Study On Galvanic Corrosion Behavior Of Titanium Alloys Under Different Protection Systems

TC4 titanium alloys are widely used in aviation thanks to its light weight and high strength.However,the contact between titanium and other metals always causes the accelerated corrosion to the metal with lower potential,like aluminum.So it is necessary to use surface protection method to prevent titanium alloys from galvanic corrosion.Phosphate fluoride chemical conversion and nitric acid passivation were applied on TC4 to study the effect for galvanic corrosion.Titanium alloys were covered by Na3TiOF5 after phosphate fluoride chemical conversion.The galvanic current of the titanium alloys treated 1-3mins was increased due to dissolution of natural passivated film.The converted film of titanium alloys approached to ideal capacitor as treating time increasing from 3mins to 6mins,the quality of this film was augmented,the galvanic corrosion was decreased.The converted film was mildly dissolved and the galvanic corrosion was slightly leveled up when the treating time exceeded 8 mins.The galvanic cell coupled by aluminum alloys and titanium alloys treated under 25?-30? was driven by electrodynamic potential,the galvanic current of titanium alloys treated over 35? increased in later phase of corrosion.The average galvanic current density of titanium alloys treated by phosphate fluoride chemical conversion process was in the range of 0.1²?A/cm²,the rank of corrosion was A^B with no need of surface protection.The mechanic of anti-galvanic corrosion was that the self-corrosion potential of converted film on titanium alloys was even less than pitting potential of aluminum,the galvanic cell was controlled by the passivity of aluminum.Simultaneously,the mitigation of galvanic corrosion was also limited by augmentation of the impedance of the surface of titanium alloys.In over view of single-variable experiment,the optimized process was that:treating time was among 4-6mins,the concentration of NaF was in the range of 10-20 g/L,and the concentration of Na₃PO₄ was within 40-50 g/L.In the process of nitric acid passivation,the passivation time and concentration of HNO₃,were controlled in the experiment.The surface of titanium alloys was in gray-white color.The titanium alloys were dissolved at the beginning,then were covered by passivated film gradually from 20mins to 100mins,and the self-corrosion potential values of passivated film and natural film were equal.The average of galvanic current density of aluminum alloys and titanium alloys treated with different time was firstly decreased and then slightly increased as treating time accumulated.The average of galvanic current density of aluminum alloys and titanium alloys treated with different concentration of nitric acid was decreased then increased while the concentration of nitric augmenting.The mechanism of anti-corrosion of passivation process was to change the surface impedance of titanium alloys.The average galvanic current density of titanium alloys treated by passivation process was between1?A/cm² and 4?A/cm²,the rank of corrosion was C,which was regarded as evident corrosion and extra surface protection was needed.Therefore,the nitric passivation process can be a pre-treatment method.In general,the optimized process was to treat TC4 titanium alloys in the nitric acid solution with the concentration of 25%-30%for 60-80mins.The galvanic corrosion rank of titanium alloys with no treatment and acid pickling treatment was D,which was regarded as extreme severe corrosion and extra surface protection was needed.In comparison,the sequence of resistance of galvanic corrosion in descending order was:phosphate-fluoride chemical conversion process?no need of extra surface protection?>nitric acid passivation process?in need of extra surface protection?>no treatment?in need of surface protection?>acid pickling process(in need of surface protection.