NEMA cabinet: galv, phosphatize, or both?

[NoHoCa]

I'm working with a municipal client who needs to install many electrical enclosures on sidewalks, along roadsides, and near fields. Their old cabinets are rusting. Their original intent was to buy standard "phosphatized" and powder coated Hoffman enclosures.

Can anyone clarify if the phosphatizing treatment is any better or worse than galvanized (under paint), or if they are best paired together?

I searched the forums and didn't find anything. I'm an ME, and didn't assume that the EE forum would be any better or worse to post than here...

 

[MintJulep]

The zinc in galvanized provides corrosion protection to the base metal by being preferentially cathodic (or is that anodic, I never can remember) to the base metal. Phosphatized is a phosphate treatment on top of a zinc coating to improve paint adhesion.

A good powder coat will then protect the whole thing from moisture, leaving the zinc to do its job only if there is physical damage to the powder coat.

The real trick is protecting the raw edges of any holes.

 

[CoryPad]

Phosphating is the process of creating a layer of phosphate (zinc or iron or manganese) on a part. Phosphating is a primer that improves the adhesion of topcoats (like powdercoating). It provides a small amount of corrosion protection, but nothing compared to that from zinc electroplating or hot dip galvanizing. For maximum corrosion protection of steel, it is best to use zinc coated steel, then zinc phosphate, epoxy basecoat, and finally an acrylic or urethane topcoat.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ScottyUK]

We're a heavy industrial site in an area with a maritime climate and some neighbours who like to fill the atmosphere with all manner of crud (chemical complex and a steelworks). Painted steel panels have a short tough life here. We have abandoned using painted steel outside and have adopted a standard of 316 stainless for all external panels. It's more expensive initially for sure, but they don't rot away. The long term cost saving is in fewer failures of the internal equipment and not having to re-house the equipment after five years or so.


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Sometimes I only open my mouth to swap feet...

 

[Ashereng]

If these cabinets are typically out of people's reach, the strength of steel (to keep people out) may not be needed.

Here's a suggestion out of left field.

Can you use painted aluminum (aluminium)?

How about fibre glass?

I don't know how thick one can make a cabinet out of aluminum, but give sufficient thickness, it may offer sufficient strength to keep out vandals?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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[NoHoCa]

Thanks for all the feedback. Sounds like zinc rich primer or galv are the best options for a low cost municipal application that must be resistant to vandals. Given a quantity of about 100, we can have Hoffman make a custom run at the paint shop.

In my previous life I was quite familiar with 316SS panels (and occasionally fiberglass) in maritime environments, however the clients and their economics are quite a bit different than public agencies delivering water...

 

[ScottyUK]

The spec quoted by CoryPad is typical for above-surface marine coatings applications. The following is from a typical North Sea spec for surface applications:

Shot Blast SA 2.5, Zink Rich Primer, 3 Coats Epoxy High Build, Polyurethane Top Coat.

We've adopted this spec for the fan motors on our cooling tower. The corrosion rate in that location is very high: cast iron mill duty motors with standard paint finish are eaten through in maybe five years. If we don't see an improvement with the new paint spec our next option is a full sub-surface marine paint job, but initial enquiries indicate that the cost for such a finish will be quite high.

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Sometimes I only open my mouth to swap feet...