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

Abstract

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.

Influence of chloride and pH on the pitting mechanism of Zn‐Ni alloy coating in sodium chloride solutions

“…[ 20,21 ] Since large pit formation is observed only in low pH solutions, it is concluded that the pH of the solution plays an active role in the propagation of the pits. A similar result has been reported by Wint et al [ 18 ]…”

“…The pits formed in the acidic solution with 0.35 moL/L NaCl and neutral solution with 0.50 moL/L NaCl have many larger holes, with diameters in the range of 80 μm‐100 μm, in comparison to the pits formed from neutral solution with 0.35 moL/L NaCl. Similar results are discussed by Wang et al, [ 14 ] Wint et al, [ 18 ] and Miao et al [ 12 ] In a solution with higher Cl − concentration (0.50 moL/L NaCl), more intense pitting corrosion with larger pit growth was observed. The impedance spectroscopy of the neutral solution with the higher 0.50 moL/L NaCl concentration as confirmed through SEM images is consistent with the high impedance in the Nyquist plots due to the formation of corrosion products (Figure 10C).…”

“…[20,21] Since large pit formation is observed only in low pH solutions, it is concluded that the pH of the solution plays an active role in the propagation of the pits. A similar result has been reported by Wint et al [18] The mechanism of the pitting corrosion process is proposed as follows: At the initial stage of the pitting, metastable pits formed, small in size, and after some time they will either re-passivate or propagate. At the beginning of the propagation, the wall of the small pits are the anodic sites on which the metal dissolution occurs.…”

“…This may be due to the formation of the passive layer on the Zn‐Ni alloy surface from corrosion and oxidation products. [ 12,18 ] Run 3 and run 10 at high NaCl concentration with neutral solution pH showed smaller pit diameters, but the actual corrosion rate increased due to the more generalized form of corrosion (uniform corrosion) on the entire surface of the coating. These samples had higher pit density (pits/mm 2 ) in comparison to other lower levels of NaCl solutions, and similar results have been reported by Wint et al [ 18 ]…”

“…[12,18] Run 3 and run 10 at high NaCl concentration with neutral solution pH showed smaller pit diameters, but the actual corrosion rate increased due to the more generalized form of corrosion (uniform corrosion) on the entire surface of the coating. These samples had higher pit density (pits/mm 2 ) in comparison to other lower levels of NaCl solutions, and similar results have been reported by Wint et al [18] 4.2 | General factorial regression: Pit diameter (μm) at 2 hours versus A, B Analysis of variance (ANOVA) is a statistical method that evaluates the outcome considering the main factors and interaction between two factors. Based on the resultant value listed in Table 5, ANOVA was carried out to justify the efficacy of the statistical models.…”