The breakpoint frequency method, which allows determination of the electrochemically active area of a coated metal in seawater, is described. A computer model is used to explain the basis of the breakpoint method, and the model is compared to impedance and visual data from epoxy-coated steel panels in ASTM artificial seawater with and without an intentional defect of known area. The breakpoint frequency method was found to be extremely useful in determining the electrochemically active area of coated steel in seawater. The equivalent circuit model used in this analysis was found capable of fitting actual data on coated steel panels with and without an intentional defect. A correlation was obtained between the breakpoint frequency and visually estimated electrochemically active area on epoxy coatings of a variety of thicknesses. This method offers a simple alternative to determination of defect areas via the use of the pseudocapacitance from difficult-to-analyze low-frequency impedance data. This approach also can detect the beginnings of coating breakdown long before visual indications are present.
The susceptibility of Cu-Ni alloys to accelerated corrosion in sea water containing low sulfide concentrations has been investigated. Test alloys included wrought 90/10 Cu-Ni, 70/30 Cu-Ni and Ni-Cu, plus cast 70/30 Cu-Ni. Sheet type specimen exposures were performed in fresh flowing sea water at a test velocity of 2.4 m/s (8 ft/s), and simultaneous polarization resistance cell exposures were conducted at a test velocity of 1.2 m/s (4 ft/s). Sulfide concentrations included 0.01, 0.05, and 0.2 ppm from addition of Na2S to flowing sea water. The test sequence included continuous sulfide exposures ranging from 1 to 1 5 days, followed by 60 days exposure in natural sea water. Gravimetric corrosion rates, depth of attack, corrosion potentials, and instantaneous corrosion rate data were developed. Results showed that wrought 90/10 Cu-Ni was susceptible to accelerated attack in sea water containing 0.01 ppm sulfides and is attacked vigorously after exposure to 0.2 ppm sulfides in sea water. Wrought 70/30 Cu-Ni is similarly susceptible to sulfide induced pitting, but appears to require higher sulfide concentration to trigger attack. Cast 70/30 Cu-Ni and Ni-Cu are susceptible to accelerated corrosion in sulfide modified sea water, but modes of attack differ from wrought 90/10 and 70/30 Cu-Ni. Interaction of exposure (or system operating) variables is seen as a key factor in assessing susceptibility of Cu-Ni alloys to sulfide induced corrosion.
Electrochemical impedance spectroscopy measurements were conducted on rotating disk specimens of pure copper and commercial 90-10 and 70-30 copper-nickel alloys over a 28 day period while immersed in an aerated aqueous 3.4% NaC1 solution, after vapor deposition of a Pd layer and finally after stripping the detachable corrosion products. The results indicate that for pure copper only the inner corrosion product layer impedes the corrosion reaction whereas for the more corrosion resistant Cu-Ni alloys, the porous, outer corrosion product layer accounts for the majority of the corrosion resistance. Contributing factors in the case of the Cu-Ni alloys are the availability of electrons and catalytic sites within the pore structure especially in the vicinity of the inner/outer layer interface and the diffusion of dissolved oxygen within the pore electrolyte to these sites, the latter being the rate-limiting step.Copper-nickel alloys are used almost exclusively for piping in all ships in the U.S. Navy and for heat exchangers and piping in many commercial ships and power plants cooled by seawater. Since 1974, severe corrosion of 90-10 copper-nickel piping has occurred in surface ships due to harbor water pollution by hydrogen sulfide, sometimes causing perforation of 0.6 cm wall pipe in as little as 30 days (1, 2). This has led to intensive research which has shown that there are several ways to prevent this corrosion. These are the use of: (i) ferrous sulfate inhibiter (3, 4), (i~) stimulated iron anodes ( 5), (iii) sodium-dimethyldithiocarbamate (SDD) inhibiter, (iv) cathodic protection (6), and (v) corrosion product films formed over a long period of time (7). Addition of ferrous sulfate for systems with large flow rates has been found to involve unacceptable consumption of the inhibiter in ship-based systems. Control problems have been experienced with iron anode systems tried on board Navy ships (8). SDD use. requires the shutdown of the systems involved, a procedure not practical in all installations. Cathodic protection of the inside of piping systems is not practical due to current throwing distance limitations. On a brighter note, the use of a corrosion product film formed over long periods of time in unpolluted seawater can improve corrosion resistance to subsequent polluted seawater exposurc. This process cannot be accelerated to time frames of use in construction due to a lack of knowledge of the nature and rate-controlling formation kinetics of corrosion product films of copper-nickel alloys. This has led to a need for more information to help determine which aspect of the corrosion product film causes corrosion resistance of these alloys, and to determine the rate-limiting step in film formation in unpolluted seawater. This should lead to a way to improve the speed of development of this pro~'ctive film, and thus, to improve the corrosion performance of copper-nickel piping in service.Until fairly recently, the corrosion mechanism of copper-nickel alloys was not well understood. The formation of a corrosion...
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