Inorganic corrosion-inhibiting coating effective on aluminum

Dr. Neil Canter, Contributing Editor | TLT Tech Beat November 2015

An economical water-based coating has been developed that better protects aluminum from corrosion caused by sunlight.

 

KEY CONCEPTS
• Temperature is often overlooked when dealing with corrosion because the degradation process roughly doubles for every 10 C increase.
• An inorganic coating based on silica has been developed that better protects aluminum from the increase in temperature that can be caused by exposure to sunlight.
• Initial testing showed that the silica-based coating exhibits low solar absorption rates and high thermal emissivity values.

THE GROWING USE OF ALUMINUM IN AUTOMOBILES and ships is leading to the need to find effective ways to protect the metal surfaces of these vehicles from corrosion. Finding a suitable corrosion-inhibiting coating for ships is particularly challenging, since they operate in an aqueous environment.

One approach that has been taken to minimize corrosion is to determine the origin of this phenomenon. A previous TLT article discusses a new model to explain pitting corrosion that involves the formation of localized holes in specific locations on the metal surface (1). The model accounts not only for the movement of metal ions in an aqueous phase in contact with the metal surface but also through the solid metal phase below the surface. In fact, the model predicts that the rate of pitting corrosion increases underneath the metal surface.

A factor that is normally not considered in evaluating the rate of corrosion is temperature. Dr. Jason Benkoski, principal scientist at the Applied Physics Laboratory of The Johns Hopkins University in Laurel, Maryland, says, “One concern about corrosion that is often overlooked is that this degradation process can accelerate as the temperature increases. For every 10 C temperature increase, the rate of corrosion roughly doubles. This leads to an increase in the chemical processes and oxidation that can accelerate corrosion. In addition, a mismatch in the coefficient of thermal expansion can cause fatigue failure because of large temperature cycles between night and day.”

The current coatings used on aluminum substrates are polymer paints based on polyurethane, acrylic or epoxy. Benkoski says, “These three resins act as binders and by themselves do not provide strong corrosion protection. Rather, additives need to be formulated into the paint to enhance corrosion protection and other characteristics such as hardness.”

Benkoski notes that one of the major problems with using polymer paints is that the polymer resins degrade due to exposure over a long period of time to ultraviolet light. He says, “Solar reflectivity can start at 90% for a polymer paint but may decline to 80% after three years. This doubles the amount of solar energy absorbed by the coating, leading to an increase in temperature and an increase in the rate of corrosion.”

A new approach is needed to develop a coating that will better protect aluminum in outside applications, particularly those in marine environments where water is present. Such a coating has now been developed.

SILICA COATING
Benkoski, in collaboration with his colleagues Christopher Hoffman, Rengaswany Srinivasan and Keith Caruso, have developed an inorganic coating based on silica. He says, “The reason we picked silica is that this substance does not degrade in the presence of ultraviolet light. From a chemical standpoint, the silicon-oxygen bonds in silicates are reversible even at room temperature, leading to the potential for the coating to migrate to a new scratch and respond to other stimuli such as the by-products of corrosion.”

The primer contains approximately 90% potassium silicate and 10% organic additives, including a monomeric polydimethylsiloxane polymer. Benkoski says, “The polydimethylsiloxane is used to impart flexibility and toughness to the coating.”

The top coat is based on polysiloxane, which does not yellow in contrast to acrylics and provides a little more flexibility to the coating. Specific additives are included in the coating to provide benefits such as a cerium III additive (corrosion inhibitor), zinc oxide (reflects visible and near infrared light) and perylene black (absorbs visible light and transmits near infrared light). Alkylsilane-filled polyurea microcapsules are present to provide the means to do self-repair of the coatings at room temperature.

Benkoski says, “We studied how conversion coatings worked and borrowed the concepts to prepare our silica-based coating.” Figure 1 shows a schematic of the coating.


Figure 1. A water-based, inorganic silica coating formulated with specific additives exhibits the ability to protect aluminum substrates from an increase in heat caused by exposure to sunlight. (Figure courtesy of The Johns Hopkins University.)

Two other benefits are that silica is a low-cost raw material and the coating is water-based, so no volatile organic compounds are required. Benkoski says, “All of the raw materials are soluble in water, which makes application easy. After mixing, the coating must be applied within four to six hours to minimize any possibility of gelling. Drying takes place overnight because it is not practical to heat large ships or other outdoor structures above room temperature.”

When the coating dries, the abrasion resistance is excellent according to Benkoski.

Initial evaluation work has been done on an aluminum-magnesium alloy. Reflectivity studies assessed the ability of the silica-based coating to reflect heat. In evaluations at wavelengths ranging from the deep infrared to ultraviolet light, the coating displayed very low solar absorption rates and high thermal emissivity values. Benkoski says, “Even for dark gray coatings, thermocouples on the back of the aluminum panel indicated that the temperature remained below 40 C on a 20 C day in the presence of direct sunlight.”

Future work will include evaluating the ability of the silica coating to provide adhesion and prevent chipping or thermal shock. Benkoski says, “We plan to run the typical suite of ASTM tests used to evaluate commercial and military paint formulations.” The researchers also intend to study additional aluminum alloys.

Additional information can be found in a recent presentation (2) or by contacting Benkoski at jason.benkoski@jhuapl.edu. The researchers wish to acknowledge funding support received from the Office of Naval Research.

REFERENCES
1. Canter, N. (2015), “Pitting corrosion model,” TLT, 71 (6), pp. 22-23.
2. Benkoski, J., Hoffman, C., Srinivasan, R. and Caruso, K. (2015), “Passive Cooling with UV-Resistant Siloxane Coatings in Direct Sunlight,” Presented at the 250th National Meeting and Exposition of the American Chemical Society, August 2015, in Boston, Mass.


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