No abstract

Localized residual strain develops within the metallurgical texture of 1018 carbon steel from metallurgical processes, such as fabrication, annealing, and shaping. This residual strain results in accelerated localized pitting due to the formation of anodic sites at these locations. Once initiated, micron-sized corrosion pits can coalesce to form sites of potential catastrophic failure. In this contribution, we focus on the localized biocorrosion which initiates and grows in areas of localized strain such as the interfaces between manganese sulfide (MnS) inclusions and ferrite grains in the steel, at grain boundaries between ferrite grains with different crystallographic orientations and at pearlite grains (intergrown cementite (Fe 3 C) and ferrite), which are readily found in 1018 carbon steel. Here we hypothesize and show experimentally that accelerated biocorrosion in 1018 carbon steel finds its roots in the electrochemical potential difference (micro galvanic cells) generated between the unstrained ferrite iron (Fe α −) and the lattice defects, dislocations and mismatches found at interfaces formed between Fe α − and secondary phases i.e. MnS inclusions, cementite lamellar structures and grain boundaries distributed throughout the 3D network of the carbon steel. This hypothesis is supported by results from multiple micro-and nanoscale imaging and analytical methods obtained from field emission scanning electron microscopy, energy dispersive spectroscopy, electron backscattered diffraction and Auger nanoprobe electron spectroscopy. The morphology and composition of grains in the steel coupons were characterized before and after exposure to suboxic and sulfidogenic environments dominated by aerobic and anaerobic marine organisms. Corrosion processes are demonstrated to initiate in localized areas of high residual strain.

a b s t r a c tA practical method has been developed for rapidly determining the depth of a corrosion micro-pit from the path lengths of X rays passing through the walls of the pit on their way to an X-ray detector. The method takes advantage of the attenuation of the Bremsstrahlung and characteristic X-ray radiation accompanying each X-ray spectrum, and the results are verified independently using AFM and the special pit geometry surrounding MnS inclusions in 1018 carbon steel. The method has general validity and is especially valuable in those cases where the pit depth-to-width ratio is too steep to measure using the conventional methods.

We report an integrated nanochannel/nanoelectrode sensor for the detection of DNA using alternating currents. We find that DNA can be detected using platinum as the metal for the detecting electrodes, with a signal to noise ratio exceeding 10. We argue that the signal is at least in part electrochemical in nature, thus holds the promise to yield a sequence-dependent signal. However, we also find that for large voltages, DNA attaches irreversibly to the driving electrodes.

Cold rolled carbon steel is a commonly used structural material in a wide variety of applications, including automotive, building construction, appliances, and even fuel storage tanks for naval ships. Although Mn is purposely added to carbon steel to getter problematic S impurities, it has long been postulated that MnS inclusions in carbon steel can act as sites of pitting initiation [1]. The propagation of pitting corrosion is relatively well understood; however, the initiation of pits is still a subject of controversy [2,3]. A careful study of pit initiation and propagation associated with sulfide inclusions has been lacking, partly because these inclusions are mostly submicron-sized and the evolution of their corrosion is difficult to monitor. An extremely small spectroscopic probe technique, such as Auger Electron Spectroscopy, capable of nanometer-scale resolution, is required to analyze and map out the elemental distributions within these submicron pits. Auger also has the surface sensitivity needed to detect very thin layers of contaminants associated with these surface-related phenomena. This presentation will review the results of Auger studies of nano-inclusions on carbon steel surfaces prior to and following exposure of the steel to corrosive solutions. The nano-inclusions were carefully characterized using a field emission Auger nanoprobe with a spatial resolution of approximately 10 nm for imaging and spectroscopy. Figure 1 shows an SEM image and Auger maps for a submicron sized MnS inclusion in carbon steel. Analysis of the inclusion by EDS showed the presence of only Mn and S while the more surface sensitive and spatially resolved Auger technique also detected Cu impurities and localized O patches (Fe and Mn oxides) on the inclusion. Thus, the chemical environment surrounding the inclusions is much more complex than originally thought. Other samples analyzed by Auger after exposure to artificial seawater provide evidence that, in corrosive environments, it is not the MnS inclusions that are initially dissolving but rather the areas surrounding the inclusions. The impact of this fundamental analysis [4] on the understanding of cold rolled steel pitting corrosion will be discussed.