I am d
I am running a Tafel Scan on an epoxy embedded Titanium Alloy sample in 3.5 wt% NaCl Solution. I allow stability time of about more than 2 hours every time I do the test.
To increase the confidence level, I repeated test on the same electrode after abrading on 220 grit paper and cleaning with ethanol and water. To my surprise, first time OCP was about -334 mV vs SCE, second time it was about -180 mV vs SCE and this time it is about -28 mV vs SCE. I am not sure where this problem could be originating.
Experimental Details:
I use a stock solution of NaCL made in glass beaker of Pyrex.
The corrosion cell volume is about 50 ml, Working electrode area is about 1 cm2 and graphite rod serves as counter electrode.
I wash both the reference electrode and graphite rod with distilled water before use to minimize any perturbations to test solutions.
Hi Hashim
Let do a standard test first to check the system and the reference electrode.
The standard test in G5. (attached).
If you do not have all in the G5 try to repeat any will known test to get the same results.
Also, here some point;
1- the scan rate should be as small as you can 0.1667mV/s is fine.
2- 220 grit paper, is quite rough, try 600 or more.
3- increase the area of the working electrode (not less than 5 cm2) and keep the solution volume to area ratio not less than 40.
4- Measure the OCP with time to fine when it becomes stable.
3- Repeat test three time if you have 5% error, double if it is 1-3%.
Good luck
Gobara
Hello Hashim Shah,
The corrosion rate for the Taifel method also depends on the type of mariculture. If it is small, it occurs early. While in a large structure corrosion occurs later. The type of structure is changed after technological processes. In case of plastic deformation (rolling) the structure of the surface is ejected and lengthens to the core. Therefore, it is very important how samples are taken for the analysis of the parent metal. In heat treatment to unify the crystalline structure, another adverse effect is observed on local sediments which alter mechanical and technological properties as well as corrosion properties.
Hi Hashim Shah, I also worked with Titanium doing corrosion tests, but I used AG/AgCl as a reference. My OCP varies between (-508 and -204mV) but for different microstructures and heattreatment; normally on the same state the OCP was very stable (on the finall tests after resolve some problems).
As you know the Titanium is very easy to oxidize and when I welded the wire (for the data adquisition) to the back of my sample, the electric contact was not very good, and many times the wire was detached, before or after the assembly in the polymer or during corrosion tests (this introduces a lot of noise). I made my welds with tin (Sn) which does not bind to the titanium surface, because the truth is a ceramic (TiO2) and not a metal.
Tips: polish or have thick stripes on the back of the sample to improve the bond between the cable and the sample.
Uses resistance spot welding (or other process), which guarantees a better join (electrical conductivity).
2) If the conductivity is not your problem, see if holes (cavities, trenches you know) are being made between the edges of the sample and the polymer (by the polishing), this causes the solution to start enter from the sides and can generate noise ... a help is to use "nail polish (enamel)" around the edges of the sample after the final polishing (obviously you would have to recalculate the final surface area, as the enamel would cover part of the exposed surface of the titanium).
3) Grit paper 220? I think it is very thick, I do not know if the roughness can also causing noise ... I used as final polishing diamond of 3 micros
Take a look at my work, I hope I've been able to help you.
https://www.researchgate.net/publication/260231744_Effect_of_the_cooling_rate_in_the_corrosion_behavior_of_a_hot_worked_Ti-6Al-4V_extra-low_interstitial_alloy
Article Effect of the cooling rate in the corrosion behavior of a ho...
Hi,
if you start immersion of titanium right after grinding, a passive layer will develop on the surface and the OCP will drift to more positive values with longer immersion time. Same will happen, when you store your fresh ground specimen in air with rH>80% for a certain time (passivation under atmospheric conditions). You will see the effect of time (and rH) on the passivation kinetics. I think titanium shows a similar behaviour to stainless steels and we described this effect e.g. in this publication:
https://www.researchgate.net/publication/230528719_The_factor_time_at_the_formation_and_development_of_the_passive_layer_in_the_atmosphere
Best regards,
Andreas
Article The factor time at the formation and development of the pass...
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