Elsevier

Corrosion Science

Volume 52, Issue 8, August 2010, Pages 2653-2660
Corrosion Science

Tuning DOS measuring parameters based on double-loop EPR in H2SO4 containing KSCN by Taguchi method

https://doi.org/10.1016/j.corsci.2010.04.015Get rights and content

Abstract

In the present work, a higher resolution than that of standard DL-EPR technique was obtained by tuning the degree of sensitisation (DOS) measuring parameters. SS304 with different sensitisation conditions was chosen. Acid and depassivator concentration, temperature, scan rate and reverse potential were defined as key factors and Taguchi L16 was applied. The optimal condition was found to be 1 M H2SO4, 0.02 M KSCN, 40 °C, 30 mV/min scan rate and 200 mV reverse potential. Main factors were acid concentration, scan rate, depassivator concentration. An increase in DOS from 18 to 42.2 in the 60 min sensitised sample was observed.

Introduction

Intergranular corrosion (IGC) is one of the most important types of localized attack in stainless steel. Although various structural changes are conductive to a selective corrosion attack, the main reason of IGC is the phenomenon called sensitisation [1]. Austenitic stainless steel may be sensitised to IGC if it is reheated in a temperature range of 500–800 °C or is slowly cooled from high temperature during solution annealing or welding operations [2], [3]. As a result, the corrosion resistance is deteriorated due to the chromium depleted zone adjacent to the chromium rich precipitates, such as Cr23C6 carbide, along the grain boundaries. These chromium depleted regions are susceptible to attack and thus the corrosion in these regions proceeds intergranularly [3]. Due to the negligible weight loss involved and to the overshadowing of depleted zone by the unaffected bulk of the grain, sensitisation measurement is relatively difficult [1]. There are a number of standard methods, such as oxalic acid test, Strauss test, Huey test, Streicher test and Copper–Copper Sulfate-50% sulphuric acid test, which can be used to assess the susceptibility to IGC. None of these methods are a quantitative and non-destructive method. Moreover, performing the above mentioned tests is time consuming. During the past decades, much research has been conducted to innovate a test method by developing the electrochemical potentiodynamic reactivation test (EPR). EPR is a quantitative method which indicates degree of sensitisation (DOS) based on electrochemical parameters obtained from the potentiodynamic polarisation curve [1], [2], [4], [5], [6], [7], [8], [9], [10]. EPR measurement tends to focus on single or double loop modes (SL-EPR and DL-EPR) as a technique used to establish the resistance of stainless steel to IGC and stress corrosion cracking (SCC) susceptibility [9], [10]. The detailed description of SL-EPR and DL-EPR can be found in the literature [9], [10]. Here in brief, it can be highlighted that the results of DL-EPR are reported as a ratio of current density (C-ratio) and/or the charge consumed during the reactivation scanning stage to the current and/or the charge created during the activation scanning stage:DOS%=iria×100orDOS%=QrQa×100where, ir is the maximum current density during reactivation scan, ia is the overall current density during the activation scan, Qr is the overall anodic charge measured during the reactivation scan and Qa is the maximum charge measured during the activation scan [10].

Although a standard procedure has been already developed for SS304 [11], improving the EPR resolution to detect the lowest possible DOS is of great demand and several researchers have tried to improve the resolution by either modifying the technique [4], [12], [13], [14], [15], or the solution constituents [2], [14], [15] for SS304 and other alloy systems. Several factors can influence the resolution of DOS measurement, including acid and depassivator concentration, temperature, scan rate, reverse potential, etc. This indicates that DOS is a multi-variant parameter and investigating the effect of individual factors is essential. Since DOS includes a significant number of factors, a full factorial design of the experiments requires a large number of runs and measurements which are all time consuming and costly. One of the well-known statistical methods to reduce the number of tests with comparable accuracy is Taguchi method. Taguchi method is a combination of mathematical and statistical techniques used in an experimental study which determines the experimental condition having the least variability as the optimum condition [16], [17]. A limited number of experiments from partial fraction which produces the most information are selected. It determines factors that have the most effects on the quality of the product with a minimum amount of experimentation [18]. Some of the main advantages of Taguchi method are simplicity in understanding and applying the method, reduction of variation without removing its causes, reduction of time and costs of the experiments and reduction of the required experimental trials [16], [17]. In this study, Taguchi method was used to evaluate the effect of important factors on DOS measurement and to tune the DOS resolution by modifying them.

Section snippets

Materials and methods

A small sample of SS304 with 1 cm2 cross sectional area was used. Chemical analysis of the steel in weight percent (%wt) is as follows: 18.13% Cr, 7.98% Ni, 0.059% C, 0.46% Si, 0.035% S, 0.045% P, 1.80% Mn, 0.12% Mo, other alloying elements <1%, and Fe bal. Specimens were held in an air atmosphere furnace at 650 °C for 10, 60 and 300 min, followed by quenching in water to produce different degrees of sensitisation. The microstructure of the samples were characterised by optical microscopy. All

Microstructure analysis

Sensitisation can be observed by optical microscopy. Fig. 1 shows the microstructure of SS304 samples in non-sensitised and sensitised situations with different sensitisation times. IGC susceptibility can be evaluated by measuring the thickness of the dark region, continuity of the ditches at the vicinity of grain boundaries and also the number of grains which have been completely surrounded by ditches. The microstructure of the non-sensitised specimen is presented in Fig. 1(a). According to

Conclusion

The optimal condition for DOS measurement based on Double-loop EPR technique in H2SO4 containing KSCN solution was obtained using the Taguchi method. The results can be summarised as follows:

  • (1)

    The new optimum condition for DOS measurement is 1 M H2SO4, 0.02 M KSCN, 40 °C, 30 mV/min scan rate and reverse potential from 200 mV. The new optimum condition provided significantly higher IGC detection, improving DOS resolution.

  • (2)

    ANOVA results showed that the main factor affecting DOS measurement is the acid

Acknowledgments

Authors appreciate the financial support from Ferdowsi University of Mashhad provision of laboratory facilities during the period that this research was conducted and A. Kamyabi Gol for his valuable assistance on ANOVA.

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