Does rust on steel slow down further rusting?

[rebar]

Hello new around here..

 

I work at a chiller plant where we have over 30000 tons of cooling capacity for the university. My boss wants to remove the rust from the inside of our steel evaporator shells which I feel is just busy work. And may also speed up further corrosion.

 

Does steel rust faster when its clean? Or does a layer of rust already formed, slow down further corrosion compared to raw steel?

 

Thanks!

[Country Boy]

In this case you are wrong and your boss is right. (Don't your just hate that?) On many metals, aluminum, say, or copper, surface corrosion (or "patina") forms a protective barrier, protecting the rest of the metal. But iron doesn't work that way. Rust actually encourages more rust and should be removed as soon as possible.

[Phi for All]

AFAIK, rust isn't slowed down by an existing layer of rust. The rust on steel kind of flakes up, exposing the rest of the steel to more corrosion.

 

Leaving the rust on merely covers up what's continuing to corrode. Best to keep it clean, and then you can see signs of further corrosion.

[studiot]

 

The rust on steel kind of flakes up

 

It can actually be much much worse than that. Ferrous metals are subject to so called pitting and pinhole corrosion where the rusting bores straight through the sheet metal forming a fine hole. You see this on car bodies.

Obviously a series of holes will compromise your evaporators long before the main sheet is rusted away.

You need to mechanically work down into pits as the active part is right at the bottom of the pit. If possible treat pits with phosphoric acid to passivate and wash out.

 

You should consider the possibility of a corrosion inhibitor if at all possible

Edited October 29, 2013 by studiot

[rebar]

Thanks guys. Darn.. I guess rust isn't the protective patina like I was hoping.

 

We have a great chemical treatment program and Iv never seen any pin hole problems unless we empty a system. Or have seen any rust out failures before the useful life of the unit was over. We nursed a few chillers from the seventies until just recently.

 

And the last time we started removing rust, my boss handed us the nastiest paint know to man. yay, crap

Edited October 29, 2013 by rebar

[studiot]

As with car bodies you don't need to remove the rust, just passivate. That means remove the loose mechanically. Then chemically treat the rest which is firmly bonded. This changes the oxide from the brown variety which is flaky and non protective to the black variety which is safe.

 

Of course you may be able to install cathodic protection.

 

http://www.epa.gov/oust/ustsystm/cathodic.htm