The relationship between alloying elements and biologically produced ennoblement in natural waters

Abstract

A range of stainless steels, nickel–chromium and nickel–chromium–molybdenum alloys were exposed to coastal seawater from Mandapam (Indian Ocean) and freshwater from a perennial pond. Biofilms from both test waters produced an ennoblement of the open circuit potential (OCP) on all alloys as expected, which was slower but substantially larger in freshwater. In both waters an interesting relationship was perceived between the plateau OCP (E_max) and the mass percentage of the major alloying elements_. In particular, iron exhibited strong positive correlations with E_max (r² ≥ 0.77; p < 0.0005), while the sum of chromium, nickel and molybdenum presented significant negative correlations (r² ≤ –0.81; p = 0.0002). Consistent with the regression analyses, Euclidean distance clustering yielded patterns where Inconel-600 and the nickel–chromium–molybdenum alloys had the smallest similarities of OCP with other alloys. The results emphatically reinforce a key role for surface passive films in the ennoblement phenomenon in natural waters.

Keywords:

• stainless steels
• nickel–chromium alloys
• nickel–chromium–molybdenum alloys
• biofilms
• ennoblement
• cathodic kinetics

Acknowledgements

The authors thank the Director, CSIR – Central Electrochemical Research Institute, Karaikudi, India for encouragement and support. Special appreciation goes to T. Jeyaram and L. Subbiah for their invaluable technical assistance at the Mandapam Laboratory.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental material

The supplemental material for this paper is available online at http://dx.doi.org/10.1080/08927014.2015.1057128.

Additional information

Funding

This work was funded by the Council of Scientific and Industrial Research, New Delhi, India under the MULTIFUN theme of the 12th Five Year Plan Programme [grant number CSC-0101F].