Concentration protective coatings

The final factor influencing the stabiHty of these three-phase emulsions is probably the most important one. Small changes in emulsifier concentration lead to drastic changes in the amounts of the three phases. As an example, consider the points A to C in Figure 16. At point A, with 2% emulsifier, 49% water, and 49% aqueous phase, 50% oil and 50% aqueous phase are the only phases present. At point B the emulsifier concentration has been increased to 4%. Now the oil phase constitutes 47% of the total and the aqueous phase is reduced to 29% the remaining 24% is a Hquid crystalline phase. The importance of these numbers is best perceived by a calculation of thickness of the protective layer of the emulsifier (point A) and of the Hquid crystal (point B). The added surfactant, which at 2% would add a protective film of only 0.07 p.m to emulsion droplets of 5 p.m if all of it were adsorbed, has now been transformed to 24% of a viscous phase. This phase would form a very viscous film 0.85 p.m thick. The protective coating is more than 10 times thicker than one from the surfactant alone because the thick viscous film contains only 7% emulsifier the rest is 75% water and 18% oil. At point C, the aqueous phase has now disappeared, and the entire emulsion consists of 42.3% oil and 57.5% Hquid crystalline phase. The stabilizing phase is now the principal part of the emulsion. 

Zinc is often used as a protective coating over iron to form galvanized iron. In industrial settings exposed to SOj and humidity, this zinc coating is subject to sufficient corrosion to destroy its protective capacity. Haynie and Upham (5) used their results from a zinc corrosion study to predict the useful life of a zinc-coated galvanized sheet in different environmental settings. Table 9-2 shows the predicted useful life as a function of SO concentration. 

GL 1] [R4] [P 2] The stability of vapor-deposited protection coatings made from nickel depends on the process conditions, particularly on the concentrations of toluene and fluorine [14]. Nickel-coated silicon micro reactors were operated for several hours for the reaction conditions given. The nickel films lose to a certain extent their adhesion to the reaction channel with ongoing processing. 

The disappearance half-life of elemental phosphorus in water also depends on the physical state of phosphorus. For example, the disappearance half-life of collodial phosphorus was 80 hours at 30°C and 240 hours at 0°C at concentrations between 10-50 mg/L (Bullock and Newlands 1969), compared to a half-life of 2 hours in solution form at 10 °C (Zitko et al. 1970). The half-life of white phosphorus in solution increased from 2 to 20 hours when the phosphorus was present in the sorbed state in sediment (Zitko et al. 1970). In anoxic water, the estimated half-life of a solid chunk of white phosphorus that was protectively coated due to oxidation/hydrolysis at the oxic zone was 2.43 years (Spanggord et al. 1985). 

Perform nondestructive testing, inspection and maintenance programs to avoid SCC precursors, such as a concentration of stresses by localized corrosion. In case of protective coatings, routine maintenance is essential since scratches could create favorable sites for initiation of SCC. 

Chromium is a white, hard, lustrous, and brittle metal (mp 1903 10°C). It is extremely resistant to ordinary corrosive agents, which accounts for its extensive use as an electroplated protective coating. The metal dissolves fairly readily in nonoxidizing mineral acids, for example, hydrochloric and sulfuric acids, but not in cold aqua regia or nitric acid, either concentrated or dilute. The last two reagents passivate the metal in a manner that is not well understood. The electrode potentials of the metal are... 

The formation of hydrolysis products, in the case of zinc phosphate, depends on the permeability of the protective coating. The permeability of the protective coating itself is influenced by the type of resin used, and in particular, by the PVC (Pigment Volume Concentration). This means that the choice of resin, pigments and fillers and the complete formulation have an important influence on the corrosion protection behavior of protective coatings containing zinc phosphate [5.69]. 

Tungsten, like molybdenum, is very resistant to the action of acids this is largely due to the formation of a protective coating of oxide. Neither aqua regia nor hydrofluoric acid dissolves the metal to any appreciable extent the best solvent for the fused metal is a mixture of concentrated nitric and hydrofluoric acids. ... 

---