After many decades investigating hundreds of paint, coating and lining failures in all sorts of industries and in many parts of the world, I have found that a very common and reoccurring problem is the dramatic failure of galvanizing plus overcoat systems. My wording here is carefully chosen: it is not a failure of the organic paint system or the galvanizing as individual items—it is the failure of the complete system!
Fig. 1: A typical dramatic failure of a galvanizing plus topcoat (duplex) coating system after four years of exposure. PHOTOS: COURTESY OF THE AUTHOR
For decades, the protective coating, painting and architectural industries have believed the promotion of what has been labeled as “duplex coating systems” (i.e., hot dip galvanizing plus an organic topcoat system). It has been widely advocated that a life expectation greater than the sum of the potential durability of galvanizing plus the life of a coating system would result if these two were combined.
My research and many investigations have indicated that this synergistic life expectation is rarely delivered, and in fact, I can show that a dramatic and sometimes catastrophic reduction in life or failure will quite likely occur if this duplex system—as it is commonly employed – is used.
What can and often does happen is that the complete coating system breaks down and it will sometimes do this in a few short years (i.e., well less than the life of just one of the coating system components, let alone anything like the sum of the individual life expectancies or any synergistic potential).
The process by which this breakdown happens is really quite simple, but it takes a bit of explaining, particularly if the chemical reactions and physical descriptions are provided in full. This article will briefly explain the basics of the process and provide suggestions on how this type of failure can be avoided.
Fig. 2: In some instances, the coating system will spectacularly self-detach from over the galvanizing.
It is a regular situation that an architect, engineer or designer wants a corrosion protection system that also has some reasonable aesthetics. A common approach is to specify hot dip galvanizing, due to its perceived completeness of cover, corrosion protection and expectation of durability. However, galvanizing is grey and dull, and it changes its appearance from a bright silvery look all too quickly and it sometimes does this in a non-uniform way depending on its exposure. The solution often proposed is to apply a thin-film overcoat system over the galvanizing finishing with a polyurethane or similar topcoat. Theoretically, this should allow for a wide choice of color, great UV stability, color fastness and uniformity of appearance.
Figs. 3 and 4: A common finding is that the coating system is drummy with very poor adhesion, and underneath the film is a prodigious amount of white zinc corrosion products. Visible rusting of the underlying steel substrate (or the zinc/iron ally layers) also quickly appears with its typical red/brown corrosion products.
In my experience, the basic misconception made by many architects, designers and coating supplier representatives is their belief that the galvanizing layer is providing the corrosion protection to the carbon steel, and that all it needs is an aesthetic topcoat system to change the color or appearance. This is on the assumption that the steel is being protected with the hot dipped galvanizing layer that is (say) 80 to 120 microns thick, plus there is a further 70 to 80 or so microns of an epoxy tiecoat and a polyurethane finish coat, for a total build of (let’s say) 150 to 200 microns. By most people’s measure, that film build is commonly perceived to be enough to adequately protect carbon steel in a moderate or harsh atmospheric exposure.
Simply put, the error in this assumption is that the carbon steel is seen as being the reactive substrate, i.e., the material that is to be protected from corrosion. In fact, with a galvanizing plus topcoat system, the reactive substrate is the top surface of the galvanizing.
Ironically, if coating system designers were asked whether an uninhibited epoxy primer and a light coat of polyurethane (or a similar topcoat) to a combined DFT of around 70 to 80 microns would provide long term protection to carbon steel in a moderate to harsh environment, many would say it this is too thin and would soon fail. My question is that if this DFT of a low-order coating system is inadequate to protect carbon steel, what makes it sufficient to protect the galvanizing?
Metallic zinc, particularly in the form presented by a hot dip galvanized layer (as opposed to a powdered zinc in a zinc-rich coating), can be quite reactive. In fact, it has a higher electrochemical potential than carbon steel or iron, which is how and why it can sacrifice itself to protect steel galvanically to which it is electrically coupled. This is based on what is called the Standard Electrode Potentials (or the half-cell potential) of the two metals, zinc (Zn) and iron (Fe).
The oxidation half-cell reaction potential for zinc (relative to the standard hydrogen potential) is:
Zn → Zn2+ + 2e– = -0.76V
The oxidation half-cell reaction potential for iron is:
Fe → Fe2+ + 2e– = -0.44V
(The negative figure to the voltages in both cases is purely convention because of the electron’s negative charge.)
Thus, the potential for the zinc to oxidize (release electrons) is numerically greater than for the iron by the quantum of 0.32V, so when these metals are electrically connected together into a galvanic couple (in the presence of an electrolyte) the zinc will have a tendency or a potential to convert from its atomic form to its ionic form—by releasing electrons—greater than iron can do the same. The zinc then becomes an anode and the iron (or steel) is the cathode and is therefore protected galvanically.
However, zinc also readily reacts with oxygen, water, carbon dioxide and chloride to form a number of reaction products such as zinc oxide, zinc hydroxide, zinc carbonate, zinc chloride, zinc oxy-chloride, and others. Collectively, these are all called zinc corrosion products and they are mostly colored either white or very light grey.
These zinc corrosion products have different levels of solubility in water. Zinc carbonate, for example, is almost insoluble; and some of the others are slightly higher in solubility. Zinc oxide is highly adherent and very protective of zinc in many environments.
To a degree, the insoluble or low-solubility zinc corrosion products can be quite strongly self-protective of an untopcoated galvanizing layer, i.e., they can shield over the zinc’s outer surface and slow down the rate of oxidation (loss of electrons) of the zinc from its atomic form to zinc ions. This is partly why galvanizing can last so long on its own without suffering from a phenomenal rate of metal loss, providing the environment is not too severe. This explains the dulling effect of weathered galvanizing in benign environments (the white zinc carbonates and zinc oxides change the color of the zinc from silver to grey); and the high build of a “coral-like” layer on galvanizing in a marine exposure.
Two things can interfere with the ability of the low-solubility zinc corrosion products to slow down the rate at which the oxidation reaction happens with metallic zinc. One of these is active and regular physical removal of the corrosion products, e.g., by wind, moving materials or flowing water; and the second is the time-of-wetness of the galvanizing. The latter item is quite critical.
We have already established that zinc is reactive in the presence of water. This is because water carries dissolved oxygen, which is the prime corrodent of metallic zinc atoms, and it also will dissolve some of the corrosion products of zinc, which will remove them from the zone of the reactions. If zinc remains almost continually wet (i.e., it has a long time-of-wetness) the rate of consumption of the atomic (metallic) zinc to ionic zinc can be prodigious.
I will now relate the above principles of zinc corrosion and its reactions with other environmental materials, to a galvanizing plus a thin overcoat combination. I will use an epoxy primer and a polyurethane topcoat material just as examples because they are very commonly used in this service.
Most polyurethane coatings are not particularly compatible with zinc materials, as the ester linkage in the polyester polyol can be attacked by the alkali zinc corrosion reaction products. This normally precludes putting a polyurethane finish coat directly over galvanizing—for the same reasons that alkyds (oil-based enamels) are incompatible with zinc-based materials. For this reason, epoxy primers are very often specified as a tie coat or primer between the galvanizing and the polyurethane. This brings the high levels of adhesion to a substrate typified by epoxy primers and their excellent resistance to alkali corrosion products to the situation, as well as the presence of large numbers of hydroxyls, which polyurethanes like to bond with.
In theory, this sounds like an excellent system to overcoat galvanizing. However, as usual, the devil is in the details!
In an attempt to keep costs reasonable (considering the general expense of the galvanizing) most specifiers try and keep the film builds of the epoxy primer and the polyurethane down to a practical minimum. Film builds of 40 or so microns of epoxy are commonly specified, followed by about the same of polyurethane, for a total DFT of maybe 80 microns. It is not uncommon to find even less than this has been applied if the coating layers have been thinned to aid flow at low film builds.
There are two things that drive this low film build initiative: the first is cost, as the colored topcoat system is seen as being additional to the expense of the galvanizing and should be practically minimized, and the second is the (incorrect) assumption that the topcoat system is just providing the aesthetics.
At a combined DFT of about 70 to 80 microns, an epoxy and polyurethane coating film is quite porous to water, oxygen and carbon dioxide. If the duplex-coated steel is in a situation where the relative humidity (RH) is frequently quite high, i.e., above about 70%, or the outer surface of the paint film is wet or damp for a reasonable length of time on a regular basis; moisture, oxygen and carbon dioxide will permeate through the film until it reaches the galvanized substrate. Here, the typical reactions between these materials and zinc will occur irrespective of the presence of the coating system. This will form some of the zinc corrosion products mentioned earlier on the outer surface of the galvanizing. These are typically white, fluffy and voluminous materials with a large collective surface area.
As these zinc corrosion products take up more room or volume than the metallic zinc, they will slowly build up beneath the paint film in the accumulation zones caused by the oxidizing zinc atoms. This will start to dislodge the coating film from the substrate by lateral adhesion loss, or undercut. This then tends to stress the film which can cause microcracks in the epoxy (in particular) which can allow for an increased rate of permeation though the paint film. This brings more moisture, oxygen, etc., to the corrosion zone which keeps the reactions fueled.
There is another significant occurrence and this relates to the more soluble zinc corrosion products such as zinc chloride and zinc hydroxide that may form beneath the coating film. As the first water reaches these materials and dissolves them, a small volume of a high concentration solution will exist on one side of a permeable membrane. This contrasts with a very low concentration on the other side, i.e., on the outer face of the coating layer.
This can set up an osmotic situation where moisture is drawn through the film by an osmotic cell. Note that this inducement or attraction of water through the film to the soluble materials at the galvanized substrate by osmosis is different from the initial stage when it was simple permeation with little or no driving force.
We then have a lot of water, oxygen, carbon dioxide and a bunch of zinc corrosion products all crowding beneath the paint film. This has another damaging consequence: this further increases the time-of wetness of the galvanizing which is already under attack, because the water and the zinc corrosion products keep the surface in a condition of near permanent wetness, irrespective of the weather or climatic conditions on the outer surface of the polyurethane.
The above process can gallop away with the consequence of each reaction seeming to make the situation somehow worse. In spite of the volume of corrosion material that builds up underneath the film, it is not uncommon to find that the coating layer still stays present, even if it is not intact or bonded. I have seen this sequence of reactions and physical conditions completely consume a full layer of galvanizing in two or three years, where adjacent untopcoated galvanizing is excellent and has only lost a few microns of film build.
The foregoing describes how a coating system that is designed to complement the corrosion resistance properties and lengthen the life of a galvanized surface can actually be fatal to the entire system. This is partly the reason why so many protective coating suppliers will not guarantee their coating products over galvanizing in the same manner that they will over good zinc-rich coatings such as zinc silicate or epoxy zinc.
It is vital to recognize that not all galvanizing is the same. Over the last few years, there has been a number of architectural and structural products that are made from continuous (strip) galvanized stock. Continuous galvanized steel is not the same as batch galvanized (hot dip galvanized) material. Not only is the form of the zinc and its means of adherence or bonding to the steel substrate different, but it is much reduced in zinc film thickness.
These materials look similar to hot dip galvanized items but because the zinc builds typically range from about 15 to 30 or so microns, they will lose this zinc thickness much quicker in most exposures. These products need the same type and at least the same film builds of coatings as described above, but the surface preparation needs to be modified in some situations as damage right down to the substrate can easily occur by blasting, sanding or grinding.
Figs. 5 and 6: The rate of consumption of the galvanizing can be incredible to the point that the only remedy is to fully abrasive blast the steelwork and re-apply a new coating system or regalvanize the structure (above). However, in other cases remnants of the galvanising layer will remain, slowly packing up beneath the coating film (below).
There are ways to successfully overcoat galvanizing so a good life expectation can be delivered, and in my long experience, these are the rules that must be followed.
Fig. 7: Often, it is the horizontal surfaces that are more affected, probably because the time-of-wetness of these is higher than on vertical members. Note the loss of topcoats on the top face of the upper and lower RHS rails, versus the uprights.
Zinc-rich coatings such as epoxy zinc and zinc silicates have quite a number of advantages where topcoating is to be performed, as these do not suffer most of the problems described above. Always seek professional and reliable advice when preparing coating specifications, especially for overcoating galvanizing and similar substrates.
Do not allow subcontractors and suppliers to amend or adjust any surface preparation or coating system specified for over galvanized substrates, however well-meaning their intentions may be. It is the usual practices that have been followed for years that are as wrong now as they were then.
Mark Dromgool is the managing director of KTA-Tator Australia Pty Ltd, based in Melbourne, Australia. He has been active in the protective coatings industry for more than 45 years. Dromgool’s experience includes 10 years as a coating application contractor and about seven working for two of the largest protective coating suppliers in Australia and New Zealand. In 1994, he formed KTA-Tator Australia as a protective coating engineering, inspection and consulting company.
A long-standing member of SSPC and NACE, Dromgool is former president of the Blast Cleaning and Coating Association (BCCA) of NSW. He has written and published many papers on coatings and linings and has lectured widely at local and international conferences.
Dromgool has qualifications as a mechanical engineer; is an ACA-certified Coatings Inspector; a NACE-accredited Protective Coating Specialist; an SSPC-accredited Protective Coatings Specialist and a NACE-certified Coating Inspector – Level 3.
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