Lubricant induced
metal corrosion An electrochemical impedance spectroscopy approach
Lutz Kogel, Tobias Asam, Boschmann Elena Société Générale de Surveillance (SGS Group Germany)
Corrosion costs are above 3.3 trillion US $ worldwide affecting infrastructure such as highways, bridges, buildings, oil pipelines and industrial machining. Corrosion so far has severe consequences like down times, maintenance costs and security issues. Concerning lubricants, a lot of research has been carried out over the last decades, by manufacturers of machines and lube formulators, to protect metals against corrosion, but each time a formulation is changed because of legislation, raw material availability or technical justifications, costly long term field tests most often have to be performed.
SGS now has implemented Electrochemical Impedance Spectroscopy (EIS), a method at the forefront of technology, to support the lubricant and metal industry identifying potential challenges of lubricant corrosiveness, corrosion tendencies of metals and additive performance.
The main advantage of EIS is speed, precision and reproducibility. We are able to measure corrosion tendencies at a broad range of temperatures from -70°C up to +240°C taking an in-depth look at decisive details such as additive activation mechanisms.
Combining electrochemical impedance spectroscopy with various methods such as REM, EDX and ICP-OES for example, we are able to correlate electrochemical
results and support our customers with the full picture R&D laboratories can draw.
Alternating current (AC) methods measuring impedance are well known since years in sensor industries and used for example as online-sensors to determine the condition of lubricants. Concerning online-sensors the impedance measurement is performed at specific frequency spots measuring properties such as conductivity and susceptibility.
Steps further depicting detailed information of corrosion tendencies EIS is used where test specimens are measured using frequencies between 0.1 Hz and 1 MHz giving a full spectrum. Performing frequency dependent measurement is making sense because impedance strongly depends on frequency. The electrochemical system reveals important information about mechanisms in lubricant formulations contributing to overall impedance by frequency response analysis.
In general EIS is very similar to FT-IR spectroscopy where vibrational excitation of covalently bonded atoms and groups are induced and absorption spectra are recorded. In case of EIS dipolar moments of molecules and charged ions interact with the electromagnetic field. Obtaining ionic movement and polar interactions EIS perfectly fits to analyze corrosion and corrosion tendencies.
Think of lubricant formulations made of several compounds containing sulfur carriers highly corrosive to copper and you would like to know if your formulation containing coin metal passivators performs well. In this case EIS is capable of measuring the charge transfer resistance Rct which is directly correlated to the corrosion tendency of the metal and lubricant under investigation. Focusing on Rct formulators are able to optimize treat rates gaining margin.
Screening temperatures between -70°C and +240°C one can focus if additives might get activated by temperature forming highly corrosive matter. Think of a formulation used at oil temperatures between +25°C and +50°C. If your formulation produces highly corrosive matter at +70°C you do not have to take care, but what if activation takes place at +40°C? Better you perform tests such as EIS to be sure and prevent severe security issues and costs.
In principle the measurement cell used is made of two metal pipes plugged together fixed into a Teflon holder. At the top of the pipes a Randel-Screw is used to fix both pipes and assure a well defined space between both electrodes. Between the electrodes the lubricant is filled in till the top of the outer pipe. Contact is taken using crocodile cramps at the top and left extends of the copper pipes. The assembly is shown in Figure 1.
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LUBE MAGAZINE NO.130 DECEMBER 2015
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