I have an observation to relate about bronze screws in oak versus PT pine

the 316 SS screws I used on the bottom 25 years ago look almost perfect, while a small percent of the new bronze screws wasted away so bad could not be used again. I am impressed with how durable the PT pine has held up. I carefully select knot free PT ground contact SYP wood, and you can cut from large pieces framing wood to avoid knots. I did paint it all before use to seal it. Doing the same this year replacing oak frames that I should have replaced back 25 years ago. These are frames running down to the keel in the deep bilge, not the curved chine or sides. If I have to replace highly curved sections, I can cut thin strips and laminate a frame section. Pain though as it uses a lot of glue.

 

Oak and Iron are bad together on my boat. Both rot. Called Iron sickness, the iron accelerates oak rot. Oak actually disintegrates, turns black and becomes like charcoal, I think due to acids developed by mixing oak steel and water. It may be worse if also exposed to oxygen. The iron eats the oak around it. The OEM pinned with iron the floors to the keel. Anywhere the iron pins and water sat, the iron disintegrated along with some of the oak, but not everywhere, some had no damage, some had lots.

 

Made good progress today checking for loose screws.
Feeling more confident the boat won't fall apart on the water.

 

My guess for the brass screws is some form of galvanic corrosion where the acidic Oak surface is oxidized forming a fine powder. A boat operates in a moist atmosphere so there may be enough vibration induced wicking into the wood interior to provide moisture and form an meniscule amount of electrolyte, between the two surfaces. Usually galvanic action is between 2 different metals, with an electrolyte, but a form of it has also been observed with nonmetals. When strips of iron and brass are merely placed on top of each other without an electrolyte, the iron surface oxidizes forming a fine powder of oxide which is kind of a similar occurrence. Stainless steel is more chemically inert than brass, so that's probably why there was less corrosion observed with SS screws. The pressure treated pine holds up better against rotting or oxidation and the chemicals used may help suppress Electro galvanic corrosion.

 

So with no treatment to the wood, my personal opinion is that the wood was already compromised 25 years ago and you just kicked the can down the road. Oak has a pretty porous vessel structure so the actual volume of degrading wood fibre extended well beyond the oversized hole. Sort of like this photo of iron sickness.
https://www.jlconline.com/deck-builder/branz-study-focuses-on-fastener-corrosion_o

 

Exactly this. The floor timbers that looked perfect on the outside were massively hollowed out inside. The astonishing thing is that the expanding rusting iron did not split the floors. On a lighter built boat, they probably would have. I may have pics, if i find them, will post.

 

That's the answer. You made a galvanic battery.

 

Maybe close together, but not many feet distant. And that does not explain why bronze screws generally disintegrate in salt water and oak framing.
If they did not rot away, no one would need to rescrew old boats. Maybe oak and bronze are not good long term just like oak and iron.

My experience tells me pine and bronze are good combination.
I have see lots of old Chesapeake deadrises, some taken apart and they used pine or juniper wood and either iron nails or SS nails. Thie iron does not rot the wood much at all, mostly stains it red and none of the SS nails cause any wood damage for decades. The iron nails do rust. I have not seen any mahogany planking damaged by iron.

 

This boat is a little over 100 yrs old on the lower Chesapeake Bay and was used as a buy boat where fisherman would bring their catches out on the water so they stayed out a long time on the water. Not made of oak.
I think the black staining is old iron fasteners. The wood though is fully sound, not affected by the rusting iron.
Boat did get a new nose piece on the front on this haul.
The paint was removed and hull slathered with epoxy only, then paint.

 

I have gone and checked 98% of all bronze screws below the water on the front starboard quarter of the boat and repaired all loose screws.
I have not yet checked a few slightly at or above the waterline yet along the stripe. I did not replace any 25 years ago and they are the old Everdure #10 screws.
A few I pulled I put in the new larger screws. The old ones are tight, and corroded bronze is holding them tight in the frames.

This part of the boat I did a poorer job on 25 years ago as I was learning.
All the other parts of the hull, I had glued in plugs into old screw holes or replaced framing, and the few I checked have been good.

I can recall back then just gluing in chopsticks to all plank screw holes frames just assuming them all to be bad. You can buy these dowels cheap at Amazon.

Cheap, good, holds screws well, never seen em rot.
Amazon.com: ROYALNA 100 Pairs Palillos UV Treated Premium Disposable Bamboo Chopsticks Sleeved and Separated (Bamboo, 100 pairs) : Home & Kitchen

 

316L is lower reactivity compared to Silicon bronze, but not by much
Regular 316 slightly anodic to SB
Not sure what the screws are exactly, but they are sold as polished 316.
Should not be much difference mixing them.
And they have to be freely wet together to make a joined cell.
All the interior framing is pretty dry, except for parts that sit in bilge water, very low to the keel.

Detailed Galvanic Table & Compatibility Chart Listing
Galvanic Table
The following galvanic table lists metals in the order of their relative activity in seawater environment. The list begins with the more active (anodic) metal and proceeds down the to the least active (cathodic) metal of the galvanic series. A “galvanic series” applies to a particular electrolyte solution, hence for each specific solution which is expected to be encountered for actual use, a different order or series will ensue.

In a galvanic couple, the metal higher in the series (or the smaller) represents the anode, and will corrode preferentially in the environment.

• Active (Anodic)
• Magnesium
• Mg alloy AZ-31B
• Mg alloy HK-31A
• Zinc (hot-dip, die cast, or plated)
• Beryllium (hot pressed)
• Al 7072 clad on 7075
• Al 2014-T3
• Al 1160-H14
• Al 7079-T6
• Cadmium (plated)
• Uranium
• Al 218 (die cast)
• Al 5052-0
• Al 5052-H12
• Al 5456-0, H353
• Al 5052-H32
• Al 1100-0
• Al 3003-H25
• Al 6061-T6
• Al A360 (die cast)
• Al 7075-T6
• Al 6061-0
• Indium
• Al 2014-0
• Al 2024-T4
• Al 5052-H16
• Tin (plated)
• Stainless steel 430 (active)
• Lead
• Steel 1010
• Iron (cast)
• Stainless steel 410 (active)
• Copper (plated, cast, or wrought)
• Nickel (plated)
• Chromium (Plated)
• Tantalum
• AM350 (active)
• Stainless steel 310 (active)
• Stainless steel 301 (active)
• Stainless steel 304 (active)
• Stainless steel 430 (active)
• Stainless steel 410 (active)
• Stainless steel 17-7PH (active)
• Tungsten
• Niobium (columbium) 1% Zr
• Brass, Yellow, 268
• Uranium 8% Mo
• Brass, Naval, 464
• Yellow Brass
• Muntz Metal 280
• Brass (plated)
• Nickel-silver (18% Ni)
• Stainless steel 316L (active)
• Bronze 220
• Copper 110
• Red Brass
• Stainless steel 347 (active)
• Molybdenum, Commercial pure
• Copper-nickel 715
• Admiralty brass
• Stainless steel 202 (active)
• Bronze, Phosphor 534 (B-1)
• Monel 400
• Stainless steel 201 (active)
• Carpenter 20 (active)
• Stainless steel 321 (active)
• Stainless steel 316 (active)
• Stainless steel 309 (active)
• Stainless steel 17-7PH (passive)
• Silicone Bronze 655
• Stainless steel 304 (passive)
• Stainless steel 301 (passive)
• Stainless steel 321 (passive)
• Stainless steel 201 (passive)
• Stainless steel 286 (passive)
• Stainless steel 316L (passive)
• AM355 (active)
• Stainless steel 202 (passive)
• Carpenter 20 (passive)
• AM355 (passive)
• A286 (passive)
• Titanium 5A1, 2.5 Sn
• Titanium 13V, 11Cr, 3Al (annealed)
• Titanium 6Al, 4V (solution treated and aged)
• Titanium 6Al, 4V (anneal)
• Titanium 8Mn
• Titanium 13V, 11Cr 3Al (solution heat treated and aged)
• Titanium 75A
• AM350 (passive)
• Silver
• Gold
• Graphite

 

Silicon Bronze vs 316 Stainless Steel: What’s the Difference? - MFG Shop

Corrosion Resistance
Corrosion resistance is crucial for materials in harsh environments. Silicon Bronze resists corrosion well, thanks to silicon and manganese, making it suitable for marine and industrial use. 316 Stainless Steel excels in resisting corrosion, especially in chloride and acidic environments, due to its molybdenum content. This makes it ideal for chemical processing, food preparation, and marine applications. Its superior corrosion resistance ensures durability and reliability in harsh environments.

 

Corrosion of Metal in Wood Products has a good description of nail sickness and similar testing in pressure treated wood to that in jehardiman's article above. An interesting phenomenon is illustrated in Table I where the corrosion of the 304 stainless took off after 3 years in one of the CCA treated samples. I would not be at all surprised if the same thing failed to happen if the test were to be repeated. Stainless can last for a very long time and suddenly corrode rapidly if the passive film is damaged or breaks down, especially in a fastener where the head is aerated and the shank is in stagnant conditions. In the table below, the driving force is illustrated by the voltage difference between the red bar (active) and blue bar (passive) for stainless steels that can occur between the head and shank of a fastener. The head can be in perfect condition and the shank badly wasted away. 316 is less prone to this, but not immune.

Chris McMullen also has a good article to read on the subject. Electrochemical Damage To Wood – the marine version of ‘leaky homes’ https://waitematawoodys.com/2015/05/15/electrochemical-damage-to-wood-the-marine-version-of-leaky-homes/

Galvanic series tables are a good start for screening metals. They typically provide data for metals in flowing, well aerated seawater, which is very useful, but does not always correspond to stagnant, poorly aerated or contaminated seawater. Since the stagnant conditions are highly variable and hard to characterize, we settle for the numbers we can measure. The table in post 26 does not have the voltage information, and the vertical scale is meaningless, since all it tells you is the order, but not the magnitude of the differences. A better chart is the old one from NSWC, La Que or whoever. I never did find the original source. It is the horizontal scale that tells the story. All the vertical scale tells you is the space between lines of type. The difference between 80-20 CuNi and aluminum bronze is much smaller than the difference between aluminum bronze and mild steel.

 

Frames sitting in stagnant salty bilge water is the only areas I have noticed minor issues with corrosion of 316 SS screws compared to nearby Silicon Bronze screws.
However, over many years prior, a few 304 SS screws I had back in the stern got major damage, and I pulled all of those.

25 years ago, I had a decently dry bilge after a major rework using Sanitred Permaflex, but in 2014 I did a haul out and tried a different coating Loctite Black roof and flashing in some areas on keel and hull which failed. I like to try different things. I noticed over years of time the coating got soft, and when I scraped off barnacles by getting into the water, the coating was scraped off in places. Worm got in, and leaks started. Of course, that filled up the low front area of the bilge with salty water, exposing both bronze and 316 screws. The SB #12. some got badly wasted away, but the few 316 SS screws I had in there came out decent enough for reuse.

Way back in 2000, I had all the bottom planks off and 75% of the OEM #10 bronze screws were done, turned basically into a nails. It was 2 years after I had bought the boat. Surprisingly the boat had not sunk. I put in over 4000 McFeeley #12 square drive SB, a lot of new frames and a box of 316 SS 3" deck screws for some framing and planks in the bow.

That in 30 years from brand new, the boat was completely trash. IF I had not fixed it, boat was a goner. I was concerned about buying an old wood boat back then, and knowing what I know now 25 years later, it was a poor decision. But I wanted a boat I felt I could afford. I was not aware how bad shape it really was.

 

+1

If the holes in the original oak were already compromised, you had to go with larger screws, there may also have been a mold or bacterial presence that penetrated deeper in some of the compromised holes than in other holes. Biological contamination would change Ph values and maybe even metabolize metals like iron and copper and bronze. The splitting is likely due to the drying out of the older timbers during repairs.

-Will