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Hot Dip Galvanizing vs. Cold Film Galvanizing
Pros of Hot Dip Galvanizing
What is so good about it?
1. Proven to Work
It has worked since 1829 and still works today as one of the best ways to protect ferrous metals from corrosion. Galvanizing has only improved since then.
The industry is almost 200 hundred years old which means it there are established companies with good reputations doing good work.
3. Provides Galvanic Protection
The name itself tell this, but it does prevent the corrosion cell from rusting the steel. The zinc sacrifices itself instead of the steel because it is less noble than the steel. This is one of the main ways that zinc provides protection from corrosion for steel.
4. Provides Barrier Protection
Galvanizing provides a pretty good barrier protection to stop an electrolyte (water) from creating a corrosion cell with the steel. Barrier protection stops one part of the corrosion cell from being present. Without one part, there can be no corrosion.
Pros of Cold Film Galvanizing
It varies by product, but there are some clear pros
1. Can Provide Galvanic Protection
In order for a coating to provide galvanic protection there needs to be more than 92% zinc pure zinc in the dry film of the coating. Cold film galvanizing system with less than 92% like organic and inorganic zinc primers do not provide galvanic protection. They only provide barrier protection, but sound like they provide galvanic protection.
2. Can provide Barrier Protection
Some of the cold film galvanizing systems are designed to provide a barrier and some are not. Read your technical data sheet carefully. If the CFG (Cold Film Galvanizing) system provides barrier protection it is a plus and can make a complete system without the need for a topcoat. If it doesn't, it will need a top coat with a recommended paint.
3. Easy Prep
Preparation for cold film galvanizing is simple. Pressure wash, sandblast, spray coating, dry. For some coatings the prep doesn't need to remove all the old paint or rust, although this is always recommended.
4. Easy Application
Cold film galvanizing systems are typically easy to apply. They are applied like most primers at a sandblasting shop or by brush or by spray can.
5. Easy touch-ups: welds, bolts, etc.
Due to the easy application it is easy to do touch-ups if anything chips, gets scratched or needs some love in the hard to reach places.
6. Dip Tank OK
It is possible to dip tank with CFGs. This is nice for fences, parts, and other porous surfaces where spraying would waste lots of paint in over-spray. Although it does create a larger setup cost, it can save money.
Cons of Cold Film Galvanizing
1. Check your Zinc
If the coating has below 92% zinc in the dry film it is ineffective for providing galvanic protection. So READ THE FINE PRINT to make sure you aren't getting something that won't work.
2. Some are Better than Others
Some can be used to recoat themselves in the future to form one solid layer again, while some are just poor coatings with poor adhesions and poor lifespans. Typically this comes down to cost.
Cons of Hot Dip Galvanizing
1. Touch-ups Aren't the Best
Hot dip often gets damaged when it is reassembled, bolted, or just put into use. This requires touch-ups. Touch-ups can't be done with the same coating since temperatures are too high. They are done with different product creating a two coat system that isn't a complete coating. It becomes a patch work of coatings. This can cause water to get between the coatings over time and cause rust.
If hot dip galvanizing wasn't the biggest and best coating system around for steel protection it would be expensive. It is a little more expensive than other CFGs.
3. High Turnaround times (Depends)
The turnaround time is increased for this. The compenents of hot dip need to be broken down into smaller components and dipped then reassembled. This isn't true for CFG systems where they are applied at a sandblasting and paint shop. Low turnaround times cost money.
4. How Much Zinc?
In reality, only about 20% of the hot dip galvanized coating is actually zinc. The rest is a bunch of alloys. Apply 10 mils, but only 2 mils of real zinc protection. This is the stuff closest to the surface of the steel which is good.
5. Limited Application Locations
Hot dip can only be done is a shop atmosphere. It needs the hot dip tanks. This isn't allowed to be mobile for environmental reasons. Cold film systems can be applied anywhere, although shop is still the best.
6. Barrier Protection Problems
One of the best parts about zinc is the salts that form on its surface when it corrodes providing a barrier against water and other corrosion causing agents. Since the zinc in hot dip is so close to the surface of the metal, but the other 80% is alloys, the zinc salts don't form to increase the protection of the coating.
7. High Preparation Required
1. Alkaline clean 2. Rinse in fresh water 3. Acid clean (pickling) 4. Rinse in fresh water 5. Dry off 6. Application of flux (powder layer) 7. Dip in zinc at 450C 8. Apply passivation (chromate layer)
Missing some Galvanizing?
Interested in Learning More?
- Cold Film Galvanizing Contact
William Y - Painting Everything with a Surface - Protective Coatings - Canada - I work at Nisku Industrial Coatings, in one of the best paint distributors in NA.
Cold Film Galvanzing
Hot Dip Galvanizing
What do you prefer?
What do you prefer? Hot dip or cold film? tell us why in the comments.
In conclusion, I think cold film galvanizing is a much better system than hot dip galvanizing.
Feel free to add your comments and any pros or cons I missed into the comments.