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Materials with an excessively high corrosion rate are generally not chosen; for those with a very low corrosion rate, no margin needs to be considered, while for materials with a moderate corrosion rate, a corrosion margin must be taken into account. It is not necessary to thicken the entire device; thickness can be increased in areas where corrosion is more severe, such as the liquid level area of the container. When gas-phase corrosion is severe, the wall thickness of the gas-phase portion increases. It can also be protected with paint. Increasing the wall thickness is feasible for some components, but difficult for others. For heat exchangers, it is feasible to thicken the shell and baffles by five or six millimeters, but thickening the heat exchanger tubes requires careful consideration. Because thickening the tube wall increases the difficulty of heat transfer and expands the space occupied by the heat exchanger. If the results of the corrosion test show significant pitting on the test pieces, but this material still has to be used, then its thickness should not be too small (it should be at least four or five millimeters). When corrosion is severe and a margin cannot be used, some effective protective measures can be taken. However, it should be noted that if the protective measures are not appropriate, they will not achieve the desired results; sometimes, they may even yield the opposite effect. For example, coating is one of the most common methods. It is very important to remove rust from the metal surface during construction; if the rust is not thoroughly removed, the coating will not adhere properly and is likely to peel off. Furthermore, designers should replace all devices with sharp corners with those with rounded corners. Because acute angles are difficult to coat, and thin layers are prone to damage. During installation, transportation, and maintenance, care must also be taken to avoid mechanical damage. The next example is failing to follow the construction and installation procedures, which is even worse than not using paint. The heat exchange tubes in the carbonization towers of alkali plants are exposed to a mixture of brine, ammonia, CO2, etc., at temperatures of 60–70°C; this environment is highly corrosive. Cast iron is generally used, and it can withstand such conditions for 3–6 years. Later, it was protected with an epoxy-phenolic coating, but perforations occurred within a year. It turns out that cast iron relies on a thick casting layer to extend its lifespan; when the coating is applied, this layer is removed. During installation, the heat exchange tubes use tight-fitting joints, which causes mechanical damage to the coating, exposing the underlying metal to the corrosive environment and thus accelerating corrosion. Thin coatings and platings always have micropores, and are generally not suitable for severe corrosive environments such as hydrochloric acid or sulfuric acid. If the coating is too thick, it is prone to peeling off. The mortar joints of the lining bricks, especially silicone mortar, are also porous; therefore, an impermeable and corrosion-resistant intermediate layer needs to be added between the steel wall and the bricks. Lining equipment should not be used in environments with rapid temperature changes, as the large difference in expansion coefficients between the lining material and the metal casing can easily cause them to separate, leading to increased cracks. Cathodic protection, such as over-protection, can cause amphoteric metals to corrode, while anodic protection, in cases of excessive passivation or failure to reach the passivation potential, can also accelerate corrosion. Corrosion inhibitors, especially anodic corrosion inhibitors, can cause severe pitting if used in insufficient amounts or in an inappropriate manner.