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Heat exchanger corrosion and protective measures
Nov 08, 2018


The application of heat exchangers in the refining industry is very extensive, and its importance is also obvious. The utilization rate of heat exchange equipment directly affects the efficiency of the refining process and the cost of the cost. According to statistics, heat exchangers account for about one-fifth of the investment in chemical construction. Therefore, the utilization rate and longevity of heat exchangers are important issues worth studying. From the point of view of the damage of the heat exchanger, corrosion is a very important reason, and the corrosion of the heat exchanger is a large number of ubiquitous, and the corrosion problem can be solved, which is equivalent to solving the fundamental of the heat exchanger damage. In order to prevent the corrosion of the heat exchanger, it is necessary to clarify the root cause of the corrosion. The reasons for the corrosion of the heat exchanger are discussed in the following aspects.


corrosion

1. The choice of materials for the heat exchanger is determined by the economics. The pipe material is stainless steel, copper-nickel alloy, nickel-based alloy, titanium and zirconium, etc., except for the case where the welded pipe cannot be used industrially. Welded tubes are used, corrosion resistant materials are used only for tube lengths, and shell material is carbon steel. 2. Metal corrosion of heat exchanger


2.1 Principle of metal corrosion Metal corrosion refers to the destruction of metal under the action of chemical or electrochemical action of the surrounding medium, and often in combination with physical, mechanical or biological factors, that is, the metal in its environment. The damage caused by the action.


2.2 Heat exchangers Several common types of corrosion damage


2.2.1 Uniform Corrosion A macroscopically uniform corrosion failure that occurs over the entire surface exposed to the medium, or over a large area, is called uniform corrosion. 2.2.2 Contact Corrosion Two metals or alloys with different potentials are in contact with each other and immersed in the electrolyte solubilizing solution. There is a current passing between them. The metal corrosion rate of the positive potential decreases, and the metal corrosion rate of the negative potential increases.


2.2.3 Selective Corrosion A phenomenon in which an element of an alloy preferentially enters the medium due to corrosion is called selective corrosion.


2.2.4 Pitting corrosion Corrosion concentrated on individual small points on the metal surface is called pitting corrosion, or small hole corrosion and pitting.


2.2.5 Crevice Corrosion Severe crevice corrosion can occur in the gaps and covered parts of the metal surface.


2.2.6 Scour Corrosion Erosion corrosion is a type of corrosion that accelerates the corrosion process due to the relative motion between the medium and the metal surface.


2.2.7 Intergranular Corrosion Intergranular corrosion is a kind of corrosion in which the grain boundary of the metal or alloy and the vicinity of the grain boundary are preferentially etched, and the grain itself is less corroded. 2.2.8 Stress Corrosion Cracking (SCC) and Corrosion Fatigue SCC is a material fracture caused by the combined action of corrosion and tensile stress in a certain metal-medium medium system.


2.2.9 Hydrogen destruction Metals in the electrolyte solution can cause damage due to hydrogen permeation due to corrosion, pickling, cathodic protection or electroplating.


3. The effect of cooling medium on metal corrosion The most widely used cooling medium in the industry is a variety of natural water. There are many factors that affect metal corrosion, the main factors and their effects on several common metals:


3.1 Dissolved Oxygen Dissolved oxygen in water is an oxidant that participates in the cathodic process, so it generally promotes corrosion. When the concentration of oxygen in the water is not uniform, a concentration cell of oxygen will be formed, causing localized corrosion. For carbon steels, low alloy steels, copper alloys and certain grades of stainless steel, melting oxygen is the most important factor affecting their corrosion behavior in water.


3.2 Other dissolved gases CO2 will cause corrosion of copper and steel in the absence of oxygen in water, but does not promote corrosion of aluminum. Trace amounts of ammonia corrode copper alloys but have no effect on aluminum and steel. H2S promotes corrosion of copper and steel, but has no effect on aluminum. SO2 reduces the pH of the water and increases the corrosivity of the water to the metal.


3.3 Hardness In general, the increase in hardness of fresh water reduces corrosion of metals such as copper, zinc, lead and steel. Very soft water is very corrosive. In this water, it is not suitable to use copper, lead and zinc. In contrast, lead is resistant to corrosion in soft water and pitting corrosion in high hardness water.


3.4 pH Steel corrosion is less in water with a pH > 11 and corrosion increases at pH < 7.


3.5 Effect of ions Chloride can destroy the surface of passivated metals such as stainless steel and induce pitting or SCC.


3.6 Effect of scale CaCO3 scale in fresh water. The CaCO3 scale layer is detrimental to heat transfer, but is beneficial for preventing corrosion.


4. The effect of heat transfer on corrosion The corrosion behavior of metals is different under the conditions of heat transfer and no heat transfer. In general, heat transfer exacerbates the corrosion of metals, especially under conditions of boiling, vaporization or overheating. The effects of heat transfer are different in different media or on different metals.


5. Anti-corrosion method Knowing the causes of various corrosion of heat exchangers and rationally selecting anti-corrosion measures, the purpose of efficient use of equipment can be achieved.


Protection

In response to the corrosion conditions discussed above, the following preservative methods are proposed:


Here mainly introduces corrosion inhibitors and electrochemical protection.


1 Corrosion inhibitor The chromate-based corrosion inhibitor is commonly used in cooling water systems. Chromate ion is an anode (process) inhibitor. When combined with a suitable cathode inhibitor, it can be Satisfactory and economical anti-corrosion effect. Chromate-zinc-polyphosphate: Polyphosphate is used because it has a clean metal surface and has corrosion inhibition ability. Polyphosphate can be partially converted into orthophosphate, which can also be formed with calcium. The colloidal cation inhibits the cathode process. Chromate-zinc-phosphonate: This method is similar to the previous method except that sodium phosphonate is used instead of polyphosphate. Carbamyl phosphate can also be used for the pH specified by polyphosphate. High occasions. The carbamate phosphonate prevents scale formation and controls the precipitation of calcium salts even at a pH of 9. Chromate-zinc-hydrolyzed polyacrylamide: Due to the dispersion of the polyacrylamide hydrolyzed by the cationic copolymer, it is possible to prevent or inhibit scale formation. 2 Electrochemical protection Cathodic protection and anode protection are used. The cathodic protection uses an external DC power source to make the metal surface become a cathode to achieve protection. This method consumes a large amount of electricity and is expensive. The anode protection is to protect the heat exchanger from the anode of the external power supply to form a passivation film on the metal surface to protect it.


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