Analysis and prevention of copper pipe corrosion

The failure of the locomotive heat exchanger due to pitting and corrosion of the T2 heat exchange copper tube is used as the engineering application background. The pitting corrosion and influencing factors of T2 copper tu were studied by weight loss method, SEM, and EDS. The optimal ratio of compound corrosion inhibitors (borax, sodium nitrite, sodium silicate, and benzotriazol) of T2 copper tube was obtained. The results show that the main reason for the pitting corrosion of T2 copper pipe is the local rupture of cuprous oxide protective film on the copper surface, resulting in the copper at the bottom of the pit as the anode and the Cu2O oxide film on the surface as the cathode to form a corrosion cell so that copper dissolves and pitting in the pit. The rupture of the Cu2O oxide film is related to excessive carbon-containing residue and the deposition of dirt (foreign matter) on the surface, mechanical attack, and defects of the material itself. The optimal ratio of compound corrosion inhibitor in 2 L pure aqueous solution is as follows: Borax content is 4.4 g, sodium silicate content is 4.23 g, sodium nitrite content is 4 g, and benzotriazole content is 0.2 g.


Introduction
The corrosion and fouling deposition of the heat exchanger will seriously reduce the heat exchange efficiency, causing the failure or even shutdown of the pipes in the heat exchanger increasing operating costs.The materials that make up the heat exchanger are mainly copper and its alloys, and the corrosion of copper and its alloys can be attributed to uniform corrosion, pitting corrosion, crevice corrosion, and selective corrosion [1][2] .It has been found that the corrosion of copper and its alloys is mainly determined by the solubility of O 2 and CO 2 in the solution, pH, temperature, microorganisms, corrosive environment (in the presence of SO 2 and H 2 S), composition of the electrolyte solution, flow rate, and microstructure potential difference (different metallurgy), as well as the content of other elements in copper and its alloys [3][4][5][6][7][8][9][10][11][12][13][14][15][16] .

Test material
The following is the selection and processing of specimens.T2 copper tube was selected, the chemical element was mainly Cu (copper content>99.9%), and the T2 copper tube was processed into Φ(10±0.03)mm× (0.6±0.03) mm, and the length was (50±0.5)mm.

Test setup
The test device is a hanging piece dynamic immersion corrosion instrument, and the schematic diagram is shown in Figure 1.The test device comprises a constant-temperature water bath, a corrosion specimen, and an electric stirrer.The device is an RCC series rotary corrosion pendant tester, dimensions: 1200×650×1100 mm, the number of test hanging pieces: 10 sets in 2 groups, temperature control range: (20-180)±1°C, and speed: (40~160)±2 r/min.

Figure 1.
Corrosion tester.The test beaker was calibrated with a 2 L volumetric flask, and the graduation mark was placed on the mural on the outside of the beaker to facilitate the addition of distilled water during the test.
Preparation of simulated fluid: According to Table 1, we weigh the drug required for the test as for the volumetric flask and place it in a 2 L beaker to let it completely dissolve after it is completely dissolved.
After the temperature of the test solution reaches the test temperature, we start the rotation system, adjust the speed to 75 r/min, and start the timing.
When the test running time reached 72 h, the rotation system was stopped, the specimen was taken out, and the appearance of the specimen was observed.Then, it was placed on a clean filter paper, blotted dry with the filter paper, placed in a desiccator for more than 4 hours, and finally weighed (accurate to 0.1 mg) and recorded the data.

Test the pilot project
(1) Corrosion weight loss: The degree of metal corrosion is measured by measuring the change in the quality of T2 copper before and after corrosion, that is, the weight loss method.The calculation is as follows: Where v is the corrosion rate, and the unit is mg/(h•m 2 ); ∆G1 is the change in the mass of the specimen before and after corrosion, and the unit is mg; A is the surface area of the specimen, and the unit is m 2 .
Scanning electron microscope morphology analysis and elemental analysis of corrosion products: Specimens with large weight loss and serious corrosion were selected to analyze the microscopic morphology by scanning electron microscope, and the microscopic morphological characteristics of different corrosion areas were observed and studied.The composition of corrosion products was further analyzed by electron microscope with EDS to study the pitting phenomenon and corrosion mechanism of specimens.
Compound corrosion inhibitor orthogonal test: According to TB/T 1750-2006 "In Coolant for Internal Combustion" No. 3 formula materials, borax, sodium silicate, sodium nitrite, benzotriazole (BTA), borax, sodium silicate, sodium nitrite, benzotriazole (BTA) and temperature were selected as test factors.According to the coolant temperature range (40-80°C) provided by the operation and maintenance party and the chloride ion content (10 mg/L) during water quality testing, the orthogonal test table L 18 (3 7 ) of 7 factor 3 levels was established.The test was carried out according to the orthogonal test table in Table 2.
We establish non-quantitative indexes, as shown in Table 3, and conduct an orthogonal analysis of the test results based on weight loss and morphological condition indexes.

Weight loss test analysis
A multi-index analysis was carried out by establishing the weightless index and morphological change as the indicators to measure corrosion, and Table 4 and Figure 2 were obtained.As you can see from Table 4 and Figure 2, whether it is the index of weightlessness or the index of morphological change, the influence of borax on these two indicators is great, considering that for the index of weightlessness, borax is larger than the temperature k value.The morphology change index is not as accurate and stable as weightlessness, so it can be determined that borax is the first influencing factor, followed by temperature.It can be seen that sodium silicate is the third factor, sodium nitrite is the fourth factor, and benzotriazole is the fifth factor.The order of the corrosion degree of T2 copper pipe specimens from large to small is borax > temperature> sodium silicate> sodium nitrite >benzotriazole.It can be seen that the change in the concentration of borax in the solution has the greatest impact on the weight loss of the T2 copper tube, which is related to the hydrolysis of borax as a strong alkali and weak acid in the water, the aqueous solution of borax is alkaline and the borax solution is a pH buffer solution so that the metal ions (such as Cu 2+ , Ca 2+ , Mg 2+ , Fe 2+ , etc.) in the solution react with acid ions (such as CO 3 2-, SiO 3 2-, PO 4 3-) to form dirt deposited on the metal surface.The pH of the solution and the Cu 2+ in the solution are almost unchanged.The decrease of concentration accelerates the corrosion of T2 copper tubes, the effect of temperature on T2 copper tube specimens accelerates the above-mentioned scaling reaction, and although sodium silicate is a strong alkali and weak salt, a large number of polysilica micelles are obtained by hydrolysis in water.The micelles are easy to combine with Ca2+, Mg2+ and Fe2+.The result of the interaction between sodium nitrite and dissolved oxygen in the solution is related to the density of the oxide film on the surface of the T2 copper tube.Benzotriazole can chelate metal ions (such as Cu2+, Ca2+, Mg2+, Fe2+, etc.).Hence, the concentration of metal ions (such as Cu 2+ , Ca 2+ , Mg 2+ , Fe 2+ , etc.) in the solution is constant, which inhibits the corrosion of T2 copper pipe to a large extent.Figure 3 plots the distribution of parameter k in the orthogonal table, representing the distribution of different factor levels.It shows the weight loss index and morphology change index of the T2 copper tube with factors.As can be seen from Figure 3, considering that weight loss is more important and reliable for measuring the severity of corrosion of T2 copper tube specimens than measuring the corrosion status of T2 copper specimens through the morphological changes observed by the naked eye, the K value is not much different for the morphology change index, so the optimal ratio of compound corrosion inhibitor in 2 L pure aqueous solution is obtained.The optimal temperature is 40°C.The worst temperature is 60°C.The best borax content is 4.4 g.The worst is 4 g.The best sodium silicate content in the solution is 4.23 g, with a worst of 4.7 g.Sodium nitrite in solution was best at 4 g and 4.4 g at worst.Benzotriazole at best 0.2 g and worst at 0.22 g.This is shown in Figure 4.

Microscopic analysis
As shown in Figure 5, the left picture is the topography of the No. 9 T2 copper tube specimen at 200 times magnification, from which it can be seen that there is a disordered circular small white spot on the surface of the specimen and one of the round small white dots is magnified and observed.The morphology of the small white spot at 2000 times is shown on the right.From the right figure, it can be seen that there are circular and irregular pits on the surface of the specimen and lumpy materials are observed in the circular pits.An EDS analysis is carried out on the corroded part of the right figure, and the analysis results are shown in Figure 6 and Table 3. Table 4 shows that the elemental composition of T2 copper pipe specimens before and after corrosion is very different.Mg, Al, K, Ca, and Fe elements not present in the copper pipe substrate appear in 34 places, and the content of O is high, indicating that oxidation has occurred in this area, resulting in corrosion.The inner side of the corrosion pit is lighter than the other parts, and the element composition at the bottom (37 places) is almost the same as that of the copper pipe substrate.In addition, Si appears in each area of the corrosion specimen, and the content of Si on the inner surface of the corrosion pit is significantly higher than in other regions.5, 6, and 4, the cause of pitting on the copper surface can be attributed to a local crack in the cuprous oxide protective film, which can be caused by mechanical damage or by defects in the material itself (including the deposition of impurities on the material).Table 5: The content of the Cu element is high in the 37 area at the bottom of the pit, while the content of the O element is very low.Around the pit (area 39), the Cu element is low, and the O is high.It can be seen that the copper at the bottom of the pit is used as the anode.The cuprous oxide film on the surface is used as the cathode, forming an electric couple that causes the copper to dissolve in the pit, and the corrosion mechanism is as follows: The water ionization in the solution yields H + and OH -, while the OH -produced in the cathode region is consumed by Cu 2+ precipitation.The formation of CaCO 3 , SiO 2 , Al 2 O 3 , and Fe 2 O 3 deposition is promoted due to the increase of the pH value of the solution around the corrosion pit, which explains why the Cu element around the corrosion pit decreases.In contrast, the content of the O element and these "unpresent" elements, such as Mg, Al, K, Ca, and Fe, increases, and the salt scale deposited around the pit further thickens.

Conclusion
(1) The factors affecting the corrosion degree of T2 copper pipe specimens from large to small are as follows: borax > temperature> sodium silicate> sodium nitrite > benzotriazole.
(2) The optimal ratio of corrosion inhibitors in 2 L pure aqueous solution was 4.4 g of borax, 4.23 g of sodium silicate, 4 g of sodium nitrite, and 0.2 g of benzotriazole.
(3) The main reason for the T2 copper pipe is the local rupture of the cuprous oxide protective film on the copper surface, which causes the copper at the bottom of the pit to the anode and the cuprous oxide film on the surface as the cathode to form electrochemical corrosion.It makes the copper dissolve in the pit and cause pitting.The rupture of the oxide film is related to the deposition of excessive carboncontaining residue and dirt (foreign matter) on the surface, mechanical attack, and defects of the material itself.
(4) It is recommended to closely monitor the water quality and temperature during the heat exchange operation to avoid impurities mixed in the aqueous solution, which will impact the copper tube of the heat exchanger and replace the unqualified hot water transfer solution in time.

Figure 2 .
Figure 2. Range comparison of weight loss index and quality condition index.

Figure 3 .
Figure 3. Change of T2 copper tube loss weight index and quality condition index with factors.

Table 1 .
Chemical reagents required for the experiment.

Table 2 .
Orthogonal test table for test.

Table 4 .
Multi-index analysis of different factors on the T2 copper sheet.

Table 5 .
Element and element content of the test piece in the designated area after corrosion.