The annual cost of steel corrosion is estimated to be over $400 billion in the United States and $2 trillion globally. Corrosion is a process where the metal can be degraded by electrochemical and/or chemical processes. This article will discuss the use of lead- and chrome-free corrosion inhibitive pigments in coatings where corrosion is primarily from electrochemical processes. Accordingly, the correct use of corrosion inhibitive pigments can be of enormous economic value.
Metals desire to be in their most thermodynamically stable state, which, in simplified terms, is the naturally occurring state of matter in its lowest energy state. Metals ordinarily exist naturally as oxides (e.g. iron oxide, aluminum oxide, zinc oxide, because oxides represent their lowest energy state. Corrosion is an electrochemical deterioration of a metal due to the reaction with its environment to transform the metal into its lowest energy state. Oxidation occurs at the anode (positive electrode) and reduction occurs at the cathode (negative electrode). Corrosion is normally accelerated by the presence of water, oxygen and salts (particularly of strong acids).
For example in the case of steel:
Corrosion inhibitive pigments can deter corrosion by multiple mechanisms. However, all of these mechanisms have the ability to disrupt the electrochemical corrosion reaction in common.
Cathodic inhibition inhibits corrosion by impeding the flow of electrons at the cathode, whereas anodic inhibition inhibits corrosion by impeding the flow of electrons at the anode. The following is the standard EMF series of metals with the more inert or cathodic metals toward the top and the more anodic or active metals toward the bottom:
Accordingly, in the above EMF series, Zn is more active than Fe. When a zinc rich primer is applied over steel, zinc will oxidize preferentially to steel and thus prevent the underlying steel from oxidizing. In this scenario, Zn is anodic (more readily oxidized) to steel and therefore protects steel from oxidation. Thus, steel is protected from corrosion by cathodic inhibition, as well as by the barrier that the zinc-rich primer provides. When choosing a corrosion inhibitive pigment, several factors must be considered.
Environmental factors that influence the rate of corrosion include moisture, pH of the moisture, wet and dry cycles, soluble salts, temperature and time.
For example, moisture, soluble salts, higher temperatures and longer exposure times all normally exacerbate corrosion in coated metal films. With these issues in mind, the evaluation criteria and test methods must be carefully contemplated before selecting corrosion inhibitive pigments.
The comparative corrosion resistance of coatings will vary dramatically depending on the test environment: Natural exposure and exposure conditions, salt spray (95% humidity/5% salt and always moist), acidic salt spray, prohesion cyclic corrosion (wet and dry cycle with 0.04% ammonium sulfate and 0.05% salt, electrochemical impedance spectroscopy, salt soak or other. Most experts agree that accelerated tests are not always a good indication of how the coated metal will perform in the real world.
Additional considerations are the metal type (e.g. steel, aluminum, galvanized), pretreatment and cleanliness of the surface. If the metal surface is not properly cleaned and prepared, the coating will lack adequate adhesion and premature failure will result.
Furthermore, the type of coating in which the pigments will be used affects the selection of appropriate corrosion inhibitive pigments. Considerations include whether the coating is solventborne, waterborne, powder, air dry or baked, and if the film will be cross-linked or thermoplastic. You should also consider the coating’s resin type and pigment volume concentration.
Corrosion inhibitive or passivating pigments promote the formation of a barrier layer over anodic areas, thus passivating the surface. To be effective, these pigments have a minimum solubility. If the solubility is too high, the pigment will leach out of the coating too rapidly- reducing the time that the pigment is available to inhibit corrosion. If the coating film is more open (e.g. air dry latex), water permeation is higher and thus the corrosion inhibitive pigment will be depleted more rapidly. To function properly, the coating must permit the diffusion of some water to dissolve the pigment. Accordingly, blister formation may result under humid conditions as the pigment dissolves. Higher Tg (glass transition temperature) and higher crosslink density binders are known to improve blister resistance.
Another prime consideration in the selection of a corrosion inhibitive pigment is the pH (EU). For example, a pigment with a high pH may have a deleterious effect on the cure of acid catalyzed systems. Conversely, a pigment with a low pH may adversely affect the stability of waterborne systems.
The vast majority of corrosion inhibitive pigments are comprised of the combination of metal ions (cations) derived from: zinc, strontium, chromium, lead, molybdenum, aluminum, calcium or barium and anions, such as those derived from phosphorous (orthophosphoric and polyphosphoric acids), chromic acid and boric acid. Although chromate and lead, containing passivating pigments, are very effective in inhibiting corrosion, their use is very limited due to a variety of environmental and toxicological regulations.
Suppliers of Corrosion Inhibitive Pigments Include:
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