The inhibitive performance of polyphosphate-based anticorrosion pigments using electrochemical techniques
Introduction
Applying organic coatings is considered as the approach most commonly employed to combat the corrosion of steel structures. To enhance protective performance of organic coatings especially when they undergo damage, anticorrosion pigments should be taken into consideration. Utilizing anticorrosion pigments is so crucial that there has been continued investigation in order to find the most effective compounds. Chromates, as one of the most efficient classes of anticorrosion pigments, have been extensively used for a long time. In fact, they belong to the group of inhibitors exhibiting excellent corrosion inhibitive performance. However, the same properties making these compounds superior corrosion inhibitors also make them dangerous. By virtue of increasing concerns for environmental protection and potential health hazard associated with chromates, their use is being restricted. The corrosion inhibition properties of numerous compounds have been examined through the years in the hope of providing effective chromate replacements [1], [2], [3], [4], [5], [6], [7], [8]. So far, the classical alternative, zinc phosphate, has been widely used. However, a chasm between zinc phosphate and chromate pigments concerning the inhibitive performance makes physical and/or chemical modifications to zinc phosphate necessary. In this sense, anionic constituent as well as cationic part could be taken into account [2], [9], [10], [11]. Polyphosphates composed of chains in which each phosphorus atom is linked to its neighbors through two oxygen atoms can be used as a substitute for simple phosphate anion. Thanks to higher phosphate content and also high chelate building potential with multivalent metal cations [12], [13], the polyphosphates appear to exhibit better inhibitive performance compared to orthophosphates [14]. Moreover, it has been found that polyphosphates have sufficient water solubility to provide corrosion inhibition [9], [10], [15]. Their inhibition mechanism has been described by migration of the positively charged complexes, formed in the presence of metallic cations, to the cathode leading to a protective film precipitated on the surface through a process of electrodeposition [12], [16], [17], [18]. In case of the cationic part, combination of various polyvalent metals has been suggested [11].
Since corrosion is an electrochemical process, evaluation of anticorrosion behavior of various systems using electrochemical techniques could provide valuable data. Electrochemical impedance spectroscopy (EIS) as a powerful non-destructive test is capable of obtaining significant quantitative and qualitative data. This technique is widely used to study effective parameters in association with corrosion process as well as different approaches for protecting metallic structures against corrosion [19], [20], [21], [22]. Taking advantage of EIS, inhibitive performance of anticorrosion pigments could be studied [5], [23], [24]. In order to interpret impedance spectra, the data should be modeled by the proper equivalent circuits. As suggested elsewhere, employing EIS along with other electrochemical tests may lead to more reliable interpretation and taking out much more information [25].
Study of corrosion behavior of the bare metal immersed in inhibitor compounds extracts is the concept employed in this work in order to have a better understanding of performance of polyphosphate-based anticorrosion pigments. Taking advantage of electrochemical tests, i.e. linear polarization and electrochemical impedance spectroscopy, as well as surface analysis, the effect of different cations incorporated into the composition of the modified anticorrosion pigments is evaluated.
Section snippets
Experimental
Table 1 shows the composition of steel panels used for different tests. Polished steel specimens, with dimensions of 3 cm × 3 cm × 0.7 cm, were degreased by acetone. In order to seal the edges and back sides of the steel panels, they were covered by a mixture of beeswax and colophony resin, leaving the apparent geometrical area of 1 cm2 unmasked.
EIS measurements
Fig. 2 displays Nyquist and Bode plots of specimens after immersion in 3.5% NaCl solution containing ZP, SAPP and ZAPP extracts for 1, 4 and 24 h. Considering the plots with one relaxation time for ZP, the relevant spectra could be modeled by the equivalent circuit shown in Fig. 3a where Rs represents solution resistance, Rct charge transfer resistance and Cdl double layer capacitance. In the case of ZAPP and SAPP, diagrams revealed quite different behavior. The circuit depicted in Fig. 3a is
Conclusion
The effect of different cations incorporated into the composition of polyphosphate-based anticorrosion pigments as different forms of the zinc phosphate modification was assessed through electrochemical tests as well as surface analysis. According to the results of EIS and polarization measurements, the modified pigments, i.e. ZAPP and SAPP, revealed greater inhibitive properties compared to conventional zinc phosphate. They appeared to act through precipitation of a layer on the surface of the
Acknowledgements
The authors would like to thank Petrochemical Research & Technology Company (NPC-RT) of Iran for financial supports.
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