Title: Defining a Framework for Comparison of Coating Technologies
Author(s): V. G. DeGiorgi, S. A. Policastro, C. R. Feng, R. W. Fonda, Naval Research Laboratory
Copper-nickel alloys are currently used in many marine system components such as seawater piping systems, heat exchangers and condensers, splash zone cladding, and boat hulls. Their frequency in marine use is due to their high level of resistance to crevice corrosion, stress corrosion cracking, hydrogen embrittlement, and innate biofouling resistance. However, despite the corrosion resistant nature of copper-nickel alloys in seawater, these alloys can be susceptible to localized corrosion, such as pitting, and fretting corrosion. Localized corrosion in shipboard piping systems can result in leaks requiring repairs that are costly in time and manpower often requiring removal of exterior lagging as well as sections of piping during the repair process. An advanced coating system which can be applied internally to piping and requiring minimal surface preparation is one step in the development of an in-situ repair system. In this work we investigate the suitability of advanced metallic coatings for copper-nickel piping system repair answering specific concerns about performance of the repair coatings.
Piping systems not only contain seawater but also must endure structural loadings. Damage can come in the form of corrosion effects such as pitting, general wasting away of material due to galvanic corrosion, mechanical deformations due to system loads or de-bonding of coating from the base metal. This work looks at both corrosion and mechanical performance characteristics. Evaluation criteria were devised in order to provide a framework to assess the performance of a coatings based directly on the outcomes of experimental processes applied.
Five commercially available coating technologies were identified and sets of coating samples were obtained for six different coatings. This represented four different coating manufacturers. Coating technologies considered were cathodic arc deposition, electrophoretic deposition (two coatings), gas dynamic cold spray, selective electroplating and metal alloy thermal spray coating.
Corrosion testing was designed to evaluate the overall corrosion resistance of the individual coatings and assess their galvanic compatibility with the base copper nickel pipe. Initial evaluations were performed in ambient aerated 0.6M NaCl solution which was intended to replicate a clean seawater environment with the added benefit of simplifying the analysis by limiting the number of electrolyte constituents. In addition, based on considerations of actual shipboard piping system operations, two acidic electrolytes were added to the test matrix to represent fluid in lower portions of pipes that can become acidified. Mechanical property investigation was focused on characterizing the coatings’ microstructure, composition, and 3-point bend performance using mechanical deformation, scanning electron microscopy and energy- dispersive x-ray spectroscopy. Results for several of the coatings tested are presented showing the wide range of characteristics possible and performance implications are discussed.