Water Journal : Water Journal March 2011
refereed paper sewer processes water MARCH 2011 75 (1) where M∞ is the maximum acid uptake in the sample mass, D is diffusivity and l is the sample thickness. Experimental Epoxy mortar coating specimens In this study four commercially available epoxy mortar coatings (A, B, C and D) were selected. The coatings were prepared and cured by the manufacturer to produce coupons with dimensions of 50 x 50mm and thicknesses of approximately 5, 10 and 20mm. Each coupon was tested to ensure it was free from pinholes and cracks, prior to acid immersion tests using a PCWI- DC30 holiday detector according to the method described in ASTM G62-87. Acid permeability by gravimetric method Acid permeation was measured gravimetrically by immersing the coating coupons in 10% (w/w) sulfuric acid for periods of up to six months. The concentration of sulfuric acid used for this accelerated testing is based on the average observed surface pH of corroding sewer surfaces. Annual H2S concentrations of over 30ppm in air have been reported to generate a corresponding surface pH measurement of 1.5 to 2.5 and a sulfuric acid concentration of 5--7%. H2S concentrations in excess of 50ppm in air have also been reported to have a pH as low as 0.50 and greater than 7% sulfuric acid concentration (Nixon 1997). The concentration of H2S found in utilities on the eastern coast of Australia varies from 5--50ppm and those on the western coast from 100--900 ppm. In this study, 10% H2SO4 was considered an appropriate approximation of the average acid concentration that would be generated in the sewer. The weight uptake was determined at various periods of time for up to six months by removing the coupons, wiping off all surface acid and immediately weighing the coupons. The acid weight uptake (Mt) was measured from: (2) where Wt and Wo are the weight of the wet and the original coupon respectively. Scanning electron microscopic studies Surface morphology of coatings was observed using scanning electron microscopy. Samples were mounted onto stubs and sputter-coated with gold in a vacuum evaporator, and were then examined using a scanning electron microscope (SEM) (Philips XL 30 CP). An Energy Dispersive X-ray Spectroscopy (EDS) linked to the SEM was used in the elemental analysis of the epoxy mortar surface. X-ray fluorescence (XRF) The bulk elemental analysis of the coatings was determined by fusing the minerals with lithium borate followed by XRF (Philips PW2404) analysis. Results and Discussion Coating morphology and filler size Scanning electron microcopy (SEM) examination of the backscattered images of epoxy mortar surfaces are shown in Figure 1 (A--D). The cracks and holes are indicative of air bubbles within the epoxy during epoxy curing. Bright spots in the image reflect high molecular weight inorganic fillers. The estimated particle size analysis and the corresponding elemental analysis of the fillers are summarised in Table 1. Diffusion co-efficients of sulfuric acid Acid uptake The sulfuric acid uptake of coating D is shown in Figure 2. As shown, the acid uptake appears to follow the Fickian behaviour. However, when plotted in Figure 3 against t0.5 according to equation Table 1: Summary of coating microanalysis by EDS and XRF. Filler Characteristics Coating Principal Fillers Shape Size Range Mean Size Filler Concentration (φ) μm μm % (w/w) A SiO2 (83%), CaO (2.9%) Spherical 5--250 50 87.1 B BaO (17%), TiO2 (5.1%), SiO2 (2.37% ) Al2O3 (1%), CaO (1%), Spherical 5--10 5 30.5 C SiO2 (7% ), Al2O3 (3%), TiO2 (1% ) Spherical 50--200 x 10-3 100x10-3 12 D SiO2 (36%), MgO (1.6%) Cubic & Spherical 10--200 100 40.8 Figure 3: Fractional uptake (Mt/M∞) vs. t1/2 for coating D in 10% (w/w) H2SO4. Figure 2: Acid uptake (Mt) for Coating D in 10% (w/w) H2SO4.
Water Journal April 2011