Corrosion Resistance Mechanism of a Chromium-Free Zinc-Flake Coating Film on Scratched Areas -
Chromium-free zinc flake coatings (zinc-rich paint) are used for automotive parts and construction parts, etc. that require a high degree of corrosion protection. The demand for this process is extremely high due in part to its being an environmentally-friendly, chromium-free process. Currently, the automotive industry and others are looking for this type of coating which exhibits superior performance. In 2008, we studied the high corrosion resistance mechanism of the chromium-free zinc flake coating film using polarization curves. Currently, we are working on studying this mechanism in greater detail, focusing on how corrosion is controlled, even on areas that are scratched.
Introduction
Basecoat (Two coats) | Topcoat | ||||
Thickness, μm | Main components | Baking condition | Thickness, μm | Main component | Drying condition |
8.0 - 10.0 | Zn, Al, Si, Ti | 260°C×30 min | 0.5 - 1.0 | Si | 100°C×15 min |
Cycles | 0 | 10 | 20 | 30 | 40 |
Zn | 54.8 | 57.8 | 49.5 | 51.6 | 51.1 |
Al | 23.8 | 4.2 | 10.4 | 8.5 | 7.0 |
Si | 7.7 | 0.9 | 1.6 | 1.2 | 2.1 |
Fe | 2.0 | 2.5 | 1.1 | 2.5 | 2.4 |
O | 8.4 | 20.3 | 29.7 | 27.7 | 26.3 |
Cl | 0.0 | 12.2 | 4.4 | 5.8 | 8.6 |
C | 3.2 | 2.2 | 3.5 | 2.6 | 2.5 |
When only the basecoat is applied (Table 2), silica and aluminum decreased, while chlorine, oxygen and zinc increased at 10 cycles. Accordingly, we see that corrosion byproducts are created from chlorine, oxygen and zinc at the beginning of the corrosion process. At 20 cycles, zinc and chlorine decreased from the levels measured at 10 cycles, while aluminum and oxygen increased. This indicates that a byproduct containing aluminum and oxygen is increasing in volume between 10 and 20 cycles.
Cycles | 0 | 10 | 20 | 30 | 40 |
Zn | 32.2 | 51.8 | 52.4 | 51.0 | 48.1 |
Al | 13.1 | 11.6 | 11.5 | 8.0 | 9.6 |
Si | 21.0 | 11.0 | 9.1 | 4.4 | 4.1 |
Fe | 2.1 | 2.8 | 2.4 | 2.6 | 2.4 |
O | 20.8 | 18.0 | 19.5 | 22.6 | 28.4 |
Cl | 0.0 | 1.1 | 1.6 | 8.3 | 4.32 |
C | 10.8 | 3.7 | 3.7 | 3.1 | 3.2 |
Cycles | 0 | 10 | 20 | 30 | 40 |
Zn | 51.2 | 56.6 | 51.8 | 50.3 | 52.0 |
Al | 22.1 | 4.1 | 8.6 | 9.3 | 6.8 |
Si | 7.2 | 1.4 | 1.5 | 1.5 | 2.9 |
Fe | 9.3 | 2.6 | 2.2 | 3.1 | 3.5 |
O | 7.4 | 22.0 | 27.5 | 28.4 | 24.2 |
Cl | 0.0 | 8.2 | 5.4 | 4.3 | 7.8 |
C | 2.8 | 5.1 | 3.2 | 3.1 | 2.8 |
Cycles | 0 | 10 | 20 | 30 | 40 |
Zn | 32.3 | 52.4 | 54.7 | 54.5 | 49.8 |
Al | 13.3 | 9.5 | 9.3 | 6.8 | 9.3 |
Si | 19.5 | 12.3 | 9.2 | 7.7 | 2.9 |
Fe | 6.8 | 3.0 | 3.1 | 2.6 | 2.2 |
O | 18.7 | 18.9 | 18.9 | 20.6 | 27.5 |
Cl | 0.0 | 0.8 | 1.83 | 3.9 | 5.5 |
C | 9.31 | 3.2 | 3.0 | 3.9 | 2.8 |
Figures 5 and 6 show the results of the SEM-EDS mapping analysis study to determine the distribution of corrosion byproducts at the
Unscratched area | Scratched area (Face side) | Scratched area (Point) | |
Zn | 57.8 | 56.6 | 60.0 |
Al | 4.2 | 4.1 | 2.66 |
Si | 0.9 | 1.4 | 5.4 |
Fe | 2.5 | 2.6 | 4.0 |
O | 20.3 | 22.0 | 21.9 |
Cl | 12.2 | 8.2 | 3.7 |
C | 2.2 | 5.1 | 2.3 |
In the point analysis of the scratched area, we see that the silica and zinc increased. There was byproduct build-up in the scratched area comprised of silica and zinc dissolved from the basecoat, and we believe that the silica from the basecoat greatly contributed to preventing the scratched area from being corroded.
When a sample was processed with both a basecoat and topcoat (Fig. 6), no iron was detected from the scratched area at 10 cycles, similar to the samples processed with the basecoat only. As can be seen from the mapping analysis, at 10 and 20 cycles, the scratched area was covered by both zinc and silica, and aluminum was barely detected. In addition, as zinc was detected over the entire area at 10 cycles, some kind of layer was formed from the zinc and silica from the topcoat. As the component distribution at 30 cycles is very similar to that of the basecoat-only sample, we presume that the corrosion protection mechanism from the topcoat was completed after 30 cycles, and that corrosion would proceed in the same manner as the samples processed with the basecoat only.
According to Fig. 7, a basecoat-only sample progressed and upon reaching 10 cycles in the corrosion protection evaluation, the normal electrode potential of the film was -0.943 VSCE. It is shifted by 0.158 V to the anodic (plus) side as compared to -1.101 VSCE at the beginning of the evaluation (i.e., at 0 cycles). This value is also more noble by 0.048 V than zinc as the standard electrode potential for this type of zinc is -0.988 VSCE. Moreover, the corrosion current density decreased from 5.0×10-5 to 5.0×10-7 A/cm2, and there was no corrosion (dissolution) area found on the anodic side of the polarization curve. Therefore, we believe that the corrosion byproduct was present in a passive form, and dissolution of the film (corrosion) was prevented as it was covered with the corrosion byproduct.
According to Fig. 8, as the corrosion test progressed, the normal electrode potential of the basecoat/topcoat sample shifted to the anodic side and the corrosion current density greatly decreased in the same way as on the basecoat-only sample.
Cycles | 0 | 10 | 20 | 40 |
Normal electrode potential, VSCE | -1.101 | -0.943 | -0.945 | -0.947 |
Open-circuit potential, VSCE | -1.001 | -0.636 | -0.636 | -0.480 |
Shift, V | +0.100 | +0.307 | +0.309 | +0.467 |
Cycles | 0 | 10 | 20 | 40 |
Normal electrode potential, VSCE | -1.181 | -0.981 | -0.980 | -0.960 |
Open-circuit potential, VSCE | -1.012 | -0.965 | -0.933 | -0.542 |
Shift, V | +0.169 | +0.017 | +0.047 | +0.417 |
In our study of silica-type zinc flake coatings, we evaluated the change in the surface condition over time in corrosion testing in order to determine the corrosion protection mechanism. Based on the results, we offer the following conclusions concerning the corrosion protection mechanism:
- Corrosion byproducts (such as Zn(OH)2, Zn2SiO4, ZnCl2, ZnO and Zn complexes) are developed when the basecoat is exposed to a corrosive environment and elements dissolved from the basecoat like zinc, aluminum and silicon react with components in the atmosphere such as oxygen gas, chlorides and hydroxides.
- Typical passivation materials in the corrosion byproducts such as Zn(OH)2, ZnO and Zn2SiO4 cover the surface of the film, and these covered films demonstrate excellent corrosion resistance as dissolution of the film is prevented.
- A layer is formed between silica, a component of the topcoat, and zinc dissolved from the basecoat film. This layer exhibits superb corrosion protection characteristics as it is very close to a passivation state.
- At scratched areas, silica, a component included in either the basecoat or topcoat, reacts with zinc dissolved in the sacrificial corrosion protection mechanism and it accumulates over the scratch areas, protecting these areas.
- M. Iijima, “High Corrosion Resistance Mechanism of Chrome-Free Zinc-Rich Paint,” in Proc. SUR/FIN 2008, Indianapolis, IN, NASF, Washington, DC, 2008; p. 345-355.
- Manual for Basic Electrochemical Measurements, Short Course, Electrochemical Society, Pennington, NJ, 2006.
________________________
Related Content
Non-PFAS Wetting Agents for Decorative Chromium(VI) Plating
This article is based on a presentation given at NASF SUR/FIN 2022, in Rosemont, Illinois, in Session 6, Responses to PFAS / PFOA. It follows the case study of three facilities’ conversion from PFAS-containing wetting agents to non-PFAS equivalents, eliminating PFAS and moving forward with a smaller and more sustainable environmental footprint. The journey of conversion from PFAS-containing wetting agents in both chromic-sulfuric etch and hexavalent decorative plating tanks can be complicated and winding due to deep rooted standard industry practices, as well as state and federal regulations. Outlined here is a clear course of action that led to eliminating PFAS from the facilities’ wetting agent strategies.
Read MoreLooking to the Future of Finishing
Products Finishing takes a look at some of the ways the finishing industry is investing in workforce development and educational initiatives.
Read MoreTake Full Advantage of Industry Events
As travel plans ramp up for the year, what industry events will you attend? Products Finishing offers a quick look at some of the upcoming opportunities for 2024.
Read MoreNASF's SUR/FIN 2023: Bringing the Surface Finishing Industry Together
SUR/FIN 2023 is an opportunity for those in the surface finishing industry to expand their knowledge, expertise and network.
Read MoreRead Next
Episode 42: An Interview with Robin Deal, Hubbard-Hall
Hubbard-Hall wastewater treatment specialist Robin Deal discusses the latest trends in wastewater management.
Read MoreThe 2024 Ford Mustang: All the Colors Available
Although Chevrolet has announced the end of the Camaro and Dodge is offering “Last Call” editions of the Charger and Challenger, the Ford Mustang is launching to its seventh generation.
Read MorePowder Coating 4.0: Smarter, Faster, More Efficient and Connected
New tools reduce cost and waste, lower manufacturing footprint of powder coating operations.
Read More