Identifying if my hull is Corten?

Corten steel is more corrosion resistant than mild steel. Therefore, the plating can be thinner. A patch of Corten on a mild steel hull would be no problem. The reverse would.

 

COR-TEN (TM) was originally formulated for railroad coal/gravel gondola cars...things that sit in the weather and really can't be kept painted. It was designed not to be painted, the alloying elements designed to give the bare surface a stable patina, even after being abraded. Original marine use was for the flaying cargo surface of bulk lighters...not the hull. Before the explosion of composites in the mid-1960's, there was some use in yacht applications...for its weight and strength, though often its "corrosion resistance" was hyped; however as a hull material in contact with water it still needs to be painted, zinced, and actually has a void activation issue. Since the mid-1970's the advances in surface coatings and other alloy steels have generally regulated "corten" steel to land based items that can stand to be unpainted for economic or aesthetic reasons.

Edit: Also in its original 1930's application, it was intended to be riveted..not welded...which is why it is a pain to weld (i.e. the alloying copper is the issue).

 

My family owned a decent sized boat built from corten from 1997until I sold it in 2020. We were able to trace it clear back to the original owner and builder. Had a unique origin story the fellow was married to the youngest daughter of a large boat builder that did a lot of Defense runs. It had two unique characteristics, the first was that it had been built very very thick from overrun material from much larger projects. Unlike most boats of its vintage and size it had no ballast, not only that it was unable to safely pack the same load as similar dimension boats were capable of. We'd haul each fall and the yard crane operator always remarked on how ridiculously heavy it was considering it's size. 2 it was built by a company with the capacity for modern complex curves, and was more similar in shape to modern steel boats than the boxy counterparts of similar vintage.

We had some patching in the lazzarette done as a stringer held water and cause galvanic corrosion from the inside our, and it had bulwark adaption from the change in fisheries that were mild steel.

We were always told to overplate the electrolysis in the laz after blasting and grinding. We did just that around 99. They were still going strong in 2020 when the boat was surveyed for the new owner. I'm not sure if that was a fisherman's olde wives tail or not I know another corrosion areas on other steel boats we usually match kind for kind up until good material And only exterior patch as a last resort. I remember where the bulwark extension was you'd get a scuff from another boat scratching paint off and then the fall you could see where the metals transitioned even though the bead had been ground flush, corten would not rust nearly as fast.

 

FWIW, COR-TEN (TM) is no longer produced in its original United States Steel (USS) form. Today a "corten" steel is sourced as ASTM A606 or A588 in the US. For those abroad, I found a very informative brochure from Nippon Steel on what they call "COR-TEN" (Note that the brochure still gives the USS trademark but says that it was first marketed in 1959....which is well after the expiry of the US patent...)

https://www.nipponsteel.com/product/catalog_download/pdf/A006en.pdf