2000 W.R. Whitney Award Lecture: Visualizing Corrosion

• Published in: Corrosion (Houston, Tex.) Vol. 56; no. 10; p. 971
• Main Authors: Isaacs, H.S., Adzic, G., Jeffcoate, C.S.
• Format: Journal Article
• Language: English
• Published: Houston NACE International 01.10.2000; NACE
• Subjects: Abrasion; Aluminum base alloys; Cathodes; Corrosion; Corrosion products; Edge effect; Gels; Localized corrosion; Metals; pH effects
• ISSN: 0010-9312; 1938-159X
• DOI: 10.5006/1.3294386

ABSTRACTUsing a broad-range pH indicator, the corrosion of Al and Al alloys in chloride-containing agar gels was studied. Distinct changes in pH were observed at low-pH anodic sites and at high-pH cathodes. There was a definite edge effect that occurred when gels were placed directly on the metal, which dominated the pattern of corrosion. Wet-abraded surfaces initially showed a general type of corrosion, whereas dryabraded surfaces showed localized corrosion under the gel. Pure Al, AA1100 (UNS A91110), AA2024 (UNS A92024), and AA5052 (UNS A95052) all showed pitting in contact with agar gels when the edge effect was prevented. Abraded AA7075 (UNS A97075) exhibited a filiform type of corrosion caused by high concentrations of corrosion product initiating corrosion ahead of the growing site.INTRODUCTIONIndicators have been used for many years to determine the location and mechanisms of corrosion. For example, in the 1920s, Evans used indicators to locate the presence of anodic and cathodic sites during the corrosion of Fe.1-3 In the 1950s, gels with indicators were used in an educational movie and its accompanying book, Corrosion in Action, to illustrate particular corrosion processes.4 Recently, pH indicators were added to paints to locate anodic and cathodic sites.5-6 In the present study, a wide-range pH indicator was added to agar gel or to saturated sucrose solutions that then were used as media for studying the corrosion of Al to locate and observe the changes in pH at anodic and cathodic sites. In-situ techniques offer many advantages in exploring corrosion processes. Differences in behavior from sample to sample arise because the location and number of corrosion sites are random. Unless in-situ methods are used, differences lead to changes in the degree and intensity of the observed corrosion, requiring many repeated experiments to resolve the mechanisms of corrosion. In-situ methods that can monitor corrosion processes locally, enabling each site to be analyzed separately, can overcome these limitations. Current density mapping of corroding surfaces have revealed the behavior of pits in Fe7-8 and stainless steels.9-10 Similar measurements were made with Al and its alloys,11-12 but a distinct sequence of events during pitting corrosion has not been obtained yet. Hence, the approach of using pH indicators is being investigated, has yielded interesting information about the corrosion of these materials, and is reported here. EXPERIMENTAL PROCEDURESGels with two different concentrations of sodium chloride (NaCl) were made up by adding 1% (wt%) of high-gel-strength agar to the boiling salt solution. When the agar had dissolved, 15% (vol%) of the widerange pH indicator (WRI) (Fisher Universal Indicator?