Fasteners in Marine Service
Authors: Thomas E.D. , Hogan E.A. , Lucas K.E. 
Published: October 2008

In this study, the galvanic effect of non-ferrous fasteners materials with similar base plate were investigated. Five candidate nickel based fastener materials were immersed in natural flowing seawater at ambient temperature and pressure. These materials were electrically attached to Alloy 625 (N06625) with greater than ten times the surface area.

Corrosion currents and open circuit potential were measured for twenty-eight to one hundred and five days. Corrosion rates were calculated from nominal current densities and are presented. Results indicated that nickel base fasteners alloys may suffer from galvanic corrosion even in cases where open circuit potentials were similar to the structural material.

INTRODUCTION

Traditional use of fasteners in the marine environment encompasses abroad range of conditions. These conditions include full immersion and atmospheric as well as the environs created at the air/water and mud/water interfaces. This is complicated by the dynamics of low or stagnant flow and seawater velocities. Each of these situations would require thorough investigation prior to selection of fastener and structural materials. Typically, the mechanical requirements of the design would limit the fastener choices. The corrosion analysis would evaluate the material for known environmental effects and compatibility with the adjoining structure. As a general rule, the fastener should be cathodic when compared to the structure to avoid an unfavorable anode to cathode surface area ratio.l This means that if corrosion does occur, the structure would be the anode and would corrode.

As corrosion resistant materials find broader use in structural applications, the choice of fastener materials are limited. The fastener material should be cathodic to the base metal and therefore should not corrode. In the case of steel structures, corrosion resistant materials such as the nickel base alloys maybe acceptable choices for fasteners. Strength requirements and resistance to corrosion may indicate the use of Monel K-500 (N0550) as a fastener material. The fastener would probably not corrode due to the cathodic protection it receives from the steel structure. When the structure requires an alloy with higher corrosion resistance, selection of the fastener material will have to account for any change in anode/cathode relationship. The fastener must be compatible with the structure so that the threads or clearance hole would not deteriorate to the point of failure. Comparison of alloys using the galvanic series in seawater may not be a good indicator of performance.

This study is part of a larger investigation to select materials for fastening nickel base alloys. Mechanical properties and other environmental effects are important aspects of the application, but this work was primarily concerned with galvanic compatibility.

EXPERIMENT

Objective

Our primary objective was to identify a fastener material which was galvanically compatible with an Alloy 625 (N06625) structure. Measurement of corrosion current between the fastener materials and the base plate was to provide the essential data. At the same time, other forms of corrosion would be monitored.

Seawater immersion

Seawater tanks constructed of polyethylene and measuring ten inches by fourteen inches with a depth of nine inches were used for exposure of specimens.