Some reasons for corrosion of plate heat exchanger

Under the scale corrosion is a special local corrosion phenomenon. The mechanism of corrosion under the scale for when the various types of impurities in the water in the defective parts of the heat exchanger scaling, will form a special environment under the scale lack of oxygen, scale outside the oxygen-rich. In this environment the formation of low-potential ferrite as the anode, high-potential iron carbide as the cathode of the corrosion battery, which leads to the occurrence of corrosion under the scale heat exchanger. Depending on the type of deposit, there are four forms of inorganic salt scale, fouling, biological slime and material deposition.

Plate heat exchanger common corrosion patterns are under the scale corrosion, chloride ion corrosion, erosion corrosion, stress corrosion and other types.

1. Under the scale corrosion

Under the scale corrosion is a special local corrosion phenomenon. The mechanism of corrosion under the scale for when the various types of impurities in the water in the heat exchanger defective parts of the scaling, will form a scale under the lack of oxygen, scale outside the special environment rich in oxygen. In this environment the formation of low-potential ferrite as the anode, high-potential iron carbide as the cathode of the corrosion battery, which leads to the occurrence of corrosion under the scale heat exchanger. Depending on the type of deposit, there are four forms of inorganic salt scale, fouling, biological slime and material deposits.

Sub-scale corrosion not only affects the flow efficiency of the heat exchange medium as well as the heat transfer efficiency, but also leads to the corrosion failure of the equipment, which has a huge impact on the work of the heat exchanger.

2. chloride ion corrosion

(1) circulation medium temperature: in the media containing chloride ion conditions, Cl- corrosion cracking sensitivity increases with temperature, higher temperatures further promote the occurrence of Cl- corrosion.

(2) Cl- concentration: with the increasing concentration of Cl-, corrosion susceptibility also gradually increased, especially when the temperature is higher, stress corrosion fracture susceptibility with the increase in Cl- concentration and increasing.

3. Scouring corrosion

Heat exchanger in the fluid flow rate is relatively high, containing solids and complex anions and cations of the media conditions, scouring corrosion has become particularly obvious. Scouring corrosion is the result of the interaction between scouring wear and electrochemical corrosion, compared to the same conditions of simple scouring wear or electrochemical corrosion, scouring and corrosion caused by the synergistic effect of material loss is much greater than the sum of these 2 factors alone.

In general, mechanical wear is dominant in scouring corrosion, so the hardness of the material chosen for the equipment itself is the most fundamental factor affecting scouring wear.

In scour corrosion, scour wear and electrochemical corrosion promote each other. The fluid inside the equipment, through constant scouring, provides a large amount of oxygen for the electrochemical corrosion process on the one hand, prompting further acceleration of the electrochemical corrosion. On the other hand, fluid scouring makes the passivation layer on the material surface gradually thinned or even disappeared under the action of mechanical forces, making electrochemical corrosion more likely to occur. At the same time, the mechanical scouring causes the material surface to produce pits, thus increasing the specific surface area of the equipment and thus promoting the occurrence of electrochemical corrosion. The continuous occurrence of electrochemical corrosion makes the surface of the material rougher, thus causing micro-turbulence and promoting scouring wear, at the same time, electrochemical corrosion can also destroy the work hardening of the material surface, not only to reduce its fatigue strength, but also to promote scouring wear.

4. Stress corrosion

Stress corrosion is the process of material damage caused by the interaction between the material and the corrosive medium in the environment under certain stress conditions. Influencing factors can be divided into the following points.

(1) stress strength: the material is subject to stress strength is an important factor influencing the occurrence of stress corrosion, when the material is subject to stress exceeds a certain range will induce stress corrosion, and the material's susceptibility to stress corrosion with the increase in residual stress and rise.

(2) operating temperature: in low conductivity, low oxygen conditions, 304 stainless steel at 200 ° C near the peak rate of stress corrosion, and the subsequent trend of decreasing, while in dissolved oxygen or conductivity of higher water, the rate increases with the temperature, but when the temperature > 150 ~ 200 ° C, the growth rate slows down.

(3) pH: the material is located in the environment of acidity and alkalinity for its sensitivity to stress corrosion has a greater impact. In the case of stainless steel, for example, when the solution pH <7, the material sensitivity increases with decreasing pH.

(4) Dissolved oxygen and anions in the solution: according to the study found that chloride ions, sulphate ions, sulphides, etc. have a more significant effect on the sensitivity to stress corrosion. At the same time, the dissolved oxygen content in a certain range on the crack expansion rate of the material also has a significant effect. When the dissolved oxygen content in the solution <200 μg/L, the crack expansion rate increases significantly with increasing content, but when the content >700 μg/L, the crack expansion rate tends to level off with increasing dissolved oxygen content.
Source: Reproduced
Disclaimers.

This article is reproduced on the internet and is copyrighted by the original author. If copyright is involved, please contact us and we will remove the content as soon as possible.