Corrosion of carbon anchor bolts 1

[Andrew88]

Hi all,

I have a case where a new outdoor mild steel structure is properly protected from aggressive environment (all epoxy coated) apart from anchor bolts and nuts... These are made of plain carbon steel with no coating so quickly got rusty finish. The location is within 10 miles from the coast. Is it sufficient to wire brush the bolts&nuts and coat it with zinc rich paint? Ideally ,would need at least 10-20 years of life before next maintenance works.

Thanks

 

[EdStainless]

I presume that you talking about a Zn rich primer with a sealer over it.
Is there any risk of water or condensation on these bolts?
If not it should be good enough.

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P.E. Metallurgy, consulting work welcomed

 

[kingnero]

Cannot you change them for HDG bolts/nuts (hot dip galvanised)?
Foolish to use bare fasteners on an epoxy coated structure.

 

[Andrew88]

This in an exterior structure exposed to rain so water could stay on the base plate.

I believe they were chemically anchored so wouldn't be easy to replace them.

 

[kingnero]

If so, do the best you can when coating them. Just a quick wirebrush won't be enough. Sandblasting might be a better option. I would be very surprised to see a coating last 20 yrs on exposed fasteners, but unlikely =!= impossible. Good luck!

 

[Tmoose]

"This in an exterior structure exposed to rain so water could stay on the base plate."

I'd wager that water will creep under the base plate and attack the part of the anchor we can't see.
The part surrounded by epoxy etc in the chemical anchor //may// be water proof.

After 10-20 years any exposed steel could be "necked down" severely.
Stress corrosion can be brutal.

https://twstatic.net/attachments/img_3054-jpg.1589404/

https://seblog.strongtie.com/wp-content/uploads/2017/05/spalled-concrete-under-concrete-bridge.png

https://www.structuraltechnologies.com/wp-content/uploads/2017/10/deteriorated-anchor-bolts-a.jpg

 

[EdStainless]

They will need to be blasted white before coating.
Use a three layer coating system.
Make sure that water can drain away. Sealing is a fools game for the long term, better to make sure that water will not stand there. Cut a drain slot or some similar method.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[SlideRuleEra]

The location is within 10 miles from the coast.

Usually, accelerated atmospheric corrosion from salt spay is a factor for half-mile or less from sea water.

Bolt diameter has not been mentioned. Corrosion is normally assumed to take place at steady rate. What's important is how much steel remains, not how much has been compromised by corrosion. A, say, 3/8" bolt will be worthless before a 1" bolt becomes unacceptable.

http://www.SlideRuleEra.net

 

[r13]

Suggest to look into cathodic protection as an alternative.

 

[0707]

brush the bolts and grease the bolts if possible seal weld a cup over each bolt, if no, just brush and grease and periodically take an eye on them.

lm

 

[mfgenggear]

once corrosion starts it's difficult to remove, best start with new coated or plated bolts, depending on the budget,
I have seen even SS 300 series parts was difficult to remove corrosion even if re-passivated.
carbon base steels are even worst,

 

[Tmoose]

Outdoor equipment with " 10-20 years of life before next maintenance work...."

https://lh3.googleusercontent.com/p...SwHKRc7HTdGCqS86aV9JhB23xNNVU4fw9fgrDUE2rfgeQ

 

[STrctPono]

Suggest to look into cathodic protection as an alternative.

What do you mean by this? Zinc rich paint is technically a form of cathodic protection.

I don't know why in the world you would ever install an anchor bolt exposed to the environment and not at least hot-dip galvanize it.

Maintaining anchor bolts is a pain. We design a lot of outdoor sign structures (sign bridges over highways) and I've seen a lot of severe corrosion and necking of anchor bolts. The biggest issues arise when the baseplate is stood off of the concrete with a leveling pad of non-shrink grout. The grout is the culprit. It always cracks and then holds water close to the anchor bolts. The bolts always corrode between the concrete and the base plate. The new State DOT policy where I am is to not use a leveling grout and to design the anchor bolts to resist this stand-off distance.

 

[r13]

STrctPono,

The bolts were epoxy coated, the zinc coat is considered by OP.