Stainless steel is not a Wulin gold bell cover iron shirt, protection also needs high-level pointing

Stainless steel contains alloy elements, can oxidize to form a passivation layer, and the passivation layer is dynamic, for which stainless steel is relatively resistant to corrosion. At the same time, stainless steel also has high strength, toughness, good processing performance characteristics, in the petrochemical industry has been widely used, such as pressure vessels, pipes, equipment. Due to the diversity of conditions in the petrochemical industry, the corrosion of stainless steel is also complex and diverse
the
. Due to long-term harsh environment, stainless steel will inevitably corrode. The corrosion of stainless steel is divided into comprehensive corrosion and local corrosion. Comprehensive corrosion of stainless steel can be predicted by data. Moreover, total corrosion is a gradual process that can be prevented in advance. However, the local corrosion process is unpredictable and relatively harmful. In order to ensure the safe and normal operation of stainless steel equipment and components, effective anti-corrosion measures must be taken. This paper focuses on the analysis of the causes of local corrosion of stainless steel, and puts forward the protective scheme and surface treatment method of coating.
Stainless steel common corrosion situation and analysis of
points of corrosion
in the stainless steel surface in a very small part of the corrosion, will extend deep, the rest of the site does not corrode or very slight corrosion, this is point corrosion, referred to as pitting. The corrosion of common chlorine ions against stainless steel is a pitting. Chloride ions have a small radius and are easy to adhere to on stainless steel surfaces. Chlorine ions can destroy the passivation film on the stainless steel surface, causing some corrosion of stainless steel. Second, with the concentration of chlorine ions in the pitting pit, causing the anode dissolution to accelerate, corrosion to the depth of stainless steel expansion, pitting intensified. This is why stainless steel components, equipment and tanks corrode in the marine environment.
stress corrosion cracking
stress corrosion cracking (SCC) is caused by the joint action of pull stress and corrosion media cracking. Stainless steel is processed forging, heat treatment, and residual stress during assembly. At the same time, stainless steel in a specific corrosion environment has been corroded. Then, under the joint action of stress and corrosion, cracks are generated inside stainless steel. Cracks can be extended by intercrystalline, intercrystalline, or hybrid. The crack eventually exceeds the stainless steel tolerance limit, causing the stainless steel to crack.
according to the concept of stress corrosion cracking, in order to avoid stress corrosion cracking of stainless steel, in addition to corrosion treatment, but also to minimize the impact of stress. Such as reducing design stress, reasonable design, reduce local stress concentration, reduce the sensitivity of stainless steel to SCC. At the same time, stainless steel alloy elements will also affect stress corrosion cracking, such as in chloride ion media, nitrogen, phosphorus will accelerate stress corrosion cracking, and silicon, nickel will improve the ability to withstand stress corrosion cracking. Therefore, the way to avoid stainless steel stress corrosion cracking is multi-ion, to combine the actual conditions and needs of reasonable choice.
between crystals
stainless steel is welded, intercrystalline corrosion often occurs at welds, causing damage. The widely accepted explanation for intercrystalline corrosion is the poor chromium theory, in which the stainless steel of austryus is heated for a long time at 450 to 850 degrees C, and the adjacent area becomes the intercrystalline corrosion sensitivity zone. In this area, chromium and carbon chemical reaction, from the solid dissolved body precipitation out, the chromium content in this area is decreasing, forming a poor intercrystalline chromium region. Corrosion continues to develop between metal or alloy grains, reducing the strength of stainless steel. It is usually difficult to find from the outer surface, and stainless steel breaks when subjected to external forces. It is not difficult to see from the principle analysis of intercrystalline corrosion that the higher the carbon content in stainless steel, the more likely intercrystalline corrosion is to occur. Moreover, the greater the chance of intercrystalline corrosion in the sensitivity temperature range of 450 to 850 degrees C.
gap corrosion
gap corrosion is also common in stainless steel in the petrochemical industry, such as the firand connection of stainless steel pipe parts, bolt threaded connection, stainless steel tank bottom plate outside the edge and base contact surface and so on. The cause of gap corrosion is thick corrosion, i.e. the concentration of metal ions or oxygen in and outside the gap. The development of corrosion is due to self-catalytic corrosion of plug-in batteries. For example, stainless steel soaked in seawater, with high concentrations of oxygen outside the crevices, is considered cathode. The continuous consumption of oxygen in the gap causes the oxygen concentration to be low and is regarded as anode. With the accumulation of metal ions formed by corrosion in the gap, the chlorine ions outside the gap are naturally attracted to the gap, and the yin and yang ions accumulate in the gap, forming a closed battery, and the corrosion of the gap accelerates.
stainless steel coating anti-corrosion measures
coating choice
stainless steel surface is smooth, with passivation layer, for this reason, stainless steel surface coating choice is different from carbon steel and low alloy steel. Generally speaking, the choice of stainless steel surface coating is mainly considered is the choice of primer, primer as the bottom layer of the multi-layer system, direct contact with the substrate, in addition to protecting the substrate from corrosion, primer or as a transition layer for subsequent coatings. Once the primer's adhesion is not good, it directly causes the coating to fall off as a whole. For special metals such as stainless steel, the choice of primers is particularly important.
(1) Epoxy paint is recommended for stainless steel surfaces after blasting, except for metallic epoxy paints, such as epoxy-rich zinc paint. Because epoxy paint is relatively high bonding strength, corrosion and applicable performance is good, but also more commonly used anti-corrosion primer. Epoxy paint combines well with blasted stainless steel, usually pulling more than 5MPa. However, epoxy zinc-rich paint is mainly the use of zinc high activity, in contact with metals, play a cathode protection role. The passivation film formed by the self-passivation of the stainless steel surface will reduce the cathode protection of the zinc-rich primer of epoxy. For this reason, epoxy zinc-rich paint cannot be used on stainless steel surfaces.
(2) for stainless steel with low surface roughness (smoothness), usually with zinc epoxy phosphate paint as primer. Zinc phosphate added to zinc epoxy phosphate primer is positive zinc phosphate Zn3 (PO4) 2,nH2O, (n=2,4). The Zn2 plus po43 plus of zinc positive phosphate react slowly with the substrate to produce a phosphate film of Me (metal)-Zn-P2O5, which is dense and adheres firmly to the substrate, ensuring a good binding force between zinc epoxy phosphate primer and stainless steel. At the same time, zinc positive phosphate also reacts with hydroxyl and carboxyl in the substrate and certain ions in the corrosion environment to form a complex, which reacts with the corrosion product and forms a tightly attached protective film on the substrate surface. These properties of zinc positive phosphate not only provide anti-corrosion properties, but also form a close combination with the substrate, for this reason, for the work parts surface treatment level requirements are not high, the use of zinc epoxy phosphate primer. About zinc epoxy phosphate primer in the principle of the utility of zinc phosphate explained quite a lot, in this not deep study, in the actual petrochemical stainless steel external corrosion-proof surface is indeed more use of zinc epoxy phosphate primer, showing good binding force. It provides the basis for the performance of the whole coating anti-corrosion system. However, because zinc phosphate is slightly soluble in water, so for the immersion environment, especially with high temperature conditions, try not to use a primer containing zinc phosphate.
(3) for the surface can not be roughed stainless steel, after surface pre-treatment, you can use phosphate primer for bottoming treatment. Phosphate primers are usually composed of polyethylene alcohol butyl, epoxy, phosphoric acid, phosphate, alcohols and body pigments, usually in two-component packaging. Phosphate primer is generally relatively thin, the thickness of the substrate surface is 8 to 12 m, can be very good combination with the smooth surface, usually used as a pre-treated primer use, for the subsequent coating combination to provide a role of upper and lower. However, its own corrosion resistance is poor, the follow-up will be applied as needed epoxy paint, polyurethane paint and so on. The viscosity of phosphate primers is small, usually by air spray, roller coating or brushing, and high-pressure airless spraying is not recommended.
(4) for stainless steel in a immersion environment, taking into account not only the binding force of the coating, but also the corrosion resistance of the coating. Combined with the harshness and complexity of the petrochemical industry, the application range of epoxy paint is generally limited, especially for low acid tolerance, in the category of coatings, better performance than epoxy paint commonly used paint is phenolic epoxy paint and vinyl ester resin paint.
phenolic epoxy paint is a linear polymer of epoxy chloropropane and linear phenolic resin synthesis, the average ecyclic oxygen capacity in each molecule is greater than 2, relative to the epoxy content is more. After phenolic epoxy paint curing, the crosslink density of molecules is higher, acid-base resistance is better, the hardness of the paint film, wear resistance, etc. will be better, generally speaking, temperature resistance can reach 230 degrees C of dry heat. Protective coatings for equipment used in chemical facilities in harsh environments as a better anti-corrosion coating.
vinyl ester resin is a thermostered resin obtained by epoxy resin and methyl acrylic reaction, the resin is usually using peroxide as the trigger for reaction, vinyl resin active crosslinking point (double bond) is located at the end of the molecule, easy to cross-link reaction, so the curing degree of vinyl resin than unsaturated polyester resin is much higher, the overall corrosion resistance is also improved. At the same time, the benzene ring in the resin provides rigidity and thermal stability, ester base to ensure alkali resistance, water resistance, good chemical stability of ether bonds, hydroxyl strengthens the resin and substrate binding force. As a result, vinyl resin cross-curing can form a stable, corrosion-resistant coating, the overall performance is exceptionally excellent. Anti-corrosion in the petrochemical industry is commonly used in bisphenol A epoxy vinyl ester resin and phenolic ethylene oxide resin, since the 1960s Shell Chemical launched the Epocrgl brand, vinyl ester resin has been rapidly developed and applied. Vinyl resin in addition to being used as a coating substrate, but also made into vinyl resin glue, vinyl resin glass and steel, is widely used in extremely harsh strong corrosion environment, such as desulfurization flue, chimney lining, strong friction, bump, acid corrosion of the flotation machine lining and so on.
in the choice of coating scheme, the tolerance of the coating must meet the requirements of use, but also to consider the actual construction performance of the coating, cost and other factors.
surface treatment
are common stainless steel surface treatment methods are the following.
(1) Pre-treatment
stainless steel before surface coating, the first need to pre-treatment, remove the surface of oil, solder slag, dust, remove corner burrs and so on. For new, intact stainless steels, the surface is cleaned, mechanically smoothed and meets other relevant requirements after pre-treatment. No subsequent surface treatment can be made under the premise that the coating adhesion can reach the requirements. If the adhesion of the coating can be improved, a slight sand sweep can be carried out, and the specific roughness can be referred to the construction instructions of the coating supplier.
(2) blasting
for stainless steel with large areas of rust, carbon, solder slag, etc. on the surface, after pre-treatment, it is recommended to take blasting treatment. Sand blasting is divided into dry blasting and wet blasting, dry blasting is easy to cause dust pollution, need to be equipped with dust protection device; To avoid flash rust caused by the presence of water, appropriate corrosion inhibitors, such as sodium nitrite, need to be added to the mixed mortar.
to prevent chlorine ions from corroding stainless steel, the abrasives used must not contain halogens. At the same time, steel sand or iron sand cannot be used, as this may cause some particles to be embedded in the stainless steel surface, causing corrosion of the electric couple. Pomegranate or brown orographia is recommended.
(3) hand-processing
For thin-walled stainless steel components, sand blasting may deform the surface, so sand blasting can not be taken, the common way is manual treatment. Can use 80 purpose sandpaper for manual grinding, the surface can not have any dirt, the surface as far as possible to have a certain degree of roughness, in order to improve the adhesion of the subsequent coating.
(4) acid wash passivation
acid wash passivation is actually two, one can clean the stainless steel surface, the other through acid washing to remove the poor chromium layer, forming a new passivation layer. The passivation layer is denser and more effective than the passivation layer of stainless steel itself, and the corrosion resistance of stainless steel is improved. According to different modes of operation, stainless steel acid wash passivation treatment mainly has paste method, impregnation method, brush method, spray method, circulation method, electrochemical method 6 kinds. One of the paste method is the acid wash passivation paste applied to the corresponding parts, such as the container welds, ladders, hanging corners and other parts, without special equipment and sites, simple and flexible operation, suitable for site construction. Specific can refer to the standard GB / T3595-94 stainless steel acid wash passivation paste.
but the ability to pickle is limited, the plasma cutting, flame cutting produced by the black oxide skin is more difficult to remove, which requires manual or sand blasting treatment.
conclusion
the rapid development of modern industry, the demand for stainless steel is also increasing, the requirements are more demanding. In this paper, the corrosion of stainless steel is mainly considered from the point of view of corrosion, especially the common local corrosion in the field. In practical use, corrosion of stainless steel may be a combination of corrosion conditions, can not be considered alone. For coating protection, a reasonable coating system is the key to stainless steel coating protection. At the same time, the surface treatment method of stainless steel is introduced to help the site construction. Taken together, coatings also have limitations for stainless steel protection, which is just one of the measures. Proper design, reasonable material selection, optimized processing process, and then with the help of coating protection program, can more effectively reduce the occurrence of corrosion.
(References)
For more information, see Coatings and Protection No. 8 of 2018, originally entitled: Common corrosion analysis and coating protection of stainless steel in the petrochemical industry
Feng Gang
(Haihong Old Man (China) Management Co., Ltd., Beijing 100011)