Selective corrosion on nickel-aluminum-bronze on Kaplan type turbine runners in a hydroelectric power plant owing to the nature of the water has been discovered. Primarily, the attack on the runners was castigated by an anodic polarization of the bronze by galvanic coupling to ennobled stainless steel which possessed a higher corrosion potential due to the growth of a biofilm incorporating a strong oxidizing species. This type of corrosion may have been anticipated due to the minerals dissolved in the water combined with the elevated potential. However, the comprehension of the consequences of galvanic coupling between nickel-aluminum-bronze and stainless steel in fresh water is still hazy. Therefore, it is imperative that the effect of the typical anions dominating the chemical composition of fresh water on the corrosion behavior and a possibly existing critical potential for specific phenomena be assessed, qualitatively at least. All attempts to assess this specific corrosion phenomenon by potentiodynamic measurements in the laboratory had failed as the attack could not be reproduced. This can be attributed to the slow kinetics of protective layer formation and stabilization of the localized corrosion, never reaching equilibrium even at slow potentiodynamic scan rates.
Recently, a team of researchers led by Professor Paul Linhardt from Institute for Chemical Technologies and Analytics at TU Wien in Vienna developed a setup for potentiostatic testing of eight specimens at different potentials in a common electrolyte pool. Their main goal was to employ the developed system to assess the effect of chloride, sulfate, and bicarbonate in fresh water-typical concentration on a nickel-aluminum bronze, assuming some anodic polarization by stainless steel with and without microbially induced ennoblement. Their work is currently published in the research journal, Materials and Corrosion.
The research method employed commenced with specimen preparation from an ingot alloy of known chemical composition. Next, the electrolytes to be used for the study were prepared from analytical grade sodium salts of bicarbonate, chloride, and sulfate by dissolving in deionized water to yield specific compositions. For potentiostatic tests with Octopoti, a cylindrical container was used as a test cell and was filled with the electrolyte. Lastly, the tests were carried out and the resulting specimens examined visually and microscopically after preparation of cross sections.
The authors observed that chloride promoted general corrosion while sulfate induces localized corrosion by attacking selectively certain phases. They also noted that the combination of bicarbonate with sufficient amounts of chloride or sulfate led to localized attack above a critical potential. Additionally, they noted that the bicarbonate exhibited an inhibiting effect. More so, they confirmed that a significant susceptibility of nickel-aluminum-bronze to localized corrosion in fresh water appeared at potentials which could be established by galvanic coupling to biofilm ennobled stainless steel.
“Predicting long term corrosion behavior from short term lab experiments is still a challenging task since complex corrosion mechanisms may not always be assessed properly in a short timeframe. Parallelization of experiments is feasible way to overcome this problem.” said Professor Paul Linhardt, lead author.
In a nutshell, Paul Linhardt and his research team successfully presented application of the Octopoti tool in carrying out systematic potentiostatic corrosion experiments. In general, they observed that the potentiostatic testing allowed for the assessment of corrosion systems with slow kinetics with much more practical relevance than potentiodynamic testing. Altogether, this relatively low cost setup saves valuable operator’s time and is easily scalable.
Paul Linhardt, Saskia Kührer, Günther Ball, Maria V. Biezma. Design of a multichannel potentiostat and its application to corrosion testing of a nickel-aluminum bronze. Materials and Corrosion, 2018, volume 69: page 358–364.Go To Materials and Corrosion