Hot Dip Galvanizing FAQs

1. How Does Hot Dip 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?
4. How long can I expect my galvanized steel project to last in service?
5. Can galvanized steel in service withstand high temperatures for long periods of time?
6. Is the zinc coating's thickness consistent over the entire piece?
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 I specify how much zinc to put on the steel?
9. What is the reason for incorporating venting & drainage holes into a project's design?
10. What is "white rust" and how can it be avoided?
11. Should I be concerned when galvanized steel comes in contact with other metals?
12. Is there any environmental impact when the zinc coating sacrificially corrodes? Is zinc a safe metal?
13. What is "cold" galvanizing?
14. Why would you want to paint over galvanized steel?

Answers to Hot Dip Galvanizing Questions

1. How Does Hot Dip Galvanizing Protect Steel from Corrosion? Zinc Metal used in the galvanizing process provides an impervious barrier between the steel substrate and corrosive environment in the atmosphere. It does not allow moisture and corrosive chlorides and sulphides 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?
   
   
   1. Caustic Tank - During this initial phase of the Galvanizing Process, untreated steel is submersed in a hot caustic tank or an acid tank to remove any, oil, grease and soluble plants.
   2. Acid Bath - Once cleaned, the steel is then immersed in a hydrochloric or sulfuric acid tank to remove surface rust, mill scale, and similar deposits. The surface of the steel emerges from the acid as a pure metallic ready to be fluxed.
   3. Pre-flux Solution - Once the steel emerges from the acid bath, it is then immersed in a hot pre-flux solution of zinc ammonium chloride, which helps prevent oxidation and makes the surface of the metal more reactive to molten zinc.
   4. Molten Zinc Dip - Once properly prepared, the steel is then immersed in a kettle filled with molten zinc at a temperature of 815° to 850° Fahrenheit. During this part of the Galvanizing Process, the zinc metallurgically bonds to the steel to form a zinc coating of uniform thickness. All surfaces are coated, including the inside of tubular structures and hard-to-reach areas.
3. How does the cost of Hot Dip Galvanizing compare to other corrosion protection systems? When compared with other 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.
5. 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.
6. 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.
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 that galvanizers may put in their zinc kettle to enhance the coating appearance by making it shiny, spangled or matte grey. The appearance of the coating (matte, grey, shiny, spangled) does nothing to change the corrosion protection of the zinc coating.
8. 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.
9. 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.
10. 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.)
11. 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.
12. 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 15mg) and necessary for reproduction. It is used in baby ointments, vitamins, surgical instruments, sunscreens and cold lozenges.
13. 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 3600psi. There is no such reaction when zinc-rich paints are applied and the bond strength is only several hundred psi.
14. Why would you want to paint over galvanized steel? Called duplex coatings, zinc and paint in combination (synergistic effect) produce a corrosion protection approximately 2 X 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 extends the life of the zinc coating.