Limestone/Steel Interaction

[kxa]

Would limestone or even slate cause premature rusting of the steel or even galvanized steel?

 

[CoryPad]

Do the limestone or slate slide or abrade the steel surface?

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[kxa]

These are for exterior steps. They are set on 1" of concrete over relatively thin metal plates that form the treads and risers. The original steps were slate and as they break, they are being replaced by limestone. There is a considerable amount of rust around both the slate steps and the limestone ones.

 

[lewtam]

Could it be the porosity of the concrete? From work I've seen from limestone quarries, their corrosion concerns are pretty much similar to most other mining activities.



LewTam Inc.
Petrophysicist, Leading Hand, Natural Horseman, Prickle Farmer, Crack Shot, Venerable Yogi.

 

[kxa]

lewtam, can you please elaborate.
Thanks,

 

[ornerynorsk]

I suspect the cracking of the stone overlay is coming from the steel substrate not being rigid enough. Limestone is naturally alkaline, so corrosion will be an issue. Just coat the steel in a good epoxy prior to the veneer stone being installed.

Also, are you cementing or mortaring the stone into place. Ordinary cement or concrete won't last as long as a good "flexible" polymerized thinset mortar.

Good luck.

 

[kenvlach]

Completely dry limestone would not be a problem.
Limestone is porous so always contains water due to humidity & rain, absorbs salts used for melting ice and also has its own ions to act as electrolyte. So, you often see rust streaks from steel in limestone.
I recall the equilibrium pH of air + water + limestone being about 8, which is not sufficiently alkaline for maximum stability [minimum solubility] of ferric hydroxide (solid). Best is a higher pH, e.g., about 10-11 as in most cured concretes.

 

[METLess]

Unless the plain carbon steel or galvanized hardware is completely embedded in the concrete with some cover, corrosion is inevitable from water and de-icing salts penetrating the limestone. Slate is significantly less porous (neglible even), but joints in the material during setting of the stone permit water (and anything else in it) to enter the assembly and initiate corrosion.

For long-term applications of this nature (50+ years), we use 304 or 316 stainless hardware.