| Popis: |
The electrochemical behaviour of copper and a high strength 70/20 copper nickel alloy, MARINEL ™ (MARINEL ™ is a registered trade make of Langley Alloys Ltd.), was investigated in sulphide contaminated and uncontaminated sodium chloride solutions (3.0%) at 293 K. Cathodic polarisation curves for the oxygen reduction reaction exhibited a well-defined plateau region between −650 and −1150 mV ( vs .SCE) prior to hydrogen evolution. In the presence of 5 ppm sulphide ions, an additional plateau region between −480 and −850 mV ( vs. SCE) is observed which is attributable to the production of hydrogen peroxide. This plateau is more prominent on copper than on the high strength alloy. The diffusion coefficient for dissolved oxygen was determined in the presence and absence of sulphide. Oxygen reduction was similar on copper nickel and copper leading to diffusion coefficients of 1.50 × 10 -5 and 1.44 × 10 -5 cm 2 s -1 respectively. In the presence of 5 ppm sodium sulphide, the diffusion coefficient values were reduced to 1.35 × 10 -5 and 1.24 × 10 -5 cm 2 s -1 , respectively. Atomic force microscopy investigations of the initiation of corrosion and the influence of passive layers on the corrosion reaction confirmed that film formation was rapid. In the presence of sulphide ions, particle size increases from 68 to 300 nm in diameter were observed for both surface films but the particle heights were of similar dimensions, 6 nm. The increase in particle size for films produced in sulphide contaminated solutions would be expected to influence film porosity and hence the corrosion resistance. |
