The use of zinc anodes on bolts to protect plates

[TainanT]

Hello,

I'm currently designing a sacrifical zinc anode protection system for a steel sleeve (ASTM A36 #12,5 plates, surface area 25 m², epoxy coated) for HDPE pipe in seawater condition. The total anode weight calculated is 330 kg.

I would like to know if bolt zinc anodes (6 kg, way larger than needed for Ø7/8''x8'' bolts) can be considered as sacrificial anode weight for the steel plates or exclusively for the bolts.

Thanks in advance.

 

[cap1a79]

No...Zinc coating ( galvanized) on bolt does not have same coorosion loop of sacrificial anode in the connected CP system.

Generally, the key factor of galvanized bolt is compliance of applicalble spec or manufacturer stanadard. While anode calculation is carried out for plain steel surface & vessel against environment (considering relevant variables such as current density, process condition etc.),[/u] bolt will be kind of at closed condition by tightening so the main reason of galvanizing will be to prevent "crevice corrosion" when some fluid or water is trapped in the bolting system. The trapped electrolyte has separate, isolated and independent corrosion loop.
For critical environment such as high sour or subsea, Zn-Ni electroplating + PTFE top coat is applicable for CS bolt as crevice corrosion would be severe, once it is agreed with client.

Lee SiHyoung,
Engineer - Material & Corrosion Specialist
WorleyParsons Oman Engineering,
cap1a79@gmail.com

 

[SJones]

Since the bolts and plates are electrically continuous, the anode coverage can't really be separated into discrete packages.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.