Galvanizing and Electroplating

[Gregorydas]

Hi all,

This question has evolved during the fabrication of 100 No's of sliding gate. Client need a guarantee of 5 Years for the corrosion. We are giving a 3 coat paint system and the paint manufacturer gives 2 year warranty on the paint. They also recommended galvanizing or electroplating before the application of the paint. The gates are to be installed on corrosive environment (SALTY AIR - BAHRAIN PORT AREA)

So I am a bit confused regarding the selection. What is the common difference between Galvanizing and Electroplating? Which would be costlier? and which would give a sufficient corrosion protection for 5 years, along with the application of 3 coat paint system (225 Microns - Epoxy Primer, Mastic Coat and Polyurethane top coat for gloss)

The gate is 4.4 Wide and 2.5 m high. With Hollow section Frame and Square bar designs + Poly carbonate Sheet and a small design made of 1.5 mm GI Sheet.

 

[racookpe1978]

It depends, really, on WHERE the rubbing and wear and scratching-of-paint/coating is going to be taking place.

For example. Imagine your gate has a standard multi-coat paint systems over plain carbon steel.

But, one area is being continuously slapped and scarred by the oily chain that pulls it open and shut.
One section is occasionally rubbed by the two steel guide rails as it retracts.
The gate itself is plain black carbon steel, but isn't being rubbed or touched.

The lock is clanging open ans shut every time the gate opens = It WILL be scarred and the edges of that scarred area WILL BE rusty. The center of the scarred area where teh latch falls will be worn away but bright and shiny because of the wear and rubbing and polishing.
The place where the chain hits will be similar: A oily/greasy spot that is "polished" surrounded by a rusty area where the paint is rubbed away, but not rubbed enough to polish the damage metal. The guide rails will have scarred the paint, and that area will be rusted.
The plain gate itself will be un-rusted. After 3-4 years, it "might be" still looking adequate (depends on the cover paint) but after 5-6 years it will be sun-burnt and "grey-looking" from ozone and UV damage.

The cover paint protects the primer. The primer protects the base metal. But if both are scarred or cut through, the base metal will rust.

If the client demands a "perfect job" never rusting in those obvious wear areas, the metal itself must be stainless under the cover paint. Or be protected by some sort of sheath or rubbing guard that itself won't rust.

 

[CoryPad]

Galvanizing and electroplating seem to have multiple definitions, especially to non-native English speakers. In general, galvanizing refers to hot dipped galvanizing. This process immerses steel into molten zinc and the resulting coating is relatively thick (40+ micrometers). Electroplating immerses steel into an aqueous solution that contains zinc ions and then uses an electrical current to deposit a thin layer (~ 10 micrometers) of zinc metal onto the steel. Cost depends on several factors (coating sheet vs. final hollow section vs. assembled gate). For your application, you could use either, they both should last 5 years.

 

[racookpe1978]

But either will rub off or get cut through if there is friction on the surface.

 

[tbuelna]

You mentioned the welded gates have hollow frame sections. This is the biggest problem I see with corrosion protection of your gates. If the gates are made from mild steel and there is even a tiny pin hole at the bottom of one of the hollow frame sections, it will quickly result in corrosion/rusting when exposed to a marine environment. The hollow frame sections must be completely sealed closed by welding, and the base plating applied must completely cover every surface including irregularities of the weld joints.

 

[hokie66]

tbuelna,
Not with hot dipped galvanizing. You have to galvanize the inside as well. Holes must be left for venting and drainage, otherwise the closed sections are prone to explosion.

 

[tbuelna]

Hot dipped galvanizing would not give thorough coverage of all internal surfaces of the weldment described in the OP. Welding everything closed, followed by phosphate coating and prime/paint might be a better approach.

 

[moltenmetal]

Sealed sections cannot be hot dip galvanized as they will distort or even burst when dunked into the molten zinc.

Plating will not work on the internal surfaces.

Welding tight to seal closed, followed by a blast and zinc rich primer, followed by a durable mid coat and then a top gloss coat, might be the best you can do here. Regrettably you will get abrasion of the parts where they rub, and the primer will not be as effective as a film of zinc at protecting the exposed metal surfaces. Design something in to protect the wear points- plastic glides or the like, and you will get a better result.

If you can have adequate venting of all closed cavities, and the part is prepped and dipped properly, hot dip galvanizing will do a decent job of protection if properly top coated with the correct transition primer and top coats.