Hot-dip galvanizing, as the name suggests, consists of dipping pieces of steel into a molten zinc bath. The molten zinc reacts metallurgically with the steel and forms four distinct layers growing outward from the steel surface. The layers consist of various alloyed combinations of zinc and iron beginning at the steel interface with a layer composed of 75 percent zinc and 25 percent iron (gamma layer). The intermediate layers (delta and zeta layers) contain progressively more zinc (90 and 94 percent, respectively) until the formation of a top layer (eta layer) of nearly pure zinc. The galvanized coating forms a strong bond to the steel that may be in the range of several thousand pounds per square inch (psi).
Galvanized steel is often not painted, because the service life of galvanizing alone typically exceeds that of conventional protective coatings. But when paint is applied to galvanizing, it is called a “duplex system.” The service life of a duplex system has been reported in some studies to be a factor of 1.5 to 2.3 times the sum of the individual service lives for galvanizing or painting alone. The factor applied depends on the exposure environment; for example, a mild exposure will have a higher factor (longer service life) and a more severe exposure a lower factor.
This column will explain the basics behind galvanizing, including the proper surface preparation and steel selection techniques, commonly used standards and testing procedures and problems that can arise on galvanized steel structures. A case study on a galvanized steel bridge will be cited as an example of some of these problems, with possible solutions discussed.
Preparing Galvanized Steel
Painting galvanized steel requires proper surface preparation, particularly for new galvanizing that has not yet been exposed to weathering elements. One concern with painting new, galvanized steel is whether various post-treatments have been used. Galvanized steel is sometimes post-treated to stop the reaction between the iron and zinc or to slow the subsequent oxidation of the zinc surface. The most common post-treatments are water quenching, chromate quenching and phosphating. In water quenching, freshly galvanized steel is dipped into a water bath to help accelerate the cooling process and stop the reaction between the iron and zinc. Consequently, water often becomes contaminated with oil dirt, which gets deposited on the zinc surface. These contaminants will interfere with adhesion if they are not adequately removed and paint is later applied.
Chromate quenching is primarily used to prevent the formation of white rust or wet storage staining when galvanized steel is closely packed during transportation or storage. White rust refers to the zinc salts that begin to form on the zinc surface as the zinc is oxidized through atmospheric exposure. Wet storage staining is an accelerated oxidation of the zinc surface from exposure to relatively high concentrations of moisture and oxygen that are trapped between pieces or sheets of steel that are closely stacked. Although the chromate treatment can prevent this oxidation, it will also interfere with adhesion if paint is later applied. Chromate treatment may be desired if painting is not intended after galvanizing, as the treatment typically results in a more uniform appearance of the zinc surface as it oxidizes and weathers. The “shiny” appearance of new galvanizing always becomes gray, duller and mottled to some degree over time as it weathers.
Phosphating steel after galvanizing forms a non-reactive zinc phosphate layer over the zinc surface. The surfaces would first be cleaned and degreased, and then immersed in a phosphating solution. The phosphating solution both prevents corrosion products from forming and promotes good adhesion with a subsequently applied paint layer.
If new galvanized steel is to be painted, it must first be cleaned and degreased in accordance with SSPC-SP 1, “Solvent Cleaning.” Following solvent cleaning, surface preparation for new galvanizing generally consists of application of a wash primer, chemical treatment or cleaning according to SSPC-SP 16, “Brush-Off Blast Cleaning of Coated and Uncoated Galvanized Steel, Stainless Steels, and Non-Ferrous Metals.” If a chromate treatment was used, the surfaces must be thoroughly cleaned and tested for chromate compounds to ensure their removal. Ideally, specifications for galvanized steel should not allow chromate treatment and instead should specify a phosphating post-treatment. The phosphating passivates the zinc surface and produces a conversion coating layer that is ideal for the application of a coating system.
Failure to properly prepare galvanizing for painting often leads to poor coating adhesion and subsequent failure. But even if galvanizing is not intended for painting, problems can develop with the zinc layer if the hot-dip process results in too thick of a layer.
One key factor in avoiding an excessively thick zinc layer, in addition to the time (duration) of dipping, is proper steel selection. The chemistry of the steel influences appearance and other properties of the galvanizing. Trace elements in the steel such as silicon and phosphorus affect the galvanizing process as well as the structure and appearance of the coating. Steels with these elements outside of the recognized ranges are known as reactive steels. General guidance for steel selection recommends levels of carbon less than 0.25 percent; phosphorus less than 0.04 percent, manganese less than 1.35 percent, and silicon levels less than 0.04 percent or between 0.15 and 0.22 percent.
|Fig. 1: Galvanized members display white zinc corrosion product on surfaces. Other areas are a uniform dark gray color that is characteristic of galvanizing on reactive steel. All figures courtesy of KTA-Tator, Inc.|
Silicon may be present as an element in many steels commonly galvanized even though it is not a part of the steel’s controlled composition, because silicon is used in the steel reduction process and is found in continuously cast steel. Both silicon and phosphorous act as catalysts during the galvanizing process, resulting in rapid growth of zinc-iron alloy layers. When the silicon content exceeds 0.22 percent, the steel is classified as reactive steel. An additional item of note with reactive steels is that the galvanizing often has a matte gray finish rather than the typical shiny surface (Fig. 1).
A commonly specified standard for hot-dip galvanizing of structural steel, ASTM A123, “Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products,” specifies a minimum zinc thickness of 3.9 mils or 100 microns (for Coating Grade 100). Note that the Coating Grade is equivalent to the zinc thickness in microns as defined for specific thicknesses ranging from 35 to 100 microns (1.4 to 4 mils). Although it is not unusual to find zinc thickness on structural steel of several mils up to approximately 10 mils, widespread thicknesses far in excess of this range are a good indication that the steel may be reactive. As will be described in the following case, the excessively thick zinc layer that is often produced when galvanizing reactive steel tends to be more brittle with a lower cohesive strength than would typically be expected. Depending on other factors, such as the exposure environment, these diminished properties can lead to the cohesive separation of the zinc layer as described for flaking zinc.
Unfortunately, industry standards for specifying steel do not limit the silicon content in steel for purposes of galvanizing. For example, ASTM A36, “Standard Specification for Carbon Structural Steel,” and ASTM A709, “Standard Specification for Structural Steel for Bridges,” both limit the silicon content to a maximum of 0.40 percent, which is well above the recommended level for galvanizing
Problems With a Galvanized Steel Bridge
One such case where galvanizing was reported to be “peeling” from the steel occurred on a small bridge near the east coast. This simple two-lane structure was composed of galvanized stringers and floor beams supporting a galvanized road deck with a small truss structure above. The bridge crossed over a small stream, which ran about 10 feet below the bottom of the structure.
|Fig. 2: The galvanized zinc layer was cohesively delaminating along many stringer and floor beam flanges. Delamination typically occurred in areas where surfaces had heavy white corrosion deposits.|
An investigation at the site observed that galvanizing was peeling or delaminating along the bottom flange of many stringers and floor beams. Examination found that the majority of the zinc layer was cohesively delaminating from the structural steel members. The galvanizing thickness, where intact, measured with an electronic gage, was 16 to 20 mils on many of the members. Where zinc was peeling, the remaining zinc thickness was typically 1 mil or less (Figs. 2 and 3).
The overall assessment of the structure noted white corrosion over parts of the bottom of the galvanized deck and supporting galvanized structural steel. At areas of heavy white corrosion, red corrosion was also observed, indicating that the steel substrate was rusting. The areas of corrosion of the galvanized deck showed a clear pattern of moisture penetration through the deck as evidenced by the amount of corrosion present at weep holes and seams of adjoining deck panels. Another obvious pattern was less corrosion in the outside bays and more corrosion in the middle bays, indicating that water penetration was greater toward the middle of the structure.
The degree of corrosion of the structural steel stringers and floor beams generally corresponded with the degree of corrosion of the galvanized deck. The white corrosion on stringers was typically heaviest directly below where the deck was corroded and likely leaking. Another observation was that galvanized surfaces without much corrosion were typically a matte or uniform darker gray color.
|Fig. 3: Large pieces of delaminating zinc could be removed from members with a thin layer of galvanizing remaining on the steel.|
Corrosion products (white zinc salts) begin to form on a zinc surface as soon as it is exposed to the environment. These products are a result of the zinc reacting with atmospheric oxygen, carbon dioxide and water. Generally, the initial zinc compounds that are formed are water soluble, porous and loosely adhered to the surface. Over time, these compounds are typically converted to a tightly adherent water insoluble film that serves to protect the zinc surface from further corrosion. However, when the environment exposes the zinc to moisture/water very frequently, the conversion to a tightly adherent film does not occur and the zinc continues to corrode, eventually consuming the zinc and allowing corrosion of the steel (red corrosion).
The moderate to heavy degree of white corrosion on the under deck steel of the structure indicated frequent exposure to moisture. By contrast, no particular issues were noted with the galvanizing on the above deck truss. The galvanized surface of the truss had the appearance of typical weathered galvanizing where an adherent and protective film had developed.
General information regarding problems such as zinc peeling and flaking is published by the American Galvanizers Association (AGA). AGA indicates that flaking, where most of the zinc layer cohesively delaminates, occurs when the outer three (of the four) galvanized layers detach from the first layer. Flaking may occur when the zinc thickness exceeds 10 mils and becomes brittle. The remaining thickness of zinc left on the steel where layers of zinc alloy have flaked off is typically near zero, indicating that only the gamma layer is left on the steel. Peeling of galvanized coating, which is a different type of problem, happens when the outer free zinc layer separates from the intermetallic layers. Peeling can occur when newly galvanized steel cools extremely slowly or if the steel is exposed to high temperatures for prolonged periods. A peeling zinc layer as described here is not necessarily related to the total galvanized zinc thickness.
A review of material test reports forthe steel used in the problem bridge structure showed that the maximum silicon content of the steel ranged from 0.24 to 0.41 percent. Additional metallurgical testing of galvanized steel samples that had been removed from the bridge confirmed a high silicon content of the steel with results of 0.40 percent. The high galvanizing thicknesses measured were consistent with what would be expected for reactive steel.
The delaminating galvanized zinc layer on the bridge was determined to be the result of a combination of an aggressive exposure environment beneath the structure due to frequent moisture exposure, in combination with a decreased cohesive strength of the galvanized zinc layer due to the high thickness that resulted from using reactive steel. The continued corrosion and degradation of the zinc due to exposure conditions resulted in accumulating stresses that even-tually caused the zinc layer to cohesively delaminate near the steel surface in many areas.
The remaining zinc layer where delamination had occurred was not considered to be adequate to provide long-term corrosion protection of the structural steel. The recommended repair option was spot surface preparation at areas of delaminated zinc to remove loose galvanizing and any red corrosion of the steel. A spot organic zinc-rich primer was recommended followed by an epoxy coat to the entire repair area. An additional epoxy or urethane finish coat was recommended to provide further protection.
Hot-dip galvanizing, when used in place of or in tandem with protective coatings, can provide a long service life for steel structures, but the performance of the galvanizing depends on a number of factors, including preparation and steel selection. For steel that will be placed in aggressive exposure environments, as seen in the bridge case discussed in this column, the factors become all the more important — otherwise, a loss in service life and potentially costly repairs can result.
About the Author
Jayson Helsel, P.E., PCS, a senior coatings consultant with KTA-Tator, Inc., manages failure investigations and coatings projects and is involved with coatings surveys and inspection of industrial structures. He holds a Master of Science degree in chemical engineering from the University of Michigan and is a registered professional engineer, an SSPC-certified Protective Coatings Specialist and a NACE-certified Coating Inspector. He has been published in the past in JPCL, Durability + Design and in the Journal of Architectural Coatings, which featured his monthly column, “Getting It Right.”
As seen on JPCL
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