In-situ Study On The Dynamic Growth Process Of Filiform Corrosion On Magnesium Alloys

Magnesium alloys have many excellent properties.However,their chemical activities are high and are prone to corrode in practical applications.The failure of magnesium alloys is almost caused by localized corrosion.Filiform corrosion as a very common localized corrosion model results in a great deal of damage.If the filiform corrosion of Mg alloys is restrained,the life time of magnesium alloys will be improved greatly.Thus,it is very necessary for us to clarify the filiform corrosion mechanism of magnesium alloys.The Mg-3Zn and ZG21 magnesium alloys were used in this investigation.The effects of microstructure and corrosive mediums on the initiation and propagation of filiform corrosion were investigated using scanning electron microscope?SEM?,scanning vibrating electrode technique?SVET?,X-ray Photoelectron spectroscopy?XPS?,electrochemistry land,immersion measurements.The control factors for the growth of corrosion filaments were determined.The reasons for the horizontal propagation of corrosion filaments were clarified.The results indicate that filiform filaments can cross grain boundaries to develop continually instead of termination at the second phase.The development of filiform corrosion is greatly associated with the existence of Cl^-in the corrosive solution.The addition of F-or raise of pH to the corrosion solution containing Cl^-can delay the initiation of corrosion filaments.The in-stiu observation by SVET confirms that the corrosion microcells change dynamically with the growth of corrosion filaments.Serious filiform corrosion is observed at the applied anode potentials while corrosion is not observed at the applied cathode potentials.The results indicate that the propagation of corrosion filaments is under anodic control.The filiform corrosion in NaCl solution and pitting corrosion in Na₂SO₄ for the Mg-3Zn alloy are compared.It is found that Cl^-move towards filament head and SO₄^2-move towards pitting bottom,Which is related to the different surface film formed in two kinds of solutions.The Cl^-can penetrate the tiny space of oxide films,and reach the metal interface and further result in the initiation of corrosion in the NaCl solution.Then loose corrosion product films can be formed around the initiation regions.Abundant Cl^-will gather these regions because of the “poisonous effect” of loose films.However,Cl^-is prone to transfer in the films instead of the interface of Mg substrate.As a result,corrosion easily propagates along the horizontal direction.The distribution of Cl^-is inhomogeneous in the damaged regions.Corrosion will extend along the direction with a higher Cl^-concentration.Filament tails can be formed after corrosion filaments expanded toward certain direction.Cl^-are easily to transferred to filament heads along the loose corrosion products in the filament tails.Thus,corrosion filaments will grow forward along the filament tails.Eventually,corrosion filaments grow along horizontal direction.