Plasma electrolytic ceramic-like aluminum oxide coatings on iron

The process characteristics of plasma-assisted electrochemical treatment of iron in aluminate electrolyte consisted of aqueous 0.1 M NaAlO2 + 0.05 M NaOH have been studied. It has been shown that in the range of DC voltages from 260 to 340 V, it is possible to deposit dense ceramic-like coatings with the thickness increased from 0.3-0.5 mu m after 1 min deposition time to 25-30 mu m for 1 h of deposition. The coatings were examined by X-ray diffractometry, Fourier-transformed infrared, X-ray photoelectron spectroscopy, scanning electron microscopy, and an electrochemical potentiodynamic voltammetry. It is shown that in pre-spark conditions (100-200 V) and on the initial stages of micro-arc anodizing the forming films contain, along with amorphous aluminum oxide/hydroxide, a substantial amount of iron oxide/hydroxide. The coatings obtained by micro-arc anodizing at 260-360 V for 1060 min represent amorphous aluminum oxide with inclusions of crystalline corundum phase. The films with a thickness of 4-15 mu m deposited at the anodizing voltage of 320-360 V exhibited the uniformity and good corrosion protection properties after sealing with common agents.