This isn't about MacGregors... but you're my friends,

[seahouse]

Mastreb – All good questions. Without going too far into a technical realm that is waayyy beyond the scope of a forum post….

The conditions that influence the corrosion of submerged metals are much more complex than just the lack of oxygen. Low temperature, lack of strong currents, organic biofilm presence, contact with other galvanic series metals, encrusting minerals, pH, alkalinity, dissolved solids, sulfur presence, high ambient pressure, are just a few that come to mind.

In reference to plain distilled water being considered corrosive…
While in theory distilled water is pH neutral, in reality it absorbs carbon dioxide from air which lowers its pH (ie. makes it acidic, since, unlike “plain” water, it has no pH buffering capacity). Ultimately, it becomes a weak solution of carbonic acid which, you guessed it, is corroding. Anyone can confirm this for themselves with litmus paper, or pool pH indicators.

Since the 70’s I’ve been distilling my own (but not hootch ) for drinking and cooking, so I’m somewhat familiar with it, and this is a well-known property of distilled water.

Also, where metals are placed in contact with distilled water, both elemental metal, and metallic ions will begin to migrate into it, down a concentration gradient, because there are no ions already in it (not unlike the osmosis that happens in FRP boat hulls). In fresh water that already is nearly saturated with minerals and ions this migration rate will be lower than with distilled water. Just two reasons (there are others) why distilled is classified by chemists as being corrosive.

Corrosion is an electrochemical process (an anode-cathode redox reaction), so the localized flow of electrical current within the pit will be a dominant factor in corrosion when oxygen is not. As a high school chemistry refresher- oxidation is defined as a loss of electrons, or the gain of an H+ ion, neither of which necessarily involve an oxygen molecule itself.

The corrosion pitting reaction in stainless steel does not reach completion and stop when oxygen is used up and thus “cut off” to it. Quite the opposite- it continues; repeated observation of corroded stainless steel parts in the field confirms this.

I’ve had little experience with saltwater-exposed boats (we stay away from them!), it’s pretty well all been with freshwater. I have many times over the years seen stainless steel bolts that have been severely pitted (cut through) from being buried in freshwater-saturated plywood backup pads for deck fittings, where oxygen has been cut off.

I’m doubtful that the points of corrosion on the fittings you mention are sufficiently cut off from oxygen for oxygen starvation to play a dominantant role in pit corrosion. If the fittings are stainless steel attached to aluminum with fasteners, then the dominant source is more likely galvanic corrosion occurring between dissimilar metals that are located at different levels on the “electrochemical series” hierarchy.

Corrosion obviously happens at a higher rate in salt water conditions, but it is certainly an issue in fresh water conditions in those areas where oxygen has been excluded. So excluding oxygen from the joints and crevices, or anywhere in stainless steel fittings is to be avoided wherever possible in both fresh and salt water environments.

Hope this helps. – Brian.

[mastreb]

Brian--Thanks for the explanation! It's astonishing how complex these things turn out to be.

[RobertB]

Judy wrote...

I think there's a good argument to be made that putting sealant around the outsaide of the swage is more likely to encourage corrosion than using no sealant at all.

The logic goes something like this: Water inside a swaged fitting becomes stagnant. Stagnant water doesn't contain enough oxygen for the chromium in the stainless steel to form a corrosion resistant coating. SS needs oxygen to oxidize and build up a molecular layer of Chromium Oxide, which gives stainless its protective barrier to oxidation. If you let water in, and the water gets stagnant (I guess low oxygen saturation levels?), the stainless steel corrodes quickly. Sealant on the outside might trap water, while simultaneously preventing oxygen from reaching it.

Stagnant water on stainless steel causes "crevice corrosion". It's the #1 cause of failure of stainless parts on sailboats.

Judy is 100% correct IMHO.

Crevice corrosion is a problem on stainless steel in fresh ("stagnant") water, too, regardless of whether or not chloride is present, although the process is accelerated in salt water conditions. While it might not be immediately “intuitive”, be aware that pure distilled water is considered to be highly corrosive as well.

It is the exclusion of oxygen that is the initiator of a small pit in the surface of stainless. Putting a protective coating or cover of any kind on stainless steel carries the risk of making corrosion worse because, in addition to excluding oxygen, pinholes or nicks in the coating will concentrate the process of pit formation.

BTW- as I’ve mentioned before, related to this; a high polish on stainless will increase its corrosion resistance by minimizing the presence of corrosion- initiating microscopic pits.

Putting tape around the shroud would not be waterproof- it is not possible, because of the contour/ texture of the metal strands, to prevent water from flowing into the joint from higher above the shroud. It would also contain the water and prevent drying, accelerate oxygen starvation, and hence corrosion, in the area. Therefore it’s not a procedure I would recommend.

The pit corrosion process in stainless steel appears to be contradictory on the surface, because it would lead you to ask “how can the exclusion of oxygen from a small area cause oxidation in that area?” The short answer is that, even though the pit area is small, there are still several different chemical reactions happening simultaneously in different regions of that pit that contribute to the erosion of the metal in that area. In none of those is the presence of chloride ion obligatory.

I would also guess that any substance used during the swaging process would be very thin, low viscosity, that would flow freely around the joint, and would have minimal lubricity, otherwise it would weaken the gripping friction of the swage.
- Brian.

I agree that wrapping the cable/swage is pretty futile if you want to prevent water getting in. The moisture will wick along the strands of the cable. If one really wanted to minimize corrosion (sounds like inspection and periodic replacement much more workable), then a penetrating sealant may work (ever use green Loctite - it will wick into the bolted connection). Also, coating the inside of the swage and the cable upon assembly may help - for example, on the F-16 aircraft, we would first coat the prepared rivet holes with paint primer and install the steel rivets "wet" in order to deal with galvanic corrosion.

[seahouse]

Good thoughts RobertB, but the wicking type of Loctite is a pretty thin fluid, and unable to fill spaces the size found in wire rope or its fittings or swages, which is something it needs to do not only to make an effective seal, but also in order to exclude oxygen, which it needs in order to set (polymerize and harden).

For an anaerobic adhesive like Loctite to set requires both the presence of metal ions (which are present here), and the absence of oxygen. But, even if Loctite could fill the voids, and thus set, it would then be serving as an indicator of the very conditions that we are trying to avoid in the joint/ area in the first place- anaerobic conditions, as discussed in above posts.

If the joint were cut off from oxygen, such as would happen if some type of barrier-protective sealant were applied, in order for it to be effective and not make the corrosion situation worse it would also have to be 100% failsafe under the conditions of internal friction from ongoing cyclical mechanical strain, and be self-healing.
Otherwise it would block the oxygen that the chromium in the stainless steel requires for its own self-healing passivation. That’s a tall order for a sealant, with little upside for using it, and a huge downside (which is an accelerated corrosion rate).

I would expect (and I’m guessing here) that a drop (or two) of light oil, applied, say, annually, might be the most I would do that would work, if I felt compelled to do anything at all (and I don’t). It’s self-healing, somewhat water-repellant/ displacing, and not heavy enough to interfere with the required oxygen migration to the surface of the stainless.

Robert -I’m guessing you’re referring to zinc chromate primer on aluminum panels (yellowy- greeny- coloured stuff) and you meant to say aluminum, not steel rivets – and “wet”, as in with water? Or did you mean steel rivets in steel parts? I'm curious.

-Brian.

[RobertB]

First, I never meant to suggest using Locktite - just illustrating a wicking type compound many of us have used. Second, it was a long time ago that I worked on the aircraft - regardless or the rivet material (and I do believe they were steel due to the high strength requirements for wing fasteners), we installed with , yes, yellow zinc cromate primer (green in some factories) and wet means they brushed it in the holes and immediately installed the rivet while the paint was still "wet". Do you really always need to be so critical, especially on details that do not really matter? The specific paint and the specific material of the rivet really were not important to the point I was trying to make - that there are ways to go overboard to try to prevent corrosion but the traditional method of inspection and periodic changeout leaves much more of my miniscule free time for sailing.

[seahouse]

Not intended to be critical, but just asking for details- sorry if I offended, Robert (or anyone)- that is not my intention.

The devil is in the details. I did suspect that water was involved in the process for zinc chromate binding to aluminum – hence my asking for more detail on the “wet” part. Procedures in the aircraft industry are often vetted “best practices” that undergo extensive critical scrutiny, so I thought it might be useful and pertinant to know what those are first-hand from someone who has worked in that industry (your profile lists your area of expertise as mechanical engineering). I like to gather information whenever, from whomever, and from wherever I can, and this forum is a source of some information not easily obtained elsewhere.

I also consider this forum to be for the exchange of information and to provide advice. The accuracy and value of that advice evolves through discussion as we fill in the gaps in each other’s knowledge (and I have my share of gaps in my knowledge to fill)!

If thread advice might be unclear to some, then that information can potentially do harm instead of good- something I think we all work toward avoiding. Ditto - so we can have more time to spend on the water.

- Brian.

[raycarlson]

as a licensed a&p whos done hundreds of repairs on Boeing,MD, and Bombardier aircraft with experience up until 2004 using SS repair parts tou would never leave any bare metal exposed to the atmosphere.All parts would be sprayed and cured twice with an epoxy primer (not zinc chromate,that went out in the 70's)then all mating surfaces would be coated with a sealant( thats the term wet,has nothing to do with water) and yes there are many steel alloy rivets( highest strength and corrosion resistence is usually Monel) and as RB stated the holes are always primed and the rivets are shot wet with sealant for anti-corrosion properties.And yes Brian we all realize you like to be the scientist and we appreciate that,but alot lot of times your simply guessing and thats not science,but it's all in fun anyway,but in this case everything robertB said was basically true and it looks like you were telling him he didn't know what he was talking about when he did.

[kmclemore]

[mod="kmclemore']Just a reminder, folks... we're ladies & gentlemen here, so let's try to avoid going to the personal level. I'd suggest keeping the discussion focused on the facts, and backing up facts with references where you feel it's appropriate. Pretend you're at the local pub, and discuss accordingly. We'd like to encourage folks to treat this place as a friendly forum, even when we disagree. Thanks.[/mod]

[raycarlson]

No need to come down on Brain so hard, thats just his nature to let everyone know he knows things, we all enjoy it very much.

[Highlander]

No need to come down on Brain so hard, thats just his nature to let everyone know he knows things, we all enjoy it very much.

Now that U two guy's r officially engaged when's the wedding !!!

J

[seahouse]

That’s the information I was asking for. Stainless steel rivets and fasteners are much more mechanically robust than those made from aluminum. So when strength is a priority the use of primers and sealers are used with stainless steel fasteners to overcome the risk of galvanic corrosion with aluminum in the aviation field. It’s somewhat contrary to conventional practice used elsewhere, where inspection intervals are much less structured, so thank you, that’s good to know.

In order to be less amiguous in my postings, I do try to specify whenever I am guessing, but, as always, anyone can feel free to ask for more details if they like, and I am happy to elaborate.

But “guessing” actually does play a large part in the progression of science, that's not to be underestimated. If you will allow, I will use one of the foremost scientists of our time, Albert Einstein, to illustrate.

Based on empirical evidence and observations he and others made, he formulated ideas, or theories, or guesses (SWAG’s ) about the world around us that were sometimes immediately rejected by his scientific peers (his quantum theory of light, for example). It was sometimes decades before the technology was developed that was able to test his guesses.

The most amazing part is, as the technology has become available to test his guesses, they have been proven to be valid, even some predictions (most recently demonstrated quantum entanglement, which he mockingly called “spooky interaction at a distance”) that Einstein himself thought had to be in error.

Note that the use of the descriptive words “theory” or “idea” or “guess” has no bearing on whether or not they accurately represent the true state of reality, but they very well may. For example, Einstein’s “theories” proved to be correct, as opposed to Newton’s “laws” which are incorrect.

BTW- that rumour that I “like” science is not true… I LOVE science! (Among other things ).

-Brian.

Hmmm. Quantum entanglement... Harnessing that would make Judy's Potter go really fast!

[seahouse]

BTW -I probably should have pointed out in my query above that I mentioned zinc chromate not out of the blue, but because of its chromium content, to suggest a possible connection in activity between it and the protective chromium oxide formed by stainless steel, which was under discussion.

-B.

[seahouse]

when's the wedding !!!

January 12, in TO, and yer invited to emcee, John!

Be there, or be square!

-B.

[CampCook]

I bet Judy wonders where her topic went. I know I do.
Dave

[raycarlson]

sorry Judy,but with all the luv being shared here we sometimes loss control of ourselves.