High-pH environment

Copolymers of propylene and tetrafluoroethylene, which are sold under the Aflas trademark by 3M, have been added to the fluorocarbon elastomer family (21—26). Also 3M has introduced an incorporated cure copolymer of vinyUdene fluoride, tetrafluoroethylene and propylene under the trademark Fluorel 11 (27). These two polymers (Aflas and Fluorel 11) do not contain hexafluoropropylene. The substitution of hexafluoropropylene with propylene is the main reason why these polymers show excellent resistance toward high pH environments (28). Table 1 Hsts the principal commercial fluorocarbon elastomers in 1993. 

The experiments also indicate that WD may be an important mechanism for producing I/S at low temperatures in nature by a transformation mechanism (56). The percentage of illite layers formed by this mechanism is proportional to the number of WD cycles, and to the layer charge of the original smectite. Simple K-exchange does not produce stable illite layers in smectite therefore, these layers probably form by WD prior to deposition in subaqueous environments. The exception is found in high pH environments where illite layers may form without WD by chemical reaction, as has been reported previously for alkaline lakes (64, 65). 

Site-specific factors can limit the effectiveness of metal-based PRB performance. In high pH environments, nitrate can form ammonium when passing through a PRB. In those same high pH environments, bicarbonate in groundwater can precipitate as carbonate, forming deposits that can reduce the permeability of the PRB. 

Systems which control pH normally employ slaked lime, Ca(OH)2, to precipitate lead and cadmium as insoluble hydroxides. A potential problem with this method is that when excess lime is used, creating a high pH environment, lead can resolubilize. To avoid this problem, the use of magnesium hydroxide, Mg(OH)2, to precipitate lead has been suggested (Turpin 1985). Magnesium hydroxide provides a buffering effect so that the potential of dissolving... 

Temperatures required to adequately produce a molten layer of the surfaces vary from 210°C-290°C. Welds may be particularly susceptible to stress cracking in strongly alkali solutions where discoloration develops as a result of dehydrofluorination. VF2/ HFP fluoropolymers exhibit improved resistance to a high pH environment, where PVDF homopolymers have been known to become brittle over time. 

A major difference between the two detection modes is that detection limits for alkali metal ions were 12-21 times lower with suppressed detection [7]. Nevertheless, cations may be separated and quantified with direct detection down to fairly low concentrations. Another advantage is that the eluent remains acidic and metal does not precipitate as easily as it might in the high pH environment of the suppressor. Figure 7.6 shows a separation of several metal cations including cesium and strontium at concentrations averaging only 1.0 ppm. 

Several materials have been proposed as alternatives to quartz gravel pack materials and have been tested for stability under steam injection conditions. Phenolic resin-coated gravel has been used in thermal wells (79). The coated grains agglomerate with temperature to form a consolidated pack that does not require a screen. Alumina-based materials have been reported to be stable in the high pH environment to over 300 °C (78), but others (80) found that dissolution of these materials can occur. Weaver and Knox (80) proposed an alternate material, SRG, whose chemical formulation is not revealed. A steel wool filter has been used as an alternative to gravel packs in thermal wells (58, 59, 81, 82). 

HMSA has been observed in different environments at concentrations as high as 300 pM near sources of S02 and HCHO (Munger et al. 1984,1986). Its formation explains the relatively high S(IV) concentrations that have been reported in high-pH environments because, we recall, HMSA is a member of the S(IV) family. In such cases the lifetime of dissolved S02(HS03 and SO2-) should be extremely short (Munger et al. 1986), so the measured S(IV) was mostly HMSA and not HSO or SO2-. 

As the Ca(OH)2 leaches from the particle and comes in contact with the unreacted AKD , it creates, with the moisture in the paper, a high pH environment, which can cause hydrolysis of the unreacted AKD and loss of the sizing effect on the PCC particle. This is then able to adsorb water, or ink, and create a desizing effect, although the fibre itself may still be sized. Obviously, the higher the entrapped lime level and the higher the filler content, the greater the effect. 

Fourier transform infrared (FTIR) analysis of the damaged epoxy suggested that phthalate linkages in the 934 epoxy resin had been disrupted, probably by nucleophilic attack by hydroxide ions in the high pH environment. (Resin 934 includes 11.2% diglycidyl orthophthalate to lower the viscosity and improve fibre wet-out). Note that most marine grade polyesters are based on phthalic acid precursors as major constituents, and thus are likely to be even more prone to this form of degradation than epoxies when within cathodically protected systems. 

AEMs containing a quaternary ammonium group as a cation exhibit higher thermal and chemical stability than other quaternary cations such as quaternary phospho-nium or tertiary sulfonium groups, although they are prone to degradation in a high-pH environment by HO nucleophilic attack [492]. 

In the hydration process the Si-O-Si and Si-O-Al bonds between the SiO and AlO tetrahedra constituting the glass phase of the ash are broken down in the highly alkaline environment, and become available for a reaction with the alkaline species of the liquid phase, yielding zeolite-type reaction products. Thus the alkahne activator not only produces the necessary high-pH environment, but also reacts with the original ash. 

Some organic polymers may be combined with inorganic cements to yield materials with unique properties. A precondition for such application is a sufficient stability of the polymer in a high-pH environment, typical for most cementitious systems. 

In the CAC-rich region setting is due to hydration of the calcium aluminate cement, which is accelerated in the high-pH environment produced by the presence of Portland cement in the mix. 

Corrosion is an electrolytic process that requires an anode, a cathode, and an electrolyte. In reinforced concrete the reinforcing steel becomes covered in a passivating layer of y-iron oxide formed in the high-pH environment ( pH 13) provided by the Portland cement. Unfortunately, the high pH can be neutralized by acidic gases such as carbon dioxide in the atmosphere. Once the pH in the concrete drops below 10, generahzed corrosion of the steel will take place in the low-pH area. 

Ramaswamy N, Mukcajee S (2010) Electrocatalysis of oxygen reduction on non-precious metallic centers at high pH environments. ECS Trans 33 1777—1785... 

Note that PVDF may be adequate for high pH environments in applications where the surface degradation and fluoride leaching is not problematic, eg, alkaline waste streams (87)... 

In addition to providing a high pH environment, concrete also has a high resistivity compared to most aqueous environments. Resistivities in water saturated concrete range from 2000 to over 300 000 I2-cm and are even higher in dry concrete [2,3], Correction for high resistance is needed in making electrochemical corrosion rate measurements. 

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