Q: "What care is taken for galvanizing threaded parts, and is rethreading them allowed after galvanizing?”
Answer:
A. Fasteners and other threaded parts are galvanized to ASTM A153/A123M, Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware. This specification is intended to be applicable to hardware items that are centrifuged or otherwise handled to remove excess galvanizing bath metal from the parts.
Bolts and similar items are galvanized in perforated metal baskets and the parts are centrifuged at a high rate of speed during withdrawal from the galvanizing bath to remove excess metal from the threads. Nuts are galvanized as “blanks” with no internal threads, and the threads are cut into the nut after galvanizing. The threads on the nuts are cut slightly oversized to allow for the buildup of zinc on the surface of the male thread on the bolt. The amount of overtap is dependent upon the thickness of the coating, which is in turn dependent upon the thickness of the steel article. (Thicker materials = thicker coating)
There will be no corrosion on the bare threads of the nut, because the steel surface is in direct contact with the galvanized coating on the bolt. The zinc on the bolt will protect the bare threads on the nut through its anodic reaction.
Section 4.4 of the specification Threaded Articles—states “the zinc coating on threads shall not be subjected to a cutting, rolling, or finishing-tool operation, unless specifically authorized by the purchaser. In order to meet overtapping allowances, tapping the threads of nuts or tapped holes after galvanizing is not prohibited.”
Q: "At what depth should someone engrave a steel plate so that, when galvanized, it can still be read? What is the minimum depth and the best depth?”
Answer:
Thanks for the question. It is a fairly common requirement that marking be legible after galvanizing for most fabricated steel structures, and it is covered by ASTM A385, Standard Practice for Providing High Quality Zinc Coatings (Hot Dip). Section 14.2 Marking for Identification states, “Satisfactory identification may be provided by welding the identifying marks on the material, by embossing the identifying marks on a steel tag of no less than No. 12 gage (0.105 in. [2.69 mm]) and securing to the material with a heavy wire such as No. 9 gage (0.148 in. [3.76 mm]), or by die stamping the identifying marks into the material with characters 1⁄2 in. [12.7 mm] high and a minimum of 1⁄32 in. [0.79 mm] deep.
The galvanized coating has a minimum thickness of 4 mils (0.004”) for material that is ¼” thick. There will be some variation in the measurement over the surface of an article, so for engraving, deeper is better to prevent the cut from filling with zinc.
Please let me know if you have any additional questions or if I can be of additional assistance.
Q: "What does galvanizing add to the thickness of steel?”
Answer:
Galvanizing is an alloy that forms when the prepared steel surface is immersed in a bath of molten zinc. The galvanizing bath temperature is @ 840-845 degrees F, and the steel must remain in the bath until the thickest part of the steel fabrication reaches the temperature of the bath. When the steel core reaches the bath temperature the reaction stops and the steel has been galvanized.
The coating reaction begins while the steel is heating and stops when it reaches bath temperature, so thin or light structures reach temperature faster than thick or heavy sections. Consequently the coating thickness will be heavier or thicker on the heavy material and the ASTM specifications reflect this difference in coating weight, based on the thickness of different materials.
There are other factors that can affect the coating, such as unusual steel chemistry, but I will keep the discussion for this based on standard mild carbon structural material.
Please refer to ASTM A123 for complete details on the minimum thickness values for different materials. The categories are Structural Shapes and Plates, Strip and Bar, Pipe and Tubing, Wire, and Reinforcing Bar. For the purposes of this question, I will discuss structural shapes and plates. The Steel Thickness Range (Measured), in. (mm) is as shown below in μm and mils.
<1⁄16 (<1.6) 1⁄16 to <1⁄8 (1.6 to <3.2) 1⁄8 to 3⁄16 (3.2 to 4.8) >3⁄16 to <1⁄4 (>4.8 to <6.4) ≥1/4 (≥6.4)
45 65 75 85 100
1.8 mils 2.6 mils 3.0 mils 3.3 mils 3.9 mils
The values in micrometres (μm) are based on the Coating Grade. The other values are based on conversions using the following formulas: mils = μm x 0.03937; oz/ft2 = μm x 0.02316; g/m 2 = μm x 7.067.
Based on this information, steels with a base metal thickness from 1/8” to ¼” will have a minimum coating thickness of 3 to 4 mils. (.003” to .004”) It is important to note that these are MINIMUM coating thicknesses. It won’t be unusual for the coating to exceed these minimums and be 4 to 6 mils in thickness.
Q: "Is there a way to predict the expected service life of galvanized steel in industrial use?”
Answer:
A. As you might expect, the answer depends on several factors, including the environment, the application and the thickness of coating.
The American Galvanizers Association (AGA) published the Time To First Maintenance Chart in 2010. ‘Time to first maintenance’ is defined as 5% rusting of the steel surface. According to the chart, in a typical industrial environment with the ASTM A 123 minimum 3.9 mil zinc coating, it will be 72 years before any maintenance is required. The coating thickness of hot-dip galvanized steel is usually between 4 and 5 mils. This means that the first need for surface maintenance may not occur for more than 90 years. Then you might need to do a little touch-up. The chart is available for download from the AGA Web site, http://www.galvanizeit.org/aga/resources/aga-publications/.
The Time To First Maintenance Chart was developed using a corrosion prediction model called the Zinc Coating Life Predictor (ZCLP). The program performs calculations based on models developed using statistical methods, neural network technology and an extensive worldwide corrosion database.
You can use the ZCLP yourself.Just click here [http://www.galvinfo.com:8080/zclp/] and put in your specific parameters. Obviously, atmospheric levels of airborne salinity, precipitation, relative humidity, sulfur dioxide, and temperature influence actual corrosion rates in specific geographic locations.
Annual average temperatures, precipitation and humidity for your state or city can be found at www.currentresults.com. You may have to estimate airborne SO2 and salinity levels for your site, unless you have actual test results. The following information might help in your estimating. According to the European Community’s LIFECON project (2003), concentrations of airborne sulfur dioxide from 60 mg/m2/day and up are considered Industrial, with 10 to 80 mg/m2/day being Light Industrial, and below 10 mg/m2/day being benign. Similarly, 60 or more mg/m2/day of airborne salinity is considered a Marine environment, with 15 to 60 mg/m2/day being Light Marine and below 15 mg/m2/day being benign. For coating thickness, since hot dip galvanizing is usually between 4 and 5 mils thick, I would go with 4.5. (Be sure to select ‘mils’ from the dropdown.)
Plug in the parameters that best describe your project and see what the ZCLP has to say. Please note that some of the parameters have drop-down selections. Make sure you choose the right ones. It will affect the results.
So, to answer your question, yes, there is a way to estimate the expected service life of galvanized steel in industrial applications. Keep in mind, the ZCLP’s predictions are estimates, not guarantees. Are they accurate? Most likely, but the fact is that hot dip galvanized steel lasts so long we may not be around to find out!
Q: "In reference to G140, the minimum figure (for coating mass) is 1.40oz/ft2. Is there a nominal figure?” "
Answer:
G-140 is covered by the specification ASTM A653/A653M-09a, titled Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process.
This specification covers steel sheet, zinc-coated (galvanized) or zinc-iron alloy-coated (galvannealed) by the hot dip process in coils and cut lengths. The product is produced in various zinc or zinc-iron alloy-coating weights [masses] or coating designations as shown in Table 1 of the standard.
Coating designation G140 does indeed indicate a coating mass of 1.40 oz/ft2 however, it is important to note that the measurement is for the total of both sides. Further, the 1.40 oz/ft2 is a triple spot test (TST) for the total of both sides. The coating thickness of a single side of G140 (TST) is approx 1.19 mils (1.4 oz/ft2 x 1.7 mils /2 = 1.19 mils)
The coating designation is the term by which the minimum triple spot, total both sides coating weight [mass] is specified. Because of the many variables and changing conditions that are characteristic of continuous hot-dip coating lines, the zinc or zinc-iron alloy coating is not always evenly divided between the two surfaces of a coated sheet; nor is it always evenly distributed from edge to edge. However, the minimum triple-spot average coating weight (mass) on any one side shall not be less than 40 % of the single-spot requirement.
The triple spot test (TST) for one side of G140 is 0.48 oz/ft2 and the single spot test as a total of both sides is 1.2 oz/ft2.
Use the following relationships to estimate the coating thickness from the coating weight [mass]:
1 oz/ft2 coating weight = 1.7 mils coating thickness,
7.14 g/m2 coating mass = 1 μm coating thickness.
Use the following relationship to convert coating weight to coating mass:
1 oz/ft2 coating weight = 305.15 g/m2 coating mass.
As it is an established fact that the atmospheric corrosion resistance of zinc or zinc-iron alloy-coated sheet products is a direct function of coating thickness (weight (mass)), the selection of thinner (lighter) coating designations will result in almost linearly reduced corrosion performance of the coating. For example, heavier galvanized coatings perform adequately in bold atmospheric exposure whereas the lighter coatings are often further coated with paint or a similar barrier coating for increased corrosion resistance. Because of this relationship, products carrying the statement “meets ASTM A653/A653M requirements” should also specify the particular coating designation.
Although the corrosion rate can vary considerably depending on the environmental factors, it is well known that, in most instances, the life of the zinc coating is a linear function of coating mass for any specific environment. That means, to achieve twice the life for any specific application, the user should order twice the coating mass.
Examples: A G60 coating mass will exhibit approximately twice the life of a G30 coating mass. A G90 coating mass will exhibit about 50% longer life than a G60 coating mass.
Products that are hot dip galvanized after fabrication are galvanized to ASTM A123, “Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products.”
This specification covers both unfabricated products and fabricated products, for example, assembled steel products, structural steel fabrications, large tubes already bent or welded before galvanizing, and wire work fabricated from uncoated steel wire. A123 specifications differ from continuous galvanizing specs in that the coating thickness or mass designation is not measured as a distribution between both sides of an article. If the specification calls for 2 oz/ft2 (3.4 mils) the coating will be as specified anywhere on the article. The total for both sides would be a minimum of 6.8 mils
I hope that this answers your question, and clarifies the mystery of “G” designations for sheet galvanized products.
Q: "What would be the best galvanized coating (i.e.G90, G140) for metal framing on an exterior wall of a high rise building on the oceanfront in Virginia Beach VA? What would be the life of your suggestion? Thank you in advance."
Answer:
Your question warrants a multi-part answer, which I will attempt to describe below.
G-90 or G-140 are both covered by ASTM A653 “Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process”. This specification covers steel sheet, zinc-coated (galvanized) or zinc-iron alloy-coated (galvannealed) by the hot dip process in coils and cut lengths. These are mill galvanized materials and are limited to coil stock that is then cut to length and formed. This is the same material that HVAC duct work is formed from.
The weight of coating is specified by the “G” designation and describes the weight or mass of the coating as the Total Both Sides, oz/ft2 of zinc. G-90 is 0.9 oz/ft2 of zinc, and G-140 is 1.4 oz/ ft2 of zinc. But again this is the TOTAL of both sides of the galvanized article. So, a G-90 coating actually has only 0.45 oz/ ft2 of zinc on any given surface.
The accepted formula for coating mass is 1 oz/ ft2 of zinc = 1.7 mils of thickness. This means that a G-90 coating has 0.45 oz/ ft2 x 1.7 mils = .765 mils of thickness on any given surface. G-140 has 0.7 oz/ft2 x 1.7 mils = 1.19 mils of thickness on any given surface. (However since the coating distribution is not exact, the coating can actually be a 60/40 distribution from side to side.)
Zinc (galvanizing) service life is a linear measure, depending upon the type of environment and the thickness of the coating. Once you know the thickness of the zinc coating and the type of exposure, the service life of the coating is easily predictable.
Two mils of zinc will last exactly twice as long as one mil of zinc in the same exposure conditions. Therefore a G-140 coating will last about 36% longer than a G-90 coating for the same application.
For a more comprehensive comparison of sheet specifications and hot dip galvanizing after fabrications please click the following link;
If you are describing exposed structural framing on the exterior wall (such as roof screen support framing or mechanical supports) you would be better suited with hot dip galvanizing after fabrication to ASTM A123 Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products. This specification covers the requirements for zinccoating (galvanizing) by the hot-dip process on iron and steelproducts made from rolled pressed and forged shapes, castings,plates, bars, and strips.
The coating thickness is also measured in terms of mass or oz/ft2, but is not a total of both sides. For example, a piece of steel with a section thickness of ¼” or greater will yield a MINIMUM coating thickness of 2.3 oz/ft2 or 3.9 mils. The coating thickness on any surface will be at least 5 times the thickness and service life of a G-90 coating. Once again, the service life will be a function of the environment that the coating is exposed to. The link below is a service life chart showing the coating life versus coating thickness for a hot dip galvanized ASTM A123 coating.
A 3.5 to 3.9 mil ASTM A123 galvanized coating, in an industrial environment can be expected to provide between 60 and 70 years of maintenance free service life.
Q: "Is it possible to have too much galvanized coating on a steel part?"
Questions like this really get the Professor's gears turning. He always has the answer or knows where to find it. The answer to this question comes from his pal Dr. Galv at the American Galvanizers Association (AGA).
Answer:
"The specifications for hot-dip galvanizing do not set a maximum coating thickness, so there is no limit on the thickness of the hot-dip galvanized coating. However, there is a requirement the coating meet the “intended use of the product,” so if there are drips and runs in an area that will cause a fit problem in the part’s final assembly, then the drips and runs must be ground smooth for the part to be acceptable.
Although the specification does not limit the coating thickness, there are some practical limits on how thick the galvanized coating should be. When the coating exceeds 10 mils or 250 microns, the coating experiences significant stress during the cool down from galvanizing temperatures. The thermal contraction of steel progresses at a different rate than the thermal contraction of the intermetallic layers of the galvanized coating. This difference in contraction rates induces a stress at the interface between layers of the coating. The stress level induced depends on the thickness of the intermetallic layers. At normal coating thickness levels, the stress is minimal and there is no issue with the coating adherence. However, if the coating is thick then the stress can be significant, and this makes the coating susceptible to flaking."
Download the complete answer, with photographic illustration, from the AGA now at www.galvanizeit.org
