Saltwater corrosion of cast iron
Saltwater corrosion of cast iron
(OP)
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
My thanks for any enlightenment,
Ted





RE: Saltwater corrosion of cast iron
RE: Saltwater corrosion of cast iron
RE: Saltwater corrosion of cast iron
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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RE: Saltwater corrosion of cast iron
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).
RE: Saltwater corrosion of cast iron
RE: Saltwater corrosion of cast iron
RE: Saltwater corrosion of cast iron
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?
RE: Saltwater corrosion of cast iron
And, yes, there was cracking as the corrosion swelled, almost from within the casting...
RE: Saltwater corrosion of cast iron
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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