Corrosion in steel: Box to plate connection

[MatthewKerr]

Good Afternoon,

I am currently asking my first question on the forums so I'll provide a brief introduction, I am currently employed as an engineer in the oil and gas industry for a company that builds building temporary equipment for use in the North Sea sector. We most commonly build containerised pump equipment.

I have been in discussion regarding corrosion with some of my colleagues and no-one appears to have a difinitive answer to the following.

The example we have been discussing is where a box section (Primary Structure for our containers) has a steel plate 5,10,15,20mm thick welded to it. For example, we have a 10mm thk plate, 200 x 100mm in width and height, welded onto the box section (not as a cap) with fillet welds. We commonly use this for equipment mounts i.e drill and tapped pad for mounting a tank, vessel etc. My query relates to the corrosion that will, or won't occur behind the fully sealed by a fillet weld steel pad (For explanation this forms a 10mm THK Pad on the box section). There will be a volume of 200mm x 100mm x ? 'pocket' behind the plate where corrosion can form between the pad and the box section.

Now I have described the example, my question is this, is this a practice we should be avoiding due to corrosion concerns behind this style of plate fixing? It has been raised as a concern that a pocket of corrosion can form and expand behind the pad, this would result in a detrimal effect to the strength and condition of the box section.

It would be greatly appreciated if someone could advise.

Thanks and regards
Matthew Kerr

 

[jayrod12]

Honestly I'd be concerned if you weren't capping your tube sections. It's more likely for water (or something more corrosive) to cause issues with an open tube compared to a closed. If I'm understanding your question correctly.

 

[MatthewKerr]

The example we were discussing was a box section to plate. However the same question could be ask of welding a plate to a channel, angle or other steel plate.

 

[kingnero]

Fully sealed means no fresh oxygen can get to it. After the initial corrosion, tbe chemical process will stop.

 

[BUGGAR]

Ditto what Kingnero says. An example is the wet sprinkler piping systems used in buildings.

 

[MatthewKerr]

Thanks folks,

This is what I thought the answer would be. It does make me consider how detrimental the corrosion behind the pad would be to the box sections surface, most of our box sections are designed with 6.3mm thick material and are generally utilised to 70-80%. What amount of corrosion would occur on the box section behind the plate? How much material would be lost in this area?

Thanks and regards

 

[jayrod12]

I'd say negligible. Surface rust basically. Likely not even a measurable percentage.

 

[dhengr]

MatthewKerr:
You certainly want to keep moisture from the exterior and environment out, thus the weld all around. Maybe you want to radius the corners of the cover plate or pad so the weld can be truly continuous around the corner. Maybe you want to provide thicker bosses for any drilled and tapped equipment attachment holes so that you do not penetrate into the void space (faying surface) at your container wall. If the interior of the container is relatively dry, maybe you want to provide a couple breather holes, and a couple of low point drain holes so that most moisture will drain or move out of the void space. With a small pad or cover pl., I think Kingnero has it about right, on 6OCT15, 18:47, ‘the chemical process will stop’ . I have usually dealt with much larger closed structures, which you try to make water tight, but they are often not completely water tight. So, we almost always put drain holes at low points in the enclosed volumes.

 

[MatthewKerr]

Thanks folks,

Great feedback, very much appreciated.