Jianguo Wang – Ion-Barrier Coatings: The Next Generation of Anticorrosion Technology

Corrosion, the gradual destruction of metals, is a significant physical and economic problem worldwide. Traditional heavy metal-based coatings used to protect metals are now viewed negatively due to their impact on the environment. Research led by Jianguo Wang of AnCatt Inc reveals why ion-barrier coatings are the next generation of anticorrosion coating technology.

Economic and Environmental Costs

Anybody who neglects oiling their steel bicycle chain, particularly after being subjected to rain, will soon notice the once shiny metal turn into a deep orange brown. This is an example of corrosion – the natural, gradual destruction of metals. Iron within the bicycle chain reacts with oxygen and moisture to form iron oxides, or rust, and the once useful properties of the steel, including strength and appearance, are modified. Left alone, this surface corrosion causes pits and cracks, finally rendering the metal useless.

According to a report issued by NACE International, metal corrosion causes $2.5 trillion USD in losses globally each year, in the order of 2–3% of gross national product (GDP). Therefore, protective coatings for metal are extremely important.

Historically, coatings containing lead and chromates have proven to be successful, but are now considered environmentally poisonous. Chromate-containing anticorrosion coatings, once used extensively within the aircraft, automotive, construction and marine industries, are now known to be human carcinogens. The EU banned their use in the automotive industry in 2007 and these coatings are under increasing scrutiny worldwide, which creates a need for non-toxic and environmentally benign alternatives. Zinc is another common metal used in anticorrosion coatings; while it is less toxic, this heavy metal is also less effective and cannot protect aluminium from corrosion.     

Insights into Polyaniline

Newly-developed coatings based on an organic polymer called ‘polyaniline’ are exciting due to their effectiveness at preventing corrosion and their relatively benign impact on the environment. Although discovered over 150 years ago, attention from the scientific community remained low until the 1980s, when the material’s high electrical conductivity was rediscovered and its anticorrosive effect was observed.

Based on its anticorrosive properties, a German company called Ormecon GmbH developed a polyaniline paint product called Corrpassiv™. Soon after, many scientists attempted to understand the corrosion protection offered by doped polyaniline based on its conductive nature. Many research papers were published and several patents were issued. Then, in 2008, Enthone Inc announced the acquisition of Ormecon GmbH, and their aims to pursue other applications of polyaniline. Unfortunately, Enthone Inc decided not to continue the development of polyaniline anticorrosion coatings, indicating that there were challenges associated with their development.   

Jianguo Wang, a chemist, began studying polyaniline coatings in 1993 at Drexel University in Philadelphia. Wang joined the DuPont Chemicals Company in 1995 (now DowDuPont Inc) and, although focused mainly on other projects, continued to study polyaniline as a side project. His aims included understanding the chemical structure of polyaniline and its anticorrosion mechanisms. He left DuPont in 2008 and founded the Ancatt Company, which has now successfully formulated anticorrosion coatings based on conducting organic polymers.

There are two basic forms of polyaniline materials – a conducting form and an insulating form, known respectively as emeraldine salt and emeraldine base. The corrosion prevention effect of the conducting form has been known since the 1980s, attributed to its metallic-like properties. However, it was more surprising to find that anticorrosive properties are also present in the insulating base form, even outperforming the salt in certain circumstances. Furthermore, chemical analysis of the emeraldine base conflicted with the structure that was accepted at the time for this compound.

Wang investigated these discrepancies and published his results in a paper in early 2002. By adding emeraldine base powder to various salt solutions, he observed that the presence of emeraldine base modifies the nature of the salt present. Emeraldine base, he realised, acts as an ‘anion’ exchanger. Anion is the name given to a negatively charged ion, whereas cations are positively charged. In simple terms, emeraldine base exchanges anions within its structure with anions in the salt. Wang concluded that emeraldine base must have a different structure to the one that was accepted, and proposed a new chemical formula and structure. He also concluded that it is the material’s anion exchange behaviour that partially explains the anticorrosion properties of the material.

Further Experiments

Later that year, Wang published research investigating further the anticorrosive properties of polyaniline. In his experiment, he applied different configurations of polyaniline coatings, of the same thickness, to steel panels and measured the corrosion protection offered in terms of the ‘pore resistance’ – a measure of how ions flow through the material.

One of the coatings applied, termed ‘bipolar’, consisted of a polyaniline layer applied directly to the steel surface and a cation-exchanging layer applied on top (a topcoat). A second ‘non-polar’ coating consisted of a single layer of the two materials mixed together, and a third consisted solely of a single layer of polyaniline.    

The ‘bipolar’ layer offered the best protection in terms of preventing corrosion, attributed to the polyaniline forming a barrier to positively charged metal cations, whilst the topcoat forms a barrier to aggressive negatively charged anions interacting with the metal. Effectively, the bipolar coating acts as an ‘electronic barrier’ to both cations and anions, and hence the ‘pore resistance’ has a high value.

On the other hand, single-layer polyaniline coatings are permeable to aggressive anions and hence have limited lifetime in preventing corrosion. With the two layers mixed in the non-polar layer, the coating is permeable to both anions and cations, and the anticorrosive ability is negligible. In a comparison of the base and salt form of polyaniline, Wang concluded that emeraldine base is a stronger anticorrosive material than emeraldine salt when a topcoat is absent.

Wang, together with Charlie Torardi and Michael Duch, fellow colleagues at DuPont, published research in 2006 that further examined the anticorrosive properties of polyaniline by applying various coatings to filter paper to make membranes. To assess the anticorrosive properties, they measured how easily charged particles move across the membranes. Wang’s conclusions from these experiments agreed with his earlier 2002 study. Bipolar layers form barriers to cations and anions and hence they have an increased ability to prevent corrosion. Mixed layers result in imperfections, reducing their ability to protect against corrosion.

A second aspect of the team’s research involved coating steel panels, as Wang had done in his 2002 study, but this time adding a ‘reverse bipolar’ coating – a cation-exchanging layer coated on the metal first with polyaniline used as a topcoat. As found in the 2002 study, single-layer polyaniline coatings impede the movement of cations but cannot prevent the passage of aggressive anions such as chlorides (found in salt). Again, they found that the bipolar layer is most effective due to its dual behaviour of preventing anions and cations permeating through.

Delamination of organic coatings, where the material breaks by fracturing into layers and separates from the metal surface, is one of the most common forms of failure and occurs when cations enter the coating-metal interface. Hence, single-layer coatings consisting solely of a cation exchange resin that is permeable to cations are ineffective, as are ‘reverse bipolar’ coatings when the cation exchange resin is applied to the metal surface first.

Next Generation Coatings

Since founding the AnCatt Company based in Newark, Delaware, Wang and his colleagues have successfully developed award-winning, heavy metal free, highly effective anticorrosion coatings for metals. AnCatt claims that their coatings are ion-barrier coatings, which combine a primer that acts as a barrier to cations with a topcoat that acts as a barrier to anions. The primers include, but are not limited to, polyaniline coatings. These ion-barrier coatings are expected to be the next generation of anti-corrosion coating technology. 

In a 2017 study, Wang and his colleague Sue Wang compared AnCatt’s ion-barrier anticorrosion coating to commercially available zinc-rich and zinc phosphate coatings. They applied different coatings to steel panels, scratched them down the middle and exposed them to a ‘salt-fog’ – a quality control widely used within the paint industry.

An independent company employing standardised corrosion rating systems evaluated the damage after 700 hours and 2000 hours of exposure. After 700 hours, the level of corrosion with AnCatt’s ion-barrier coating applied was almost equal to that observed with commercially available zinc-rich paint, but corrosion was notably less after 2000 hours. In fact, there was essentially no difference in corrosion between 700 hours and 2000 hours when employing the ion-barrier coating.

Using the standardised measuring systems, all the coatings received a perfect 10 for ‘Red rust rating’ and ‘Blister rating’ but the ion-barrier coating performed 1–3 points higher (on a scale of 1 to 10) for ‘Scribe rating’ than the commercially available coatings.

Top: Bare panels exposed to the atmosphere for 7 months. Bottom: AnCatt coated panels exposed to the atmosphere for 1 year.

A Lustrous Future

Through over 25 years of leading research starting with the organic polymer polyaniline, Jianguo Wang has provided insights into ion barrier coatings, and the reasons behind their effectiveness as an anticorrosive coating for metals. As we move away from the use of traditional heavy metal-based coatings due to their deleterious impact on the environment and human health, ion-barrier coatings have a bright, lustrous future.