Natural Surface Films

Many studies have shown that unsaturated fatty acids, common constituents of aquatic organisms, are unstable to sunlight in a surface environment. For example, Wheeler (1972) showed in an early investigation that soft glass-filtered ( 320 nm) wavelengths promoted the formation of oxidation products from linoleic acid (4), linolenic acid (5), and the lipid extract from the diatom Thalassiosira fluviatilis. The 

Effects of microlayer constituents or other surface-active agents on photochemical reactions in aqueous media have not been widely studied. Zadelis and Simmons (1983) demonstrated that the photodecomposition of naphthalene in Lake Michigan microlayer material was slower by a factor of about 2 relative to that in lake water. Larson and Rounds (1987) also showed that a 30 mM concentration of a surface-active material (sodium dodecyl sulfate) significantly decreased the photolysis rate of 1-naphthol at pH 7. In contrast, Epling et al. (1988) showed that borohydride-promoted photodechlorination of polychlorinated biphenyls was significantly increased in the presence of ca. 100 mM concentrations of the surface-active agents Brij-58 (a polyethoxyethanol) and sodium dioctyl succinate. The mechanisms for these observed rate effects are unknown. 

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Fig. 3. Left side SIR-C/X-SAR images of the same spot of the North Sea acquired during the first shuttle mission on April 12th, 1994, and showing signatures of natural surface films (image dimensions 7 by 9 km). The images were acquired at L-, C-, and X-band (from top to bottom), W-polarisation. On the bottom right, damping ratios obtained from SIR-C/X-SAR images of various natural slicks at low to moderate wind speeds (< 7 m s 1) are shown. Diamonds and circles denote HH and W polarisation, respectively...

Since the 1960s natural surface films ( sea slicks ), that tend to exhibit thicknesses of one molecule only, have been in the focus of interdisciplinary research that required input by various disciplines such as oceanography, meteorology, physics and chemistry. Albeit the thickness of such monomolecular surface films is small compared to that of mineral oil films their wave damping capability and, thus, their influence on air-sea interactions is comparable. Consequently, they are still often mixed up with mineral oil films ( oil spills ), particularly in the Same of remote sensing applications. It is the aim of the present book to provide a scientific basis that allows avoiding such misinterpretation in the future. 

Deposition of Lime-rich Dielectric Nature surface Films on the... 

Characterization. The proper characterization of coUoids depends on the purposes for which the information is sought because the total description would be an enormous task (27). The foUowiag physical traits are among those to be considered size, shape, and morphology of the primary particles surface area number and size distribution of pores degree of crystallinity and polycrystaUinity defect concentration nature of internal and surface stresses and state of agglomeration (27). Chemical and phase composition are needed for complete characterization, including data on the purity of the bulk phase and the nature and quaHty of adsorbed surface films or impurities. 

Alloys having varying degrees of corrosion resistance have been developed in response to various environmental needs. At the lower end of the alloying scale are the low alloy steels. These are kon-base alloys containing from 0.5—3.0 wt % Ni, Cr, Mo, or Cu and controlled amounts of P, N, and S. The exact composition varies with the manufacturer. The corrosion resistance of the alloy is based on the protective nature of the surface film, which in turn is based on the physical and chemical properties of the oxide film. As a rule, this alloying reduces the rate of corrosion by 50% over the fkst few years of atmosphere exposure. Low alloy steels have been used outdoors with protection. 

Films Once corrosion has started, its further progress very often is controlled by the nature of films, such as passive films, that may form or accumulate on the metallic surface. The classical example is the thin oxide tilm that forms on stainless steels. 

Pure aluminum cannot be used as an anode material on account of its easy passivatability. For galvanic anodes, aluminum alloys are employed that contain activating alloying elements that hinder or prevent the formation of surface films. These are usually up to 8% Zn and/or 5% Mg. In addition, metals such as Cd, Ga, In, Hg and T1 are added as so-called lattice expanders, these maintain the longterm activity of the anode. Activation naturally also encourages self-corrosion of the anode. In order to optimize the current yield, so-called lattice contractors are added that include Mn, Si and Ti. 

Since the formation nature and breakdown of protective surface films depends on both material and environmental parameters such influences on erosion corrosion will be discussed together. Particular attention will be paid to the copper/seawater and carbon steel/water (steam) systems. 

Tin when made anodic shows passive behaviour as surface films are built up but slow dissolution of tin may persist in some solutions and transpassive dissolution may occur in strongly alkaline solutions. Some details have been published for phosphoric acid with readily obtained passivity, and sulphuric acid " for which activity is more persistent, but most interest has been shown in the effects in alkaline solutions. For galvanostatic polarisation in sodium borate and in sodium carbonate solutions at 1 x 10" -50 X 10" A/cm, simultaneous dissolution of tin as stannite ions and formation of a layer of SnO occurs until a critical potential is reached, at which a different oxide or hydroxide (possibly SnOj) is formed and dissolution ceases. Finally oxygen is evolved from the passive metal. The nature of the surface films formed in KOH solutions up to 7 m and other alkaline solutions has also been examined. 

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

The highest concentrations of naturally occurring dissolved and particulate organic matter in the oceans are normally found in the surface films. When organic pollutants are present, they too, tend to accumulate in this surface film, particularly if they are either non-polar or surface active. Much of the available information on these surface films is reviewed by Wangersky [8,9]. 

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