Why is this exposed steel not rusting?

[KelseyB]

I recently bought a 30 foot steel sailboat and am refitting her. I know that exposed steel is a huge problem because it flash rusts, but I've noticed that there are some pretty big areas of exposed metal in the cockpit that haven't rusted at all, but have been exposed to plenty of humidity and rain for at least a month. It is definitely steel and definitely rusts because there's surface rust on the inside that I've been addressing, but that's also because the boat hasn't been opened up in at least ten years, so that's not surprising. But in terms of those exposed areas, I was wondering if there are any variations of steel that don't rust as quickly? She was built in 1974 in the Netherlands, so maybe there's a certain alloy or treatment they used to use or that they used in Europe? Anyone have any guesses?

(The attached photo is of one of the areas in the cockpit that I'm specifically asking about)

[hpeer]

Our big boat was built in 1987 in UK. Her entire exterior was flame coated with aluminum. It doesn’t look like that from you photo but it is something to consider.

[roverhi]

Looks like galvanized plate. They may have used hot dipped zinc galvanized piece of plate in that area expecting it to get a lot of abrasion/abuse.

[psk125]

Went bicycling in Normandy and came across a German pillbox with a rifled, maybe 88mm cannon inside. It had been blown off it’s mount by an aerial bomb through the roof. No rust anywhere. 1944 was a good while ago, but they used good steel.

[Midnight Son]

Originally Posted by hpeer

Our big boat was built in 1987 in UK. Her entire exterior was flame coated with aluminum. It doesn’t look like that from you photo but it is something to consider.

Same process, but zinc used instead. I have heard of it being done, one other boat on these forums, and this is only the second example I have heard of.
Cheers

[Cheechako]

That looks too dark grey for plain steel to me. Primer? or a coating of some sort?

[mvmojo]

Look up Corten steel

[Captn_Black]

Originally Posted by hpeer

Our big boat was built in 1987 in UK. Her entire exterior was flame coated with aluminum. It doesn’t look like that from you photo but it is something to consider.

Funny you should mention that, as when I was cleaning the deck near the aluminium toe rails (yes, I hate them on a steel ship too, they will be gone one day) there was a layer of aluminium rust over the adjacent deck that had a similar effect. Perhaps something like that is going on for OP?

[Cheechako]

Originally Posted by mvmojo

Look up Corten steel

Actually CorTen surface rusts quickly in a manner that the rust becomes it's protective layer... kinda like Aluminum creates Aluminum Oxide on it's surface. If you see a big power pole that is that dark rust orange, it's probably made of CorTen.

[Paul Howard]

Might be an inorganic zinc rich primer coat.

[Midnight Son]

I haven't seen any that where as hard looking as this??? I can be pleasantly surprised though.
Cheers

[Gaia]

Originally Posted by Paul Howard

Might be an inorganic zinc rich primer coat.

Interesting question and answers. Inorganic zn primer is wonderful stuff. We used it on our boat's exterior above water surfaces and it has stood up well for many years. We had a small area delaminate from its epoxy overcoat and it showed some whitish areas which I think is the zinc hydrolysing. So if that is an inorganic zn coat exposed to air and moisture would you not think it whitish in places Paul?

And yes Corten not coated looks like and is rusted steel. Normally nice and uniform broem.

Jim sv GAIA

[peter57]

That is definitely not bare steel asd it is a coating like other posters have said. most likely a zinc coating but it looks like there is rust starting in little spots. To fix these spots do not grind off the coating that has worked so well, use a die grinder burr in a battery drill so you only do the rust spots and make sure all the rust, which leaves a surface that rich zinc paint will adhere too. There are many articles on the net for how to clean the new bare steel after removing the rust out with the burr.

The Burrs can be purchased off of the net in a variety of shapes very cheap. Be careful with the burr swarf, best vacuumed up as they are nasty to remove from your skin.

[Captn_Black]

Originally Posted by peter57

Be careful with the burr swarf, best vacuumed up as they are nasty to remove from your skin.

Can confirm

[SVTatia]

Originally Posted by KelseyB

I recently bought a 30 foot steel sailboat and am refitting her. I know that exposed steel is a huge problem because it flash rusts, but I've noticed that there are some pretty big areas of exposed metal in the cockpit that haven't rusted at all, but have been exposed to plenty of humidity and rain for at least a month. It is definitely steel and definitely rusts because there's surface rust on the inside that I've been addressing, but that's also because the boat hasn't been opened up in at least ten years, so that's not surprising. But in terms of those exposed areas, I was wondering if there are any variations of steel that don't rust as quickly? She was built in 1974 in the Netherlands, so maybe there's a certain alloy or treatment they used to use or that they used in Europe? Anyone have any guesses?

(The attached photo is of one of the areas in the cockpit that I'm specifically asking about)

Holland has the best method to properly build and protect a steel boat. One of the reasons I heard is that fiberglass was so expensive there that steel boats were more economical to build - not sure if this still the case.
The boats were very well built. While my steel boat had rust stains after every passage, my neighbour on a steel Dutch boat had his paint intact and always sparkling....

Anyway, most boats were sand blasted and flame zinc sprayed within a couple of hours, and then coated with other paint systems. The one drawback is that paints do not adhere very well to the sprayed zinc, and eventually lift with time, hence the flaking you notice on your boat.
I would not sand or grind that area very aggressively, but clean it up and epoxy it. The zinc coat is very thin but enough to act as an anode when the paint is chipped - this is the idea, and the reason you don’t see rust streaming from the exposed metal.