During routine maintenance inspection of some of the water wells owned/operated by San Francisco Public Utilities Commission, severe corrosion was observed on the pump columns and on the stainless-steel pump shafts. Due to a phenomenon known as ennoblement of stainless steel the potential of stainless steel in water can become positive, in the range of +300 mV (CSE) which can result in serious galvanic corrosion of the pump column. Casing-to-water potential profiles of the wells were conducted. Our results showed that this ennoblement was occurring in eight out of 13 of the well locations. Ennoblement can cause galvanic corrosion of the pump column if it is electrically connected to the stainless steel casing. Cathodic protection of the pump column and the well casing was considered a feasible option to mitigate galvanic corrosion as well as MIC. Cathodic protection (CP) has subsequently been installed on some of the wells. The corrosion analysis, design considerations and results of the CP survey are included in this paper.
The subject wells are part of a project by San Francisco Public Utilities Commission (SFPUC) to develop a groundwater supply in the South Westside Basin of San Francisco for use during drought conditions for Daly City, San Bruno, and the South San Francisco District. The project includes construction of 13 groundwater wells with a total capacity of 7.2 million gallons per day (mgd). During routine maintenance inspection of two of the wells in Daly City, severe corrosion was observed on the pump columns and on the stainless steel pump shaft. A study was subsequently conducted to determine the cause(s) of the corrosion found on the pump columns and on one stainless steel casing and based on these findings to determine appropriate remedial action to mitigate this on-going corrosion problem. Investigations were conducted to determine how pervasive these corrosion problems are on other wells owned and operated by SFPUC as a part of this drought year groundwater supply project. Casing-to-water electrical potential profiles of the wells were conducted to determine if the stainless-steel casings are being ennobled which could be contributory to the corrosion problem. Tests were also conducted to determine the degree of electrical continuity between the casings and pump columns to establish if galvanic corrosion is contributing to the accelerated corrosion occurring to the pump column and casing. Better understanding these complex issues provided information necessary to the development of a road map to remediating these corrosion problems
