*School of Technology and Design, University College Northampton, Northampton NN2 6JD E-Mail :
+Centre for Corrosion Technology, Sheffield Hallam, University, Sheffield, SD1 1WB E-mail :
Keywords: Electrochemical techniques, anti-corrosive coatings, scribe protection
At Sheffield SRET (Scanning Reference Electrode Technique) has been used and modified to study a variety of corrosion processes [5,6]. With the ability to give localised, quantitative information about anodic and cathodic processes it has proved a very useful tool. Particularly useful for coatings evaluation is the rotational configured SRET as application is easy and the problem of height variation is minimised. A Project in early 1998 at Sheffield looked at a Polyaniline coating on steel using SRET. As expected from previous work with intact coatings, the high resistance of the coating made it difficult to "pick up" anodic and cathodic areas at least during the initial stages of exposure. However when used to examine deliberately created circular defects (created fairly crudely using a drill) results proved quite promising.
The general aim of the present work is to develop a fast laboratory test method to assess the effectiveness of inhibitive materials within a coating to protect the substrate at a break. A further objective of the work is to gain a mechanistic understanding of the coating behaviour where protection takes place. Defects are being investigated in several solvent-borne and water-borne primers on steel using the Electrochemical Noise method. Reported here are results for two proprietary primers, an epoxy (CM) and an alkyd (RH). Red Lead in linseed oil has also been investigated along with a control (water-borne unpigmented epoxy). All apart from the control contain inhibitive pigments in the coatings. Defects were produced mainly by scribing. Resistance (Rn) measurements were monitored as a function of time in either de-mineralised water or 10% Harrison's solution. Some results in other environments will be given in the full paper. It was found that none of the inhibitive coatings were successful in preventing corrosion upon immediate immersion in 10% Harrisons solution. So coatings were given a pre-soak by immersing for a time of 24 or 96 h in de-mineralised water. SRET has also been used to examine scribed (deliberately damaged) areas in the same solutions over a similar period of time. The expectation is that SRET would be able to provide useful information about the initial stages of passivity, or of substrate damage i.e. corrosion, at the (initially) bare metal area.
Most of the Noise results on CM and RH coatings were obtained from the same samples used for SRET (i.e. cylinders). The beaker method (as described in reference ) was employed. Fresh scribes were made either with or without re-coating before SRET measurements. Noise results from the red lead and IC 152 were obtained from Q panel samples using the bridge method (1). Blanking was achieved using a 75 / 25 mix of beeswax and colophony resin. The scratch area was typically 0.02cm2 (10mm x 0.2mm). The scribes were produced by a novel mechanical removal technique using a specially designed tool which removes the area of coating rather than displacing it by plastic/elastic deformation. The cutting edge of the blade resembles a small lathe (part-off tool) when viewed under magnification. This lifts the coating and shears the edges of the cut to give a square section scribe with an easily calculable area and leads to more reproducible results. Potassium ferricyanide indicator was used throughout this work to establish if corrosion was actually taking place at a particular point at a particular time. The indicator is very sensitive to Fe++ ions, a blue colour can be detected even when only traces of iron ions are present.
Electrochemical Noise was measured using a zero resistance ammeter and data logger with dedicated software supplied by ACM. Measurements are generally made every half-second for a period of 300 s. The current between the two working electrodes is monitored and at the same time the voltage of the pair is measured with respect to the reference electrode. From the standard deviations of the resultant 300 data points, the voltage noise and current noise are calculated. By dividing the former by the latter, the parameter resistance noise (Rn) is derived. Further details can be found [1,4].
SRET results were obtained by mapping over an area of 1cm by 1.5cm which included most of the scribe. The specimen was rotated at 100rpm and the measurements were taken over a period of up to about thirty hours. Further details on the experimental set-up can be found [5,6].
Although not given here intact coatings were investigated and gave Rn values which were considerably higher than the scribe results. Thus all values here are attributed to the scribe. Figure 1 shows the comparison between the clear epoxy and the CM coating after just 2 h in solution. It can be seen that the Rn value of the CM coating is approximately an order of magnitude higher than the unpigmented epoxy. Figure 2 shows the effect of time over a 24 h period for both CM and RH coatings in 10% Harrisons solution. Initially the CM coating inhibited (corresponding to an increase in the Rn value). However within 24h both coatings were failing to inhibit although the nature of the corrosion product was somewhat different in the two cases. Some degree of reduction in corrosion rate based on Rn beyond that shown by the unpigmented epoxy was evident though. Results for the red lead coating are shown in Figure 3. When immersed in 10% Harrisons solution it did not inhibit very effectively as shown by the ferricyanide test and by low Rn value. However when pre-soaked for 96 h in water and then transferred to Harrisons solution , it maintained a sustained high value of Rn and no visible corrosion was seen at the scribe. This graph also shows that when the Harrisons solution was increased to 55% of full strength this overwhelmed the inhibitor and visible corrosion ensued along with a drop in the Rn value to the levels of the non-inhibiting situation. The final graph Fig 4 shows a comparison of Rn value between red lead and the CM and RH coatings. Even after a 24 h soak in de-mineralised water the latter two primers were failing to afford much long term (more than 24 h) protection at the scribe. The preliminary soak in de-mineralised water (unlike the red lead) did not seem to have made any significant difference (value of Rn being as low as if it had been immersed immediately in 10% Harrisons solution. However (as already seen in figure 3) the red lead maintained a high Rn value.
SRETOnly results for the CM coating are reported in detail here. A sequence obtained over a period of some 24 h is shown in Figure 5. After preliminary immersion in de-mineralised water for 2-3 h the solution was changed to 10% Harrisons. Another set of timed results after straight immersion in Harrisons solution is shown in Figure 6. Corrosion can be seen to increase /decrease over that time and corresponds approximately with the Rn results for the CM coatings shown in Figure 2. Note that SRET is providing a spatially resolved measurement of electrochemical activity over the period of immersion.
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Figure 4 (Editor's note: in this and following figures, the scribe corresponds to the central vertical 'shadow')
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