Water Journal : Water Journal March 2011
sewer processes refereed paper technical features 76 MARCH 2011 water (1) the acid uptake does not follow Fick's law but demonstrates a history-dependent diffusion. Diffusivities of sulfuric acid through coatings A, B, C and D that were measured according to equation (1) for short periods are compared in Figure 4. Film thickness and polymer orientation Film thickness is one of the coating properties that could affect its permeability. As shown in Figure 4, the diffusion co-efficient of sulfuric acid increased with increasing coating thickness. This is less apparent with coating C, where only a minor increase was observed when coating thickness increased from 8mm to 18mm. Higher rates of eluent diffusion with increasing thickness of coating have been attributed primarily to polymer morphology (Mensitieri et al. 1994, Pongjanyakul et al. 2008, Yang et al. 1986). Higher diffusivity of eluent with thicker films has been attributed to the presence of reduced paracrystalline aggregates (Yang et al. 1986). The degree of order and the orientation of these paracrystalline aggregates vary with film thickness (Isoda et al. 1981, Sroog 1991). Thinner films are characterised by a higher chain alignment in the plane of the film, resulting in higher tortuosity effects compared to thicker films, resulting in reduced diffusivity (Mensitieri et al. 1994). Whereas the coating thickness increases, a looser polymer matrix is observed (Pongjanyakul and Puttipipatkhachorn 2008). In coating C, it appears the degree of chain alignment is maintained within its various thicknesses. The observed higher diffusivity of sulfuric acid with thicker coatings has considerable implications to sewer protection. Thicker coatings are currently recommended when corrosion conditions are severe. In considering the results obtained here, the level of protection offered by thicker coatings needs to be further re-examined. Filler properties The sulfuric acid permeability (k) of the coatings is indirectly related to the tortuosity (τ) imposed by the filler (k ~ 1/τ). Tortuosity is defined as the actual distance travelled in the presence of the filler (d') to the shortest distance travelled through the coating in the absence of the filler (d). This assumes the fillers are inert to the permeating acid and are relatively non-porous. Figure 5 shows a simple model of tortuosity path provided by the filler in the coating matrix. Parameters including the filler orientation, concentration (φ) and dispersion and aspect ratio (L/W, where L is the thickness and W is the width of the filler) all define Figure 7: Effect of filler size on sulfuric acid diffusivity in various coating thicknesses in equation 4. Figure 6: Effect of filler concentration on sulfuric acid diffusivity in various coating thicknesses. Figure 5: Tortuosity model (Lu et al. 2005). Figure 4: Apparent diffusivities of sulfuric acid in epoxy mortar coatings.
Water Journal April 2011