Comments to
- M.M. Attar, J.D. Scantlebury
- Polyaniline as a possible inhibitor for the corrosion of mild steel
J. Corr. Sci. Eng., Vol 1, paper 8
Date: Sun, 15 Feb 1998 17:41:11 +0100 (MET)
From: "Dr. Bernhard Wessling" @zipperling.do.eunet.de>
Subject: Re: what is passivity? (2)
At 17:02 13.02.98 BST, David Scantlebury @FS1.CP.UMIST.AC.UK> wrote:
>...
>
>Corrosion protection of Mild Steel by Coatings containing polyaniline;
>Lu, Elsenbaumer and Wessling, Synthetic Metals, 71, 2163-2166, (1995)
>
>In this paper ... drilled a whole ... in NaCl solution ...
>The doped polyaniline, started at -0.87V SCE and moved to -0.85V SCE
>after 8 weeks.
>
>Looking at Pourbaix, it is clear that at these potentials in neutral
>sodium chloride, it cannot be claimed that the steel was passive and
>that this conclusion drawn by Wessling in the final and abstract
>sections of the paper is wrong.
1) you forgot (?) to cite my paper in Adv. Mater. 6, No 3, 226 (1994). Here
- measuring with a completely coated metal surface - we showed that the
corrosion potential, defined as the potential, at which corrosion current
starts to increase exponentially, was shifted by about 800 mV to the
positive (more noble) range. What we measured here, was a kind of "real"
corrosion potential, i.e., the potential at which an environment would have
to be for becoming able to corrode the system investigated.
2) in the a.m. paper, we used a different technique and have determined (a)
the electrochemical corrosion potential of the pure metal surface (beneath
the oxide layer) ---- nobody will change the potential of an iron surface.
(b) the corrosion current --- and you forgot (?) to mention this part of the
results: the corrosion current was reduced by orders of magnitude just above
the corrosion potential, and also even furthermore at higher potentials.
(3) Iron has a potential at which it will release electrons (in a certain
electrochemical environment). This is an electrochemical fact. Corrosion is
the process during which electrons are removed at a CERTAIN RATE (per day,
per cm²). Whenever a coating can reduce this current significantly,
corrosion will be prevented.
(4) Polyaniline, our Organic Metal, when incorporated in primers, is
shifting the potential at which a corrosion current would flow (i.e., at
which corrosion would TAKE PLACE). It does in fact also initiate and
catalyse the formation of a alpha-Fe2O3-layer. The potential, at which a
primer/top coat system with an without Organic Metal would show the same
corrosion current per cm² are different by about 800 mV!
(5) You can repeat such measurements as shown in Adv. Mater. 6, No 3, 226
(1994), or even better (as we do now) by Scanning Volta Potential (Scanning
KElvin Probe). We will show results of such measurements shortly in a new
paper. Also here, you see a potential shift by about 500 - 800 mV.
This is passivity (passivation).
Dr. Bernhard Wessling
