Hofmann (Beijing) Engineering Technology Co.,Ltd
Add:6F, East International Building, No.2 DongZhiMen Outside Street, DongCheng District, Beijing
1. Reduce the corrodibility of the medium.
Removing the dissolved oxygen and oxidant from the medium can control stress corrosion and reduce the concentration of Cl-in the medium. Also, strictly control the sulfur concentration is also an effective way.
2. Improve the structure of the heat exchanger.
In order to avoid the retention of deposits and residual liquid, adopt double-sided butt welding and continuous welding, not spot welding or overlap welding. In the welding process, the wire should be filled to 100% to ensure well-formed welded joint.
3. Reduce residual stress.
Based on practical experience, stress corrosion cracking is mainly caused by residual stress which mainly consists of the internal stress induced by cold working and welding. Heat treatment to the cold working parts and the weldments will help to eliminate residual stress, thus preventing the stress corrosion, generally by stress annealing heat treatment or other methods like hydraulic test, vibration aging and hammering.
4. Electrochemical protection.
Stress corrosion cracking is the anodic dissolution process under the stress action, which can be controlled by electrochemical protection. This method can also limit the crack extension in the conditions of appropriate selection in protection parameters. The method may be carried out by sacrificial anode protection or spraying corrosion-resistant metal on the surface.
5. Add corrosion inhibitors.
It can prevent stress corrosion cracking of stainless steel. The cracking occurs in the activation potential region, thus it is necessary to use adsorption-type corrosion inhibitor to cover and form a dense adsorption film on the metal surface to inhibit the metal corrosion process.
6. Clean heat exchanger.
One way is circulation cleaning which is to pump the cleaning fluid into the circulating water system for internal circulation and this way has a better effect on cleaning the scaling. The other is disassembly cleaning which is used when the heat exchanger channels are blocked by impurities. Cleaning the heat exchanger generally use the cleaning agents that has strong penetration, stripping, washing and inhibition ability to perform the cleaning process before operation, thus the corrosion accidents can be greatly reduced.
The impurities, salts, gas and microorganisms all will corrode the tube, the plate and the weld. Research shows that industrial water, no matter fresh water or sea water, contains various ions and dissolved oxygen, of which the changes in the concentration of chloride ions and oxygen have a great effect on the shape of the metal corrosion.
Further, the complexity of the structure also affects the metal corrosion forms. The one that centers on the individual dots of greater depth on the metal surface is called pitting corrosion or pitting. And the crevice corrosion refers to the corrosion that centers on the slit of the metal surface. Therefore, the corrosion of the tube and the weld mainly includes pitting and crevice corrosion. From the outside, there is much corrosion product and precipitate on the surface with unequal-sized bubbles.
Galvanic corrosion will occur in the case of seawater as the medium. "Bimetallic corrosion" is also a common phenomenon. According to the engineer on the anti-corrosion coatings in Beijing ZSWH Chemical Co., Ltd., the common types of heat exchanger corrosion include: crevice corrosion, stress corrosion and wear corrosion. The chemical or electrochemical action of the surrounding medium, together with the action of physical, mechanical or biological factors, will corrode the metal, that is, the metal is corroded by the environment.
1. Stress corrosion. The stress corrosion on the heat exchanger requires a certain corrosive environment sufficient tensile stress. And Cl- is another major factor. The Cl-has small radius and strong penetrating force which can help to penetrate the tiny pores of the protective membrane, destroy the local passivation membrane and enter the crack tip to generate HCl and the auto-accelerated catalytic corrosion process. Meanwhile, H+ is released from the tip and infiltrates into the leading edge, making the metal embrittled. Temperature is an important factor to destroy the stress corrosion. The higher the temperature, the lower the concentration of the Cl-, then more prone to stress corrosion cracks.
2. Electro-chemical corrosion, which is a more common metal corrosion of the heat exchanger. When the metal inside the cooling water system stays in moisture air or aqueous solution for a long period, a microbattery will form on the metal surface, also known as corrosion cell. Oxidizing reaction occurs on the anode and the anode dissolves meanwhile a reduction reaction occurs on the cathode, leading to continuous metal corrosion in the cooling water system and threatening its safe operation.
3. Chemical corrosion. Temperature has a great impact on the chemical corrosion of heat exchanger. Steel is not easy to corrode in dry air, but it easily gets oxidized to generate oxide skin at high temperature, of which the cementite prone to decarbonizing reaction, reducing the surface strength and fatigue limit.
4. Biological corrosion. The medium (such as the circulating water in the cooling water system) in contact with metal surface is the main reason for biological corrosion of the heat exchanger. The organism takes organic inhibitor as food, then the biological metabolism generates acid and destroys metal anti-corrosion protective layer. Also, the oxygen consumption caused by the biological metabolism leads to uneven concentration of O2on metal surface and then oxygen concentration corrosion.
After years of research and development and combined with much on-site practical experience of heat exchanger anti-corrosion, various ceramic high-temperature anti-corrosion coatings have been developed, which are high-temperature resistant and can form dense ceramic glaze with the hardness of 7H at high temperature after being brushed on the heat exchanger.
The coating is smooth, self-cleaning, anti-impact, alkali-proof and acid-proof in long term, strong adhesion, waterproof and oil-proof, and organic-solvent resistant. Such anti-corrosion coatings can withstand the longtime immersion of strong solvents and mixed solvents, the oxidation corrosion of concentrated sulfuric acid and concentrated hydrochloric acid, the corrosion of strong acid, the corrosion of high-temperature chlorine, hydrogen corrosion, high-temperature seawater corrosion, high-temperature (up to 1000 °C) corrosion. Besides, it is waterproof，wear-resistant and self-cleaning.