Corrosion Failure Analysis Of Water Coolers' On Vacuum Tower Overhead And Corrosion Rule Of Materials

Corrosion of atmospheric and vacuum units is widely considered to be one of the most serious problems in the refineries since it affects the long period operation, security, production quality and economic benefits etc. Therefore, it is necessary to study the corrosion mechanism and solve the corrosion problem for safe and efficient prosuction. Main work in this paper is as follows:(1) In order to find the reason for water cooler's corrosion on the overhead of vacuum tower, the morphology, composition and hardness of heat exchanger tube and support beam were characterized by the microscope and scanning electron microscope (SEM), electron energy spectrum and Rockwell hardness. The results showed that corrosion failure of heat transfer tubes is pitting, while the main failure reasons of support beam are pitting and cracking. It is found that the material composition of the heat exchanger tubes corresponds with the 304 stainless steel, which only appears pitting in the HC1-H2S-H2O acidic environment. However, the Ni, Cr element contents of support beam are lower than minimum content standards and its hardness is 32 HRC, exceeding the equipment design requirements of 22 HRC. The metallurgical features show that the cracks distribution of support beam is similar to "step" and develop along the grain. In addition, the corrosion product of support beam contains S and Cl elements. For the complex crude oil refinery and unstable desalination, the content of HC1, H2S increase and form a HC1-H2S-H2O acidic environment with condensate water, resulting in a "stepped" cracking, namely, a typical characteristic of hydrogen embrittlement failures. The higher hardness of support beam due to bad solid solution treatment is likely to cause hydrogen induced cracking in the HC1-H2S-H2O environment.(2) The effects of chloride ion concentration and sulfide concentration on the 2205 duplex stainless steel were taken into consideration to make out the corrosion law. The results show that chloride ion concentration was negatively correlated with pitting temperature, whereas pH was associated positively with pitting temperature. Sulfide was proved to change the growth process of passive film making the corrosion easier happen by the EIS test. The higher the sulfide concentration, the more serious the damage. Besides, the effect of pH and temperature prove that acidic condition and high temperature promote the corrosion, respectively. Hence, the conclusion has significance on reducing the risk of equipment corrosion by controlling chloride concentration and pH.Corrosion-related research in this paper has some guiding meaning and application value to improve the reliability of atmospheric and vacuum systems, extending the safe operation and ensuring the production safety and economic benefits.