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The Interface Zinc-Coated Steel-Paint J. F. H. van Eijnsbergen Dutch Galvanizing Institute, The Hague, Holland
The increasing uses of both air-drying and stoving paints and lacquers on continuously and job-galvanized steel, as well as on steel surfaces electroplated with zinc, have created a number of paint adhesion problems, both as initial failures and after weathering or immersion. Zinc surfaces are even more reactive in moist atmospheres than steel surfaces, and extreme care must be given to surface conditions and surface preparations. The majority of cases of paint adhesion failures are due to an insufficient knowledge of this reactivity. Galvanized steel surfaces can be divided in two different groups: (i) continuously galvanized steel strip or sheet and (ii) general- or job-galvanized steel articles. Each of these immersion processes yields a rather different surface. The size and distribution of the zinc spangles (also called zinc flowers) are irregular and variable on job-galvanized surfaces because of local differences in cooling time, zinc flow, galvanizing conditions, steel composition a t the surface, and the presence of very small amounts of tin or antimony. On continuously galvanized steel strip the zinc crystals are uniform in size and distribution. Their size may be reduced by air-jet cooling immediately after galvanizing. Often the surface is smoothed mechanically by skin-passing the galvanized strip between roughened rolls, thus obliterating surface crystals and producing a matte amorphous surface. Because zinc is very reactive, any zinc surface in contact with the surrounding air will oxidize immediately. A t normal galvanizing temperatures (440-470 “C) a film of zinc oxide grows upon the surface a t a rate of 1-2 Alh, whereas a t room temperature the oxidation speed is approximately 1AI100 h. Over 200 8, this oxide film becomes visible to the naked eye. A molecular layer of zinc oxide forms within 1 s a t room temperature and thus a relatively dense oxide film is present after the galvanized part has cooled to room temperature. Since ZnO is water insoluble and the very thin oxide film adheres well to the galvanized steel surface, its presence is not detrimental to paint adhesion, as will be discussed in the next paragraph. This first or oxidation period is immediately followed by the second or salt-formation period. During this period two processes may occur, depending on the environmental conditions, especially the microclimate. When the zinc (oxide) surface has access to free moving air, a chain of reactions will gradually develop, finally resulting in the formation of the insoluble zinc patina, basically written as 2ZnC0&3Zn(OH)2. Oxygen, water (vapor), and carbon dioxide, all present in the atmosphere, are the main reactants in this process. The zinc patina protects zinc from a too-rapid corrosion. Its thin well-adhering film has a whitish-grey color, which after 6-12 months obliterates the zinc crystals. However, when the zinc (oxide) surface has almost no contact with moving air with varying moisture content, such as in densely packed galvanized sheets, angle bars, pipe bundles, etc., zinc corrosion products (sometimes, but wrongly, called “white rust”) are white, fluffy, and much more voluminous than the zinc patina. Under such circumstances various modifications of Zn(OH), will be formed. Since this hydroxide remains rather stable under these conditions, zinc ions constantly leave the metal lattice to be bound by water, thus causing accelerated corrosion. The speed of this reaction is relatively high and-especially on continuously galvanized sheet or strip-may result in premature rust formation and even perforation of thin strip or sheets. 0019-7890/78/1217-0183$0.100/0
In the presence of atmospheric contaminants such as SO2 and NaC1, secondary reactions take place, leading toward the formation of sulfates, basic sulfates, chlorides, and oxychlorides. I t is emphasized that the presence of such products has to be determined by chemical analysis or by Rontgen fluorescence or Auger spectrometric analyses. Colors differ little and various crystalline forms are difficult to determine (except the needle-like zinc sulfate crystals). No other quick and practical method is available a t present for a rapid determination of these zinc reaction products. Apart from the materials formed upon weathering, there are other surface contaminants to be considered, viz. those products left on the zinc surface after the galvanizing procedure. On general-galvanized steel parts traces of zinc ashes (zinc oxide) and fluxes (mostly ammonium chloride and zinc chloride, occasionally also potassium chloride or flourides) may be present. I t should be noted that the formation of white corrosion products-all other parameters being equal-is more pronounced on continuously galvanized surfaces. Engelbrecht has shown that the formation of detrimetal white corrosion products reaches a maximum with 0.20-0.22% A1 in the zinc coating, whereas a t 0.05% A1 chemical reactivity was a t a minimum. Thus continuously galvanized strip is more reactive in the atmosphere than general- or job-galvanized steel surfaces. Apart from the chemical composition of zinc surfaces exposed to the atmosphere, the crystalline form of the galvanized sheet or strip has to be taken into consideration. In a recent ILZRO/NCCA research project, it is stated that minimized spangle strip generally yields better paint adhesion, in comparison to a bright regular spangle and-even more-temper rolled material having no spangle a t all. This has also been confirmed by other investigations. Also, orientation of zinc crystals can influence paint adhesion. The best paint adhesion was found on zinc coatings with the majority of grains oriented within their basal planes parallel to the surface. Such a correlation does not exist between the paint adhesion and zinc grain orientation after temper rolling. In previous ILZRO work-using Auger spectroscopy in analyzing the outer surface layers of 5-10 A-it has been found that adsorbed carbonaceous material on the galvanized sheet surfaces unfavorably influences paint adhesion, whereas oxygen in the surface increased paint adhesion. Aluminum seems to exert a favorable influence on paint adhesion. I t should be noted that aluminum in zinc baths tends to concentrate in the upper layers of the zinc coating (up to 40 A). It was also shown that higher sulfur concentrations in the zinc surface are detrimental to paint adhesion. On several silicon-killed or semi-killed steels the upper layer of the coating system, obtained after general galvanizing, mainly consists of the {-alloy layer. On galvannealed steel strip the whole coating consists of a 6 1-layer only. It has been stated by several authors that on such alloy layers adhesion of paint coatings is much better than on the pure zinc (oxide) layer, all other factors being equal. On electroplated zinc strip surfaces, almost the same conditions exist. However, electroplated steel strip is manufactured with a phosphate plus chromate aftertreatment, thus avoiding adhesion failures in many cases. When a paint is applied to galvanized steel, the large molecules of its polymer binder will, in the first stage, migrate to Q 1978 American Chemical Society
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the interface through macro-Brownian motion. Polar groups or groups which can form a hydrogen bond through microBrownian motion will be attracted by similar groups of the zinc surface. However, after the evaporation of solvents, this movement is slowed down and only on local molecular spots will contacts be made. In the second stage, an adsorption equilibrium is established, provided no paint ingredients, not participating in the cross-linking process, will accumulate a t this interface. At distances between 2 and 5 8, from the interface the van der Waals, or secondary atomic attraction forces, will act. These forces consist of dispersion, induction, and orientation forces, of which the dispersion (London) forces are the most important. It should be noted that the dispersion forces generally are an order of 100 weaker than hydrogen bonds (e.g., "3, H20, HF) or very polar groups (e.g., HCN, NOa), and that hydrogen bonds are approximately 10 times weaker than the primary valence bonds. Induction forces are always very small in comparison to the dispersion forces and the orientation (dipole/dipole) forces. When water diffuses toward the interface, hydrogen bonds may be destroyed and osmotic pressure will aid in reducing or even destroying adhesion on these areas. On polar substrates such as ZnL, reactions between ZnL or Zn and a group of a polymer chain, a pigment, or another ingredient in the coating may raise the total adhesive force 5-10 times over the total van der Waals forces, even when only one primary chemical bond on 100 secondary bonds is formed. Such bonds may be obtained by the judicious use of small amounts of coupling agents, containing hydrolyzable groups,
e.g., nitrogen-containing silane compounds, substituted methylsiloxanes, or complex organic titanium compounds. Not only the paint vehicle but also the pigments can influence adhesion considerably. Apart from calcium orthoplumbate, basic zinc chromate, zinc phosphate, and zinc phosphite will also influence paint adhesion on zinc surfaces, and especially the PVC of the paint film often is limited within a narrow margin. The solvent balance also exerts an influence, although less than the other paint ingredients, on adhesion. Solvents such as cyclohexanone, ethylene-glycol ether acetate, and diacetone alcohol can improve adhesion. It is also possible to formulate paints in which zinc surface conversion agents have been incorporated, such as is the case with wash-primers. Rhombic Hopeit, Zn3(P0&4H20, formed upon phosphating zinc surfaces, offers an excellent paint base. Also 0.1-0.5 pm thick adsorbed chromate layers, free from silicates, can be useful in upgrading paint adhesion. Increased knowledge and understanding of adsorption and reaction phenomena at the zinc-paint interface will bring forth improved formulations of paints and lacquers. Some practical examples of the use of various observations, mentioned above, in formulating such paint products will be presented at the conference.
Presented as part of the Symposium on Interfacial Phenomena in Corrosion Protection at the Division of Organic Coatings and Plastics Chemistry, 173rd National Meeting of the American Chemical Society, New Orleans, La., March 1977.
