17-4 Corrosion

[Hush]

We have some equipment which has corroded faster than expected and I wonder if anyone might be able to provide some insight?

Matl: 17-4PH H1150 HRC 28 with a liquid nitride treatment
Exposed to atmosphere on offshore platform in warm climate. Severely pitted/corroded within 4 months.

We have hundreds of these parts out there and several on the same platform without problems. The only thing I can think of (and I'm grasping at straws) is: Could bird droppings in a salt spray, hot & humid environment cause severe corrosion of 17-4PH?

 

[unclesyd]

I don't know about bird droppings but I've found out that liquid nitrided 17-4 PH is not a equal in corrosion resistance as untreated 17-4. All my data was derived from contact will HNO3, Formic, and H2O2/mixed organic acids.


The bird droppings would probably be in the form of phosphates, or H3PO4, or NH3/nitrates/nitrites which may go after the nitrided surface. It would be hard to do a lab test as guano isn’t available any more. If you have time I would mount a few strips near where there are a lot of droppings to monitor.

Are the components seeing any other media?
Were they liquid nitrided by the same people?
I had a fabricator substitute gas nitriding for liquid and had a total disaster

 

[Hush]

I had the gas nitride experience with 410 some years back, the stuff was pitting on the shelf within a week so I've already checked with the heat treater and looked at some stock, definately liquid nitride. Did PMI, definately 17-4. The part is a bare stem hanging out over the edge of a production platform in the China Sea, about 15 feet off the ocean so it's probably constantly wetted by salt spray and dried by hot sun. The only other media it would see is hydraulic oil when the valve is stroked. Where it's sitting and the angle it's at would make a perfect roost for a seagull which is why the thought of bird manure occurred to me.

 

[Metalguy]

Bird manure or nearly any other substance can concentrate impurities--like Cl. Even microscopic bacteria (gallionella) concentrate Cl in their droppings. Pitting quickly follows.

Don't you guys do any trapshooting?!