The corrosion resistance of technical passive alloy is often limited by chemical or structural heterogeneities that might act as potential initiation points for localized corrosion. Micro- and nanoelectrochemical techniques are powerful methods to study the local phenomenas occurring at such heterogeneities. This paper gives a overview of local electrochemical techniques, which are applied to investigate initiation and propagation processes of localized corrosion. A local technique which uses glass microcapillaries as electrochemical microcell is presented. The possibilities and the limitations of the technique are demonstrated. The results of pit initiation studies at sulfur inclusions on 304 stainless steels demonstrate the potential of local corrosion experiments. Corrosion measurements in different scales showed different information. Electrochemical experiments using capillary with a tip diameter in the range of 100μm allowed evaluating the corrosion behavior of sites that contained a single weak point. Electrochemical measurements using capillary with a tip diameter in the range of 1μm allowed investigating different spots of a single weak point. Hence the corrosion behavior of the weakest zone can be determined.
The highly kinetic nature of the corrosion process often leads to the formation of metastable corrosion product layers and alloy surface compositions sometimes vastly different from that of the bulk. In order to understand the corrosion process it is often necessary to combine surface and interface compositional and structural studies alongside more traditional electrochemical kinetic studies. In this review we discuss corrosion research work performed in our laboratory which has utilized laboratory based as well as synchrotron spectroscopic techniques. We shall discuss issues related to the strengths and weaknesses of several spectroscopic techniques using examples of our work related to stainless steels and aluminum-copper alloys.
Sulfidation behavior of a thermal barrier coating consisted of CoNiCrAlY and YSZ (ZrO2-8mol%Y2O3) coating, prepared by low pressure and air plasma sprayings, respectively, on a nickel based heat resistant alloy was investigated in a H2S-H2 atmosphere with a 10-0.65Pa sulfur partial pressure at 1173K for 43.2ks. The corrosion product with a network structure was identified by X-ray diffraction and electron probe microanalyses to be Zr3Y4S3.5O9.4, which was formed from the Zr3Y4O12 by replacement of oxygen by sulfur, having the elongated c-axis and the shrunken a-axis in contrast to those of the Zr3Y4O12. Sulfur and/or hydrogen sulfide gases were assumed to diffuse through flaws such as cracks and voids across the YSZ coating and reacted with Cr to form Cr-sulfide surrounding the aluminum-rich oxide layer at the YSZ/CoNiCrAlY interface.
The anodic dissolution mechanism of copper in weak alkaline carbonate solution containing chloride ion was investigated using channel flow multi-electrode. Anodic polarization curves of copper indicated three Tafel regions. The region I has a slope of 60mV/decade in the potential range from-0.3 to-0.1V and the current in this region depended on pH (∂log i/∂pH=1). The region II has a slope of 60-120 mV/decade in the potential range from -0.1 to 0.1V and the current in this region depended on concentration of chloride ion (∂log i/∂log [Cl-]=1). Active dissolution mechanism of copper in weak alkaline carbonate solution containing chloride ion was proposed on the basis of these results. The chronopotentiograms indicated that film formation of Cu (I) in region I and Cu (II) in region II occurred during dissolution of Cu+ and Cu2+at steady state.
The all volatile treatment (AVT) of boiler feed water where the dissolved oxygen content is maintained at levels of less than 7 ppb and the hydrogen ion content at pH 8.5-9.6, is typically used for the majority of once-through boilers in Japan. In the past several years, some extraordinary high rate of corrosion damage have been reported to occur on the inside wall surface of tubes which carry the AVT boiler feed water at elevated temperatures. In this study, the effect of pH on the corrosion rate of carbon steel at 120°C in flowing AVT feed water was examined with the goal of elucidating the mechanism which operated for this type of unusual corrosion. A jet-in-slit test apparatus was used, in which the so-called erosion-corrosion as well as corrosion accelerated by the gap in flow velocity could be reproduced on a model system. At pH 9.0, the specimen surface, after a 40h test was covered with a thick corrosion product layer of magnetite (Fe3 O4) to an extent that the so-called erosion-corrosion, as well as the corrosion accelerated by the flow velocity gap, had occurred. At pH 10.0 the entire surface had the original metallic polish even after 40h of flowing test liquid, suggesting that a perfect passivation film had been deposited on it. At pH 9.5, the major portion of the specimen surface was covered with a passivation film, while numerous narrow dark lines radiated from the center of the specimen. These lines were revealed to be grooves filled with magnetite which appeared to be produced at a high rate (over 1mm/y) by a type of oxygen concentration cell corrosion. In conclusion, AVT boiler feed water, at pH 9.5, should be avoided.