ABSTRACT: This paper describes the most serious corrosion forms, influencing factors, mechanism and harm of copper alloys. Key words: Copper Tube Condenser, stress corrosion cracking, dezincification, pore corrosion foreword the most serious corrosion forms of copper alloys are three: Stress Corrosion cracking (SCC) , removal of alloy elements, and pore corrosion. It is very important to understand the factors, mechanism and harm of these three corrosion forms for the corrosion protection of copper alloys.

1. Stress Corrosion cracking (SCC) occurs in metals that are sensitive to SCC under tensile stress and corrosion environment. Therefore, a metal failure due to SCC must have all three factors at the same time: Stress Corrosion cracking sensitivity, tensile stress and corrosion environment. The stress causing stress corrosion cracking can be either external stress or residual stress. Pipe bracket spacing is too large, often due to the weight of the pipe itself and the weight of coolant caused by pipe droop and external stress. When the pipe is installed, there will be external stress. During the rolling process, the tube is stretched or flexed, and the external stress also appears. In short, the appearance of external stress is often the result of poor engineering design quality or improper installation. Most of the residual stresses that cause stress corrosion cracking are generated during the forming process and remain inside the metal. For example: due to machining carelessness and indentation or rolling the shape of the bell and so that the surface deformation, also easy to produce alloy residual stress. In addition to the effect of residual stress, the corrosion environment plays an important role in the process of SCC of sensitive alloys. After the ammonia is dissolved in the oxygenated water, such as in the condensate on the steam side, the sensitive copper alloy is corroded due to the rupture of the passivation film and forms a soluble cuprous-ammonia complex. In the air removal section of surface condenser, local area often contains high concentration of ammonia, air leakage is suitable for SCC environment. In this area of the condenser, navy brass has a limited life. It is well known that SCC can be caused by atmospheric pollutants such as wet sulfur dioxide and wet carbon dioxide. Some common copper alloys for condenser tubes are listed in Table 1. As you can see, brass is susceptible to SCC, and then aluminum bronze. CU-NI alloys, like copper alloys with high copper content, are not affected. A-BRASS is susceptible to SCC. Under the action of tensile stress, the oxide film partially cracked and then corroded in the environment of navy brass cracking. On the surface of the alloy, the activity of grain boundary (anode region) is higher, and the dissolution rate is higher than that of grain boundary (Cathode region) . The corrosion rate of the solution at the crack tip is significantly different from that of the aqueous solution. At the crack tip, the metal ions are hydrolyzed to produce acidic conditions that promote dissolution and fracture.

2. This form of corrosion is formed by the selective dissolution of the more active components in the alloy by electrochemical processes. The most common example is the selective leaching of zinc from brass, known as dezincification. Dealumination is the loss of aluminum. Nickel removal is the loss of nickel. Cast Iron Water tank also appears to remove the alloy elements, at this time the iron in cast iron is selectively dissolved, leaving graphite layer. This is called graphitization. However, the most common example of removal of alloying elements is dezincification. There are two types of dezincification of brass: layer dezincification and plug dezincification. Layered dezincification is also called Uniform Dezincification, in which the zinc is stripped away from the brass surface like strips. Embolus Dezincification, also known as local dezincification, forms pitting on the local surface of brass due to dissolution of zinc. Pits are sometimes covered with corrosion products. In the process of layer dezincification of brass, the surface layer becomes a copper layer with lower mechanical strength due to the dissolution of active component zinc. When subjected to water pressure or external stress, it will crack and suffer damage. Brass with high zinc content will appear this corrosion form in low hard and low pH still water, which is aggravated by the presence of atmosphere ions and sulfate ions. Differential aeration of oxygen in the sediment also contributes to this corrosion. When the corrosion occurs, the corrosion product is brittle, porous copper residue, can be retained in situ, or may be washed away by water and lead to material perforation. The pitting produced by this localized corrosion is usually of significant depth in the direction perpendicular to the metal surface. Aluminum brass with low zinc content is easy to be corroded in neutral, weak acid or alkaline medium. It can be seen that dezincification corrosion is also a kind of harmful corrosion type. There are two mechanisms of DEZINCIFICATION: (L) theory of selective dissolution of zinc: selective dissolution of zinc occurs in the alloy surface layer, leaving a loose copper layer. (2) solution-deposition theory: When corroding, copper and zinc enter the solution in the form of ions at the same time. Since the precipitation potential of copper ions is higher than the corrosion potential of the alloy, therefore, the copper ions are rapidly redeposited as pure copper on the brass surface near the melting point. Depending on the circumstances, both mechanisms are likely to work at the same time. Dezincification corrosion of industrial brass often causes serious consequences. For the brass tube of heat exchanger, because the tube wall is thin, the corrosion hole caused by Embolus dezincification becomes more serious when the penetration is deep. For tubesheet, as the tubesheet is thicker, the same corrosion appears to be less serious. Layer dezincification causes more metal loss on the whole surface than plug dezincification, but it is not serious. There are many factors affecting the dezincification of brass. There are still wet areas in the drain pipe where dezincification corrosion can occur. This corrosion is facilitated by the reduced oxygen supply in stagnant water. Therefore, dezincification occurs easily in the gap and in the wet area under the accumulation layer. In Brackish water or sea water with high chloride ion content, the dezincification of brass occurs because of its strong conductivity and the chloride Ion's easy penetration of oxide film. On the surface of metal, the hydrolysis of copper salt will produce acidic environment and preferentially dissolve zinc. The porous sediment restricts the oxygen entry and forms the oxygen differential charging cell, where the dezincification corrosion occurs continuously. The increase of temperature greatly accelerates the dezincification corrosion, especially on the hot spot. Addition of alloying elements as, P or SB to naval brass can significantly reduce dezincification corrosion. Dezincification corrosion can be avoided by adding alloying element as to aluminum brass. The dezincification corrosion can be further controlled by adding chemical inhibitors into cooling water. Wow class corrosion inhibitors are extremely effective in controlling this corrosion.

3. Pitting corrosion pitting corrosion is a kind of quite hidden and harmful corrosion form. It occurs in individual small areas, but the total metal loss is negligible. However, its penetration rate may be so great that it causes perforation in a relatively short time. Even though most of the pipe surface is unaffected, it can no longer carry cooling water. There are two main factors affecting the pitting corrosion of condenser copper tubes. One is the pitting corrosion caused by the metal ion concentration cell produced by the deposition of silt and silt. The other is that the high temperature part (thermocouple cell) moves directly from the hot anode region to the cold cathode region through the copper ion and produces the local corrosion. In addition, the quality of the water flowing through the copper tube of condenser, especially whether there is hydrogen sulfide, is one of the causes of the pinhole corrosion. Hydrogen sulfide accelerates the corrosion by changing the surface film of the Cuprous oxide. Compared with soft water, hard water causes the pitting corrosion of the copper alloy to a greater extent, and bicarbonate and sulfate ions can affect the pitting corrosion. One view is that chloride ion is the most important material to affect the pitting corrosion of copper tubes. Chloride ions are directly involved in the formation of protective Cuprous oxide, because the initial corrosion product on the metal surface in the presence of chloride ions is copper(I) chloride, copper(I) chloride Cuprous oxide: Zcu '+ ZCI-- ZCuCI-CuZO + ZHCI according to Lucey' . Pinhole corrosion occurs at any particular location where the rate of CUCI formation is increased or the rate of CUCI consumption by hydrolysis is reduced. Chlorination accelerates the formation of the initial corrosion product, copper(I) chloride; Deposition. Such as silt, silt, calcium scale and so on because of the influence of the diffusion of the initial corrosion products to inhibit the hydrolysis of copper(I) chloride, for example, the condenser on the surface of the deposits on the site became a place of corrosion pitting. The main step in the formation of pitting is the formation of a porous and partially detached Cuprous oxide film from the metal surface. This occurs where the balance of the copper(I) chloride is disturbed. The Cuprous oxide film formed at these sites is covered with small solid copper(I) chloride, which are oxidized to copper ions.CATHODIC reaction of copper ion reduction takes place on the outer surface of the membrane. These cuprous ions are formed by the oxidation of cuprous ions by dissolved oxygen in aqueous phase, and the source of cuprous ions is partly supplied by diffusion through the film from the inside of the etching hole, and partly reused by cuprous ions produced by Cathodic reduction on the outer surface of the film. The lumps formed at the top of the pits are composed of alkaline copper salts (I. E. Green Malachite Cuco) . Cu (OH)2). Inside the pits, copper, copper(I) chloride and Cuprous oxide coexist to produce a solution with a Ph of 2.5 to 4.0. As a result, the base metal is perforated by Autocatalysis and acidic conditions. 

Source: Chinanews.com, by Yang Xiaoling

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