Discolouration of architectural arch

[RobWard]

I hope someone here can answer some questions for someone with pretty much no knowledge of stainless steel.

We're a small family company who have recently had a new factory built. We had a nice stainless steel and glass arch installed over the office entrance. It's about 6m long and fabricated from 8" diameter round tube.

In the space of less than one year it has become quite discoloured, looking as though it is rusting. The fabricators have said that it is water-staining and not their fault, and the builders seem uninterested.

They have offered to get a team in to repolish it at our cost (£600 per day) and then to coat it in a varnish.

The stays that help support it haven't discoloured, nor have the stainless steel bollards in car park.

Could someone explain to me what the water-staining is (or point me to a reference source), and why it hasn't happened to the other stainless steel items (I'm guessing it's a result of different grades being used.)
Also what amount of staining is common, or indeed acceptable, on your average bit of architectural steel, and would you concur with the varnish idea (which worries me: I think the varnish is more likely to harbou moss and mould in the long term)

Sorry about the long post. Many thanks for any advice you can offer.

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[mcguire]

Tell us the alloy and the surfaace finish. The use of a coarser polished finish such as #3 or #4 generally results in this type of discoloration due to corrosion when there is any salinity around and when rain doesn't cleanse the steel frequently.
Repolishing with aa mechanical technique will be quickly followed by a recurrence. A different finish doesn't sound feasible. The only possible solution I know of is to try to a chemical polish, such as sulfuric/peroxide which may duplicate the beneficial effect of electropoolishing.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[RobWard]

Thank you for your comments.
I'll have to ask the questions to the fabricators.
We just went for a brushed finish stainless steel. We assumed that as the fabricators and builders do this all day every day they'd know their onions.
But we all know that too much assumption spoils the broth...


"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[EdStainless]

The surface may be too rough as already mentioned.
It also may not have been properly cleaned and passivated after the original polishing.
Any contact with steel (scaffolding, tools, chains and so on) will leave small amounts of iron embedded in the surface and lead to rust staining.

I would suggest a good scrubbing with soap (do not use a liquid soap) water. Use stiff brushes, and ScotchBrite for the tough areas. Then finish with a citric acid wash.

Did the same firm do all of your stainless work? They screwed up.

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Plymouth Tube

 

[CoryPad]

What is your environment? Near the coast? Near a steel mill or coal-burning electricity generating plant? If it is a severe environment, the alloy may not be suitable (unlikely). If the environment is not severe, then the surface treatment was not sufficient (likely).

Regards,

Cory

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

 

[RobWard]

Thank you for your replies.
According the the fabricators:
The material is 304 stainless steel.
The finish is 240 grit dull polished finish.

We are nowhere near the coast or steel mills or coal burning electricity plant. We're based in Bradford, in Yorkshire in the UK. (If you know it, pity us).

EdStainless: you say not to use a liquid soap. Why is that, and is there anything we should look out for in a soap's composition to particularly include or exclude.

I'm wondering if the glass canopy is preventing the steel being washed by the rain as mcguire alluded to. The stays are above the glass, shining and lovely. They glisten and gleam in the precise way that the rest of the arch doesn't.

The actual fabrication is lovely. Really nice welding and fit. Just a real shame that the finish is looking so lousy!

Again thank you for taking the time to reply.






"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[EdStainless]

Almost all liquid soaps contain salt, it is used as a viscosity adjuster. Any little residue would result in fairly rapid pitting.
I think that McG has hit this one. The surface is too rough and washed too little. This allows airborne debris to settle on it and build up.

Is this outdoors enough that you could have someone hose it off once a week? Once you clean it up that might be enough to keep it from re-staining.

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Plymouth Tube