Kunigahalli L. Vasanth

A novel corrosion inhibitor for aluminum alloys using the beef lipids

Vigorous efforts are being made at many research institutes and academic institutions to find a substitute for chromates which are extremely good inhibitors for mild steel and aluminum alloys but are known to be carcinogenic due to Cr(VI) ions. The Environmental Protection Agency (EPA) is the main regulator of chromate uses and emissions through several different acts including the Clean Water Act, the Comprehensive Environmental Response, Compensation and Liability Act (CRCLA) and Toxic Substances Control Act (TSCA) [1]. Many of the organic inhibitors that are being studied are really promising as they have low toxicity and are readily biodegradable. Fatty acids are chains of covalently linked carbon atoms, bearing hydrogen atoms, which terminate in a carboxyl group that is responsible for their properties as acids. These fatty acids, also known as monocarboxylic acids, are known to be extremely good corrosion inhibitors for mild steel and aluminum alloys, particularly in neutral media [2, 3]. These carboxylic acids get absorbed on the aluminum surface and raise the pitting corrosion resistance [3]. Lipids are a heterogeneous group of substances which occur in biological materials [4]. The lipid family is comprised of 1) fatty acids; 2) neutral fats; 3) phosphatides; 4) glycolipids; 5) aliphatic alcohols and waxes; 6) terpenes and 7) steroids [5]. Since lipids contain both saturated and unsaturated fatty acids, it was decided to study the effect of these lipids on the localized corrosion resistance of aluminum alloys. In order to separate lipids from the beef, about 500 g of fresh beef fat (raw) was cut into small pieces and placed in a 2-liter clean beaker. To this one liter of a mixture of chloroform and methanol (both AR grade) was added. The mixture in the beaker was constantly stirred and the beef fat was allowed to dissolve for 3 h. After this period the supernatant solution was filtered off and the solvent was evaporated using the rotary evaporator. The lipids (44 g) that collected were stored in a glass vial with a stopper and kept in a freezer to avoid any decomposition. A 0.1 wt.% solution was prepared from these lipids in a suitable alcohol and all the dilutions were prepared from this stock solution using the same alcohol. This solution was stored in a cooler. Electrochemical corrosion studies were conducted on high purity aluminum (Al 99.99%, Aldrich Chemical Company, Inc.) sheet (50× 50× 1 mm) and Al 6061 T6 sheet (75× 75× 3 mm) by using a GAMRY CMS/100 system. These specimens were polished mechanically up to 1500 grit, cleaned in soap solution and degreased in acetone before the commencement of the experiments. These experiments were conducted by using a EG&G PARC flat cell where 1 cm2 area of the specimen is exposed to the electrolyte. The corrosion potential measurement was carried out for 30 min. The electrochemical impedance spectroscopic (EIS) experiments were conducted in the frequency range of 5 kHz to 10 mHz at open circuit potential (OCP) by applying a alternating current (AC) signal of 10 mV peak-to-peak. In order to determine the pitting potential (Epp), potentiodynamic anodic polarization experiments were conducted at a scan rate of 1 mV/s in aerated solutions at room temperature until the current increased monotonically. All the potential measurements of the working electrode were conducted against a saturated calomel electrode (SCE). The lipids were studied in the concentration range of 0.0002 wt.% to 0.05 wt.% comprising of two decades of concentration change. These solutions were prepared by taking the required aliquots from the stock solution and diluting them in a 50:50 mixture of a buffer solution (pH= 7.0) and an alcohol (which was used to prepare the stock solution) along with the addition of 200 ppm of AR grade NaCl to them. The profiles of the corrosion potential for pure aluminum and Al 6061 are shown in Fig. 1a and b. The corrosion potential profile at 0.001 wt.% could not be determined for pure Al. It was observed that there was rapid ennoblement of the corrosion potentials in the initial 30 min for both the alloys indicating a passivating tendency of the lipids. Aluminum 6061 showed more ennoblement at higher concentrations of lipids as very active initial OCP values were noted at these concentrations. More active OCP values suggested a better chemisorption of the inhibitor molecules on the electrode/electrolyte interface. From the EIS