As the name implies, hot-dip galvanizing involves dipping steel sheets into a bath of molten zinc. The molten zinc reacts metallurgically with the steel and forms four distinct layers that grow outward from the steel surface. These layers consist of various alloy combinations of zinc and iron, starting from the steel interface, layers consisting of 75% zinc and 25% iron (gamma layer). The middle layers (δ and ζ layers) gradually contained more zinc (90% and 94%, respectively) until the top layer (η layer) of almost pure zinc was formed. Galvanized coatings form a strong bond to steel, which can be in the range of several thousand pounds per square inch (psi).
Galvanized steel is usually left unpainted, as the service life of galvanizing alone usually exceeds the service life of traditional protective coatings. However, when the paint is applied to galvanizing, it is called a "two-sided system". Some studies report that duplex systems last 1.5 to 2.3 times longer than the sum of the individual lifetimes of galvanized or painted systems. alone. The factors applied depend on the exposure environment; for example, mild exposures will have higher factors (longer lifetime) and more severe exposures will have lower factors.
This column will cover the basics behind galvanizing, including proper surface preparation and steel selection techniques, common standards and testing procedures, and problems that can arise with galvanized steel structures. A case study of a galvanized steel bridge will be cited as an example of some of these problems and possible solutions discussed.
Prepare galvanized steel
Painted galvanized steel requires proper surface preparation, especially for new galvanized that has not been exposed to the weathering elements. One issue with painting newly galvanized steel is whether or not various post-treatments are used. Galvanized steel is sometimes post-treated to stop the reaction between the iron and zinc or to slow down the subsequent oxidation of the zinc surface. Common post-treatments are water quenching, chromate quenching and phosphating. In water quenching, freshly galvanized steel is dipped into a Water Bath to help speed up the cooling process and stop the reaction between the iron and zinc. As a result, the water is often contaminated with oil, which deposits on the zinc surface. These contaminants can interfere with adhesion if not adequately removed, and the paint will be applied later.
Chromate quenching is mainly used to prevent the formation of white rust or wet storage pollution when galvanized steel is packed tightly during transportation or storage. White rust is when zinc begins to form on zinc surfaces as the zinc is oxidized through atmospheric exposure. Wet storage staining is the accelerated oxidation of the zinc surface due to exposure to relatively high concentrations of moisture and oxygen trapped between tightly packed sheets or plates of steel. Although chromating prevents this oxidation, it can also interfere with adhesion if the paint is applied later. If you are not going to paint after galvanizing, you may need a chromate treatment, as the treatment often results in a more uniform appearance of the zinc surface as it oxidizes and wears away.
Phosphated steel after galvanization forms a non-reactive zinc phosphate layer on the zinc surface. The surface is first cleaned and degreased, then immersed in a phosphating solution. Phosphating solutions both prevent corrosion product formation and promote good adhesion to subsequently applied paint layers.
If new galvanized steel is to be painted, it will need to be cleaned and degreased first in accordance with SSPC-SP 1 "Solvent Cleaning". After cleaning solvents, surface preparation for freshly galvanized typically includes use of a wash primer, chemical treatment, or cleaning in accordance with SSPC-SP 16, "Brush Cleaning of Coated and Uncoated Galvanized Steel, Stainless Steel, and Non-ferrous Metals." If used Chromate treatment requires effective cleaning of the surface and testing for chromate compounds to ensure they are removed. Where desirable, specifications for galvanized steel should not allow chromate treatment but specify a post-phosphating treatment. Phosphating passivates the zinc surface and produces a conversion coating, which is suitable for the application of paint systems.
Failure to properly prepare galvanized for painting often results in poor coating adhesion and subsequent failure. But even if galvanizing is not used for painting, problems can arise with the zinc layer if the hot-dip process results in a layer that is too thick.
steel selection
Aside from the time (duration) of impregnation, a key factor in avoiding an overly thick zinc layer is proper steel selection. The chemical properties of the steel affect the appearance and other properties of the galvanizing. Trace elements in steel such as silicon and phosphorus can affect the galvanizing process as well as the structure and appearance of the coating. Steels with these elements outside the recognized ranges are called reactive steels. General guidelines for steel selection suggest less than 0.25% carbon; less than 0.04% phosphorus, less than 1.35% manganese, and less than 0.04% or 0.15% to 0.22% silicon.
Silicon can be present as an element in many steels, usually galvanized, even though it is not part of the steel's controlled composition, because silicon is used in the steel reduction process and is present in continuously cast steel. Both silicon and phosphorus act as catalysts in the galvanizing process, leading to the rapid growth of the zinc-iron alloy layer. When the silicon content exceeds 0.22%, the steel is classified as reactive steel. Another consideration with reactive steel is that galvanized usually has a matte gray finish rather than the typical bright finish (Figure 1).
The usual standard for hot-dip galvanizing of structural steel, ASTM A123, "Standard Specification for Zinc (Hot-Dip Galvanizing) Coatings on Iron and Steel Products," specifies a minimum zinc thickness of 3.9 mils or 100 microns (coating grade 100). Note that coating grades are equivalent to zinc thickness in microns, with specific thicknesses ranging from 35 to 100 microns (1.4 to 4 mils). While it is not uncommon to find zinc thicknesses on structural steel ranging from a few thousand mils to about 10 mils, a wide range of thicknesses well beyond that range is a good indicator that the steel may be reactive. As noted below, excessively thick zinc layers often produced when galvanizing reactive steels tend to be more brittle, with lower cohesive strength than would normally be expected. Depending on other factors, such as the exposure environment, these reduced properties can lead to cohesive separation of the zinc layer, as described for exfoliated zinc.
Unfortunately, industry standards for specifying steel do not limit the silicon content in steel for 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%, well above the recommended level for galvanizing.
Problems with galvanized steel bridges
An incident of galvanizing was reported on a small bridge off the east coast. This simple two-lane structure consists of galvanized stringers and floor beams supporting a galvanized road deck with a small truss structure above it. The bridge spans a small stream about 10 feet below the base of the building.
An investigation at the site found that the galvanizing was peeling or delaminating along the bottom flanges of many of the stringers and floor beams. Inspection found that most of the zinc layer was bonded to the structural steel members. The complete galvanizing thickness measured with an electronic gauge is 16 to 20 mils on many components. In the case of zinc stripping, the remaining zinc thickness is typically 1 mil or less (Figures 2 and 3).
An overall assessment of the structure indicated white corrosion on portions of the bottom of the galvanized deck and supporting galvanized structural steel. In areas of severe white corrosion, red corrosion was also observed, indicating rust on the steel substrate. Corroded areas of galvanized decks show a clear pattern of moisture penetration through the deck as evidenced by the amount of corrosion in the scuppers and seams of adjacent deck panels. Another clear pattern is less corrosion in the outer bays and more corrosion in the middle bays, indicating greater water infiltration in the middle of the structure.
The degree of corrosion of structural steel stringers and floor beams generally corresponds to the degree of corrosion of galvanized decking. White corrosion on stringers is usually heaviest directly below where the deck is corroded and likely leaking. Another observation is that galvanized surfaces without much corrosion are usually matte or a uniform dark gray.
Once exposed to the environment, corrosion products (white zinc salts) begin to form on the zinc surface. These products are the result of zinc reacting with atmospheric oxygen, carbon dioxide and water. Typically, the initial zinc compound formed is water-soluble, porous and loosely adhered to the surface. Over time, these compounds usually transform into a tightly adhered, 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 adhered 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 severe white corrosion of the structure's below-deck steel indicates frequent exposure to moisture. In contrast, no particular problems were found with the galvanizing on the deck trusses described above. The galvanized surface of the trusses has a typical weathered galvanized appearance where an adhesive and protective film has formed.
General information on issues such as zinc stripping and spalling is published by the American Galvanizing Association (AGA). AGA shows that spalling occurs when the outer three (four) galvanized layers separate from the first layer, with most of the zinc layer cohesively delaminated. Spalling can occur when zinc is thicker than 10 mils and becomes brittle. The residual thickness of zinc left on steel from which the zinc alloy layer has flaked is usually close to zero, indicating that only the gamma layer remains on the steel. Delamination of the galvanized coating occurs when the outer free zinc layer separates from the intermetallic layer, which is a different type of problem. Debonding occurs when newly galvanized steel cools very slowly or when the steel is exposed to high temperatures for an extended period of time.
result
A review of material test reports for the steel used in the bridge structure in question indicated that the maximum silicon content of the steel ranged from 0.24% to 0.41%. Additional metallurgical testing of a galvanized steel sample that had been removed from the bridge confirmed the high silicon content of the steel, with a result of 0.40%. The measured high galvanized thickness is consistent with that expected for reactive steels.
The delaminated galvanized zinc coating on the bridge was determined to be the result of a combination of the aggressive exposure environment beneath the structure due to frequent moisture exposure, and the reduced cohesive strength of the galvanized zinc coating due to the high thickness. From using active steel. Continued corrosion and degradation of the zinc due to exposure conditions leads to accumulated stresses which even lead to adhesive delamination of the zinc layer near the steel surface in many areas.
The remaining zinc layer where delamination occurs is not considered sufficient to provide long-term corrosion protection for structural steel. The recommended repair option is spot surfacing in areas of delaminated zinc to remove loose galvanizing and any red corrosion on the steel. An organic zinc-rich primer is recommended, followed by an epoxy coat over the entire repaired area. An additional epoxy or polyurethane topcoat is recommended for further protection.
in conclusion
Hot-dip galvanizing, when used in place of or in tandem with protective coatings, can provide long service life to steel structures, but the performance of galvanizing depends on many factors, including preparation and choice of steel. For steel that will be placed in aggressively exposed environments, as in the case of bridges discussed in this column, these factors become even more important - otherwise, loss of service life and potentially costly repairs may result.
