Early corrosion detector

Dr. Neil Canter, Contributing Editor | TLT Tech Beat February 2010

Researchers have developed a ‘smart’ epoxy coating that can detect early steel corrosion. 

KEY CONCEPTS
• Early detection of corrosion can be achieved through activation of a fluorescence probe in a process known as chelation-enhanced fluorescence.
• A probe based on spirolactam has been found to fluoresce reversibly with ferric ions.
• This probe can detect corrosion at an earlier stage than seen with the naked eye.

With the challenges of stopping corrosion, a number of technologies have emerged to deal with it through a stimulus-generated response. This means that an external mechanism impinges on the specific area suspected to be corroding, and a response is generated to assist with retarding the process.

In a previous TLT article, the concept of self-healing coatings was discussed (1). A new approach to fix an epoxy coating on a metal substrate was described that involved incorporation of microcapsules of a monomer and catalyst into the coating. These capsules open when the coating is ruptured and exposes the substrate. The monomer and catalyst react to form a self-healing coating that provides protection to the section of substrate exposed.

Dr. John Tsvalas of the Nanostructured Polymers Research Center at the University of New Hampshire, in Durham, N.H., says, “Self-healing polymers is just one of several techniques that are in the field of stimuli response. Other approaches include ion sequestration technology and release of rust inhibitors. Missing from these technologies is a way to detect corrosion at a very early stage.”

Corrosion is a process that initially starts on the molecular level and is very difficult to identify at an early stage. Before this electrochemical process makes a major impact in destabilizing a specific substrate, detection at an early stage enables maintenance to be conducted to minimize further damage.

One strategy that has been looked at is to use a pH-triggered mechanism in which a specific substance is incorporated into a coating and changes either color or fluorescence. Tsavalas says, “Compounds sensitive to changes in pH have worked with acrylic and polyurethane coatings. But a compound sensitive to pH changes has not worked well in epoxy coatings.”

The reason for the problem with epoxy coatings is the raw materials used in their preparation. One of the precursors used to prepare epoxies is an amine that exhibits basic pH values. Residual amines in epoxy coatings tend to prematurely trigger the currently known pH indicators, leading to misleading results.

Epoxy-based coatings are widely used to protect steel substrates. A new approach is needed to develop early detection technology that will not prematurely activate. According to UNH researcher Dr. Weihua “Marshall” Ming and Tsavalas, “Key characteristics that are required for a fluorescence indicator include no premature reaction with the coating during application; ability to remain in a dormant, non-fluorescence state when no corrosion is present; effectiveness to fluoresce at low concentrations when corrosion is taking place and to maintain effectiveness, even in the presence of other ingredients such as pigments.”

Such a technology has just become available. Drs. Tsavalas and Ming along with fellow UNH researcher Anita Augustyniak have developed a “smart” epoxy coating that can detect early steel corrosion.

FLUORESCENCE INDICATOR 
In looking through the literature, the researchers noted that a number of fluorescence probes have been developed for use in bioimaging. In particular, a series of probes could be activated in the presence of specific metal cations in a process known as chelation-enhanced fluorescence (CHEF).

They focused their interest on a probe based on a spirolactam that fluoresces reversibly when chelating ferric ions. This molecule is known as FD1 and has appeal as a corrosion detector because it fluoresces selectively with ferric ions.

The researchers initially worked with the CHEF in a solution environment where ion mobility is high. They found that it worked well and then embedded FD1 into epoxy coating films to check its efficacy through titration with ferric ions.

A laser scanning confocal microscope was utilized to image the fluorescence, which is observed between wavelengths of 520 and 660 nanometers with the maximum emission near 580 nanometers. The coating thickness ranged from 30 to 40 microns.

These experiments went well, and the researchers finally incorporated FD1 into an epoxy coating placed on a 1018 steel substrate. Two procedures were used to evaluate the effectiveness of FD1. Augustyniak first scribed a coated panel with a razor blade and placed the coating on top of a beaker containing a 0.5 molar sodium chloride solution for 40 hours. The coating was then placed in deionized water for 22 hours and submerged in the sodium chloride solution for an additional 30 minutes.

Fluorescence generated by the CHEF was observed at the end of this procedure. More important, no corrosion was observed with the naked eye until the coated steel was left in the sodium chloride solution for an additional 90 minutes.

A second evaluation technique used was to deliberately generate a defect in the coating to see if FD1 can detect undercoat corrosion. The researchers generated the defect by applying a silicone oil on the steel substrate prior to application of the FD1-incorporated epoxy coating.

The defective part of the coating was then exposed to a 5% sodium chloride solution at ambient temperature to accelerate the corrosive response. Fluorescence was detected after two days, but corrosion was not seen by the naked eye until after three days. Figure 2 shows the difference in using fluorescence vs. the naked eye.


Figure 2. A fluorescence probe detected corrosion on a surface (left) two days after exposure to a 5% sodium chloride solution. Corrosion on the same surface was seen (right) by the naked eye after three days. (Courtesy of the University of New Hampshire)

Ming says, “The key performance feature of FD1 is that we can detect fluorescence at a very early stage before it can be seen with the naked eye. Initial work with FD1 was done at a concentration in the coating of 0.5% by weight. We believe that FD1 is sensitive at lower treat rates, and we will evaluate its efficacy in the future.”

The CHEF technique also has been found to be more sensitive than pH detection. FD1 does not appear to leach out of the coating and exhibits stability for at least one year in a dormant state prior to the onset of corrosion, according to Tsavalas and Ming.

Future work will involve learning more about how broadly FD1 can be used as a fluorescence detector. The researchers indicate that they will look into other steel alloys and nonferrous metals.

Additional information can be found in a recent article (2) or by contacting Ming at w.ming@unh.edu. 

REFERENCES
1. Canter, N. (2009), “Coating: Heal Thyself,” TLT, 65 (6), pp. 16–17.
2. Augustyniak, A. Tsavalas, J. and Ming, W. (2009), “Early Detection of Steel Corrosion via “Turn On” Fluorescence in Smart Epoxy Coatings,” ACS Applied Materials & Interfaces, 1 (11), pp. 2618–2623.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.