Galvanizing is a process in which a coating of zinc is applied to a steel surface from a bath of molten zinc. This can occur in several ways. Coil steel, such as steel used to make automobile bodies, is commonly galvanized in a continuous roller process. The sheet of steel is fed into a bath of liquid zinc where the zinc is deposited on the surface. Alternatively, structural pieces and hardware are galvanized in a batch process by immersing them in a bath of molten zinc. This is called hot-dip galvanizing. Chemical reactions occur between the iron in the steel and the molten zinc. (See sidebar.) An outer layer of pure zinc covers the surface as the piece is withdrawn from the bath.
The corrosion protection of unpainted galvanizing comes from the formation of a thin, invisible layer of insoluble corrosion products. Zinc, being an active metal, reacts with oxygen in the air. Zinc oxide starts forming 24 to 48 hours after galvanizing. It takes about a year for the zinc oxide to cover the entire surface. The zinc oxide converts to zinc hydroxide upon exposure to moisture in the form of rain, dew, or high humidity. The final step is the reaction of zinc oxide and zinc hydroxide with carbon dioxide in the air to form zinc carbonate. This requires free-flowing air. Zinc carbonate is the dense, insoluble material that forms the protective layer (sometimes called the patina). Zinc oxide and zinc hydroxide are water soluble and not very dense. They adhere loosely to the surface, so painting over zinc oxide or zinc hydroxide does not provide good adhesion of the coating to the surface. The practical problem is that zinc oxide, zinc hydroxide, and zinc carbonate are all white. They give the galvanized surface a dull, matte grey appearance, and there is no way to determine visually what form of zinc compound is present. Knowing the compound is important because some forms are not suitable for painting over.
Each of the galvanized surface conditions: newly galvanized, partially weathered, or fully weathered have different cleaning and/or profiling requirements. Therefore, properly identifying the surface condition is important.
Newly galvanized steel has been exposed to the atmosphere for no more than 48 hours and has few zinc compounds on the surface. The coating can be bright and shiny, indicating an all-zinc outer layer, or dull gray, indicating a zinc-iron intermetallic outer layer, or a combination of both. Newly galvanized surfaces have few or no zinc compounds on the surface simplifying the cleaning, but are relatively smooth so profiling is necessary to ensure adherence.
Partially weathered galvanized surfaces have a build-up of zinc compounds and, possibly, organic contaminants such as dirt, dust, oil, or grease. The compounds become attached to the zinc coating by electrostatic forces, and can be expected to release from the surface over time. The zinc compounds, mostly zinc oxide and zinc hydroxide, must be removed before painting. Partially weathered galvanized steel is the most common galvanized surface condition when painting, and also the most difficult to prepare. The partially weathered surface condition is present from two days to about one year after galvanizing, depending on temperature variation and humidity factors.
Fully weathered galvanized steel has zinc compounds covering the entire surface. The main compound in the fully weathered surface is zinc carbonate. Zinc carbonate is tightly adhered to the surface, is not water soluble, and does not wash off the surface when water hits the part. In this state, the zinc compounds should not be removed, as the paint performs better when the compounds are left on the surface. Fully weathered galvanized coatings are the simplest surface condition to paint, as only mild cleaning is necessary. This condition is present from one year of exposure to the atmosphere until all the zinc coating has been consumed by protecting the underlying steel, which will take decades.
When cleaning a galvanized surface prior to painting, the goal is to remove any dirt, grease or oils. At the same time, care must be taken not to remove too much of the galvanized coating. Alkaline cleaning, ammonia cleaning and solvent cleaning are the most common ways of removing dirt from a galvanized surface. As some cleaners may react differently with different paint systems, the paint manufacturer should be consulted for specific reaction problems. Oil, grease and dirt can be removed by using an alkaline solution in the pH range of 11 to 12, but not greater than 13 as this will damage the zinc coating. Most alkaline cleaning solutions are nominally 2 to 5 percent sodium compounds with small additions of emulsifying or chelating agents. The solution can be applied through dipping, spraying or brushing. If brushing is used, apply the solution with a soft bristle brush, preferably of nylon, definitely not copper or steel bristle brushes. If dipping or spraying the solution, the temperature range that works best is between 140 and 185 F. For newly galvanized steel, a water-based emulsifier can be used to remove contaminants. After cleaning, thoroughly rinse the surface with hot water and allow to dry.
In order to provide a good adhesion profile for the paint, the galvanized surface must be flat with no protrusions and slightly roughened to provide an anchor profile. During the removal of the galvanized article from the zinc bath, the excess zinc runs down the edges of the part and can sometimes build-up at a protrusion or irregular edge. The zinc can also form tears at the edge where it drains off the part. These high spots and tears must be removed before painting as they will be very difficult to coat. The high spots and tears are usually ground off with hand tools or power grinders. Care must be taken when performing this operation to insure that the galvanized coating is not removed below the specified thickness.
Fig. 1- Sweep Blasting of Galvanized Steel Surface
In order to roughen the typically smooth galvanized surface after cleaning, an abrasive sweep or brush blast may be used. Care should be taken to prevent removing too much of the zinc coating. Particle size for a sweep blast of galvanized steel should range between 200 and 500 microns. Aluminum/magnesium silicate has been used successfully in the sweep blasting of galvanized steel as seen in Fig. 1. Organic media such as corn cobs and walnut shells or minerals such as corundum, limestone and sands with a Mhos hardness of five or less may also be used.
Coatings For Galvanizing
Many types of coatings can be applied over galvanizing. The choice sometimes depends, in part, on how the surface was prepared. However, there are certain types of coatings that are not recommended, especially oil-based alkyds. Most coatings can be applied by brush, roller, or spray. Check with the coating manufacturer on application methods. The most common coatings applied directly to galvanizing are epoxies and water-bornes.
Epoxies come in many varieties. Those selected must be specifically formulated for use over galvanizing. Make sure the manufacturer’s technical data sheet for a particular product says it can be applied to galvanizing. Epoxies generally chalk in sunlight, so a topcoat would be needed for outdoor exposure. Aliphatic urethanes give good appearance and chemical resistance, so they are commonly used for this purpose.
Water-borne acrylic coatings are fast drying. They make good topcoats and are easy to maintain. Acrylic latexes are preferred. Other water-borne latexes have properties similar to acrylic latexes. However, although the water-borne latexes dry quickly, they can take two to four weeks to thoroughly cure. Therefore, their adhesion and abrasion resistance will initially be low. This can present a problem for new construction because of possible handling damage.
Chlorinated rubbers and vinyls have been used in the past, but environmental regulations concerning volatile organic compound (VOC) content have severely limited their availability and use. They are best used on galvanizing that has been prepared by sweep blasting or with a wash primer.
Acrylics can be applied directly to a galvanized surface, although a wash primer is preferred. However, some acrylics contain ammonia, and if the pH of the paint is high, the ammonia may react with the zinc, causing the paint to peel.
Galvanized steel that is to be buried is typically painted with bituminous materials or coal tar epoxy. Coal tar epoxies are best applied over a sweep blasted surface or a wash primer.
Organic zinc-rich primers, such as an epoxy zinc-rich, can also be applied directly to galvanizing. However, not all organic zinc-rich primers are formulated for application to galvanizing, so check the product data sheet or with the manufacturer.
Oil-based and alkyd paints are not recommended unless they are specifically formulated for use on galvanizing. The problem is that zinc is an alkaline material. The zinc and the binder react in the presence of alkalinity to form a metal soap. (Sometimes, you can actually form lather by adding a little water to this soap.) This process, which is called saponification, breaks the bond between the galvanizing and the paint. Saponification requires moisture, so it occurs in a well-adhered coating film only after moisture starts permeating the film. The coating then falls off, or it can be completely removed in large sheets. Experience has shown that it takes six months to a year in outdoor exposure before the peeling occurs.
Finally, if sweep blasting is used to prepare the surface, it should be painted within 24 hours because the blasting exposes fresh zinc that will start converting to zinc oxides. Wash primers and acrylic passivators will give you more time to apply the paint, but check with the coating supplier on how much more time you have.
- American Galvanizers Association, Painting Over Hot Dip Galvanized Steel, data of access: 22 September 2016, www.galvanizeit.org/images/uploads/articles/paintsteel.pdf
- American Galvanizers Association, Preparing HDG for Paint, data of access: 22 September 2016, http://www.galvanizeit.org/specification-and-inspection/specifying-duplex-systems/preparing-hdg-for-paint
- Paint Square, Cleaning and Painting Galvanized Steel, data of access: 22 September 2016, http://www.paintsquare.com/library/articles/Applicator_Training_Bulletin__Cleaning_and_Painting_Galvanized_Steel.pdf
- ASM Handbook Volume 5, Batch Hot Dip Galvanized Coatings, 1994