Greetings,
Recently I just finish analyzing the impedance data collected on coatings which has been immersed in a stagnant artificial seawater inside laboratory and in real seawater at the beach (I basically threw the coated metal into the ocean).
So, what I have discovered is that, the coatings in the lab (artificial seawater) has been degraded severely as compared to the one immersed in the real site.
What is the reason of this phenomena?
Temperature has a great influence on the corrosion rate. If the temperature of the water in the laboratory was higher than the temperature of the water in the sea, then your result is well explained. At the same water temperature, corrosion in the sea should proceed faster than in the laboratory, since the saturation of the sea water with oxygen is higher.
Dear Wan, there could be tons of reasons. Mass transport is not the culprit. Assuming you have the same initial pH, You know what is in the artificial sea water. You do not know what is in the real site. You might check how contaminated is the real site sea water. Check for organic matter.
If the mechanism of corrosion is a complexation of metal, the difference in the concentration of soluble CO2 can explain this behavior
Dear Dr. Wan Mohamad Ikhmal,
your question is very interesting and it is quite difficult to give an absolute answer, I will mention only two examples below.
Regarding some metals or alloys (for example Aluminium based materials) corrosion results obtained in artificial seawater, prepared in the lab from distilled water and the aimed concentration of dissolved inorganic salts, are known to be generally different (more severe corrosion) from those obtained in natural seawater, even if the quantity and nature of dissolved salts is close to that of natural seawater.
In lab, all the biological components that are involved in the corrosion behaviour of metals/alloy in contact with seawater are missing in artificial seawater.
By comparing the degradation of coatings in natural and artificial sea water, it was shown that the kinetic of degradation was faster in artificial medium while the water absorption was larger in natural seawater. The results highlight the harmful impact of seasonal temperature variation in natural medium which could be explained by the development of internal stresses into the coating, as said in:
“Ageing of marine coating in natural and artificial seawater under mechanical stresses”
Narjes Fredj, S. Cohendoz, Feaugas Xavier, Sébastien Touzain
Progress in Organic Coatings 74(2) – 2012
In a very simple way I would affirm only that in the laboratory, using artificial solutions, I expose the samples in a controlled and defined exposure situation while in a situation of real type, the conditions are multiple (maybe not simulated in the laboratory in the impedance tests, eg influence solar radiation) and can also be very variable.
My best regards, Pierluigi Traverso
the question in incomplete in many aspects. it doesnt provide the base line specifications/ composition (physical, chemical and bilogical content) of both seawaters. the duaration of exposure is also not mentioned@
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