Under-scale corrosion in coolers

Introduction to Under-Deposit Corrosion 1. Definition: Under-deposit corrosion refers to the corrosion that occurs as a result of deposits on the metal surface. 2. Corrosion mechanism: It is a special form of localized corrosion; its mechanism lies in the fact that, due to the geometry of the equipment as well as corrosion products and deposits, the flow of the medium over the metal surface and the diffusion of dielectrics are restricted. This results in significant differences in the chemical composition of the medium within the blocked cavities compared to the medium elsewhere, leading to large changes in the pH value of that medium. Such conditions give rise to occluded cell corrosion, and the electrode potential at the tips decreases, thereby causing corrosion. Based on their corrosion mechanisms, they can be divided into acidic corrosion and alkaline corrosion; generally, the corrosion that occurs beneath scale in circulating cooling systems is acidic corrosion. Scaling refers to the process by which scaling-forming components present in cooling water accumulate on the metal surfaces on the water side, while fouling is a collection of solid substances including scale. Common types of dirt include sludge, dust, and sand particles, corrosion products, as well as natural organic organisms. Deposits such as debris, alumina, aluminum phosphate, iron phosphate, and other kinds of dirt are also present. The dirt in cooling towers comes from the following sources: ① Dirt originating from the make-up water. ②From air dirt. ③Dirt from the system itself. Microorganisms are small organisms that are often invisible to the naked eye. The types of microorganisms include bacteria, algae, fungi, and protozoa. When they multiply in large numbers in cooling water systems, they can cause the water to turn black and produce an unpleasant odor. Damaging the environment, it also generates large amounts of sludge that reduces the cooling efficiency of the cooling tower; the increased head loss due to this reduced efficiency, along with the fungi that accumulate on the metal surfaces, can cause severe under-scale corrosion. All of these factors prevent the cooling water system from operating safely over the long term, affecting production and resulting in economic losses. Therefore, the threat posed by microorganisms and scale corrosion to cooling water is of equal importance; among the three, controlling the threat from microorganisms should be a top priority. The microorganisms in cooling water include the following types: fungi, sulfate bacteria, reducing bacteria, autotrophic bacteria, heterotrophic bacteria, sulfur bacteria, iron bacteria, nitrifying bacteria, and algae. Algae are simple green plants that lack the differentiation of stems and leaves; they are also known as prothallus plants. The main difference between algae and fungi is that algae possess pigments in the form of chloroplasts, which enable them to carry out photosynthesis. Nutrients are produced by photosynthetic autotrophic organisms. In closed cooling water systems, the main types of algae that appear are cyanobacteria, green algae, and diatoms. In the cooling water tanks and in the illuminated parts of cooling towers, algae grow, reproduce, and then die; these dead algae end up as part of the sediment in the cooling system. Undermetal corrosion occurs due to the autocatalytic effect associated with the electrochemical corrosion process itself, while acid corrosion is caused by the depolarization of hydrogen (2H++2e→H2). The corrosion products are mainly soluble salts, and the hydrolysis of these salts further increases the acidity of the medium, thereby accelerating metal corrosion. The corrosion of metal surfaces by water is primarily electrochemical corrosion; in the corrosion cell, the cathodic reaction is mainly the reduction of oxygen, while the metal in the enclosed area beneath the scale acts as the anode, and the anodic reaction is the dissolution of iron. The corrosion reaction of carbon steel in water is as follows: Anodic reaction: Fe → Fe2+ + 2e-. Hydrolysis of ferrous ions: Fe2+ + H2O → Fe(OH)2 + H+. This further lowers the pH of the medium beneath the scale, accelerating corrosion. Cathodic reaction: O2 + 2H2O + 4e = 4OH–. Thus, corrosion beneath metal deposits occurs because the electrochemical corrosion of the metal itself has a self-catalytic effect that accelerates the corrosion of the metal. The electrochemical mechanisms of pitting and crevice corrosion are similar to this. The scale that forms on the heat transfer surfaces of heat exchange equipment typically has uneven density, thickness, and chemical composition. This uneven accumulation of scale leads to electrochemical inconsistencies on the metal surface, which easily facilitates chemical corrosion reactions. Additionally, the scale causes certain corrosive substances present in water, such as H+, OH-, Cl-, Mg2+, S2-, etc., to accumulate on the metal surface beneath the scale, thereby triggering chemical corrosion. As a result of this corrosion, the metal in those areas is damaged and thinned out; in severe cases, the corrosion can penetrate the steel plates of the equipment, leading to leaks. Existing research indicates that the formation of underscale corrosion requires certain conditions and a incubation period; the main conditions are the presence of aggressive anions and oxidants in the medium. The corrosion area formed by the blockage in the battery is very small, and it is difficult for the electrolyte within this corrosion area to convect and diffuse with the surrounding medium. Inside the blocked battery, as the anodic reactions proceed, the hydrolysis of metal ions leads to an increase in H+ activity, further acidifying the medium in the corrosion area and increasing the driving force for the corrosion reaction. Inclusions such as MnS in metals, surface defects, rolling oxide scales, and surface attachments (especially loose sulfides), stagnant media, Cl‑ in the media, and high temperatures will promote the formation and development of blocked cells; the presence of oxygen will increase the rate of pitting by 1 to 2 orders of magnitude. Gravity causes the holes at the bottom of the container to develop much faster than those on the vertical and downward surfaces, resulting in the bottom of the container perforating first. Sulfides in the medium form a non-compact sulfide film of a certain thickness on the steel surface, which helps to retain moisture in the blocked area and prevents the medium in that area from spreading outward. This accelerates the formation of blocked cells and the development of pitting; such blocked cells can typically cause perforation in steel plates 3 to 24 months after their formation. A smooth, clean surface helps prevent the formation of blockages in the battery; once such blockages occur, cleaning the surface can help slow down the process of corrosion. 3. Hazards: In the production processes of the petrochemical and thermal power industries, scaling in coolers is a common problem; scaling can reduce the heat transfer efficiency of the equipment and lower its production capacity ； It increases the resistance to medium flow, raising the energy consumption of the conveying equipment. Scale can also cause under-scale corrosion in equipment, reducing its service life ； In severe cases, it causes equipment blockages, affecting the smooth operation of the device and even leading to shutdowns. Currently, scaling in coolers is mainly addressed through periodic shutdown cleaning (chemical cleaning or high-pressure cleaning). This is a post-event solution that cannot address issues such as reduced efficiency and under-scale corrosion caused by scaling during the cooler’s operation. Impurities such as sludge carried along tend to deposit and form scale, leading to corrosion beneath the scale. The higher the mass concentration of oxygen in the solution, the greater the difference in mass concentration between the water phase and the scale phase, and consequently the larger the potential difference between the anode and cathode. This makes it easier for iron to react with oxygen and undergo corrosion. The thicker the scale layer, the more likely corrosion will occur beneath it, progressing deeper until perforation occurs.