What corrosion process is going on in this steel deck?

[stillbuilding]

A steel project boat which was painted with simple household enamel primer and topcoat as a purely temporary measure three years ago so we could move it to a boatyard - meant to be sand blasted and painted within two months but did not happen.

Left on for a year, the paint started to show bubbles with rust underneath (hardly surprising). A further year finds the enamel thoroughly underrun with bubbles containing rust core. Again no surprise.

What IS surprising to me is that when we knock off the bubbles with a needle gun we find a bit of soft rust with a proliferation of really hard material as core. The core actually stands up proud of the steel surface such that it leaves a mound of steel-like compound in a divot of powdery rust.

I have taken pics of the untouched painted surface lesions, after needle gun exposure and after grinding flat.

Anyone care to hazard a guess on what material comprises the core metal and why it stands proud of the surface?

[jimbunyard]

Likely just different forms of iron oxide (according to Wikipedia there are 16 different kinds), but your pictures of it could be hung on a gallery wall...

[stillbuilding]

Originally Posted by jimbunyard

Likely just different forms of iron oxide (according to Wikipedia there are 16 different kinds), but your pictures of it could be hung on a gallery wall...

I had wondered if the paint contributed some component.
I expected some oxidation of course because is just a thin enamel coat.

[jimbunyard]

Could have, as could have minerals dissolved in rainwater; it would be rare for such a chemical reaction to occur pristinely, but I'm guessing the corrosion is mostly oxidized iron. Hard to know what else without some kind of chemical analysis.

[stillbuilding]

So would some of the iron oxides be approximately as hard as steel and be less dense and therefore "stand up" from the steel surface?

[UNCIVILIZED]

Look at the metal as multiple layers of dough stacked together. Then picture air being blown underneath of, & in between the layers. Which is essentially what you're seeing happen in the steel. And it's easy to see such layers peeling off of steel in small sheets, or even playing card sized pieces, on old boilers or car bodies left out to rot. So that prior to the eruptions being easy to readily peel off by hand alone, they begin (visibly) as the ugly acne that you're describing/pictured. Some flakes & eruptions can be microscopic, others macroscopic. Depends on the steel's (chemical & molecular) makeup, ditto that of the contaminants, local conditions, etc.

[jimbunyard]

Originally Posted by stillbuilding

So would some of the iron oxides be approximately as hard as steel and be less dense and therefore "stand up" from the steel surface?

Three different types of iron oxides. Note the large differences in hardness.

Hematite ( Fe2 O3) has a microhardness of ~ 1030 D.P.H., is red in color, and is not soluble in acid.
Magnetite (Fe3O4) has a microhardness of ~ 420-500 D.P.H., is black in color, and is not soluble in acid.
Wustite (FeO) has a microhardness of ~ 270 -350 D.P.H., is blueish in color, and is soluble in acid. Wustite is the phase that makes up the innermost scale on the bars or rods.

In my layman's understanding of it (maybe one of you chemists can advise?), the reason the rust 'stands proud' is because of the actual physical volume gained by the combination of an oxygen molecule (atom?) with an iron molecule (atom?), resulting in a (more voluminous) iron oxide molecule...

[stillbuilding]

Originally Posted by jimbunyard

Three different types of iron oxides. Note the large differences in hardness.

Hematite ( Fe2 O3) has a microhardness of ~ 1030 D.P.H., is red in color, and is not soluble in acid.
Magnetite (Fe3O4) has a microhardness of ~ 420-500 D.P.H., is black in color, and is not soluble in acid.
Wustite (FeO) has a microhardness of ~ 270 -350 D.P.H., is blueish in color, and is soluble in acid. Wustite is the phase that makes up the innermost scale on the bars or rods.

In my layman's understanding of it (maybe one of you chemists can advise?), the reason the rust 'stands proud' is because of the actual physical volume gained by the combination of an oxygen molecule (atom?) with an iron molecule (atom?), resulting in a (more voluminous) iron oxide molecule...

Yup, that makes sense. Seems likely to be a mixture of the oxides
Thanks Jim.

[stillbuilding]

Thanks Uncivilised,
That seems to be a very apt description of the observed changes.

Very helpful in understanding the process.

Now a further question if I may follow the line of thought :

Do you think Corten steel is more resistant to this oxidation corrosion in air ? Not thinking of underwater.

[UNCIVILIZED]

Originally Posted by stillbuilding

Thanks Uncivilised,
That seems to be a very apt description of the observed changes.

Very helpful in understanding the process.

Now a further question if I may follow the line of thought :

Do you think Corten steel is more resistant to this oxidation corrosion in air ? Not thinking of underwater.

I'd say that it's corrosion resistance is the primary reason that Corten Steel gets used. Be it bridges, boats, or shipping containers. Since it's far from cheap, or easy to work, as compared to some other steel alloys. Especially as the most serious corrosion in steel generally occurs in things made from it that ain't submerge. Given that steel that's immersed most always has lots, & lots of coatings applied with plenty of prayers or swearing accompanying the application of same. The swearing being more during the prep before the coatings get put on

As to the description of rust in steel. You're welcome. And also consider how corrosion forms on other metals. Aluminum for example. Where it develops into a protective layer on the metal's surface, instead of creating layers or flakes that sluff off as they "grow". This, when in theory, due to it's makeup/being a "galvanic slut", an aluminum airplane should melt in a rain shower.

Galvanic slut being the opposite of electrical noblity.


Edit: In case it wasn't clear in my earlier description before, the "type" of corrosion that happens in steel (or to anything else, metal or other material), also depends upon what substance is interacting with the material that's "corroding".

So that the corrosion caused by Sodium Chloride mixing with steel will be different than that caused by Sulfuric Acid. And even then, what's formed depends on the ratios of the involved molecules, minerals, & elements. Since the elements in an alloy of steel are far from equally distributed throughout it's makeup. At least on a super small scale.

Thus you can have a dozen (or 100+) kinds of corrosion occurring in a patch of exposed steel that measures all of 1 square centimeter. Rust ain't faithful, like one hopes their spouse is. Rather, it's quite opportunistic, & yes, slutty. With no two types of corrosion in steel behaving in exactly the same manner.

[stillbuilding]

Thanks Uncivilized. Finely expressed. I enjoyed the graphic description.

[stillbuilding]

For dessert, I surmise that there is a snowball's chance in Hades of completely excising the divots of iron oxide in this steel deck and there must remain the potential for future corrosive "pox" and this may be one of the rare situations where painting with a "rust converter" after sand blasting may be appropriate.

[Lodesman]

Rust converters use acids to preferentially replace the O in iron oxide (rust) and turn it into a stable chemical compound such as iron phosphate or ferric tannate. The black nubs in your photos look like a phosphate, but could be just about anything. Something in the environment and/or paint could have reacted with the steel underneath.

Corten isn't really corrosion resistant, but rather alloyed to "rust" in such a way that the rust forms a protective barrier for the steel underneath.

[Russ]

It is just rust. Needle guns are great but slag like this only responds to a hammer. The type that comes to a wedge/ point. Or a cold chisel Hammer till it cracks off and yes you may go through the deck but better now. I've owned a steel cutter for 15 years now. Tuss

[S/V Alchemy]

Originally Posted by stillbuilding

So would some of the iron oxides be approximately as hard as steel and be less dense and therefore "stand up" from the steel surface?

Without knowing the composition of the temporary paint, the nature of the treatment (if any) from the steel mill or even the humidity on the day you put the paint on, it's very hard to diagnose even with your decent photos.

Rust of any kind, however, never sleeps.