Water Well Journal

July 2016

Water Well Journal

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is the electrochemical process at the surface of the metal. Small differences in metal composition can create anodes and cathodes that facilitate the corrosion process. The basis of many forms of corrosion is the electrochemical reaction involving an anode and a cathode, and an electrolyte producing the components for ion transfer. In a groundwater well system, the anode and cathode are provided by the vari- ous metals of construction and the electrolyte is the water. Metals are arranged into a series with the less noble metals at the top and the more noble ones at the bottom. This type of arrangement is known as the galvanic series (Figure 1). In the electrochemical reaction, the less noble metals will become the anodic site and sacrifice their metal ions to the more noble metals at the cathodic site. As noted in the series, low carbon steel is less noble than stainless steel and will sacrifice ions to the stainless steel ca- thodic site if the electrolyte—water—provides an adequate electrical connection. This activity is referred to as dissimilar metal corrosion and can be addressed in the material selection process of the well design. Concentration cell corrosion, also referred to as under de- posit or oxygen cell corrosion, actually occurs in the absence of oxygen. This electrochemical reaction develops underneath deposit buildup on the metal surface and produces a localized pitting damage. This type of corrosion is the most rapid and damaging to the well structure and equipment and is greatly influenced by the ion concentration of the aquifer water. Understandably, periodic removal of deposits from the metal surfaces within the well will reduce the occurrence of this type of corrosion. Microbial influenced/induced corrosion is material degra- dation caused by bacterial activity. This includes their produc- tion of acids and enzymes, pitting, and the most common degradation by iron oxidizing bacteria. In relationship to the prior-discussed concentration cell or under deposit corrosion, the predominant cause of this type is bacterial growth. As a residual problem within the water well environment, it is not only the material degradation of iron oxidation but also the accumulation of this oxidized iron into the bacterial slime and fouling the flow paths within the well structure. Evaluating Corrosion Within the groundwater industry we have standardized our initial evaluation of corrosion potential on the saturation index of the aquifer water. The Langelier Saturation Index is the most common calculation used. However, the Ryznar Index is similar and uses the same set of parameters to make the evaluation, but Dr. John Ryznar modified the Langelier calculation based on actual field results he studied over time. The end result of these calculations is a determination if the water is saturated and will form mineral scale, or if it is undersaturated and will be aggressive or corrosive. With the understanding these saturation indexes require laboratory analysis to generate some of the parameters required by the index calculation, a fast and simple field test can be per- formed to assess general corrosion of the aquifer water, the Nail Test. This test uses a low carbon nail placed in a glass container of aquifer water and observed for 24 hours. If a pink color de- velops within the water or around the nail in the first three to five hours, the water is determined to be fairly corrosive. Defining Disinfection Disinfection is the application of energy or chemistry to kill pathogenic organisms. The need for this process was initi- ated around 1900 when the science community established the cause of worldwide epidemics of typhoid fever, cholera, and dysentery was waterborne bacteria. The response to this need was the application of oxidation chemicals. Although there are a variety of biocidal chemicals avail- able, not all are applicable to the potable water industry and therefore oxidation chemicals have evolved as the standard form of water disinfection. The oxidation chemicals seen in the groundwater industry to varying degrees are chlorine Figure 1. Figure 2. OXIDATION from page 25 waterwelljournal.com 26 July 2016 WWJ

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