Physical and Structural Characterization
The impact of the rolling is to impart non isotropy in the surface with peaks and valleys running in a direction parallel to the rolling direction, Fig. 1a–f. Similar surface structures are observed with the ECCS and TCCT substrates as this is predominately dictated by the finishing roller surface characteristics, Fig. 1a–d. There is no discernible change in the topological characteristics beyond which can be attributed to measurement position within the sample. The surface of the tinplate is somewhat smoother with areas of lower gradients in the valleys, associated with tin reflow process which occurs after tin deposition, but prior to chrome passivation, Fig. 1e, f. The impact of thermal exposure is minimal with each substrate showing only subtle changes in structure between the surface uncoated and thermally treated to 200 °C. No measureable difference in surface finish (as defined by Ra) was observed between the unheated and heated samples beyond the variability observed between areas on the same sample, Fig. 1g.
Further SEM (Scanning electron microscopy) microstructural examination of the substrates shows subtle changes between the untreated and heat-treated surfaces. This was not deemed to be significant given the natural local variations between areas on the substrate, Fig. 2. Generally, the changes observed were less noticeable than natural variations observed between individual topographic areas on any sheet.
The primary instance of differences between the pretreated and untreated substrates was observed on the 311 tinplate, Fig. 3a. Thermal treatment results in some evidence of FeSn2 growth at the peaks through the tin surface.
There is a greater, but not large, discrepancy between the untreated and treated subsurface structure Fig. 4. For ECCS, Fig. 4a, there is a slight structural change in the unit cell with the peak at 45° increasing and the peak at 65° decreasing in size relative to the peak at 82°. On the TCCT substrate, the Cr and iron peaks at 45°, 65° and 82°, respectively, remain more or less constant, Fig. 4b. The TCCT substrate does, however, experience a structural change in the surface with the carbide response increasing at 38° and decreasing at 80°. This implies that the meta-stable material which is only partially crystalline and that the effect of the temperature to increase crystallinity. For the tinplate substrate, there is a reduction of the peak for tin at 32° due to the reflowing of the material alongside a slight decrease in the tin peak at 62°, Fig. 4c.
For the current substrates there is a minimal change in the physical characteristics of the ECCS substrate and only subtle changes in the tinplate substrate due to some evidence of FeSn2 growth. The primary finding for the developmental substrate is the growth of the carbide layer near the surface.
Surface Chemical Characterization
Surface species provide the binding surface for the organic lacquer and thus plays an important role in determining lacquer adhesion . Each substrate experiences some changes in surface chemistry over the temperature range considered, Fig. 5. For clarity, compounds with proportions less than 2% are omitted while the labels “OH” and “Oxide” represent the hydroxyl and oxidic species which are associated with the chrome [Cr2O3 and Cr(OH)3], respectively.
The surface of the ECCS substrate is dominated by hydroxide species, Fig. 5a, b. In air, the structural hydroxide within the chromium hydroxide remains stable until around 100 °C and then decreases as the temperature is increased. At 200 °C, the chromium hydroxide is reduced by around 12% with a complimentary increase in the surface oxide species. The water of hydration within the chromium structure is activated via thermal treatment, leading to the creation of a coordinated –OH network on the surface of the material for subsequent bonding within a chromium network. This olation process at the surface is followed by an oxolation process, with increased formation of Cr2O3 species . The imposition of vacuum results in lower initial chromium hydroxide levels on the surface and earlier transition of the chromium hydroxide to chromium oxide, Fig. 5b. Thus, both temperature and partial pressure play a role in determining the thermodynamic stability of the chromium hydroxide species and the process of olation and oxolation to form the chrome oxide. There is significantly lower chromium hydroxide with the novel passivated TCCT material compared to the ECCS substrate, Fig. 5b. The rate of hydroxide to oxide conversion is relatively low at temperatures below 200 °C but becomes rapid beyond 200 °C. For the 311 tinplate, oxidation of the tin is clearly evident with a rapid increase in tin oxide growth as the temperature increases, Fig. 5c. The growth in oxide is a result of the thermal dehydration of the hydroxide which undergoes a corresponding reduction in relative abundance .
While the surface dictates the lacquer/substrate bonding, adhesion failure can be a result of changes in the subsurface, e.g. growth in brittle metal oxide layers. For this reason, further analysis of the near subsurface via depth profiling XPS through the substrate at room temperature and at 200 °C was carried out. For each substrate, the thermal regime has an impact on the subsurface chemistry, Fig. 6. By examining the change in chrome level (shown in green) between the unheated and heated samples then it is possible to identify the extent to which the high temperature exposure alters the near surface chemical composition. For ECCS there is a reduction in chrome level to 8 nm below the surface with a greater oxidation in the near surface layers, Fig. 6a, b. For the ECCS substrate, the results at room temperature show a similar structure to that measured in the literature  although this work provides a more detailed through layer composition map. The change in chrome layer between the unheated and heated samples for the TCCT substrate is proportionally larger than that observed with the ECCS substrate, Fig. 6c, d with the TCCT also exhibiting chrome oxidation. The primary difference between the ECCS and TCCT substrate in the sub surface layers is the presence of significant carbide proportions in the near surface area with the TCCT which are not present with the ECCS. With the TCCT, there is also some evidence of appreciable iron between 5 and 10 nm and at 200 °C which would tend to indicate some iron diffusion is occurring.
Exposure to 200 °C results in small changes in the subsurface chemical composition with the 311 tinplate, Fig. 6e, f. The dominant effect is again an increase in the chrome oxide levels near the surface region. A small amount of migration of iron from the black plate and its oxidation to iron oxide is also evident at a depth of 15 nm within the tinplate substrate.
The tinplate substrate exhibits solid state diffusion of iron into the tin which starts at 200 °C. This represents an interesting result as the nominal reflow temperature for tin is around 260 °C . The passivation layer, however, tends to stay relatively stable. This is believed to be due to the very low level of metallic chromium actually deposited on the surface during the 311 passivation process. Hence, the passivation has no driver for oxidation of the metallic chromium to oxidic chromium. It does, however, show an increase in the thickness of the tin oxide layer and overall a larger proportion of hydrated chromium segregated at the surface of the material. The untreated tinplate measurements are in line with those in the literature [9, 17] although this dataset provides improved spatial and elemental resolution.
Thus, while the changes in surface chemistry are not hugely significant there is evidence that exposure to 200 °C does begin the process of removal of the hydroxide surface groups. These changes in surface and near surface chemistry will likely have an impact on the wet adhesion of any lacquer system , and is also likely to impact the corrosion mechanics for processes such as cathodic delamination and filiform corrosion  as it would likely affect the rate of water update on the surface of the materials.
The result of the chemical changes observed is that the thermal pretreatment also leads to an increase in the surface energy of each substrate, Fig. 7. In each instance, the dispersive element dominates surface energy and there is an increase in the overall surface energy of around 5 dynes/cm as a result of the thermal treatment. This increase in the surface energy can be related to the change in the surface oxide species and is also a reflection of the removal of the DOS oil which evaporates during the thermal treatment. Both mechanisms would be beneficial to the lacquer adhesion.
The net result of the changes in surface chemistry is to effect a change in the lacquer adhesion, Fig. 8. In all instances, the highest adhesion is observed with the epoxy phenolic coating, whether in transparent (vitalure) or white anhydride pigmented form. On the ECCS and TCCT substrates, the adhesion for the BPANI polyester materials is only around 50% of that observed with current generation epoxy phenolic materials. The poorest adhesive performance is observed with the tinplate substrate while the next generation substrate improves the dry adhesive performance by around a further 20% compared to the current ECCS substrate. This relatively lower adhesion strength on the tinplate substrate is attributed to an alternative failure mechanism which was observed for the tinplate with failure occurring within the brittle tin oxide layer below surface chrome/chrome oxide layers. The exact mechanism by which a small increase in adhesion is observed with heating may be associated with the presence of the DOS oil during film formation and some increased surface oxidation which improves adhesion. There is no clear trend observed between the adhesion and the presence of the pigment with adhesion behaviour. Both epoxy materials are similar in performance and thus the impact of pigment on the adhesion can be considered minimal.
The impact of the heat treatment is to increase the adhesion by around 13% for the ECCS, 10% for the 311 tinplate (although in absolute magnitude, this is less) and 10% for the TCCT substrate. From a dry adhesion perspective, the 200 °C pretreatment therefore has a positive impact on the adhesion properties. From the surface analysis and macro adhesion measurement, it is proposed that the primary mechanism in the improvement in adhesion is associated with increase in the number of oxidic bonding sites on the surface.