffusion and thus inhibit the dissolution of the metal. The corrosion resistance of metals in industrial water depends to a large extent on the formation of a protective film on the metal. Generally speaking, the protective film formed by direct oxidation provides good protection, whereas the secondary products formed by corrosion do not offer sufficient protection. Additionally, certain corrosion inhibitors can be artificially added to form a protective film on the metal surface. The corrosivity of circulating cooling water is related to the corrosive substances present in the water. As follows: 1. pH value. If an acid-soluble metal oxide is formed, corrosion increases as the pH value decreases. For metal oxides of both sexes, a moderate pH level is conducive to protection, while too low or too high pH levels accelerate corrosion. Oxides of precious metals are insoluble at any pH value and are inert. 2. Salts in water. The presence of chlorides can destroy the metal oxide film and accelerate corrosion. Certain salts of calcium, magnesium, and aluminum can form a protective deposit layer after precipitation. 3. Gases dissolved in water. Dissolved oxygen in water acts as a cathodic depolarizer, promoting corrosion; carbon dioxide dissolved in water forms carbonic acid, which also facilitates corrosion; ammonia introduced by polluted water causes selective corrosion in copper-based materials. If H2S enters the water cooling system, it causes the pH level to drop, thereby accelerating corrosion. On top of that, the corrosion products formed, namely iron sulfide, have a high potential and act as a cathode for iron; this in turn leads to galvanic corrosion and further accelerates the corrosion process. Chlorine is generally added to cooling water systems to inhibit microorganisms, but it can produce hypochlorous acid and hydrochloric acid, which lower the pH value and increase corrosion, as well as prevent certain corrosion inhibitors from forming protective films. 4. Suspended solids. Suspended solids come from pollutants carried in the air, as well as sediment, dust, and other particles present in the water used to replenish the cooling water system. Differential gas filling in the deposition areas of these substances can lead to the formation of differential cells, accelerating corrosion. 5. Microorganisms. The role of microorganisms is to block water flow channels, increase flow resistance, reduce heat exchange efficiency, and cause corrosion and perforation. Common aerobic bacteria in water cooling systems include sulfur-oxidizing bacteria, iron bacteria, fungi, nitrifying bacteria, etc. Anaerobic bacteria include mesophilic and thermophilic sulfate-reducing bacteria.
The corrosion characteristics of recirculating cooling water are always related to the properties of the water medium used in the cooling system and the characteristics of the materials. For example, copper tube condensers exhibit the following forms of corrosion. 1. Uniform corrosion. In environments containing corrosive carbon dioxide or acidic corrosive media, uniform corrosion occurs, characterized by the exposure of the base metal on the surface; there is very little greenish corrosion product deposition, the surface lacks luster, and appears slightly uneven. 2. Dezincification corrosion. Zinc loss corrosion occurs when brass is used in circulating water; this corrosion takes place in water that is slightly acidic or slightly alkaline and contains a small amount of dissolved oxygen ； The water flow rate is relatively low ； The pipe is at a high temperature; there are permeable deposits on the inner surface. Brass pipes containing more than 15% zinc are prone to zinc loss; the presence of iron and manganese in the copper alloy accelerates this zinc loss, while arsenic, antimony, and phosphorus can inhibit it. The arsenic-containing brasses commonly used today generally have an arsenic content of 0.02% to 0.03% or more, and such brasses do not suffer from zinc loss in cooling water; if magnesium is present in the copper tubes, it can counteract the effect of arsenic. In cases of severe water pollution, zinc loss still occurs in copper pipes even when they contain more than 0.03% arsenic; in arsenic-added brass, the effect of arsenic in preventing zinc loss decreases when the temperature exceeds 60–70°C; zinc loss corrosion is likely to occur when the mass concentration of chloride ions is below 10 mg/L; the least zinc leaching occurs in neutral water, while it is more prone to zinc loss in slightly acidic or slightly alkaline water. 3. Erosive corrosion. Usually, there is a protective film on the surface of the copper tubes in the condenser, preventing direct contact between the metal and water. If the intense agitation caused by water flowing through the pipes, or the impact of air bubbles mixed in with the water, damages the protective film on certain areas of the copper pipe surface, horseshoe-shaped corrosion pits can form in those areas. Erosion pits are directional, aligning with the flow of water, which is a characteristic of erosion. Erosion usually occurs near the cooling water inlet end. Erosive corrosion is related to the flow rate of water inside the pipe; the critical velocity at which several copper alloys begin to suffer from erosive corrosion is higher, and erosion occurs more easily when the flow rate exceeds this critical value. 4. Sand erosion. Also known as slurry erosion, it is a type of copper tube corrosion caused by suspended sand particles in the cooling water scouring the tube walls. The characteristic of this corrosion is the presence of a small number of punctate depressions on the inner surface of the copper tubes; sometimes these depressions are arranged in rows. There are generally no corrosion products on these depressions, and at times a metallic luster can be seen in those areas. When the sand content in water is less than 30 mg/L, sand erosion does not occur. 5. Corrosion under sediments. There are two scenarios. In the first, poor fluidity leads to the deposition of substances, creating an isolated area beneath the deposits. The area through which water flows at the edges of these deposits forms gaps, which serve as anodic areas prone to corrosion; corrosion grooves form at these edges, while the metal surface near the deposits acts as a cathodic area. In the second scenario, when the deposits are corrosion products that are loose and porous, many pitting defects appear on the metal surface beneath those deposits. 6. Ammonia corrosion. NH3 and hydrazine are typically added to boiler feedwater to control pH and oxygen levels, thereby preventing corrosion in water and steam systems. In the air-cooled area of the condenser, high concentrations of ammonia occur due to the accumulation of NH3 and non-condensable gases; these concentrations can reach 10^4 mg/L, causing the pH of the condensate water to be between 9 and 9.4. Together with the presence of oxygen and moisture, this area is prone to corrosion. To prevent ammonia corrosion, electroplating a layer of nickel with a thickness of 30 um on aluminum brass tubes proves to be effective. Recently, due to the use of steel tube feedwater heaters in the generator sets, the pH value of the feedwater has increased (pH > 9). The pH value of the condensate water in the air-cooled area of the condenser has reached 10–10.4, resulting in localized pitting corrosion on the nickel-plated aluminum brass tubes as well. To solve this problem, aluminum brass tubes or titanium tubes lined with 18-8 stainless steel can be used. 7. Corrosion in polluted seawater. Groundwater is commonly used in coastal areas to replenish cooling water, but with the development of industry, the large amount of wastewater discharged is causing increasing pollution of groundwater there. Tubes corroded by polluted circulating water are covered with a sticky deposit high in sulfides, beneath which pitting corrosion occurs. Due to the presence of organic matter in polluted water, when its concentration is high, it creates an oxygen-deficient environment. Sulfate and organic matter in circulating water generate H2S as a result of the metabolism of sulfate-reducing bacteria (anaerobic bacteria), causing the pH to drop to as low as 3.2, which leads to severe corrosion of metal pipes.
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