Stainless cracking. Metalurgists, puzzel this one

[dlj]

Originally Posted by skipperpete

Hmmm,….propeller shafts????

Yeap, they need cathodic protection - that's why we put zinc anodes (or the appropriate alloy) on them...

dj

[s/v Jedi]

Originally Posted by skipperpete

Hmmm,….propeller shafts????

Mine is Monel so that I don’t get any trouble

Really it’s all a matter of cost savings. Why aren’t propellers made from stainless steel? Maybe the pitting on thin tips would reduce lifespan too much?

[s/v Jedi]

Originally Posted by dlj

Yeap, they need cathodic protection - that's why we put zinc anodes (or the appropriate alloy) on them...

dj

Cathodic protection is against galvanic corrosion, not crevice corrosion.

Okay, here is clear list of why other metals are better (ai aided)

Stainless steel, while generally resistant to corrosion, is not always the best choice for applications submerged in seawater due to several reasons:

1. **Chloride-Induced Corrosion:** Seawater contains high levels of chlorides (salt), which can lead to pitting and crevice corrosion in stainless steel. These localized forms of corrosion can cause significant damage even if the rest of the metal remains intact.

2. **Crevice Corrosion:** Stainless steel is prone to crevice corrosion in seawater, especially in areas where the water can stagnate or where there are joints or welds. This type of corrosion occurs because the oxygen concentration in these areas is lower, which disrupts the protective oxide layer on the metal's surface.

3. **Galvanic Corrosion:** When stainless steel is in contact with other metals in seawater, galvanic corrosion can occur. The different metals can create a galvanic cell, where one metal corrodes faster than the other. Stainless steel can either be the anode or the cathode, depending on the metals involved.

4. **Cost and Maintenance:** While there are grades of stainless steel designed for marine environments, such as 316L, these are more expensive than other materials. Additionally, maintaining the protective passive layer on stainless steel in aggressive environments like seawater can require special treatments or coatings, adding to the maintenance costs.

For these reasons, other materials, such as marine-grade aluminum, titanium, or specialized alloys, might be preferred in specific applications where long-term exposure to seawater is expected.

[wyb2]

I haven’t read the whole thread, so apologies if I missed something, but:

The question is why are the bolts failing right under the heads. The bolt is mounted with the head facing down (unlike say, most deck hardware). Apparently “glass reinforced acetal washers” were used to isolate the bolts from the aluminum plate.

Based on those facts, I’m guessing the polymer washer is creating a good seal between the bolt head and aluminum plate (like the softer copper or aluminum crush washer on a car’s oil drain bolt), and any water that gets into the joint is getting trapped right against the underside of the head. Add time and get crevice corrosion.

[dlj]

Originally Posted by s/v Jedi

Cathodic protection is against galvanic corrosion, not crevice corrosion.

Just for technical accuracy.

From “Corrosion Handbook” by H. H. Uhlig in the chapter on Fundamentals of Cathodic Protection, page 923:

“Definition
Cathodic protection is the use of an impressed current to prevent or to reduce the rate of corrosion of a metal in an electrolyte by making the metal the cathode for the impressed current.”

You should easily be able to find that definition through the lovely AI....

In fact, by using zincs you are creating galvanic corrosion – the applied zinc is consumed, it becomes the preferred anode where corrosion takes place so the material it is attached to, does not corrode.

Originally Posted by s/v Jedi

Okay, here is clear list of why other metals are better (ai aided)

Stainless steel, while generally resistant to corrosion, is not always the best choice for applications submerged in seawater due to several reasons:

In metallurgy, the devil is in the details. You use the term submerged. There are specific zones, splash zone, tidal zone, submersed zone. In this later there are levels, but for our purposes on boats, that's sufficient.

Originally Posted by s/v Jedi

1. **Chloride-Induced Corrosion:** Seawater contains high levels of chlorides (salt), which can lead to pitting and crevice corrosion in stainless steel. These localized forms of corrosion can cause significant damage even if the rest of the metal remains intact.

2. **Crevice Corrosion:** Stainless steel is prone to crevice corrosion in seawater, especially in areas where the water can stagnate or where there are joints or welds. This type of corrosion occurs because the oxygen concentration in these areas is lower, which disrupts the protective oxide layer on the metal's surface.

3. **Galvanic Corrosion:** When stainless steel is in contact with other metals in seawater, galvanic corrosion can occur. The different metals can create a galvanic cell, where one metal corrodes faster than the other. Stainless steel can either be the anode or the cathode, depending on the metals involved.

4. **Cost and Maintenance:** While there are grades of stainless steel designed for marine environments, such as 316L, these are more expensive than other materials. Additionally, maintaining the protective passive layer on stainless steel in aggressive environments like seawater can require special treatments or coatings, adding to the maintenance costs.

The above is nothing more than a list of potential degradation mechanisms that can occur related to corrosion and stainless steel, well except point 4. I note in your point 4 it does point to using 316L (it could have as easily said 316) as an appropriate alloy to use. You should run this same AI search for all alloys you may wish to discuss.

Just a note, in metallurgy, google is not your friend. AI is not much better. One actually has to understand metallurgy to successfully use google or AI. There is way too much – hmmm – was going to use the term mis-information – but that's not exactly correct. There is way too much apparent conflicting information due to the fact that the required details are far to often not included in whatever is being stated.

Originally Posted by s/v Jedi

For these reasons, other materials, such as marine-grade aluminum, titanium, or specialized alloys, might be preferred in specific applications where long-term exposure to seawater is expected.

316 is a specialized alloy (noted in your list above).

Marine grade aluminum's have their place. Titanium can also have a place. But specifically an aspect of titanium that no one is talking about here in the marine industry is the fact it also suffers from crevice corrosion as does stainless steel.

I quote from “Titanium – A Technical Guide 2nd edition published by ASM International."

Chapter 13 Corrosion Resistance
Page 123
“The major corrosion problem with titanium alloys appears to be crevice corrosion, which occurs in locations where corroding media are virtually stagnant."

Page 127
“The mechanism for crevice corrosion of titanium is similar to the process for stainless steels in which oxygen-depleted reducing acid conditions develop within tight crevices.”

Now, certainly titanium is more resistant that the stainless steels in this respect, but it's not immune. There are numerous other aspects of titanium that make it an inappropriate material in a number of applications on boats. But we aren't talking titanium here...

There is no such thing as a “magic bullet” - that magical material that simply works everywhere for everything....

All materials have their application where they are the best choice. But they are always a compromise.

dj

[dlj]

Originally Posted by wyb2

I haven’t read the whole thread, so apologies if I missed something, but:

The question is why are the bolts failing right under the heads. The bolt is mounted with the head facing down (unlike say, most deck hardware). Apparently “glass reinforced acetal washers” were used to isolate the bolts from the aluminum plate.

Based on those facts, I’m guessing the polymer washer is creating a good seal between the bolt head and aluminum plate (like the softer copper or aluminum crush washer on a car’s oil drain bolt), and any water that gets into the joint is getting trapped right against the underside of the head. Add time and get crevice corrosion.

That is exactly the original question - why right under the heads.

Actually, without having the bolts, and more, to analyze, one really can't say exactly why. But it appears more likely that the bolts are connected to the carbon fiber in the hull and they are experiencing galvanic corrosion. Your geometry description of retained water under the head at the washer would likely make that part of the bolt suffer the galvanic corrosion effects much more so than the rest of the bolt. Best guess really..

dj

[Carlr]

I’m a boater and a metallurgist. I’d be happy to discuss the problem further and offer some alternative solutions. I’m happy to connect by phone. Carl

[ferhomme96]

Looks like crevice corrosion, a little known problem with SS, but very common. Stagnant water or
just captive water between a bolthead and other materials will cause it.
it is NOT electrolysis.
Offshore subsea equipment does not use SS for this reason and use Titanium of Superduplex instead.
Stainless piping systems are used by some Navies but only when they have a minimum flow of 5m/sec to avoid stagnant water which can cause this.

[b.needalman]

Crevice corrosion is the likely cause. 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.

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.

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.

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.

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

[dlj]

Originally Posted by ferhomme96

Looks like crevice corrosion, a little known problem with SS, but very common. Stagnant water or
just captive water between a bolthead and other materials will cause it.
it is NOT electrolysis.
Offshore subsea equipment does not use SS for this reason and use Titanium of Superduplex instead.
Stainless piping systems are used by some Navies but only when they have a minimum flow of 5m/sec to avoid stagnant water which can cause this.

I always love these answers "looks like crevice corrosion"....

The only way to positively identify crevice corrosion is through metallurgical examination, although for reasons I'll never understand, here in the marine world it's the number 1 reason every time someone sees any corrosion product on a stainless steel part...

And your statement "NOT electrolysis" - well perhaps you should read the whole thread... And by the way - the definition of electrolysis: production of chemical changes of the electrolyte by the current flowing through an electro-chemical cell. Nobody here was talking about the electrolyte....

dj

[wyb2]

Originally Posted by dlj

That is exactly the original question - why right under the heads.

Actually, without having the bolts, and more, to analyze, one really can't say exactly why. But it appears more likely that the bolts are connected to the carbon fiber in the hull and they are experiencing galvanic corrosion. Your geometry description of retained water under the head at the washer would likely make that part of the bolt suffer the galvanic corrosion effects much more so than the rest of the bolt. Best guess really..

dj

Ah, yes, so I had missed the carbon discussion a little bit up the page. But seems to align with the location of the failure possibly being a result of the orientation and the washer material trapping water at an atypical spot.

[BobMc]

Originally Posted by Jammer

If the bolts are thought to be significantly stronger than necessary, then one choice would be to switch from A2 to 316.

Seems to me that the major factor is the lower grade SS. I wouldn't put anything other than 316 SS on a boat!

[b.needalman]

Pitting indicates crevice corrosion, while the presence of white powder indicates galvanic corrosion.

Putting stainless directly against aluminum will result in galvanic corrosion, but you should see the corrosion product. Using a thin layer of Tef-Gel or electrician's compound containing zinc will prevent galvanic corrosion. Actually any grease will work adequately.

[s/v Jedi]

Originally Posted by dlj

Just for technical accuracy.

Just a note, in metallurgy, google is not your friend. AI is not much better

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.

Here is the take on using bronze instead of stainless steel:

Bronze can be a suitable material for submerged seawater applications, and in some cases, it may be better than stainless steel. Here's why:

1. **Corrosion Resistance:** Bronze, particularly silicon bronze and aluminum bronze, is highly resistant to corrosion in seawater. It forms a stable patina that protects the metal underneath from further corrosion. This patina is generally more resistant to seawater than the passive oxide layer that forms on stainless steel.

2. **Resistance to Biofouling:** Certain types of bronze have biocidal properties, which can inhibit the growth of marine organisms like barnacles and algae. This makes bronze a popular choice for marine fittings, propellers, and underwater bearings.

3. **Galvanic Compatibility:** Bronze has a lower tendency to cause galvanic corrosion when in contact with other metals, compared to stainless steel. This makes it a safer choice in assemblies where multiple metals are used.

4. **Mechanical Properties:** Bronze generally has good mechanical properties, including high strength and ductility. It's also resistant to wear and can be cast into complex shapes, making it versatile for various marine applications.

However, there are some drawbacks to using bronze:

- **Cost:** High-quality bronze alloys can be expensive, sometimes more so than certain grades of stainless steel.
- **Strength:** While bronze is strong, it is generally not as strong as some high-grade stainless steels. For structural applications requiring high strength, stainless steel might still be preferred.

In summary, bronze is often better suited for seawater applications where corrosion resistance and reduced maintenance are critical. However, the choice between bronze and stainless steel ultimately depends on the specific requirements of the application, including mechanical strength, cost considerations, and the potential for galvanic corrosion.

[s/v Jedi]

And titanium vs either:

Titanium is often considered superior to both stainless steel and bronze for submerged seawater applications. Here's a comparison:

### Titanium vs. Stainless Steel

1. **Corrosion Resistance:** Titanium exhibits exceptional corrosion resistance in seawater due to the stable and protective oxide film that forms on its surface. Unlike stainless steel, which can suffer from pitting and crevice corrosion in chloride-rich environments, titanium is virtually immune to these forms of corrosion.

2. **Weight:** Titanium is significantly lighter than stainless steel, offering high strength-to-weight ratios. This can be a critical advantage in applications where weight savings are important, such as in marine vessels and offshore structures.

3. **Strength:** Titanium alloys offer excellent mechanical properties, including high strength and toughness. Certain grades, such as Grade 5 (Ti-6Al-4V), are as strong as some of the highest grades of stainless steel while being lighter.

4. **Cost:** Titanium is generally more expensive than stainless steel, both in terms of raw material and processing costs. However, its longevity and reduced maintenance requirements can offset the initial expense in long-term applications.

5. **Galvanic Corrosion:** Titanium has a very low electrochemical potential, making it highly resistant to galvanic corrosion when in contact with other metals. This makes it a good choice in mixed-metal assemblies.

### Titanium vs. Bronze

1. **Corrosion Resistance:** Titanium's corrosion resistance in seawater is superior to that of bronze. While bronze can resist corrosion well and form a protective patina, it can still suffer in certain aggressive seawater environments where titanium would remain unaffected.

2. **Strength and Weight:** Titanium is much stronger and lighter than bronze, making it more suitable for high-strength, weight-sensitive applications. Bronze, while tough and durable, is heavier and generally not as strong as titanium alloys.

3. **Biofouling Resistance:** Both materials can resist biofouling, but titanium does not have the biocidal properties that some bronze alloys possess. However, titanium's resistance to biofouling can be enhanced through coatings or surface treatments.

4. **Cost:** Like bronze, titanium is more expensive than many other metals, including stainless steel. However, the long-term benefits of using titanium, such as lower maintenance costs and longer service life, can justify the higher initial investment.

5. **Galvanic Compatibility:** Titanium's low tendency to cause galvanic corrosion makes it a suitable choice in assemblies with other metals, often more so than bronze.

### Conclusion

Titanium is generally the best choice for submerged seawater applications requiring excellent corrosion resistance, high strength-to-weight ratio, and longevity. Its main drawback is the high cost, but this can be offset by reduced maintenance and longer lifespan. Bronze can still be a viable option for certain applications, especially where cost or specific mechanical properties (like wear resistance) are prioritized, but titanium often offers superior overall performance.