Fiberglass burried chainplate recommended metals

[boatrus]

I am replacing marine chain-plates on a 28 yo boat that are encapsulated in the hull.

The original plates corroded in two places --typical anaerobic SCC-- on questionable 316 SS (unknown chemistry). So my question is, what better metal choices do I have for fiberglass encapsulated metals in a marine environment?

Some choices I have looked at include:

1. Nitronic 50
2. Grade 5 Ti
3. Monel 400

I have seen a flurry of topics on other 'possible' alloy metals, so there's a typical set of needs, such a rigging loads, costs, availability, and machining, welding, etc. (The original 13mm 316 handled the loads without issue for 28 years, with some elongation in the clevis pin area of approximately 1 to 2%.)

While it is true that the original 316(?) did last ~25 years, I would still like to look at perhaps better replacement possibilities as it is possible this boat will be around for quite some time. (BTW, outside chain-plates are not an option.)

-Thanks.

More Info:

The plates are 46mm x 13mm and 105mm x 13mm bar with butt-end welds for hull anchor arms and legs, with a neck length of ~650mm.

Originally embedded in a polyester resin but will be replaced with either vinyl-ester or epoxy resin bedding.

A new two-part design will be used with an overlapping part (originally one-piece) that will overlap on the glass with the embedded part, assembled with custom flanged cap hex 'pin' shoulder bolts held in-place with tacked-on flanged nuts, whereby the threads only support clamping pressure and not shear (placed on the shoulder pins only).

These custom made bolts and nuts will be made with the same material as the plate.

With the two-piece design, the top part can be removed for inspection while the anchor part will be sealed in the hull. To avoid overlap of metal parts, the fiberglass bedding should prevent crevices formation between the two overlapping metal parts.

A two-part design should also permit totally sealing the embedded part from moisture whereas the original design allowed deck water to invade down to the embedded hull anchor, since it was a common compartment from hull up to the deck opening.

If you have Google Sketchup8 (download for free at google.com) or another CAD system, you can use my SKP file attached to view the proposed 3D design.

 

[JamesCKelly]

Simplest, and available/weldable, choice for seawater is one of the 6% molybdenum stainless grades. AL-6XN is stocked by Rolled Alloys in many sizes and forms, as is 254 SMO by Outokumpu. Better crevice corrosion resistance than 316.

Duplex 2205 is better than 316, widely available; more economical but not nearly as good as 6% moly stainless.
A superduplex 2507 or Zeron 100 could be more economical wrt inital material cost but does require more sophistication in welding.

 

[EdStainless]

One of the 6% Mo superaustenitics would be a good option.
Weld, anneal (professionally), form, pickle, install.

Monel 400 is a very choice as well but not as strong and harder to weld.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[boatrus]

I am finding 'new' use of Nitronic 50 (tm) in the marine industries as chainplate (and historically used as rod rigging) material on new production build sailboats and mega yachts (replacing 300 series metals). But looking at the chemistry, it is not quite up to the 6% Mo:

Carbon 0.06 max
Chromium 20.5 - 23.5
Iron Balance
Manganese 4 - 6
Molybdenum 1.5 - 3
Nickel 11.5 - 13.5
Niobium 0.1 - 0.3
Nitrogen 0.2 - 0.4
Phosphorus 0.04 max
Silicon 1 max
Sulphur 0.03 max
Vanadium 0.1 - 0.3

I have also found one supplier in Texas, US, that can build the requested chainplates from Nitronic 50 (tm) as it is used quite often in the Oil Rig/ Offshore Oil Platform industry, and readily available in the correct sizes.

What's interesting is the metal reaction lists I have found for Nitronic 50 do not show it as having an active and passive state; these lists show the galvanic potential of 'stainless' (300 series) on scale in both passive and active states. But none show Nitronic 50 on scale as having both states. (In fact, many show the galvanic potential of Nitronic 50 just under Titanium --or at least very near or the closest to...)

Since the issue is when the metal is encapsulated in the fiberglass it will more than like go in to an active state, would the indication that Nitronic 50 (tm) not having an active state potential still make it a 'good' choice, even though it is lacking in chemistry % Mo? (I would think that only the 400 series metals could have this type of property...)

In the meantime, I am searching for other potential suppliers who have built chainplates using the higher % Mo metals such as the 254SMO and AL6XN... (May have found a AL6XN value-added supplier with chainplate experience also in the Texas, US area --but I still need to make contact.)

On an interesting side-note, I have found info that having a lower nickel content is best as most of the issues in the old chainplates were related to stress corrosion cracking and then more intergranular failure. Since I would like to keep the metal nonmagnetic, and therefore in the austenic range, it would seem 310-AL6XN (*70% probability) is a better choice than 254SMO (*~87%)? Also noted relationship of the alloying agents as:

*Chromium: Oxidation Resistance
*Nickel: Austenite former - Increases resistance to mineral acids
*Molybdenum: Increases resistance to chlorides
*Copper: Provides resistance to sulfuric acid
*Manganese: Austenite former - Combines with sulfur
*Sulfur: Austenite former - Improves resistance to chlorides

So it seems nickel has some role for preventing failure issues, but perhaps lower %Ni is better(?)

(*See downloaded reference material in attached HTTP pointer for more source info and credits... Note that page 15 seems to have contradictory statements about SCC and %Ni content. %Probability Values given in the chart on page 14, but interestingly, that Table on page 14 show Titanium as subject to SCC in a 'marine' environment --coastal atmosphere? Document seems biased to selling 310-AL6XN.)

-Thanks.

 

[boatrus]

Here is another reference that I found, now showing 254SMO a better solution (based on the report's testing criteria and conclusions) as a potential replacement for 300 series metals. It seems the copper content helped preserve the passive resistance capability in 254SMO as compared to AL6XN.

Based upon some of these references, it seems the PRE formula is a good indicator for how the metal will handle being buried in fiberglass:

For Austenitic X5CrNiMo17-12-2 PRE=25 seems to be about the highest PRE value, as the relationship to tensile strength, and ratios of Cr, Ni, and Mo % are important.

 

[JamesCKelly]

If you are saying that Nitronic 50 fails by stress corrosion cracking (SCC), then I would use AL-6XN/254 SMO, whichever is available.

If in fact Nitronic 50 WORKS and is available, I'd seriously consider using it.
Yeah, the 6%Mo grades are better but AVAILABILITY of the parts you want + good service experience are important points.

So far as chloride SCC is concerned you need either NO nickel, quite impractical, or a fair amount of Ni. Both AL-6XN and 254 SMO have proven resistant to SCC in seawater environments. I don't know about Nitronic 50 in that regard, but from chemistry would expect it to be inferior. It is not suited for long time seawater service, specifically not for use in off-shore oil drilling equipnemt.

Fiberglass encapsulation would, I suspect, make a nice crevice to cause crevice corrosion in this environment. Whether the 6%Mo alloys are resistant depends upon operating temperature, i.e., hot is bad. Disremember the temperatures but there is experience to show how hot they can be used. Again, either 6%Mo grade better than Nitronic.

Nitrogen, as in Nitronic 50 and both 6Moly alloys, improves chloride pitting and crevice corrosion resistance. It adds to the effect of molybdenum and is the "N" in AL-6XN. (Metallurgical Rumour suggests a variety of much more innovative origins for alloys, such as Rene 41 for a healthy secretary at GE named Rene, or A-286 being the designation of the particular heat that worked during development of that age hardening stainless at Special Metals in New York state, &c.)