As thechemical or electrochemical effects of atmosphereand water, petrochemical equipment and pipelines often appear “run, emit, drop, drain” and other corrosion appearances.
Equipment and pipeline corrosion will cause a large number of raw materials and finished products loss, affecting product quality, environmental pollution, and even cause equipment explosion, fire and other major accidents. It’s very important to take good anti-corrosion measures.
Reasons for corrosion of petrochemical equipment
70% of metal structures in the petrochemical production are exposed to the atmosphere, the plant contains a large amount of atmospheric SO2, SO3, CO2 and a large number of nitrogen oxides.
There will produce a large number of salt and acid and alkaline sewage in the production process of petrochemical.
Petroleum, intermediates and products are corrosive.
Anti – corrosion measures
Reasonable selection of materials
The selection of petrochemical equipment plays a decisive role in the corrosion resistance of the equipment.
Selection of materials must first consider the corrosion resistance of the material itself, in addition, we must consider the nature of the media, temperature, and pressure, equipment type and structure, and product requirements.
Several petrochemical equipment commonly used materials
Carbon steel is the most commonly used structural material, which may account for 80% of the total steel used in the refinery.
Distillation columns, separators, heat exchangers, tanks, most pipelines, all structural parts are generally made of carbon steel.
Low alloy steel is widely used to make flange bolts, valve parts, pumps and shaft or rod of compressors.
Cr-Mo Cr-Mo steel
Besides stainless steel, Cr-Mo steel is the only one steel which is able to withstand high temperature of 648 ℃.
Cr-Mo steel with a chromium content of less than 4% is more resistant to corrosion than carbon steel.
These materials are suitable for applications requiring high temperature strength, creep resistance and high temperature and high pressure hydrogen erosion. Steel with a molybdenum content of 1/2% or more can achieve the highest creep strength.
In refineries cryogenic processes, for example, 2-1 / 4 Ni and 3-1 / 2 Ni steels have been used in propane refrigeration systems.
The use of nickel steel in refineries is generally limited to process conditions where the operating temperature is below – 50 degrees.
Martensitic stainless steel
Using temperatures below 371 ° C (700 ° F) for typical applications in refineries, includes pump parts, fasteners, spools, turbine blades, and tray valves and other tray components in the distillation column.
In the desulfurization device, 410 stainless steel is commonly used in lining protection tower, heat exchanger and other pressure vessels to prevent the occurrence of high temperature sulfide corrosion.
Austenitic stainless steel
316 and 317 stainless steel are two commonly used austenitic stainless steel gradeswith excellent resistance to pitting and acid corrosion.
These steels are commonly used in the processing of naphthenic crude oil refineries to prevent the occurrence of naphthenic acid corrosion.
Improve the corrosive environment
Using of electric desalination, neutralizerinjection, inhibitorinjection, water injection and other processes can effectively improve the corrosion environment of refinery equipment.
Crude oil desalination
The crude oil desalination plant is the first process of petroleum processing, which removes salt, water and other impurities from the crude oil.
Electrode desalination is not only as an anti-corrosion means, but also as an effective means of removing alkali metals and heavy metals from crude oil in order to ensure the performance of the catalyst used for subsequent processing.
Demulsifier is a high molecular weight nonionic surfactant, whose role is to destroy the oil and water emulsion of electric desalination tank, to promote oil and water separation.
Oil-soluble demulsifier: good demulsification effect,less dosage, expensive.
In addition, the oil-soluble demulsifier exists in the oilafter the electric desalination device , can reduce the difficulty of sewage treatment.
Water-soluble demulsifier: general effect in breaking the cream, large dosage, cheap.
The water-soluble demulsifier is dissolved in water after being desalted by electricity, and enters the sewage treatment system with the discharged water, which increases the difficulty of the sewage treatment system.
Injection of decalcifying agent in the process of electric desalination can remove some of the organic calcium in crude oil to reduce the impact of calcium on the subsequent catalytic cracking unit catalyst.
The mechanism of decalcifying agent is mainly the application of chelating precipitation reaction.
Neutralize the corrosive acid of the tower top,improve the PH value of condensate, slow down the corrosion of equipment.
Amine and ammonia are mainly used in the refinery tower top system, ammonia is cheap, easy to get, low cost; organic amine is good performance in neutralizationbut expensive.
At present there are three options in the refinery:
Full organic amine neutralizer: After the full organic amine neutralizer is injected into the top of the column, it can rapidly enter the initial condensate and neutralize the condense HCl, effectively slow down the corrosion of the initial condensate area.
At the same time the use of full organic amine neutralizer can avoid under scale corrosion caused by ammonia salt scale.
Ammonia and neutralizing corrosion inhibitor: Full organic amine neutralizer is expensive, often adopts ammonia and neutralizing corrosion inhibitor.
As the two in one neutralizing corrosion inhibitor contains a certain amount of organic amine neutralizer, can use with ammonia, on the one hand it can reduce costs, on the other hand it can play a part of the role of organic amines.
Ammonia and corrosion inhibitor: Ammonia is poor in neutralization ability, needs over-injection, ammonia salt scale can easily lead to under scale corrosion. The program is currently less used in refineries.
Inject corrosion inhibitor
Film-forming corrosion inhibitor can be adsorbed on the metal surface, forming a layer of hydrophobic protective film, cutting off the contact path of corrosive medium and metal, so as to achieve the purpose of reducing corrosion.
The main components of commonly used film-forming corrosion inhibitor include alkyl pyridine quaternary ammonium salt, alkyl amide, alkyl imidazoline quaternary ammonium salt, film-forming agent and additives.
In water-soluble corrosion inhibitor and oil-soluble corrosion inhibitor, water-soluble corrosion inhibitor is cheap, large amount of injection; oil-soluble corrosion inhibitor is expensive, small injection.
At present there are three options in the refinery:
Oil-soluble corrosion inhibitor: oil-soluble corrosion inhibitor is injected into the top of the evaporation line, some parts can be refluxed into the tower, to expand the scope of corrosion protection.
Neutralizing corrosion inhibitor (two in one): the organic amine neutralizer and water-soluble corrosion inhibitor, and play a dual role in neutralization and corrosion inhibition, need to use with ammonia.
The program is currently heavily used in refineries.
Water-soluble corrosion inhibitor: Water-soluble corrosion inhibitor needs to be used with ammonia.
The program is currently less used in refineries.
Water injection at the top of the tower is an often used means in refineries anti-corrosion.
At the top of primary towerin atmospheric distillation tower, the top of the atmospheric tower and the top of the vacuum tower, the top of the fractionation tower for catalytic cracking, before the fractionation tower for catalytic cracking,after the air compressor, water injection process is generally used before the hydrocracking of the high pressure air.
The position of the initial condensation zone is controlled and adjusted by water injection.
Water injection can inhibit ammonia fouling, to avoid the occurrence of under scalecorrosion.
The PH of the initial condensation zone is increased by injecting water to dilute the acid in the initial condensation zone.
The main purpose of water injection at the catalytic cracking of the fractionation tower top, before the catalytic cracking of the fractionation tower, before the hydrocracking of the high pressure air cooling, is to dissolve the ammonia salt, to eliminate scaling, to avoid under scale corrosion.
Metal surface is covered with a layer of dense protective layer which can be closely integrated with the metal , and isstablein the medium, then the corrosion will make no damage to the metal.
The protective layer is divided into two types: metal and nonmetal.
Metal protective layer
The commonly seen metal protective layers are chrome, nickel, galvanized and lining stainless steel lining and so on.
Electroplating: The metal is precipitated from the electrolyte by means of direct current and deposited on the surface of the workpiece to obtain a metal protective layer.
Common electroplating materials are: zinc, nickel, chromium, cadmium, copper and tin alloy.
Spraying: Molten metal is atomized into particles by high speed airflow and spray onto the surface of the workpiece to form a metal protective layer.
Common metal spray materials are zinc, aluminum and stainless steel.
Hot dip plating: The protected steel workpiece is immersed in a tank containing molten metal or passed through the molten metal tank at a constant speed so that the surface of the workpiece is coated with a metal protective layer.
The metal materials used for hot dip plating are aluminum, zinc, tin and alloys thereof.
Surface metallization: the use of infiltration, diffusion and other processes can get the alloy surfaceof the protective workpiece surface.
Metal lining: the metals with high corrosion resistance, such as lead, titanium, aluminum and other lining are lined in the equipment to prevent corrosion.
Lacquer: with the virtues of acid resistance, solvent resistance, water resistance, oil resistance, wear resistance and strong adhesion and so on.
Disadvantages are intolerant alkali and strong oxidants, toxic.
Using temperature is 150 ° C.
Phenolic resin paint: change the shortcomings of raw paint which are big toxicity, slow drying, poor construction.
But it’s intolerant to ultraviolet light, and can not be set for a long time.
Phenolic resin paint: a good electrical insulation and oil resistance, can bear medium corrosion of 60% sulfuric acid, hydrochloric acid, a certain concentration of acetic acid and phosphoric acid, most salts and organic solvents.
Epoxy-phenolic paint: has good mechanical properties, alkali resistance, acid resistance, solvent resistance and electrical insulation.
Epoxy coating: has good corrosion resistance, especially alkali resistance, and has good wear resistance, the film has good flexibility and hardness, low shrinkage.
The using temperature is generally 90 ~ 100 ℃.
Perchlorethylene paint: has a good resistance to industrial atmosphere, resistant to seawater, acid, oil, salt spray, mildew, and anti-burning performance.
But not resistant to phenols, ketones, lipids and benzene and other organic solvent medium corrosion.
The maximum operating temperature is about 70 ℃.
Furan resin paint: with excellent acid, alkali, temperature resistance, a wide range of raw materials sources, low prices.
Inorganic zinc-rich paint is simple onstruction, cheap, with good water resistance, oil resistance, solvent resistance and resistance to dry and wet alternately of salt spray.
Epoxy coal asphalt: high mechanical strength, high adhesion, chemical resistance and corrosion resistance of coal tar pitch and so on.
Using temperature of coating is between -40 ~ 150 ℃. There will be no changes after long-term immersionin the acid, alkali, salt, water, gasoline, kerosene, diesel and other general thinner .
The substance of plus current cathodic protection is that the entire metal device is polarized cathodeby the plus currentand the auxiliary electrode is the anode.
Cathodic protection way in recent years in China has begun to apply to petrochemical production, which are mainly used to protect the cooling equipment and a variety of pipelines in seawater, river corrosion,such as halide crystallization tank, salt evaporation equipment.
Anode protection is to connectthe protected equipment to the anode DC power supply,to generate passivation film on the metal surface, which plays a protective role.
Anode protection can be only applied when the metal passivates in the media, and the technology is complex, not much use.
The heat exchanger uses anodic protection which are mostly used in process material side.