Metal Corrosion and the Efficiency of Corrosion Inhibitors in Less Conductive Media

These methods can help answer the questions about the corrosion resistance of metallic materials, the corrosion aggressiveness of different environments, and the efficiency of corrosion inhibitors in different environments. The main advantage of these methods is that they can be applied in both aqueous and non-aqueous environments. This method can be useful in the automotive and fuel industry for study of the corrosion effect of ethanol gasoline blends on different construction materials within fuel systems.

Demonstrating the procedure will be Lukas Matejovsky, a member of my team. To test the static immersion corrosion of metal liquid systems, begin by adding 100 to 150 milliliters of the tested liquid corrosion environment to a 250-milliliter bottle equipped with a hook for hanging an analyzed sample, and use 1, 200-mesh sandpaper to grind and polish the surface of the metallic sample under running water to achieve an even adjustment of the surfaces. Next, thoroughly degrease the sample surface with about 25 milliliters of acetone and about 25 milliliters of ethanol.

After drying, weigh the sample on an analytical balance to an accuracy of four decimal places, and hang the metallic sample within the bottle so that the sample is emerged within the liquid, but does not lie on the bottom of the bottle. Then, close the bottle tightly enough to prevent liquid evaporation and air entry. Remove the metallic sample from the bottle at regular intervals for rinsing with about 25 milliliters of acetone, using pulp tissue to remove any excess corrosion products from the surface.

Then weigh the sample to four decimal places, and return the sample to the bottle. When equilibrium is achieved within the metal liquid system, terminate the experiment. For a dynamic corrosion test, add 500 milliliters of the tested liquid corrosion environment into the four-necked flask of the storage part of the apparatus, and lubricate the ground glass joints of the flask with a silicone grease.

Fix a reflux cooler, thermometer, suction capillary connected to a pump, and the overflow connected to the tempered part, on the necks of the flask. And set the cryostat connected to the cooler to 40 degrees Celsius. Fill the closed cooling circuit with ethanol.

Using the capillary for the fuel pumping, connect the pump to the preheating spiral of the tempered part of the flask that brings preheated fuel via the bottom of the measuring cell. Set the desired pump fuel flow rate to 500 milliliters per hour, and the thermostat for the tempered part of the flask to 40 degrees Celsius. Once the tempered part of the flask is filled with fuel, and the fuel starts to flow via the overhead part back into the storage flask, open the measuring cell that consists of two parts connected via a ground glass joint, and hang the ground, polished, degreased and weighed sample on the hanger.

Use a pressure vessel to connect the frit to the tube for the air supply via a pressure regulator and a flow meter, and set the desired gas flow rate on the flow meter to 20 to 30 milliliters per minute. Then, remove the metallic sample from the tempered part of the flask and rinse, polish and weigh the sample to determine the sample's surface loss over time, as just demonstrated. For a static immersion corrosion test, add 200 to 300 milliliters of the liquid test sample into the tempered flask, and hang a ground, polished, degreased and weighed metal sample on the hook of the reflux cooler.

Lubricate the ground glass joint to the cooler with the silicone grease, and fix the cooler into the flask. Connect the frit to the tube for the air supply with a pressure vessel via a pressure regulator and a flow meter, and set the desired gas flow rate to 80 milliliters per minute on the flow meter. Then, set the temperature to 80 degrees Celsius on the thermostat for flask tempering, and do 40 degrees Celsius on the cryostat connected to the cooler.

After the appropriate experimental exposure period, remove the metallic sample from the apparatus, and rinse, polish and weigh the sample to determine the sample's surface loss over the time, as just demonstrated. For electrochemical measurements in a two-electrode arrangement, first, remove the electrode system from the measuring cell, and unscrew the system. Adjust the surface of the electrodes, as just demonstrated, and reassemble the electrode system.

Fill the measuring cell with 80 milliliters of the tested liquid corrosion environment, and close the measuring cell through the electrode system. Place the whole cell into a grounded Faraday cage and connect the galvanostat and potentiostat to the electrode system, so that one electrode of the system acts as a reference electrode, and the other electrode acts as a working, and an auxiliary, electrode, at the same time. In the measurement software, set the sequence containing the open circuit potential measurements and the electrochemical impedance spectroscopy measurement, and perform the stabilization for at least 30 minutes to minimize the potential change.

Then, acquire the electrochemical impedance spectroscopy measurement at a sufficiently high amplitude, according to the conductivity of the corrosion environment, and at a sufficient range of frequencies to allow evaluation of the low and high-frequency parts of the spectra. For electrochemical measurements in a three-electrode arrangement, adjust the measuring part of the working electrode from the tested metallic material, as demonstrated, and screw the part onto the electrode extension. Fill the measurement cell with 100 milliliters of the tested liquid corrosion environment, and close the cell with a cap, through which the working electrode from the tested material, and the auxiliary electrode from the platinum wire, are led.

Make sure that the auxiliary electrode is twisted around the working electrode. Insert the reference electrode with a bridge through the side entry of the cell, so that the reference electrode is as close to the working electrode as possible, without the electrodes touching each other. Insert the cell into a grounded Faraday cell, and connect the electrodes, via a cable system, to the galvanostat and potentiostat equipped with the appropriate software.

Then, in the software of the measuring devices used, set the measuring sequence, containing the measurement of the open circuit potential, for at least 20 minutes, the electrochemical impedance spectroscopy in the range of about one megahertz to one millihertz, and an amplitude value of five to 20 millivolts, and the polarization characteristics between 200 to 500 millivolts to the corrosion potential. In a static corrosion test, 1, 200 hours are sufficient to achieve stabilization of the mild steel E10 and E85 fuel systems, while 340 hours are required for stabilization within the dynamic corrosion system. The efficiency of the corrosion inhibitor is also evident in both of the fuels, as substantially lower material losses are observed when the inhibitor is applied.

The removal of the surface corrosion products by pickling enables the acquisition of real material losses that are important for calculation of the efficiency of the corrosion inhibitors. When the conductivity of an environment is low, the spectrum consists of only one high-frequency half circle, making it possible to evaluate the properties that characterize the used environment only. When the conductivity of an environment is high enough, the spectrum consists of both high and low-frequency regions that form two, relatively well-separated, half circles.

Here, the Tafel curves of mild steel in the environment of the aggressive E85 fuel without the inhibitor before, and after, the potential loss drop compensation, as well as in the presence of an amine-based inhibitor, are shown. While attempting these methods it's important to remember to minimize the Wang error and to carefully perform the adjustment of the sample surface. For the static and dynamic tests, the corrosion resistance of the metallic materials, or the corrosion aggressiveness of the different environments, can be evaluated based on the corrosion rate of the tested materials during the testing.

For the electrochemical methods, the corrosion resistance of the metallic materials, or the corrosion aggressiveness of the different environments, can be evaluated based on the polarization correctory sticks. All of the presented methods enable us to test the efficiency of corrosion inhibitors.