Advanced electrochemical methods for food can evaluation

The scanning vibrating electrode technique (SVET), electrochemical impedance spectroscopy, and salt spray testing are used to investigate the ability of Zn rich sol-gel coatings to provide sacrificial protection to carbon steel. Three types of coatings (containing either Zn powder, a coloured pigment, or both) are applied to steel. Intact coatings are shown to act as barrier layers through which electrolyte ingresses over time.Under conditions where the substrate is exposed by an artificial coating defect, SVET is used to investigate the extent to which different coatings offer sacrificial protection when the defect size is systematically changed. The total anodic current, as derived using SVET, doubles when the defect covers 25 % of the total area compared to when 12 % of the area is exposed. This finding is consistent with efficient sacrificial protection of the steel by the zinc based coating. This sacrificial protection is observed for up to 24 hours for cases where the defect constitutes up to 52 % of total area. The protection offered in the presence of a coloured pigment is delayed and it is proposed that the pigment restricts the ability of Zn to couple with the underlying steel.

Section: Discussionmentioning

confidence: 99%

The Sacrificial Protection of Steel by Zinc-Containing Sol-Gel Coatings

Wint

Wijesinghe

Yan

et al. 2019

“…to study weld-metal corrosion, 28 localized corrosion of stainless steels, 29,30 carbon steel, 14 and galvanized steels, 18,31 as well as pit initialization. 32 Its principle attractive feature is that it can map out regions of localized corrosion while the sample is still immersed within the test solution.…”

Section: Scanning Reference Electrode Technique-sret Has Been Usedmentioning

confidence: 99%

Investigations of the Mechanism of Corrosion Inhibition by Polyaniline. Polyaniline-Coated Stainless Steel in Sulfuric Acid Solution

Gašparac

Martin

2001

114

The objective of this work was to elucidate the mechanism by which polyaniline ͑PANI͒ films passivate stainless steel surfaces in highly corrosive H 2 SO 4 solution. A variety of experimental methods, including measurements of the open-circuit potential, Auger depth profiling, and the scanning reference electrode technique ͑SRET͒, was used. These studies have shown that passivation is achieved because the oxidized and protically doped emeraldine-salt form of PANI holds the potential of the underlying stainless steel electrode in the passive region. Because of this electrostatic mechanism of corrosion inhibition, the entire stainless steel surface does not have to be coated with PANI in order to achieve passivation. Auger depth profiling experiments show that, in analogy to the case of a bare stainless steel surface whose potential is potentiostatically maintained in the passive region, the stainless steel surface beneath the PANI film is enriched in Cr after exposure to the H 2 SO 4 solution. Finally, SRET was used to explore passivation of the stainless steel in regions exposed to solution by pinholes intentionally cut through the PANI film. SRET shows that the PANI film surrounding the pinhole can heal the stainless steel in the pinhole region.Polyaniline ͑PANI͒ is one of the most intensively investigated of the electronically conductive polymers. 1-7 In 1985, DeBerry found that PANI films electrodeposited on stainless steel passivated the surface to corrosion in sulfuric acid solution. 8 Since then there have been numerous studies of the corrosion inhibition properties of PANI 9-14 and other electronically conductive polymers. 15 Nevertheless, there is no universally agreed upon mechanism by which these polymers provide anticorrosion properties. We have investigated corrosion inhibition by chemically synthesized PANI films coated onto stainless steel surfaces in aqueous sulfuric acid solution. The emeraldine-base form of the polymer was coated onto the electrode surface; however, exposure to the H 2 SO 4 solution converted this to the protically doped emeraldine salt form.A variety of experimental methods, including measurements of the open-circuit potential ͑OCP͒ Auger depth profiling, and the scanning reference electrode technique ͑SRET͒, was used to explore the mechanism by which the PANI film passivates the underlying stainless steel surface. These studies have shown that passivation is achieved because the PANI film holds the potential of the underlying stainless steel electrode in the passive region. The Auger results lend especially strong support to this conclusion. In analogy to the case of a bare stainless steel surface whose potential is potentiostatically maintained in the passive region, the stainless steel surface beneath the PANI film is enriched in Cr after exposure to the H 2 SO 4 solution. Because the PANI film passivates the electrode by simply holding the potential in the passive region, the entire stainless steel surface does not have to be coated with PANI in order to achieve passivation;...

“…The metal-solution interface.-In unstirred aerated aqueous electrolytes containing high concentrations of chloride ion both iron 31 and zinc 6,32,33 typically exhibit highly localized corrosion behavior. Under these circumstances metal dissolution remains focused at a number of anodic sites which together comprise only a small fraction of the total metal surface in contact with electrolyte.…”

Section: Discussionmentioning

confidence: 99%

Kinetic and Mechanistic Studies of Rare Earth-Rich Protective Film Formation Using In Situ Ellipsometry

Böhm

Greef

McMurray

et al. 2000

The deposition of rare earth metal (REM) rich (hydr)oxide films on pure iron and zinc surfaces has been studied using in situ ellipsometry. Iron and zinc surfaces freely corroding in near-neutral 0.86 mol dm Ϫ3 aerated aqueous sodium chloride were found to be covered with 6-15 nm thick native (hydr)oxide films. The kinetics of REM-rich film growth for one redox active [Ce(III)] and two redox inactive [Y(III) and La(III)] cations were measured. Addition of 2.5 ϫ 10 Ϫ3 mol dm Ϫ3 REM chloride salts to the electrolyte resulted in rapid growth of REM-rich (hydr)oxide films on both metal substrates. Similar film growth rates were observed for each REM cation. REM-rich film deposition proceeds via the precipitation of REM hydroxides [M(OH) 3 ] produced by cation hydrolysis proximal to the substrate-solution interface driven by increased interfacial pH resulting from cathodic oxygen reduction. When iron was cathodically polarized to near the free corrosion potential of zinc (Ϫ1.05 V vs. SCE) the pre-existing iron (hydr)oxide film was removed within 400 s and no deposition of REM-(hydr)oxide films was observed. On the basis of these findings it is proposed that REM-(hydr)oxide deposits only nucleate efficiently on the native (hydr)oxide covered metal surfaces.