“Understanding the properties of the underlying material is the basis of understanding the cleaning, surface preparation and ultimately the advanced corrosion resistance only available by the experts at PowerGroup”
Steel is an alloy of iron and carbon in varying percentages. Its high strength and malleability account for its wide use in the fabrication of durable goods. Steel is typically milled to thickness (gauge) and width through one of two processes; cold rolling, where initially red hot steel is allowed to cool through subsequent sizing and hot rolling where steel is kept red hot throughout the process. Hot rolled steel picks up impurities during this process that leads to mill scale and this scale must be recovered before the phosphating process.
Steel corrodes easily to its oxide of red rust. This is one reason why steel is usually preserved with rust preventing materials if it is to be stored, or if it is to be top coated with paint.
Steel can be cleaned using several types of cleaning compounds. Strong acids will attack steel and cause etching and promote corrosion. The use of strong acids and/ or heat may also cause a subsequent cleaning problem by increasing the surface layer concentration of carbon smut. Removal of this smut is best accomplished using sequestrates, chelating materials, and spray impingement.
One of the light metals, aluminum is characterized by its high strength to weight ratio. Used in several industries, it has a great role in the aerospace, automotive and building materials.
Aluminum is very resistant to atmospheric corrosion because the oxide layer developed, unlike ferrous metals, is very dense and tenacious. This tight oxide layer is usually self-limiting and formation slows after initial development. Aluminum oxide is recognized by its white appearance, giving rise to its name, white rust.
Though virtually impervious to atmospheric conditions, aluminum is extremely reactive to highly alkaline and acidic solutions. Alkalis can be inhibited by silicates and/or berates in cleaning solutions which “plate-out’ on the surface and act as a barrier to prevent further attack of the substrate. Temperatures above 160 degrees F will also promote corrosion of aluminum, even if the solution is only water. Mild acids may be used if the soil dictates. The key to preparing the substrate, however, is removing the oxide layer, providing a pure metal “surface” for the coating to bond to. This usually is accomplished with a fluoride source in solution as it reacts quickly with the oxide and dissolves it.
Zinc substrates, like aluminum, are light metals usually found in the form of extrusions, castings, or in the form of galvanized steel. Galvanized steel is a metal substrate comprised of a layer of zinc bonded to steel, through either hot dipping or electro coating. Hot dipped galvanized layers not only contain zinc but other elements as well. Electrogalvanized is generally pure zinc.
Zinc behaves similarly to aluminum in its corrosion protection (i.e. the tight oxide phenomenon and the reduction to white rust is evident). The same concerns and cautions afforded the cleaning and pretreating of aluminum can apply to zinc as well.