Polycarboxylic acid, a scale control inhibitor has demonstrated adsorption properties on stainless steel surfaces. An electrochemically-based technique has been used to assess the extent of film formation. Calcium concentration in the solution and the cathodic electrochemical activity at the metal surface have been shown to enhance the inhibitor film formation by promoting the transport and adherence of the inhibitor. Practical implications of this findings are discussed.
Formation of mineral scales, such as barium sulfate, calcium carbonate and calcium sulfate, on heat exchangers, reverse osmosis membrane surfaces and petroleum production equipment surfaces is a persistent and a e x~pensive problem in cooling water systems, boilers, secondary oil recovery and desalination plants -. Deposition of these materials can lead to loss of system efficiency, shutdown and ultimately equipment failures 4.
The inhibition of precipitation and deposition of sparingly soluble salts, is important in many fields. Polyelectrolytes type additives are commonly applied to retard the unwanted deposition of mineral salts 5-7. Such inhibitors have been identified as crystal habit modifiers and nucleation preventers for the common oilfield scales. They are primarily designed for their ability to form coordinative bonds with the cations at the crystal surface. Benton et al. 8 showed that it is only necessary to have as little as 5% of the surface area covered by adsorbed rnolecules in order for complete inhibition of BaSO 4 growth to occur. Crystal nucleation and growth inhibition can exhibit complex mechanisms, in the solution as well as at the equipment surface.
Several workers 9'1° have reported that organic compounds demonstrate a tendency to form a protective film at metallic surfaces. This property is associated with the ability of polyanionic compounds to inhibit corrosion. It is often shown that these compounds can display dual properties, as scale control inhibitors as well as corrosion inhibitors I H3. There is a lack of information in the literature to explain the inhibitor mechanisms transport to the metallic surface. As stated by Mueller et al., the adsorption quality displayed by polyelectrolytes can explain their scale inhibition properties (especially on nucleation and agglomeration of scale) at metallic surfaces ~.
Formation of an inhibiting film has been investigated previously using mainly spectroscopy techniques, Auger spectroscopy 12, Fourier-transform infrared spectroscopy z4 and X-ray Photoelectron Spectroscopy 12, Raman Spectroscopy ~5. The contact electrical resistance technique (CER) allows inorganic and organic adsorption to be studied by following the electrical resistance of metal surface layers 15. AC impedance was used by Neagle to give a semi-quantitative assessment of the film formation rate with time ~4. These techniques listed above allow the presence of a film to be determined and can identify the main elements present a the surface. In the authors' knowledge, none of these methods quantify the film coverage at the surface or its properties in terms of the ability to block diffusion of species such as dissolved oxygen to the surface. The electrochemical technique presented in this paper directly addresses the issue relating to the relationship between the film coverage and film formation conditions.
The work reported in this paper uses an electrochemical technique, which has been introduced and described in full elsewhere 1637. The technique uses analysis of the oxygen reduction reaction at a rotating disk electrode (RDE) surface under potentiostatic control, to investigate the initial stages of film formation in the presence
