The Pemcoat process: a new process that simplifies direct enamelling
Christian Schlegel - Pemco Brugge, Belgium
Koen Lips - Pemco Brugge, Belgium
Frits Lamote - Pemco Brugge, Belgium

Abstract
This new process, through the use of a new type of substrate and adapted enamels, enables for instance the enamelling of direct white without the preliminary pickling and nickeling. So, the process isn't only a simplification but it also has an ecological impact.
The application of this process only requires light investments, and can consequently easily substitute existing enamelling technique.
Another important thing: the new enamels show the same chemical and mechanical characteristics as the steel enamels currently used.
In this way, as for the explanation that follows, we will not only try to describe the different points previously mentioned but also the adherence mechanism, the results until today and the approach we are following in this project.

Introduction
The use of enamels in the field of coatings is on the one side dependent on its performances and on the other hand on its application; the latter has to be as simple as possible. For this reason, we have tried and succeeded in implementing the "simplified enamelling", which we, Pemco Brugge, call the "PEMCOAT technology" (patent PCT/EP 99/03491). When you say simplification, you say economy! Economy through a substantial reduction of the pre-treatment phases, especially for the white enamels and direct pastel colours.
Also economy because it is possible to do without adherence oxides, such as cobalt, nickel, copper, etc… In fact the introduced new technology recommends nothing of all this. In contrary, the technology only aims at the use of another substrate that is well-known in the automobile industry, namely the zinced or galvanised sheet steel.
However, in order to succeed in enamelling a galvanised sheet steel, a certain amount of preliminary precautions are required, such as the quality of the galvanised sheet, its pre-treatment, the enamels to be used or the firing. At the same time we may not forget the problems that welding or edge covering can bring about.

The galvanised sheet steel
There are numerous qualities, as the table borrowed from a steel plant (figure 1) shows; these qualities distinguish themselves through the quality of the steel substrate, the composition of the zinc film, the way of deposition of this film or its thickness; another parameter than can intervene is the thermal treatment, which the galvanised sheet steel has to undergo at the steel plant; in this case we do not talk about a "galvanised" quality but about a " galvannealed " quality, which in the technical lan-guage of the steel plants is called "annealing pro-cess".

The composition of the zinc film
The zinc film can be doped through certain elements, such as aluminium, copper, nickel, cobalt, iron, silicon or magnesium; according to our studies, these elements do not necessarily influence the fact if a galvanised sheet steel can be enamelled or not; it should also be noted that we have only made few investigations concerning the zinc films doped with lead or antimony (flower designs), since rejected from the beginning for ecological reasons.
The way of deposition of the zinc film This does not really influence the enamelling; in fact, if the zinc layer is deposited by means of "hot dipping " or "electro dipping" (electrolytic), we have hardly noticed any difference, besides the fact that we experienced more difficulty to develop adherence on an " electro dip " quality.

Thickness of the zinc layer
According to the demand, the galvanised sheet steel can be supplied with zinc thicknesses varying between 3 and 30 microns; it is nevertheless important that it has a similar thickness over the whole coil; it will indirectly intervene in the enamelling.
When the zinc layer is too thick, the latter will melt and will flow from its support in case of vertical firing.
When the zinc layer is too thin, there is the risk of an insufficient adherence, even no adherence (a too intense iron migration).

The steel substrate
The latter does not intervene in the actual enamelling phase, but we give priority to enamelling steels that present good mechanical characteristics (good deformation) and a low carbon content.

Preliminary trials
These trials were useful to define the enamelling limits of the galvanised sheet; in fact a galvanised steel sheet can only be enamelled when it has undergone a preliminary thermal treatment to transform its zinc layer into an inter-metal Fe-Zn phase (figure 2); this does not at all mean that a "galvanised" quality, not thermally pre-treated, cannot be enamelled: it requires to be fired in a complete flat position, since when one approaches the fusion temperature of zinc or of the created inter-metal Fe-Zn, a vertical firing (as already previously mentioned) will cause the separation of the zinc film or the inter-metal Fe-Zn from its metal support, by dragging along the enamel layer. Therefore we can say that:

only the "galvannealed" quality can be enamelled

The substrate itself has also its importance; in fact it has to be of a quality that can be enamelled, since contrary to the zinc layer, the inter-metal Fe-Zn layer obtained after thermal pre-treatment, is no longer a barrier against the hydrogen and consequently we risk the appearance of fish scales, particularly in case of double sided enamelling. We can also say that:

only an enamellable steel substrate quality is suitable

Thanks to its very good deformation and especially because the majority of the steel manufacturers have it at their disposal, we prefer an IF quality (Ti or Ti/Nb).

The thermal pre-treatment or "annealing process"
This pre-treatment, which is the key of the enamellability of the galvanised sheet steel, has in fact a very clear purpose, namely:

the transformation of the zinc layer into an inter-metal Fe-Zn phase

figure 2 - Binary diagram Fe Zn

figure 3 - Thermal pre-treatment = annealing process

The diagram of figure 3 shows the progressive transformation of the zinc layer into an inter-metal Fe-Zn layer in function of the temperature; in this diagram you can notice that only the inter-metal Fe-Zn of a Fe₄Zn₉ configuration satisfies by its stability.
The diagram has been elaborated in the laboratory and can therefore display differences between the transformation phases of the zinc layer and the temperatures, depending on the fact if the pre-treatment has been carried out at the steel plant or at a customer who has industrial means of firing at its disposal; nevertheless we have no further doubt about the fact that we only register adherence in the presence of an inter-metal Fe-Zn layer (Fe₄Zn₉ ), which is characterized by a strong roughness, even a certain porosity (figure 4).

figure 4 - Bonding mechanism

In this way, the thermal pre-treatment proposes 2 options:

The adherence mechanism
As in every enamelling system, the adherence remains one of the essential conditions to be guaranteed. This adherence, as we have seen in the previous chapter, is closely connected to the inter-metal Fe-Zn phase formation and particularly to the intended Fe₄Zn₉ formation.
What does this inter-metal Fe-Zn present in particular? As seen under the optical microscope, this inter-metal Fe-Zn phase, which is the result of the iron migration from the substrate into the zinc layer during the thermal pre-treatment, is extremely rough in comparison to the initial zinc layer. (figure 5 - photoes 1 and 2).

figure 5

In the same way, the photoes 3 and 4 of figure 6 show an enamelled surface; at the interface we notice the formation of a network of shrinkage cavities connected to each other by micro channels; these shrinkage cavities and channels are not empty; in fact the analyses through an electronic microscope (figure 7) have clearly shown the presence of "soft glasses" and more particularly zinc borates even zinc iron silicates in the cavities; further to our registered results for those various optical and electronical microscopic examinations, today we rather prefer a physico-chemical than a chemical adherence, a bit as the one noted for the cast iron.

figure 6

Photo 5 : Image BSE of the interface (Electronic microscope - 3000 X)
Spectrum of the zone indicated on photo 5
EDAX ZAF Quantification (Pure Elements)
Element Normalized
Element	Wt %	At %	Z	A	F
O K	27.97	55.83	1.1167	0.4394	1.0012
Al K	0.50	0.59	1.0385	0.4820	1.0023
Si K	13.08	14.88	1.0733	0.6029	1.0001
Ba L	0.60	0.14	0.7645	1.0249	1.0169
Ti K	0.32	0.21	0.9422	0.9727	1.0147
Mo K	0.61	0.36	0.9209	0.9930	1.0536
Fe K	2.04	1.16	0.9375	0.9963	1.0061
Zo K	54.88	26.81	0.9028	1.0008	1.0000
Total	100.00	100.00
figure 7

The enamels
All the previously mentioned preliminary trials that have allowed us to define the enamelling on galvanised sheet steel were made with a white enamel opacified with titane; these trials have been repeated with transparent, semi opaque, mat systems, so without adherence oxides; the obtained results are similar.
Consequently, the enamels for the enamelling of zinced sheet steel are in no way dependent on metallic oxides, such as cobalt, nickel or other oxides to guarantee a good adherence; these oxides, if they are present, have a colouring function.
On the other hand, other series of trials have shown that those new enamel families present a better suitability for pyrolysis, thermal shock, sometimes dielectric; most of the time they also present more restricted but especially more uniform bubble structures; moreover at a purely application level, they have no difficulty to be reworked in case of defects.

The enamelling and firing techniques
The new enamelling technologies generally disturb the enamelling plants: they require new investments, often also a new know-how, a more qualified personnel, etc. In our case, nothing of all this, because the enamelling of galvanised sheet steel can with some adaptations be done with all the existing application techniques.

figure 8 - Comparison between the different chemical pre-treatment stages through spraying required for the direct white enamellings (DWE), 2 coat/1 fire and Pemcoat

The advantages
The enamelling of the zinced sheet steel is economically very promising; it presents a multitude of advantages, such as:

(thinner enamelling = less fragile in case of mechanical shocks). The exact evaluation of these advantages is difficult; it will be different for every customer. By basing ourselves on the productive diagrams in figure 9, we have tried to evaluate the costs (figure 10) as good as possible and this b taking these costs 100 % into account for the enamelling of "Direct white" (DWE); this clearly proves that the PEMCOAT, thanks its proposed simplifications is a promising technology.

figure 9 - Productive parameters served as basis for the cost evaluation

The disadvantages
As every technology, the galvanised sheet steel enamelling technology also
presents its disadvantages and in particular:

It should among other things permit the fire temperature reduction as we have hardly any difficulty to develop an adherence at 750 °C; in this case all the problems, connected with piece deformation are minimised and this to the point where it will be possible to think about finer sheet steel thicknesses (e.g. 0,6 mm).
This remains our goal, but first of all we have to succeed in industrialising this new technology, which we hope will not take as much time as the direct white enamelling, which has taken several years before being really operational.

figure 10 - Cost evaluation and comparison