Corrosion tests randomization

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. 

In a chemical plant nine aluminum alloys were tested for their resistance to corrosion. Four locations were chosen inside the chemical plant for this experiment and at each such place one plate of all nine alloys was placed. Exposure to chemical corrosion lasted one year. After the experiment four researchers examined the plates randomly and gave a mark from 1 to 10 to each plate depending on the observed resistance to corrosion. The experiment was aimed at asserting which of the offered aluminum alloys had the best resistance to corrosion at one or at all locations of the chemical plant. It was also interesting to see how much the researchers agreed or disagreed in their estimates of resistance to corrosion. Thus, the experiment had included three factors nine aluminum alloys, four locations and four researchers. Experimental results are shown in the following table ... 

In another study [35], the electrochemical emission spectroscopy (electrochemical noise) was implemented at temperatures up to 390 °C. It is well known that the electrochemical systems demonstrate apparently random fluctuations in current and potential around their open-circuit values, and these current and potential noise signals contain valuable electrochemical kinetics information. The value of this technique lies in its simplicity and, therefore, it can be considered for high-temperature implementation. The approach requires no reference electrode but instead employs two identical electrodes of the metal or alloy under study. Also, in the same study electrochemical noise sensors have been shown in Ref. 35 to measure electrochemical kinetics and corrosion rates in subcritical and supercritical hydrothermal systems. Moreover, the instrument shown in Fig. 5 has been tested in flowing aqueous solutions at temperatures ranging from 150 to 390 °C and pressure of 25 M Pa. It turns out that the rate of the electrochemical reaction, in principle, can be estimated in hydrothermal systems by simultaneously measuring the coupled electrochemical noise potential and current. Although the electrochemical noise analysis has yet to be rendered quantitative, in the sense that a determination relationship between the experimentally measured noise and the rate of the electrochemical reaction has not been finally established, the results obtained thus far [35] demonstrate that this method is an effective tool for... 

As indicated by e.g. Allamilla Sosa (2008) and NORSOK M-506 (2005), there are reasons to believe that the lack of knowledge and uncertainty in the influencing factors in the field is the main contributor to the imcertainty in the prediction of the deterioration. With other words, the uncertainty in z(f) will be much larger than the uncertainty of g(), hence we will start to focus on the uncertainty in z(i). Melchers (2005) suggests that even if all test samples in a corrosion experiment are exposed to the same environment, a random behavior ofX f) is experienced. In this paper we suggest to explain this randomness as a result of an unknown z, such that this situation is a variant of the second condition. 

Studies have been conducted on the effects of corrosive salt-spray environment on bondlines of different bonded systems. The system variations included clad and bare alloys, surface treatments, adhesive primers, and adhesives. Five specimens were fabricated for each of the bonded systems. The specimens were then placed in a salt-spray environment of 5% NaCl at 35 °C. The change in wedge-test crack length of each specimen was recorded periodically. At the end of 1 month, one specimen was randomly selected from each bonded system and opened for visual inspection of the bondline condition, both in the stressed zone (crack-tip zone) and in the unstressed zone. The same procedure was carried out after 2, 3, 6, and 12 months, when the last specimen was removed from test. The conclusions were as follows... 

That corrosion depends little on the height above ground was shown by tests on a television tower at Sutton Coldfield near Birmingham (Table 2.4). This result was confirmed by a survey in the late 80s by Van Eijnsbeigen on weathered Dutch galvanized transmission towers, which showed a residual 90 p.m at the top, 10 p.m in the middle, and 100 p.m at the bottom— all within the possible limit of random error. 

The time to failure measured on non-precracked specimens represents the sum of the initiation and propagation times of a crack. Unfortunately, crack initiation, which often is a slow step, is a random phenomenon. As a result, the reproducibility of corrosion fatigue tests earried out with non-precracked specimens is usually mediocre. Furthermore, to determine a Wohler eurve, a large number of specimens is needed. Statistical methods are usually applied to determine the endurance limit in corrosion fatigue tests. 

With the use of desktop computers and work stations now common, it is possible for corrosion researchers to put in their data and come out with analysis of variance tables for factorial experiments and multiple linetir regression analysis tables for uncontrolled experiments. Most such tables include a statistical test of significance for observed effects. That test (such as a F test) is a measure of the probability that an observed effect either exists or is caused by random error. 

Under conditions of free corrosion and constant amplitude (S-N curves), a tolerable stress amplitude results about 20% lower than in air. With cathodic polarisation, the air exposure values are generally achieved once again. In tests with exposure to random stress loads as well, cathodic polarisation extends the fatigue life. 

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