FAQ

The following is a list of frequently asked questions about hot-dip galvanizing. Click on the question to be taken to the answer listed further down on the page. If you do not see your question listed here, try searching using the links above, the search engine on the top right of the page, or contact the AGA for assistance.

  1. How does galvanizing protect steel from corrosion?
  2. What are the steps in the galvanizing process?
  3. How does the cost of hot-dip galvanizing compare to other corrosion protection systems, such as paints?
  4. How long can I expect my galvanized steel project to last in service?
  5. Does the galvanized steel coating of zinc resist abrasion?
  6. What causes wet storage stain and how can it be prevented?
  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?
  8. Can galvanized steel in service withstand high temperatures for long periods of time?
  9. Why would you want to paint over galvanized steel?
  10. What are the specifications governing hot-dip galvanized steel?
  11. Isn’t galvanizing more expensive than paint?
  12. What if the article to be galvanized is larger than the dimensions of the galvanizer’s kettle? Can it still be galvanized?
  13. What is the difference between hot-dip galvanized fasteners and zinc-plated fasteners?
  14. How long will hot-dip galvanizing protect my steel from corrosion?
  15. Are there any special design and fabrication considerations required to make steel ready for hot-dip galvanizing?
  16. Where are galvanized steel products used?
  17. What are the size limitations of steel that is to be galvanized?
  18. What types of products can be galvanized?
  19. Sometimes the galvanized coating is shinier in some places than others. Why is that?
  20. Is the zinc coating’s thickness consistent over the entire piece?
  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?
  22. Can I paint right over the galvanized coating? If so, what procedure should be followed?
  23. How much weight will my material gain from galvanizing?
  24. Are slip-critical connections a concern when the steel is to be galvanized?
  25. I’m interested in specifying hot-dip galvanizing for reinforcing steel. Are there any concerns with fabricating rebar after galvanizing?
  26. Can I specify how much zinc to put on the steel?
  27. What does it mean to ‘double-dip’ steel?
  28. What is the reason for incorporating venting & drainage holes into a project’s design?
  29. If I stitch-weld, will there be uncoated areas after galvanizing?
  30. What is ‘white rust’ and how can it be avoided?
  31. Is there a way to provide for intentionally ungalvanized areas?
  32. Is there any environmental impact when the zinc coating sacrificially corrodes? Is zinc a safe metal? 
  33. Should I be concerned when galvanized steel comes in contact with other metals?
  34. What is the difference between hot-dip galvanizing after fabrication and continuous hot-dip galvanized sheet?
  35. What is a G90 or A60 coating?
  36. Is a salt spray test in a laboratory appropriate to estimate the corrosion rate of zinc coated steel?
  37. Can galvanized steel in service withstand high temperatures for long periods of time?
  38. Can I specify how much zinc to put on the steel?
  39. What is ‘cold’ galvanizing?

 

1. How does galvanizing protect steel from corrosion?


Zinc metal used in the galvanizing process provides an impervious barrier between the steel substrate and corrosive elements in the atmosphere. It does not allow moisture and corrosive chlorides and sulfides to attack the steel. Zinc is more importantly anodic to steel – meaning it will corrode before the steel, until the zinc is entirely consumed.

2. What are the steps in the galvanizing process?

There are four steps:

  • Pre-inspection – where the fabricated structural steel is viewed to ensure it has, if necessary, the proper venting and draining holes, bracing, and overall design characteristics necessary to yield a quality galvanized coating
  • Cleaning – steel is immersed in a caustic solution to remove organic material such as grease and dirt, followed by dipping in an acid bath (hydrochloric or sulfuric) to remove mill scale and rust, and finally lowered into a bath of flux that promotes zinc & steel reaction and retards further oxidation of the steel… (steel will not react with zinc unless it is perfectly clean)
  • Galvanizing – the clean steel is lowered into a kettle containing 850 F molten zinc where the steel and zinc metallurgically react to form three zinc-iron intermetallic layers and one pure zinc layer
  • Final inspection – the newly galvanized steel is sight-inspected (if it looks good, it is), followed up by measurement of coating thickness with a magnetic thickness gauge

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3. How does the cost of hot-dip galvanizing compare to other corrosion protection systems, such as paints?

When compared with paint systems, hot-dip galvanizing after fabrication has comparable initial application costs and, almost always, lower life-cycle costs. In fact, the lower life-cycle costs of a hot-dip galvanized project make galvanizing the smart choice for today and tomorrow.

4. How long can I expect my galvanized steel project to last in service?

Hot-dip galvanized steel resists corrosion in numerous environments extremely well. It is not uncommon for galvanized steel to last more than 70 years under certain conditions. To get a good idea of how long your project will last, see the service-life chart.

5. Does the galvanized steel coating of zinc resist abrasion?

The three intermetallic layers that form during the galvanizing process are all harder than the substrate steel and have excellent abrasion resistance.

6. What causes wet storage stain and how can it be prevented?

Zinc on newly galvanized steel is very reactive and wants to form zinc oxide and zinc hydroxide corrosion products that eventually become the stable zinc carbonate. When galvanized steel is tightly stacked or stored in wet boxes that don’t allow for free flowing air, the zinc forms excessive layers of zinc hydroxide, otherwise known as wet storage stain. Most wet storage stain can be easily removed with a cleaner or nylon brush. To prevent wet storage stain, store galvanized steel indoors or block it so that there is ample free flowing air between each galvanized article.

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?

The steel chemistry is the primary determinant of galvanized coating thickness and appearance. Continuously cast steel produced by the steel companies has a wide variety of chemistries, thus the different coating appearances. There are several different additives galvanizers may put in their zinc kettle to enhance the coating appearance by making it shiny, spangled or matte gray. The appearance of the coating (matte gray, shiny, spangled) does nothing to change the corrosion protection of the zinc coating.

8. Can galvanized steel in service withstand high temperatures for long periods of time?

Constant exposure to temperatures below 390 F (200 C) is a perfectly acceptable environment for hot-dip galvanized steel. Good performance can also be obtained when hot-dip galvanized steel is exposed to temperatures above 390 F (200 C) on an intermittent basis.

9. Why would you want to paint over galvanized steel?

Called duplex coatings, zinc and paint in combination (synergistic effect) produce a corrosion protection approximately 2X the sum of the corrosion protection that each alone would provide. Additionally, duplex coatings make for easy repainting, excellent safety marking systems, and good color-coding. Painting over galvanized steel that has been in service for many years also extends the life of the zinc coating.

10. What are the specifications governing hot-dip galvanized steel?

Structural steel (plate, wide-flange beams, angles, channels, pipe, tubing) are galvanized to ASTM A 123/A 123M. Fasteners and small parts that fit into a centrifuging basket are galvanized to ASTM A 153/A 153M. Reinforcing steel is galvanized to ASTM A 767/A 767M.

11. Isn’t galvanizing more expensive than paint?

Depending on the product mix, square feet per ton, and condition of the steel surface, galvanizing is often less expensive on an initial cost basis. However, as with any purchase, the lifetime costs should be considered when making a project decision on the corrosion prevention system to utilize. And, with galvanizing, the life-cycle cost, i.e. the cost per year to maintain, is almost always less than a paint system. Paint systems require maintenance, partial repainting and full repainting several times over a 30-year project life. The costs can be staggering, making the decision to paint a costly one in the long run.

12. What if the article to be galvanized is larger than the dimensions of the galvanizer’s kettle? Can it still be galvanized?

Galvanizers can progressively dip such a fabrication or article of steel. They dip one half in the molten zinc bath, remove it, turn it around or over and immerse the other half in the zinc. This method is often erroneously referred to as ‘double dipping’.

13. What is the difference between hot-dip galvanized fasteners and zinc-plated fasteners?

Hot-dip fasteners generally have about 10 times as much zinc on the surface and are suitable for use in all exterior and interior applications. Zinc-plated fasteners will provide a disappointing performance if used outside, especially when used to connect hot-dip galvanized structural steel members.

14. How long will hot-dip galvanizing protect my steel from corrosion?

The corrosion rate of zinc and how long it will provide protection is a function of the coating thickness and the amount of corrosive elements in the atmosphere. For example, in rural settings where there is less automotive/truck exhaust and plant emissions, galvanized steel can easily last for 100 – 150 years without maintenance. Industrial and marine locations contain significantly more aggressive corrosion elements such as chlorides and sulfides and galvanized steel may last for 50 – 100 years in those cases. The relationship between coating thickness and atmospheric conditions is contained in a popular graph developed by the AGA. Please see the publication Hot-Dip Galvanizing for Corrosion Protection: A Specifier’s Guide.

15. Are there any special design and fabrication considerations required to make steel ready for hot-dip galvanizing?

Yes. Specifically, fabricated steel must allow for easy flow of the cleaning chemicals and molten zinc metal over and through it. This means that gussets must be cropped, holes put in the proper location for draining and venting of zinc from tubular configurations, weld flux removed, overlapping surfaces must be seal-welded, and light gauge material temporarily braced. The details of design and fabrication are contained in the AGA publication The Design of Products to be Hot-dip Galvanized After Fabrication.

16. Where are galvanized steel products used?

First of all, the variety of things galvanized is broad. Structural steel (angles, channels, wide-flange beams, I-beams, H-beams), grating, expanded metal, corrugated sheets, wire, cables, plate, castings, tubing, pipe, bolts & nuts. The industries that utilized hot-dip galvanized steel range from bridge & highway (reinforcing steel for decks and column concrete, girders, stringers, light and signposts, guardrail, fencing), water & wastewater treatment plants (walkway grating/expanded metal, handrails) architectural (facades, exposed structural steel, lentils), parking garages (reinforcing steel for concrete decks, exposed structural steel columns and barriers), pulp & paper plants (structural steel, walkways, handrail), OEMs (motor housings, electrical cabinets, frames, heat exchanger coils), electrical utilities (transmission towers, distribution poles, substations, wind turbine poles), communication (cell towers), rail transportation (poles, switchgear, miscellaneous hardware), chemical/petro-chemical (pipeline hardware, manufacturing buildings, storage tanks, walkways), recreation (boat trailers, stadiums, arenas, racetracks), and many more.

17. What are the size limitations of steel that is to be galvanized?

The hot-dip galvanizing process can accommodate various different shapes and sizes of steel. Kettle sizes vary in dimensions from one galvanizer to the next. You can view the online listing of all the galvanizers in North America and their kettle sizes.

18. What types of products can be galvanized?

Numerous different fabrications for a variety of applications are galvanized each year. To view a list of the different types of products that have been hot-dip galvanized click here.

19. Sometimes, the galvanized coating is shinier in some places than others. Why is that?

The galvanized coating appearance may either be bright and shiny resulting from the presence of an outer layer of pure zinc, or duller, matte gray as the result of the coating’s intermetallic layers being exposed. Performance is not affected. Coating appearance depends on the amount of zinc in the coating.
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20. Is the zinc coating’s thickness consistent over the entire piece?

Coating thickness depends on the thickness, roughness, chemistry, and design of the steel being galvanized. Any or all of these factors could produce galvanized coatings of non-uniform thickness. Members of the American Galvanizers Association galvanize to ASTM standards, which define minimum average coating thickness grades for various material categories.
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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?

Minimizing potential warpage and distortion is easily done in the project’s design stages by selecting steel of equal thicknesses for use in every separate subassembly that is to be hot-dip galvanized, using symmetrical designs whenever possible, and by avoiding the use of light-gage steel (<1/16” / 1.6 mm). Some structures may benefit from the use of temporary bracing to help maintain their shape and/or alignment.
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22. Can I paint right over the galvanized coating? If so, what procedure should be followed?

Galvanized coatings can be easily and effectively painted, not only for aesthetics but also to extend the structure’s service life. The age and extent of weathering of the galvanized coating dictate the extent of surface preparation required to produce a quality paint system over galvanized steel. ASTM D 6386, Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting, should be consulted for suggested surface preparation methods for galvanized coatings of varying ages.

23. How much weight will my material gain from galvanizing?

As an average, the weight of the article will increase by about 3.5% due to zinc picked up in the galvanizing process. However, that figure can vary greatly based on numerous factors. The fabrication’s shape, size, and steel chemistry all play a major role, and other factors like the black weight, the different types of steel that get welded together, and the galvanizing bath chemistry can also have an effect.

24. Are slip-critical connections a concern when the steel is to be galvanized?

When galvanized parts are used for slip-critical connections, they must either be brushed, abrasive blasted, or painted with zinc-silicate paint to increase the surface roughness and, thus, the 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 is commonly fabricated after galvanizing. In order to minimize the possibility for coating damage, avoid bending the rebar at a radius of more than 8 times its radius. ASTM A 767, Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement, has a table that provides maximum bend diameters for various-sized rebar.
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26. Can I specify how much zinc to put on the steel?

No, the steel chemistry and surface condition are the primary determinants of zinc coating thickness. Leaving the steel in the molten zinc a little longer than optimal may have one of two effects: 1) it may increase the coating thickness, but only marginally; 2) it may significantly increase the coating thickness and cause a brittle coating.

27. What does it mean to “double-dip” steel?

“Double-dipping” is the progressive dipping of steel that is too large to fit into the kettle in a single dip. Double-dipping cannot be used to produce a thicker hot-dip galvanized coating.
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28. What is the reason for incorporating venting & drainage holes into a project’s design?

The primary reason for vent holes is to allow otherwise trapped air and gases to escape; the primary reason for drain holes is to allow cleaning solutions and molten zinc metal to flow entirely into, over, and throughout the part, and then back into the tank or kettle.
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29. If I stitch-weld, will there be uncoated areas after galvanizing?

When stitch-welding is used, there is a possibility of gas release between gaps, which will prevent the galvanized coating from forming in these areas. By leaving at least a 3/32” (2.4 mm) gap between the contacting surfaces, gases are allowed to escape and cleaning solutions and molten zinc are allowed to flow in between the surfaces for a complete and uniform coating.
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30. What is “white rust” and how can it be avoided?

“White rust” is the term mistakenly applied to wet storage stain, which actually is a milder corrosion product than white rust. Wet storage stain can be avoided by properly stacking freshly galvanized articles, avoiding unprotected exposure to wet or humid climates, or by using a surface passivation treatment after galvanizing. Wet storage stain typically weathers away once the part is in service. (True “white rust” is most commonly associated with galvanized cooling towers.)
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31. Is there a way to provide for intentionally ungalvanized areas?

Yes, but because masking or stop-off materials may not be 100% effective, contact your galvanizer for suggestions.
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32. Is there any environmental impact when the zinc coating sacrificially corrodes? Is zinc a safe metal?

There are no known studies to suggest zinc corrosion products cause any harm to the environment. Zinc is a naturally occurring element (25th most abundant element in the earth), and necessary for all organisms to live. It is a recommended part of our diet (RDA 15 mg) and necessary for reproduction. It is used in baby ointments, vitamins, surgical instruments, sunscreens and cold lozenges.

33. Should I be concerned when galvanized steel comes in contact with other metals?

Zinc is a noble metal and will sacrifice itself (i.e. corrode, give up its electrons and create a bi-metallic couple) to protect most metals. So, it is recommended to insulate galvanized steel so that it doesn’t come in direct contact with dissimilar metals. Rubber or plastic, both non-conductive, are often used to provide this insulation.

34. What is the difference between hot-dip galvanizing after fabrication and continuous hot-dip galvanized sheet?

The process steps are similar but the production equipment is very different. After fabrication galvanizing is a more manual process where structural steel (fabricated plate, wide-flange beams, angles, channels, tube, pipe, fasteners) is suspended by wire, chain or hook from crane hoists and immersed in the cleaning solutions and zinc. Continuous sheet galvanizing involves uncoiling sheet, passing it through the cleaning steps and molten zinc bath at speeds up to 500 feet per minute, drying and recoiling. The uses of after-fabrication galvanized steel are usually exterior in nature because the zinc coating is relatively thick (3.0 – 6 mils, 75 – 150 microns, 1.7 to 3.6 oz/sq. ft.) and will protect steel from corrosion in most atmospheric conditions for 50 to 100 years. Galvanized sheet is suitable for interior applications because of the relatively thin coating (0.45 oz on each side), unless it is painted after galvanizing.

35. What is a G90 or A60 coating?

G90 is a grade of galvanized sheet produced to ASTM A653. It has 0.90 oz/sq. ft. of zinc overall or 0.45 oz/sq. ft. per side. A60 is also a grade, has 0.30 oz/sq. ft. per side, and has been annealed after galvanizing to produce a surface that promotes good adhesion of paint.

36. Is a salt spray test in a laboratory appropriate to estimate the corrosion rate of zinc coated steel?

In order for zinc to develop its protective patina of zinc carbonate that is very stable and non-reactive, it requires a wetting and drying cycle like that produced by nature. Salt spray tests keep the zinc wet and essentially wash the zinc corrosion products off as they develop, inflating the corrosion rate of zinc. This lab test is not reflective of real-world performance of zinc coatings.

37. Can galvanized steel in service withstand high temperatures for long periods of time?

Constant exposure to temperatures below 390 F (200 C) is a perfectly acceptable environment for hot-dip galvanized steel. Good performance can also be obtained when hot-dip galvanized steel is exposed to temperatures above 390 F (200 C) on an intermittent basis.

38. Can I specify how much zinc to put on the steel?

No, the steel chemistry and surface condition are the primary determinants of zinc coating thickness. Leaving the steel in the molten zinc a little longer than optimal may have one of two effects: 1) it may increase the coating thickness, but only marginally; 2) it may significantly increase the coating thickness and cause a brittle coating.

39. What is “cold” galvanizing?

There is no such thing as cold galvanizing. The term is often used in reference to painting with zinc-rich paint. Galvanizing by definition means a metallurgical reaction between zinc and iron to create a bond between the zinc and the steel of approximately 3600 psi. There is no such reaction when zinc-rich paints are applied and the bond strength is only several hundred psi.