Stainless cracking. Metalurgists, puzzel this one

[s/v Jedi]

I see a lot of repeat recommendations for 316L stainless steel, so let’s consider that too:

(Note: my recommendations for titanium and bronze score better than even 316L, also for splash zones)

316L stainless steel, often referred to as "marine grade" stainless steel, is a popular choice for marine and coastal applications due to its relatively good corrosion resistance, including in environments with chloride exposure like seawater. However, in the specific context of the splash zone, 316L stainless steel has certain limitations.

### 316L Stainless Steel in the Splash Zone

**Advantages:**
1. **Corrosion Resistance:** 316L stainless steel has good general corrosion resistance, particularly in low-chloride environments. The addition of molybdenum improves its resistance to pitting and crevice corrosion compared to lower-grade stainless steels.
2. **Availability and Cost:** It is widely available and generally less expensive than materials like titanium or nickel-based alloys.
3. **Mechanical Properties:** It offers a good balance of strength, ductility, and toughness, suitable for various structural applications.

**Disadvantages:**
1. **Pitting and Crevice Corrosion:** Despite its improved resistance over other stainless steels, 316L can still suffer from pitting and crevice corrosion in environments with high chloride concentrations, such as the splash zone.
2. **Not Ideal for High-Chloride Environments:** The splash zone can concentrate chlorides due to evaporating seawater, which may exceed the tolerance levels of 316L, leading to accelerated corrosion.
3. **Maintenance:** To maintain its appearance and prevent corrosion, 316L often requires regular cleaning and maintenance, especially in aggressive environments like the splash zone.

### Position in the List

Considering the specific challenges of the splash zone, the materials might be ranked as follows:

1. **Titanium**
2. **Nickel Alloys (e.g., Hastelloy)**
3. **Duplex and Super Duplex Stainless Steels**
4. **Bronze**
5. **Copper-Nickel Alloys (Cu-Ni)**
6. **316L Stainless Steel**
7. **Aluminum Alloys**
8. **Fiber-Reinforced Polymers (FRPs)**

**Rationale:**
- **Below Duplex and Super Duplex Stainless Steels:** Duplex stainless steels provide superior resistance to pitting and crevice corrosion compared to 316L, which is crucial in the splash zone's high-chloride environment.
- **Below Bronze and Copper-Nickel Alloys:** These materials generally offer better overall corrosion resistance and durability in marine environments, particularly in resisting biofouling and dealing with varying chloride concentrations.
- **Above Aluminum Alloys and FRPs:** While 316L stainless steel can suffer from localized corrosion, it generally has better mechanical properties and is more readily used in structural applications than aluminum alloys or FRPs in harsh marine environments.

316L stainless steel can be a practical and cost-effective choice for many marine applications, including some splash zone scenarios, but it may require careful maintenance and may not perform as well as more corrosion-resistant materials like duplex stainless steels, bronze, or nickel alloys in the most aggressive environments.

[dlj]

Nice material summary (a bit lacking in actual detail - for example, if we are truly considering all material options, then you are missing cobalt alloys, and one of my favorites - Rex 734. Plus the generalized families of alloys is not sufficiently specific for my tastes).

Now, if you take that list, and put availability of parts and price - that may be functionally more useful. Remember, you said cost is one of the variables needed to be considered... Cost and availability are both useful considerations.

dj

[Icarus]

Crevice corrosion

[OneBoatman]

So, this one is like so many others. It starts outs as a guy curious to maybe learn why what the mfgr may have used on his boat for the most part all failed in the same manner but then turned into open season on what he should use to replace the unknown. Today even Boeing's purchasing agents seem to get duped now and then even though the board and the stockholders might encourage them to save money whenever they can, (provided they never fly in Boeing's plane) as that might be a good enough reason not to responsible to check to see if the certs they receive represent the truth. We have come to know that the self cert thing doesn't really fly.

[dlj]

Originally Posted by OneBoatman

So, this one is like so many others. It starts outs as a guy curious to maybe learn why what the mfgr may have used on his boat for the most part all failed in the same manner but then turned into open season on what he should use to replace the unknown. Today even Boeing's purchasing agents seem to get duped now and then even though the board and the stockholders might encourage them to save money whenever they can, (provided they never fly in Boeing's plane) as that might be a good enough reason not to responsible to check to see if the certs they receive represent the truth. We have come to know that the self cert thing doesn't really fly.

Oh yeah, years ago I was working for a company that supplied supposedly 316 parts in an exterior application. The sales rep for the area got a call - Hey why are all these parts rusting? We pulled some parts out and discovered they were NOT 316... That single event justified buying a XRF gun for the QC department to perform spot checks in incoming material to make sure they were getting what was claimed ..

dj

[Mrcruisine]

A clear case of crevice corrosion as many here have correctly mentioned. On my old Westerley 36 foot ketch purchased 18 months ago there was tell tale rust weeping from under chainplate bolt heads. Something the surveyor didn't seem to notice or understand. When I removed and replaced each one the bolt heads literally exploded off and the images were much like yours. The original bolts had been installed back in 1978, and they were probably 304 or similar although no bolt markings were on them. Where the bolt head contacted the stainless chainplates crevice corrosion had obviously started even though inside and not exposed to weather. New 316 bolts were installed and under the bolt head where it passed through the chainplate I have used Tef gel Ultra to minimize any corrosion activity. Consider using that as its great stuff for use on your application.

[thinwater]

The question was never if it was crevice corrosion or some related mechanism specific to stainless. This is assumed, since the structure is very conservatively designed and the bolts are not highly stressed by sailing. The questions were:

• Why right under the bolt heads and not farther in, as is more common on chain plates and thru-hull bolts?
• Why some Farrier designs and not others, because the hinge design is very similar? From the information we are gathering, the problems seem to be with DIY built boats from plans and not factory boats.

A few plausible reasons seem to be:

• A different washer material was suggested as a possible reason, but different materials were used under failed bolts (304 SS and composite).
• These boats use far more carbon in the beam area. Carbon is very noble and could contribute if not well isolated. But we are not sure the bolts penetrated the carbon.
• Factory boats used 316 SS bolts and the DIY boats seem to have used 304 S bolts. But we're not sure of this.

[dlj]

Originally Posted by thinwater

The question was never if it was crevice corrosion or some related mechanism specific to stainless. This is assumed, since the structure is very conservatively designed and the bolts are not highly stressed by sailing. The questions were:

• Why right under the bolt heads and not farther in, as is more common on chain plates and thru-hull bolts?
• Why some Farrier designs and not others, because the hinge design is very similar? From the information we are gathering, the problems seem to be with DIY built boats from plans and not factory boats.

A few plausible reasons seem to be:

• A different washer material was suggested as a possible reason, but different materials were used under failed bolts (304 SS and composite).
• These boats use far more carbon in the beam area. Carbon is very noble and could contribute if not well isolated. But we are not sure the bolts penetrated the carbon.
• Factory boats used 316 SS bolts and the DIY boats seem to have used 304 S bolts. But we're not sure of this.

You don't have enough information to answer your question beyond conjecture.

dj

[s/v Jedi]

Y’all need to up your game

Here’s the correct and complete answer as to why those bolts failed:

In splash zone conditions, a stainless steel bolt failing with the head falling off is typically indicative of corrosion-related issues. Several likely causes include:

### 1. **Stress Corrosion Cracking (SCC)**
SCC is a type of localized corrosion that can lead to sudden and catastrophic failure of metal under tensile stress, even if the stress levels are below the material's yield strength. In the case of stainless steel, especially in the presence of chlorides, SCC can cause cracking that propagates over time. The splash zone, with its frequent wetting and drying cycles, can create the conditions that accelerate SCC.

### 2. **Hydrogen Embrittlement**
Hydrogen embrittlement occurs when hydrogen atoms penetrate the metal and cause it to become brittle. This can happen in environments where moisture and certain chemicals are present, such as in the splash zone. If the bolt was subjected to processes that introduced hydrogen (like improper galvanic processes or welding without appropriate precautions), it might become susceptible to this type of failure, leading to a brittle fracture where the head falls off.

### 3. **Galvanic Corrosion**
If the stainless steel bolt is in contact with a more noble metal (a metal with a higher corrosion potential) or a less noble metal in the presence of an electrolyte (such as seawater), galvanic corrosion can occur. The stainless steel may act as the anode or cathode in this process, depending on the metal it’s in contact with. Galvanic corrosion could lead to significant material loss at the joint, weakening the bolt and leading to the head detaching.

### 4. **Crevice Corrosion**
Crevice corrosion can occur in the small gaps between the bolt and the material it is fastened to, especially in environments with chlorides. The splash zone is particularly prone to this type of corrosion due to the presence of oxygen and chlorides. This localized corrosion can severely weaken the bolt at the crevice area, potentially leading to the head falling off.

### 5. **Pitting Corrosion**
Pitting corrosion is another localized form of corrosion that can create small pits on the surface of the stainless steel. In the splash zone, where chloride concentrations can be high, 316L stainless steel and similar alloys can suffer from pitting. Over time, these pits can grow and lead to structural weaknesses, potentially causing the head of the bolt to fall off.

### 6. **Fatigue Corrosion**
Repeated mechanical loading in conjunction with a corrosive environment can lead to fatigue corrosion. In the splash zone, the combination of mechanical stresses (even low loads) and the corrosive action of seawater can initiate and propagate cracks. Over time, these cracks can grow, leading to the bolt head detaching.

**Conclusion:**
In the splash zone, the most likely cause of a stainless steel bolt failing with the head falling off is a combination of stress corrosion cracking and other localized corrosion mechanisms, such as crevice or pitting corrosion. Factors like the presence of chlorides, repeated wetting and drying, and potential galvanic coupling with other metals can exacerbate these issues. Preventative measures include selecting appropriate materials, using protective coatings, and ensuring proper installation practices to minimize crevices and galvanic interactions.

[s/v Jedi]

And about this happening when carbon fiber is involved:

When stainless steel bolts in a structure with carbon fiber-reinforced fiberglass fail more frequently compared to those in fiberglass-only structures, it often points to issues related to **galvanic corrosion**. Here's why this happens:

### **Galvanic Corrosion Between Carbon Fiber and Stainless Steel**

1. **Electrochemical Potential Difference:**
- Carbon fiber, especially when used in reinforced composites, is electrically conductive. In a wet or moist environment, such as the splash zone, this conductivity can create a galvanic cell if carbon fiber is in contact with stainless steel.
- Stainless steel and carbon fiber have significantly different electrochemical potentials. In the presence of an electrolyte (like seawater or even moist air), the carbon fiber can act as a cathode and the stainless steel as an anode. This leads to accelerated corrosion of the stainless steel, especially at the anode, where material loss occurs.

2. **Accelerated Corrosion Rates:**
- The splash zone's environment—with its frequent wetting and drying cycles, high chloride concentrations, and ample oxygen—can exacerbate galvanic corrosion. The more noble carbon fiber material can cause the less noble stainless steel to corrode more quickly, particularly at points where the stainless steel is in direct contact with or close proximity to the carbon fiber.

3. **Localized Corrosion and Mechanical Weakening:**
- The result of galvanic corrosion in this context is often localized pitting or crevice corrosion at the points of contact. This localized attack can significantly weaken the stainless steel, leading to premature failure such as the heads of bolts falling off.

### **Mitigation Strategies:**

1. **Isolate Materials:**
- To prevent galvanic corrosion, it’s crucial to electrically isolate the stainless steel from the carbon fiber. This can be done using non-conductive barriers, such as gaskets or washers made from insulating materials, to break the electrical path.

2. **Material Selection:**
- Consider using more galvanically compatible materials with carbon fiber, such as titanium or high-nickel alloys, which are less susceptible to galvanic corrosion in contact with carbon fiber.

3. **Coatings and Treatments:**
- Protective coatings on stainless steel bolts can provide a barrier to electrolytes, reducing the risk of galvanic corrosion. However, care must be taken to ensure the coatings are intact and effective in the challenging environment of the splash zone.

4. **Regular Inspection and Maintenance:**
- Regular inspection for signs of corrosion and maintenance to replace affected components can help mitigate the risk of catastrophic failure.

**Conclusion:**
The increased frequency of stainless steel bolt failures in structures incorporating carbon fiber-reinforced fiberglass compared to fiberglass-only structures is primarily due to galvanic corrosion. The conductive nature of carbon fiber exacerbates corrosion when in contact with metals like stainless steel, especially in aggressive environments such as the splash zone. Mitigating this issue requires careful consideration of materials, isolation techniques, and protective measures.

[dlj]

Like I said - if you really want a serious answer - you need more information. The above is simply conjecture.

Nothing in the above has included the potential for manufacturing deficiencies in the base metal, it has not included specific washer interaction if that exits. It does not account for potential off axis head loading, which is likely present looking at two of the bolt head fractures.

But hey, it's a great AI search... Oh and no answer as to why right under the head...

dj

[s/v Jedi]

Originally Posted by dlj

Like I said - if you really want a serious answer - you need more information. The above is simply conjecture.

Nothing in the above has included the potential for manufacturing deficiencies in the base metal, it has not included specific washer interaction if that exits. It does not account for potential off axis head loading, which is likely present looking at two of the bolt head fractures.

But hey, it's a great AI search... Oh and no answer as to why right under the head...

dj

It is a complete answer and you can’t handle it. You move goalposts continuously when this happens. It isn’t conjecture because it lists every possible cause within the context it has. I expanded that context with splash zone, with carbon fiber etc. and when this results in perfect answers and conclusions, you cry and come up with something else that the AI supposedly missed. It doesn’t miss anything, it is up to the human interacting with it to add to the context.

Thing is that every point I made before involving ChatGPT is fully confirmed by science regardless of your supposedly solid references to the opposite. You called me out for just voicing my opinion while you have solid data and I have showed that I was correct and you were wrong in those accusations. It appears that what I write is based upon the knowledge I have built up over years of study and solid references for my statements are abundantly available.

Of course I’m not going to expand context anymore. Everyone can afford to pay for their own ChatGPT subscription and learn how to use it effectively. I hope that readers of this thread realize the potential of it despite the haters trying to ridicule it.

[dlj]

Originally Posted by s/v Jedi

It is a complete answer and you can’t handle it. You move goalposts continuously when this happens. It isn’t conjecture because it lists every possible cause within the context it has. I expanded that context with splash zone, with carbon fiber etc. and when this results in perfect answers and conclusions, you cry and come up with something else that the AI supposedly missed. It doesn’t miss anything, it is up to the human interacting with it to add to the context.

Thing is that every point I made before involving ChatGPT is fully confirmed by science regardless of your supposedly solid references to the opposite. You called me out for just voicing my opinion while you have solid data and I have showed that I was correct and you were wrong in those accusations. It appears that what I write is based upon the knowledge I have built up over years of study and solid references for my statements are abundantly available.

Of course I’m not going to expand context anymore. Everyone can afford to pay for their own ChatGPT subscription and learn how to use it effectively. I hope that readers of this thread realize the potential of it despite the haters trying to ridicule it.

Hahahaha, too bad you prefer to have such a confrontational relationship with me. It could actually be quite useful to show both the advantages and limitations of using AI.

I never "called you out" for voicing your opinion. But I did correct statements that were technically wrong, for example when you wrote that anodes only protect against galvanic corrosion... In the subsequent link pointing to the British Web site addressing crevice corrosion that stated cathodic protection was one of the methods used to eliminate crevice, at that time, I said nothing. Some of your statements have been wrong at fundamental levels, I have nicely corrected those. I have been civil and accurate.

It seems to me the problem lies with you. With a more civil interaction with me, we both could benefit as could the rest of the folk who may be reading. But, that's your choice.

dj

[seandepagnier]

Originally Posted by s/v Jedi

I see a lot of repeat recommendations for 316L stainless steel, so let’s consider that too:

(Note: my recommendations for titanium and bronze score better than even 316L, also for splash zones)

Why do you keep repeating "bronze" when it is one of the worst possible things you can bolt to aluminum? Then posting chatgpt bot results that anyone can use for free when we should all know by now that a lot of the stuff that it says is wrong or misleading.

The obvious answer is titanium with some kind of grease as well as replacing all isolation delrin etc.

[notoldbilbo]

I'm one of 'the other people reading this' with acute interest. I'd like to 'add some context' and seek some Guru Guidance, if I may.

I'm looking to bolt a pair of 95mm titanium pad eyes onto the quarters of my 40 y.o. boat, from which to hang a JSD. According to Don Jordan's teachings, I can expect a Maximum Wave Strike to impart a very occasional ~6000lbf/26700kn 'thump'. Much more than that, and I'll need a 'Beam me up, Scottie' badge.

The question has been reinforcing 'adequately' the unknown grp layup of about 10-12mm locally, to cope. I've queried this before and our good friend Zonker was kind enough to advise, wisely, that I ignore the 'known unknown' of the original layup and add sufficient multi-layers of biaxial carbon cloth inside. I'm happy to do that in my cack-handed way and live ( or die ) with the result.

Or... I could use many more layers of biaxial glasscloth. Example in pic below.

I plan to use a 132mm x 3mm bronze plate as 'plate washer' on the inside, giving an interesting 'stew' for the metallurgists among us. The bolts are 4 x M12 BUMAX 88 ISO10642. Their heads and the titanium will be in the 'splash zone'. The inner threaded ends, in contact with the carbon cloth and bronze plate, will not.

The question now is how best to mitigate the risks of corrosion outlined by s/v Jedi in #85 above. Would Tefgel and butyl tape likely suffice? Epoxy?

Then there is the vexed question of 'Preload'....