The Missouri S&T Coatings Institute and its staff want to wish you a Happy Holiday Season. 2004 was an excellent year for the Coatings Industry and the Missouri S&T Coatings Institute. Prices are rising and VOC is dropping. The ICE show will now be every other year so regional events will be where we will meet this coming year. We would like to thank the Coatings Industry for its support of our program and its students through donations, project funding and scholarships for our undergraduates as well as for short course participants. It is through your generosity that our program has been able to thrive for the past 48 years. The Missouri S&T Coatings Institute does not receive any state funds and must rely on donations and short course income to keep our facilities at the state-of-the-art level. We hope you enjoy the new news letter and the information they provide. We want to keep educating tomorrow’s formulators, researchers and developers in coatings and polymer sciences with state-of-the-art equipment and information. We look forward to the coming year and working with you. Have a very successful and happy new year.

Best wishes,

Technical Insights on Coatings Science

Corrosion 102: Surface Coatings
By Thomas Schuman

In Corrosion 101: The Oxide Layer, we discussed the potential energy of corrosion and how the oxide layer reduces the rate of what would otherwise be a very fast reaction. The spontaneity of corrosion is evidenced by a number of observations: First, the natural ore of most metals is the metal in an oxidized state. As mined, extensive amounts of energy must be added to convert them from the oxide ore to the useful metallic form. Thus, the value of recycling is not just in the conservation of materials but the reduction of energy input to process the used metal versus that needed for reduction of the natural oxide ore. The electrochemical energy input is defined as the number of electrons to cause reduction of the metal, as a reduction current, times the electrochemical potential required to push the reduction reaction.

The oxide layer on a metal substrate, often referred to as a passive layer, is generally protective of the underlying metal. Physical and/or chemical dissolution can damage the oxide layer, e.g., by abrasion or strong acids/bases, respectively, increasing air or water exposure to the bare metal to allow oxidation or corrosion. Oxidizing agents are responsible for a cathodic reaction to help power the oxidation of the metal surface. Corrosive agents, on the other hand, aid dissolution of the oxide layer and/or the metal to enhance corrosion rate. For instance, chloride forms soluble salts upon combining with metal ions or most metal oxides. When the now-soluble oxide layer is dissolved, the bare metal hiding beneath is exposed to enhance its contact with water and oxygen. Enhanced contact increases the rate of electron transfer from the metal atoms to the water or oxygen, i.e., oxidizing agents, and thus the corrosion rate. Thus, the goals of coatings design for corrosion resistance are to provide additional protection beyond that provided by the oxide layer, to physically protect the oxide layer, to chemically stabilize the oxide coating interface or the nearly bare metal itself against chemical attack, and/or to mitigate the contact of water and oxygen or other corrosive agents with the nearly bare metal.

We can now summarize different types of coatings and their effect on corrosion protection. Some coatings systems, much like the oxide layer, offer passive protection as a hindering obstacle to corrosion reactions, while other coatings impart active protection, responding against corrosion. Most coatings are simply passive barriers to corrosion.

One way to enhance the protective nature of the natural oxide layer is to make the oxide layer thicker, which increases the amount of “passive layer¿? that must be damaged before allowing oxidative damage of the underlying metal. Anodizing is the electrochemically induced growth of an oxide layer. Anodizing is typically performed in concentrated sulfuric or phosphoric acid, which are incorporated to some extent into the anodize coating. Sulfuric or phosphoric acids are commonly used because their continued presence does not diminish coating performance.

Anodizing forces a fast, corrosive buildup of a stable oxide. Contrary to some beliefs, the oxide layer is not deposited onto the existing metal but into the metal, that is, converting metal into oxide from the solution interface inward. For aluminum, a two-layer structure results. Anodize coating growth results in ~0.25µm diameter, hexagonally shaped cells surrounding a 250Å central pore. Just beneath this outer anodize coating is a thin metal oxide layer called the barrier layer. The thickness of the inner barrier layer is proportional to electrical potential, i.e., voltage, used to cause anodizing, whereas the overall anodized coating thickness, equal to the barrier layer plus porous layer, is controlled by treatment time and electrical current density of the applied voltage.

The porous, outer coating grows in thickness at the expense of the metal surface and the pore represents transport of corrosion products away from the anodic dissolution of the metal during the anodize process. The underlying or barrier layer, between the metal and anodize layers, is a rearranged aluminum oxide film, i.e., of different structure compared to the natural oxide. The process can be conducted under either constant current or constant voltage control.

The as-is product of anodizing is a thick, but porous, metal oxide coating. The pores can then be filled through a process called sealing to provide additional protection. Common sealing treatments include coloration with dyes, immersion in boiling water or steam, borate, nickel salts, phosphate or other anion salts, or may include inhibitors such as cobalt, molybdate, lithium, rare earth elements, or chromate. Sealing agents can add active corrosion response to the otherwise passive layer. Dyes adsorb and complex into the oxide pores through a charge-induced adsorption as affected by ionization of the amphoteric metal oxide surface at extremes of pH. A pH greater than the isoelectric point produces negative surface charge while pH less than the isoelectric point produces surface protonation and a positive surface charge. Due to the reactivity of the sealing bath, oxide layer dissolution as well as filling of the pores occurs during sealing.

Another coating design that can improve the quality of metal protection compared to the original oxide layer is the formation of a conversion coating. Conversion coating converts the natural oxide coating to that of a different metal oxide composition. The new oxide, as a surface coating layer, should form a consistent, dense film and be more chemically resistant to dissolution by corrosive species, at extremes of pH, and at service temperatures. For instance, chromating has been the principle aluminum pretreatment since the 1940’s due to its stable chromic oxide and capability to migrate to damaged regions for self-healing under the influence of pH. However, chromium, and in particular the soluble chromium (VI) ions, is a reactive, toxic, carcinogenic transition metal and subject to environmentally-minded disposal restriction as a hazardous solid, solution waste material, or an airborne contaminant. Other conversion coating systems commonly used on aluminum surfaces include sealing of an anodized film, phosphate, borate, lead-tin, cobalt, titanium, zirconium, manganate, silicate, and rare earth based conversion systems. Coatings over steel can also include zinc. Conversion coatings may also be sacrificial, to corrode preferentially compared to the metal underneath. Sacrificial consumption of the protective layer may impart a limited lifetime. These sacrificial coatings include galvanization and zinc rich primers as well as some aluminum pigmented systems.

Phosphating is the conversion of the metal oxide to a poorly soluble, mechanically strong, stable metal phosphate salt coating or passive layer. As such, the phosphate layer acts as the outer protective layer but is also commonly used to improve adhesive strength of organic top coating to the metal surface. Common phosphating types include chromium, nickel, iron, aluminum, and zinc depending on the type of metal to be protected. Each chemical process has its own environmental challenges in application, disposal, or regeneration to prevent or minimize pollution.

Since conversion coatings are often thin and hence minimally protective under normal environmental exposure conditions, and even less protective under extreme conditions such as aqueous salt, the conversion coating is typically supplemented with an organic top coating. Organic coatings employed include epoxy, urethane, acrylic, alkyd/ester, melamine, amide, silicone, halogenated, phenolic, cellulose, or a combination of one or more of these polymer types as applied from solvent, water, 100% solids, or powder technologies. Since there are many styles of coatings from which to choose, one should select the type(s) that fits a designed need regarding physical performance, such as tensile strength, flexibility, surface wetting, adhesion, chemical and light resistance, and the prevention of transport of water and/or oxygen to the surface.

Selection of a polymer system should be made while keeping in mind the earlier lesson on oxide stability, controlling variables such as pH, temperature, and corrosive agents. Epoxies provide superior barrier properties towards oxygen and corrosive ions compared to other coating types. They offer formulation flexibility and ambient temperature cure as thermosetting systems. The surface oxide layer can be further stabilized from dissolution by matching the isoelectric point of the coating system to the oxide on the metal surface and by protection of the oxide from corrosive agents. An excellent example is the topcoating of a galvanized substrate. If the coating possesses an isoelectric point pH similar to zinc oxide, ~10.5, the interface is stable and long life is observed. If the coating is significantly more acidic or basic than zinc oxide, the coating proceeds to corrode the zinc at the interface between the zinc and the coating. Ultimately, coating delamination from the zinc surface occurs. Similar processes occur over other metal surfaces but at a slower rate compared to zinc.

Pigments, in addition to bestowing the cosmetic or optical features of pigmentation, can adjust the local pH through their isoelectric point and/or can act as a barrier to transport of oxygen, water or ions. Pigments can also add an active corrosion inhibiting response. They can supply reagents that heal damaged regions, for example, supplying chromate ions to a damage site in order to regenerate the lost chromate coating. These activities remain possible even with epoxies since no organic coating is a perfect barrier, permitting transport of ions to some extent even in the absence of a scratch.

Good coatings properties support the protection mission, to prevent metal contact with water, oxygen, or corrosive species. A durable surface prevents damage to an underlying oxide layer. Adhesion and surface wetting avoid the formation of a porous interface between the coating and metal, which precludes lateral transport of corrosive species. Minimizing porosity through dense crosslinking improves barrier performance. However, extremes in crosslinking are not warranted since coating flexibility and toughness help prevent fracture under mechanical impact. Chemical and light resistances similarly prevent damage to the film and degradation of its protection.

Similar to conversion coatings, any coating can be thought of as converting the pre-existing surface to one that is less reactive, more inert. The methodologies for treating nearly all metals are similar; though the coating compositions are necessarily different as treatments that work on one metal, being inhibitive of corrosion, may accelerate corrosion for another, e.g., through differences in isoelectric point or corrosion potential mismatch.

The oxide layer of metals is their natural defense against corrosion and can constitute a fairly effective protection, e.g., the durable bare aluminum drink can. A surface conversion coating not only functions as a barrier to the corrosive agent transport to/from the metal surface, but better resists chemical dissolution and can impart additional mechanical strength for the adhesion of organic coatings compared to the native oxide. Surface conversion coatings are, in turn, physically and chemically supported by organic topcoatings (primer coatings), which in turn may be protected by additional topcoatings. Inhibitive pigments, such as chromates or molybdates, can provide additional chemical support as transportable, precipitating species that deposit and heal damaged areas.

Is there a topic you would like discussed? Contact us by e-mail at coatings@mst.edu.