Corrosion of Subsurf Utility Transformer SS Tank

[2banee]

Gents,
I'm not a corrosion engineer, I'm an EE, but recently we've had a couple of stainless steel (409) subsurface transformer tanks corrode through and fail. The transformers are only 3.5 yr old. They're located in a concrete vault and have standing water around them pretty much 24/7. I'm not sure whether to install CP and/or change our spec to require a higher grade stainless. Water tests haven't pointed out anything abnormal:

CL 20 ppm
EC 4000 ohms/cm
pH 7.2 neutral

the soil conditions are corrosive to mildly corrosive. Does anyone have any suggestions or know of a corrosive engineering firm with experience in utility based corrosion of this nature???

 

[TVP]

Type 409 stainless steel is completely inadequate for this type of envionment. A very experienced guy at Armco onced described it to me as "barely stainless", due to its limited alloying content, and hence performance. One of the other members will give you some advice on suitable grades, CP, etc.

 

[EdStainless]

CP is an option. It will need to be monitored and maintained.
The other option is a higher grade of stainless. My suggestion would be a lean duplex grade like LDX2101 or AL2003. These grades have great strength (make the tanks thinner) and have good corrosion resistance even in the as-welded condition.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

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

 

[stevenal]

Forget cathodic protection. I don't think you will want to revisit these locations until there is a change or failure. Talk with your transformer suppliers about what they'd suggest and are able to supply.

 

[2banee]

Unfortunately I'm probably going to have to do both. I have approximately 250 to 300 of these same SS URD transformers installed in the field already. In order to protect our investment and prolong their life cycle I’m forced into doing CP on them. In the mean time I need to come up with a different Engr Spec for future purchases that address the best material to be used in this environment.

I've talked with the manufacturer and they’ve been reluctant to suggest anything with respect to the type or grade of stainless we should be using. I assume due to liability reasons. I have sent off material samples of one of the failed transformers for failure analysis along with water samples and I’m waiting for the results. FYI. I’ve also posted this thread in Electric Power Engineering forum for input as well.