Cerium compounds have been identified as leading candidates to replace hexavalent chrome as conversion coatings on aluminum alloys to improve corrosion resistance. Cerium also shows promise for use as an inhibiting pigment in paint systems. The cerium conversion coatings can be deposited using either spontaneous or nonspontaneous electrolytic processes. In both cases the protective cerium oxide film forms by a precipitation mechanism that is very dependent on electrochemical potential and pH. The oxidation state and phase of the condensed cerium has been shown to be an important aspect of the corrosion protection properties provided by the film. Because of the strong influence of the solution chemistry and operating parameters on film performance, a basic knowledge of the system stability is essential. Toward this end, a revised E-pH diagram was developed for the normalCe­H2O system. The normalCe­H2O­HClO4 system was chosen as an example system in which hydroxy ions are the only significant complexing species for the Ce ions. A stability diagram was constructed using more recent thermodynamic data on cerium species not available for the original diagram published in Pourbaix’s atlas. Significant differences were noted between the previously published normalCe­H2O diagram and the one presented here. Precipitation tests were carried out to verify the trends indicated in the new diagram. The importance of the updated E-pH diagram in understanding the formation processes of cerium conversion coatings are discussed. © 2002 The Electrochemical Society. All rights reserved.

The use of rare-earth metal compounds and oxides to improve the corrosion resistance of aluminum alloys in aircraft applications continues to show promise as an attractive alternative to chromate conversion coatings. Cerium conversion coatings used for metal protection can be deposited by either spontaneous or nonspontaneous processes. In both cases the protective cerium oxide film forms by a precipitation mechanism that is electrochemically initiated. As a result, the process is very dependent on the potential and pH of the solution. In an initial study, a revised E-pH diagram was developed for the Ce-H 2 O system ͓J. Electrochem. Soc., 149, C623 ͑2002͔͒. Precipitation studies were also conducted to verify selected regions of the diagram. In the actual cerium conversion coating processes, an oxidizer, such as hydrogen peroxide ͑H 2 O 2 ͒, is normally used to convert Ce͑III͒ to Ce͑IV͒. Therefore, an E-pH diagram for the Ce-H 2 O-H 2 O 2 system is essential to understand and predict the factors in the cerium precipitation in the presence of H 2 O 2 . In this paper, the trends and pH regions of oxidation/reduction reactions of Ce͑III͒ and Ce͑IV͒ with H 2 O 2 were calculated and evaluated.

SynopsisPhase diagrams for monomer (methyl acrylate containing microemulsions) are shown and described. Polymnrizations of a series of microemulsified monomer reveals the expected linear dependence of 1/DP vs. [S]/[M] for pentanol acting as a chain transfer agent and giving a value of 5.1 X for C,. No break in molecular weight behavior was shown as a result of micelliition at higher water contents. A comparison of molecular weights obtained by various classical methods (solution, emulsion, bulk) are also given. A or about 1/1000 the size of normal emulsions. These systems are usually prepared with water, surfactant, cosurfactant, and a hydr~carbon.~?~We considered it of interest to expand the area of polymerization research to thermodynamically stable microemulsion systems that are colloidal dispersions of water in an organic medium.6-8 The nature of these microemulsion systems and their structure has been discussed intensely over the last five years after it was realizeds that the earlier treatment, which emphasized negative interfacial tension,l@-12 was an oversimplification. The phase diagram ~o n c e p t~J~-~~ and the more complete theoretical treatment of these phenomena16J7 in later years have provided a sufficiently reliable basis for the use of microemulsions in polymerization systems.Recently we reportedls on the polymerization of water-in-oil microemulsion systems that contain methyl methacrylate. This article shows typical phase regions for thermodynamically stable W/O microemulsions of methyl acrylate for polymers. It also gives the molecular weights of polymers obtained in different parts of the microemulsion region and presents preliminary conclusions on the influence of the polymerization of the association aggregates present in the microemulsion. A recent contribution from our combined research efforts which deals with the polymerization of liquid crystalline materialslS has been expanded for presentation in an accompanying article.20

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SYNOPSIS:The emulsion polymerization of methyl methacrylate initiated by ultrasound has been studied at ambient temperature using sodium lauryl sulfate as the surfactant. The investigation includes the: (1) nature and source of the free radical for the initiation process; (2) effects of different types of cavitation; and (3) dependence of the polymerization rate, polymer particle number generated, and the polymer molecular weight on acoustic intensity, argon gas flow rate, surfactant concentration, and initial monomer concentration. It was found that the polymerization could be initiated by ultrasound in the emulsion systems containing methyl methacrylate, water, and sodium lauryl sulfate at ambient temperature in the absence of a conventional initiator. The source of the free radical for the initiation process was found to come from the degradation of the sodium lauryl sulfate, presumably in the aqueous phase. The weight average molecular weight of the poly(methyl methacrylate) obtained varied from 2,500,000 to 3,500,000 g mol Ϫ1, and the conversion for polymerization was up to 70%. Deviations from the Smith-Ewart kinetics were observed. The polymerization rate was found to be proportional to the acoustic intensity to the 0.98 power; to the argon gas flow rate to the 0.086 power; to the surfactant concentration to the 0.08 power, with the 0.035M-0.139M surfactant concentration range; and to the surfactant concentration to the 0.58 power, with the 0.139M-0.243M surfactant concentration range. The polymerization rate was found to increase with increasing initial monomer concentration up to a point where it became independent of initial monomer concentration. The polymer particle number generated per milliliter of water was found to be proportional to the acoustic intensity to the 1.23 power; to the argon gas flow rate to the 0.16 power; to the surfactant concentration to the 0.3 power, with the 0.035M-0.139M surfactant concentration range; and to the surfactant concentration to the 1.87 power, with the 0.139M-0.243M surfactant concentration range. The polymer weight average molecular weight was found to be proportional to the acoustic intensity to the 0.21 power, and to the argon gas flow rate to the 0.02 power. It was found to be inversely proportional to the surfactant concentration to the 0.12 and 0.34 power, with the 0.035M-0.139M and the 0.139M-0.243M surfactant concentration ranges, respectively. The polymer yield and polymerization rate were found to be much larger than those obtained from an ultrasonically initiated bulk polymerization method. The polymerization rates obtained at ambient temperature were found to be similar to or higher than those obtained from the conventional higher temperature thermal emulsion polymerization method. This investigation demonstrated the capability of ultrasound to both initiate and accelerate polymerization in the emulsion system, and to do this at a lower temperature that could offer substantial energy savings.

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