Common corrosion analysis and coating prevention o

Common corrosion analysis and coating protection of stainless steel in petrochemical industry

common corrosion analysis and coating protection of stainless steel in petrochemical industry

November 1, 2018

stainless steel contains alloy elements, which can be oxidized to form a passivation layer, and the passivation layer is dynamic. Therefore, stainless steel has relatively good corrosion resistance. At the same time, stainless steel also has the characteristics of high strength, strong toughness and good processing performance, and is widely used in the petrochemical industry, such as pressure vessels, pipelines, equipment, etc. Due to the diversity of working conditions in the petrochemical industry, the corrosion of stainless steel is also complex and diverse. Due to the long-term harsh environment, stainless steel will inevitably be corroded. The corrosion of stainless steel is divided into overall corrosion and local corrosion. The overall corrosion of stainless steel can be predicted by data. Moreover, overall corrosion is a gradual process, which can be prevented in advance. However, the local corrosion process is unpredictable, and the potential harm is relatively greater. In order to ensure the safe and normal operation of stainless steel equipment and components, effective anti-corrosion measures must be taken. This paper mainly analyzes the causes of local corrosion of stainless steel, and puts forward the protection scheme and surface treatment method of coating

common corrosion conditions and analysis of stainless steel

pitting corrosion

the corrosion that occurs in a small part of the stainless steel surface will extend longitudinally to the depth, and there is no corrosion or slight corrosion in the rest parts, which is called pitting corrosion, referred to as pitting corrosion for short. The corrosion of common chloride ions on stainless steel belongs to pitting corrosion. The radius of chloride ion is small, which is easy to be adsorbed on the surface of stainless steel. Chloride ion can destroy the passive film on the surface of stainless steel, causing pitting corrosion of stainless steel. Second, with the enrichment of chloride ions in the pitting pit, the anodic dissolution accelerates, the corrosion extends to the depth of stainless steel, and the pitting corrosion intensifies. This is why stainless steel components, equipment and storage tanks are corroded in the marine environment

stress corrosion cracking

stress corrosion cracking (SCC) is the cracking caused by the joint action of tensile stress and corrosive medium. Stainless steel will produce residual stress during processing, forging, heat treatment and assembly. At the same time, stainless steel in a specific corrosive environment has been corroded. Then, under the joint action of stress and corrosion, cracks are formed in stainless steel. Cracks can propagate through intergranular, transgranular or mixed modes. The crack finally exceeded the bearing limit of stainless steel, resulting in the cracking of stainless steel

according to the mechanism of stress corrosion cracking, in order to avoid stress corrosion cracking of stainless steel, in addition to anti-corrosion treatment, we should also try to reduce the impact of stress. For example, reduce the design stress, design reasonably, reduce local stress concentration, and reduce the sensitivity of stainless steel to SCC. At the same time, stainless steel alloy elements will also affect stress corrosion cracking. For example, in chloride medium, nitrogen and phosphorus will accelerate stress corrosion cracking, while silicon and nickel will improve the ability to resist stress corrosion cracking. Therefore, there are many ways to avoid stress corrosion cracking of stainless steel, which should be reasonably selected in combination with actual working conditions and needs

intergranular corrosion

austenitic stainless steel after welding, intergranular corrosion often occurs at the weld, causing damage. The widely accepted explanation for intergranular corrosion is the chromium poor theory. Austenitic stainless steel is heated at 450 ℃ ~ 850 ℃ for a long time, and the adjacent area becomes the intergranular corrosion sensitization zone. In this area, chromium and carbon react and precipitate from the solid solution. The chromium content in this area is decreasing, forming an intergranular chromium poor area. Corrosion will continue to develop between the grains of metal or alloy, and the strength of stainless steel will be reduced. Before the dream of multi-dimensional material map is completed, it is usually difficult to find from the outer surface that when subjected to external forces, stainless steel breaks. Through the analysis of the principle of intergranular corrosion, it is not difficult to see that the higher the carbon content in stainless steel, the easier intergranular corrosion occurs. Moreover, in the sensitization temperature range of 450 ~ 850 ℃, the greater the probability of intergranular corrosion

crevice corrosion

crevice corrosion is also common in stainless steel in the petrochemical industry, such as the flange connection of stainless steel pipe fittings, the threaded connection of bolts, the contact surface between the outer edge of the stainless steel tank bottom plate and the foundation, and so on. The cause of crevice corrosion is concentration corrosion, that is, there is a concentration difference between metal ions or oxygen inside and outside the crevice. The development of corrosion is due to the autocatalytic corrosion of occluded batteries, which can not meet the needs of every premature baby. For example, stainless steel soaked in seawater has a high concentration of oxygen outside the gap, which is regarded as a cathode. The continuous consumption of oxygen in the gap leads to low oxygen concentration, which is regarded as anode. With the enrichment of metal ions formed by corrosion in the gap, chloride ions outside the gap are naturally attracted to the gap, where cation and anion ions accumulate, forming an occluded battery, and the gap corrosion accelerates

coating anti-corrosion measures for stainless steel

selection of coating

the surface of stainless steel is relatively smooth and has a passivation layer. Therefore, the selection of coating on the surface of stainless steel is different from that of carbon steel and low alloy steel. Generally speaking, the choice of stainless steel surface coating mainly considers the choice of primer. As the lowest layer of multi-layer system, primer is in direct contact with the substrate. In addition to protecting the substrate from corrosion, primer is also used as the transition layer of subsequent coatings. Once the adhesion of the primer is poor, it will directly cause the overall coating to fall off. For special metals like stainless steel, the choice of primer is particularly important

(1) epoxy paint is recommended for stainless steel surface after sand blasting, except for epoxy paint containing metals, such as epoxy zinc rich paint. Because epoxy paint has relatively high bonding strength and good corrosion prevention and application performance, it is also a commonly used anti-corrosion primer. Epoxy paint can be well combined with stainless steel after sand blasting, and the drawing strength usually exceeds 5MPa. However, epoxy zinc rich paint mainly uses the high activity of zinc to play a cathodic protection role after contacting with metal. The passive film formed by self passivation on the surface of stainless steel will reduce the cathodic protection effect of epoxy zinc rich primer. For this reason, epoxy zinc rich paint cannot be used on the surface of stainless steel

(2) for stainless steel with low surface roughness (smooth), epoxy zinc phosphate paint is usually used as primer. The zinc phosphate added to the epoxy zinc phosphate primer is zinc orthophosphate zn3 (PO4) 2 ¥ nH2O, (n=2,4). Zn2+ and po43+ of zinc orthophosphate react slowly with the substrate to form a phosphating film of me (metal) -zn-p2o5. This phosphating film is dense and firmly attached to the substrate, ensuring the good adhesion between epoxy zinc phosphate primer and stainless steel. At the same time, zinc orthophosphate also reacts with hydroxyl and carboxyl groups in the base material and some ions in the corrosive environment to form a complex, which can react with corrosion products to form a closely attached protective film on the surface of the substrate. These properties of zinc orthophosphate not only provide corrosion resistance, but also form a close combination with the substrate. Therefore, under the working conditions with low requirements for the surface treatment grade of the workpiece, epoxy zinc phosphate primer is often used. There are many explanations about the utility principle of zinc orthophosphate in epoxy zinc phosphate primer, which will not be studied here. Epoxy zinc phosphate primer is indeed used on the external anti-corrosion surface of stainless steel in actual petrochemical industry, showing good adhesion. It provides a basis for the performance of the whole coating anti-corrosion system. However, as zinc phosphate is slightly soluble in water, primer containing zinc phosphate shall not be used for soaking environment, especially under high temperature conditions

(3) for stainless steel whose surface cannot be roughened, phosphating primer can be used for priming after surface pretreatment. Phosphating primer is usually composed of polyvinyl butyral, epoxy resin, phosphoric acid, phosphate, alcohol and constitutional pigments, and is usually packaged in two components. Phosphating primer is generally thin, with a thickness of 8~12 µ m on the surface of the substrate, which can be well combined with the smooth surface. It is usually used as a pretreatment primer to provide a connecting link for the later coating. However, its corrosion resistance is poor. Subsequently, it will be coated with epoxy paint, polyurethane paint, etc. as required. The viscosity of phosphating primer is small, and it is usually sprayed by air, roller or brush. High pressure airless spraying is not recommended

(4) for stainless steel in soaking environment, in addition to considering the adhesion of the coating, we should also consider the corrosion resistance of the coating. Combined with the harshness and complexity of working conditions in the petrochemical industry, generally speaking, the scope of application of epoxy paint is limited, especially the low resistance to acid. In the field of coatings, phenolic epoxy paint and vinyl ester resin paint are commonly used with better performance than epoxy paint

phenolic epoxy paint is a linear polymer synthesized from epichlorohydrin and linear phenolic resin. Its average epoxy functionality in each molecule is greater than 2, which is more relative to the content of epoxy group. After curing, phenolic epoxy paint of Chinese paint has higher molecular crosslinking density, better acid and alkali resistance, and better hardness and wear resistance of paint film. Generally speaking, the temperature resistance can reach 230 ℃ dry heat. As a better anti-corrosion coating, it is used for the protective coating of chemical facilities and equipment in harsh environment

vinyl ester resin is a thermosetting resin obtained by the reaction of epoxy resin and methacrylic acid. This kind of resin usually uses peroxide as the initiator of the reaction. The active cross-linking point (double bond) of vinyl ester resin is located at the end of the molecule, which is prone to cross-linking reaction. Therefore, the curing degree of vinyl ester resin is much higher than that of unsaturated polyester resin, and the overall corrosion resistance is also improved. At the same time, the benzene ring in the resin provides rigidity and thermal stability, the ester group ensures alkali resistance and water resistance, the chemical stability of the ether bond is good, and the hydroxyl group strengthens the adhesion between the resin and the substrate. Thus, the crosslinking and curing of vinyl ester resin can form a stable, strong corrosion-resistant coating with excellent overall performance. Bisphenol A epoxy vinyl ester resin and phenolic epoxy vinyl ester resin are commonly used in the petrochemical industry. Since the launch of the epocrgl brand by shell chemical in the 1960s, vinyl ester resin has been developed and applied by leaps and bounds. In addition to being used as the matrix of coating, vinyl ester resin is also made into vinyl ester resin mastic and vinyl ester resin FRP, which are widely used in extremely harsh and strong corrosive environments, such as the lining of desulfurization flue and chimney, the lining of flotation machine with strong friction, bump and acid corrosion, etc

when selecting the coating scheme, the resistance performance of the coating must meet the use requirements, and the actual construction performance, cost and other factors of the coating should also be considered

surface treatment

the common treatment methods of stainless steel surface are as follows

(1) pretreatment

before surface coating, stainless steel needs to be pretreated to remove oil, welding slag, dust, burrs, etc. on the surface. For new and intact stainless steel, the surface shall be cleaned, mechanically leveled and meet other relevant requirements after pretreatment. On the premise of ensuring that the adhesion of the coating can meet the requirements, subsequent surface treatment can be omitted. In consideration of improving the adhesion of the coating, a slight sand sweeping treatment can be carried out, and the specific roughness can be referred to the coating for reference