Corrosion on stainless steel

[gonzo]

These bolts came out of a 1972 Swan. They were on the toe rail. It is mainly crevice corrosion but I think that there may be also electrolysis caused by the two SSB radios they used to have.

[michael pierzga]

They were exposed to water...electrolyte. When SS is deprived of oxygen in a wet area, electrolyte, ... ......such as in a keel bolt, a rudder shaft, ss fastener in a wooden hull , a propeller shaft—it’s suffers pit corrosion, . In this pitted state the surface is no longer "passivated" ...it becomes ACTIVE and as a result is much less noble on the Galvanic Scale. This is why you don't use ss fasteners in wooden boat construction below the waterline.

[gonzo]

Unless something horrible happened to the boat, toe rails are usually above the waterline.

[michael pierzga]

Read again.....IN THE PRESENTS OF WATER.. anerobic......depassivation....Galvanicly active. WATERPROOF THE JOINT.

[gonzo]

Capital letters means you are yelling at me. You did that on the previous post too. Learn some manners, and grow up boy.

[baeckmo]

Would you have any idea about what SS quality it´s all about, Gonzo? And how far from each other were they mounted? One observation I make is that the corrosion occurrs at ~the same depth in both bolts; is there any kind of "moisture-carrying" structure at that level? You also see some free ferro-oxide near the bolt head, indicating a possible galvanic potential along the bolt length, also supporting the thought of differing oxygen concentration in the moisture at varying levels.

[michael pierzga]

Once again...when in doubt..consult literature.

http://www.mcnallyinstitute.com/04-html/4-1.html

[gonzo]

It is A4/316 stainless. They were about 5" apart. The boat had at least two SSBs installed with through-deck insulators with bare connection nuts on top. The corrosion is at the base of the toe rail, where water intrusion is most likely.
Michael: what is your disagreement with my statement that this is crevice corrosion with maybe electrolysis too?