Corrosion Resisnat Secondary Steel Elements

[Everynameistaken]

Hi all,

We are designing a steel building structure for and industrial facility.

The key industrial process requires moving large amounts of water through open troughs. These troughs must be kept out of the direct light without adding rain so we need to provide rain weather shield. Not a full building envelope with insulation.

This is where my specific question comes in.

Typically on a metal building we would design the cladding system with sheet metal girts and purlins and then an exposed fastener metal cladding. The secondary elements would be inside the building envelope and moisture barriers with HVAC to keep the moisture under control. The girts and purlins are usually from galvanize coil bent into either C or Z sections, bolted to the main steel and then the cladding screwed into them.

The challenge with our current situation, is the environment is very moist, there is no HVAC or moisture barriers, and we expect condensation to be significant with quite a bit of water accumulating on the secondary members.

Does anyone have any experience with the durability of traditional galvanized coil secondary elements in a moist environment?

Has anybody specified these elements to be taken for a secondary coating (paint) to be applied?

Are there any other products people have used for this application that have better durability than sheet metal?

Thanks for your thoughts!

 

[JStephen]

They do make galvanized bolted tanks for water storage applications. The idea on these is generally that they'll last 20 years or whatever, then you replace the whole tank. The tank design codes do specify minimum thicknesses (partly for erection purposes), something like 0.094" or so. On the bolted tanks, the bolt holes are all punched prior to galvanizing, not field-drilled or punched.

They do use lined/coated carbon steel tanks for water storage. Typically, the liner is an epoxy lining, field applied. The idea is that the tank is "forever" but the lining/coating will need to be periodically redone, typically 10-20 year cycles. Minimum thickness will be 0.1875" to 0.2500". For maximum performance, all interior joints and connections will be seal-welded so that there are no hidden surfaces exposed to the tank contents. If I remember correctly, the Structural Welding Code has some provisions that limit the use of all-around welding like this in structural steel.

For water or various chemical storage, it is possible to add a "corrosion allowance" to the minimum or design thicknesses to compensate for some future corrosion.

In carbon steel water tanks, we do have customers who specify galvanized, stainless steel (304 or 316), or fiberglass interior ladders or other appurtenances. The use of these items is usually based on the individual customer's experience or theories of what will work well, and I am not aware of any definitive source that says what is or is not good practice. Some customers will specify insulated gaskets at the connections to carbon steel items, some do not. Galvanized and stainless steel items are not normally coated with the tank lining system.

In most of these cases, the tanks are for potable water and the water is chlorinated but otherwise clean.

 

[human909]

Based on the description galvalized should be fine.

Galvanised is used across the world in warm humid environments and it lasts. You start having issues if its is HOT, humid and contains electrolytes or other corrosion accelerators in the air. I've done ducting and structural steel out of stainless in such applications.