Increasing the longevity of galvanized steel fence in production

[reliableair]

I need help with a galvanized steel fence that will be powder coated black. I am ordering it from China and I don't know what specs to ask for in order to make sure it lasts 25+ years easy without maintenance.

I need ideas to fullproof the longevity factor on this fence. I am aiming to get to 50+ years maintenance free because factories in China do cut corners so if they do I hope to get the initial 25 which is why I am looking for other methods.

Typically what I see offered is G60 galvanized (which basically means the steel is dipped in zinc once I think) and I am sure the term G60 conforms to some other standards as well but in China they will dip it once and call it G60 whether it meets standards or not.

I've also looked at Class 1 vs Class 3 Galvanization for steel wire fence and have seen the claim where Class 3 is dipped twice in zinc and gets triple the life compared to Class 1 being dipped once. Is that true?

I need to know what additional precaution I should take that will have the largest effect in longevity for this fence. I can have them dip it twice in the zinc (perhaps then it becomes G90?) which seems like a good option. Is there a certain zinc or specs I can request to be dipped in?
It will be powder coated black. I can also have them use an epoxy powder coat (Instead or in addition to the original powder coat). This costs more but I don't know how much better epoxy powder coating is for outdoor conditions than regular powder coating).

Does anyone know what outcomes any of these ideas will have or have any other ideas to increase steel's longevity outdoors?

Also, does anyone here know about crevice corrosion and whether that applies to powder coatings? Will it differ with an epoxy powder coat?

[Moderator: excessive bold removed.]

 

[anorlunda]

I think highways are the biggest consumer of outdoor galvanized steel. They need it for guard rails and bridges. They are also very interested in lifetime.

In New York, that I'm most familiar with, I know that the DOT uses local sources to hot dip the steel. Purchase of the steel and the dipping are two contracts. It's possible that they dismount and re-dip the steel during its lifetime.

I would call the Department of Transportation in your state and ask to speak to their engineers. They may have a wealth of data relevant to you.

Powder coated galvanized steel is new to me.

Also thinking of highways, corrosion and salt and galvanic action and all sources of corrosion would be important to you. What do you know about corrosive substances in the location of your fence?

 

[reliableair]

Thank you for the idea. I will also try to find a company that does only the dipping part to find out how many times it is dipped.

From what I've read is that it's dipped twice during production; not during it's life, at least for powder coated fencing. I don't know if they let it cool and then dip again but that is something I need to find out.
Most outdoor steel fencing you see that is black is powder coated. It is very common. I don't know if crevice corrosion applies to powder coatings.

As far as corrosives, There should be little to no corrosives around the fence, maybe some salt around the gates which have an entrance during winters but nothing other than that and that would be limited to the entrances only.

 

[Lnewqban]

I need help with a galvanized steel fence that will be powder coated black.

I don't know the reason, but during many years I have seen paint that has been applied over galvanized surfaces to peel-off sooner than later.
Perhaps these two types of coating that you mention can hold to metal longer and better.

There is always the corrosion that starts up from inside the metal, especially if the process and material are less that perfect.
I have even seen stainless steel produced by non-reputable manufacturers rusting from its core in less than a year of outdoor exposure.

I am telling you this in order for you to weight costs of super-finishing versus eventual replacement.

 

[reliableair]

Good point, however I am not too worried about the powder coat peeling off. It can easily be touched up but I am more worried about the lack of oxidation or crevice corrosion. As long as it doesn't rust and it oxidizes, touching up with paint will not be an issue.

If it does rust then it will be very hard to refinish/might have to be replaced. I want the steel to maintain it's integrity. Eventual replacement isn't something that is common to see as these fences should last over 25 years without maintenance and some have lasted well over 50.

 

[tech99]

I have been involved with steel radio towers which were hot dip galvanised fifty years ago. They were painted at intervals, which is probably not necessary. The zinc was 3 thousands of an inch. That is a good spec. You don't want anyone using electroplated zinc, which is very thin.
In the event of corrosion in a crevice, the zinc will protect it. But for this to happen you need a large area of exposed zinc so that cathodic protection can occur. Painting does not allow this to happen so I am unsure about painting galvanised steel. Be careful of any damage during installation - cut ends or site welds etc. One of the best ways to protect this is to apply a zinc rich paint (UK=Galvafroid for instance). This paint needs to have very high percentage zinc. Obviously avoid anywhere that water can pool; all surfaces must shed water and drain holes may be needed.
Any bolts should be spun galvanised not electroplated.

 

[reliableair]

Very helpful, thank you.

If not electoplated zinc, what other kinds are there? I am not familiar with the terms.

These will be welded and then dipped in zinc I believe. The company said they offer 50 g/m2 thickness of coating and 400 g/m2 of coating. Do you have any idea how to convert this to thousands of an inch or vice versa? Is there perhaps a table? Is 400 g/m2 good? I am assuming 50 won't cut it.

I specifically asked for it to be welded and then dipped for that very reason. I want to minimize the painting I will have to do when it arrives here since their powder coating process should be better than my paint process. I will use a high zinc paint. Thank you.

 

[anorlunda]

If not electoplated zinc, what other kinds are there?

Plating makes much thinner layers than hot dipping. I'm sure that no kind of plating is of interest for your fence.

 

[jrmichler]

The zinc was 3 thousands of an inch. That is a good spec.

and 400 g/m2 of coating.

Time to grab the calculator. Zinc weighs 7,100 kg/m^3, or 7,100,000 g/m^3.

400 g/m^2 / 7,100,000 g/m^3 X 1000 mm/m = 0.056 mm = 0.0023 inch.

I have read that the life of a zinc coating is proportional to the thickness of the coating, and the harshness of the environment. I have also read that hot dip galvanized is much better than electroplated for outdoor use.

 

[reliableair]

Thank you, definitely would not have been able to calculate that myself.

Looks like tech99 proposed .003 thickness of coating and the company in China can offer .0023 which is about a third less of the thickness proposed.
Their other (standard) option was 1/8th of the .0023!(at 50 g/m2). Just imagine how quickly their standard option would deteriorate.

Does anyone have suggestions for how I can verify the thickness of the coating when I receive it? They can claim 400 g/m2 or any number for that matter but they can send something completely different, and it will be powder coated on top so I am worried they will try to pass off something that has a much thinner coating but I don't know what tools I could use to measure it.

This is becoming a great discussion I appreciate all the insight this is valuable for more than just fencing at this point.

 

[tech99]

This website will tell you everything! https://www.galvanizing.org.uk/galvanized-steel-coating-thickness/
I think that 3 thousands of an inch is about 600g/m2.
They do recommend slightly thinner coatings for thinner steel sections.
There used to be a magnetic gadget like a pen or tyre gauge which would measure thickness.

 

[anorlunda]

This website will tell you everything! https://www.galvanizing.org.uk/galvanized-steel-coating-thickness/

That's a useful link. The document linked offers other important points other than coating thickness.

In addition to steel thickness, the composition of the steel alloy itself is also relevant. Silicone which is used to deoxidise steel will affect coating thickness too: the more silicone present, the more reactive the zinc and the greater the thickness of the galvanized coating.

How this microscopic coating then weathers is predictable but complex and fluctuates with atmospheric conditions. In most dry inland situations this can be infinitesimally slow, less than 1 micron per year, and unlike other coatings, a galvanized coating deteriorates in a linear fashion. As long as the environment remains stable, case studies show that the coating throws up no unpleasant surprises and can be relied upon.

Moreover, in the UK and Ireland, as sulphur dioxide levels have fallen, corrosion protection has been extended. It does indeed pay to know your microns.

 

[Merlin3189]

EN ISO 1461: coating minimum masses/ thickness on articles that are not centrifuged

Centrifuging removes excess liquid zinc and reduces thickness.

Hot dip galvanising produces an alloy layer, but I don't know about electroplating.

 

[tech99]

There is another process called Sherardizing, where the item is tumbled at 500C in a drum containing zinc dust. This produces an intimate bond between the two metals by diffusion. https://en.wikipedia.org/wiki/Sherardising

 

[arydberg]

Plating makes much thinner layers than hot dipping. I'm sure that no kind of plating is of interest for your fence.

EN ISO 1461: coating minimum masses/ thickness on articles that are not centrifuged
View attachment 263054
Centrifuging removes excess liquid zinc and reduces thickness.

Hot dip galvanising produces an alloy layer, but I don't know about electroplating.

 

[arydberg]

Here on the east coast they make lobster traps out of steel wire that is coated with vinyl plastic. These withstand submersion in the ocean and seem to do quite well.

 

[tech99]

I think the difficulty with coating with vinyl is that the underlying structure cannot be inspected and if there is a break in the coating anywhere it will cause rapid failure. There is also the issue of how to make corrosion resistant connections.

 

[reliableair]

Thanks everyone for all the replies and information.

I have received a sample and now in the process of trying to verify the thickness of the galvanization, and I saw a cheap option would be the "PosiPen". It has a 10% margin for error which is large but will still work for my application. The other options were too expensive.

Does anyone know a lab or testing center that can test it for me? If not I will purchase the PosiPen and measure it myself. Any tips or recommendations for tools other than the PosiPen in the $300 range?

Someone mentioned the amount of silicone in the zinc, etc. but I don't see how I will be able to test that.

As noted before, the sample is powder coated on top of the galvanized steel, but seems fairly easy to remove. Not sure if it is bonded properly.

 

[tech99]

Adhesion of paint to zinc is an issue. Maybe an etch primer is required.

 

[tech99]

Adhesion of paint to zinc is an issue. Maybe an etch primer is required.

We used to leave the galvanised steel outdoors for a few months before painting, so it was roughened by the weather.

 

[reliableair]

I will look into the etch primer and see if they can apply it, but not sure if I can trust them to do it properly or use the correct one. A lot of the time they will say they will use a certain brand (AkzoNobel powder coat) for example but in reality they use some chinese brand you've never heard of and you won't know how good the quality of the product is.

I would leave it outside if I had the time and then paint it myself, but that is a good idea for a future project.

 

[reliableair]

One more question, one factory said they will use raw steel tubes for the fence and the other said they need to use low carbon steel tubes, because they will hot dip galvanize them. Is a certain type of steel better for bonding/adhesion?

Also they have the pre galvanized G60 that I mentioned earlier which has a SUPER light coat, that would not be a good tube to hot dip because it won't adhere correct?