Frequently Asked Questions
Frequently Asked Questions
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The following is a comprehensive guide to frequently asked questions about hot-dip galvanizing. Click on any question to jump to the answer further down the page. If your question isn't listed here, use the search bar at the top right or contact the AGA for help.
1. How does galvanizing protect steel from corrosion?
In the galvanizing process, zinc provides a barrier between the steel and the atmosphere, preventing moisture and corrosive elements from reaching the steel. Zinc is anodic to steel, so it corrodes first, protecting the steel.
2. What are the steps in the galvanizing process?
The steps include:
- Pre-inspection: Examining the steel to ensure proper design and venting.
- Cleaning: Immersing in caustic solutions and acid baths to remove impurities.
- Galvanizing: Dipping the clean steel in molten zinc at 850°F to form a coating.
- Final inspection: Visual and thickness inspections ensure quality.
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3. How does the cost of hot-dip galvanizing compare to other corrosion protection systems, such as paints?
Hot-dip galvanizing has comparable initial costs to paint systems and, generally, lower life-cycle costs, making it cost-effective for the long term.
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4. How long can I expect my galvanized steel project to last in service?
Hot-dip galvanized steel can last over 70 years under certain conditions. For more details, check the service-life chart.
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5. Does the galvanized steel coating of zinc resist abrasion?
Yes. The intermetallic layers formed during the galvanizing process are harder than the steel and resist abrasion.
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6. What causes wet storage stain and how can it be prevented?
Wet storage stain occurs when galvanized steel is stored without sufficient airflow. The zinc forms zinc hydroxide layers. Prevent it by storing indoors or allowing air circulation. It can generally be cleaned with a nylon brush or cleaner.
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7. Why do galvanized steel appearances differ from project to project and galvanizer to galvanizer and is there any difference in the corrosion protection offered by the different appearing coatings?
Different steel chemistries primarily determine coating appearance. Additives used by galvanizers may result in shiny, spangled, or matte finishes, but these do not affect corrosion protection.
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8. Can galvanized steel in service withstand high temperatures for long periods of time?
Yes. It performs well under temperatures below 390°F (200°C) constantly and intermittently above 390°F.
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9. Why would you want to paint over galvanized steel?
Painting over galvanized steel (duplex coatings) provides enhanced corrosion protection, ease of repainting, and effective marking systems. It also extends the life of older zinc coatings.
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10. What are the specifications governing hot-dip galvanized steel?
Specifications include ASTM A 123/A 123M for fabricated products, ASTM A 153/A 153M for fasteners, and ASTM A 767/A 767M for reinforcing steel.
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11. Isn’t galvanizing more expensive than paint?
Initially, the cost is often comparable or even less than paint systems. Additionally, galvanizing requires significantly less maintenance over time, reducing overall life-cycle costs.
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12. What if the article to be galvanized is larger than the dimensions of the galvanizer’s kettle? Can it still be galvanized?
Yes. Large items can be galvanized through progressive dipping, commonly known as "double dipping."
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13. What is the difference between hot-dip galvanized fasteners and zinc-plated fasteners?
Hot-dip galvanized fasteners have thicker zinc coatings and are suitable for all environments. Zinc-plated fasteners are best for indoor use and do not offer the same level of protection.
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14. How long will hot-dip galvanizing protect my steel from corrosion?
The lifespan of galvanized steel depends on coating thickness and environmental conditions. It can last 100-150 years in rural environments and 50-100 years in harsher conditions.
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15. Are there any special design and fabrication considerations required to make steel ready for hot-dip galvanizing?
Yes. Design considerations include ensuring proper flow of cleaning solutions and zinc. Details are available in the AGA publication "The Design of Products to be Hot-dip Galvanized After Fabrication."
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16. Where are galvanized steel products used?
Galvanized steel is used in various industries, including construction, transportation, utilities, and more.
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17. What are the size limitations of steel that is to be galvanized?
The process can accommodate various sizes, contingent on the dimensions of the galvanizer’s kettle. A list of galvanizers and their kettle sizes is available online.
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18. What types of products can be galvanized?
A diverse range of fabrications are galvanized annually. Click here to see a list of these products.
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19. Sometimes, the galvanized coating is shinier in some places than others. Why is that?
The coating appearance can vary due to different layers of zinc. The protective efficacy remains unaffected by these visual differences.
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20. Is the zinc coating’s thickness consistent over the entire piece?
Coating thickness can vary based on steel characteristics but is standardized by ASTM requirements.
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21. What can I do to minimize possible warping & distortion? Is it possible to determine — prior to galvanizing — which pieces might be prone to this occurrence?
Minimize warping through careful design using uniform thicknesses and symmetrical shapes. Temporary bracing may also help maintain shape.
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22. Can I paint right over the galvanized coating? If so, what procedure should be followed?
Yes. Proper surface preparation as specified in ASTM D 6386 ensures a quality paint system over galvanized steel.
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23. How much weight will my material gain from galvanizing?
Typically, the weight increases by about 3.5% due to zinc coating, though this can vary based on multiple factors.
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24. Are slip-critical connections a concern when the steel is to be galvanized?
For slip-critical connections, galvanized parts should be brushed, abrasive blasted, or painted with zinc-silicate paint to improve surface roughness and slip factor.
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25. I’m interested in specifying hot-dip galvanizing for reinforcing steel. Are there any concerns with fabricating rebar after galvanizing?
Rebar can be bent after galvanizing with minimal coating damage. ASTM A 767 provides guidelines for maximum bend diameters.
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26. Can I specify how much zinc to put on the steel?
No, zinc coating thickness is determined by the steel’s chemistry and surface condition. Prolonged immersion in zinc may cause brittle coatings.
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27. What does it mean to “double-dip” steel?
“Double-dipping” refers to sequential immersion of large steel items in molten zinc, not to creating thicker coatings.
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28. What is the reason for incorporating venting & drainage holes into a project’s design?
Venting allows trapped air and gases to escape, and drainage lets cleaning solutions and zinc flow freely, ensuring complete coating.
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29. If I stitch-weld, will there be uncoated areas after galvanizing?
Gas release from gaps in stitch-welded joints can prevent zinc coating formation. Leaving gaps allows for full coating.
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30. What is “white rust” and how can it be avoided?
“White rust” is a mild corrosion product known as wet storage stain. Prevent it by stacking properly and avoiding exposure to humid environments.
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31. Is there a way to provide for intentionally ungalvanized areas?
Yes, contact your galvanizer for masking techniques, though total efficacy is not guaranteed.
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32. Is there any environmental impact when the zinc coating sacrificially corrodes? Is zinc a safe metal?
Zinc is environmentally safe and essential for life. Its corrosion products do not harm the environment.
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33. Should I be concerned when galvanized steel comes in contact with other metals?
Zinc sacrifices itself to protect other metals. Use non-conductive materials like rubber or plastic for insulation.
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34. What is the difference between hot-dip galvanizing after fabrication and continuous hot-dip galvanized sheet?
After-fabrication galvanizing is manual, involving the immersion of structural steel. Continuous sheet galvanizing involves high-speed processing of steel sheets. Both processes protect against corrosion, with after-fabrication galvanized steel usually used for exterior applications and galvanized sheet suitable for interior unless painted.
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35. What is a G90 or A60 coating?
G90 and A60 are grades of galvanized sheet to ASTM standards. G90 has 0.90 oz/sq. ft. of zinc, and A60 has 0.30 oz/sq. ft. per side.
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36. Is a salt spray test in a laboratory appropriate to estimate the corrosion rate of zinc coated steel?
No. Salt spray tests do not reflect real-world conditions as they wash away zinc corrosion products, inflating the corrosion rate. Real-world performance involves cyclic wetting and drying.
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37. Can galvanized steel in service withstand high temperatures for long periods of time?
Yes. It can withstand temperatures below 390°F (200°C) constantly and perform well in intermittent exposure above that.
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38. Can I specify how much zinc to put on the steel?
No, zinc thickness is determined by steel chemistry and conditions. Excess immersion can cause brittleness in the coating.
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39. What is “cold” galvanizing?
There is no cold galvanizing. The term often refers to zinc-rich paint, which does not form the metallurgical bond like true galvanizing.
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Stainless steel vs. galvanized
I think the lights themselves would probably handle the 25' span without help—they are very heavy-duty cable. What I'm trying to do is use a "main line" to span about 200', and then have 25' zigzag segments between the house and the "main line." I'm saying "main line" because I have no idea what this would be called in architectural/civil engineering terminology... it's not a guy wire, but that flavor. It may not work, and I might have to install some posts instead. I'm just wary of the posts because they will be more work, cost more, could be more unsightly, and might destabilize my retaining wall (since the below-ground portion of the posts would be levering against the retaining wall unless they are very deep). Decided to use 1/4' SS eye bolts to anchor into the house at the second-floor girder. Not sure if this will work well, but again, the load isn't high, and nobody's life is at stake. Was considering carabiners at all connection points but decided to go with turnbuckles just in case. Crap, just realized I didn't order the clamps. Trying to decide between simplex and duplex clamps—any advice? Simplex seems sufficient, but duplex is more bang for the buck. Simplex
Duplex
Thanks, guys. Greg's final comment notwithstanding, I went with stainless steel 3/16 uncoated at twice the price of 1/8 galvanized, vinyl-coated. The biggest issue I have with the galvanized is that I have no idea if it's "quality"—hot-dipped vs. electroplated, etc. Stainless steel has the same problem as I don't always know the specific SS grade, but I hope the quality is there. This isn't a huge project, so either way, the expense is minimal—I just want to "do it right".
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