An Investigation into the Factors and Mechanisms Underlying Corrosion Failure of B10 Copper Tubes in Shipboard Pipelines

The present study investigated the corrosion behavior and mechanism of B10 copper tubes used for shipboard pipelines through various analytical techniques, including macroscopic inspection, chemical analysis, electrochemical impedance, corrosion product morphology, and physical phase and electron microscopic observation. The research findings revealed that the surface corrosion product film on the B10 copper tube in seawater mainly consists of Cu2O. Furthermore, the corrosion behavior under actual working conditions was attributed to crevice corrosion, non-electric coupling corrosion, and the corrosion mechanism was found to be the result of the combined effect of oxygen concentration difference cell and occlusion cell autocatalytic effect. These observations provide valuable insights into the corrosion performance of B10 copper tubes in seawater, which could potentially aid in improving their durability and reliability in marine applications.


Introduction
In recent years, China's marine ship industry has undergone rapid development due to the implementation of the strategy of strengthening the country by sea.However, corrosion has emerged as a significant challenge and is considered the leading cause of equipment failure and accidents according to research conducted by the Naval Research Institute.It is also identified as the primary factor contributing to reduced combat readiness, maintenance capability, and rapid response and deployment of equipment.The issue of seawater piping system corrosion failure has long plagued the shipbuilding industry, and the localization of essential materials has

B10 copper tube failure background and macroscopic phenomenon
During the B10 copper tube overhaul process, the shipyard discovered corrosion on the inner surface of the B10 copper-nickel pipe within the connector, as depicted in Figure 1(b).The connecting pipe had been in service for five years, with an expected service life of eight years.Figure 1(b) shows that the B10 copper tube was connected using a pipe connector with a seal.Upon disassembling the pipe connector, no corrosion was observed on the inner wall of the connector, as illustrated in Figure 1(c).However, green corrosion products were detected on the inner wall of the seal, as shown in Figure 1(d).Observations of the B10 copper tube's corrosion area indicated that the corroded section was concentrated at the bonding parts between the pipe connector seal and the copper tube, as presented in Figure 1(e).Macroscopic inspection of the outer surface of the copper tube revealed noticeable corrosion pits, perforations, and the presence of corrosion material.The severe corrosion resulted in the complete disappearance of the pipe's edge.

Chemical composition analysis
The composition of the failed B10 copper pipe was analyzed using ICP and carbon sulfur meter, and the results were compared with the requirements outlined in the GB/T 5231-2012 standard for processing copper and copper alloy grades and chemical composition.The findings are presented in Table 1.The analysis confirms that the B10 copper pipe's composition complies with the specified requirements for each element content outlined in the GB/T 5231-2012 standard.wall corrosion pit high times morphology.

Corrosion products -phase structure analysis
To ascertain whether the phase structures of the corrosion material present on the inner and outer walls of the copper tube are consistent, the inner wall corrosion material was stripped, crushed into powder, and subjected to XRD testing.Similarly, the outer wall corrosion material (taken from the Figure 1(d) seal ring inner wall corrosion residue) was also pulverized and analyzed.The findings are presented in Figure 6.From the results, it can be observed that the phases of the copper tube's inner and outer wall corrosion materials are cuprous oxide (Cu2O) and alkaline copper chloride (Cu2(OH)3Cl), respectively.This observation indicates that seawater is the corrosion medium for both the inner and outer walls of the copper tube.Additionally, the sand (SiO2) phase peak was observed in the outer wall corrosion material phase.The evidence further suggests that seawater penetrated the gap between the copper pipe's outer wall and the seal's inner wall, forming an "occlusion" area due to the difficulty of seawater flowing out of the gap.Consequently, sand particles could not be discharged with the seawater, leading to their accumulation within the pipe.

Electrochemical analysis
Based on the previous analysis combined with the corrosion environment and the theory of metal corrosion [13-14]   , it is initially inferred that corrosion in the copper pipe connection gap is more likely.However, it cannot be excluded that the seal ring may have aged after prolonged use, resulting in seawater leakage from the seal ring, leading to galvanic corrosion between the outer wall of the copper pipe and the stainless steel pipe connector, thereby accelerating the outer wall's corrosion rate.Hence, electrochemical analysis was conducted for both B10 copper pipe and pipe connectors.The polarization curves were measured to obtain the self-corrosion potential of the two materials to determine whether there was galvanic corrosion between the copper pipe and stainless steel pipe connectors.
Figure 7 illustrates the polarization curves of B10 copper and stainless steel pipe connectors.It can be observed that the self-corrosion potential of the stainless steel pipe connector is approximately -350mV, while that of B10 copper is around -230mV.This suggests that the self-corrosion potential of B10 copper is significantly higher than that of the stainless steel pipe connector.If there were galvanic corrosion between B10 copper and stainless steel pipe connector, B10 copper should be protected as the cathode, whereas the stainless steel pipe connector would corrode preferentially as the anode.However, Figure 1(c) shows that the stainless steel pipe connector wall did not corrode.Therefore, it can be inferred that there was no galvanic corrosion between B10 copper and stainless steel pipe connector, indicating that the outer wall's corrosion failure was not caused by the galvanic corrosion mechanism.

B10 copper corrosion failure mechanism
After conducting sample composition analysis, scanning electron microscopy, XRD physical phase test, and electrochemical testing on the failed site of the outer surface of the B10 copper tube, it can be concluded that the corrosion failure of the copper tube occurred because the corrosion rate of the outer wall was higher than that of the inner wall.The analysis further infers that the corrosion failure of the outer wall resulted from metal crevice corrosion.Metal crevice corrosion typically occurs within the gaps formed between metals and non-metallic materials (such as plastic, rubber, glass, fiberboard, etc.) when in contact with flange connection gaskets, as well as between different metals [15] .In this case, there were gaps between connected copper pipes, as well as between the outer wall of the copper pipe and the seal, which are consistent with the characteristics of crevice corrosion [16] .
The detailed process of crevice corrosion leading to outer surface crevice corrosion failure in the copper tube can be explained and illustrated in Figure 8.The gap corrosion mechanism is the outcome of the combined action of the oxygen concentration difference cell and the autocatalytic effect of the occlusion cell.The detailed process is as follows: (1) In the initial stage of crevice corrosion, both the inner and outer surfaces of the copper tube within the crevice experience anodic dissolution of Cu→Cu 2+ +2e -and cathodic reduction of O2 + 2H2O+4e -→4OH -.
Although only a minor cathodic current flows out from the crevice, the entire surface of the copper tube (including both inner and outer surfaces within the crevice) remains in the equipotential state, i.e., in a passive state, as depicted in Figure 8(a).
(2) Subsequently, as the oxygen inside the crevice is depleted during the incubation period, its diffusion-dependent replenishment from the solution within the crevice becomes challenging.Consequently, it suspends the cathodic reduction reaction of oxygen within the crevice, leading to the formation of a macroscopic cell between the outer surface of the copper tube within the crevice and the exposed copper tube's outer surface outside it.The region with low oxygen concentration within the crevice becomes the anodic region with lower potential, while the area outside the crevice that readily accesses oxygen becomes the cathodic region with higher potential.This results in the dissolution of copper within the crevice, resulting in an increase in Cu2 + .It attracts chloride ions (Cl -) present in seawater outside the crevice to transfer into the crevice for maintaining charge balance, as illustrated in Figure 8(b).
(3) As the chloride ion concentration within the crevice increases, the protective film of cuprous oxide inside the crevice hydrolyzes in seawater, producing insoluble alkali copper chloride and free acid.The reaction equation can be represented as follows: Cu2O+Cl -+2H2O Cu2(OH)3Cl+H + +2e -.This results in the reduction of the pH value within the crevice as a result of the combined action of chloride ions and acid.Consequently, the surface of the copper tube becomes activated, accelerating crevice corrosion and eventually leading to the development stage of corrosion.
(4) The excess copper ions within the crevice facilitate the movement of chloride ions into the crevice, leading to the continuous hydrolysis of the protective film and metal ions.This reduces the pH value within the crevice further, accelerating the dissolution of copper within the crevice.This process is akin to that of an autocatalytic pitting failure observed in occlusion cells.However, unlike pitting corrosion that occurs due to material's passive state or local destruction of the protective layer, crevice corrosion results from concentration differences in the medium.Typically, the crevice corrosion rate is significantly higher than that of pitting corrosion by more than an order of magnitude.This often leads to the development of perforations at the corrosion site, which ultimately results in the failure of the outer surface of the copper pipe due to crevice corrosion.Furthermore, pitting corrosion often results in a deeper and more extensive corrosion pattern compared to crevice corrosion, which is consistent with the corrosion pit shape observed in Figure4.

Conclusions
Based on the case study of corrosion failure of B10 copper pipe used for ship pipelines and the analysis of macro-observation, material analysis, microstructure and morphology analysis of corrosion area, corrosion product analysis and electrochemical analysis, the causes and mechanisms of corrosion failure of B10 copper pipe under actual working conditions were finally found out, and the following conclusions were drawn： (1)The B10 copper pipe connection process is very important.If the B10 copper pipe is poorly combined with the connector, there will be gaps in the connection, resulting in the non-uniform dense protective film on the outer wall of the copper pipe being destroyed and gap corrosion, resulting in the failure of the B10 copper pipe. ( The self-corrosion potential of the B10 copper tube is -230mV and that of the stainless steel pipe connector is -350mV.According to the theory of metal corrosion, the B10 copper tube is protected as the cathode and the stainless steel pipe connector as the anode.There is no galvanic corrosion between the B10 copper pipe and the stainless steel pipe connector.The failure of the outer wall corrosion of the B10 copper pipe is not caused by galvanic corrosion mechanism. (3)Corrosion failure of B10 copper tube is caused by metal gap corrosion, and its corrosion mechanism is the result of the co-action of oxygen concentration cell and block cell autocatalytic effect.

Figure 1 .
Figure 1.B10 copper tube failure site and failure corrosion macroscopic photos

Figure 2 (
Figure 2(a) clearly illustrates the presence of corrosion perforation on the surface of the B10 copper tube.Moreover, the cut samples and panel samples were observed using SEM to obtain a cross-sectional view, as shown in Figure2(b).A careful analysis of the cross-sectional morphology indicates that the corrosion rate of the outer wall of the B10 copper tube is considerably higher than that of the inner wall.This observation leads to an inference that the corrosion failure of the copper tube occurred because of the excessively rapid corrosion rate of the outer wall.

Figure 2 .
Figure 2. corrosion perforation macroscopic photos and microscopic cross-sectional morphologyFigure3is the corrosion area and not corroded area of B10 copper tube cross-sectional morphology.Figure3(a) reveals the presence of a complete and dense protective film on the uncorroded area's outer wall of the copper tube, with an uneven thickness that can reduce the corrosion rate of the copper tube.However, in the corrosion area of the outer wall, the protective film was destroyed, and only a thin and incomplete layer of corrosion material remains.EDS analysis of this area identified the main components as Cu, O, Cl, C, Ni, Fe, and other elements (refer to Figure4).Meanwhile, observations of the inner wall of the copper tube revealed the presence of a thicker layer of incomplete corrosion material interspersed with an incomplete thin protective film.

Figure 3 .Figure 4 .
Figure 3. Cross-sections of the corroded and uncorroded areas: (a) Cross-section of the outer wall of the copper tube in the uncorroded area; (b) Cross-section of the outer wall of the copper tube in the corroded area; (c) Cross-section of the inner wall of the copper tube

Figure 5 .
Figure 5. copper tube inner wall and outer wall corrosion pit morphology: (a) inner wall corrosion pit low times morphology; (b) inner wall corrosion pit high times morphology; (c) outer wall corrosion pit low times morphology; (d) outer

Figure 6 .
Figure 6.B10 copper pipe inside and outside the wall corrosion XRD pattern

Figure 7 .
Figure 7. B10 copper pipe and pipe connector polarization curve