Galvanizing presents many challenges to painting operations. While the zinc coating can add substantial life to steel surfaces, its surface chemistry and electrochemical activity can result in conditions leading to adhesion failures of applied organic coatings. The key to successful painting lies in recognizing the many surface conditions presented by new and weathered galvanizing and then employing effective cleaning operations to mitigate their effect. Success is ultimately achieved by selecting coating products that will develop a strong and enduring bond to the galvanized surface.
***As seen in the November 2016 Edition of JPCL***
WHAT IS GALVANIZING?
Galvanizing is a general term for the metallurgical process that applies a layer of zinc metal over a steel surface in order to prevent corrosion of the steel. In the corrosion reaction, the zinc layer will sacrifice itself to protect the underlying steel substrate. In this electrochemical reaction, the zinc performs as the anode and the steel becomes the cathode. As long as there is adequate zinc, the steel will be protected from corrosion.
TYPES OF GALVANIZING
Structural steel, piping and many other steel shapes are coated with zinc using a hot-dip galvanizing (HDG) process. This process involves cleaning the steel using a combination of hot acid baths to remove rust and scale followed by fluxing and immersion into a molten bath of pure zinc heated in a range of 820-to-860 F. The zinc thickness achieved by HDG is in the range of 2-to-8 mils for many steels and is dependent on immersion time in the bath. However, an HDG thickness in the range of 20-to-30 mils can be achieved with some reactive steels.
The HDG process creates a dense coating of zinc that is metallurgically bonded to the steel surface. As the molten zinc diffuses into the steel surface, it creates four distinct layers that vary in their zinc-to-iron ratio. They range from 100-percent zinc at the outermost layer to 75-percent zinc and 25-percent iron at the steel substrate.
Other methods are also used to apply a zinc layer to steel that include electroplating and sherardizing (dry galvanizing named for the British metallurgist Sherard Osborn Cowper-Coles). Electroplating deposits zinc on a steel surface immersed in a chemical solution of zinc salts and exposed to electrical current. Sherardizing is a process that rotates steel parts and fasteners with zinc dust in a heated drum to diffuse the zinc into the steel surface. Both methods apply much thinner layers of zinc than HDG.
HOW LONG WILL ZINC PROTECT STEEL?
Zinc is not immune to corrosion. When zinc is exposed to the atmosphere it enters into chemical reactions with oxygen, moisture, carbon dioxide and soluble airborne salts like chlorides and sulfates. The rate of zinc corrosion will depend on the severity of the environmental exposure. A rural exposure results in far less corrosion than do seaside or industrial exposures. The American Galvanizers Association (AGA) has data showing that a galvanized steel surface with 2 mils of zinc will only develop 5-percent rusting after approximately 35 years of exposure in an industrial environment.
HOW DOES ZINC DETERIORATE?
The corrosion of HDG begins immediately after the molten zinc layer has cooled. The appearance of HDG can vary. Typically, the zinc surface is smooth with a pattern of random spangles resembling snowflakes in zinc. When the steel contains trace elements of silicon and phosphorus outside of recommended ranges, it is considered a “reactive” steel. The HDG of reactive steel produces a matte finish after cooling. After exposure to atmospheric oxygen and moisture, the zinc surface begins to corrode, developing a thin layer of zinc oxide. Continued atmospheric exposure results in the formation of zinc hydroxide. Eventually, after about two years of exposure, the zinc surface layers have been converted largely to a metallic gray patina composed of insoluble zinc carbonate and zinc oxide. At this point, the zinc surface has essentially stabilized and continued atmospheric corrosion is substantially reduced.
WHY DO WE PAINT GALVANIZING?
To Replace Lost Corrosion Protection
While the corrosion rate of atmospherically stabilized zinc is relatively low, the thickness of the zinc layer will continue to be reduced with time. Both the original zinc thickness and the severity of the atmospheric exposure will affect the life of the galvanizing. As the zinc layer is reduced through corrosion, its ability to protect the underlying steel substrate is diminished and the galvanic protection afforded by the zinc is lost. At this point, the application of a polymer overcoating may be considered to extend the life of the structure.
To Extend Galvanizing Life with a Duplex System
There are situations where new HDG may be immediately overcoated to extend the life of the structure beyond the longevity afforded by the zinc layer alone. When new HDG is overcoated with an organic coating, a synergistic effect is created that extends the life of the galvanizing. The organic overcoating becomes a barrier to penetration by moisture that slows down the corrosion of the zinc layer. A duplex coating system can extend the life of a galvanized coating by a factor of from 1.4-to-2.7 times the sum of the lives predicted for each coating layer alone.
To Improve Appearance
Because the natural appearance of metallic gray spangling may be objectionable, HDG is often overcoated with an organic coating to improve its appearance or impart a specific color required by an architectural rendering. In these situations, the addition of an overcoat will extend the life of the galvanized layer.
PROBLEMS WITH PAINTING GALVANIZING
Galvanizing can be a difficult surface for the successful retention of an organic paint film. Difficulties arise when surface contamination is not recognized and removed before the galvanizing is coated. There are many possible types of surface contamination on galvanizing that must be removed before painting is attempted.
Oil and Grease
Oil and grease can be removed using an aqueous alkaline cleaning solution followed by a hot water wash; mineral spirit solvent followed by wet steam flushing; or wet steam with the injection of a biodegradable detergent. To avoid damaging the zinc carbonate film, water pressure cleaning should not exceed 1,450 psi.
Zinc oxide is the natural corrosion product that starts to form when the HDG has cooled and is exposed to air. The oxide layer is very thin and forms slowly. It is not considered to be problematic to painting within the first 48 hours of galvanizing.
Wet Storage Stain
Wet storage stain (also known as white rust) primarily consists of zinc hydroxide. The white rust forms as the galvanizing is exposed to moist air without sufficient air circulation between the parts. White rust is hygroscopic and if not removed, will affect paint adhesion. Light deposits of white rust can be removed by scrubbing with an ammonia solution followed by a water flush. Heavy deposits of white rust can be removed by applying such materials as acetic acid (vinegar), citric acid, or a commercial rust remover and scrubbing with a soft bristle brush. Flushing with warm water should follow removal. Avoid using any concentrations of hydrochloric (muriatic) or sulfuric acid as cleaning agents. Both will attack and remove any sound galvanizing.
Chromate Conversion Coatings
Chromate conversion coatings may be applied at the galvanizing facility to protect new galvanizing from excessive growth of zinc oxide and zinc hydroxide (wet storage stain). The chromate is slowly removed by natural weathering, but may persist for up to two years. When in doubt, a standard test (ASTM B201, “Standard Practice for Testing Chromate Coatings on Zinc and Cadmium Surfaces”) can be performed to determine the presence of chromate. The test involves placing a drop of lead acetate solution onto the galvanizing. If the zinc surface turns black, there is no chromate present. If there is no color change, chromate is present. The chromate coating must be removed prior to any paint application. Methods to remove the chromate layer include weathering for six months, sanding or sweep blasting. Liquid solutions are not recommended for removing chromate conversion coatings.
If sweep blasting is used, the abrasive media must be chosen with care to provide a cleaning action without removing excess zinc layers or fracturing one of the subsurface zinc/iron diffusion layers that form the total galvanized layer. Nozzle movement should be rapid with rates in excess of 1,200 square feet per hour. Removal of up to 1 mil (25 microns) of the uppermost 100-percent zinc with a profile in the range of 1-to-2 mils is acceptable.
Before using sweep blasting, the contractor should consult SSPC-SP 16, “Brush-Off Blast Cleaning of Coated and Uncoated Galvanized Steel, Stainless Steels, and Non-Ferrous Metals.” One of the abrasives that has been used successfully is aluminum/magnesium silicate. Particle size should be in the range of 8-to-20 mils (200-to-500 microns). Other abrasives that can be used are silica-free soft mineral sands with a mineral hardness of 5 MOHs or less, organic media such as corn cobs or walnut shells, corundum and limestone. The use of mineral slags should be avoided. Blasting nozzle pressures should be in the range of 20-to-40 psi.
Rusted areas may be present on heavily weathered galvanizing. In order to facilitate painting, the rusted areas must be cleaned to a level compatible with the coating product to be applied. If cleaning to a tight rust condition is acceptable, hand-tool cleaning (SSPC-SP 2), power-tool cleaning (SSPC-SP 3) or brush-off blast-cleaning (SSPC-SP 16 ) can be used.
Newly Galvanized Steel
Newly galvanized steel (exposed to the atmosphere for no more than 48 hours) has few zinc compounds on the surface simplifying the cleaning, but is relatively smooth so profiling is necessary to ensure the adhesion of an overcoating.
Before profiling, the galvanizing must be cleaned to remove any grease or oil contamination. Following cleaning, create a profile in the zinc compatible with the coating product to be applied. Brush-off blast cleaning (SSPC-SP 16) is a viable method of profiling. When blast-cleaning, caution must be taken to avoid removing too much of the galvanizing or damaging the metallurgical bond of the zinc to the steel.
PARTIALLY WEATHERED GALVANIZED STEEL
Partially weathered galvanized steel (exposed to the atmosphere from 48 hours to one year) can contain a wide range of the surface contaminants previously described. The problem lies in identifying each contaminant and applying appropriate remedial action before paint application. The amount of contaminants will be a function of the duration of weathering, the severity of the weathering environment and the metal treatments applied immediately after galvanizing. Surface contaminants present could include the following.
Oil and grease.
Chromate conversion coatings.
Zinc hydroxide (white rust).
Methods for dealing with each contaminant have been previously described with the exception of rust, as found on galvanized surfaces with atmospheric exposure in excess of several days. This involves cleaning solutions, wash primers and acid-based acrylic solutions. Depending on the condition, mechanical means may also be necessary to clean and profile weathered galvanized surfaces.
Phosphate Conversion Coatings
Following initial cleaning to remove oil and grease contamination, iron phosphate and zinc phosphate coatings are frequently used to improve paint adhesion. The conversion coatings can be applied by dipping, spraying or with a soft-bristled brush. After a reaction time of 3-to-6 minutes, the surfaces are flushed with clean water and allowed to dry before painting. Product manufacturers should be consulted regarding the specific application time and temperature requirements for their products.
Basic Zinc Chromate-Vinyl Butyral Wash Primer
One treatment for partially weathered galvanizing is a basic zinc chromate-vinyl butyral wash primer (SSPC Paint No. 27). When applied within the recommended DFT range of 0.3-to-0.5 mils, it cures rapidly to a resin finish that neutralizes zinc corrosion products and retards their formation. The bonding ability of the converted zinc surface will accept many paints. Paints that contain ketone or alcohol solvents develop the best bond. Once the wash primer is applied, it dries in about 15 minutes and should be overcoated in about 30 minutes. It is extremely important to apply the wash primer within the range of 0.3-to-0.5 mils DFT. Exceeding 0.5 mils will lead to delamination failures after overcoating. The wash primer also etches the zinc surface and eliminates the need for sweep blasting.
Acidic Acrylic Pretreatment
Another surface treatment for partially weathered galvanizing is an acidic acrylic solution. When applied, the solution first reacts with the zinc surface forming a passivating conversion coating, to suppress further corrosion while simultaneously forming an acrylic coating suitable for subsequent painting on top of the passivation layer. The underlying conversion coating provides strong adhesion to the galvanized surface while the thin film acrylic layer provides barrier protection, inhibiting corrosion and providing a highly compatible surface for the application of organic paint films. The coating is approximately 0.04 mil (1 micron) thick. Painting is possible at any time during a period of four months after application, as long as the surface remains free of visible zinc oxides or white rust. The pretreatment also etches the zinc surface and eliminates the need for sweep blasting.
FULLY WEATHERED GALVANIZED STEEL
After approximately one year of outdoor exposure, weathered galvanized steel will develop a stable and finely etched surface of zinc carbonate that can remain on the surface and is favorable to coating adhesion. Zinc carbonate is insoluble, tightly adhered to the surface and should be preserved. However, before painting, any oil or grease must be removed and the zinc surface modified to enhance paint adhesion. Modification can be accomplished by either using one of the previously described chemical wash primers to promote paint bonding or by sweep blasting to create an acceptable profile in the galvanized surface. Because zinc oxide forms rapidly on blasted surfaces, paint application should take place within 30 minutes of cleaning.
COATING CHOICES FOR GALVANIZING
There are some alkyd formulations that are recommended for painting galvanizing. These coatings incorporate a modified alkyd that integrates Portland cement into the binder.
Coating products, such as epoxy or polyaspartic, require higher levels of cleaning such as a minimum of commercial blast cleaning (SSPC-SP 6). In these cases, only the necessary profile should be created in the zinc, taking care not to remove too much of the surrounding sound galvanizing or damage the metallurgical bond between the zinc and the steel.
The saponification reaction forms water-soluble soaps at the interface between the coating and the galvanizing. The soaps become soft, sticky and dissolve when exposed to moisture. The moisture can be present in the original wet coating or as atmospheric humidity that passes through the cured coating film and reaches the galvanized substrate.
Saponification can be destructive to paint films. Oil-based paints that utilize alkyd esters, epoxy esters or linseed oil derivatives in their composition can saponify when placed on an alkaline surface and galvanizing is such an alkaline surface. Some polyvinyl acetate (PVA), water-based latexes can also be broken down by saponification. Latex acrylic paints resist saponification. Other acceptable coating choices include solvent-based epoxies, urethanes and polyaspartic formulations.
There are documents available that describe methods of preparing surfaces of new and weathered hot-dip galvanized steel for painting.
In addition to SSPC-SP 16 referenced earlier, ASTM International has published Standard Practice D6386-10, “Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting.” The document describes methods of preparing surfaces of new and weathered hot-dip galvanized steel for painting. These resources should be consulted before any painting of galvanizing is attempted.
ABOUT THE AUTHOR
E. Bud Senkowski E. Bud Senkowski, P.E. is a senior consultant with KTA-Tator, Inc., where he has been employed for over 24 years. He is a registered professional engineer in several states, an ANSI-Certified Nuclear Coatings Inspector, an SSPC-certified Coatings Specifier and a NACE-certified Level 3 Coating Inspector. Senkowski has over 45 years of coating engineering, inspection, training and project management experience. He has an extensive background in the engineering, application and inspection of coatings and linings used in nuclear and fossil-based service, water and chemical storage facilities, and is the primary instructor for KTA’s Nuclear Inspection and Pipeline Coating training courses. Senkowski holds an MBA from Drexel University and a Bachelor of Science degree in fuel engineering from Pennsylvania State University.