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Mar 2012

Volume 68, Issue 3 (partial)

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Assessment of Coating Adhesion Degradation by Atomic Force Microscopy Scratching

J. Seong and G. S. Frankel

Corrosion 68, 032501 (2012); doi:http://dx.doi.org/10.5006/1.3688500 (4 pages)

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The effects of environmental exposure on polymer coating degradation were studied using atomic force microscopy (AFM) scratching. Adhesion was assessed by the coating removal rate during controlled-force contact mode scratching in the AFM. Low-carbon steel coupons were coated with an ultra-high molecular weight epoxy resin and exposed to environments with high relative humidity and exposure to ultraviolet light. AFM scratching was performed both in air and in water to assess the effect of water on adhesion degradation. The average coating removal rate increased when samples were first degraded by environmental exposure and by scratching in water. AFM scratching is shown to be a quantitative method for the assessment of adhesion and adhesion degradation for coatings less than about 4 μm in thickness.
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Effect of S-Phase Dissolution on the Corrosion and Stress Corrosion Cracking of an As-Rolled Al-Zn-Mg-Cu Alloy

D. K. Xu, N. Birbilis, and P. A. Rometsch

Corrosion 68, 035001 (2012); doi:http://dx.doi.org/10.5006/1.3688501 (10 pages)

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The effect of two specific solution heat treatment procedures on the corrosion and stress corrosion cracking (SCC) susceptibility of an as-rolled Al-Zn-Mg-Cu alloy was investigated. It is shown that for a stepped solution treatment with a final temperature of 495°C, the SCC resistance of subsequently retrogressed and re-aged (RRA) samples is remarkably improved compared with that of samples solution-treated at a stationary temperature of 475°C. Since the investigated alloy is probably only partially homogenized or cooled with a very slow industrial cooling rate after homogenization, the mechanistic aspects of this are related to the dissolution of Al2CuMg and the Cu content in the grain boundary precipitates after the RRA treatment.

Implementation of Electrochemical Frequency Modulation to Analyze Stress Corrosion Cracking

A. Rauf, W. F. Bogaerts, and E. Mahdi

Corrosion 68, 035002 (2012); doi:http://dx.doi.org/10.5006/1.3691836 (9 pages)

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The electrochemical frequency modulation (EFM) technique has been used to investigate its ability to monitor stress corrosion cracking (SCC) of sensitized stainless steel U-bend specimens in sodium thiosulfate (Na2S2O3) and sodium chloride (NaCl) solutions. Type 304 (UNS S30400) stainless steel was sensitized for different times to vary the SCC susceptibility. With EFM, a potential perturbation composed of two sine waves generates current responses at multiple frequencies. Because of the nonlinear nature of a corroding system, the alternating current (AC) response contains nonlinear components at harmonic and intermodulation frequencies. Analysis of these components provides information about the corrosion behavior of the system under investigation like the uniform corrosion rate. SCC has been investigated by measuring the so-called “causality factors,” which are calculated from the ratio of the current components in the AC response. A mathematical model is developed based on the causality factors during SCC. Both theoretical and experimental results show that the causality factors change when the system goes from uniform corrosion to SCC.
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