Metals in contact: zinc, stainless, and zinc-alume coated steel

[NorthCivil]

Structural engineer here.

I have a project in a moderate/mild climate, coastal area. Fairly sheltered - approx 500m from the shore which is a sheltered harbour. 4km or so from the open/rough sea.

some fairly fancy cladding is specified. zinc cladding panels, fastened with stainless steel fasteners, to steel studs. the studs are coated with a zinc-alume coating.

the material set: steel - stainless - zinc.

The body of zinc will be large, as will the body of steel. the stainless steel connections which bridge the two metals will be small bodied.

the stainless steel is the most noble, and is the smallest component, which bridges the steel and zinc. zinc is the least noble.

My hypothesis is the flow of electrons though this assembly will only cause the zinc to corrode. I'm not concerned about corrosion of the stainless, though potentially there may be some atmospheric corrosion of the steel, from airborne salts, at cuts ends and other areas where the coating is damaged.

forgive my elementary knowledge. I have studied and understand nobility, the theory of large vs small cathodes & anodes. but with 3 metals in contact, not sure if other natural phenomena will come into play.

 

[TugboatEng]

The stainless will cause all of the other materials to corrode. The rate of corrosion will be relative to the exposed surface areas. Lots of zinc plated steel and a few small stainless fasteners won't present much trouble but a lot of stainless fasteners could cause the problem to be worse. This assumes the climate is anything but dry. Under dry conditions galvanic corrosion will barely exist.

The proximity to a coastal climate will cause unique issues for the stainless steel.

 

[EdStainless]

You say sheltered, does that mean under cover? (no rain on it)
If this is the case then things will just happen faster as the salt concentrations will be much higher.
Either way the first thing will be the Zn under the fasteners going away.
That could take weeks to months.
Then you will get some corrosion of the underlying steel.
I doubt that the amount of corrosion itself will be an issue, but the rust staining might be.
In coastal situations the windward side tends to fair better since it gets washed with rain more.
The lee side, even if it is facing inland usually takes a beating.
The same is true for open but under cover structures, they see heavier corrosion.

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P.E. Metallurgy, consulting work welcomed

 

[blacksmith37]

The materials will have limited life in the location, depending on which "stainless steel" . What is the intended life ? There are various data sets for seacoast exposure such as INCO /Huntington but I can not give a web site.

 

[EdStainless]

LAQUE CENTER FOR CORROSION TECHNOLOGY used to a lab of INCO, then it was privately run.
It was in Wrightsville Beach NC.
They did testing submerged in seawater as well as at three exposure lots.
As I recall the lots were 50', 500', and 5,000' above mean high tide.
The lab is closed, and I believe that only the 5,000' lot remains.
When I last saw it about 10 years ago there were still test panels in it from the 1950's.
There are many papers from the 1960-1980 time frame on the work there.
You might also find papers from the old Seahorse Corrosion conf.
This was all marine and coastal exposure work.

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P.E. Metallurgy, consulting work welcomed

 

[NorthCivil]

Thanks Tugboat, Ed, blacksmith.

In regards to definition of sheltered: sheltered from direct sea-spray. the cladding will be open to the surrounding environment, exposed to weather/rain. though far enough from the seashore (4km from exposed coastline) that airborne salts, while likely present, wont be extreme.

The stainless i am intending to use is A470.

The detail i am intending to use is similar to the attached sketch.

Intended life, approx 20 years to first serious maintenance/overhaul.

For corrosion around the fastener points at the exterior. this can be inspected, cleaned and if the materials have been degraded, repaired/replaced.

what concerns me most is the fastener connection points "hidden" from the exterior, which cannot be seen. if the zincalume corrodes with the stainless steel, staining is fine, however if the metals corrode enough that the screws no longer hold, there is potential for the cladding to come flying off the building.

As these are sheet metals, there is not much material for aggressive corrosion to eat through, and render the fasteners useless.
staining etc is OK, flying cladding, not so.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1658787477/tips/eng-tips_zinc_aoxrtb.pdf[/url].

 

[hokie66]

Why do you need the stainless washers? Screws with neoprene separators should work. If you need the washers for load, you can get stainless washers with bonded separators.

 

[NorthCivil]

Hi Hokie

Yes i would likely spec separators.

Though there is still contact between the screw and the base metal where the fastener engages to steel. this point of contact is where corrosion will likely manifest. which if it develops quickly, will render the fastener useless. this is my concern.

 

[TugboatEng]

The screw into the steel stud should be dry if your design is correct and there cannot be galvanic corrosion.

 

[NorthCivil]

Hi Tugboat, thanks for the response.

you are correct about that screw, which connects steel batten to steel stud.

the screw which concerns me is the screw from cladding to the steel batten (drawn red). and possible corrosion of the steel batten, at the engagement of screw to batten.

this may see some wetting, and is exposed to exterior air

 

[EdStainless]

You said SS fasteners, but then A470 as a spec.
The 470 grades that I am familiar with are not SS, they do not have enough Cr.
Which stainless alloy are you planning on using?
And I second the neoprene separator approach.


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P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

A4-70 is an ISO designation for type 316 SS bolts.

The neoprene separator should also seal the connection to minimize any chance of moisture getting to the threads.

There are metal roofing screws available with bonded washers and everything. They're self drilling which means a 400 series stainless which will not be as corrosion resistant. I see 410 is typical.

I do suggest EPDM instead of Neoprene for the washer. It is more resistant to sunlight and aging and is less likely to crack.

 

[EdStainless]

Thanks Tug

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P.E. Metallurgy, consulting work welcomed

 

[hokie66]

Like this...

https://monsterbolts.com/products/self-drill-hwh-epdm-410-10

 

[NorthCivil]

Thanks Hokie

the thing with these fancy screws, is though the head of the screw is isolated from the substrate metals, the shank of the screw will still be in contact with both base metals that are being fastened together.

maybe it is possible to avoid it in a lab, by creating a passhole on the upper metal. but when asking trades to fasten 1000's of screws to bond 2 different sheet metals, the passhole perimeter will inevitably contact the screw in a lot of instances. creating a bridge

 

[NorthCivil]

Thanks to all for the help.

I've been in touch with various people in the local industry. coil coaters, sheet metal technical gurus, etc. They are all adament against fastening steel sheet metals with stainless fasteners.

The problem with this corrosion is that because the sheet metal is so thin, corrosion quickly can destroy the thin steel and render the fastening point useless.

 

[TugboatEng]

The automotive industry has gone to Dacromet nd similar cooated fastened. You probably still want the rubber faced washers as they prevent moisture getting to the threads where the coatings will be broken. There are tons of options for self drilling coated fastened nowadays.

 

[Dhurjati Sen]

Instead of using the ss316 fasteners, steel fasteners with zinc alume coating seems to be a better design.

N.B. I have assumed plain carbon steel / mild steel as referred by you as "steel" above.

DHURJATI SEN
Kolkata, India

 

[TugboatEng]

Lawd, that's an embarrassing number of typos/autocorrects in my previous comment. I wish I could edit but I think my point carried through.