A Study On The Characteristics Of Plasma Electrolytic Oxidation Of Carbon Steel And The Coating Formation Mechanism

Plasma electrolytic oxidation（PEO）is also named micro-arc oxidation（MAO）.Which is an advanced surface treatment technique for the forming of ceramic coatings on the so called valve metals（Al,Mg,Ti and Zr,etc.）and their alloys.But the non-valve metals,such as iron,copper and nickel and their alloys.Which are diffi cult for the application of PEO technology.However,a few researchers directly attempt to fabricate PEO coatings on carbon steel and make some progress.Some protective properties of PEO coatings were fabricated in aluminate based electrolyte and silicate based electrolyte respectively.But the coating formation mechanism and plasma breakdown behavior need to make an in-depth investigation.In this paper,the coating formation mechanism and accompanying PEO spark discharge behaviour of Q345 Carbon steel in aluminate based electrolyte and silicate based electrolyte are further investigated.In order to inverstigate the different electrolyte components on the PEO behavior of Q345 Carbon steel in aluminate electrolytes,the PEO coatings can be successfully prepared in 8 g/l NaAlO₂+2 g/l NaH₂PO₄,the high current density is benefit for coating formation in 8 g/l NaAlO₂,It only leads to the dissolution of the metal substrate in 2 g/l NaH₂PO₄.It has been found that the formation of a layer of aluminum phosphates（AlPO₄and AlPO₄·2H₂O）or alumina aluminum phosphate（Al₂O₃·AlPO₄）at the initial PEO stage plays an important role for the plasma generation and hence PEO coating formation on Q345 carbon steel.The surface roughness（polished or unpolished）and electrolyte state（stagnant electrolyte or agitated electrolyte）of the samples are adjusted in 8 g/l NaAlO₂+2 g/l NaH₂PO₄,Which are associated with the stability of the aluminum phosphates or alumina aluminum phosphate layer on the Q345 carbon steel.The duty cycle has some influences for the coating formation,It is clear that the coating formed at5%duty cycle shows a thicker thickness.This dielectric layer of aluminum phosphates lead to the plasma breakdown behaviour.The dielectric breakdown of valve metals may be more suitable for Q345 Carbon steel than gas breakdown theory.It was found that the PEO film formed on polished carbon steel at 5%duty cycle has best corrosion resistance and the corrosion current was reduced to 1.474×10^-6 A?cm^-2.In order to inverstigate the coating formation on the PEO behavior of Q345 Carbon steel in silicate electrolytes,the PEO coatings can be successfully prepared in 32 g/l Na₂SiO₃·9H₂O+2 g/l NaH₂PO₂·H₂O and 32 g/l Na₂SiO₃·9H₂O,the formation of an initial silicon phosphate mixtures(SiP₂O₇、Si₃（PO₄）₄ and Si₅P₆O₂₅)is critical to the establishment of plasma discharge.The thickness of the film prepared by Q345 carbon steel in 32 g/l Na₂SiO₃·9H₂O has a trendency from increasing to decreasing,but the high current density caused the film damaged.The decomposition of silicate ions in discharge process and the initial deposited layer of SiO₂ are essential for coating growth.The state of the electrolyte was changed in two different solutions,the agitation of electrolyte promotes the mass transfer of particles.It was found that the PEO film formed on Q345carbon steel at moderate current density(0.772A?cm^-2)has better corrosion resistance than other conditions,but their corrosion resistance was poorer than the PEO coating formed in32 g/l Na₂SiO₃·9H₂O+2 g/l NaH₂PO₂·H₂O.