Study On Localized Corrosion Characteristics And Mechanisms For Typical Petrochemical Equipments

Corrosion in petrochemical equipments of manufacturing process is mostly dependent of some environment factors, such as machining raw materials properties, production of manufacturing process, temperature, pressure, machining and position of equipment. The different corrosion modes are caused by different corrosive environment, microbiological induced corrosion (MIC), stress corrosion cracking (SCC) and pitting corrosion are three primary corrosion modes. MIC in heat exchange equipments is very severe, corrosion of seventy percent of heat-exchange tube bundles is induced by microorganisms, sulfate-reducing bacteria (SRB) and iron-oxidizing bacteria (IOB) are the dominating reasons which cause MIC of cooling circuit water system in oil refinery. Stress corrosion cracking of steels in wet hydrogen sulfide is the principal corrosion failure in the refining process of high sulfur crude oil. SSCC is the most serious corrosion mode in acidic environment. At present, corrosion perforation of stainless steel induced by maleic acid medium has become the most important question which needed to resolve in the producing equipment of maleic acid anhydride. Thus, it is very significative in the aspect of theory and engineering application to study MIC in cooling water system, SSCC of welded joint and corrosion of maleic acid medium. Five innovative research results are achieved:1. The stress corrosion cracking susceptibility of welded joints of 20 carbon steel in wet hydrogen sulfide environment was investigated by constant load technique. The results show welded metal and base metal indicated lower SSCC susceptibility, heat affected zone demonstrated higher SSCC susceptibility. The critical stress (σth) of welded specimen of 20 carbon steel was determined. The crack mechanism of welded specimens in wet hydrogen sulfide medium is the mixed crack mechanism of sulfide stress corrosion crack and stress-oriented hydrogen-induced crack, fracture mode is transgranular quasi-cleavage crack with hydrogen brittlement. The SSCC process for welded samples is composed of three periods: nucleation, steady propagation, and terminal propagation periods. It is proposed first that the steady-state elongation rate (iss/m·s-1), obtained from the corrosion-elongation curve is an important parameter for the prediction of time-to-failure (tf) and as a criterion for the assessment of their SSCC susceptibility.2. Localized corrosion characteristic and mechanism of 316L stainless steel was investigated in the presence of anaerobic SRB and aerobic IOB isolated from cooling water system of an oil refinery plant using bacterial analyses, electrochemical measurement and surface analysis. The formed biofilm mechanisms were opened out in four bacterial solutions, they changed the electrochemical properties of stainless steel surface, induced the reduction of corrosion resistance. The presence of SRB demonstrated higher corrosion rates than IOB. The combination of SRB and IOB yielded more higher corrosion rates than SRB or IOB alone. The synergy of 0.01M NaCl+SRB+IOB yielded the highest corrosion rate. 3. The main reason of stainless steel corrosion induced by SRB is put forward according to corrosion behavior of SRB, which is acidification, corrosion products film and biofilms arosed by SRB metabolic activity on the stainless steel surface. Through the analysis of IOB corrosion characteristic, the main reason of stainless steel corrosion induced by IOB is put forward that is oxygen concentration cell formed by IOB metabolic activity on the stainless steel surface, the corrosion mechanism is the mixed mechanism of pitting corrosion and crevice corrosion. These results can provide not only credible theoretic foundation for MIC of SRB and IOB in cooling water system, but also possess engineering direction function in the control of MIC.4. Corrosion behavior of 316L SS in the synergy of SRB, IOB and Cl- anion was researched first, the synergic corrosion mechanism is put forward, which shows microorganisms (SRB and IOB) attach to surfaces and interact in complex ways to form mixed biofilm, biofilm can promote H2S induced by SRB cumulation near metal surface, then form high undissolved FeS to accelerate the anode reaction. Moreover, IOB can oxidize ferrous cations produced from SRB to the deposit of ferric hydroxide, which increases the oxygen concentration grads and allows the transport of aggressive anions (chloride) to the SS surface, but prevents the transport of metal ions away from the surface, thereby assisting the development of the concentrated local chemistry required for pit propagation. These research results possess not only innovative meaning in theory, but also provide theoretic basis and engineering direction for understanding MIC questions in cooling water system.5. The corrosion behavior of 316L stainless steel in the maleic acid anhydride equipment is investigated first using weight loss technique, electrochemical measurement and surface anlysis technique. The results show corrosion tendency of 316L decreased with the increase of concentration of maleic acid and immersion time in maleic acid medium, and corrosion rate of stainless steel increased with the increase of concentration of chloride anion and immersion time in the combination solutions of maleic acid, H2SO4 and NaCl. Maleic acid mitigated corrosion course of 316L SS to a certain extent was put forward. The synergy of Cl- and SO4- anion accelerated the corrosion course of stainless steel, induced enlargement and deep of corrosion pits, leaded to the mixed-mode corrosion of pitting and general corrosion on stainless steel surface. When H2SO4 or NaCl alone was entered into maleic acid medium, the corrosion can't be accelerated, but H2SO4 and NaCl together were entered into, corrosion rate increased rapidly. Thus it can be found that the synergy between maleic acid, H2SO4 and NaCl accelerated corrosion course of 316L stainless steel. These research results provide theoretic basis and engineering direction for settling corrosion questions in maleic acid anhydride equipment, possess important reference worth to the development of corrosion theory about other metal materials in maleic acid solution.