Resistance to corrosion damage (i.e. from rain and seawater)
Construction of terrestrial and marine sensors that are resistant to the corrosion is essential to the functionality of the sensor network deployed in the Galapagos. Two options for materials that adequately protect these instruments are available, as well as one method for protecting the wiring and specialized components themselves, and one method that works for both purposes. Regardless of what surface material is used, the surface will, if possible, also be painted or protected by some other coating.
The first option is using aluminum instead of steel for the sensor's casing. Steel, being formed from iron, oxidizes much more easily than aluminum.(E0 of Fe --> Fe3+ + 3e- is 0.037 and E0 of Fe --> Fe2+ + 2e- is 0.447, while E0 of Al --> Al3+ + 3e- is 1.662 => a substantial increase. (Vanysek) Even with its added carbon, steel is still more easily corroded than aluminum.) Aluminum is also easily workable, and has a long history of use which allows for a great knowledge about its properties. It is more expensive than steel, but not restrictively so. (Steel Costs approximately 38 cents per pound, and aluminum costs $1.52 per pound (U.S. Steel), but this cost is almost negligible when compared to the cost of the equipment housed inside the shell of each individual sensor platform.
A second option would be to use high-grade, corrosion resistant steel, called which sells for only 1% more than the cost of ordinary carbon steel. This alloy is known as UNS S31803 and also as Alloy 2205, made by Carpenter Technology Corporation. It is currently being used in support struts for a new bridge in Brooklyn, and the struts are predicted to remain stable enough to support the enormous weight of the bridge for at least the next 100 years. (Stainless Steel World)
A third option looks to recent technology. At MIT, Paul Laibinis developed a technology in which a substance would form a crystalline "self-assembled monolayer" around a certain metal. This outer layer is hydrophobic, and prevents the metal from oxidation in terrestrial and marine environments (Neither H2O nor O2 reach the metal, and therefore cannot react with it.) The monolayer of protective "barrier film" exists on the scale of angstroms, 1/1000 the thickness of traditional polymer coatings, and is more effective than the thicker polymer because of its ordered crystalline structure. Another advantage of this technology is its ease of application -- one needs only to dip the sample into a solution containing the compound that will become the monolayer, and the monolayer automatically adheres to the surface in a crystalline structure. However, this technology is still in development, and more work needs to be done on making it able to be effective over an extended period of time and across a wide range of metals. Prof. Laibinis' experiments used copper and the class of organic compounds called alkanethiols. At the present time, this research seems to provide a logical solution for the protection of the internal components of the probe, since the wiring will be made of most likely be made of copper, providing a direct application of Laibinis' research. (Jennings, G. K., Munro, J. C., Yong T., and Laibinis, P. E.)
Whatever method for exterior protection is chosen, protective paints/coatings will also be applied to the sensor exteriors. The supplier of these paints will be Sigma Coatings, which supplies the U.S. Navy with paint and corrosion protection products, and the preferred paint is Sigma Balamastic (Sigma Coatings, specifications at http://www.skcm.nl/skcm/sites/site31/download/00_7104.pdf). This is because paint for ballast tanks must be very corrosion-resistant and require little maintenance, much like the sensors that are planned to be put in place on the Galapagos.
"TTH" plating, developed by the U.S. Navy, could also be used to protect the internal components. It has been extremely successful as a corrosion retardant in lab tests, (as shown at http://www.sabritec.com/inthenews/TTHPlatedConnectors.htm). TTH has properties that go beyond corrosion-resistance: it is impact- and abrasion-resistant, has good electrical conductivity, and its range of safe operating temperatures runs from -65 to 260C. In summary, it would be highly recommended over traditional nickel or cadmium plating, for internal to provide the sensors' internal components with the ruggedness necessary for field work. (Engineering Talk)
Conclusions:
The outer casing of the marine sensors will be made of aluminum and coated with Sigma Balamastic. The coating, designed for ballast tanks, will provide extremely effective protection on its own. Should all go well, the corrosion-resistance of the aluminum will never need to be tested. However, it is unwise to depend entirely on one system of protection in such a costly endeavor as field research, so the metal underneath should be resilient as well. Aluminum was chosen because it is more corrosion-resistant than steel, and saltwater is a much more corrosive environment than a terrestrial landscape. It is believed that this combination of materials will make the marine sensors hearty enough that only minimal maintenance is needed.
Terrestrial sensors also need protection from the elements. Therefore, they will be made of corrosion-resistant S31803 steel, coated with Sigma Balamastic for increased protection from the rain and humidity.
The internal components (power supplies, wiring, and everything else essential to the operation of the sensor) of both terrestrial and marine sensors will use TTH plating, taking advantage of its corrosion-resistance and general ruggedness. The self-assembled monolayer technology is intriguing, and should be monitored for future sensor improvements. However, it is not presently marketed, and therefore is not as feasible an option as the TTH plating method, which is already manufactured and widely used.
Sources:
Engineering Talk. Plating system avoids the use of cadmium. Retrieved 11/28/04 from http://www.engineeringtalk.com/news/ica/ica100.html.
Jennings, G. K., Munro, J. C., Yong T., and Laibinis, P. E. (1998). Effect of Chain Length on the Protection of Copper by n-Alkanethiols. Langmuir 1998, 14, 6130-6139. Retrieved 11/27/04, from http://pubs.acs.org/cgi-bin/archive.cgi/langd5/1998/14/i21/pdf/la980333y.pdf.
Key to Steel. Structure of Plain Steel. Retrieved 11/27/04 from http://www.key-to-steel.com/Articles/Art3.htm.
Sigma Coatings. Sigma Balamastic. Retrieved 11/28/04 from http://www.skcm.nl/skcm/sites/site31/download/00_7104.pdf.
Skolnik, A. M., Hughes, W. C., & Augustine, B. H. (2000). A Metallic Surface Corrosion Study in Aqueous NaCl Solutions Using Atomic Force Microscopy (AFM). The Chemical Educator, Vol. 5, No. 1. Retrieved 11/27/04 from http://chemeducator.org/sbibs/s0005001/spapers/510008ba.htm.
Stainless Steel World: News Archive. New Bridge in Brooklyn. Retrieved 11/27/04, from http://www.stainless-steel-world.net/projects/search_detail.asp?NewsID=3457.
Swales, Shannon. Sabritec Delivers Connectors with New TTH Plating System. Retrieved 11/28/04 from http://www.sabritec.com/inthenews/TTHPlatedConnectors.htm.
U. S. Steel. Cost and Benefit Analysis: An Aluminum vs. Steel Truck Hood Assembly. Retrieved 11/28/04 from http://ussautomotive.com/auto/steelvsal/hood.htm.
Vanysek, Petr. (1991). Electrochemical Series. David R. Lide (Ed.), CRC Handbook of Chemistry and Physics: 71st Edition. (pp. 8-16 - 8-23) Boston: Chemical Rubber Publishing Company.
