Volume 2 Paper 24

Metal containing ceramic coatings as an anti-corrosion treatment for steel

M. Walsh and J. D. Scantlebury



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JCSE Volume 2 Paper 24 Submitted 15th October 1999 Metal containing ceramic coatings as an anti-corrosion treatment for steel mailto('michael.walsh1','talk21.com','M. Walsh') & mailto('scantlebury','manchester.ac.uk','J.D. Scantlebury') Corrosion & Protection Centre, UMIST, PO Box 88, Manchester M60 1QD §1 Abstract The use of Zinc Ethyl Silicate shop primers and their desired properties have been explained. Three primers with zinc contents of 46% by weight, 36% by weight and 18% by weight have been studied. A comparison of change of potential with time when immersed in 3.5%w/w sodium chloride solution has found the curves show similar characteristics to literature work but in a shorter time period. Immersion in artificial Manchester rainwater (AMR) has shown the potential initially moves toward that of steel before stabilising when pH approaches neutrality and the solution becomes less aggressive. The behaviour of the three coatings is much closer with immersion in AMR but only the 18% coating has shown any visible sign of steel corrosion and that is in 3.5%w/w sodium chloride solution. SEM images show the size, distribution, connectivity and clustering of zinc within the coatings. This allows for evidence of the protective action to be studied later. §2 Keywords:� Zinc Ethyl Silicate, Shop Primers, Cathodic Protection, Barrier Properties, SEM, Immersion, Potential Introduction §3 A variety of reasons exist to ensure the success of anti-corrosive coatings within the ship building industry. To commercial producers and users these range from economic and image issues to the importance of safety. A comprehensive understanding of the processes involved in the protective properties of these coatings is necessary. This study is particularly concerned with the first available opportunity to protect the steel, as the earlier protection can be applied the better the it becomes in the later stages [1]. The initial coating applied to the steel is known as the �shop primer� which utilises low levels of zinc metal giving �weldability� to the primer. §4 Modern ships are built using �blocks� and these blocks are formed from steel that has been primed (Figure 1). The primer protects the steel while it is stored in the marine atmosphere where the majority of shipyards are located. The primer also forms part of the completed coating system. On delivery the steel plate that forms the blocks can be covered in millscale. This is undesirable due to the rapid way millscale delaminates when exposed to heat and thus removing any paint applied over it. The millscale is removed by abrasive blasting which gives the steel a blast profile of ~70mm [2]. This freshly cleaned surface wishes to oxidise therefore the blasting and primer application are carried out on an automated continuous line [3]. The primer should be compatible with cutting, bending and welding providing corrosion protection by a thin coating. The coatings should provide short term corrosion protection from the atmosphere in marine environments. §5 Early shop primers were poly vinyl butytral (PVB) types and iron oxide pigmented epoxy paints. The shop primers used today are zinc silicate coatings with successive generations having reduced zinc content. It is this content compared to the level of success of corrosion protection stipulated by Evans & Mayne [3,4] that is not fully understood and the purpose of this study is to examine this adventitious performance. §6 Figure 1 Progress of steel at shipyard Experimental Name Metallic Zinc Content (Weight %) Pigment:Binder Mix Ratio Mean Dry Film Thickness (mm) ZS63 63% 2.62 : 1 22.5 � 2.8 ZS46 46% 2.49 : 1 22.0 � 3.0 ZS36 36% 2.40 : 1 26.5 � 2.2 ZS18 18% 2.26 : 1 21.8 � 3.1 §7 Table 1 Coating Properties §8 Three experimental coatings provided by International Coatings Ltd have been used in this study. The coatings are based on commercially available products that provide corrosion protection when used. The coatings are of the two-pack type. The coatings were prepared using the manufacturers mix ratios and applied to Q-panels (151mm � 101mm � 1.5mm) using a draw bar method. Table 1 identifies the coatings under test and their properties. §9 The coatings become touch dry within 5-10 minutes and were allowed to cure in an atmosphere of 20-25�C, 75-85%RH for 10 days. Assessment of cure was carried out using a solvent resistance test. The pass mark was no loss of film thickness. §10 Contacts were attached to the panels, which were then edged and backed with beeswax: colophony resin mixture (4:1). The panels were then immersed continuously in 3.5% weight/weight sodium chloride solution and artificial Manchester rainwater solution [5]. Potential versus time was measured using a saturated calomel electrode. SEM was carried out using an AMRAY 1810 operating at �20KeV in secondary electron and backscatter electron detection. Results and Discussion §11 In order to build up an understanding of this type of coating it was decided to monitor potential with respect to time by immersion in 3.5%w/w sodium chloride solution. This solution was chosen to allow comparison with work reported in the literature. It was also decided to monitor the potential at open circuit rather than imposing a current because of the low levels of zinc would have a shorter protection time [6]. §12 The electrolyte solution is particularly aggressive with regard to low-level zinc silicate shop primers that were not designed for immersion in such solutions. Artificial Manchester Rainwater (AMR) was selected to evaluate performance in acidic conditions. Again, the coatings were not designed for immersion in such solutions but nevertheless results will be useful in understanding coating performance. The use of AMR fits in with a program of work that is explained later in this paper.   Time to exceed EProt (Hours) Coating 3.5%w/w NaCl Solution AMR ZS18 60 115 ZS36 490 170 ZS46 650 290 ZS63 Not exceeded Not exceeded §13 Table 2 Coating Performance §14 Table 2 shows the time taken in hours for the system potential to reach the protection potential of steel at �780mV/SCE [7]. It would be expected that increasing zinc content would have increasing protection time. §15 As previously observed in the literature [8], the potential vs time curve exhibits an initial drop in potential before a steady rise and levelling towards the potential of steel (Figure 2).� The extent of the initial part of the curve indicates the extent of the cathodic protection. §16 Initially the curve shows an activation of the zinc while the potential moves to that of zinc plate. This is followed by a stabilisation period as corrosion products begin to form.� As the galvanic activity slows the potential begins to move towards that of steel [8-14].� It should be said that this model concerns zinc rich coatings but these zinc silicate shop primers have similar curves in a shorter time scale. Table 3 shows a selection of coating systems and the time to reach the protection potential of steel. §17 The behaviour of the coating system in AMR is what would be expected. The potential vs time curve shows that the trend of higher zinc content giving extended lifetime is observed (Figure 3). The corrosion rate is accelerated at the extremes of pH [15]. The zinc corrosion reaction takes part in a neutralisation of the pH which moves from pH 4.5 to pH 6.5 within 24 to 30 hours. The potential vs time curves appear to stabilise following a sharp movement to more noble values. §18 Visual observations of the coated panels under immersion in both electrolytes show that at the reading taken at 1200 hours ZS18, ZS36 & ZS46 had sites of iron rust. ZS18 began to exhibit iron rust at 650 hours while ZS36 showed sites at 700 hours (Figure 4). ZS63 coated panels have a �patchy� appearance where the colour is a slightly different tone in places compared to pre-exposure at 1200 hours. §19 Figure 2 � Potential vs Time for ZS18, ZS36, ZS46 & ZS63 under continuous immersion in 3.5% w/w sodium chloride solution comment(20)Figure 3 � Potential vs Time for ZS18, ZS36, ZS46 & ZS63 under continuous immersion in AMR solution §21 Figure 4 � ZS36 after 700 hours continuous immersion in 3.5%w/w sodium chloride solution Zinc Content (%w/w) Coating Thickness (mm) Time to EProt (Days) Reference 84* 60 � 5mm 60 [9] 70* 70 � 5mm 180 [10] Zinc Rich Epoxy 65 � 10mm 6 [11] Zinc Rich* 50� 5mm 40 [12] Zinc Rich Epoxy 48mm 65-70 [13] 84* 75mm 30 [14] §22 Table 3 - * indicates coating of ethyl silicate type. None of the coatings are shop primers pH Corrosion Rate (mils/year) 2 >200 4 45 6 15 8 5 10 <1 12 5 14 120 §23 Table 4 � pH / corrosion rate of zinc §24 AMR solution is an industrial environment based formulation [5] that is used at the Corrosion & Protection Centre, UMIST. A marine system would be more relevant to this coating system. However, with regard to pH it is in line with some marine environments measured at shipyards around the world (Table 5). Although useful, continuous immersion is not an ideal exposure condition for this type of coating. Further work is moving towards a cyclic immersion coupled with prohesion and not salt spray which has been discussed by Mitchell [22]. Location pH Reference Coimbra (Portugal) 4.75 [16] Kobe (Japan) 4.34 [17] Pusan (Korea) 5.99 [18] Mace Head (Ireland) 5.2-5.4 [19] Average UK 5-6 [20] AMR 4.5 [5] §25 Table 5 pH of rainwater §26 SEM observations and analysis of the unexposed coatings indicate the arrangement of the zinc particles and the situations of sites and types of corrosion. Arrangements within the coatings would show pathways for both electronic and ionic conduction. Ionic conduction pathways would take the form of cracks, binder and voids. Electronic conduction pathways would take the form of particle to particle to steel connectivity as illustrated by Morcillo [21]. §27 Figures 5-8 show zinc x-ray maps acquired using the SEM show the situations that the zinc finds itself in over the four coatings. ZS63 (Figure 5) shows a relatively high density of particles which coincides with zinc to zinc connectivity. This higher density makes the gaps in the coating more apparent. The figure provides a good example of the conduction pathways discussed above. It can be seen from the ZS46 map (Figure 6) that the zinc is in clusters with an apparent inter-cluster connectivity. The is also evidence of connectivity between the clusters and the substrate. The connectivity is not as great as observed in zinc rich coatings [21]. Figures 7 & 8 show significantly less zinc and connectivity for the ZS36 & ZS18 coatings respectively than observed with ZS63 & ZS18. §28 Work carried out by Morcillo [21] suggests that zinc in clusters which affords cathodic protection will show localised attack whereas isolated zinc will be surrounded by a layer of zinc corrosion product. These SEM images show that there is a certain degree of zinc to zinc to steel contact that can give cathodic protection.� This can be partly observed with figures 9-11.� Figure 9 shows a SEM photograph of a large particle in a ZS46 coating that has undergone localised attack.� The outer �ring� that can be observed contains chlorine, as shown by elemental analysis. This is indicative of corrosion product but is by no means an absolute method of determining the existence of corrosion product. Visually, it is difficult other than in this case to differentiate between silicate binder and corrosion product.� It can be said at this point further analysis is needed to determine the exact nature of the �ring� though current work indicates the possible existence of corrosion product. §29 Figure 10 shows a ZS36 coating that contains a number of voids along with a zinc particle in contact with the substrate and exhibiting localised corrosion.� As yet it has not been possible to determine whether any corrosion product is left in the voids.� This could be due to the type of exposure i.e. continuous immersion where the corrosion product is dissolved into the electrolyte. The coatings do have a number of voids present before exposure and observations indicate an increase in number with exposure. §30 Figure 11 shows localised attack on a small particle in a ZS63 coating and again there is evidence of the outer �ring� which also contains a level of chlorine indicating corrosion product. §31 At this stage the work appears to disagree with that of Morcillo [21] where corrosion product is found around zinc particles that are or have been in contact with other particles and the substrate along with localised corrosion on the zinc. This could be explained by the fundamental differences in the coatings (zinc-rich and low zinc); by the thickness of the dry film (50-60mm compared to 20mm); or by the protective mechanism of the coating. �This piece of work forms part of the evidence in investigating the last point. comment(32)Figure 5 � ZS63 coating, Zinc X-Ray map Figure 6 � ZS46 coating, Zinc X-Ray map Figure 7 � ZS36 coating, Zinc X-Ray map  Figure 8 � ZS18 coating, Zinc X-Ray map  §33 Figure 9 � SEM photograph of ZS46 coating after 240 hours exposure §34 Figure 10 � SEM photograph of ZS36 coating after 240 hours exposure §35 Figure 11 - SEM photograph of ZS63 coating after 240 hours exposure Conclusions §36 The behaviour of the coatings when immersed in 3.5%w/w sodium chloride solution has been shown to be similar in character albeit in a smaller time frame to literature work. Improvements in coating technology driven by the desire to increase ship building time may have lead to current shop primers performing better when welded and exposed than previous experience with low-level zinc coatings. It must be remembered that these coatings were not designed for immersion in aggressive solutions. §37 The use of AMR shows that corrosion occurs faster initially before stabilising. As the electrolyte is not refreshed the pH increases to neutral values to create a less aggressive environment. The range of exposures to be studied will in future include cyclic immersion and prohesion/spray to simulate rain pooling and rainfall. Alongside potential vs time, the barrier development with time, SEM observations with time and zinc content with time will be studied. §38 The SEM images have shown the distribution, size, connectivity and clustering of zinc particles in the coatings.� This establishes the type and number of electronic and ionic conduction pathways in the coatings. Localised corrosion of zinc particles has been observed which according to Morcillo [21] indicates the mechanism of cathodic protection. If this is the case then the extent of this protection in providing barrier properties is to be investigated. Acknowledgements §39 The Author would like to thank Dr P A Jackson and Dr L M Callow of International Coatings Ltd for their involvement and sponsorship of the project. The author would also like to thank the Engineering and Physical Science Research Council for sponsorship. 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