Stainless chain - WARNING !!

[By Invitation]

Got a call this moring from my Aunt telling me that "Aurora" our Herreshoff 15 was not on her mooring. My heart sank ! I called my brother John "Loith" to see if he had taken her for a sail - No, he was still in Boston - again my heart sank.

Apparently during the storm last night she broke her mooring and was spotted at 9pm going out of Narragansett bay in RI. I arranged a Mo boat with a friend, called the coast guard and started our search. My fear was that she had gone up on the rocks - there is nothing but rocks here - and been beat up during the night.

We found her in about 45 minutes on the rocky west shore of Jamestown. She was listing about 30-40 degrees to starboard. Juli and I got in the water and swam to her, I couldn't believe it, she was lodged between two large rocks by her keel, no hull breach and no apparent damage. We used her main halyard to pull her over further and pulled with the motor boat and she slid back off the rocks into clear water. I can't believe our luck here, there is not a scratch on her. We found her at high tide which was the only time we would be able to get her off. She is now back on a friends mooring and tomorrow we will dive on our mooring to figure out how she broke loose. First sign is that a new shackle we just put on 3 weeks ago failed. We have had this boat on this mooring for 24 years and never has this happened.

I don't know who to thank but thank God she's OK.

I don't want to wake up again like this! EVER !

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Found our problem: Stainless shackle and stainless chain

Know the solution: Never buy stainless shackles or chain again !

I never thought that changing from standard galvanized chain to stainless would be such a bad decision. We have always been very careful with our mooring. My brother dives on it all the time to check everything. Look at the photo below:

This is a shackle which was 3 weeks old !! Our boat only weighs 2,000 lbs and this shackle had no wear marks, it just snapped an let go !





Now for our chain - the 1/2" long link chain is one year old. Has been on the bottom for this entire time. Normal galvanized chain lasts 3-4 years.
Look at the links where they were welded, the stuff just ate itself.





Yes this was in Salt Water. An yes we also have a long penant to reduce the shock load. I just shot this chain with an alloy analyzer ( it's what I do for a business - I make them ) and the chain is 304SS the shackle is 316SS and the welds were done with 321SS. The mooring is a large cast iron train wheel.

ACCO chain from now on ! HT 1/2" on the bottom with HT 3/8 to the surface. ALL GALVANIZED !!

[ssullivan]

This is just incredible. I'm glad your boat's ok after ghosting around Narragansett. You are indeed very lucky about it not being damaged. I actually can't believe the boat made it out of Narragansett and all the way over to Jamestown without damage. WHEW!

That chain and shackle issue is crazy. Who is the manufacturer?

[exposure]

The chain is classic stress corrosion cracking. It occurs with austenetic stainless steels (316, 304, 317) in high chloride concentrations (sea water). It will occur in high stress and/or heat affect zone areas first. This is why you don't see boat hulls made from stainless. It's also one reason for rudder stock failures.

The shackle is a little harder to explain. I am not sure that I would call it SCC with only a three week life. However, an existing pit or crack would certainly accellerate the process. Maybe others have more experience with this.

This does go to show that you need to inspect any SS anchoring hardware regularly for signs of cracking or pitting.

[By Invitation]

Sean,

I agree with your assesment of the boat "ghosting". She was spotted at 9p.m. friday going past the Bonnet Shores cliffs - outbound. My only assumption is the the tide and wind carried her out than back in again and finally on the rocks in Austin Hollow on Jamestown. She very luckily lodged her keel in between to large rocks and just sat there till I found her.

Damn I can't believe our luck on this one - NEVER AGAIN !!

[Rick]

I've recovered many S/S items diving in marinas. Depending upon the chemical make up of the water and bottom there can be amazing deterioration in 3 days with S/S in contact with the bottom. Regardless of the quality of the stainless it WILL begin pitting when the chemistry is such that the stainless is positive refered to the elements around it.

Stainless is great with intermittent emersion in salt water if normally it is dry or at least in an oxygenated environment for all surfaces. Otherwise it does not belong below the water line without anodic protection (like your prop shaft).

Stainless anchors and chain don't make sense for moorings or anchoring. A waste of good money for sure.

[By Invitation]

Rick,

I have a SS 44Lb Bruce anchor suddenly for sale - wanna but it?
I am now looking for a galvanized "Bruce" brand 66lB Bruce for my Cheoy Lee 41.

[exposure]

Upon further thought, SCC is a high temperature process. More likely its Intergranular Corrosion. This would explain the corrosion in the weld zone...


Quoted:
Intergranular corrosion is a form of relatively rapid and localised corrosion associated with a defective microstructure known as carbide precipitation. When austenitic steels have been exposed for a period of time in the range of approximately 425 to 850°C, or when the steel has been heated to higher temperatures and allowed to cool through that temperature range at a relatively slow rate (such as occurs after welding or air cooling after annealing), the chromium and carbon in the steel combine to form chromium carbide particles along the grain boundaries throughout the steel. Formation of these carbide particles in the grain boundaries depletes the surrounding metal of chromium and reduces its corrosion resistance, allowing the steel to corrode preferentially along the grain boundaries. Steel in this condition is said to be "sensitised".

It should be noted that carbide precipitation depends upon carbon content, temperature and time at temperature. The most critical temperature range is around 700°C, at which 0.06% carbon steels will precipitate carbides in about 2 minutes, whereas 0.02% carbon steels are effectively immune from this problem.

It is possible to reclaim steel which suffers from carbide precipitation by heating it above 1000°C, followed by water quenching to retain the carbon and chromium in solution and so prevent the formation of carbides. Most structures which are welded or heated cannot be given this heat treatment and therefore special grades of steel have been designed to avoid this problem. These are the stabilised grades 321 (stabilised with titanium) and 347 (stabilised with niobium). Titanium and niobium each have much higher affinities for carbon than chromium and therefore titanium carbides, niobium carbides and tantalum carbides form instead of chromium carbides, leaving the chromium in solution and ensuring full corrosion resistance.

Another method used to overcome intergranular corrosion is to use the extra low carbon grades such as Grades 316L and 304L; these have extremely low carbon levels (generally less than 0.03%) and are therefore considerably more resistant to the precipitation of carbide.

Many environments do not cause intergranular corrosion in sensitised austenitic stainless steels, for example, glacial acetic acid at room temperature, alkaline salt solution such as sodium carbonate, potable water and most inland bodies of fresh water. For such environments, it would not be necessary to be concerned about sensitisation. There is also generally no problem in light gauge steel since it usually cools very quickly following welding or other exposure to high temperatures.

It is also the case that the presence of grain boundary carbides is not harmful to the high temperature strength of stainless steels. Grades which are specifically intended for these applications often intentionally have high carbon contents as this increases their high temperature strength and creep resistance. These are the "H" variants such as grades 304H, 316H, 321H and 347H, and also 310. All of these have carbon contents deliberately in the range in which precipitation will occur.
End Quote...

It looks like the low carbon SS is required for use in salt water.