Month: October 2021

Corrosion is a process where the metal can be degraded by electrochemical and/or chemical processes. Metals desire to be in their most thermodynamically stable state, which, in simplified terms, is the naturally occurring state of matter in its lowest energy state. Metals ordinarily exist naturally as oxides (e.g., iron oxide, aluminum oxide, zinc oxide, because oxides represent their lowest energy state). As corrosion is normally accelerated by the presence of water, oxygen and salts (particularly of strong acids), the function of a protective coating system is to maximize protection of metal substrate from these forces.

Oxidation occurs at the anode (positive electrode), and reduction occurs at the cathode (negative electrode). Corrosion is normally accelerated by the presence of water, oxygen and salts (particularly salts of strong acids).

Corrosion process in the case of steel (Figures 1 and 2) below.

This article will consider the influence of the following formulating factors on the corrosion resistance of a coating.

1. Type of metal
2. Pigment level and selection
3. Corrosion inhibitive pigments
4. Service environment considerations and new innovations

1. Type of metal

According to the EMF series, aluminum and zinc are more active than iron and oxidize more quickly when exposed to oxygen and water. However, the oxides of uncoated zinc and aluminum form a tightly bound layer to the metal surface that decreases the rate of corrosion of the underlying metal. Whereas when carbon steel rusts, the corrosion product is ferric oxide that is loosely attached to the surface that is prone to more rapidly oxidize.

In the EMF series (figure 3 below), Zn is more active than Fe. When a zinc-rich primer is applied over steel, or in the case of galvanized steel, zinc will oxidize preferentially to steel and thus prevent the underlying steel from oxidizing. In this scenario, Zn is anodic (more readily oxidized) to steel and therefore protects steel from oxidation. Thus, steel is protected from corrosion by cathodic inhibition, as well as by the barrier that the zinc-rich primer provides.

2. Pigment considerations

The PVC (pigment volume concentration) of a system is defined as the volume percentage of solid particles in the system after film formation, when all volatile ingredients such as solvents and water have evaporated. The level and type of pigment used in a primer not only affects initial coating adhesion, but also its longevity while in service. Most primers are formulated at or slightly below Critical Pigment Volume Concentration (CPVC) to maximize topcoat adhesion (rougher primer surface and higher free energy) as well as many other coating properties (Figure 4).

PVC and the relationship between PVC to Critical Volume Concentration is a critical consideration and governs not only mechanical properties, but also influences moisture and oxygen permeation through the coating film to the metal substrate. Depending on the application and the desired mechanical properties (e.g., substrate adhesion, flexibility, topcoat adhesion, sandability), corrosion-resistant primers are formulated at various PVC’s and contain a variety of inert pigments.

The use of more polar pigments may provide ease of wetting during the pigment dispersion process, but may degrade long-term adhesion as they are more susceptible to moisture migration and disbondment at the coating-substrate interface. Plate-like pigments and pigments that have very low or no water-soluble components also enhance longevity.

Pigment particle size, shape and structure can influence moisture and oxygen permeation and ultimately corrosion resistance. Pigments with platelet-shaped particles can reduce permeability, especially if they are aligned parallel to the coating surface. Mica, micaceous iron oxide and metal flakes are a few examples of such pigments. These platy pigments provide a more torturous path water, soluble salts and oxygen to reach the metal surface. Other pigments that contribute to corrosion resistance include Platy aluminum silicate and Wollastonite (calcium silicate).

3. Corrosion inhibitive pigments

As detailed above, PVC and selection of select inert pigments influence barrier properties of a coating and enhance corrosion resistance. Corrosion inhibitive pigments impact the rate of corrosion by two main mechanisms, cathodic and anodic inhibition. Cathodic inhibition inhibits corrosion by impeding the flow of electrons at the cathode, whereas anodic inhibition inhibits corrosion by impeding the flow of electrons at the anode.

When choosing a corrosion inhibitive pigment, several factors must be considered. Environmental factors that influence the rate of corrosion include moisture, pH of the moisture, wet and dry cycles, soluble salts, temperature and time. With these issues in mind, the evaluation criteria and test methods must be carefully contemplated before selecting corrosion inhibitive pigments. Corrosion inhibitive or passivating pigments promote the formation of a barrier layer over anodic areas, thus passivating the surface. To be effective, these pigments have a minimum solubility. If the solubility is too high, the pigment will leach out of the coating too rapidly, reducing the time that the pigment is available to inhibit corrosion. If the coating film is more open (e.g., air dry latex), water permeation is higher, and thus the corrosion inhibitive pigment will be depleted more rapidly. To function properly, the coating must permit the diffusion of some water to dissolve the pigment. Accordingly, blister formation may result under humid conditions as the pigment dissolves. Higher Tg (glass transition temperature) and higher cross-link density binders are known to improve blister resistance.

The vast majority of corrosion inhibitive pigments are comprised of the combination of metal ions (cations) derived from zinc, strontium, chromium, lead, molybdenum, aluminum, calcium or barium and anions, such as those derived from phosphorous (orthophosphoric and polyphosphoric acids), chromic acid and boric acid. Although chromate and lead, containing passivating pigments, are very effective in inhibiting corrosion, their use is very limited due to a variety of environmental and toxicological regulations.

Another prime consideration in the selection of a corrosion inhibitive pigment is the pH. For example, a pigment with a high pH may have a deleterious effect on the cure of acid-catalyzed systems. Conversely, a pigment with a low pH may adversely affect the stability of waterborne systems.

4. Service environment, considerations and new innovations

The relative corrosion resistance of coatings can vary dramatically depending on the test method and exposure conditions. Common test methods include salt spray (95% humidity/5% salt and always moist), acidic salt spray, prohesion cyclic corrosion (wet and dry cycle with 0.04% ammonium sulfate and 0.05% salt), electrochemical impedance spectroscopy and salt soak. Most experts agree that accelerated tests are not always a good indication of how the coated metal will perform in the real world.

Additional considerations are the metal type (e.g., steel, aluminum, galvanized), pretreatment and cleanliness of the surface. If the metal surface is not properly cleaned and prepared, the coating will lack adequate adhesion and premature failure will result.

Furthermore, the type of coating in which the pigments will be used affects the selection of appropriate corrosion inhibitive pigments. Considerations include whether the coating is solvent-borne, waterborne, powder, air dry or baked, and if the film will be cross-linked or thermoplastic.

Other formulating factors that have a profound influence on substrate corrosion include the degree of hydrophobicity of the coating. Surface and volume hydrophobicity can be increased by the use of surface modifiers of specially designed/structured pigments as well as the addition of hydrophobic additives that minimize moisture permeation of the coating and thus decrease the rate of corrosion.

It is our experience that a coating with a high contact angle and volume hydrophobicity will also provide excellent retention of adhesion after accelerated testing such as salt spray or condensing humidity.

Two-component polyester urethane with a 155-degree contact angle and excellent volume hydrophobicity formulated in the laboratory of Chemical Dynamics, LLC

A sampling of suppliers of Corrosion Inhibitive Pigments include:

• Buckman
• Grace
• Halox (EU)
• Heubach (EU)
• Nubiola (EU)
• SNCZ (EU)

The previous article titled Remain Bug Free with Antimicrobial Coatings described fundamental aspects of Antimicrobial (AM) coatings as well as AM agents. This article will provide an update on AM coatings technology in the form of paint additives and technology approaches that act to kill microorganisms or minimize their growth on the coated surface.

According to the Grand View Research report, the compound annual growth rate (CAGR) is expected to be 13.1% from 2021 to 2028, with a global market size of 8.1 billion USD in 2020. Major market areas include:

• Medical
• Heating, Airconditioning and ventilation (HVAC)
• Food processing and sanitary facilities
• Mold remediation

Antimicrobial materials can function to kill or combat the growth of bacteria, viruses, fungus and algae on the coating surface. Control of microbes can be achieved through the use of antimicrobial technologies that keep microorganisms from multiplying or growing, providing hygienic surfaces in hospitals and the food industry and preserving the integrity of paint films.

This article will focus on antimicrobial materials and approaches to design AM paint films. Applications where AM agents are used in coatings include the following microbe classifications:

• Fungi
• Mold (form of fungus)
• Bacteria
• Algae
• Virus

Most biocides used in paints are migratory as they function by releasing the active ingredient to the surface of the coating when exposed to moisture. The longevity of the AM modified paint film depends on the rate of release of the biocide as the concentration of the active ingredient decreases with time.The effectiveness of an AM additive in a paint film is dependent on concentration, resin system, gloss, PVC, coating surface structure and the environment to which it is exposed. The choice of AM agent depends on the desired function in the AM coating system. In addition, before selecting the AM, carefully review the MSD and TDS for safety, environmental acceptability and compatibility prior to incorporation in a paint.

Examples of AM agents

• Mold/Fungi
  • IPBC (3-iodo-2-propenyl-butyl carbamate)
  • BBIT (N-butyl 1,2 benzisothazolin-3-one)
  • Zinc dimethyl thiocarbamate
  • 2-n-octyl-4-isothiazoline
• Bacteria
  • Tetrahydro-3,5-dimethyl-2h-1,3,5-thiadiazine-2-thione
  • Zinc oxide/1,2-benzuisothiazol-3-(2H)-one
  • Zinc Pyrithione
  • Silver-zinc Zeolite
  • Carbon-based materials (graphene, carbon nanotubes etc.)
• Algae – many of the AM agents that are effective for Mold and Fungi are also effective to control Algae growth
• Virus
  • Silver zeolite, silver compounds and silver nanoparticles
  • Copper and copper alloys
  • Carbon-based materials (graphene, carbon nanotubes etc.)

How do AM Agents Function in Coatings?

• Metal and Metal Compounds and Metal Nanoparticles

The use of metals such as silver, copper (and many copper alloys ) and zinc in various forms in paints can be effective antimicrobial additives. There are several mechanisms by which silver acts as an antimicrobial. One such example is that silver ions react with the thiol group in enzymes leading to cell death. The mechanisms through which copper acts to destroy cells include the generation of hydrogen peroxide in the cells or excess copper can also bind with proteins resulting in the breakdown of the protein into nonfunctional sections. Zinc pyrithione/2-propynyl butylcarbamate acts both as a preservative and as a fungicide. The EPA oversees the regulation of antimicrobial agents and materials and determined that copper alloys kill more than 99.9% of disease-causing bacteria within just two hours when cleaned regularly. Copper and copper alloys are unique classes of solid materials as no other solid touch surfaces have permission in the U.S. to make human health claims. Accordingly, the EPA has granted antimicrobial registration status to 355 different copper alloy compositions. Metal nanoparticles, including PVP and polysaccharide-coated silver nanoparticles, MES-coated silver and gold, have also demonstrated promise as antiviral agents. Copper nanoparticles have demonstrated antimicrobiological activity with Ecoli, fungus and bacteria.

• Quaternary Ammonium Compounds

Some examples include dimethyloctadecyl (3-trimethoxysilyl propyl) ammonium chloride, alkyldimethylbenzylammonium chloride and didecyldimethylammonium chloride. Some silanes form a needle-like surface structure by the bonding of 3-(trihydroxysilyl) propyldimethyloctadecyl ammonium chloride to the surface to destroy microbes by rupturing their outer membrane as they come in contact with surface spikes.

• Carbon-Based Materials (CBMs)

Graphene materials (GM) such as graphene oxide, reduced Graphene Oxide (rGO) and Carbon nanostructures (CNSs) such as fullerene and carbon nanotubes (CNTs). There is not total agreement on how these materials function as AM agents (bacteria); however, the unique physicochemical characteristics such as particle size, morphology and surface structure properties of CBMs provide nanoknives, oxidative stress and wrapping/trapping of microbes.

• AM Smart Hydrogels

Hydrogels are comprised of 3D networks of crosslinked hydrophilic polymers that are responsive to changes in environmental stimuli such as pH and temperature that result in the destruction of microbes.

• Cationic Polymers                        

Cationic polymers are defined as polymers with an electropositive charge on the polymer or AM backbone. They have efficacy in use in AM coatings and are unique in the fact that they have the ability to kill microbes on contact. As opposed to conventional bioactive materials, appropriate cationic polymers and functionalized molecules can be effective without the release of AM chemicals. Such chemicals are currently used in biomedical applications and include ammonium, phosphonium, sulfonium, pyridinium salts and guanidines. Many of these materials with cationic salt functionality have broad antibacterial activity.

• Self-Cleaning Surfaces

There are three categories of self-cleaning surfaces, superhydrophobic, photocatalytic and superhydrophilic. Superhydrophobic surfaces (contact angle > 150 degrees are water-shedding and thus repel dirt. As many superhydrophobic coatings have a low water-roll-off angle (ROA), this characteristic also provides self-cleaning properties. The surface structure of SH coatings is characterized by a needle-like micro-structure coupled with components that provide a low surface tension. Such surface structures also have efficacy in reducing the ability of microbes to adhere to the surface, thus imparting antimicrobial activity. Photocatalytic surfaces degrade surface deposits when exposed to light. Superhydrophilic coating surfaces (contact angle < 10 degrees) enable dirt and water to easily slide off the surface.

Final thoughts

The future of AM coatings technology will include a combination of technologies that will maximize their effectiveness and longevity. This may include the incorporation of AM agents in Self-cleaning coatings, the addition of slow-release AM nanomaterials and that are absorbed or adsorbed on high surface area particles. Smart AM materials that respond to environmental stimuli such as fluctuations in pH and/or temperature as well as have a surface structure that can rupture the offending cell membrane.