Applications of Electrodeposition Technology to the Inner Walls of Pipe Fittings

Due to the advantages of simple equipment, dense coating, controllable composition and thickness, and strong winding plating, electrodeposition coating technology has been widely used for modifying and protecting the inner walls of pipe fittings (especially special-shaped parts). However, compared to the outer surface, inner wall electrodeposition technology for pipe fittings faces greater challenges, including technical difficulties such as a large cathode/anode area ratio, difficult coating inner corners, uneven coating thickness, and partial overheating or temperature gradients. In engineering, refined pictographic anode design and reasonable pre-treatment methods are often used to optimize electrodeposition process parameters (such as current density, current efficiency, plating solution temperature, and stirring rate) to achieve a uniform and dense coating.
 

In fields such as chemical engineering, machinery manufacturing, energy production, and national defense, pipe fittings are indispensable structural and functional components that perform functions like material storage and transportation, as well as medium containment, such as in oil pipelines, oil pump barrels, internal combustion engine cylinder sleeves, chemical pipelines, fuel supply pipelines, and naval gun tubes. However, the inner walls of these pipes are often easily damaged, leading to functional failure due to harsh working conditions, such as high temperature, high pressure, gas-liquid corrosion, particle wear, and oxidation. Therefore, it is urgent to strengthen the surface and develop surface modification technologies and processes with wear resistance, oxidation resistance, and corrosion resistance, such as thermal processing, C/N infiltration, ion implantation, and coating plating. Among these technologies, due to the advantages of simple equipment, dense coating, controllable composition and thickness, and strong winding plating, inner wall functional film or coating technology has been widely used for pipe protection.
 
Compared to the outer surface, inner wall coating technology for pipes faces greater challenges:
(1) The inner wall space is limited, making operations inconvenient, and coating technology or equipment designed for the outer surface cannot be directly used. 
(2) The treatment medium is unevenly distributed inside the pipe, making it difficult to ensure uniform coating.
(3) The amount of treatment medium used per unit time is relatively small, which may result in poor coating on the inner wall.
(4) The inner wall is difficult to analyze and detect, making it challenging to intuitively judge the coating quality.
 
Therefore, to meet various needs, researchers have developed numerous inner wall coating technologies for pipe fittings, including physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), sol-gel, spraying, mechanical alloying (MA), and electrodeposition (i.e., electroplating). The pipe fittings used include stainless steel pipes, Ni pipes, Al pipes, and Cu pipes, with deposited coatings such as Al, Ni, Al-Cr, VC, diamond-like carbon (DLC), Si, Mo, and TiZrV. Each technology has its own advantages and disadvantages and has been applied to different treatment processes. Ensinger and others used magnetron sputtering technology to coat the inner wall of a slender aluminum pipe with a carbon film tens of nanometers thick, ensuring uniform deposition by rotating the pipe during the process. Baba and others used coaxial electron cyclotron resonance microwave discharge to generate acetylene plasma, which was used to prepare a DLC coating on the inner wall of a 1-meter-long stainless steel pipe, significantly improving its corrosion resistance.

CVD technology, a widely used coating method, can also be applied to treat the inner wall of pipe fittings. Berreth and others applied CVD technology to prepare a silicide coating on the inner wall of a T91 steel pipe, enhancing its thermal oxidation resistance. To address the challenges of high deposition temperatures and limited aspect ratios in inner wall CVD technology for pipe fittings, Shaosong Huang and others developed metal organic chemical vapor deposition (MOCVD) technology and related equipment for low-temperature coating of slender metal pipes, achieving uniform, strong-bonding VC coatings with good integrity on inner walls of high-aspect-ratio pipes.

In contrast, the simple equipment, dense coatings, controllable composition and thickness, and strong plating resistance make electrodeposition technology more promising for treating the inner walls of pipe fittings. Electrodeposition is, in fact, one of the earliest technologies used to protect the inner surface of workpieces. With continuous technological advancements, particularly in the recycling of electroplating waste liquid, electrodeposition technology has been widely applied in the modification and protection of the inner walls of pipe fittings, especially for special-shaped parts such as rotating bodies, square tubes, and semi-enclosed tubes (containers).

Therefore, considering the role of electrodeposition technology in treating the inner walls of pipe fittings, this article briefly summarizes its technical characteristics, influencing factors, and engineering applications, aiming to provide reference and guidance for related basic research and engineering practice.
 

In addition to the challenges of the previously mentioned inner wall coating technologies, electrodeposition on the inner wall of pipe fittings presents the following technical difficulties:
 

(1) The current on the inner wall is easily shielded.
During electrodeposition, if the anode is placed outside the pipe fitting, the current on the inner wall can be easily shielded, making deposition difficult. Thus, an internal anode is often used to resolve this issue.

 

(2) The anode is easily passivated.
When the anode is placed inside the pipe fitting, the cathode-to-anode area ratio becomes large, resulting in higher current density through the anode than the cathode, which can easily cause anode passivation. To address this, the anode's surface area needs to be maximized. Typically, the anode's surface area can be increased by knurling, using a hollow anode, or a porous anode.

 

(3) Plating the inner corner is challenging.
The inner corner of the pipe fitting has a small curvature, making it difficult for the electric field to reach. As a result, the current density is low, leading to a thin and uneven coating near the inner corner, and possibly even leakage plating.

 
(4) Uneven Coating Thickness
In industry, vertical suspension is commonly used for electroplating long pipes. In the conventional aqueous electrolyte electrodeposition process, the hydrogen evolution reaction typically occurs on the cathode surface, and the generated bubbles overflow, creating a strong stirring effect. This causes the cathode polarization resistance and electrolyte resistance to vary along the length of the pipe, leading to differences in current efficiency between the upper and lower ends, ultimately resulting in uneven coating thickness. In high-temperature molten salt and ionic liquid electrodeposition, the absence of water generally prevents changes in cathode polarization resistance and electrolyte resistance. Additionally, the potential drop caused by the resistance of the long anode creates a voltage gradient between the anode and cathode at different ends, leading to uneven current distribution along the cathode surface and resulting in uneven coating thickness at both ends of the pipeline.

(5) Partial Overheating or Temperature Gradient
Compared to the outer surface, the flow space for the plating solution inside the pipe is limited, which increases the current density and, consequently, the heat generated during the electroplating process. If the plating solution does not circulate smoothly, it may cause partial overheating, affecting the quality of the coating. Additionally, for long pipes, the uneven distribution of electrolyte and cathode resistance along the pipe length creates a temperature gradient during the electrodeposition process, leading to variations in coating thickness, grain size, and microstructure.

(6) Inconvenient Loading and Unloading
Loading and unloading the inner anode during electrodeposition of the pipe’s inner wall can be challenging. When installing the inner anode, factors such as the concentricity of the cathode, the distance between the anode and cathode, and the self-weight deflection of the anode must be considered. After electrodeposition, direct contact between the inner anode and cathode must be avoided during disassembly, as it can easily cause scratches and damage to the coating.