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45 NACE INTERNATIONAL: VOL. 55, NO. 12 MATERIALS PERFORMANCE DECEMBER 2016 Thiobacillus sulfur-oxidizing bacteria (SOB), capable of oxidizing H 2 S to H 2 SO 4 , are ubiquitous in nature and always pres- ent on the crown (internal top surface) of sewage pipes. Because of the aerobic condi- tions normally prevailing above the waste water in sewers, the SOB oxidize the H 2 S to H 2 SO 4 . The latter, being a strong acid, then attacks the concrete, cast iron, or steel. Numerous methods for prevention and control of sewer corrosion are available and discussed in the technical literature. Those methods include injection of oxygen, hydrogen peroxide (H 2 O 2 ), iron and zinc salts, potassium permanganate, sodium ni- trate (NaNO 3 ), ozone, bacterial culture and enzymes, and sodium hydroxide (NaOH). 5 These methods are temporary and typically only last for a few hours to a few months. The crown spray process was first in- vented by Edward Esfandi in 1986. 6 In 1989, a 24% NaOH solution was sprayed to in- crease the sewer crown pH, but test data showed that it dropped back to pH 4 or lower in only 60 days or less. Since 1994, 50 to 60% magnesium hydroxide [Mg(OH) 2 ] alone has been used by public agencies in the United States to deactivate sulfur and control H 2 SO 4 on the pipe crown surface. The purpose of the crown spray process is to leave residual alkalinity on the sewer crown. It is known that the effectiveness of the c u r re n t c o n t r o l m e t h o d — 5 0 t o 6 0 % Mg(OH) 2 alone using the crown spray pro- cess—was measured, which found that the sewer crown surface pH rose from approxi- mately 2 before control to 9 after control and then gradually decreased with time to the original pH. This demonstrated that the lifetime of Mg(OH) 2 alone is only maximum about seven to nine months. 1,7 The basic crown spraying system con- sists of a spray head mounted on a f loat. The f loat is pulled through the sewer at a controlled rate to spray the crown at a pre- determined application rate. The spray head receives its chemical from a chemical feed tank located aboveground at an up- stream or insertion manhole. A chemical feed pump delivers the chemical through a hose from the feed tank to the spray head. FIGURE 1 Sewer corrosion due to H 2 S oxidation. FIGURE 2 Equilibrium diagram of H 2 S concentration—pH dependence.