Steel Corrosion

Date posted: October 7, 2014

Steel plays a major role as a structural and fabricating material in the construction industry. Steel, a processed metal alloy made from iron with carbon, and other metallic compounds, is prone to corrosion, or otherwise known as rust, when subjected to environmental elements.

steel corossionCorrosion of steel:

Steel corrosion, or rust, is an oxidation or chemical reaction that occurs when active metals become exposed to moisture, salt, carbon dioxide and oxygen rich environments.
There are five different climates in external environments that cause corrosion:

  • Dry (cold polar or hot desert). These areas are characterized by very low corrosion.
  • Normal (town and rural). This will be characterized by higher humidity than dry climate and so faster corrosion but still slow.
  • Coastal (with chloride contamination). Much faster corrosion.
  • Industrial. Characterized by acid pollution and faster corrosion.
  • Tropical. An increase of temperature of 10C will double the speed of a chemical reaction such as corrosion.

See http://www.aicoatings.com.au/home/steel-coatings-new/coating-solvent-borne/ for more information on the corrosive environment categories.

Steel without any surface protection will rust very quickly. If the rust is left untreated, it will eat through the steel, until its atomic structure completely breaks down.

Treatment of corroded steel

Depending on the extent to which steel has been corroded, there are two main methods for treating and protecting steel from further corrosion.

  1. The most preferred method is to remove the rust, and expose the layer of uncorroded steel by abrasive blasting, grinding or abrading. Once the rust has been removed a protective primer coating can be applied. See Australian Standards AS1627.1 – 4 “Preparation and pretreatment of surfaces”.
  2. An alternative method, and often opted for when the steel is heavily rusted, is converting the rusted steel surfaces using a rust convertor, to a passive hydrophobic layer providing a firm substrate for a primer coat to be applied to. A&I Coatings have a rust convertor called the Envirothane 1980 (or “E1980″) Rust Convertor. You can download the E1980 technical data sheet here…

The Ultimate Protection of Steel

Two of the factors involved in protective steel coatings are: protection of the steel from rust by the primer and protective intermediate coat, and the decorative and UV protective role of the polyurethane or fluoropolymer topcoat.

The ultimate protection of steel is using the typical “3 coat system”, as illustrated in the diagram below:

steel coatings

(Source: www.steelconstruction.info )

The roleplay of each coat is further explained below:

Primer-

The primer coat is the bonding coat that assists with long term adhesion of the overall system to the substrate of which you are applying to. This concept is used for most two pack systems, on a variety of substrate material (concrete, steel, timber, etc). The primer coat can work in two ways: etch priming, where the primer has a low level phosphoric acid in the coating etch the surface, or bonding with a high build primer coat. High build primers are often manufactured from a hard epoxy resin with excellent adhesive performance. High build primers with high contents of zinc phosphate are often used as a primer and intermediate coat.

Intermediate coat-

The intermediate coat is often used to create film build. Increasing film build in the coating system, simply increases the systems longevity in the environment, preventing corrosion from reaching the substrate for a longer period of time. Permeable topcoats can allow moisture to penetrate, and reach the primercoat, so the intermediate coat is often a very high content of zinc phosphate, or micaceous iron oxide.

Micaceous Iron Oxide coatings work on the interlocking particles of iron oxide forming an “armour” coat, preventing moisture transmission.

Finish Coat (Topcoat)-

The topcoat is most often the decorative pigmented coating providing the overall system with an aesthetical appearance. The topcoat is often formulated for other functions such as anti-graffiti protection, water repellent, most commonly UV protection. UV protective topcoats such as the polyurethane and fluoropolymer have greater gloss and colour retention than that of the cheaper single pack acrylics.

At A&I Coatings, we have a team of technical experts, who will be very keen to recommend a protective coating system. If you have any questions on how best to protect your project, contact us today. For more information, you can also download our 2013 Product Guide.