Data analysis, corrosion, surface

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

In the laboratory portion of the project, the students quantify iron in real and artificial surface water samples by UV-Vis spectroscopy. The iron is complexed to the o-phenanthroline (phen) ligands in a buffered solution to create a highly colored orange complex, [Fe(phen)3] The intensity of the complex color is proportional to the concentration, following Beer s Law. Students create a standard series and prepare a surface water sample using modified standard protocols (21). We use autodispensers to dispense corrosive reagents and provide the stock iron solutions this equipment reduces exposure and ensures that the experimental work can fit within the three hour laboratory period. Students measure of the absorbance of their standard series as well as their surface water sample on a spectrometer at X = 508 nm. Students complete the experimental write-up, calculations, data analysis, and assessment during the subsequent laboratory period. 

Crack closure can strongly affect fatigue and CF [45]. This phenomenon is based on crack surface contact during unloading, critically at stress intensity levels above zero and apphed-positive values. Crack wake contact is caused by corrosion debris, plasticity, crack path roughness, or phase transformation products each mechanism may be sensitive to aqueous environmental reactions [6]. To account for closure, dfl/dN is correlated with an effective stress intensity range that is defined operationally as the difference between appUed and the level where surface contact is resolved (see Data Analysis and Evaluation in this chapter). 

Randomness, independence and trend (upward, or downward) are fundamental concepts in a statistical analysis of observations. Distribution-free observations, or observations with unknown probability distributions, require specific nonparametric techniques, such as tests based on Spearman s D - type statistics (i.e. D, D, D, Z)k) whose application to various electrochemical data sets is herein described. The numerical illustrations include surface phenomena, technology, production time-horizons, corrosion inhibition and standard cell characteristics. The subject matter also demonstrates cross fertilization of two major disciplines. 

The purpose of this section is to show, by example, how the concerns of technique selection, potential problems, data acquisition and analysis have been applied for several different corrosion problems and techniques. Examples of fundamental research work and industrial problem solving have been included to show the range of applicability of the techniques. In most cases, more than one technique was used to solve the problem. Frequently, a surface analysis technique was used in combination with one or more other types of analysis method. These examples are not comprehensive it is hoped that sufficient references have been supplied to enable the reader to find other work of relevant interest. 

Metal corrosion is a superposition of metal dissolution or the formation of solid corrosion products and a compensating cathodic reaction. Both processes have their own thermodynamic data and kinetics including a possible transport control. Furthermore, metals are generally not chemically and physically homogeneous so that localized corrosion phenomena, local elements, mechanical stress, surface layers, etc. may play a decisive role. Therefore, one approach is the detailed analysis of all contributing reactions and their mechanisms, which however does not always give a conclusive answer for an existing corrosion in practice. 

The analysis of experimental data shows that the average value 111 g/m2 of all corrosivity data (improved by rejecting outliers) corresponds to the value 140 40 g/m2 indicated in the standard. For the evaluation of the expanded combined uncertainty U with factor k=2 the corrosivity measurement gives the value of 215 g/m2 (at 95% confidence). It means that our data uncertainty is five-times higher than that specified in the standard as the data scattering interval 40 g/m2 and seven times as wide compared the statistic confidence interval in our own experimental data corrosivity (Table 2a and 2b). The main components of the combined uncertainty are mass loss and surface area determination. 

In the case of metallic adsorbates (metal deposits, underpotentially deposited upd-layers, catalytically active metal deposits), the type of coordination to surface sites (one-, two- or three-fold) and the distance to these sites may be of interest. Vice versa the same type of data may be of importance in the case of adsorbed ions on metal electrodes or about the atomic environment of a given atom/ion in an interphase. Analysis of the fine structure of X-ray absorption (EXAFS, XANES) close to the X-ray absorption edge of the species (atom) of interest will yield this data provided the sample can be prepared in a very thin layer in order to exclude unwanted bulk interference. Otherwise the experiment can be done in reflection (SEXAFS). Information about the distance between the atom of interest and its first and sometimes even second shell of surrounding species can be derived from the spectra [95]. Availability of a suitable light source, generally a synchrotron (for details see p. 15), is an experimental prerequisite. The method has been applied in studies of passive and corrosion layers on various metals [96-102] and of molecular and ionic adsorbates on single crystal surfaces [103]. 

In the advanced state of delamination, two zones of different activity can be located underneath the organic coating. The shift of the corrosion potential close to the rather anodic potential of the intact interface marks the front of the advancing cathode. Behind this cathodic area the steep slope marks the front of anodic undermining. Fiirbeth and Stratmann proved this interpretation of the SKP data with cross-sectional and surface analysis of the delaminated area [86]. 

A retrieval analysis of Kuntscher intramedullary rods (Cook et al. 1990) has shown that significant surface corrosion, inclusion content and carbon content occurred on early materials, which had remained in situ for 10 years or longer (maximum 23 years). Significant relationships were obtained for surface corrosion score vs. thin globular oxide inclusion content, and for surface corrosion score vs. sulphide inclusion content. Figure 9.2 shows the data obtained for the former correlation. 

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