AC Impedance Study of Cu and Cu‐Ni Alloys in Aerated Salt Water: I . Pd Coating and Corrosion Product Stripping

Abstract

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...

Investigation of the first stages of the localized corrosion of pure copper combining EIS, FE-SEM and FE-AES

“…In the CuO layer also hydrated forms can be found. In some cases more complex combinations of Cu(I) or Cu(II) with Cl and OH − ions are reported [50,[73][74][75]. As well soluble products are formed.…”

An electrochemical impedance spectroscopy study of copper in a bentonite/saline groundwater environment

“…This prohibits spectra fitting by using a simple equivalent circuit. The shape (two time constants feature) of the impedance spectrum indicates that a bi-layer corrosion product film has been formed on the copper surface, which is well known for copper and copper alloys in different systems [18][19][20][21]. In this study, it is presumed that a thick porous corrosion product layer (mainly cuprite, and possibly other compounds) is formed on top of the thin protective cuprite film.…”

“…As judged from the Q-values the interfacial capacitance for copper after 3 days exposure is significantly higher compared to platinum. There are ample evidences that a thin protective cuprite film forms on copper in aerated aqueous solutions at neutral pH [18][19][20][21]. In the presence of chloride the film is relatively rough and exhibits a higher capacitance as found from electrochemical impedance measurements.…”

The inhibition of copper–nickel alloy corrosion under controlled hydrodynamic condition in seawater

“…as construction materials for seawater piping, heat exchangers and condensers in ships, desalination plants, power plants, as protective layers on oil platforms and ship hulls etc. This extensive use has triggered numerous investigations of these alloys with the aim of improving their performance under various conditions [1][2][3][4][5]. Many applications of cupronickels involve flow of the corrosive fluid and these alloys are known to suffer accelerated corrosion when exposed to flowing electrolyte [6].…”