M.M. Attar and J.D.Scantlebury
Corrosion and Protection Centre, UMIST, P.O.Box 88, Manchester M60 1QD, United Kingdom
Many current corrosion control methods use coatings and conversion layers which contain toxic and environmentally hazardous material, in particular hexavalent chromium compounds. There is a great need to find a non toxic replacement that is compatible with current industrial technologies.
Polyaniline has been found to have many interesting electrochemical properties, with some electronic conductivity on doping with acid. Its potential use in protecting metals from corrosion has been explored. with a number of papers on the electroactive polyaniline(PANI), with some claiming corrosion inhibition.(1-7).
Polyaniline exists in several different oxidation states, of which only the emeraldine salt form is conductive. Each of the forms can be reversibly interconverted through electron transfer, thereby enabling selective colour change and the switching from conductive to non conductive forms. The conductive form of polyaniline is the emeraldine salt, a green black powder. All other forms are insulating polymers.
The base form of the polymer in the emeraldine oxidation state (Y = 0.5) is :
Which contains equal numbers of alternating reduced,
repeat units which can be protonated by dilute aqueous acids such as HCl to produce the corresponding salt ( A = anion ) :
which it is believed exists as the polysemiquinone radical cation (8).
The purpose of the study reported here is to explore the corrosion inhibiting properties of saturated aqueous extracts of PANI in neutral and slightly acid electrolytes on mild steel. Such methods have traditionally been used as the starting point for the investigation of the mechanism of inhibitive pigments for paints (9). A commercial chromate containing pigment, strontium chromate was used for comparative purposes.
Square (1 x 1 cm) mild steel samples from Q Panel Co. were prepared by degreasing with ethanol. PANI was prepared at 0, 42 and 50% protonation using chemical synthesis; oxidation was by ammonium persulphate in hydrochloric acid according to (2).
The electrochemical methods exploited in these studies were the measurement of open circuit potential ( OCP ) and linear polarisation resistance (LPR) for bare mild steel after immersion in the saturated solutions and blanks. The equipment used was " AutoLPR " from ACM instruments.
The saturated solutions were 0%, 42%, 50% protonated polyaniline and strontium chromate in 0.01 molar NaCl.
Adding different states of polyaniline to 0.01 molar NaCl solution can cause changes of pH. A saturated solution of 0% protonated polyaniline produces a pH = 6.0, while the pH of the electrolyte changes for 42% and 50% protonated polyaniline to 4. Therefore blank solutions were made according to different acidic environments in order to compare with the PANI saturated solutions.
Testing solutions were 0.01 M NaCl at pH 6.0 its as- prepared pH and also a blank solution was prepared at pH 4.0. PANI solutions were prepared by shaking PANI powder in neutral 0.01 M NaCl until no further dissolved. The solutions were used in an unfiltered and stirred state.
Open Circuit Potential (OCP), Linear Polarisation Resistance (LPR) and also visual observations for mild steel immersed in different media are shown in figures 1 to 2 and the accompanying colour plates. Visual observation made on the samples after eight hours exposures are summarised in Table 1 and are to be seen in the accompanying images. Of the samples exposed to NaCl only strontium chromate gave complete protection against corrosion. The corrosion process with 0% protonated polyaniline appeared to be localised.
Table 1: visual observations after 8h corrosion testing
|0.01M NaCl, pH6,||yellow-orange film covering steel surface|
|0.01M NaCl, pH4||deep yellow film covering steel surface|
|0.01M NaCl + 0% protonated PANI||bright steel with pits|
|0.01M NaCl + 42% protonated PANI||brown film covering steel surface|
|0.01M NaCl + 50% protonated PANI||deep brown film covering steel surface|
|0.01M NaCl + strontium chromate||shiny uncorroded steel surface|
In addition to visual observation two approaches of corrosion measurement were made during the exposure period. Measurements of corrosion potential suggest that the presence of the polyaniline on or near the surface of mild steel mild steel somehow seems to affect the corrosion process. Figure1 shows the corrosion potentials at various times for the samples exposed to the six test environments.
In all cases, after 8 hours exposure the samples immersed in strontium chromate, 0%, 42% and 50% protonated PANI exhibited corrosion potentials more noble than the corresponding control samples. The largest shifts in corrosion potential of polyaniline environments with exposure time were observed for the sample immersed in 0% protonated polyaniline, and were almost 100 mV. The samples immersed in 42 and 50% protonated PAn showed shifts to slightly more noble potentials with exposure time.
LPR measurements for the six samples after 8 hours exposure are shown in figure 2. As expected the LPR measured clearly show a substantial increase in polarisation resistance (some gave a twenty times increase) for the samples immersed in strontium chromate.
It can be seen that after 8 hours exposure the sample exposed to 0% protonated PANI exhibited corrosion resistance a little higher than the blank. However for the 42% and 50% protonated PANI, LPR is lower than the corresponding control sample.
Thus the evidence so far does not suggest that the unprotonated PANI is a
successful corrosion inhibitor with an Rp increase of approximately double.
Even so, substantial changes in the nature of the corrosion process may be
seen, from uniform to pitting. With the other PANI systems studied (42% and
50%), there was a reduction in Rp implying a corrosion stimulating effect.
There was also a colour change in the uniform corrosion product. This may be
due to the PANI in suspension altering the morphology of the rust deposit.
Although this work does not substantiate the previous claims of PANI being a corrosion inhibitor, it might be that when dispersed in a paint binder having different solubility characteristics, a different behaviour might be seen.
As expected, the strontium chromate pigment gave excellent performance in this investigation. The 42 and 50% protonated PANI far from providing inhibition, actually appeared to increase the corrosion rate of mild steel. The 0% protonated PANI showed little inhibitive effect in terms of polarisation resistance, however its major effect was in the appearance of the corrosion processes, changing from uniform to a pitting regime. It may be that from this compound insufficient is dissolved in aqueous solution to provide complete inhibition and some means may be needed to place a higher concentration of this material in contact with the steel surface. The use of an organic polymeric dispersing medium is an obvious possibility.
(1) Wessling, B. Synth. Met. 1991,41-43, 907
(2) MacDiarmid, A.G.; Chang,J.C.; Richter, A.F.; Somasiri,N.L.D.; Epstein, A.J. In "Conducting polymers", Alcacer, L., Ed., Reidel publishing Co.: Holland. 1987, 105.
(3) DeBerry, D.W.,J.Electrochem. Soc. 1985, 132,5, 1023
(4) Wrobleski, D.A.; Bencewicz, B.C.; Thomson, K.G.;Bryan,C.J.,Proc ACS, March 1996, Extended Abstr. 256
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(6) MacDiarmid,A.G., Mol.Cryst.Liq. Cryst., 1985,121,173
(7) F.Wudl,R.O.Angus.F.L.Lu,P.M.Allen and, D.J.Vachon, M.Nowak, Z.X.Liu and A.J.Heeger, J.Am.Chem.Soc.,109,3677 (1987)
(8) MacDiarmid et al., Mol.Cryst.Liq.Cryst.,1988, 160,.151
(9) Mayne, J.E.O and Rawshaw, E.H., J.Appl.Chem. (1960) 10 419
Image 1, after immersion for 8h in strontium chromate
Image 2, after immersion for 8h in 0% protonated PANI
Image 3, after immersion for 8h in blank solution pH 6
Image 4, after immersion for 8h in blank solution pH 4
Image 5, after immersion for 8h in 42% protonated PANI
Image 6, after immersion for 8h in 50% protonated PANI