Mechanisms and differences between dew point corrosion, pitting corrosion, and crevice corrosion

Thread Content

This post was last edited by Wang Genrong on 2024-7-27 08:34. Dew point corrosion, pitting corrosion, and crevice corrosion: Dew point corrosion – In general terms, dew point corrosion occurs when the process gas reaches its phase transition point as it cools down, resulting in the formation of liquid droplets, i.e., dew. Conversely, the temperature at which a liquid rises to the point of vaporization – boiling – is called the boiling point. The temperatures of the two are the same, but the temperature level differs by the latent heat of vaporization. At this time, if there are some acidic substances in the medium (such as SOx, HCl, NOx, etc.), they will accumulate in the condensed water to form acids; the corrosion resulting from this is known as dew point corrosion. For example, the most common form of dew point corrosion occurs in the flue gases emitted by boilers; the main acidic substances are sulfides – sulfuric acid and sulfurous acid (with small amounts of chlorides and nitrogen oxides present as well). Their concentration can reach 85%, causing severe corrosion of metals, especially stainless steel (and sometimes even stress corrosion). Pitting (punctate corrosion) and crevice corrosion: Pitting, also known as small-hole corrosion, is a severe form of localized corrosion. After corrosion pits form on the metal surface, they spread deeper at a rate that is greater than or equal to their rate of spread sideways. As a result of corrosion, pits or small holes are formed in the metal, while most of the metal remains uncorroded or only suffers slight corrosion. This form of corrosion is known as pitting or hole corrosion. The mechanism of pitting (corrosion at points) and the mechanism of crevice corrosion are essentially the same, but there are differences between the two. Pitting does not require the presence of an actual gap; it can generate pits spontaneously. Generally, in certain media, metals prone to pitting are also susceptible to crevice corrosion; however, systems prone to crevice corrosion (including both the metal and the medium) do not necessarily suffer from pitting. Factors affecting it: (1) Composition of the solution: Most pitting is caused by Cl-. (2) Flow velocity of the medium: Since stagnant liquid is a prerequisite for pitting corrosion, in media with flow or when the flow velocity is increased, pitting corrosion is often mitigated. (3) Factors related to the metal itself: Metal alloys with self-passivation properties are more susceptible to pitting corrosion.