ABSTRACT

Environments where a continuous electrolyte does not exist can be a problem in providing adequate cathodic protection to a pipeline or other structure. A method where the anode and electrolyte are integrated with the structure to be cathodically protected solves this problem. This paper discusses a project where the system was installed onto a coated steel pipeline and the assembly installed along a rocky mountainous right-of-way.

INTRODUCTION

Cathodic protection is defined as either (1) the reduction of corrosion rate by shifting the corrosion potential of the electrode toward a more negative oxidizing potential by applying an external electromotive force or (2) the partial or complete protection of a metal from corrosion by making it a cathode, using either a galvanic or an impressed current. Cathodic protection is accomplished by causing a flow of direct current (DC) between another electrode (called the anode) and the structure (called the cathode). In all cases, both electron and ionic currents are involved in cathodic protection. That is, both a continuous common electrolyte and a metallic connection between the anode and cathode are required. Soil, water, or other conductive medium provides the common electrolyte. In cathodic protection practice, anodes are installed in the electrolyte where the anodic reactions take place. If a separation in the electrolyte occurs between the anode and cathode, the transfer or flow of current will stop, and cathodic protection will not occur. Conditions where the electrolyte becomes discontinuous between anode and cathode can occur in practice. Examples of situations where this could occur include: backfill materials that are too coarse to permit intimate contact of the electrolyte with the structure surface (e.g., gravel, rocks), where fine backfill is washed away from the environment surrounding the structure from by groundwater flow leaving voids against the structure containing only air, and in exposed situations where fine backfill is not possible or practical to place.

The operator was faced with a problem as was just described with the need to replace 1,960 ft (597 m) of 8-in. (20.3 cm) steel petroleum products pipeline situated in rocky terrain in north central Pennsylvania. The existing coated pipeline was cathodically protected by a remote anode impressed current system and pipe-to-soil potential measurements indicated that the pipe was protected. However, the rocky terrain left many areas of the pipeline not in contact with the soil (Figure 1). This meant that these areas were not receiving cathodic protection because they were shielded from receiving adequate cathodic protection by the rocks, or were subjected to possible atmospheric corrosion. Despite the rocky environment, corrosion was still a problem. Previous weld repairs to dense and deeply pitted areas were evident at one end of the pipeline. One possible solution was to use fine soil as backfill around the pipe, but in this mountainous terrain, the cost of hauling soil to the site is very high. Another problem where steep slopes are involved is keeping the fine soil in place. It had been Sun Pipeline's experience that the soil would wash away, thus exposing the pipe again. A better method of applying cathodic protection was needed.

DISCUSSION

Fabrication

Since the problem was one of maintaining a continuous electrolyte between the pipe and anode that will stay in place and be economical to install, the use of a pre-installed anode in a concrete jacket was explored. The type of concrete jacket used as a weight coating for submerged pipe was considered as the appropriate medium for the anode. The concrete serves as the common electrolyte between the ano

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