Feared Corrosion Problem 2

[Bernie101]

For a specific kind of valve used in the cooling system of locomotive diesel engines, 6061-T6 aluminum is being chosen for the material of the valve body for several good reasons. The question arises as to how best to aleviate the corrosion problems inherent in such an application. The cooling water, at 180ºF max, may or may not have corrosion inhibitors in it and may be hard water taken from local wells and streams. The valve body is connected by bolts and flanges to steel pipes. Electroless nickel is being considered as a coating but doing this to aluminum is complicated and costly. Hard anodize can serve as an insulator against galvanic corrosion with the steel pipes. The bolt threads do not touch the cooling water but they will make an electrical connection. How good is hard anodize against chemical corrosion with the water? The valve is expected to operate for 20 years. I would appreciate some words of wisdom from you folks.

 

[TVP]

Electroless nickel is not a good option for this type of environment. Anodizing will probably be a good option, but may not be sufficient. Without knowing a little more about the specific operating environment for the valve body, I can only make some initial recommendations. Due to the aggressive environment and long life requirement, I would investigate a robust coating system such as the following:

1. Anodize (sulfuric acid, ASTM B 580 Type A or B or MIL-A-8625 Type II)

2. Nickel acetate seal

3. Epoxy primer (2 coats of high-solids type MIL-PRF-23377 or MIL-P-53022)

4. Some type of water-resistant topcoat paint. Polyurethanes are typically used over epoxies when the coating will be exposed to direct sunlight. However, I am not sure about the long-term water resistance of typical PU coatings such as those specified in MIL-PRF-53039.

This type of system provides outstanding corrosion performance on aluminum components. It would insulate the surface from galvanic corrosion with the steel pipes. I would contact some paint vendors to discuss this type of application. You can obtain the MIL standards that I mentioned, along with the list of approved suppliers, by using the following link:

http://assist.daps.dla.mil/quicksearch/

Just substitute QPL for MIL-P to find the approved suppliers and contact info.

 

[Bernie101]

Thank you,TVP, for your in-depth reponse. What issues do you see with the electroless nickel coating?

 

[EdStainless]

I hate to say this, but given the system and potential corrosion issues, the selection of an Al alloy may simply not be appropriate.
That said, I have seen parts treated as TVP suggests hold up very well. In the long term the limit will be the durability of epoxy at 180F in water. You will reach a service limit.

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Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[TVP]

Electroless is cathodic to aluminum, and if there are ANY discontinuities in the coating due to poor adhesion, scratches, nicks, etc., then the anode to cathode ratio is locally very low and this promotes extreme corrosion at defect. Achieving perfect coatings is a fallacy, and acheiving this level of quality with EN on Al is exceedingly difficult. Do a keyword search on this site and finishing.com for more info.

 

[joeswoes]

try composite valves to eliminate galvanic-related problems

 

[rorschach]

I had an aluminum injection molded thermostat hose neck on my 82 Jeep Cherokee. It experienced SEVERE pitting failure due to galvanic corrosion couple with the steel of the block. Coating it with epoxy solved the problem and it worked without any additional corrosion until I sold it. Sometimes JB Weld is your friend....=D Yep, I jury rigged it (it was a sunday evening and nobody that had a replacement was open and I had to have the vehicle running Monday morning.). after scrubbing the heck out of it with a wire brush, I filled the pits (that went all the way through) and slathered the entire wetted surface with it. I figured I'd replace it the next weekend. got busy, forgot to change it. went back 6 months later and couldn't find a speck of corrosion, so I put it back together and left it alone.

Ed is right, if you can avoid Aluminum, do so. If not, coat it with epoxy and electrically isolate it fo cut down on the galvanic coupling.