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Simulation results of the Al fin-tube heat exchanger assembled with an anodic joint in lowconductivity electrolyte: (a) potential distribution, (b) current density distribution, and (c) current density distribution at the tube.
Source publication
The multi-galvanic effect of an Al fin-tube heat exchanger was evaluated using polarization tests, numerical simulation, and the seawater acetic acid test (SWAAT). Determination of the polarization state using polarization curves was well correlated with numerical simulations using a high-conductivity electrolyte. However, the polarization results...
Citations
... The use of allaluminium alloys in heat exchanger construction is also advantageous in terms of corrosion resistance, effectively addressing the issue of galvanic corrosion present in conventional RTPF heat exchangers. However, despite the passive nature of aluminium alloys and the use of all-aluminium materials to mitigate galvanic corrosion, instances of corrosion in MCHE systems have been reported [1][2][3][4][5][6][7][8]. The majority of heat exchanger failures occur due to harsh operating environments. ...
All-aluminum microchannel heat exchangers have recently gained popularity in the heating, ventilation, and air conditioning industry. Despite their attractive thermal performance design, these heat exchangers make coils used in automotive, commercial, and residential applications prone to crevice corrosion. This study uses high-fidelity conjugate heat transfer simulations to model a micro channel heat exchanger system that includes fins and tubes with crossflow to compare their thermal effectiveness to gain insight into potential crevice corrosion of the MCHE alloy. This study considers three fin geometries (louver, step, and saw) with the same tube and circular shape microchannel and identifies the corrosion hot spot and thermal effectiveness. A predicted flow field also identifies crevices between fins and tube surfaces as critical corrosion hot spots often associated with low-velocity regions. The crevice volumes for the louver, step, and saw fin shapes are calculated as 2.719 × 10 −5 inch3 , 3.297 × 10 −5 inch3, and 3.508 × 10 −4 inch3 respectively. Results also show that the same circular microchannel louver shape fin has higher effectiveness than the step and saw shape fin. The thermal effectiveness for microchannel tubes with louver, step, and saw shape fins are 0.337, 0.20737, and 0.2895, respectively.
... There is a belief that the use of only aluminum alloys in microchannel exchangers prevents galvanic corrosion, which occurs in the case of classic shell and tube structures made of dissimilar materials. However, the experience so far shows that also this design does not completely reduce the risk of corrosion [1,13,[15][16][17][18][19][20]. Most exchanger failures appear due to the aggressive environment that occurs in the heat exchangers during operation. ...
... The causes of corrosion of exchangers are mainly localized forms of corrosion [20]. Kim et al. [18] showed that the emerging galvanic effect is dependent on the conductivity of the electrolyte. Yuan et al. [17] point out the important role of copper and silicon diffusion on the development of corrosion. ...
As part of the work, three used microchannel heat exchangers from different manufacturers were analyzed. For comparison purposes, the new heat exchanger was also subjected to the NSS salt spray test. The material of the Al-Si system used for brazing, as well as the material of the core, were subjected to electrochemical tests. In laboratory tests, polarization curves were determined for the Al-Si material used for brazing, as well as for the core material. On the basis of exchanger research, it was found that a critical problem in the use of microchannel exchangers is galvanic corrosion occurring in the areas of brazed joints. Electrochemical tests have shown that the Al-Si alloy used for brazing has a lower value of corrosion potential than the core material. The existing potential difference is sufficient for galvanic corrosion. Electrochemical corrosion is initiated in the eutectic structure. The soplid solution is the anode and is particularly susceptible to corrosion, which led to its preferential dissolution in the entire volume of the eutectic mixture
... In a study by Kim et al., the multi-galvanic effect of an Al fin-tube heat exchanger with cathodic or anodic joints was evaluated using polarization tests, numerical simulation, and the seawater acetic acid test (SWAAT) [6]. Determination of the polarization state using polarization curves was well correlated with numerical simulations using a high-conductivity electrolyte, thus envisaging a novel approach to improve the design of products subject to multi-galvanic corrosion. ...
During the last few decades, an enormous effort has been made to understand corrosionphenomena and their mechanisms, and to elucidate the causes that dramatically influence the servicelifetime of metal materials [...]
p>This study investigated the tri-metallic galvanic coupling of different metals in the tubes, fillers, and fins of a heat exchanger. The goal was to prevent corrosion of the tubes using the fin as a sacrificial anode while ensuring that the filler metal has a more noble potential than the fin, to avoid detachment. The metals were arranged in descending order of corrosion potential, with the noblest potential assigned to the tube, followed by the filler metal and the fin. To address a reduction in protection current of the fin, the filler metal was modified by adding Zn to decrease its corrosion potential. However, increasing the Zn content of filler metal also increases its corrosion current. The study examined three different filler metals, considering their corrosion potential, and kinetics. The results suggest that a filler metal with 1.5 wt.% Zn addition is optimal for providing cathodic protection to the tube while reducing the reaction rate of the sacrificial anode.</p
div class="section abstract"> Open Circuit Potential (OCP) is the potential established between the working electrode (the metallic surface to be studied) and the environment, with respect to a reference electrode, which will be placed in the electrolyte close to the working electrode. It is very important to measure the electric potentials of various metallic materials in order to know their corrosion behavior in various environments. But perhaps if there is brazing joint involved, as in the case of aluminium heat exchanger components, there are some challenges involved in knowing the potential of the fillet part which includes the selective masking of the areas within the same sample for testing. Firstly, getting reliable & accurate data due to masking process is one concern. Secondly, about the aluminium heat exchanger sample, the variation in OCP values within a single sample are possible due to various factors i.e. presence of cladding on the surface of the materials, presence of sacrificial Zn coating on the microtubes, variation in brazing parameters etc.
In this study analytical correlation is made to OCP values of individual postbrazed child part samples w.r.t the entire brazed joint sample. This is being done in order to calculate & estimate the electric potential of the brazing fillet part. The individual currents at various areas within the sample w.r.t total specimen area are calculated & evaluated in order to arrive at a correlation between the observed potential vs. the sample area. From the obtained results it was found that the predicted theoretical fillet potential was in close agreement with the measured value. However, there is a scope of improvement in refining the simulation methods used in this study, in order to obtain more repeatable & accurate measurement OCP values.
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This paper presents the failure analysis of a gas-gas heater (GGH) tube in a coal-fired flue gas desulfurization (FGD) system. The GGH was operated at a temperature below those of the hydrochloric acid dew point as well as sulfuric acid dew point. Leaks were detected in the U-bend of the failed GGH tube. Corrosion products and ashes were attached on the surface of the tube, and uniform and localized corrosion were present throughout the surface. Corrosion products were mainly shown in reddish brown, orange and black color which were identified as hematite (Fe2O3), magnetite (Fe3O4), ferric sulfate (Fe2(SO4)3), ferrous sulfate heptahydrate (FeSO4·7H2O) and ferrous carbonate (FeCO3). In addition, S and Cl were observed in the corrosion products. This is a clear evidence that both sulfuric acid dew point corrosion and hydrochloric acid dew point corrosion occurred simultaneously. Further, when Cu and Cr are alloyed together, Cu segregates Cr into the grain boundary, forming a Cr depletion region, and localized corrosion occurs in this area. In the case of the fin-tube joint corrosion, the galvanic corrosion occurred at the filler metal because the filler metal acts as an anode with a middle potential.
Researchers and engineers have used cathodic protection (CP) to prevent corrosion in reinforced concrete (RC). Boundary element method (BEM) is a promising numerical technique to evaluate the effectiveness of CP on RC structures. However, some parameters that might affect the system or the potential distribution, such as mesh ratio, still need to be studied further. This paper aims to study the effect of mesh ratio on the potential distribution of RC installed CP using BEM. Simple 3D beam model of RC was used for the case study. The mesh for the model was triangular element and six variations of the mesh ratio were selected for the study. The mesh ratio obtained from the comparison between the size of the concrete element and the anode or cathode element. Simulation results show that the distribution of potential for all mesh ratios is within the protection criteria (⩽ −850 mV vs Cu/CuSO 4 ). However, the difference between maximum and minimum potential value becomes smaller when the mesh ratio increased. Hence, it shows that the mesh ratio has an effect on the distribution of potential of RC installed CP. However, it is still tolerable since the potential within protection criteria.
