The corrosion types and corrosion degree of petroleum refining equipment have a great relationship with the material and content of crude oil. As the market continues to increase demand for oil and many oil fields enter the middle and late stage. The quality of crude oil is deteriorating, the amount of crude oil, sulfur content, salt content, heavy metal content and acid value are all rising, these adverse factors have caused the corrosion of the equipment to become more serious. Therefore, the economic and effective protection measures have become an important and urgent task for the refineries.
In recent years, the sulfide corrosion of many refinery equipment in China has deteriorated significantly. This is mainly due to the high sulfur content of crude oil extracted from many oil fields in our country and the high sulfur content of imported crude oil. The sulfide corrosion of the chemical equipment is mainly caused by chemical corrosion, hydrogen drum, sulfide stress corrosion cracking and high temperature sulfur corrosion. Also in the process of crude oil processing, due to high temperature, high pressure and catalyst, some sulfide starts to break down at 120 ℃. Some non-reactive sulfur is constantly changing to reactive sulfur, which also results in the occurrence of sulfur corrosion.
a. The corrosion and protection of H2S-HCI-H2O type
In equipment near normal temperature, such as: in the initial distillation tower of the constant pressure relief device, the condensing cooling system of the normal decompression tower and the top of the tower, the hydrolysis of the crude oil produced HCI, as the light components and moisture in crude oil evaporate and condensate together, when the pH value of the acid is 2-3, the strong corrosion of the equipment will be caused, and its role belongs to the category of electrochemical corrosion. The corrosion form is the general reduction of carbon steel. Corrosion resistance to stainless steel like Cr13; The stress corrosion cracking of 1cr18ni9ti.
H2S and HCI are ionized to reduce the concentration and turn into protective film. However, HCI and the reaction damage the protective film, in this way, the corrosion is aggravated by the mutual promotion of HCI in the corrosive environment. In addition, because of its existence, it also accelerates the corrosion damage caused by hydrogen. For this type of corrosion, most of the protective measures taken by the refinery are "one of four injection" (desalting, alkali injection, injection of ammonia, corrosion inhibitor and water injection), and reasonable selection of corrosion resistant materials, etc.
b. The corrosion and protection of H2S-HCN-H2O type
There is also nitrogen in the crude oil. In the pyrolysis temperature, not only complex sulfide is decomposed into H2S, but also the sulfur element and hydrocarbon reactions generate H2S, which results in the occurrence of this type of corrosion. The main parts are liquefied petroleum gas canisters, hydrogenation devices, desulfurization devices, catalytic condensing cooling systems and absorbing desorption systems. The corrosion morphology shows that carbon steel is uniformly reduced, hydrogen bubbling and sulfide stress corrosion cracking, and austenitic stainless steel is the stress corrosion cracking.
In H2S-HCN-H2O system, HCN is a hydrogenation promoter, which can dissolve the protective film FeS to produce white water-soluble colloidal materials. Fe2[Fe(CN)6] can be oxidized into Prussian orchid Fe2[Fe(CN)6]3 at the time of shutdown, and it will accelerate the corrosion of the equipment. The method of reducing the corrosion of this kind of media has the method of washing, removing HCN and H2S, improving the alkalinity of the medium and controlling the impurities of the material. The addition of Cu and AI in steel blocks the diffusion of hydrogen to steel and the corrosion inhibitor.
c. The corrosion and protection of polysulfuric acid (H2SxO6)
The FeS generated in the high temperature environment are exposed to moist air when the equipment is turned on, and the sulfuric acid is formed to cause the sulfide stress corrosion cracking SSCC.
First, the sulfuric acid causes intergranular corrosion and then cracks under the joint action of stress concentration and polysulfuric acid. The stress corrosion of intergranular stress originates from the thermal influence zone of welding, the method to avoid corrosion of such medium is to prevent air contact or nitrogen protection when the device is parked. In general, the corrosion occurs only within a pH of less than 5, therefore, the alkaline solution can be properly cleaned, but sodium ions must be removed, and sodium can destroy the hydrogenation catalyst. In addition, corrosion resistant materials and improved welding technology can be used.