Electrochemical Impedance Spectroscopy (EIS) is a powerful electrochemical technique that uses a small amplitude, alternating current (AC) signal to probe the impedance characteristics of a cell. EIS is widely used as a standard characterization technique in corrosion, biosensors, batteries, fuel cells, coatings, physical electrochemistry and more.
For protective coatings, EIS is a quantitative method used for the accelerated evaluation of anti-corrosion performance. In the case of corrosion, it is essential to be able to predict it and understand the damage mechanisms because of the significant challenges corrosion presents to so many industries in the world, not to mention the disasters and massive financial losses it can cause. EIS measurements provide more reliable data to predict the long-term performance of a protective coating within shorter testing times compared with traditional corrosion test methods.
The Basics of the EIS Test
The EIS test is not a routine process – rather it requires an EIS expert to execute the test and analyze the data through modelling simulation. Ximing Li, PhD has been studying EIS for years, and is an expert in the field at Lubrizol. The EIS test is run using a Potentiostat/Galvanostat, which uses a small amplitude, AC signal to probe the impedance characteristics of a tested cell.
Lubrizol application labs use Gamry Reference® 600 Potentiostat/Galvanostat instruments and many glass test-cells to perform EIS testing.
Compared with traditional accelerated corrosion tests, such as salt fog, EIS provides an objective, faster and more reliable, quantitative test for protective coatings. It is also a non-destructive test method, as opposed to the scribes made on test panels in salt spray testing, to monitor the coating damage process and understand the failure mechanisms.
Protective Coatings and Additives
The EIS method can provide the mechanisms to help understand the structure/property relationship to accelerate and improve the development process of protective coatings in direct-to-metal (DTM) applications, and Lubrizol takes full advantage of this capability. There are two important ways EIS technique helps with improving polymers. First, EIS provides much faster performance evaluation—at least 10 times faster compared with other common test methods, like salt-spray. Second, the modeling analysis of EIS helps understand the failure mechanisms (barrier, adhesion, and passivation) and the damage evolution of a new polymer, which provides valuable inputs. That is something common test methods cannot do.
"To improve dispersant performance for DTM coatings, Lubrizol has used EIS to study the effect of different types of dispersants in white (TiO2) paint on cold rolled steel", shares Ximing. "We designed different models to differentiate two protective properties: barrier and scribe adhesion. This helps to understand how a dispersant interacts with pigments in a polymer."
Ximing continues, "the EIS technique is not just for DTM coatings and additives, but the system must be able to connect with an electrochemical test cell, which is why it is widely used in applications of corrosion, biosensors, batteries, fuel cells, metal coatings and physical electrochemistry. For example, we have used EIS to evaluate rust preventatives and anti-flush inhibitors."
In summary, EIS is an important and powerful tool to improve polymers and dispersants for DTM applications, and in various other electrochemical related areas. In protective coatings, the EIS technique offers several advantages in predicting real world corrosion resistance more accurately and faster over other testing methods, like salt-fog testing, due to the poor repeatability and reproducibility of those other methods.
Contact us to learn more about EIS testing or other electrochemical test methods or to discuss how our work in this area brings reliable performance you can trust for your coating challenges.