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Mica and its anti-corrosive properties

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

The products from the MicaFort portfolio with differing particle size distribution, aspect ratio and chemical composition are being evaluated to highlight the best solution for recommendation as an anti-corrosive pigment in two component epoxy type of coatings.

Metals have the natural tendency to oxidize, as they want to return to their original state. As this is not desired for man-made steel or metal structures, these need to be protected against corrosion. “It is estimated that the costs for corrosion exceed USD $2.2 trillion on an annual basis, equal to approx. 3% of the global GDP,” says Mark Smit, Research & Development Manager at LKAB Minerals and in charge of this project.

In the past, extensive and time consuming salt spray testing were the norm which could last up to 60 days, not ideal for fast development of improved formulations. More recently, Electrochemical Impedance Spectroscopy (EIS) has been introduced 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.”

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.

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.

Mark Smit is 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.

The experimental part

To be able to evaluate the differences of the lamellar fillers tested the formulations were adapted with only slight variations in dispersant agent due to the filler’s characteristics. Each formulation was optimized for the amount of dispersant concentration (Anti Terra U). In order to ensure obtaining proper dispersions, the formulations were prepared 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, which 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 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:

> 108 Ω cm2 : excellent
> 107 Ω cm2 < 108 Ω cm2 : sufficient
> 106 Ω cm2 > 107 Ω cm2 : doubtful
< 106 Ω cm2 : bad

Mica for the best anti-corrosive protection: The results

The results of the testing made clear that the type of mineral filler being used has a strong influence on the anti-corrosive properties of the coating. All formulations have been compared to a reference where no lamellar filler has been used at all. “Some fillers show a very similar anti-corrosive performance as the reference, but 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) and an aspect ratio of 34 which is similar to another grade in the portfolio of LKAB Minerals for other applications.

A clear advantage using MicaFort MM75 in intermediate coats in anti-corrosion or protective coating systems are the increased performance on the one hand but also 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 20% overall weight saving. As most paints are sold by volume, a total cost saving of 21% can be achieved.

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