Atmospheric and vacuum corrosion protection

Generally, the corrosion mechanisms at the top of vacuum distillation columns are quite similar to those of atmospheric pressure columns; the corrosion mechanisms and anti-corrosion measures remain the same. 1. Corrosion mechanism: In the top of the vacuum tower as well, the main corrosive agents in low-temperature areas are the HCl-H2S-H2O system (T≦120°C). During the processing of crude oil, H2S generated from various sulfides and HCl produced by the hydrolysis of salts volatilize along with the light components and moisture in the crude oil; they then accumulate in the low-temperature areas of the light oil zone at the top of the tower, especially at the places where the gas-liquid phase transition occurs. When water vapor condenses, H2S and HCl dissolve in the condensed water; once their relative concentration reaches around 100 ppm, the pH value drops to 2–3, creating a severe electrochemical corrosion environment. The resulting hydrochloric acid can not only react directly with iron, but also destroy the ferrous sulfide corrosion product film that forms as a result of the reaction between H2S and iron; this film provides some protective effect, and its destruction leads to pitting and underscale corrosion. Overall, corrosion in the HCl-H2S-H2O system is relatively severe; generally, corrosion is mild in the gas phase while it is more severe in the liquid phase, with the most severe conditions occurring at the gas-liquid transition zone, namely the so-called \"dew point\" area. Carbon steel primarily suffers from uniform corrosion and pitting, producing large amounts of corrosion product FeS, while austenitic stainless steels experience pitting and stress corrosion cracking (SCC). 2. Corrosion prevention measures: To address the low-temperature corrosion in the overhead system of vacuum distillation towers, common preventive measures include the \"one removal and two injections\" process (ammonia injection and alkaline water injection), material upgrades, anti-corrosion coating applications, and Ni-P electroless plating. These measures have yielded excellent results, effectively controlling the corrosion in the overhead systems of distillation units.