01. Stress Corrosion cracking of copper alloys and cases copper has a face-centered cubic crystal structure, easy processing and forming, with high conductivity and thermal conductivity. Copper is a positive metal. When Cu2 + and CU2 + are ionized, the standard electrode potentials are 0.337V and 0.521V respectively. Therefore, copper has good corrosion resistance and is the most widely used Non-ferrous metal. There are 4 kinds of copper and copper alloys in common use: pure copper, brass, bronze (tin bronze Cu-Sn, aluminum bronze Cu-A1 and silicon bronze Cu-Si) and white copper (Cu-ni) . Pure copper refers to industrial copper with a mass fraction of not less than 99.5% . Binary alloys of CU and Zn in brass system. Brass with zinc content less than 30% ~ 40% has phase and a small amount of phase, so the strength, plasticity and corrosion resistance are improved. SCC can occur in alloys such as brass and bronze, and in pure copper in ammoniacal media. 

02. Mechanism of SCC IN COPPER AND COPPER ALLOYS A. Rupture mechanism of surface film. According to the E-pH diagram of copper, the CATHODIC reaction of hydrogen evolution does not occur when copper is corroded in aqueous solution, so SCC is not the mechanism of hydrogen-induced cracking. The SCC mechanism of the surface film is that the "tarnish" (CU2O oxide film) is formed on the surface of copper and copper alloy in the medium containing ammonia, and the film is formed preferentially at the grain boundary of copper alloy. Figure 4-197b. The dull film is Brittle and ruptures under tensile stress. According to Suzuki et AL, the dull film on pure copper cracked on the grain, and the dull film on brass cracked on the grain boundary, figure 4-197c. The solution corrodes the grain boundary at the breaking point, figure 4-197d, then slowly reforms the film and grows along the grain boundary, figure 4-197e, when the newly formed film grows to a certain thickness, the deformation variable can cause the opaque film to break, and the new film breaks again, figure 4-197f. And so on and so forth, causing the SCC. Transgranular fracture occurs in pure copper and intergranular fracture in brass. The fracture is discontinuous and the fracture surface should be in the form of a step, Fig. 4-197g. The fracture surface is zigzag, figure 4-198.

Although Cu-30Zn brass may have intergranular and transgranular fracture types when SCC in aqueous solution containing ammonia, the actual brass encountered (formerly known as Season cracking) are intergranular fracture with film. The relationship between SCC sensitivity and tensile stress induced by corrosion in brass is studied. It is suggested that the formation of passive film or loose layer in SCC results in an additional tensile stress, so that dislocations can be emitted and moved under low external stress, mechanism of microcrack nucleation leading to SCC. During SCC, the Zn layer on the surface will increase continuously, and a tensile stress will appear at the interface between the Zn layer and the substrate. The in-situ TEM observation shows that the crack tip of SCC emits dislocation first, and the dislocation-free zone (DFZ) will be formed when the constant displacement is maintained, sCC micro-cracks are then nucleated at the tip of the original crack or DFZ (which has been passivated into a sharp notch) . The results show that there are two stress peaks at the tip of the notch and a point in the DFZ after forming the DFZ. The tensile stress of the stable interface is 0.2 times of the yield strength. When the applied stress is high, the peak stress can be close to or equal to the atomic bonding force, and the microcrack will nucleate at the top of the DFZ or the original crack. It can be concluded that the passive film or loose layer formed in the process of corrosion will produce an additional tensile stress, so that the dislocation can emit and move under the lower external stress, and form DFZ, and then the stress peak in DFZ will be equal to the atomic bonding force under the lower external stress, leading to SCC micro-crack nucleation. The Zn in copper alloy accelerates the formation of surface film, and the stress caused by passivation film or de-Zn loose layer changes with the PH value. The results show that the stress caused by the porous layer increases with the increase of Ph value, see Fig. 4-199,201[2] .

03. Influencing factor a of copper SCC. Effect of alloy composition. The SCC of brass with Zn content less than 20% is not usually produced in natural environment. The higher the Zn content is, the greater the SCC sensitivity is. The SCC of brass can be alleviated by adding Al, Ni and SN. The effect of stress. SCC OF BRASS parts occurs under residual stress (or even no load) . The residual stress of cold-processed deformed brass parts, which have not been annealed after processing, is large, and is easy to cause SCC in corrosive medium. When the stress is reduced, the fracture time is greatly prolonged. Brass appears to be stable when the stress is less than about 98 MPA. Influence of environmental media. Ammonia and substances that can derive ammonia (or NH) , as well as Sulfides, are most likely to cause SCC in copper alloys. SCC OF BRASS under tensile stress may occur in fresh water, high temperature and pressure water and steam, and in all media containing ammonia (or Nh) . Even trace amounts of ammonia can cause SCC in brass under tensile stress. Water or moisture, oxygen, SO2, CO2 and cyanide all accelerate the breakdown. A solution of mercury salts can also cause brass to corrode and fracture. High-stress brass will fracture in a mercury salt solution in a matter of seconds. The most commonly used reagent is 1%-10% HgNO3 with a mass fraction of 1% HNO3. The concentration of the latter is closely related to the breaking time. Figure 4-202. H2S Can aggravate the corrosion of copper and copper alloy, carbon steel and alloy steel, especially accelerate the pitting corrosion of copper alloy tube of condenser, the corrosion rate of condenser copper alloy tubes cooled by seawater heavily contaminated with H2s was 20 times higher than that of condenser copper alloy tubes cooled by clean seawater, but H2S was not corrosive to aluminum alloys; the effect of d.ph value on the brass cracking time was investigated. As shown in Fig. 4-203, the breaking time is shorter in alkaline solution; the breaking time is the shortest when Ph = 7.3 and the surface is covered with shiny black Cu2O; when Ph ≤4, the breaking time is increased sharply, and when Ph = 2, the breaking time is not broken for 1000h. As a result, Season cracking's sensitivity to Ph ranges from about 5 to 11.

E. Effect of different hydrogen content on the stress of dezincification layer. The additional stress caused by the dezincification layer was measured on the SSRT curve in the air before and after the formation of the dezincification layer. 04. MEASURES TO PREVENT SCC A. Stress Reduction and relief. Improve the structure design, avoid or reduce the local stress concentration of the structure form. The structure design should avoid the crevice and the dead corner which may cause the corrosion liquid to remain as far as possible, to prevent the concentration of the harmful substance. In the process of machining, manufacturing and assembling, large residual stress should be avoided as far as possible. Stress Relief Annealing is the most important means to reduce residual stress, especially for welded parts, annealing treatment is particularly important. B. Control Environment. To improve the use condition, firstly, the environment temperature should be controlled and the temperature should be lowered when the condition is allowed. In addition, reduce the temperature difference, to avoid repeated heating, cooling, to prevent the harm of thermal stress. Avoid contact with any form of ammonia and ammonium salts. Add corrosion inhibitors, such as Benzotriazole, to inhibit SCC. Protective Coatings, organic coatings that insulate the surface from the environment, or coatings that use environmentally insensitive metals as sensitive materials can reduce susceptibility to SCC. Electrochemical protection, because SCC occurs in 3 sensitive potential intervals, it can be prevented theoretically by Cathodic or anodic protection through control potential. C. Improve the texture. When other conditions (performance, cost, etc.) are met, the materials that have not been SCC in this environment should be selected as far as possible, or the materials available for selection should be tested and selected for optimal use. Smelting process and heat treatment process control, using new metallurgical process to reduce impurities in materials, improve purity, avoid SCC is beneficial. It plays an important role in reducing SCC susceptibility by heat treatment to change microstructure, eliminate segregation of harmful substances and refine grain size. Case 1. H65 BRASS TUBE cracking [3] H65 brass tube after bending without withdrawal treatment, use less than 1A, in the elbow cracking. A large number of circumferential cracks were found to extend from the inner wall to the outer wall, and a large number of light green corrosion products were accumulated in the inner wall. The microstructure of H65 copper tube was single phase by metallographic examination. The crack propagates through the grain, see figure 4-204

Case 2: Stress Corrosion of brass tube heat exchanger tube bundles [4] low pressure heater (JD-270) in a fertilizer plant resulted in SCC of copper tube bundles, which caused huge economic loss. The Heat Exchanger is a vertical U Tube Bundle Heat Exchanger, the tube body is made of Q345r, the Tube Bundle is made of Q345III, 112mm thick tube sheet and 19 groups of 610 u tubes made of HSn70-1, the copper tube is cold-formed, the specification is 20mm 1mm. Equipment parameters, equipment design pressure 2.12 MPA (Tube)/0.66 MPA (Shell) , working pressure 1.96 MPA (tube)/0.54 MPA (Shell) , design temperature 160 °c (tube)/277 °c (Shell) , working temperature 150 °c (tube)/261 °c (Shell) , working Medium Water (pipe)/Saturated Steam (Shell). When the equipment is manufactured, after the shell side water pressure is qualified, it is transferred to the shell side ammonia infiltration process. Due to the lack of the condition of c method ammonia seepage, the constructors carried out B method, resulting in ammonia seepage 2 ~ 3 hours later to find a large area of pipe head leakage, and found that the external surface of the pipe bundle is light blue, the straight pipe section and the bent pipe section have the penetrating crack of different degree, the crack can be torn by hand. The corrosion appearance is shown in Fig. 4-207. The brass tube fragments are shown in Fig. 4-208. metallographic examination shows that the crack extends along the grain boundary as shown in Fig. 4-209.

There is residual stress in the tin brass tube after drawing and forming. After water pressure test, the inner part of the tube can not be really dried and dried, as a result of ammonia infiltration by B method, tin brass tube formed a kind of ammonia, ammonium salt coexistence of highly corrosive humid environment. The content of Zn in tin-brass tube is 28% , and it is easy to be corroded and broken in humid atmosphere and ammonia. Case 3. Cracking analysis of H62 brass tube for heat exchanger of sugar machine [5] H62 brass tube is used for heat exchanger of sugar machine, the tube size is 45mm 2mm. During commissioning and trial production, more than 200 tubes burst longitudinally. Most of the rupture sites are in the middle of the pipe and some are near the pipe joints. The medium in the tube is 0.3 MPA steam, and the outer surface of the tube is in contact with the syrup. During the sugar-making process, sulfur is fumigated many times. Copper tubes work in a stress and corrosive environment. The macroscopic residual stress on the outer surface of copper tube is 156.9 MPA, and the circumferential tensile stress is much higher than the working stress, which is the main source of SCC of copper tube. The microstructure of brass tube is composed of + two phases. The microstructure is obviously inhomogeneous, one side is finer and the other side is coarser, the phase is elongated along the deformation direction and distributed in a continuous network, and the phase is surrounded by one or more grains, and the thickness of the phase is also very inhomogeneous, as a result, the stress distribution in the alloy is not uniform. Chemical etching tends to deepen in areas where local stress is concentrated, such as in some bulk phases. In the metallographic examination, micro-cracks were also observed on the interface of the phase. These cracks were almost in the same direction and were dendritic in shape. Phase is rich in zinc phase, relative to the phase is the anode phase, so the corrosion starts from the phase interface. The cracks are formed by phase anodic dissolution, and the orientation of the cracks is perpendicular to the tensile stress Axis. The SEM fracture analysis showed that there were two long and low concave corrosion source areas at the outer surface edge of brass tube, and the corrosion crack developed into deep arc corrosion area. It shows that the crack is produced from the outer wall of the Tube and extends to the inner wall of the tube. Energy Spectrum analysis shows that the small particles on fracture surface are Cu2O, the small massive particles are Znso4, and the villous phase is zinc-rich (dezincification product) . In addition, there are sulfate, carbonate and a small amount of chloride and so on. 04. CONCLUSION: A. The cracking of BRASS TUBE IS SCC plate cracking, which is caused by the outer wall of Tube and extends to the inner wall of Tube. Under the combined action of static tensile stress and corrosive medium, preferential corrosion occurs along the interphase and cracks are produced. Subsequently, the crack is stress-oriented and propagates through the corrosion of phase and grain boundary. The internal stress in brass tube is inhomogeneous, and there are continuous phase network or discontinuous interphase thin layer in the microstructure, which is the result of incorrect annealing process and the internal cause of SCC in brass tube.

 Source: Industrial South Point

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