How To Choose Piping Materials In Seawater Desalination？

The main influencing factors of seawater corrosion

Seawater is a complex natural balance system, a corrosive electrolyte solution, with high salt content, conductivity and biological activity. Therefore, seawater is highly corrosive, and its corrosion process and phenomenon are extremely complicated. The corrosion laws of metals in different regions of the seawater environment are very different, and the corrosion characteristics shown by the multiple effects of different factors are also different.

The Institute of Metal Research of the Chinese Academy of Sciences conducted mathematical modeling through the corrosion data obtained, and studied the influence of seawater environmental factors on carbon steel and low-alloy steel. It showed that the main influencing factors of seawater corrosion are dissolved oxygen concentration, pH value and average salinity.

The corrosion of most metals in seawater is a depolarization reaction of dissolved oxygen. The cathode process of the electrochemical reaction of carbon steel in seawater is usually controlled by the oxygen diffusion rate, and the oxygen supply is sufficient and the corrosion rate is fast. Seawater is an electrolyte solution with a high salt content. CI is an active anion. As the concentration of CI increases, the exchange current density of the metal anode dissolution reaction increases, causing local damage to the passive film, causing local corrosion such as pitting corrosion and crevice corrosion: When the pH value of seawater is weakly alkaline, it is not very destructive to the metal oxide protective film, thus reducing the corrosion of seawater.

Anti-corrosion method

An effective way to resist seawater corrosion is to passivate the surface of the metal surface treatment technology passivation to form a protective dense passivation film on the surface of the metal or alloy pipe wall, thereby improving the corrosion resistance of the material: the other is Coating protection technology for pipe wall coating or lining. In the seawater environment, pitting corrosion, crevice corrosion and stress corrosion cracking mainly occur. The corrosion resistance of some pure phase metals comes from their own surface passivation, while the corrosion resistance of stainless steel depends on the reaction of chromium and oxygen to form an oxidized passivation film: carbon steel and low alloy steel are not passivated in seawater environments , The inner wall can be coated with antiseptic, sherardizing, aluminizing, and hot-dip aluminizing technology to form a dense protective film on the surface of the steel pipe.

Seawater corrosion resistant material-Hastelloy

According to the literature (Chinese Journal of Corrosion and Protection-Huangguiqiao-The relationship between corrosion resistance ofstainless steelin seawater and corrosion points), the order of seawater corrosion of stainless steel after seawater immersion experiment is consistent with the order of positive and negative corrosion potential. The corrosion potential of stainless steel with strong passivation ability tends to be stable in seawater for a long time, the steady-state potential is more positive, and its corrosion resistance is better. The local corrosion resistance of stainless steel is also often measured according to the pitting corrosion resistance coefficient in seawater. The different stainless steel grades are listed below, and their chemical composition is shown in the table| (arranged in the order of increasing passivation ability).

Hastelloy is a nickel-chromium alloy. Nickel-based alloys have the ability to resist general corrosion, pitting corrosion and crevice corrosion in seawater, and can be competent for general stainless steel and other unsolvable carmine seawater corrosion problems! . The pitting corrosion resistance and corrosion resistance of dual-phase steel are better than ultra-low carbon alloy steel (316L), and the country is vigorously researching, producing and using this kind of special steel.

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