Material Compatibility Issues Subsea Flanges & Fittings 1

[JT100]

Could anyone provide any guidance on any material compatibility issues with specifying A453 Grd660 fasteners for use in a marine environment on Inconel Alloy fittings? I have checked the coefficients of expansion and there isn't a major difference so I wouldn't of thought this would/could lead to excessive bolt stresses or flange connection integrity issues.

 

[CoryPad]

SAE J2280 SHIP SYSTEMS AND EQUIPMENT—FASTENERS—SELECTION AND IDENTIFICATION REQUIREMENTS states that nickel alloy components that are submerged should be joined with fasteners composed of the following materials:

NiCu Alloy 400/405
NiCuAl QQ-N-286
Alloy 625+
Alloy 725
Titanium

 

[TVP]

ASTM A 453 Grade 660 fasteners are not suitable for this application for a several reasons. First, galvanic compatibility with Ni-based alloys in seawater is not good. Second, without cathodic protection (CP), Grade 660 (also called A286, X5NiCrTiMoVB25-15-2, or 1.4606) is susceptible to pitting and crevice corrosion in subsea applications. Ni-based alloys like Alloy 625 or 725 are high strength fasteners that are suitable for subsea use without CP.

 

[JT100]

there will be CP provided, however it sounds like i'll be better going with something else. is there a reference for the galvanic compatibility issues? i read in ISO 10423 that A453 Gr 660 fasteners are good for use across the board for subsea use. thanks

 

[TVP]

JT100,

I assumed that CP was not being used based on the materials you mentioned, which was obviously a bad assumption on my part. As long as the full length of the fastener will be exposed to CP, then ASTM A 453 Grade 660 is acceptable in contact with Ni-based alloys. In addition to ISO 10463, heere is an article that discusses the topic of fasteners for seawater use, galvanic coupling, etc.

http://namis.alionscience.com/conf/tscc/search/pdf/AM022833.pdf

 

[CoryPad]

JT100,

The SAE document I referred to shows Grade 660 as compatible with nickel alloys in wet environments, but not compatible for submerged conditions.