Following are excerpts from an article that appeared in January 2022 issue of European Coatings Journal written by Dr. Cathy Cooper, Applications Scientist at Lubrizol.
Corrosion of metal surfaces can be a significant problem for sustainability—decreasing the product lifecycle and wasting the energy and resources that have gone into making the metal structure. This has a direct economic impact: a report by the National Association of Corrosion Engineers (NACE) estimates the worldwide cost of corrosion at US$2.5 trillion, equating to 3.4% of global gross domestic product(1). The costs can be split into two types: corrective and preventative. Corrective costs include labor hours and materials for replacing corroded items, as well as product lost as a result of leaks or accidents involving corroded infrastructure. Preventative costs incorporate methods for reducing corrosion on metal surfaces, including the option of applying a surface coating.
Corrosion is a chemical or electrochemical process between a metal and the surrounding environment that causes deterioration of the surface(2). The information that follows focuses on steel surfaces where the iron acts as the anode.
The role of a protective coating is to provide a barrier between the anode and the cathode or surrounding electrolyte. An ideal coating system needs to be impermeable to water and oxygen, and not allow for transport of ions through the coating layer. Since a protective coating is designed to form a barrier between the metal surface and the surrounding environment, the nature of this environment must also be considered. Traditionally, protective coatings for areas with the harshest conditions were provided by multi-layer solvent-borne systems, but there has been increasing research into alternative coatings that have a lower volatile organic content (VOC)(3). One way to lower the VOC of a coating is to employ a water-borne system.
The Need for Superior Dispersion
Scientists at Lubrizol investigated how direct-to-metal (DTM) water-borne coatings could be optimized to increase their use across the full range of corrosion environments—and to develop a novel technology for water-borne DTM coating applications.
The major components of a water-borne coating formulation are the polymeric resin binder, the pigment and a dispersant for ensuring that the pigment remains sufficiently stabilized in the system. Optimizing the dispersant will yield an even, defect-free dry film where the pigment is a suitable particle size and not agglomerated. A coating where the pigment is sufficiently well dispersed is essential for delivering acceptable corrosion performance.
To investigate developing novel DTM water-borne coating dispersant technology, Lubrizol scientists applied their in-depth understanding of dispersion and corrosion pathways. The goal was to develop dispersants for the Solsperse™ Hyperdispersant product range that would balance dispersion performance with inherent corrosion resistance.
Scientists produced multiple water-borne formulas. To evaluate performance, the impact of a dispersant on corrosion properties was evaluated by means of electrochemical impedance spectroscopy (EIS) and salt spray exposure. A rutile titanium dioxide pigment was used, with a millbase pigment loading of 63.3% and a dispersant agent on weight of pigment (AOWP) of 2.7%, unless otherwise stated. The pigment volume concentration was 11% and the gloss of this formulation was approximately 85 at 60°.
Once the formulations had been produced, they were coated on “QPanel” S-46 steel panels with the aid of a 10 ml well applicator. This yielded a final coating of approx. 85 μm thickness, with 0.85% dispersant in the film after it had dried at room temperature for a week. These panels were used intact for EIS measurements performed at room temperature with a 5 wt.% sodium chloride solution.
A screening formulation, in which the acrylic resin was not optimized for corrosion resistance, was used for the EIS studies. These experiments gave data on the extent to which the coating on the panels provided a barrier to ion transport. This technique measures the electrical resistance of the coating at different frequencies, and mathematical modelling is employed to obtain a resistivity value, Rp. A higher Rp denotes a better coating.
Analysis of EIS data and salt spray experiments on the scribed panels to determine the link between pigment-dispersion quality and corrosion resistance showed that film defects or overdosing of a dispersant caused a greater degree of ion transport through the film.
Analysis of the EIS data also revealed that the novel dispersant yielded a resistivity of 4.35 x 106 ohm/cm2 after 10 days exposure to the salt solution, compared with the market standard value of 7.21 x 105 ohm/cm2. This indicates significantly less transport of ions through the coating that contained the dispersant, and therefore less corrosion.
For the best corrosion resistance, optimization of the dispersant structure is required in addition to optimization of the loading level. It was found that the novel dispersant technology enabled milling of the titanium dioxide at 5 % AOWP in the corrosion screening formulation, yielding a resistivity of 7.95 x 106 ohm/cm2. The impact of this inhibition of ion transport in the film on the corrosion performance can clearly be seen in the images below, which show the film after four days exposure to the salt solution. The market standard exhibits significant blistering and surface rust, even after loading optimization.
Excellent Corrosion Resistance
Results in the fully-formulated corrosion panels were analyzed after four weeks of exposure to salt spray. The panel with no dispersant showed some scribe growth and blistering around the scribe, but also significant blistering and film defects in the field area. This was due to the insufficient stabilization of the pigment dispersion in this system, and this would not yield a suitable paint film for a DTM application. The panel with the market standard dispersant shows a significant deterioration of corrosion performance relative to the panel without any dispersant additive. This is potentially due to the hydrophilic nature of the stabilizing groups on a dispersant. The film quality was much improved from the panel without dispersant, but the blisters did extend away from the scribe. The scribe itself grew as a result of corrosion over the course of the four-week exposure, yielding a final scribe growth of 2.19 mm.
The panel with the novel dispersant shows a significant improvement over the market standard in terms of corrosion, and over the no-dispersant panel in terms of film properties. Although there are some blisters surrounding the scribe, these are fewer in number than for the market standard. Scribe growth was only 0.90 mm and there were no adverse effects on the film properties. Overall, the novel dispersant yielded a good balance of suitable film properties and corrosion resistance.
(1) Nace International Impact Report, International Measures of Prevention, Application and Economics of Corrosion Technologies Study, March 2016
(2) Revie, R. Winston. Corrosion and corrosion control: an introduction to corrosion science and engineering. John Wiley & Sons, 2008
(3) Faccini, Mirko, et al. Environment ally Friendly Anticorrosive Polymeric Coatings, Applied Sciences 11.8 , 2021, 3446