ASNE Day 2016 - Technical Paper Session 3 : Thursday, March 3, 2016 0945-1100
Ship Structures
Authors: Virginia DeGiorgi, Steve Policastro, Jerry Feng, Richard Fonda
Title: Corrosion and mechanical evaluation of coatings for Cu-Ni piping
Abstract:
A shipboard piping system can be considered analogous to the transport and distribution system of a land-based piping system. The intakes and supply pumps provide the initial uptake and head pressure while the piping system provides the distribution throughout the rest of the ship. However, unlike most land-based systems, a shipboard piping system has few straight runs, thus requiring many direction changes, causing pressure differentials to arise and flow conditions to be altered in areas throughout the distribution system. These changes in flow conditions, corrosion, and externally applied stresses age and eventually deteriorate the piping system.
From (1), the lifetime reliability of a shipboard piping system can be sketched as shown in Figure 1 with system integrity in percent on the y-axis and time on the x-axis. Once the system is installed and seawater, for example, is pumped through it, the interior surfaces and joints equilibrate over time to reach a stable state with the pipe system’s internal and external environments. Disruptions to the internal environment, e.g., due to turbulent flow, abrasion, pit nucleation, applied stresses, etc. leads to the initiation of microscale damage. The damage continues to propagate over time, eventually growing large enough to be visually detected, then to the formation of leaks, and finally to complete failure, requiring immediate, extensive repairs.
The degradation that occurs on the interior of the pipe is difficult to detect and remedy. Current inspection approaches require removal, and replacement of, insulation/lagging and a certified ultrasonic assessor. In addition, these inspections can only be executed pier side and are impacted by the limited number of qualified assessors. Hidden or undetected corrosion and erosion of shipboard piping systems that lead to leaks result in the implementation of damage control procedures while underway and/or full repair/replacement of this piping once pier side. The development of an inspection method that could be inserted directly into the piping system at a distant access point and could determine the damage state of the piping system prior to the development of visible damage or leaking would permit either the use of an in-situ repair technique or selective removal and repair of the damaged section to mitigate the damage and extend the life of the damaged area without the need for wholesale pipe removal and replacement.
While several options are possible for repair, one approach is to clean and prepare the damaged pipe surface and apply a metallic coating to the damaged area. The benefits of a metallic coating over an organic coating such as an epoxy are that the metallic coatings are generally more resistant to abrasive wear and have less surface roughness to engender turbulent flow than organic coatings. However, metallic coatings, depending on their composition, can be galvanically incompatible with the pipe system material – with both ferrous and copper-nickel alloys used for pipe system components. An unintentional galvanic coupling between repaired section and the parent piping would result in the possibility of acceleration in damage, especially near edges of repairs, after the repair is completed. It is therefore critical to understand the interaction between repair coating and parent piping. The focus of this paper is the evaluation of possible metallic coatings for repair of Cu-Ni piping.
The results of a corrosion and materials evaluation sequence on six coatings, labeled A, B, C, D, E, and F, and five coating application techniques that is designed to predict compatibility of the coatings with a copper-nickel seawater piping repair process is presented. The corrosion evaluation was focused on anodic corrosion and galvanic interactions between each coating and Cu0.7Ni0.3, a common piping material, in 0.6M NaCl, 0.6M NaCl + 0.01M NaOH, 0.6M NaCl + 0.01M HCl, and 0.6M + 0.005M H2SO4 electrolytes. The materials evaluation was focused on microstructural characterization and mechanical response.
The outcome of the evaluation sequence is the ranking of relative merit of these coatings. Results indicate that coatings applied using cathodic arc and electrophoretic deposition techniques, respectively, were most compatible for use in seawater piping repair of copper- nickel piping.
Works Cited
1. Bond, L. J., et al., et al. Advanced Inspection and Monitoring Technologies for Fresh and Salt Water Pipe Systems. Center for Nondestructive Evaluation, Iowa State University. s.l. : Office of Naval Research, 2014.
2. Failure monitoring and asset condition assessment in water supply systems. Misiunas, Dalius. Lund University, Lund, Sweden : s.n., 2005, Ph.D Dissertation.
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