Corrosion on cable trays. 6

[martinrelayer]

Hello,

In a wastewater plant, there´s installed HDG cable trays with what seemed good HDG bolts (same supplier)
No some years passed and only see the bolts rusted.
What could cause this in particular and what can be done?
Replacing bolts from same material Hot dip galvanized seem to have the same future as now.
This rustiness is only three yeear operation.




In this other picture is the bonding for the frame of the machine, this is tin plated copper terminal and copper cable over stainless steel. What do you recommend to make the bonding to this material in such environment? you can see terminal looks totally rusted.


Thanks in advance,
Martin.

 

[EdStainless]

So why do you care about the bolts rusting?
Have any failed?
You buy double galvanized (not twice, just double weight) or fluoropolymer coated fasters if you want less rusting.
Fasteners are notorious for this because installing them often damages the Zn.
Unless you remove the bond wire you won't know if there is any corrosion in the joint.
What material is the stud and washer?
It should all be bronze.

 

[waross]

We always used Spray on Electrical Insulating Varnish on all ground connections.
The bolt corrosion appears to be atmospheric corrosion.
Try Stainless steel bolts.

 

[MintJulep]

I see what look to be HDG bolts and nuts on pipe flanges and conduit clamps in your pictures without corrosion.

That suggests the problem is with the specific hardware from the cable tray contractor.

 

[TugboatEng]

You must not use split lock washers or any type of locking washer as it provides a path of water ingress to corroded the connection. If you use no washer, the outside of the connection may appear corroded but the copper ring terminal acts as a gasket and seals the inside.

For encapsulating exposed grounds we use a flowable silicone product such as Dow Corning 734

For the rest of us, do not use split lock washers... ever.

As for your fasteners, it appears that they may have been electroplated galvanized and not hot dipped.

If hot dip is not practical, inorganic zinc/aluminum flake coatings such as Dacromet offer similar performance and price over a wider range of applications. PTFE coatings such as Xylem are the next step in performance and price.

I do not suggest stainless fasters as they will quickly consume the zinc coating in their immediate vicinity and then the framing after that.

 

[3DDave]

There should be photos from when the plant was commissioned to determine if that hardware was zinc plated to begin with or if someone ran out of the plated hardware and used non-plated.

While I expect corrosion is possible from installation damage I would expect it to mainly be at the bolt and nut corners and fading from there, but it is clearly visible on the threaded portion that should have had no particular contact during the install. Instead it all looks very uniform.

Maybe this corroded hardware was improperly plated or not plated at all, but if it is replaced with properly plated hardware I would not expect a recurrence.

 

[TugboatEng]

Zinc plated is the problem. This hardware was likely zinc plated and that's why it's failing. Hot dip galvanizing is a separate process from plating and much more durable. Zinc plating is never acceptable for damp environments. Even automotive manufacturers no longer use zinc plated fasteners for anything exterior.

 

[martinrelayer]

Hi all, thanks you very much for your comments.

All bolts are hot dip galvanized. You can see some of them are not rusted so I suppose is the fixing of the bolts and nuts that breaks the coating? Or should I thing it is more of a problem on the galvanization process?
Would it be that after installation, spray paint should be enough in this particular case?

Also the bonding connection after replacing it, paint could be a good measure?

Regards,
martin.

 

[3DDave]

A product I have seen used on unprotected steel in an environment where saturated salt solution is sprayed on is called "Fluid Film"

See https://www.fluid-film.com/ for more info.

If I say they are not a sponsor, you'll know which YouTube channel I refer to.

There are probably better coatings, but I am not sure paint is a good choice. Unless the items are carefully cleaned and the paint properly applied, it can be brittle; it also has a different thermal expansion coefficient. The combination is that any defect in the paint film will be extended by thermal cycles and will allow moisture to collect under the paint, focusing corrosion. The Fluid Film just remains a flexible, compliant coating.

Again, to emphasize, there may be other industrial coatings that can be applied that also don't harden or crack that will perform just as well.

As an example, I got a hitch attachment for the car. It was powder coated. The thickest layer of rust is where the powdercoat was as it cracked and held the moisture in place for longer times. The interior opening, which was never coated has some rust, but a small fraction of that where the powder coat was applied.

One thing I would worry about is that zinc oxidizes readily and the oxide doesn't bind tightly so if the parts are painted, that paint may stick to the oxide layer and not be well bonded. This would make painting a problem. I looked at instructions from paint suppliers for galvanize painting and I would stick with a non-paint coating.