Inhibition of Methanol Cracking of Ti-6Al-4V Foil: Effect of Pretreatments with Aqueous Solutions of Electrolytes

Pretreatment of Ti-6Al-4V foil with aqueous solutions of chlorides for one hour can greatly inhibit the initiation of stress corrosion cracking of the foil in methanol containing 0.2% by volume of water and 0.01M in NaCl. The cations of six different elements are particularly effective, and half of these six elements occupy adjacent positions in the fifth period of the periodic table. With hydrogen ions as the common cation, the most effective anion was nitrate. Dilute equivolume mixtures of nitric and hydrochloric acid are very effective inhibitors of stress corrosion cracking. The experimental results are explained on the assumption that the surface oxide layer of untreated alloy contains microcrystallites of colloidal dimensions, which in the stress corrosion test are the locus for the attack of the base metal by methanol. The effect of the pretreatment is to destroy in some manner the colloidal character of the microcrystallites and to make a surface film more resistant to methanol attack.