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David C. Silverman |
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Example in a Low Corrosion Rate Environment  (227k).
Cocoyl glutamate was discovered to prevent staining of magnesium and aluminum alloys under alkaline conditions. The question was how this staining was being prevented, corrosion inhibition vs. removal of corrosion products (e.g. some corrosion acceleration). Solutions of various concentrations of cocoyl glutamate were formulated at a pH of 9.5. Corrosion of aluminum 7075-T6 and magnesium AZ31B were measured in these solutions by generating polarization resistance curves as a function of time over 24 hours and exposing these alloys to the test environments for 30 days to obtain corrosion rates by mass loss. The polarization resistance curves were analyzed by non-linear regression of the equations (2) and (3) against the data. 
(2)and 
(3)The symbols are explained in the section entitled Summary of Polarization Resistance Technique in this tutorial. This figure )
shows the plot of 1/Rp versus time for magnesium
and this figure )
shows the same plot for aluminum. Triethanolamine was
used as a control because it is present in many metalworking fluids and is thought
to provide corrosion inhibition to at least steel. When Tafel slopes are
difficult to estimate, the reciprocal of the polarization resistance can often
be used to represent the corrosion rate for screening purposes. The inherent assumption
was that the Tafel slopes did not change as a function of conditions, an assumption
thought to be reasonable in this study.
Corrosion rates were estimated by assuming a reasonable value of "B" in 
(4)The estimate was made in this case assuming a value of 0.025V for both aluminum and magnesium. The corrosion rate of magnesium was estimated to be in the range of 0.5 to 1 mpy (0.01 to 0.02 mm/y). The corrosion rate of aluminum was estimated to be less than 0.1 mpy (<0.0025 mm/y). Mass loss results suggested somewhat higher corrosion rates for magnesium in the range of 2 to 3 mpy. The aluminum corrosion rates from mass loss were less than 0.1 mpy (<0.0025 mm/y). The results suggested that the cleaning of magnesium was by removal of stains by corrosion of discoloration products while the cleaning of aluminum was through relatively complete passivation of aluminum. Further discussion of the reasons for this conclusion can be found in D. C. Silverman and T. K. Hirzel, "Divergent Effects of N-Acyl Glutamates on Corrosion of Aluminum and Magnesium Alloys", Corrosion, Vol. 58, p.99 (2002) 1 (227k).
Note that the agreement of corrosion rates for magnesium between techniques is good but not great. Assuming that the choice of 0.025V is within a factor of two of the actual value, the discussion elsewhere in this tutorial suggests that the discrepancy could be an artifact of the procedure for generating and analyzing the polarization resistance scans especially for alloys exhibiting very low corrosion rates. This figure )
shows a typical scan for magnesium and this
figure )
shows a typical scan for aluminum, both generated after 24 hours of
exposure. At that time, the corrosion potential measured as an open circuit potential
was stable, changing at less than 1 mV per hour for either alloy.
The abscissa for each plot is the voltage relative to that voltage registering as zero current on the polarization resistance scan not as the voltage relative to the open circuit potential measured prior to starting the scan. The reason is that equation (2) is derived from the fact that no current is applied at the corrosion potential. To use equation (2) to estimate polarization resistance and Tafel slopes requires that when V=Vcorr, iapplied = 0. The fact that Vcorr,true measured at open circuit and Vcorr,apparent as estimated from polarization resistance plots are slightly different means that polarization to -20 mV relative to Vcorr,true upset the corrosion process enough that it could not recover during generation of the scan. The process being examined electrochemically is slightly different from the process existing under open circuit potential even under the low polarization required by this technique. This artifact does not negate the usefulness of the polarization resistance technique. It does indicate that care is required when interpreting results especially for more passive alloy-environment systems. | |||
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Previous Page: Example (Nickel in Strong Acid) 1 © NACE International publication and year shown in citation above. All rights reserved. Displayed with permission from NACE International, Houston, TX (http://www.nace.org). Published in Corrosion, in the month and year shown in the citation above. | |||
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David C. Silverman, Ph.D. - Primary Consultant |