Open circuit potential coatings

The photoelectrochemical behavior of ZnSe-coated CdSe thin Aims (both deposited by vacuum evaporation on Ti) in polysulflde solution has been described by Russak and Reichman [112] and was reported to be similar to MIS-type devices. Specifically, Auger depth profiling showed the ZnSe component of the (ZnSe)CdSe heterostructures to convert to ZnO after heat treatment in air, thus forming a (ZnO)CdSe structure, while the ZnO surface layer was further converted to a ZnS layer by cycling the electrode in polysulfide electrolyte. This electrochemically generated ZnS layer provided an enhanced open-circuit potential compared to CdSe alone. Efficiencies as high as 5.4% under simulated AM2 conditions were recorded for these electrodes. 

Absolute correlations between service performance and electrochemical measurements do not appear frequently in the literature. Based on 300 test systems. Bacon and coworkers (, correlated electrochemical resistance with exposure time. Recently, Mills ( also observed a correlation between salt fog corrosion and electrochemical resistance. We have found open circuit potential measurements to be extremely useful for the routine evaluation of high-nitrile polymer-based photocured coatings. 

Twelve different Bonderite 40 coated steel (B40) panels were examined to provide a statistically valid value for the open circuit potential. Their average rest potential was -0.578 V (vs. SCE) with an average deviation of 20 mv. After recording the open circuit potential, polarization curves were obtained (Figure 1). 

Figure 2. Changes in open circuit potential (OPC) and rust rating (RR) with temperature for post-thermally treated Bonderite 40 coated steel (B40) panels and Barex 210/N-vinylpyrrolidone (B210/NVP) photocured coatings on B40 panels.

B40 panels were also subjected to heat treatment. As shown in Figure 2, no dependence of the open circuit potential on post-thermal treatment temperature is observed. This suggested that the open circuit potential measurements reflected changes at the coating/B40 panel interface and not in the B40 panel alone. 

High nitrile photocured coatings on B40 coated steel panels exhibit corrosion resistance when their open circuit potentials are either more noble or less noble than the B40 coated steel panel (Figure 3). At first, we assumed that enhanced nobility should give rise to enhanced corrosion resistance, i.e. the galvanic series. 

In conclusion, we have shown that a simple, fast, electrochemical measurement, the open circuit potential, can be extremely useful in assessing the effect of diluent for high nitrile photocured coatings. Further work is underway to elucidate the underlying reasons behind the temperature induced changes observed in corrosion performance. 

Coated specimens were placed in an electrochemical cell. After 4 hours of temperature, open-circuit potentials were measurements were made on duplicate samples, in a salt spray test cabinet (ASTM B117-73) for 1, 17 and 96 hours respectively and their surfaces photographed in order to calculate the percentage of surface covered by corroded spots and blisters (ASTM D610-68). 

The activities of CNTs have been evaluated by Girishkumar et al. [7] using ex situ EIS. Their study was conducted in a three-compartment electrochemical cell using a GDE electrode (a carbon fibre paper coated with SWCNTs and Pt black as an anode or cathode). Electrophoretic deposition was used to deposit both the commercially available carbon black (CB) for comparison and the SWCNT onto the carbon Toray paper. Commercially available Pt black from Johnson Matthey was used as the catalyst. In both cases, the loading of the electrocatalyst (Pt), the carbon support, and the geometric area of the electrode were kept the same. EIS was conducted in a potentiostatic mode at either an open circuit potential or controlled potentials. 

Multiwall carbon nanotube (MWCNT)-reinforced hydroxyapatite composite coatings (80% HAp/20% MWCNT) were deposited on austenitic stainless steel AISI 316L by laser surface alloying (LSA) with a 2.5-kW CW Nd YAG laser (Kwok, 2007). EIS of unprotected AISI 316L and HAp/MWCNT-coated steel obtained at open circuit potential are shown in Figure 7.60 after immersion in 0.9% NaCl solution for 2 h. The Bode plot shows that the total impedance Z has noticeably increased for the steel substrate coated with HAp/MWCNT. While the thin passive oxide film on the stainless steel surface was rendered less protective... 

Figure 7.60 Electrochemical impedance spectra (Bode plots) of unprotected surgical AISI 316L stainless steel and HAp/MWCNT-coated AISI 316L steel obtained at open circuit potential after corrosion in 0.9% NaCI solution for 2 h. ((Modified after Kwok (2007).)...

The open circuit potential as a function of time is a qualitative method that evaluates sacrificial coating stability. The sacrificial metal removal rate in the corroding solution is controlled by the potential difference of the underlying metal and the sacrificial coating, corroding solution, and sacrificial coating thickness. The open circuit potential changes to the protected metal when the sacrificial metal is completely removed. 

Open circuit potentials also pointed in the same direction. Since OCP values decreased with time to salt spray chamber exposure, there seems to be no passivation effect induced by the coating, and the active electronic barrier model seems to be a more reasonable explanation for the mechanism of protection. 

If the metal is polarized slightly beyond the open-circuit potential, of the anode, the corrosion rate remains zero. Net current flows from the electrolyte to the metal hence, metal ions cannot enter the solution. Current in excess of the required does no good, however, and may damage amphoteric metals and coatings. In practice, therefore, the impressed current is kept close to the theoretical minimum. Should the applied current fall below that required for complete protection, some degree of protection nevertheless occurs. For example, if the corrosion potential is moved from to a in Fig. 5.15, by appUed current e-b, the corrosion current decreases from Icon to b. As the applied current e-b is increased, the potential a moves to more active values, and the corrosion current b becomes smaller. When a coincides with < )x, the corrosion current becomes zero, and the applied current for complete cathodic protection equals /appi. 

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