A method and apparatus comprising a sol-gel solution. Zirconium n-propoxide is aged in a solvent to form a first part for the sol-gel solution. The first part is combined with deionized water to form a second part for the sol-gel solution. Aminoaryltrialkoxysilane is combined with an alcohol to form a third part for the sol-gel solution. The third part is combined with the second part to form a mixture for the sol-gel solution. The deionized water, or the alcohol, or a combination thereof is combined with the mixture to form a solution. The solution is aged to form the sol-gel solution comprising a mole balance ratio of approximately 1:5 between the zirconium n-propoxide and the aminoaryltrialkoxysilane.