Introduction to the corrosion and protection of heat exchangers

Heat exchanger in the refining industry is very widely used, its importance is also obvious, the high or low utilization rate of heat exchanger equipment directly affects the efficiency of the refining process and the cost of the cost of the problem. According to statistics, the heat exchanger in chemical construction accounts for about 1/5 of the investment, therefore, the utilization rate and life of the heat exchanger is an important issue worth studying. By the heat exchanger damage causes, corrosion is a very important reason, and the corrosion of the heat exchanger is a large number of widespread, can solve the problem of corrosion, it is the same as solving the fundamental damage to the heat exchanger. To prevent the corrosion of the heat exchanger, we must clarify the root causes of corrosion, now the causes of corrosion of the heat exchanger from the following aspects of the discussion.

1-heat exchanger with the choice of materials The decisive factor in the use of what materials is its economy, tube materials are stainless steel, copper-nickel alloy, nickel-based alloys, titanium and zirconium, etc., in addition to industry can not use welded tubes are used in addition to the case of welded tubes, corrosion-resistant materials only for the tube process, the shell process materials are carbon steel. Wuxi Dongjin production spiral plate heat exchanger, affordable prices, good corrosion resistance!

2 - heat exchanger metal corrosion

2.1 The principle of metal corrosion metal corrosion is the role of chemical or electrochemical in the surrounding medium, and often in and physical, mechanical or biological factors in the common role of metal damage, that is, the metal in its environment under the action of the damage.

2.2 heat exchanger several common types of corrosion damage

2.2.1 uniform corrosion in the entire surface exposed to the medium, or in a larger area produced, the macroscopic uniform corrosion damage called uniform corrosion.

2.2.2 contact corrosion of two different potentials of metals or alloys in contact with each other, and immersed in the electrolyte solution of soluble matter, there will be current between them, the potential of positive metal corrosion rate decreases, the potential of negative metal corrosion rate increases.

2.2.3 Selective corrosion Alloy in a certain element due to corrosion, the priority of entering the medium phenomenon known as selective corrosion.

2.2.4 pore corrosion concentrated in the metal surface of individual small points on the depth of corrosion is called pore corrosion, or small hole corrosion, pitting corrosion.

2.2.5 crevice corrosion in the metal surface crevices and covered parts will produce intense crevice corrosion.

2.2.6 scouring corrosion scouring corrosion is due to the relative movement between the medium and the metal surface and the corrosion process accelerated a corrosion.

2.2.7 intergranular corrosion intergranular corrosion is the priority corrosion of metal or alloy grain boundaries and grain boundaries near the region, and the grain itself is relatively small corrosion of a corrosion.

2.2.8 stress corrosion rupture (SCC) and corrosion fatigue SCC is in a certain metal a medium system, due to corrosion and tensile stress caused by the joint action of material fracture.

2.2.9 hydrogen damage metal in the electrolyte solution, due to corrosion, pickling, cathodic protection or electroplating, can produce damage caused by hydrogen seepage.

3-Cooling medium on the impact of metal corrosion Industrial use of super cooling medium is a variety of natural water.

Many factors affect the corrosion of metals, the main factors and their impact on several commonly used metals: Spiral plate heat exchanger is one of the main products of Wuxi Dongjin Chemical, good quality and affordable price!

3.1 dissolved oxygen dissolved oxygen in the water is to participate in the cathodic process of oxidant, so it generally promotes corrosion. When the concentration of oxygen in the water is not uniform, will form the oxygen concentration difference battery, resulting in local corrosion. For carbon steel, low alloy steel, copper alloys and some grades of stainless steel, molten oxygen is a super important factor affecting their corrosion behavior in water.

3.2 other dissolved gases in the water without oxygen CO2 will lead to corrosion of copper and steel, but does not promote the corrosion of aluminum. A small amount of ammonia corrosion of copper alloys, but no effect on aluminum and steel. H2S promotes the corrosion of copper and steel, but no effect on aluminum. SO2 reduces the pH of water, increasing the corrosion of water on metals.

3.3 hardness Generally speaking, the hardness of fresh water increases on the corrosion of copper, zinc, lead and steel and other metals is reduced. Very soft water is very corrosive, in this water, it is not suitable for copper, lead, zinc. On the contrary, lead in soft water corrosion resistance, in the hardness of the water produces pore corrosion.

3.4 pH steel in pH>11 water corrosion is small, pH <7 corrosion increases.

3.5 The effect of ions chloride ions can damage the surface of stainless steel and other passivated metals, inducing pore corrosion or SCC.

3.6 The effect of scale CaCO3 scale in fresh water. CaCO3 scale layer is unfavorable to heat transfer, but helps to prevent corrosion.

4-The effect of heat transfer process on corrosion The corrosion behavior of metals is different under the conditions of heat transfer and no heat transfer. Generally speaking, heat transfer to the corrosion of metals intensified, especially in the conditions of boiling, vaporization or superheating more obvious. In different media, or to different metals, the impact of heat transfer is also different.

5 - anti-corrosion methods know the various causes of heat exchanger corrosion, the reasonable choice of anti-corrosion measures to achieve the purpose of efficient use of equipment. For the above discussion of the corrosion situation, the following anti-corrosion methods.

Talk about the heat exchanger corrosion and protection

Here mainly introduce corrosion inhibitors, electrochemical protection.

① corrosion inhibitor chromate as the main component of the corrosion inhibitor is commonly used in cooling water systems, chromate ion is an anodic (process) inhibitor, when it is combined with a suitable cathodic inhibitor, can get a satisfactory and economical anti-corrosion effect. Chromate - zinc - polyphosphate: polyphosphate is used because it is the role of cleaning the metal surface, corrosion inhibition, polyphosphate can be partially converted to orthophosphate, they can also generate large colloidal cations with calcium, inhibit the cathodic process. Chromate - zinc - phosphate: This method uses sodium phosphonate instead of polyphosphate, similar to the previous method, aminomethine phosphate can also be used for higher pH than that specified for polyphosphate. Aminomethylphosphonate can prevent scale and control the precipitation of calcium salts even at a pH of 9. Chromate - zinc - hydrolyzed polyacrylamide: Due to the dispersion of cationic copolymer hydrolyzed polyacrylamide, it can prevent or inhibit the generation of scale into fouling.

② electrochemical protection using cathodic protection and anodic protection. Cathodic protection is the use of external DC power supply, so that the metal surface into a cathode to achieve protection, this method consumes a lot of electricity, high cost. Anodic protection is to protect the heat exchanger connected to the external power supply anode, so that the metal surface to generate passivation film, so as to get protection.