Causes of Failures of Copper Alloy Globe Valve Seals

During the voyage of a ship, it was found that there were leaks in the sprinkler system. After inspection, it was found that the valve disc of the copper alloy globe valve in the system was corroded, and the valve disc and the valve seat were not sealed well, resulting in leakages in the pipeline's sprinkler system. The schematic diagram of the sealing is shown in picture 1. The valve of this system has been put into use for less than 3 years, and many similar situations have occurred. In order to find out the cause of the failure of the copper alloy globe valve seal, third-party failure analysis of a globe valve disc taken from the site was conducted.

1. Analysis methods and results
1.1 Physical and chemical testing
1.1.1 Chemical composition
Use a drill press to take chemical chips from the uncorroded area of the valve disc. The chemical composition was detected by ICP full-spectrum direct-reading plasma emission spectrometer, and compared with the composition of ZCuSn5Pb5Zn5 in GB/T 1176-2013 "Casting Copper and Copper Alloys". The results are shown in Table 1. It can be seen from Table 1 that the chemical composition of the valve disc meets the requirements of the ordering standard GB/T 1176-2013.

Figure 1 The schematic diagram of the sealing of the globe valve

Table 1 Test results of chemical composition(w) %

Items	Sn	Zn	Pb	Cu	Fe
Globe valve disc	4.90	4.24	4.79	86.09	Less than and equal to 0.010
GB/T 1176-2013	4.0 to 6.0	4.0 to 6.0	4.0 to 6.0	4.0 to 6.0	The rest

1.1.2 The analysis of mechanical properties
A round tensile specimen of 11 mm × 60 mm was cut from the disc of the globe valve, and an Instron 5982 10kN tensile testing machine was selected to test the mechanical properties at room temperatures. The test results are shown in Table 2. It can be found from Table 2 that the relevant tensile properties of the globe valve disc meet the requirements of GB/T 1176-2013 for the yield strength, tensile strength and elongation of ZCuSn5Pb5Zn5. Using CV-430DAT Vickers hardness tester, the microhardness test was carried out on the corroded area and the uncorroded area of the sealing surface of the globe valve disc. The test load was 300g, and the hardness value was HV0.3; the microhardness results are shown in Table 3. It can be seen that there is no significant difference in the microhardness values near the corroded area and the uncorroded area in similar positions.

Table 2 Tensile properties at room temperatures

Items	Rp0.2/MPa	Rm/MPa	A/%
Globe valve disc	127	295	41
GB/T 1176-2013	Greater than and equal to 90	Greater than and equal to 200	Greater than and equal to 13

Table 3 Micro hardness kg/mm2

Positions	1	2	3	Mean values
Near corroded areas	111	110	108	109
Un corroded areas	116	111	107	111

1.2 Analyses of metallographic structure
Figure 2 shows the appearance of the sealing surface of the globe valve disc, ZEISS Observer Z1. m metallographic microscope was used to observe the metallographic structure of the polished and eroded state of the corrosion area of the sealing surface.

Figure 2 The sealing surface of the globe valve disc

1.2.1 Observation of the polished state
Figure 3(a) shows the metallographic structure of the overall polished state of the cross-section of the corrosion drop area. It can be seen from the figure that a large number of point-like particles are dispersed and distributed on the matrix of the valve disc, and no metallurgical defects such as microscopic porosity are found in the entire matrix morphology. The corrosion drop area in Figure 3(a) is magnified and observed. The entire area is covered with corrosion products, and decomposition areas coexist with corrosion products. The morphology is shown in Fig 3(b). The magnified observation of the non-corroded part also found that there was a corrosion product, and there was a red area caused by de-composition corrosion partially. However, it can be seen that compared with the part where corrosion and falling occur, the decompositing in this area is relatively small, and the specific morphology is shown in Figure 3(c).

Figure 3 The metallographic morphology of the polished state

1.2.2 The observation of the corroded state
The polished sample was corroded with the ferric nitrate alcohol solution, and it was observed that the metallographic structure of the surface of the corrosion drop area and the surface far from the surface and the surface of the corrosion drop area were all dendritic α-Cu matrix plus the dispersion distributed Pb phase; the morphology is shown in Figure 4. In addition, after corrosion, no decompositing area was found on the surface accompanied by corrosion products. It can be inferred that the bonding strength of the decomposing area and the matrix structure is low, and all of them have fallen off after corrosion.

Figure 4 The morphology of the corroded state

1.3 The morphology observation and corrosion product analysis
1.3.1 Macro-morphological observation
It can be seen from Figure 2 that most of the outer surfaces of the globe valve disc were turquoise; the sealing surface of the valve disc is corroded and the thinned area is gray as a whole, with metallic luster in part. The arc length of the thinned area is about 120mm, accounting for about 30% of the outer circumference of the disc sealing surface, and the maximum corrosion depth of the sealing surface is about 4 to 5mm. A KEYENCE VHX-600E microscope was used to observe the macroscopic morphology of the corroded parts of the valve disc, and the morphology is shown in Figure 5. On the side of the valve disc corrosion area, it can be found that the maximum corrosion depth is about 4.5mm, and there are many corrosion pits of different sizes on the outer surface of the valve disc sealing surface. Obvious metallic luster can be observed in the corrosion pit, and it can be speculated that corrosion may preferentially occur from the outer surface of the valve disc sealing surface. The specific morphology is shown in Figure 5(a). Observed the front of the corroded part of the valve disc, and there were metallic lusters in partial areas, which are uncorroded and missing raised parts. Most of the concave area is blue-gray. There are pits of different sizes at the bottom. The bottom of the pit is relatively smooth, and purple-red can be observed partially. The specific morphology is shown in Figure 5 (b).

Figure 5 The morphology of the corroded area of the disc of the globe valve

1.3.2 Microscopic morphology observation
The corroded part of the valve disc was placed in an FEI Quanta 650FEG field emission environment, and its microscopic morphology was observed by a scanning electron microscope. The low magnification morphology of the corroded area is shown in Figure 6(a). It was further magnified and observed; there are pits of different sizes on the surface, and the entire surface is covered by corrosion products; and cracks can be observed in some areas of the corrosion products. There are approximately uniformly distributed pits on the surface after magnification, and the morphology is shown in Figure 6(c). The magnified observation of the area with metallic luster in macroscopic observation shows that the partial edge of the area is relatively flat and there are friction marks. It is found that there were a large number of pits on the surface of the area with metallic luster after magnification, and the size and distribution of pits are the same as that covered with corrosion products. In addition, some particles are still observed inside the pits. It can be inferred that the pits are formed after the particles fall off. The morphology is shown in Figure 6(d).

Figure 6 The morphology of the corroded part of the disc of the globe valve

Table 4 shows the energy spectrum analysis results of the corrosion products attached to the surface. Among them, the content of corrosive elements Cl and S is relatively high, and they also contain relatively high elements of O; the corrosion products are oxides and chlorides. Energy spectrum analysis was performed on the non-pit area in the area with metallic luster and the particles in the pit. The energy spectrum analysis results are shown in Table 5 and Table 6. It can be seen from Table 5 that the metallic luster area without pits contains a small amount of C and O. Compared with the chemical composition of ZCuSn5Pb5Zn5, the main difference is that it does not contain Pb element, and the content of other elements is the same; no corrosive elements such as S and Cl were found. It can be seen from Table 6 that the particles in the pits contain much Pb, and the content of the corrosive element Cl is also significantly higher than that in other areas.

Table 4 Energy spectrum analyses of corrosion products attached on the surface %

Elements	w	x
C	8.47	24.81
O	14.06	30.13
Mg	1.16	1.63
Al	0.59	0.75
Si	1.23	1.50
S	2.16	2.31
Pb	2.50	0.41
Cl	8.17	7.91
Sn	9.67	2.79
Ca	1.00	0.86
Fe	0.60	0.37
Cu	46.67	25.19
Zn	3.71	1.95

Table 5 Energy dispersion spectrum results of metallic luster region %

Elements	w	x
C	8.04	30.08
O	3.20	9.00
Sn	5.69	2.15
Ca	0.31	0.35
Cu	78.18	55.28
Zn	4.58	3.15

Table 6 Energy dispersion spectrum of particles in pits in metallic luster regions %

Elements	w	x
C	14.94	50.52
O	5.04	12.79
Cl	12.84	14.71
Cu	19.86	12.70
Pb	47.33	9.28

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