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Volume 2 Extended Abstract 21

Submitted 26th August 1999

The Effectiveness of Chromate-free Inhibiting Pigments in Coil-coated Galvanized Steel

I.M.Zin, S.B.Lyon, S.J.Badger*, J.D.Scantlebury, V.I.Pokhmurskii 
Corrosion and Protection Centre, UMIST, P. O. Box 88, Manchester, M60 1QD, UK *Uniscan Instruments Ltd, Sigma House, Burlow Rd, Buxton, Derbyshire, SK17 9JB, UK
E-mail:  

Keywords: inhibition, galvanized steel, impedance, cut-edge corrosion, SVET

Introduction

Intensive investigations with the aim of finding substitutes for chromate inhibitors are currently being investigated around the world with phosphates one possible alternative. They are usually classified as non-oxiding anodic inhibitors in which the presence of oxygen is required for corrosion protection [1,2]. However under certain conditions, in the presence of metal ions, phosphates may also act as cathodic inhibitors where precipitation of a phosphate film on a metal surface provides barrier inhibitive properties. Uhlig at al [3] found that the enhanced efficiency of polyphosphate in reducing the corrosion rate, obtained by cojoint use of the divalent Ca2+ or Zn2+ ions, is connected with the formation on the cathodic areas of a diffusion barrier layer impeding access of oxygen to the iron surface. However, it is well known, that the function of soluble inhibitors in corrosive solutions differs from their behaviour as inhibiting pigments distributed in a organic coating. The objective of this work was to study the mechanism of corrosion inhibition by pigments used in coil-coated cladding with focus on chromate substitutes able to provide phosphate and calcium ions. Such pigments include commercial compounds eg: Shieldex (Ca2+ ion exchange silica) and Actirox (Zinc phosphate/molybdate).

Experimental

The corrosion of galvanized steel was investigated using electrochemical impedance spectroscopy, and potentiodynamic polarization. All electrochemical measurements were carried out in artificial acid rain solution with additions of the pigments Actirox, Shieldex, strontium chromate and the double extract Actirox/Shieldex. The ability of the various pigment extracts to reduce galvanic current between zinc and steel at a cut edge corrosion was evaluated using a model cell [4] consisting of a "sandwich" of steel sheet, plastic film and zinc foil with the galvanic current measured by the ZRA method. Finally the Scanning Vibrating Electrode Technique (SVET), was used to make measurements of local potential gradients (and hence the localised corrosion activity) at defects made in the pigmented polyester coatings. Coated samples were prepared from Hot Dip Galvanized (HDG) mild steel sheet, with a zinc layer of 15 m m. An inhibitor-containing polyester primer (5 m m) and polyester top-coat (20 m m) were applied using a draw-bar technique. Four different primer formulations were used, namely RH5 with Actirox 106, RH2 with Shieldex CP 47394, RH3 with Strontium Chromate and RH1 with blend RH2/RH5. A laser-ablation was used to place a 1 cm long scribe through the zinc, primer and topcoat layers. The laser spot size (and hence scribe width) was 180 m m, and the depth was 50 m m.

Results and discussion

EIS of galvanized steel both in single extracts of the pigments and in the two component extract (Actirox + Shieldex) shows a depressed semicircle. It was established (Figure 1), that the initial charge transfer resistance is in about 10 times higher for galvanized steel in chromate and the Actirox/Shieldex blend compared to single solutions of the chromate-free pigments. This clearly shows a synergetic protective effect, because the sum of impedance values of galvanized steel observed in extracts of single pigments is about 5-10 times less compared to the impedance in the blend extract. DC polarization show, that cathodic control of corrosion reaction prevails over the first hours of exposure for all saturated pigment solutions. In addition, the chromate-containing solution shows an anodic shift in corrosion potential. Further, during 7 days of exposure, clear cathodic inhibition is observed in all solutions. However, at the same time increasing passivation of the surface is evident for strontium chromate and the Actirox 106/Shieldex CP 47394 combination extracts. It was established that all pigment solutions to some extent decrease the galvanic current in the model cell compared with the blank solution. The strontium chromate solution has the best inhibitive properties with the current decreasing to 0.5 mA after only 100 minutes exposure. Other pigments (Actirox 106 and Shieldex CP 47394) during the test reached only a level of 4 mA and their inhibitive efficiency is about 50%-60% compared to the corrosion of the metal in the blank acid solution. The acid solution, saturated with both Actrirox 106 and Shieldex CP 47394, is close in efficiency to chromate solution with a minimal current of 0.8-1.0 mA. The distributions of DC potentials around the scratches, upon initial exposure and after 7 days, are obtained as 2-dimensional area maps for each inhibitor formulation. An example of the map is given for the sample RH1 (Actirox/Shieldex) in Figure 2. As indicated by the shaded scale below the map, relatively dark areas on the map represent local anodic sites, whereas the lighter areas represent cathodic sites. The map for initial exposure of RH1 shows intense corrosion activity at defect with an uneven distribution of relatively negative (anodic) and positive (cathodic).  

In order to make a quantitative assessment of activity the total anodic potential measured at every data point was summed. The results for a 7 day immersion period indicate that all samples reach approximately the same level of activity after 72 hours. However, though the initial anodic activity upon immersion for Actirox/Shieldex was the highest of all the samples the corrosion activity in this sample decreased rapidly and remained lower than strontium chromate for the majority of the remaining exposure period.

Conclusions

The combination of zinc phosphate/molybdate (Actirox 106) and calcium ion exchange silica (Shieldex CP) has significant synergetic anticorrosion effect for galvanized steel in the acid rain solution compared to the mentioned pigments used alone. It was established by using a model galvanic cell that the acid rain solution saturated by the pigments mixture Actirox 106/Shieldex CP47394 has similar inhibiting efficiency for cut edge corrosion of galvanized steel as the acid solution saturated by strontium chromate. The SVET technique was able to map and quantify discrete anodic and cathodic sites along the defect, which were due to the inherent localised corrosion behavior of zinc. By summing the anodic potentials measured using SVET for each scan, a quantitative comparison was made of the chromate and chromate-free inhibitors. The best corrosion inhibiting performance was offered by the blend of two chromate-free formulations (Actirox+Shieldex).

References

1. M. Pryor, and M.Cohen, J. Electrochem. Soc., 98, p 263 (1951). 

2. U.R.Evans. Corrosion and Oxidation of Metals, 1960, p.136. (London: Arnold). 

3. H.H.Uhlig, D.N.Triadis and M. Stern, J. Electrochem. Soc., 102, p 59 (1955). 

4. R.L.Howard, S.B.Lyon and J.D.Scantlebury, Proceeding of 13th ICC, Melbourne, Australia, 26-30 November 1996, Paper 022. Figure 1: Time dependence on charge transfer resistance of various pigmented coatings during immersion.         

 

 

Figure 2: SVET maps of local solution potentials for Actirox/Shieldex combination: left picture initial exposure, right picture after 7 days.

 

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