Marine Corrosion Performance of Copper Alloy UNS C69100

ABSTRACT

Tungum alloy (UNS C69100) is an aluminum-nickel-silicon brass and is reported to have a good corrosion performance in marine environments (fully wetted, splash zone and atmospheric conditions). In order to gain an in-depth understanding of the marine corrosion performance of this alloy, electrochemical test methods including open-circuit potential, electrochemical impedance spectroscopy, potentiodynamic polarization, and zero-resistance ammetry were used for corrosion investigation of UNS C69100 in a 3.5 wt.% sodium chloride aqueous testing solution, in combination with optical microscopy and scanning electron microscopy. The corrosion properties of UNS C69100 obtained by electrochemical methods were also compared with six alternative alloys: UNS S31603, UNS S31254, UNS S32750, UNS N04400, UNS N08904 and UNS C36000. Galvanic coupling behavior of wrought UNS C69100 bar and seamless tubing against these six alloys in a 3.5% NaCl solution for 30 days immersion are also reported in this paper.

INTRODUCTION

Tungum alloy⁽¹⁾ (UNS C69100) is an aluminum-nickel-silicon brass (chemical composition: 81-84% Cu, 0.70-1.20 Al, 0.8-1.40 Ni, 0.80-1.30 Si, with the remainder Zn) and is reported to have good corrosion performance in marine environments (fully wetted, splash zone and atmospheric conditions) from many years' service experience. UNS C69100 tubing was reported to have best localized corrosion resistance among six metallic materials (316L, Alloy 825, 317LMN, 254 SMO, Alloy 625 and UNS C69100), judged by crevice corrosion and pit depths when exposed to a cyclic salt fog environment (ASTM D 5894 – alternating 1 h wet and 1 h dry conditions, with temperatures varying between 35 °C and 45 °C).¹ In the same report, eight alloys (316L, 317 LMN, Duplex, 254 SMO, Alloy 825, Alloy 625, Alloy 400 and UNS C69100) underwent field trials on two offshore platforms for one year. Again C69100 performed well, although there was evidence of shallow pits but with a relatively high pit density, the UNS C69100 outperformed 316L (30 µm vs. 90 µm seen on 316L). Overall, C69100 was recommended as a good alternative to 316L for small bore tubing in marine applications.¹ To date, there appears to be scant data available about the corrosion behavior and galvanic performance of UNS C69100 alloy in marine environments. La Que² has ranked the open-circuit potentials of many metals and the effects of coupling common alloys in seawater. This ranking on its own is often not necessarily sufficient to enable a satisfactory materials selection free from galvanic corrosion since it neither considers cathodic efficiency nor the underlying corrosion mechanisms of the couple components.³ The marine corrosion behavior of the least noble or anodic component is thus important. For instance, unlike stainless steels, copper alloys in seawater do not have a truly passive layer but rely on a tenacious protective oxide film, which limits the rate of metal dissolution, although copper-based alloys generally suffer general corrosion which is increased when coupled to a more noble metal / more corrosion resistant alloy. Wallen and Andersen⁴ reported that most copper alloys when coupled to equal areas of stainless steel have increased corrosion rates by a factor of seven in natural seawater. However, since the free corrosion (single metal / uncoupled) is generally very low, the increase can often be acceptable, especially for thick walled components.