Corrosion refers to the process of loss and damage (including metals and non-metals) under the action of surrounding media (water, air, acid, alkali, salt, solvent, etc.). A very important task of circulating water treatment is anti-corrosion.
1. Electrochemical Corrosion
Electrochemical corrosion refers to the damage caused by the electrochemical reaction between the metal surface and the ionically conductive medium. An electric current is generated during the reaction, and there are cathodes and anodes on the surface of the corroded metal.
The anodic reaction is that the metal atoms lose electrons and become ionic states and transfer to the medium, which is called the anodic oxidation process. The cathodic reaction is the absorption of electrons from the anode by the depolarizer in the medium, which is called the cathodic reduction process.
These two reactions are carried out independently and simultaneously, which is called a pair of conjugation reactions. A short-circuit battery is formed by the cathode and anode, and current is generated during the corrosion process. For example, the corrosion of metals in seawater, soil and acid, alkali and salt solutions belongs to this category.
2. Polarization and Depolarization
Polarization:
During metal corrosion, current flows between the anodic and cathodic sites, indicating a potential difference between the anodic and cathodic sites. If the water does not contain oxygen, the electrons in the anode corrosion reaction react at the cathode, and the generated atomic hydrogen and gas cover the surface of the cathode. The circulating water treatment and cooling water treatment produce a voltage opposite to the corrosion potential, called the overvoltage. , so that the potential difference in the circulating water treatment has changed, preventing the flow of current, that is, stopping the corrosion process. This change in potential difference caused by the reaction product is called polarization. In the treatment of circulating water, polarization plays a role in the corrosion process, and the polarization plays a role in inhibiting the corrosion process.
Depolarization:
When there is dissolved oxygen in the water, due to the oxygen participating in the reaction, the atomic hydrogen and the surface of the cathode are taken away, so the polarization of the gas is destroyed. The effect of excluding polarization is called depolarization, oxygen plays a role in the corrosion process, and the depolarization plays a role in promoting the corrosion process.
3. Galvanic Corrosion
Many production units are made of dissimilar metals or alloys that are in contact with each other. Due to the difference between the potentials of different metals, a galvanic battery is formed in an aqueous solution (dielectric). The more active metal with a negative potential is the anode, and the corrosion rate is higher than that when it is not coupled. The metal with a more positive potential is cathodic protected, Corrosion rate decreases or stops. In the system, the common galvanic corrosion is iron and brass, iron and stainless steel, aluminum and steel, pound and steel, and zinc and brass, etc. In either case, the former metal is corroded.
4. Oxygen Concentration Corrosion
Oxygen concentration corrosion battery is the most common and most harmful corrosion battery when metal is corroded in water, but it is also the most difficult to prevent corrosion battery. The oxygen concentration cell is the most potential difference caused by the influence of the medium concentration on the cathode reaction.
There are two types of common oxygen concentration cells, one is an oxygen concentration cell that produces an oxygen concentration gradient due to different dissolved oxygen concentrations in water at different depths, such as waterline corrosion; the other is common in cooling water systems. , is also the most dangerous corrosion under fouling or called sediment corrosion. Crevice areas are formed under the deposit, and it is very difficult for oxygen to be replenished in the solution in these crevices; while the solution on the metal surface outside the crevice has a sufficient supply of oxygen, so outside the crevice is an oxygen-rich zone—a cathode , and the gap is the oxygen-depleted area-anode. The corrosion site of the oxygen concentration cell formed in the crevice area is within the crevice, or under the deposit.
5. Crevice Corrosion
Crevice corrosion is a form of localized corrosion of covered parts of metal surfaces in certain environments. Corrosion perforation of a large number of heat exchangers, of which the main cause is corrosion under fouling - a type of crevice corrosion.
There are two conditions for crevice corrosion: one is the presence of hazardous anions (Cl); the other is that there must be a stagnant crevice as a corrosion site, and the crevice must be wide enough to allow the liquid to enter, but narrow enough to be able to enter. Maintain a hold-up area.
Widths of a few thousandths of an inch (under 1 mil) are generally considered to cause corrosion, and widths above 1/8 of an inch (0.3 mm) are considered rare.
6. Pitting Corrosion
Pitting corrosion is a special kind of localized corrosion, which leads to the formation of small holes on the metal. If "P" is used to represent the depth of the corrosion hole, "d" is the width of the corrosion hole. When P/d≤1, it is called localized corrosion.
When P/d>1, it is called pitting corrosion. The main reason for pitting corrosion is the deposition of ions or slime in the water on the metal surface. These deposits cover the metal surface so that dissolved oxygen and corrosion inhibitors in the water cannot diffuse to the metal surface, resulting in localized corrosion.
Cl- in water also has an impact on pitting corrosion. Pitting corrosion often occurs in the high temperature area of the heat exchanger and where the deposition occurs at a slow flow rate. Increasing the flow rate of the water is conducive to the diffusion of oxygen and the repair of the passivation film. Take away the deposits on the small holes, which is beneficial to control the occurrence of pitting corrosion.
Pitting corrosion is the most latent and destructive type of corrosion. Pitting corrosion is a large cathode and a small anode, which has autocatalytic properties. Corrosion inside the small hole, so that the surrounding area of the small hole is cathodically protected. The smaller the hole, the larger the area ratio of the cathode to the anode, and the faster the perforation.
Pitting corrosion sometimes tends to start on one side of the material and expand the perforation on the other side, making detection difficult. Due to the extremely destructive nature of pitting corrosion, it has now attracted more and more attention.
7. Stress Corrosion
Stress corrosion refers to the damage of metals in corrosive media under the action of tensile stress. This corrosion is generally through the grain, the so-called transgranular corrosion. Stress corrosion is the process of material failure resulting from the combined action of strain and corrosion caused by residual or applied stress. The fracture of materials caused by stress corrosion is called stress corrosion cracking. Stress corrosion is generally considered to have two types of anodic dissolution and hydrogen-induced cracking.
Common stress corrosion mechanisms:
Under the action of stress and corrosive medium, the oxide film on the surface of the part or component is corroded and damaged. The damaged surface and the undamaged surface form an anode and a cathode, respectively. The metal at the anode becomes ions and is dissolved, resulting in a current flowing to the cathode.
Since the area of the anode is much smaller than that of the cathode, the current density of the anode is high, further corroding the damaged surface. Coupled with the action of tensile stress, cracks are gradually formed at the failure point, and the cracks gradually expand with time until they break. Such cracks can develop not only along metal grain boundaries, but also through grains.
8. Abrasion and Cavitation
Abrasion is the accelerated damage or corrosion of the metal due to the relative motion between the corrosive fluid and the metal surface. This type of corrosion is often related to the degree of turbulence on the metal surface.
Turbulent flow makes the agitation of the liquid on the metal surface more intense than that in laminar flow, which makes the contact between the metal and the medium more frequent, so it is usually called turbulent corrosion. Turbulent flow corrosion is actually the result of a combination of mechanical wear and corrosion.
The abrasive appearance is characterized by grooves, grooves, corrugations, holes, and valleys, and often shows directionality. In the factory, parts with large changes in flow velocity, such as pump blades, valves, elbows, elbows, turbine blades, and nozzles, are prone to abrasion.
Cavitation, also known as cavitation corrosion, is a special form of abrasion caused by the generation and collapse of vapor bubbles in the liquid near the metal surface. This type of corrosion is prone to occur in equipment with high flow rates of liquids and pressure changes, such as hydro turbines, marine propellers, pump impellers, etc. The appearance of cavitation corrosion is very rough and the porosity is closely distributed, which is caused by both corrosion and mechanical action.
9. Microbial Corrosion
Microbial corrosion is a special type of corrosion, which is due to the direct or indirect participation of microorganisms in the metal destruction effect of the corrosion process. Microbial corrosion generally does not exist alone, and often occurs at the same time as electrochemical corrosion, and it is difficult to separate the two.
The microorganisms that cause corrosion are generally bacteria and fungi, but there are also algae and protozoa. The influence of microorganisms on corrosion is mainly through the change of electrode potential and concentration cell to indirectly participate in corrosion. The methods are roughly divided into the following categories:
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1. The slime formed by bacterial reproduction is deposited on the metal surface, destroying the protective film and forming a local battery
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2. Oxygen and other compounds are consumed by bacterial metabolism, forming aeration cells and concentration cells, and depolarization occurs in local cells
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3. Caused by the action of bacterial metabolites
(1) Affect pH or acidity
(2) Affect the redox potential
(3) Change the chemical state of the environment (including other ions such as ammonia, nitrate, nitrite, sulfate, sulfide, etc., which play a catalytic role in the reaction)
(4) The concentration of oxygen is affected by the generation or consumption of oxygen.
Microbial corrosion is a localized corrosion, and there are almost all signs of pitting corrosion, and its harm is extremely serious.
