Extracts of juice from the common date palm fruit may prove to be a greener way to protect against corrosion in the strong, lightweight aluminum alloys being used to make aircraft, cars, and industrial machines. The extracts, developed at Turkey's Duzce University, could replace anti-corrosion coatings made with potentially toxic chemicals.
Although aluminum's oxide layer makes it naturally resistant to corrosion, it can corrode in some extreme environments, including sea water. The wrought aluminum–zinc–magnesium–copper AA7000 alloys commonly used in load-bearing structures and other structurally demanding aerospace applications are especially susceptible to local corrosion from intermetallic phases.
An extract of juice from the fruit of date palms like the ones shown here in Morocco could be used as the basis of a nontoxic anti-corrosive agent for aluminum alloys commonly used in aircraft structures. (Source: Wikimedia Commons/Erg Chebbi)
Inorganic corrosion inhibitors form an oxide film or hardly soluble salt on the metal's surface. The most common inorganic inhibitors used for aluminum and aluminum alloys are chromates, which are highly toxic. Organic inhibitors, such as amino acids and hydroxy carboxylic acids, protect the metal by adsorption on the oxide film. The Duzce University research is looking at nontoxic alternatives to both types.
Previous research suggested the anti-corrosion effect of extracts of leaves of the date palm (Phoenix dactylifera L.). Husnu Gerengi, a researcher in the department of chemistry of the corrosion research laboratory at the university's Kaynasli Vocational College, decided to examine extracts from date palm fruit juice. The fruit is used in several forms as food, as well as in traditional medicine.
Gerengi detailed his findings in an article in Industrial & Engineering Chemistry Research (subscription required). He investigated the influence of date palm fruit juice on AA7075 aluminum alloy in a 3.5 percent sodium chloride solution by Tafel extrapolarization and electrochemical impedance spectroscopy. He found that the juice acted as a slightly cathodic inhibitor. The efficiencies of this inhibition increased as the juice's concentration in the solution increased from 100ppm to 2,000ppm.
Previous studies have shown that compounds containing sugars can inhibit corrosion. Date palm fruit juice contains several sugars, including glucose, xylose, and fructose. Gerengi's research suggests that a reaction between the metal and these sugars, especially glucose, forms an anticorrosive film on the alloy's surface.
The juice adsorbed into the aluminum's surface, obeying the Temkin adsorption isotherm. The types of adsorption that Gerengi found indicate "that the inhibition acts due to the electrostatic interaction between the charged molecules and the charged metal," he wrote. The results suggest "the spontaneity of the adsorption of the inhibitor molecules and the stability of the adsorbed layer on [the] Al surface."
Given what sugars and fruit juice can do to the enamel on one's teeth, this result is a surprise to me. I suppose teeth are far more porous than aluminum, which could explain some of the differences. Nevertheless, using juice extracts as a way to prevent corrosion as opposed to toxic materials--you can't argue with that kind of green development.
@ Ann: Do you have the name of the tree (common or scientific)? A friend just came back from Morocco and mentioned a tree that's being promoted heavily for export products. I'm curious to see if it's the same tree.
Beth, that is an interesting development. Using non-toxic organics will probably result in production processes that are less toxic as well. Then the issue is can we grow enough of the organic compound to keep up. I doubt that is an issue with dates.
Beth, it's not just a difference in porosity between tooth enamel and aluminum, it's the fact that they are entirely different physical and chemical structures. What happens between aluminum and the fruit juice extract is an electrostatic interaction, which is very different from what occurs between that extract and enamel.
Lou, you're right--date palms are grown very widely. OTOH, depending on how many it takes to produce the extract, that could interfere with production of food crops and medicine crops.
@Ann: If you have access to the article, can you tell us what the Icorr is, with and without the coating?
Icorr is the corrosion current (usually measured in mA/cm²), and is directly related to the corrosion rate. For example, if Icorr without the coating is 4 times more than Icorr with the coating, then the coating reduces the corrosion rate by a factor of 4.
In an article released earlier this year, Dr. Gerengi showed that the extract of a certain South American hardwood tree could reduce the corrosion rate of low carbon steel in hydrochloric acid by as much as factor of 3.
While this research into "green" corrosion inhibitors is interesting, the effects documented so far have been relatively modest. Many commercial corrosion inhibitors and coatings can reduce corrrosion rates by a factor of 10 or more.
Also, there are cultural issues that might hamper the adoption of such "green" corrosion inhibitors. If I suggested to management that we begin using fruit juice as a corrosion inhibitor for aluminum, I doubt it would be taken seriously.
Dave, corrosion current density values (Icorr) appear several times in different contexts. Without an inhibitor it's 3.47, at 100 ppm it's 2.32, at 500 1.84, at 1500 1.25 and at 2000ppm it's 0.98. Regarding management not taking to the idea of using fruit juice, I think that has more to do with our Western culturally-induced industrial paradigms. People in other areas of the world, usually with a less complex economic infrastructure and greater reliance on local natural resources, take this research and its potential applications seriously. We have much to learn, or re-learn, it seems to me.
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