Abstract
The behaviour of 90/10/1. 5% Fe Coppernickel alloy in 'live' seawater and synthetic solution i. e 3. 4% NaCl solution was investigated by a combination of electrochemical and surface analysis techniques. Laboratory scale tests were carried out in seawater, aerated and exposed to appropriate lighting levels in order to maintain a population of micro-organisms throughout the exposure. In addition to the observation of corrosion, the effect of the ecological variation of living seawater was studied. A number of test pieces were exposed at controlled electropotential in 3. 4% sodium chloride solution and seawater. The development of the corrosion product with time in these solutions was reasonably well understood and, after studies by XPS, Auger electron and X-ray spectroscopies. The mechanisms of corrosion in seawater and 3. 4% sodium chloride solutions were found to be different. Apart from the mineral deposits formed in the two media, there were organic metabolic products in the organic and mineral structures completely interwoven within the surface films formed in seawater. Natural products were detected by a broad carbon peak which was quite characteristic and totally dissimilar to that from exposure in synthetic media. When samples were passivated in sodium chloride, the film formed was essentially inorganic in composition. In the seawater, the layer hierarchy within the mineral products was considered together with the presence and interaction of nitrogen, magnesium, calcium, sodium, sulphur and the alloying elements. The main component of the layer which was formed in sodium chloride at anodic potential was 'copper-chloride' with excess chloride precipitation. At -150 mV (SCE) which was slightly anodic to the rest potential, an enriched layer of iron compounds was found in 3. 4% NaCl but not in seawater. However, the passivating film owed nothing to iron but was derived from other corrosion products which normally lie below the iron layer when this was present. Since the true passivating film was fragile, we could assume that the iron rich film gives a mechanical protection and this finding could be important for the general protection of the alloy surface. Polarization studies showed that the passivating layer did not influence the anodic behaviour but it considerably inhibited the cathodic reaction. In seawater, due to incorporation of organic substances in the corrosion product this inhibition was lost when the sample was exposed to less oxygenated water. Different ecological varieties showed significant differences in the electrochemical behaviour of the passivated samples i. e loss of inhibition was less pronounced in highly filtered seawater in low oxygenated environment. The loss of inhibition was prevented by pretreatment and preformation of a protective film, built up in a synthetic solution or in high temperature seawater. The effect of temperature on the corrosion of the alloy in seawater was studied. It was found that under controlled electropotential conditions, an iron and nickel containing layer was formed at 40 C, but not at room or lower temperatures. The film formed at high temperature was well adhering, tenacious and protective. No cathodic inhibition loss effect was observed on these samples in low oxygenated media. Since it was our interest to study and find a common rule in behaviour of the alloy in seawater, a survey of cupronickel condenser tubes from various location was initiated. This survey showed interesting divergence in the texture, colour and composition of tubes from different sources. However, no strong correlation between these diversities was found nor could any be correlated with a particular pattern of corrosion.