Risk of seawater corrosion to the bolt hole and screw?

[Jackfeng]

An equipment made of carbon steel will be used about 30 meters under seawater. The wetted surfaces are coated and cathodically protected.

Bolts are used to connect a cap into the body. The bolts heads are exposed to seawater. One concern is that the seawater can get into bolt hole through the mating surfaces and may cause corrosion to the bolt hole and scew.

Anyone can tell me if this risk is high and how to provent it? Thanks a lot.

 

[boo1]

Yes this is a common problem, we typically wet install the fastener with Pro-seal sealant.

 

[Jackfeng]

Thank you very much.
Can you introduce more about this corrosion mechnism and how to select Pro-seal sealant?

 

[kclim]

It sounds like Crevice corrosion would be the primary concern, as Ugwue already outlined. However, I would've thought if your structure was cathodically protected (impressed current) and the whole structure is immersed, then the bolting would also be protected. Maybe someone can back me up/correct me on this one.

 

[ugwuegbuchima]

Like boo1 rightly said,sealants provide the best method of protecting the bolt holes against corrosion.Sealants makes water impermeable to the bolt holes thus preventing water from entering the bolt holes.Sealants can be inform of gaskets etc.Care must be taken while slecting sealants to be used in other not to use poros sealants.

 

[boo1]

Use care with gaskets and bolts, leakage can occure. The sealant we use is Pro-Seal 870.

 

[MikeBurns]

A serious concern with bolting materials used subsea under cathodic protection is hydrogen-stress cracking, sometimes called hydrogen embrittlement.

We have seen a number of failures of bolting materials with hardness in excess of 35 Rockwell C (HRC). 35 HRC should be specified as a maximmum for all steel bolting materials. (No Grade 8 bolts, no ASTM A 574 socket head cap screws, etc.)

Beyond that, the combination of cathodic protection and coating should be sufficient to prevent general corrosion. A good thread compound is helpful for reasons other than corrosion control.

mburns@metallurgical.com