Is corrosion caused by flowing water severe? Is still stagnant water (non-flowing) causing severe corrosion?

It’s likely that the lack of flow leads to severe pitting corrosion, which is uneven

It is recommended that you replace the fire hose. There are multiple leakage points, indicating that the pipeline is severely corroded

For carbon steel, chloride corrosion is not severe. This is mainly other forms of corrosion. As you asked in the title, still water, water that is not flowing, is more corrosive than flowing water ； Flowing water is more corrosive than water in a full pipe.

It cannot be simply attributed to chloride ion corrosion; it is also necessary to determine the acidity of the medium in which chloride ions are present. For example, neutral saline does not cause corrosion to carbon steel pipelines.

It cannot be simply said that flow rate is the only factor affecting corrosion; corrosion is the result of multiple factors interacting together. It is influenced by the properties of the medium, its composition, temperature, pressure, the material used, its state, flow rate, and so on. It is a systematic phenomenon, and under different operating conditions, one factor may play a dominant role. If flow rate is the main factor causing corrosion, then the faster the flow rate, the faster the corrosion occurs – but in such cases, the corrosion is of an erosive type； And if the medium is highly corrosive, coupled with the material having poor resistance to pitting corrosion, then a stationary medium will cause even greater corrosion of the material. This is just my personal opinion; please feel free to point out any mistakes: lol

Chloride ions have a depolarizing effect, that is, they can destroy the oxide layer on the metal surface; meanwhile, if water does not flow, concentration cell corrosion is likely to occur in corners and gaps. If it is filled with water, corrosion occurs more slowly; if there is only half water in the tube, corrosion accelerates because air is present in the areas without water, which facilitates oxygen-induced corrosion. An increase in oxygen concentration also leads to the same type of concentration cell corrosion. In addition to scouring, flowing water helps ensure uniformity of the water, reducing concentration cell corrosion. Moreover, since flow creates pressure in enclosed spaces, it drives out the oxygen from the water, thereby reducing the corrosiveness of the water. PS. In this case, chloride ions do not decrease as corrosion progresses, because due to their reactivity, they merely act as catalysts in the corrosion process. Therefore, the idea that corrosion is accelerated due to the continuous supply of chloride ions is somewhat misleading; of course, this assumption needs to be re-evaluated if the water itself contains low levels of chloride ions. For ordinary cast iron parts, using the ZS-1034 acid-alkali resistant anti-corrosion coating or the ZS-711 inorganic anti-corrosion coating can provide excellent protection.

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The wastewater excluded from the chemical treatment process is not only a problem related to chloride ion levels; there are usually also some hidden corrosive ions present. Exactly this portion of water was used for the fire suppression system. We know that fire-fighting water is often not in use, and under such conditions the factors in the water that cause metal corrosion lead to further acidification, creating an environment conducive to metal corrosion. The presence of chloride ions here accelerates the corrosion of the metal. Moreover, water that is in constant flow has a higher pH value than still water, and causes less corrosion. This means that with the same corrosive medium, still water is more corrosive than flowing water; metal corrosion yields different results in different environments. Therefore, corrosion occurs in the pipes, and significant corrosion takes place in the stagnant areas of the water tanks. If this water is to be used, it serves to prevent corrosion of the metal surface.