PAINTSQUARE BLOG

Globally, the corrosion of metals accounts for trillions of dollars in annual costs. Some environments are especially aggressive: for example, direct exposure to saltwater via immersion or spray in marine shipping and offshore applications such as oil rigs or wind farms. In general, corrosion is particularly problematic in the architectural use of both structural iron and steel and metallic hardware such as balcony railings, especially in coastal areas.

Architectural Metals

Only the noble metals, such as platinum, silver and gold, possess such low reactivity that they can be considered effectively non-corroding. Stainless steel, which combines iron with other alloys, is generally slow to corrode, while pure iron and steel can corrode quickly to form iron oxide, otherwise known as rust. While marine and coastal areas are more corrosion-prone due to salt, the natural atmospheric combination of oxygen and moisture alone is sufficient to cause gradual corrosion, thus reducing the service life and physical strength of the metal. This is true for both outdoor- and indoor-exposed metals. This “atmospheric corrosion” is the most prevalent of several corrosion forms for architectural metals.

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While marine and coastal areas are more corrosion-prone due to salt, the natural atmospheric combination of oxygen and moisture alone is sufficient to cause gradual corrosion, thus reducing the service life and physical strength of the metal.

From an architectural product standpoint, the most prevalent method of protecting a metal from corrosion is by the application of an organic coating to insulate the metal from the agents that cause the electrochemical reduction-oxidation (ReDox) reactions we know as corrosion. However, other coating types, such as thermoplastic encapsulation and zinc-silicate inorganic coatings, are also used. Further, surface treatments such as the plating and galvanizing of steel, or anodizing of aluminum, are also used to protect the surface. These various treatments can perform well, provided the coating or surface treatment remains undamaged from the effects of normal use and wear, or degrades from the effects of weather exposure.

The Process of Corrosion

Most corrosion processes are electrochemical reactions where electrons flow and current is generated. Three components are required: an electron donor (anode), an electron receptor (cathode) and a conductive path between them. This conductive path is primarily water, which contains electrolytes. These electrolytes can come from seawater salts, atmospheric pollutants such as sulfur dioxide, dry deposition of particulates followed by wetting, etc. This explains why salts, in combination with wet surfaces, accelerate corrosion rates. Removing any one of these, or blocking the electron transport with a continuous film coating, provides corrosion protection. In the case of surface treatments such as galvanizing or plating, a reactive surface such as iron or steel is replaced with a less reactive one and the corrosion process is inhibited.

Corrosion is often accelerated by alternating wet-dry cycles rather than a continuous steady state, and this is the condition for most architectural metals. For organic film-forming protective coatings, moisture absorption and desorption can occur at quite different rates depending on the resin chemistry. Moisture uptake can swell an organic coating while drying results in shrinkage; the resulting mechanical stress can result in microcracks and other defects in the coating, and result in concentration changes of the electrolytes. This is one reason why the widely used steady-state salt spray (fog) corrosion tests, such as ASTM B117 and ISO 9227, often don’t correlate to field performance as well as more advanced wet-dry cyclic tests.

Barrier Coatings

Most anti-corrosion coatings for architectural applications provide passive protection, which functions by coating the metal with a corrosion protection system that forms a barrier to eliminate exposure to water, oxygen and electrolyte salts. The lower the permeability of the anti-corrosion coatings to these, the better the protection afforded.

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Most corrosion processes are electrochemical reactions where electrons flow and current is generated. Three components are required: an electron donor (anode), an electron receptor (cathode) and a conductive path between them.

As with any paint formulation we have four primary constituents:

• Binders, the resins which form the paint film;
• Pigments, which provide color and can provide function if metal based;
• Diluents, the base solvents and thinners; and
• Additives.

In anti-corrosion barrier coatings, the chemical nature of the binder and the type of pigments typically provide the greatest functional contributions. Most effective treatments are multilayer coatings systems, consisting of one or two applications of a primer over a properly prepared surface, followed by one or two finish coats. The purpose of the primer is mostly to protect the surface while that of the topcoats is essentially to protect the primer. Due to the higher pigment volume concentration of the primer, it inherently has higher porosity to moisture, so the finish coats are formulated to provide the moisture barrier properties to protect the primer/substrate.

Cathodic Protection

The protected surface is the electron receptor, the cathode. Since a reaction between two different metals can offer corrosion protection, cathodic protection uses a sacrificial electron donor, the anode. This is most commonly zinc as it is a metal with a relatively slow corrosion rate (more negative electrochemical potential) as compared to iron; your hot water heater typically has a sacrificial anode.

The difference in potential between the two metals means the sacrificial anode material corrodes in preference to the substrate. This effectively stops the oxidation reactions on the protected metal. Magnesium and aluminum alloys are also used as sacrificial anodes in some coatings. Zinc chromate is often used as a pigment in primers as it reacts with rust and passivates the surface. Zinc and magnesium powders are also used for this, typically at PVC of around 65-90 percent loading, and the primer binder is often epoxy or alkyd based. As epoxy is not UV-resistant, tending to yellow and chalk in one to two years, other binders are used for the finish coats. Epoxies are often preferred where corrosion is severe, and they are very abrasive-, chemical- and solvent-resistant. Epoxy-based systems comprise nearly half the total market for anti-corrosion coatings.

Polyurethane binders also form very good corrosion-resistant coatings, are not affected by UV and are often preferred for exterior application topcoats over epoxy primers for architectural protection. Acrylic formulations have started to appear and are likely to grow due to their environmental friendliness. Alkyds are preferred by some contractors due to their ease of use, but usually are not as high-performing or durable as the other chemistries.

Active Protection with “Smart” Coatings

Newer anti-corrosion coatings technologies are emerging and are often called “smart” or “self-healing.” Some of these have an active corrosion inhibitor substance contained in nano-scale reservoirs in the coating system for release in the event of damage, or have mechanisms to entrap corrosive ions. For example, one product works by chemically altering the top layer of carbon steel, making it nonreactive, rather than by forming a sacrificial layer; it also forms an inert ceramic layer over the alloy.

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Epoxies are often preffered where corrosion is severe, and they are very abrasive-, chemical- and solvent-resistant. Epoxy-based systems comprise nearly half the total market for anti-corrosion coatings.

The introduction of environmental restrictions on the use of Cr(VI)-containing compounds spurred research aimed at developing substitutes for such coatings. Polymer layers, metallic and ceramic layers, silica-organic layers and conversion layers are now being used as self-healing coatings and the development is likely to continue.

Non-organic Anti-Corrosion Coatings

Fluoropolymer resins are also effective anti-corrosion binders owing to their impermeability and excellent durability, especially now that field-applied chemistries have become available, but are in more limited use largely due to cost and application requirements.

One well-known non-organic anti-corrosion coating is based on zinc silicate. Here, a large amount of zinc metallic dust is mixed with a zinc silicate, which also contains high-build polysiloxane or a hybrid topcoat and primer made from inorganic zinc. Most of the zinc-based coatings are formulated with either epoxy resin binders or inorganic silicate, although organic polyurethane has also been used. Zinc silicate coatings are also known as inorganic zinc-rich coatings because they usually contain a zinc silicate binder. They do require a totally clean substrate in order to work effectively.

When applied, the binder keeps the zinc particles in contact with the steel surface. This results in the zinc particles acting as the sacrificial anode to the steel cathode, affording some protection to the substrate. This was originally developed by NASA to protect space launch gantries at the Kennedy Space Center, but perhaps the most iconic use was to protect the inside of the Statue of Liberty from corrosion in the mid-1980s.

Rust Conversion Coatings & Powder Coatings

Proper surface preparation is a critical requirement for most anti-corrosion coatings. However, some product technologies are designed to cope with some existing rust by interacting with the corrosion products of the steel. There are several mechanisms by which this works. For example, they can impregnate rust, inactivate soluble salts, or convert iron oxide rust to magnetite or other products. This last type is known as a rust conversion coating, reacting directly with the rusted surface to form a water-insoluble matrix that can then be finish coated.

© iStock.com / FelixStrummer

One well-known non-organic anti-corrosion coating is based on zinc silicate. This was originally developed by NASA to protect space launch gantries at the Kennedy Space Center, but perhaps the most iconic use was to protect the inside of the Statue of Liberty from corrosion in the mid-1980s.

Certain powder coatings also offer a degree of corrosion protection, primarily through their low moisture transmission rate, or the use of zinc phosphate or other pigments. However, most of the issues with corrosion occurring in architectural building products, such as balcony railings, are due to the use of the wrong resin (polyester is prone to moisture hydrolysis and acrylics have an acid rain sensitivity, for example), pinholes and other defects during application or in-service mechanical damage. Coastal areas with aggressive climates such as Florida prove to be particularly challenging for some powder coatings.

Summary

While there are other specific forms of corrosion such as the galvanic corrosion of dissimilar metals in contact, and localized corrosion due to stress concentration, the primary mechanism for building products is generalized atmospheric corrosion. While there are various surface treatments, the most common method of protection is through the application of coatings. Coatings systems comprise primer layers, usually containing metallic pigments such as zinc compounds, to act as sacrificial anodes to the cathode steel substrate, while the topcoats primarily serve as barrier layers to salts and water. These systems are often organic polymer resin-based, although fluoropolymer and inorganic zinc silicate, and other systems are also used. And the newest line of defense employs active surface modification protection, often coupled with self-healing properties for extended service life protection.

ABOUT THE THE BLOGGER Allen Zielnik Allen Zielnik has 42 years of experience in both chemical and physical instrumental methods of analysis of materials. He has been with Atlas Material Testing Technology for the past 22 years, specializing in the effects of solar radiation, weather, and the environment on the durability and performance of materials and products, including coatings. A frequent speaker at various worldwide technical symposia, he is the author of more than 120 publications and conference presentations. Zielnik has degrees in electronics engineering and analytical chemistry. SEE ALL CONTENT FROM THIS CONTRIBUTOR