Cuprous oxide films

Little work has been carried out on the mechanism of inhibition of the corrosion. of copper in neutral solutions by anions. Inhibition occurs in solutions containing chromate , benzoate or nitrite ions. Chloride ions and sulphide ions act aggressively. There is evidence that chloride ions can be taken up into the cuprous oxide film on copper to replace oxide ions and create cuprous ion vacancies which permit easier diffusion of cuprous ions through the film, thus increasing the corrosion rate. 

Figures 6 and 7 IS) summarize several important results obtained with cuprous oxide films at 200° and 20° respectively. Exposure to oxygen at a few microns pressure is seen to be accompanied by an abrupt fall in resistance (i.e., rise in conductivity) showing that the concentration of current carriers—positive holes in the case of CU2O—has increased. Oxygen is therefore being adsorbed as negative ions. The fact that recovery of the original conductivity is possible on evacuation at 200°, but not at 20°,...

R.P.M. Procter, G.N. Stevens, Formation of cuprous oxide films on a a-brass stress-corrosion fracture surfaces, Corros. Sci. 15 (1975) 349—359. 

Copper Corrosion Inhibitors. The most effective corrosion inhibitors for copper and its alloys are the aromatic triazoles, such as benzotriazole (BZT) and tolyltriazole (TTA). These compounds bond direcdy with cuprous oxide (CU2O) at the metal surface, forming a "chemisorbed" film. The plane of the triazole Hes parallel to the metal surface, thus each molecule covers a relatively large surface area. The exact mechanism of inhibition is unknown. Various studies indicate anodic inhibition, cathodic inhibition, or a combination of the two. Other studies indicate the formation of an insulating layer between the water surface and the metal surface. A recent study supports the idea of an electronic stabilization mechanism. The protective cuprous oxide layer is prevented from oxidizing to the nonprotective cupric oxide. This is an anodic mechanism. However, the triazole film exhibits some cathodic properties as well. 

Copper is not ordinarily corroded in water unless dissolved oxygen is present. In nearly pure aerated water, a thin, protective layer of cuprous oxide and cupric hydroxide forms. Oxygen must diffuse through the film for corrosion to occur. 

The electrode potential behaviour of copper in various solutions has been investigated and discussed in considerable detail by Catty and Spooner . According to these workers a large part of the surface of copper electrodes in aerated aqueous solutions is normally covered with a film of cuprous oxide and the electrode potential is usually close to the potential of these film-covered areas. The filmed metal simulates a reversible oxygen electrode at... 

The volume ratio (see Section 1.9) for cuprous oxide on copper is 1 7, so that an initially protective film is to be expected. Such a film must grow by a diffusion process and should obey a parabolic law. This has been found to apply for copper in many conditions, but other relationships have been noted. Thus in the very early stages of oxidation a linear growth law has been observed (e.g. at 1 000°C) . 

At medium and high temperatures copper ultimately follows the parabolic law " . It has been shown " using radioactive tracers that the diffusion of copper ions in cuprous oxide is the rate-determining step at 8(X)-1 000°C, and there is considerable evidence in favour of the view that metal moves outwards through the film by means of vacant sites in the oxide lattice . 

Passivation under a wide range of operating conditions and in all boiler designs is a goal of all BW treatment programs. It is achieved through the formation of a stable, continuous, uniform, self-limited, and nonporous magnetite film. (This objective also applies to the maintenance of cuprous oxide, the copper equivalent of a film of passive iron.)... 

NOTE Cupric copper (Cu2+) is a catalyst for the hydrazine-oxygen reaction, as well as a catalyst for sulfite, DEHA, erythorbic acid, and hydroquinone. Cuprous copper (Cu+) acts as a complexing agent in the desirable formation of protective, pasivated copper oxide films. 

APPENDIX 1. PREPARATION AND ANALYSIS OF STEARYL THIOGLYCOLATE MONOMOLECULAR FILMS ON CUPROUS OXIDE MIRRORS... 

A 2 cm length of isopropanol-washed copper wire was vapor-deposited onto 1 X 3 sections of silicon wafer at a pressure of about 10 5 Torr. The composition of the copper films was determined via XPS to be 99% cuprous oxide. The mirrors were then stored in a desiccator for up to 1 month without any change in composition. Next, a 0.01 M solution of stearyl thioglycolate in isopropanol was prepared. A single, isopropanol-rinsed copper mirror was then immersed into 200 ml of the stearyl thioglycolate solution for 10 min. The mirror was then removed from the solution, air-dried horizontally, and finally immersed into a swirling bath of pure isopropanol for about 30 s. After being air-dried, experimental analysis was then conducted on the prepared films. 

Organophosphorus -compounds Supported copper-cuprous oxide island film mg L 1 [350, 351]... 

Copper, being a noble metal, has good resistance to corrosion. A thin adherent film of cuprous oxide and cupric carbonate is formed due to corrosion. Passivation is not a prominent process. The dissolved copper in solution affects the electrode potential such that the increase in velocity of the solution in contact with the metal results in increasing attack of the metal. Thus cuprous oxide is produced under dynamic flow of the solution. The thickness of the oxide film is about 500 nm. 

Plates of copper covered by neutral solutions of cupric sulphate always gain weight, becoming coated with a film of cuprous oxide. 

---