How to consider corrosion allowance and anti-corrosion measures

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Materials with excessively high corrosion rates generally are not selected. For those with very low corrosion rates, no allowance needs to be considered; for those with moderate corrosion rates, a corrosion allowance must be taken into account. It’s not necessary to thicken the entire equipment; thickening can be considered for areas with severe corrosion. For instance, the liquid level zone of a vessel needs to be thickened. When gas-phase corrosion is severe, the wall thickness in the gas-phase section is increased. 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 tube sheets by five to six millimeters; however, thickening the heat exchanger tubes requires careful consideration. Because thickening the tube walls increases the difficulty of heat transfer and enlarges the space occupied by the heat exchanger. In the results of corrosion tests, if significant pitting is observed on the test specimens but this material still has to be selected, then its thickness should not be too small (it should be 4–5 mm or more). When corrosion is relatively severe and it is not possible to apply a margin, 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 have 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 cleaned, the coating will not adhere properly and is likely to peel off. Additionally, designers should change all devices with sharp corners to rounded corners. Because acute angles are difficult to coat, and thin layers are prone to damage. During installation, handling, and maintenance, care must also be taken to avoid mechanical damage. The following example illustrates a failure to follow construction and installation procedures; this is worse than not using any paint at all. 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 an corrosive environment and thus accelerating corrosion. Thin coatings and platings always have micropores; they generally should not be used in 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. Lined brick equipment is not suitable for environments with drastic temperature changes, as the coefficients of thermal expansion of the lining material and the shell metal differ significantly; this can easily lead to separation and an increase in 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 inhibitors, if used in insufficient amounts or improperly, can cause severe pitting corrosion.