Stainless cracking. Metalurgists, puzzel this one

[Seacod]

Given exposure to chlorides and structural loads, 304 was really not appropriate and example of similiar problems see this:


https://www.iims.org.uk/stainless-st...sion-cracking/

[s/v Jedi]

And for completeness I asked about any other materials, see response below.

Note that my recommendations for bronze, titanium and Monel are fully supported y expert opinion like I already suggested before.

Several materials can be considered comparable or even superior to titanium in certain aspects for submerged seawater applications, depending on the specific requirements such as corrosion resistance, strength, weight, and cost. Here are a few notable alternatives:

### 1. **Duplex and Super Duplex Stainless Steels**

- **Advantages:** These are highly alloyed stainless steels that offer excellent resistance to chloride-induced stress corrosion cracking and pitting, better than standard stainless steels. They provide a good balance of strength, toughness, and corrosion resistance.
- **Disadvantages:** While better than regular stainless steel, they can still suffer from crevice corrosion in stagnant seawater conditions. They are also more expensive and harder to work with than standard stainless steels.

### 2. **Hastelloy and Other Nickel Alloys**

- **Advantages:** Nickel-based alloys like Hastelloy are extremely resistant to corrosion in harsh environments, including seawater. They can withstand very high temperatures and are immune to most forms of chemical corrosion.
- **Disadvantages:** These alloys are typically very expensive and can be challenging to machine and fabricate. They are usually used in specialized applications where extreme corrosion resistance is required.

### 3. **Aluminum Alloys**

- **Advantages:** Certain aluminum alloys, especially marine-grade ones (e.g., 5083 and 6061), offer good corrosion resistance in seawater, are lightweight, and have a favorable strength-to-weight ratio.
- **Disadvantages:** Aluminum is not as strong as titanium or high-strength stainless steels, and its corrosion resistance, while good, is not as high as that of titanium or nickel alloys. It can also suffer from galvanic corrosion when in contact with other metals.

### 4. **Copper-Nickel Alloys (Cu-Ni)**

- **Advantages:** These alloys offer excellent resistance to corrosion and biofouling in seawater. They are also resistant to stress corrosion cracking and are good thermal conductors, making them suitable for heat exchanger and piping systems.
- **Disadvantages:** Copper-nickel alloys are heavier than titanium and are not as strong. They are also relatively expensive and can suffer from galvanic corrosion when in contact with more noble metals.

### 5. **Fiber-Reinforced Polymers (FRPs)**

- **Advantages:** FRPs, including carbon fiber and glass-reinforced plastics, offer excellent corrosion resistance and are very lightweight. They are increasingly used in marine environments for structures, vessels, and piping.
- **Disadvantages:** While very corrosion-resistant, they generally lack the strength of metals like titanium or duplex stainless steel. They can also be more costly to produce and repair.

### 6. **Tantalum and Zirconium Alloys**

- **Advantages:** These are highly resistant to corrosion, including in very aggressive environments such as seawater. They are used in extremely specialized applications, such as in chemical processing equipment.
- **Disadvantages:** These metals are extremely expensive and are not typically used unless absolutely necessary due to their cost.

### Conclusion

Titanium remains one of the most versatile and reliable materials for submerged seawater applications due to its combination of corrosion resistance, strength, and relatively low density. However, for specific applications, other materials like duplex stainless steels, nickel alloys, or copper-nickel alloys may offer advantages, depending on factors like cost, specific mechanical properties, and environmental conditions. The best choice often depends on the precise requirements of the application, including mechanical loads, environmental exposure, and budget constraints.

[dlj]

Originally Posted by b.needalman

Crevice corrosion is the likely cause.

It is never a good idea to assign a specific cause without having actual data to support that. This immediate allocation appears to be the main reason that here in the marine industry, it seems crevice corrosion is the only cause attributed to any and all corrosion issues seen in this industry when talking about stainless steel.

In my opinion, that is doing a major disservice to the marine industry.

It is so widespread at this point, that anyone suggesting that it may be something other than crevice corrosion is immediately discarded as wrong. That's a shame...

Originally Posted by b.needalman

When water sits on stainless for a long period of time, it denies the stainless the oxygen needed to maintain the protective chromium oxide layer. The same effect is seen whenever the installation keeps air away from the stainless. Stainless chainplates embedded in fiberglass often suffer crevice corrosion.

Yes, this is a good elementary explanation of crevice corrosion.

Originally Posted by b.needalman

Bolts fail at the head because the forging process used to make them work hardens and embrittles the metal at the point that the head meets the shaft. Right angles in metals are where stresses are highest.

While this can be true in some senses, some of your description is inaccurate. Work hardening does not necessarily “embrittle” this material. Work hardening of austenitic stainless steels is used to increase the tensile strength of those materials. The term embrittlement typically is used in metallurgy to mean that the material has been compromised. It would be a stretch to apply this term to these alloys. But that would require a fairly long discussion.

Right angles are indeed detrimental, but bolts are designed to have a radius under the head to not have that problem. From the photos posted earlier, you can see that those bolts did have that radius.

Additionally, most fasteners of this size post forging - when they are forged, not all are - undergo an annealing step to remove excessive work hardening. Here, one would have to have the actual bolts that broke and do a metallurgical assessment of how the bolt was fabricated to begin to understand more fully if that had anything to do with the broken heads.

Originally Posted by b.needalman

316 is a common alloy that stands up better than other stainless to crevice corrosion. 316 bolts are readily available for a slight premium. There are more resistant alloys, but you will likely have trouble finding a bolt made from them.

There's the rub – there are a number of alloys that could perform better, but finding off the shelf bolts would be difficult. So keeping the solution within the constraints of what is easily obtainable is what is desired.

Originally Posted by b.needalman

Never use 5200 as a bedding compound. It is a permanent adhesive that is difficult to remove. Polysulfide is a better choice. Butyl tape is an alternative.

This is good advice. In this specific case there may be other solutions rather than using a sealant. Although that may also be a viable addition to the solution. However, in this case, it is more likely that removing contact with the fastener to the carbon fiber as it passes through the hull should have a much bigger positive impact on the fix.

Originally Posted by b.needalman

https://masteel.co.uk/news/a-quick-g...l/#:~:text=For

I've copied below what your link says about crevice corrosion. It's a nice overview.

There are fundamental concepts found in the below that I would like to point out. First in the What Causes Crevice Corrosion – note how it says the chromium layer “stops forming”. Now, in normal practice, that layer stops forming once it is formed initially. That layer is actually more like a ceramic than a metal. So the real question is what is causing this protective layer to need to be reformed? The answer to that question is one that must be answered in order to have a successful “repair”. There are many reasons this can be happening.... That is what becomes the root cause – identify that root cause and you solve the problem. Crevice corrosion is not the root cause – that is the mechanism causing the part to break. Those are two different things.

The solutions listed in Preventing Crevice Corrosion include cathodic protection, coatings, and sealants. Here one has to look at what method will work best in the specific application. That can become a pretty big task.

dj

Crevice Corrosion

Pitting corrosion can be seen upon the surface of stainless steel. However, crevice corrosion occurs when the spaces found between various surfaces begin to hold water or a different mixture. When these gaps lose oxygen, its conditions become acidic. This is when corrosion can occur and reduce the protection provided by the defensive layer of the stainless steel.

What Causes Crevice Corrosion?

Crevice corrosion appears when there are low amounts of oxygen within small gaps or crevices. As a result, the stainless steel’s chromium layer stops reforming. This type of corrosion can also occur due to the presence of moisture.

Signs of Crevice Corrosion

It can be difficult to see the damage that crevice corrosion inflicts upon stainless steel. However it can quickly spread within this material. Therefore, it is important to keep an eye on stains of rust. As internal devices, such as tubing clamps, can be affected by crevice corrosion, they may need to be taken out of the overall system.

The Effects of Crevice Corrosion

If left untreated, stainless steel can quickly deteriorate because of corrosion. The consequences of this corrosion are normally worse than those from pitting corrosion. This is due to the gaps being more extensive and thinner. Moreover, the thickness of stainless steel components can be affected by crevice corrosion and may break under weight. Cracks may also form as crevice corrosion can increase specific concentrations of stress.

Preventing Crevice Corrosion

The issues from crevice corrosion stem from the stainless steel having gaps within it. Creating a design for a device that incorporates fewer gaps between surfaces, such as those found next to corners and bolted connections, is one way to deal with crevice corrosion. Should this not be possible, the following techniques can help:

Applying cathodic protection*

Incorporating coatings and inhibitors

Using seal crevices, such as polysulfide sealants.

[sailorladd]

I suspect that these bolts are not 316L SS, rather, they appear to be 304 which may make them susceptible to electrolysis...

[dlj]

Originally Posted by s/v Jedi

I use AI to help me formulate things better and to dig up (factual) data. I use ChatGPT 4, which is a paid for version and it will actually search for and present me any scientific data I want or even does that to aid in formulating what I ask it to do.

Don’t try the “AI doesn’t know” angle, these new GPT’s outperform most experts.

You are taking my statement out of context. I did not say AI doesn't know - what I said is you have to have a solid understanding of metallurgy and the application of the information coming out of AI. The other point, is that AI will pull up all sorts of "reports" but many of those reports have inadequate background data to rely on the information being presented.

I have worked with ChatGPT 4 - I am very aware of what it does and does not do.

As to the rest of the multiple posts you just made regarding several of these alloy systems - note that you were using "submerged" often.

While we could have long conversations about each of those searches - they are a great starting point. But, submerged is not the application relevant to the fasteners in this thread.

dj

[dlj]

Quick note to all the posters saying the bolts are not 316 - that's correct - they were 304.

Indeed one of the first recommendations was to go to 316.

dj

[dlj]

Originally Posted by s/v Jedi

### Conclusion

Titanium remains one of the most versatile and reliable materials for submerged seawater applications due to its combination of corrosion resistance, strength, and relatively low density. However, for specific applications, other materials like duplex stainless steels, nickel alloys, or copper-nickel alloys may offer advantages, depending on factors like cost, specific mechanical properties, and environmental conditions. The best choice often depends on the precise requirements of the application, including mechanical loads, environmental exposure, and budget constraints.

Excellent conclusion - but we aren't talking submerged application here.

But the last sentence "The best choice often depends on the precise requirements of the application, including mechanical loads, environmental exposure, and budget constraints."

That conclusion is applicable to ... everything.... LOL

dj

[OneBoatman]

You might want to determine if you can if they used structural hex bolts, or hex head caps screws? This could help explain. One indicator is the presence or lack of a radius where the head joins the shank. The former is hot forged and the latter is typically cold forged. Hex bolts which are hot forged if they are of 304 they must be cold carbide annealed to retain necessary corrosion resistance. They will exhibit a radius where the head transitions to the shank. Hex head cap screws are usually cold forged and depending on the number of stages the strength is most typically lower than the former in the vicinity of the processing. If in the manufacturing process either turning is done to reduce or nearly eliminate the radius I speak of, as potentially creating an undercut that also could account for why so many failed in that location. Others factors contribute to premature failure besides alloy and processing. among them galvanic corrosion which will usually be at the very same low current density location just under the head, you have a cell there, the steel alloy, moisture, and Aluminum. The area just below head also sees nearly fifty percent of whatever the fastening stress is. However, I don't think we are looking at crevice stress corrosion as a singular cause.

[dlj]

Originally Posted by OneBoatman

You might want to determine if you can if they used structural hex bolts, or hex head caps screws? This could help explain. One indicator is the presence or lack of a radius where the head joins the shank. The former is hot forged and the latter is typically cold forged. Hex bolts which are hot forged if they are of 304 they must be cold carbide annealed to retain necessary corrosion resistance. They will exhibit a radius where the head transitions to the shank. Hex head cap screws are usually cold forged and depending on the number of stages the strength is most typically lower than the former in the vicinity of the processing. If in the manufacturing process either turning is done to reduce or nearly eliminate the radius I speak of, as potentially creating an undercut that also could account for why so many failed in that location. Others factors contribute to premature failure besides alloy and processing. among them galvanic corrosion which will usually be at the very same low current density location just under the head, you have a cell there, the steel alloy, moisture, and Aluminum. The area just below head also sees nearly fifty percent of whatever the fastening stress is. However, I don't think we are looking at crevice stress corrosion as a singular cause.

The bolts no longer exist - they were thrown out. However, if you look very carefully at the original photos – there does appear to be the radius indicating they were likely structural hex bolts and likely hot forged, and therefore, likely would have gone through the annealing step you mention. There is no way to know if they were subsequently machined, but the visible radius seems to indicate no they weren't.

I completely agree with you that crevice corrosion is not the singular cause. There may not even be any crevice corrosion present to any degree – but – not having the bolts, it's all just guessing...

dj

[Seahunter]

Stainless is only one thing, stainless. Not rust/corrosion proof, not exceedingly strong, but brittle because of its carbon/chromium composition. You don't need a degree in metallurgy to understand why its susceptible to cracking. On top of that stainless is as susceptible to corrosion failure as any ferrous metal. Sometimes it goes without notice until it fails.

[Tillikum]

Don't have time to read it all, but looks like a brittle fracture to me. Stainless in general is prone to work-hardening and embrittlement. Could be the problem originated during manufacture in a certain part of the world where QC tends to be the responsibility of the customer.

The forming of the head of a bolt via forging is where the greatest movement of material occurs and therefore where the greatest embedded stresses are found, and where the greatest load occurs in use. Improper heat treating during manufacture or wrong alloy?

Would have been smart to take them to a scrap yard or university lab which has an x-ray spectrometer and test their actual metallic composition as well.

[Shaneesprit]

Actually it is not the bolts that take the full force of the heeling or at least it should not be, the bolts provide the clamping force between the plate and the hull.
The friction between the 2 surfaces then takes the load, this is why correct tightening torque of bolts is critical.
Over tightening a bolt causes bolt stretch and so the bolt feels tighter but the clamping force is reduced, leading to bolt failure.
Not saying that is the cause here but it’s an important point to understand when assessing bolt failures.

[jjjacq]

Originally Posted by sailorboy1

Well to me that seems a shear or bearing failure instead of a pitting corrosion issue. That suggests that bolt strength is too low, either from the material of size.

Can't be, the shear plane is not below the head , it is where the two plates slide against each other. This is inter-granular corrosion of a bolt in tension.

[s/v Jedi]

Okay so all the info I post is irrelevant because we are talking about a splash zone instead of submerged. More work for my gpt:

The considerations and material choices do change when dealing with the splash zone, compared to continuous submersion in seawater. The splash zone, which includes areas exposed to both air and seawater due to tides, waves, or splashing, presents unique challenges due to:

1. **Frequent Wetting and Drying:** This can accelerate corrosion, especially for materials that form protective oxide layers, as these layers can be repeatedly broken and reformed.
2. **Higher Oxygen Levels:** The presence of both air and water increases the oxygen availability, which can enhance certain types of corrosion, like pitting and crevice corrosion.
3. **Chloride Concentration:** Evaporation in the splash zone can lead to higher concentrations of chlorides, exacerbating corrosion issues.

Given these factors, here's how the materials compare for splash zone applications:

### 1. **Titanium**

- **Advantages:** Titanium remains highly resistant to corrosion in the splash zone, as its protective oxide layer is very stable and self-healing even in aggressive environments. It is one of the most reliable choices for resisting corrosion due to both chlorides and oxygen exposure.
- **Disadvantages:** Cost remains high, and its performance advantage over other materials might not justify the expense in all cases.

### 2. **Duplex and Super Duplex Stainless Steels**

- **Advantages:** These alloys perform better than standard stainless steels in splash zones due to their higher resistance to pitting and crevice corrosion. They also have good mechanical strength.
- **Disadvantages:** They can still suffer from localized corrosion if there are areas of stagnant water or if the protective film is repeatedly disrupted.

### 3. **Nickel Alloys (e.g., Hastelloy)**

- **Advantages:** Excellent resistance to all forms of corrosion, including in the splash zone. Nickel alloys are particularly resistant to crevice and pitting corrosion.
- **Disadvantages:** Very high cost and difficulty in fabrication can limit their use to highly critical applications.

### 4. **Aluminum Alloys**

- **Advantages:** Aluminum's lightweight nature is beneficial, and certain marine-grade aluminum alloys can perform well in the splash zone with appropriate coatings.
- **Disadvantages:** Without protective coatings, aluminum can suffer from localized corrosion. It is less durable than titanium or high-grade stainless steels in this environment.

### 5. **Copper-Nickel Alloys (Cu-Ni)**

- **Advantages:** Good corrosion resistance in marine environments, including the splash zone. They are also resistant to biofouling.
- **Disadvantages:** Heavier and less strong than some other options. They can also suffer from galvanic corrosion when in contact with more noble metals.

### 6. **Fiber-Reinforced Polymers (FRPs)**

- **Advantages:** Excellent corrosion resistance and lightweight. FRPs do not corrode in the traditional sense and are immune to chlorides.
- **Disadvantages:** Mechanical properties can be lower than metals, and they can suffer from UV degradation if not properly protected. Repairing damage can also be more complex.

### Conclusion for Splash Zone Applications

In the splash zone, the dynamic nature of the environment—constant wetting and drying, high oxygen, and high chloride concentrations—requires materials that can withstand these conditions without significant degradation. Titanium and high-performance nickel alloys (like Hastelloy) generally provide the best resistance but at a high cost. Duplex stainless steels offer a good compromise between performance and cost. For less critical applications where weight and cost are more significant concerns, aluminum alloys or fiber-reinforced polymers may be suitable, provided they are adequately protected with coatings or treatments.

The specific choice of material will depend on the balance between performance requirements, environmental conditions, and budget considerations.

[s/v Jedi]

Naughty gpt left out bronze as it thinks it’s old fashioned

So I force it to consider it:

In the context of the splash zone, bronze would generally fall into the following considerations:

### Bronze in the Splash Zone

**Advantages:**
1. **Corrosion Resistance:** Bronze, particularly silicon bronze and aluminum bronze, has good corrosion resistance in marine environments, including the splash zone. It forms a protective patina that helps guard against further corrosion.
2. **Biofouling Resistance:** Bronze has some natural resistance to biofouling, which can be an advantage in marine environments.
3. **Mechanical Properties:** It offers good strength and durability, with decent resistance to wear and fatigue.

**Disadvantages:**
1. **Susceptibility to Erosion-Corrosion:** In the splash zone, where the action of waves and splashing water can be intense, bronze can suffer from erosion-corrosion, where the protective patina can be mechanically worn away, exposing fresh metal to corrosive elements.
2. **Galvanic Corrosion:** Bronze can suffer from galvanic corrosion if it is in contact with more noble metals in the presence of an electrolyte, such as seawater. This is a consideration in mixed-metal constructions.
3. **Cost and Availability:** Bronze is generally more expensive than standard stainless steels, although it is usually less expensive than titanium and nickel-based alloys.

### Position in the List

Bronze would typically be placed 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. **Aluminum Alloys**
7. **Fiber-Reinforced Polymers (FRPs)**

**Rationale:**
- **Above Copper-Nickel Alloys (Cu-Ni):** While both bronze and copper-nickel alloys have good corrosion resistance and similar applications, bronze is often more robust in terms of mechanical properties and has better resistance to stress and wear. However, copper-nickel alloys may have the edge in specific resistance to seawater, particularly concerning biofouling and uniform corrosion resistance.
- **Below Duplex Stainless Steels:** Duplex and super duplex stainless steels generally offer better resistance to localized corrosion types, such as pitting and crevice corrosion, which are critical concerns in the splash zone. They also have higher strength-to-weight ratios compared to bronze.
- **Cost Consideration:** Bronze can be a more cost-effective choice than nickel alloys and titanium while still offering significant corrosion resistance and durability.

Bronze is thus a viable option for many splash zone applications, particularly where a balance of corrosion resistance, mechanical strength, and cost is required. However, it may not always be the first choice in the most aggressive or highly critical applications where materials like titanium or high-performance nickel alloys are justified despite their higher cost.