One of the biggest threats to aviation could be combated with the juice from date palms, researchers have revealed.
Aluminium alloys are extensively used in aircraft construction – including the fuselage, wings and individual parts – but serious corrosion of the metal can result in structural or equipment failure. This can cause serious emergencies in flight or in the worst cases even total loss of an aircraft.
Aluminium corrosion has been blamed for some of aviation’s most serious incidents.
In 1988 an Aloha Airlines B737-200 had 18ft of skin ripped off, causing the death of a stewardess and injury to passengers in the explosive decompression.
The resulting investigation found that corrosion combined with metal fatigue and stress fractures had damaged the aircraft’s skin.
Corrosion was also blamed when two engines fell off an EL AL B747-200 freighter – which crashed into an Amsterdam apartment block, killing all four on the aircraft and 47 people on the ground.
Unlike steel, aluminium has a natural corrosion protection from its oxide layer – an extremely hard, protective coating that prevents degradation of the underlying metal.
However, this film layer can become damaged or broken down if exposed to harsh environments such as salt water, due to the presence of chloride ions.
Jetliners fly over the sea and in and out of coastal areas every day across the world, raising the risk of problems setting in.
To combat this, corrosion inhibitors are widely used in the aerospace industry to reduce the rusting rate of key metals and alloys.
One of the most common inhibitor types are chromates, however these are highly toxic and raise the risks of environmental or ecological problems arising from use.
Now scientists have found that a much greener solution could come from an unusual source – juice from the date palm – one of the most cultivated palms around the world, especially in North African and the Middle East.
The date palm (Phoenix dactylifera L or PDL.) is used as food, confectionery and even traditional medicine.
Researcher Dr Husnu Gerengi, from the Department of Chemistry at Duzce University, Turkey, found that date palm juice inhibited corrosion of an aluminium alloy called AA7075, widely used in aerospace and other applications.
He tested the juice on the alloy in a 3.5 per cent salt solution (NaCl) to mimic the salinity of sea water.
Electrochemical measurements were all carried out in a three-electrode type cell with separate compartments for the reference electrode, and the counter electrode was platinum plate.
It was found that PDL juice – at a concentration of 2,000 parts per million – had an inhibition efficiency of 63 per cent compared to no inhibitor.
Dr Gerengi noted that while an extract from date palm leaves is a known anticorrosive, this was the first test of the fruit's juice.
The juice, which he reported adsorbed into the aluminium's surface, contains a number of sugars and he says that these react with aluminium to form an anticorrosive film on the metal's surface.
Writing in the American Chemical Society’s journal Industrial & Engineering Chemistry Research, Dr Gerengi said: “It was found that PDL fruit juice acted as a slightly cathodic inhibitor, and inhibition efficiencies increased with the increase of PDL fruit juice concentration.
“It was observed that both the cathodic and the anodic curves showed lower current densities in the presence of the juice derivatives than those recorded in the solution without PDL fruit juice derivatives.
“A decrease in the cathodic current density is more than the anodic part.
“This indicates that PDL fruit juice inhibits the corrosion process.
“The results also suggested that the studied natural inhibitor affects both anodic and cathodic corrosion processes, hence, it reveals properties of a mixed type inhibitor.
The data clearly showed that the electrochemical corrosion rate of AA7075 decreased in the presence of PDL fruit juice.”
Dr Gerengi added: “The inhibition mechanism of the tested inhibitor is a combination of surface blockage and electrostatic repulsion between adsorbed surfactant layer and chloride ions.
“The adsorption density of inhibitor depends on the inhibitor concentration.
“The increase in efficiency of inhibition of PDL fruit juice indicates that the inhibitor molecules are adsorbed on the AA7075 alloy surface with higher concentration, leading to greater surface coverage.
“The date palm PDL fruit juice was found to be a green inhibitor for AA7075 alloy corrosion in 3.5 per cent NaCl solution and adsorb on the AA7075 alloy surface according to the Temkin adsorption isotherm.
“Results showed that this investigated compound acted as a slightly cathodic inhibitor for the corrosion of AA7075 alloy in 3.5 per cent NaCl solution, and the inhibition efficiency of this alloy in this environment increases with an increasing concentration of PDL fruit juice.
“There is a good potential for the use of PDL fruit juice as a nontoxic (green) inhibitor in 3.5 per cent NaCl environments.”