R. Arrabal

Transmission electron microscopy of coatings formed by plasma electrolytic oxidation of titanium.

Transmission electron microscopy and supporting film analyses are used to investigate the changes in composition, morphology and structure of coatings formed on titanium during DC plasma electrolytic oxidation in a calcium- and phosphorus-containing electrolyte. The coatings are of potential interest as bioactive surfaces. The initial barrier film, of mixed amorphous and nanocrystalline structure, formed below the sparking voltage of 180 V, incorporates small amounts of phosphorus and calcium species, with phosphorus confined to the outer approximately 63% of the coating thickness. On commencement of sparking, calcium- and phosphorus-rich amorphous material forms at the coating surface, with local heating promoting crystallization in underlying and adjacent anodic titania. The amorphous material thickens with increased treatment time, comprising almost the whole of the approximately 5.7-microm-thick coating formed at 340 V. At this stage, the coating is approximately 4.4 times thicker than the oxidized titanium, with a near-surface composition of about 12 at.% Ti, 58 at.% O, 19 at.% P and 11 at.% Ca. Further, the amount of titanium consumed in forming the coating is similar to that calculated from the anodizing charge, although there may be non-Faradaic contributions to the coating growth.

Electrochemical Estimation of the Corrosion Rate of Magnesium/Aluminium Alloys

The corrosion rate of AZ31, AZ80, and AZ91D magnesium/aluminium alloys immersed in 3.5 wt.% NaCl was determined comparing gravimetric and electrochemical measurements. The findings revealed that, for all investigated materials, a fraction of the metallic surface exposed to the corrosive medium did not reveal a normal electrochemical response to the applied signal. This may be associated with phenomena such as partial disintegration of specimens into fine metallic particles, electrochemical formation of Mg

XPS Study of Chemical Changes on the La/Ce Treated Surface of A361 Aluminium Alloy Exposed to Air at Temperatures up to

The chemical changes that take place on the rare earth treated surface of the A361 aluminium alloy exposed to air at temperatures between 100 and have been examined using X-ray photoelectron spectroscopy (XPS). The most notable features discussed in this work are the disappearance of Mg and Si signals at the tested temperatures and disappearance of the Ce signal at temperatures of 400–. The biphasic microstructure of the A361 alloy, constituted by close to 12 wt% Si and the Al matrix, plays an important role in many of the results obtained. The notable growth of aluminium oxide across the conversion coating in the case of the Ce-treated surface is related to the structural transformation experienced by the cerium oxide coating at 400–.

Corrosion of AISI 316 stainless steel containing Cu and Sn in acid media

Abstract The influence of Cu and Sn additions on the corrosion resistance of AISI 316 stainless steels has been evaluated in various acid media (boiling 50 wt-%H3PO4, boiling 65 wt-%HNO3, 3 wt-%HCl at 25°C and 30 wt-%H2SO4 at 50°C). A kinetic study of the corrosion process was carried out using gravimetric tests. The nature of the corrosion products was analysed by SEM, X-ray mapping and X-ray photoelectron spectroscopy both before and after accelerated corrosion tests. The steels displayed high chemical resistance to the nitric, hydrochloric and phosphoric acid media. An increase in Cu and Sn concentration did not significantly affect the corrosion resistance in boiling 50 wt-% phosphoric acid, boiling 65 wt-% nitric acid or 3 wt-% hydrochloric acid. The behaviour of AISI 316 in sulphuric acid solution was greatly improved by increasing the Cu concentration, whether alone or in combination with Sn. The effect of Sn addition in decreasing the corrosion rate was minor in comparison with that of Cu.