No abstract

Corrosion protection of aluminum metal-matrix composites (MMC) by anodizing treatments was investigated. Electrochemical behavior of MMC without protection also was investigated. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements were used to characterize the properties of protective surface layers. Materials studied were Al 6061/SiC (UNS A96061), alloy A356/SiC (UNS A13560), Al 2009/SiC (UNS A92009), Al 2014/Al 2 O 3 (UNS A92014) and Al 6061/Al 2 O 3 with various reinforcement concentrations. The MMC had similar corrosion (E corr ) and pitting (E pit ) potentials as the matrix alloy. The cathodic current density for oxygen reduction in 0.5% N sodium chloride (NaCl) increased for Al 6061/SiC MMC with reinforcement concentration, which was attributed to electrochemically active interfaces between the matrix and the reinforcement particles. Anodizing and hot-water sealing were less effective for MMC than for the matrix aluminum alloys. The reinforcement particles produced a more porous structure of the anodized layer for MMC. Improved results were noted for dichromate sealing, where chromium (Cr 6+ ) in the pores of the outer oxide acted as an inhibitor. The effectiveness of corrosion protection methods decreased with increasing reinforcement concentration and was a function of the matrix alloy but not of the reinforcement material. The observed reduction in corrosion protection was believed to result from corrosion-susceptible interfaces formed between the reinforcement particles and the matrix.

Al/SiC MMCs, this galvanic problem might be less severe because of the insulating nature of SiC.Anodizing of Al alloys is an electrochemical method of converting aluminum metal into aluminum oxide by applying an external current in an acid electrolyte. The most widely used electrolyte is sulfuric acid. There are two types of sulfuric anodizing: conventional anodizing, which is performed at room temperature and provides about 7 to 15 µm of oxide thickness and a fairly hard surface, and hardcoat anodizing, which is performed at around 0°C and provides about 50 µm of oxide thickness with extreme hardness. The oxide film consists of a thin, continuous barrier layer below a thick, porous layer. The structure of the porous layer was characterized by Keller et al., 5 as a closed-packed array of columnar hexagonal cells that contain a central pore normal to the substrate surface. The porous layer can be sealed in hot water or a dichromate solution to close these pores. EXPERIMENTAL APPROACH AND DATA ANALYSISThe materials studied were Al 6061 and Al 6061/ SiC. The Al/SiC MMC (provided by DWA Composites Specialties) contained 25 vol% of 10 µm SiC particulates, which were mixed with Al 6061 powder and processed with an extrusion method. Anodic coatings are produced by anodic oxidation in an acid bath to form an oxide layer. The procedures used in this study consisted of the following steps:ABSTRACT Sulfuric acid anodizing with a hot water seal does not produce the same corrosion resistance for a Al/SiC metal matrix composite as for Al 6061. The corrosion resistance of hard-anodized Al/SiC is less than that of conventional anodized Al/SiC. This result is considered to be due to the presence of the SiC particulates that interfere with the formation of a continuous barrier layer. A new mechanism for the formation of anodized layers on Al/SiC MMCs is proposed.

A 16-year old thoroughbred mare was presented with dysphagia and food being ejected from the mouth and nostrils. Clinical signs were exhibited for three weeks before it was euthanased on humanitarian grounds. Post mortem examination revealed a soft haemangioma measuring 7 cm X 5 cm suspended from the roof of the medial compartment of the left guttural pouch.