Over the past half century, use of zinc-rich silicate coatings has been among the effective methods to prevent corrosion of steel structures in corrosive atmospheres. Moreover, micaceous iron oxide pigments are widely used in barrier anti-corrosion coatings. In this study, a number of waterborne zinc-rich silicate coatings were formulated based on potassium silicate modified with silica nanocolloids and zinc dust, in which some portion of zinc pigment was replaced with various levels of micaceous iron oxide. The prepared coatings, after applying on the steel panels, were evaluated by use of atomic-force microscopy (AFM), electrochemistry impedance spectroscopy, free corrosion potential measurement and salt spray test. The results indicated that, although the above-mentioned replacement reduced the period of cathodic protection of steel by the zinc-rich coating, it, in return, led to significant anti-corrosive properties in the salt spray test. The results also indicated that micaceous iron oxide can be used to control the activity rate of zinc-rich coatings.
Keywords: Zinc-rich silicate, micaceous iron oxide, cathodic protection, barrier, free corrosion potential, electrochemistry impedance spectroscopy, activity
Zinc-rich silicate coatings are anti-corrosion coatings that benefit from the sacrificial behavior of zinc particles for the steel substrate cathodic protection. As the name implies, the outstanding characteristic of such coatings is large portion of zinc metal pigment in them. In conventional coatings, pigments adhere together by a small amount of resin, insufficient to embed and wet all pigments. The aim here is to facilitate the electrical conductivity through direct binding of zinc particles. Driving zinc-rich coatings to sacrifice themselves relies on their direct contact with the substrate, which makes them usable in multilayer systems as primers. Shortage of resin has caused zinc-rich silicate coatings to have porous and permeable structures; unlike other coatings, however, such pores do not inhibit performance in such coatings. With the electrolyte penetrating into the pores and reaching the common coating/substrate surface, anodic reaction of zinc particles as well as cathodic protection of steel occurs. Over time, accumulation of zinc corrosion products that have low electrical conductivity causes the surface-active of zinc particles to decline and thus, gradually interrupting the contact zinc particles have with each other and with the substrate. Ultimately, pores are partially filled with insoluble/low soluble zinc corrosion products and cathodic protection is replaced by barrier property.
Therefore, the protective behavior of zinc-rich coatings is divided into two periods of cathodic and barrier protection [1-4].
Application of alkaline zinc-rich silicates that historically were the first category of the zinc-rich coatings gradually declined; however, recent strictness in applying the environmental rules and regulations in Europe and the United States once again has diverged the attention of manufacturers and research centers from solvent-based zinc-rich coatings to alkaline zinc-rich silicates, as they are waterborne, non-volatile organic eco-friendly compounds .
Over the last few decades, in the field of zinc-rich silicates, study on the effect of replacing portion of zinc dust with co-pigments has been widely taken into consideration [4, 6, 12]. The reason could be that the only a small percentage of zinc undergoes corrosion during the cathodic protection period, as compared to the initial metal zinc in the coating . On the other hand, use of co-pigments can reduce the risk of mudcracking, topcoat bubbling of surface layers and zinc release into the environment, as well as improve weldability and reduce relatively high costs of zinc dust .
Micaceous iron oxide is the most widely used lamellar pigment in anti-corrosion coatings with the barrier mechanism, which is non-toxic and non-reactive with high thermal stability. The lamellar pigment particles create a barrier against the electrolyte penetration into the substrate by parallel orientation with the substrate .
In this study, a number of waterborne zinc-rich silicate coatings were formulated based on potassium silicate modified with silica nanocolloids (it should be noted that modification of potassium silicate with silica nanocolloids has significant influence in most properties of the coating ), zinc dust as the main pigment and micaceous iron oxide as the co-pigment. The coatings, after applying on the steel substrate, were experimented to evaluate the corrosion performance.
2. Materials and method
2.1 Raw materials
All raw materials used in the preparation of the coatings were of commercial grade that were domestically produced. The 40 wt% silicate potassium solution with the silica (SiO2) to alkali metal oxide (K2O) molar ratio of 1:3.29 was prepared from the Iran Silicate Industries and the 30 wt% acid silica nanocolloids solution (pH=4) with the particle size between 10-20 nm was obtained from the Sharif Nanopigment Company. In order increase the molar ratio of silica to alkali metal oxide, certain amounts of the two solutions were perfectly mixed together until a resin with the molar ratio of 4:1 was obtained. In addition, the micaceous iron oxide pigments (with more than 85% of the particles smaller than 44 μm in size) obtained from the Sormak Mining Company along with zinc dust with the particle size of 4-6 μm obtained from the Pars Zinc Dust Company were used.
2.2 Samples preparation
A number of blasted SAE 1010 steel panels (15×8×0.2 cm³) with the average roughness of 25 μm were used as the substrate and were defatted with acetone prior to applying the coating. The coating various components were completely mixed together, and the coatings were applied on the substrates in two series with the thickness of 70±10 μm (Series I) and 100±10 μm (Series II) using the adjustable film applicator. In order to ensure that the curing process was complete, the samples were kept under laboratory conditions 10 days before beginning of the tests.
2.3 Laboratory tests
To study the topography of the coatings surface, the Easyscan 2 AFM (Nanosurf AG, Switzerland) was used. The accelerated corrosion test (salt spray) in the B.AZMA CTS-114D Cabin constructed in Iran was conducted on the Series I coatings for 1000 h according to the standard ASTM B117-03. Moreover, to evaluate the protective capability of the coatings, x-cut scribes were created on them that were ended into the substrate. Finally, the coatings performance was evaluated based on the blistering rate, rust and rust creepage in the scribes and areas outside them in accordance with the standards ASTM D1654-08, ASTM D610-01 and ASTM D714-02. Afterwards, a defect- and rust-free area was selected from each of the Series I coatings and undergone the electrochemistry impedance spectroscopy test on the initial and end day of a 1000-hour period of immersion in a 3.5% sodium chloride solution (Merck). The Series II coatings were only undergone the electrochemical impedance spectroscopy experiment that lasted for 1800 hours under the same electrolyte exposure.
The area exposed to the electrolyte in the test was equal to 2 cm². In a standard three-electrode cell system, the coated samples played the role of the working electrode, a silver-saturated silver chloride played the role of the reference electrode and a platinum bar played the role of the counter electrode.
The test was carried out with the use of Autolab PGSTAT 302N device and FRA2 frequency response analyzer around the free corrosion potential with the amplitude of 10 mV and in the frequency range of 10-100 kHz. The free corrosion potential was also recorded during the electrochemical tests period by using the same cell and equipment. The electrochemical cells of the Series II coatings were discharged of electrolyte after 1800 hours, and the coatings were given one week to naturally dry. Then fresh electrolyte was poured into the cells and potential measurements were taken 24 hours after the new electrolyte was poured.
3. Results and discussion
3.1 Surface topography study
Figure (1) indicates the optical microscope image of the II-A80 coating surface from above. In this figure, spherical zinc and lamellar micaceous iron oxide pigments that are randomly scattered in the structure can be seen.
Figure 1. The optical microscope image of the II-A80 coating surface from above
Figure 2 presents the two- and three-dimensional atomic-force microscopy (AFM) images from the same coating surface. In these images, the bright peeks are the pigments developed beyond the coating surface, and the dark areas are the pores. The high roughness of the coating surface is due to the shortage of resin to embed the pigments.
3.2 Free corrosion potential
Time dependent potential changes of the Series I and II coatings are shown in Figs. 3 and 4, respectively. As can be seen in these two figures, as the time passed, potential of the Series I coatings, except for the I-B80, and all the Series II coatings until 1800 hours (with fluctuation) gradually changed toward less negative values. This behavior is usually attributed to the synergistic effect of filling the pores with insoluble corrosion products (that play a barrier role and interrupt electric connection in the coating conductive network) on the one hand and corrosion of zinc (and thus decreased ratio of zinc to steel active area and reduced share of zinc in the complex potential) on the other.
Figure 2. The AFM images of the II-A80 coating surface
Figure 3. The potential changes of the Series I coatings over time
Moreover, the other process that is often true is that, by the increase in percentage of micaceous iron oxide in the formulation, the potential changed toward positive values. This behavior can be attributed to placing micaceous iron oxide particles between zinc particles, which leads to interruption of the electric connection in some of these particles.
Figure 4. The potential changes of the Series II coatings over time
One of the most widely-used measures of cathodic protection is to have negative potential of -780mV compared to the calomel (-735 mV compared to silver / silver chloride). According to this metrics, all the Series II coatings have preserved the ability to maintain cathodic protection of steel. In opposite, the Series I coatings with smaller amounts of zinc in the formulation, (I-B 80, I-A80 and I-A70) lost parts of their cathodic protection in testing. It should be noted that the Series I coatings were of lower thickness and had passed the salt spray test before electrochemical evaluation. Generally, it can be said that duration of cathodic protection is reduced by increasing micaceous iron oxide. The potential of the Series II coatings has changed towards more negative values after passing a wet cycle (drying and re-exposure to electrolyte). This potential decrease can be resulting from changes in the corrosion products during the wet cycle or reactivation of zinc particles which were partly made passive by local increase of alkalinity in the cathodic reduction reactions or as a result of exposure to fresh electrolytes . This indicates that cyclic immersion can change duration of cathodic protection.
3.3. Electrochemical Impedance Spectroscopy
Nyquist plots of the Series I coatings in 1000 and 24 hours and of the Series II coatings in 22 and 1800 hours are shown in Figures 5-8, respectively. In order to analyze the impedance spectra in Figure (7), the plot equivalent to Figure (9-A) was used. The plot equivalent to Figure (9-B) was also used to analyze the impedance spectrum in Figures (5), (6), and (8).
Figure 5. Nyquist plot of the Series I coatings after 24 hours of immersion
Figure 6. Nyquist plot of the Series I coatings after 1000 hours of immersion
Figure 7. Nyquist plot of the Series II coatings after 100 hours of immersion
Figure 8. Nyquist plot of the Series II coatings after 1800 hours of immersion
In the equivalent circuits, Rs, Rc, Qc, Rct and Qct are electrolyte resistance, coating capacitance and resistance, charge transfer resistance and capacitance of two electrical layers. Q is constant phase element which is substituted with the ideal capacitor (c) for better interpretation of distributed time constant. Figure 7 shows that the Series II intact coatings had only a time constant in the early of immersion.
Since different electrochemical elements are expected in the immersion of a zinc-rich coating, it seems that the responses of different elements overlap. However, this capacitive loop in this study is attributed to the dissolution of zinc. This finding is consistent with the results obtained by Novoa et al.  and Abreu et al. [1, 14]. Over time, another capacitive loop was gradually appeared in a high-frequency zone for the Series II coatings. In this study, the loop is generally attributed to coating features.
Figure 9. Equivalent circuits for analyzing impedance curve in Figures (7-A), (5-B), (6-B) and (8-B)
Several research studies suggest that accumulation of corrosion products is partially responsible for increased coating resistance or the diameter of the capacitive loop in high-frequency zone over time [1, 2, 4, 14, 16-21]. The process is also observed for the Series II coatings.
From the beginning of the impedance spectroscopy test, the Series I coatings had two completely-segregated capacitive loops. More interestingly, the high-frequency loop resistance decreased over time for all coatings. It is supposed that corrosion products accumulated in pores and on the surface during the salt spray test have undertaken some changes (such as dissolution or conversion from one type to another) caused by immersion. If the ratio of final resistance to the initial resistance is calculated, I-B80 has the lowest value. This means that corrosion products and barrier layer have undergone more changes in this coating than in other. I-B80 is a sample which had revealed potential decrease over time. This can be justified according to descriptions provided on potential and findings obtained from this section.
Figure 10. The Series I coatings after 1000-hour salt spray test
3.4 Salt Spray Test
Figure (10) displays the Series I coatings after 1000-hour salt spray test. There was observed no sign of pin point or general iron rusting or even rust growth (creep) on none of the samples. Thus, the score 10/10 was assigned to coatings based on the standards ASTM D1654- 08 and ASTM D610-01. In some points, some coatings were puffed and blistered. To separate these sections, the coating surface was slightly polished with 220-grit sanders and evaluations were then carried out based on the standard ASTM D714-02.
Replacing zinc spherical pigments with the layered pigments of micaceous iron oxide increases coating porosity because micaceous iron oxide has greater surface-to-weight ratio and requires more resin to get wet. A more porous structure contains a higher amount of corrosion products and the migration of corrosion products and hydrogen emission occur more easily outside the coating. On the other hand, the activity rate is the production speed of corrosion products, reducing as zinc decreases. Concerning these explanations and knowing that zinc corrosion products occupy a greater volume than zinc, it is not clear why coatings with higher percentages of zinc and less porosity are most destroyed.
3.5 Microscopic studies of cross-sections
Figure (11) shows optical microscopic images of cross-sections in coatings II-B90 (11-A and B) and II-A70 (11-C and D). Figures (11 -A) and (11-C) are the cross-section of two coatings exposed to the laboratory atmosphere for 1824 hours and Figures (11 -B) and (11-D) are the cross-section of two coatings exposed to sodium chloride for 1824 hours. These images, along with resistance values of electrochemical tests indicate that II-B90 finally had a large number of intact zinc particles developing an effectively cathodic protection; however, it had higher corrosion and activity rates (lower zinc dissolution resistance during testing. On the other hand, II-A70 had lower activity rate during testing and its zinc particles were mainly corroded and their cathodic protection was to be terminated. In general, it can be claimed that the substitution of zinc with iron oxide decreases activity rate and duration of cathodic protection.
Figure 11. Optical microscopic images of cross-sections in the coatings II-B90 (11-A and B) and II-A70 (11-C and D)
1. Substitution of zinc pigments with micaceous iron oxide in waterborne zinc-rich silicate coating formulation decreases the activity rate and duration of cathodic protection.
2. Cyclic immersion may change the duration of cathodic protection in zinc-rich coatings.
3. Diameter of high-frequency capacitive is affected by condition governing corrosion products.
4. Substitution of zinc with micaceous iron oxide provided acceptable results in the accelerated corrosion test.
5. The coating activity can be controlled by using micaceous iron oxide in zinc-rich coatings and the costs imposed by zinc powder can be somewhat reduced.
The authors would like to express their gratitude to Sarmak and Pars Zinc Dust Mining Companies for preparing pigments.
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