New insights are presented on the speciation of surface oxide phases on fine inert gas atomised (GA, ,45 and ,4 mm) and water atomised (WA, ,45 mm) stainless steel AISI 316L powders. Xray photoelectron and Auger electron spectroscopy, scanning electron microscopy, Raman spectroscopy, and cyclic voltammetry were applied for the characterisation. Oxidised manganese was strongly enriched in the outermost surface oxide of the GA powders (13 and 47 wt-%), an effect increasing with reduced particle size. Manganese and sulphur were enriched in oxide nanoparticles on the surface. Oxidised silicon (59 wt-%) was enriched on the WA powder surface. Tri-or tetravalent manganese oxides were observed on the GA particles in addition to a-Fe 2 O 3 , and Cr 2 O 3 . The oxide of the WA powder revealed in addition the likely presence of a silicate rich phase, mainly consisting of tetravalent Si, di-and/or trivalent Fe, and hexavalent Cr, which was confirmed not present as chromate.
Localized phase selective corrosion of turbine runners made from the bronze CuAl10Fe5Ni5 was observed in a hydroelectric power plant. Routine investigation of the corrosion system did not provide a satisfactory explanation for this specific type of corrosion which seems uncommon to occur in freshwater. Corrosion potential measurements of the stainless steel runner rings, galvanically coupled to the runners, indicated ennoblement, creating an effective galvanic couple driving the bronze to corrode. This mechanism was verified by on‐site, in situ electrochemical experiments. Biofilm formation by diatoms was identified as the cause of ennoblement.
A setup for potentiostatic testing of eight specimens at different potentials in a common electrolyte pool was developed. The potentiostat requires only one reference electrode and one counter electrode to control all working electrodes and its design is easily scalable with respect to the number of channels. The system is applied 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. Chloride and sulfate activate corrosion, bicarbonate has a passivating effect, and its combination with chloride and sulfate promotes localized corrosion. The susceptibility of nickel‐aluminum bronze to localized corrosion may be high in fresh water when polarized anodically. Potentiostatic corrosion testing may yield practically relevant results for systems with slow kinetics.
Manganese oxidizing microorganisms are known as ubiquitous species in soil and fresh water. Their ability to extract dissolved manganese even at minute concentrations from the water and to biomineralize it as manganese(III/IV)oxides makes them potentially relevant for corrosion processes in technical systems carrying freshwater. These oxides are known as strong oxidants and may act as catalyst for the oxygen reduction reaction. Thus, they are cathodically active, possibly driving anodic metal dissolution processes. The personal experiences over two decades from failure analysis related to these organisms have indeed shown that manganese oxidizers may appear in all kinds of freshwater systems. This paper summarizes observations and conclusions drawn from these cases and provides an overview on the methods found useful in their investigation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.