Saltwater corrosion of cast iron

[sailorted]

Help me to understand something outside my area of expertise (computers)... I have a boat with two marine diesel engines both of which have had decades of seawater cooling. Three years ago we disassembled, sandblasted and painted one of the water cooled cast iron exhaust manifolds (after extensive freshwater flushing) before putting the boat in dry storage. Three years later... the untreated manifold is perfect while the treated one has completely "rotted" with corrosion. Both have the same history (salt water usage followed by months of freshwater) and environment (dry storage). What happened and why?
My thanks for any enlightenment,
Ted

 

[micalbrch]

Any photos available? Here in this forum are some corrosion experts (not me!) who can for sure tell you what happened when they see a photo.

 

[sailorted]

Just a couple of poor quality photos. Sorry we didn't take any shots before pealing away the rust. The corrosion took place on the underside of the manifold/water jacket, where the water would naturally pool. My assumption is that hot salt water could suffuse somehow over the years into the cast iron. But what would kick off such rapid corrosion in a dry environment after sandblasting?

 

[EdStainless]

In cast iron with seawater there is a rapid onset of corrosion, and then the corrosion product forms a surface layer and corrosion will nearly stop.
By cleaning the surfaces they exposed fresh metal, probably a more reactive surface than the original casting had, and the process starts all over again.
Often parts such as these do better if they start life in fresh water to form the surface layers, and then are exposed to more severe conditions.
Yes it seams counter intuitive but cleaning can often make corrosion more serious.

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[Metaljon]

When cleaning cast iron you will remove the natural mill scale (magnetite) which provides some protection to the cast iron against corrosion. The process of sandblasting will create quite a reactive surface as mentioned by Edstainless. Consequently to prevent further corrosion the surface has to be painted to seal the surface. If this is not practical then follow this procedure to passivate the surface.

1. Sandblast surface to remove all corrosion deposits to bare metal. 2. Wet the surface and then allow it to rust for a couple of days. 3. Brush on two coats of a combined rust converter/sealer. Note use the type that contains ferric tannate, not phosphoric acid. It will turn it black and glossy, and lasts for years and looks good.

This process will inhibit corrosion but you still need to keep surface dry (ie no pooling water).

 

[metengr]

Simply painting the cast iron exposed to seawater was the root cause of corrosion. You have a surface that is essentially passive (cathode) after painting with the substrate active (anode). If any defects were present in the painted layer, this will create a corrosion cell based on the ratio of the anode to cathode area that will result in corrosion pitting from galvanic attack in pooled water. Either leave the cast iron unpainted in this service or follow the recommendation above for coating, and get rid of pooled water.

 

[sailorted]

Thanks to all for your input. I realize that my actions in cleaning up the old cast iron accelerated the corrosion. What I am having trouble understanding is why the corrosion erupted at such a rate in a completely dry environment. A minimum of 1/4 inch of solid metal gone completely to rust in a three year period! If I understand galvanic corrosion, it requires an electrolyte which isn't there... unless it exists in the porosity of the metal itself.

 

[metengr]

sailorted;
Was the boat in dry storage for 3 years total or was it used periodically in service? Are you sure of dry storage conditions? Third, from a closer look of the photo it would appear that the manifold had sections of it cracked and ruptured. Could water have pooled and froze in this manifold leading to the conditions observed in the top photo?

 

[sailorted]

Metengr, it was stored for three years straight, never used, never subjected to any moisture, never subjected to any temperatures below about 55F or above 90F... of this I am sure. The manifold casting was dry both before and after the storage period.
And, yes, there was cracking as the corrosion swelled, almost from within the casting...

 

[EdStainless]

No, it wasn't dry. Somewhere along the line this was exposed to moisture. Salts trapped in the porous metal will suck up water just from the air.
Cast iron has pieces of graphite (carbon) in the iron matrix. There is nearly always some moisture that wicks along these boundaries.
Blast away the protective film, allow the salt to absorb moisture, and it is all over.
This is why relics retrieved from the ocean need to be soaked for years before they can be dried out.

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