Production of Anti-Corrosion Coatings on Light Alloys (Al, Mg, Ti) by Plasma-Electrolytic Oxidation (PEO)

Abstract

Voltage and current density High Low Deposition rate Fast (1~2μm/min) Slow (~0.3μm/min) Oxidation mechanism Chemical/ electrochemical and plasma chemical reactions Chemical/electrochemical reactions Coating on selected alloys Practical for various kinds of Al, Mg and Ti alloys Limited (not used for 2000-series alloys, high zinc or silicon Al alloys and Al casting alloys) Microstructure Amorphous and crystalline phase / Inner dense layer and outer porous layer Amorphous / Columnar porous layer and very thin barrier Corrosion resistance (Relative) Excellent (5) Good (1) Hardness High (~Hv1600) Low (Hv600 max) Wear resistance (relative) Excellent (30) Fair (2) Thermal protection Excellent Good Electrolyte Alkaline solution Acid solution Dielectric strength Excellent Fair

Corrosion protection of thickness reduced plasma electrolytic layers on AZ31

“…The formulation of principle equations of the reaction is hard to identify and is sometimes contradicting. (see, e.g., [16,24]). Nevertheless, it can be assumed that in an alkaline solution (pH > 11) the early stage is characterized by passive film formation and oxygen gas evolution according to Eqs.…”

“…The region after the breakdown is characterized by a high number of tiny sparks of short life time. The discharge events are coupled with a local high temperature rise which dehydrates the magnesium hydroxide [16] according to:…”

“…With ongoing voltage and time the sparks will be brighter and the life time will increase to milliseconds and the sparks growth up to micro-arcs (stage III). Due to the high local temperature (%10 4 K [25]) and the high field strength (%10 6 V/m [24]) the discharge process additionally creates molten magnesium oxide on the bottom of the channels according to [16]:…”

“…In the presented results Mg 2 SiO 4 phases are clearly detectable only in the oxide formed in the electrolyte 5 g/L KOH þ 10 g/L Na 2 SiO 3 . Hussein et al [16] assume that the magnesium silicate phase is formed by a combination of dissolved magnesium cations and silicate anions according to Eq. (5):…”

“…In the present paper, PEO is the term used for all of these processes. The state-of-art on magnesium is summarized by Blawert et al [12,14] and very recently by Hussein et al [15,16]. Current magnesium technologies (e.g., Keronite 1 or Magoxide-Coat 1 ) operate in alkaline electrolytes, free of chromates or fluorides [12].…”