Anti corrosion coating - EIS testing

The natural mineral supplier LKAB Minerals is currently evaluating various mica products.

This article will highlight the products from the MicaFort portfolio. We evaluate products with a differing particle size distribution, aspect ratio and chemical composition. As a result, you can select the best solution for the recommendation as an anti-corrosive pigment in a two-component epoxy type of coatings.

Corrosion protection with MicaFort

Metals have the natural tendency to oxidize, as they want to return to their original state. However, we do not desire this for man-made steel or metal structures. Therefore we protect these against corrosion.

“The industry estimates that the costs for corrosion exceed USD $2.2 trillion on an annual basis. This amount equals approx. 3% of the global GDP,” says Mark Smit, Research & Development Manager at LKAB Minerals and in charge of this project.

Modern corrosion testing

In the past, extensive and time-consuming salt spray testing was the norm. These tests could last up to 60 days, not ideal for fast development of improved formulations. More recently, the industry introduced Electrochemical Impedance Spectroscopy (EIS) as a fast and reliable method to assess the anti-corrosive properties of coating systems.

Smit explains, “EIS is an electrochemical measuring method quantifying the protective behaviour of coatings. Even short periods of testing using EIS provides reliable data to predict real-life performance.”

How to improve anti-corrosive behaviour?

In order to improve the anti-corrosive behaviour of coatings, platy minerals like talc, micaceous iron oxide (MIO) or mica are chosen fillers for this coating type. Flat, platy particles overlap in a film and prevent water, oxygen or other chemicals ingress through the coating, thereby increasing, on the one hand, the diffusion pathways and on the other hand, the time for corrosion stimulators to reach the substrate.

Experience in Mica

LKAB Minerals have been manufacturing high quality, high aspect ratio Mica products for over 30 years offering both Muscovite and Phlogopite. “LKAB Minerals has recently changed to the brand name MicaFort which encompasses our high quality, high aspect ratio and very low silica Muscovite and Phlogopite grades,” explains David Högnelid, Marketing & Communications Manager at LKAB Minerals.

We are currently evaluating various MicaFort products which differ in processing method, aspect ratio and origin to understand their influence on the anti-corrosive properties of a two-component epoxy coating.

Experimenting with MicaFort fillers

We adapted the formulations with only slight variations in the dispersant agent due to the filler’s characteristics. This allowed us to evaluate the differences between the tested lamellar fillers. Each formulation was optimized for the amount of dispersant concentration (Anti Terra U).

In order to ensure obtaining proper dispersions, we prepared the formulations with the purpose of matching

  • an equal ratio of PVC/CPVC to the reduced PVC Δ = 0.80
  • a solids volume concentration of 75% ± 5
  • a remaining VOC < 300 g/L

For all formulations, first, the solvents and epoxy resin were homogenized. Next, this was followed by additive incorporation. Subsequently, the solid materials were added and dispersed at 300 rpm for 1 hour. Directly prior to curing the hardener was added.

The paints were applied on steel panels with a wet thickness of 200 microns. This was done in order to obtain a dry film thickness (DFT) of approximately 150 microns. The panels were left to cure for seven days at room temperature and 75% relative humidity. Subsequently, a PVC plastic tube was sealed on the surface and filled with a salt solution (3.5% NaCl).

The corrosion resistance of the panels was measured by EIS immediately, after one day, seven and 21 days of exposure. A working electrode of 1 cm2 was used, with a graphite counter electrode and a Ag/AgCl reference electrode. The impedance is measured over a range of 1 MHz to 0.01 Hz using a 50 mV amplitude sinusoidal voltage. The impedance of the electrical circuit is a guideline of the protective behaviour of the coating after the following rule:

Excellent: > 108 Ω cm2
Sufficient: > 107 Ω cm2 < 108 Ω cm2
Doubtful: > 106 Ω cm2 > 107 Ω cm2
Bad: < 106 Ω cm2

Mica for the best anti-corrosive protection: The results

The results of the testing made clear that the type of mineral filler you will use, has a strong influence on the anti-corrosive properties of the coating. We compared all formulations to a reference where no lamellar filler has been used at all.

“Some fillers show a very similar anti-corrosive performance as the reference. However, there is one grade that exceptionally increased the corrosion resistance of the coating: MicaFort MM75” explains Mark Smit. This is a very fine grade with a particle size top cut of 75µm (D98). The grade has an aspect ratio of 34 which is similar to another grade in the portfolio of LKAB Minerals for other applications.

Advantage using MicaFort MM75

A clear advantage of using MicaFort MM75 in intermediate coats in anti-corrosion or protective coating systems are the increased performance on the one hand. And in addition the lower density, lower weight and most importantly lower costs, compared to intermediate coats based on a commonly used platy filler. The MicaFort based coating has a 25% lower density, resulting in a 20% overall weight saving. The industry sells most paints by volume, allowing a total cost saving of 21%.

For further information, please contact LKAB Minerals. You can receive the article as published in the June issue of PPCJ upon request.