Galvanic Corrosion of CuNi-Sheathed Steel Pilings in Seawater

Abstract

This paper presents the results of a study conducted to determine the extent of galvanic corrosion that would be caused by copper-nickel sheathing of offshore steel platforms. The study investigated the effects of selected steel-to-CuNi area ratios on the magnitude and distribution of the galvanic current. One year seawater exposure tests utilizing segmented CuNi-steel pilings were carried out near Ocean City, New Jersey. The results of the study suggest that, for piling configurations and lengths of copper-nickel sheathing similar to those included in the study, the maximum galvanic corrosion rate of the steel will be between 0.2 and 0.3 mm/y, immediately next to the sheathing. The galvanic attack will be confined primarily to the first few meters of piling near to the sheathing. Beyond this, the intensity of galvanic attack will decline markedly. INTRODUCTION In order to achieve better corrosion control, steel pilings are being sheathed with copper-nickel in the tide or splash zone of offshore platforms. The use of copper-nickel sheathing will promote galvanic corrosion of the steel piling. The intensity of the galvanic attack and the length of steel piling over which it might occur are uncertain. As a result, a study was undertaken to investigate the severity of galvanic corrosion that might be expected on a copper-nickel sheathed steel piling. The specific objectives of the study were:To determine the intensity of attack as a function of various steel-to-CuNi area ratios.To determine the effective length of steel piling that would be involved in the galvanic cell.To provide a basis for estimating cathodic protection requirements for a copper-nickel sheathed steel piling. EXPERIMENTAL APPROACH Conceptual Design To develop data that would provide a meaningful basis for the galvanic corrosion likely to occur on a copper-nickel sheathed offshore steel platform, studies were conducted utilizing segmented test pilings immersed in natural seawater. Figure 1 shows the general arrangement of the test piling. This arrangement provided the ability to investigate various anode-to-cathode (steel-to-CuNi) area ratios and the distribution of the galvanic current. Different anode-to-cathode area ratios were investigated by exposing differing lengths of copper-nickel and/or steel. The distribution of the galvanic current was determined by measuring the current flow from individual steel segments. As shown in Figure 1, the segmented piling consisted of a length of copper-nickel pipe (Lc) joined to nine segments of steel pipe. To determine more accurately the corrosion rate and current distribution immediately adjacent to the copper-nickel cathode, the first five segments were only 1/5 the length of each of the last four segments. Six pilings were tested. Table 1 summarizes the piling configurations selected for testing. Piling Construction The test pilings were constructed from nominal 5-cm diameter 90-10 copper-nickel and mild steel pipe. Prior to assembly, all of the mild steel test sections were abrasive blasted to remove all mill scale and provide a consistent initial surface condition. The segments were joined by nonconductive PVC couplings.