Thoughts on 304L and 316L corrosion in the HA area.

By Kix

Date 12-13-2011 16:04 Edited 12-13-2011 19:04

Here's a rundown on the product. Product is a turbine inlet coil with 304L and 316L frames. The complete coil will get a heresite coating before installation. Customer is requiring all heat affected areas be brushed or blasted so they will not rust under the coating. What are your thoughts on the heat affected area's on 304L and 316L stainless steel corroding when they are not brushed to a shiny silver finish? Thoughts as in will it rust really easy or are we talking 200 years from now? I'm talking about the colored area on the backside of the weld or even on the same side as the weld. This is only colored and may get a dull gray, but has not reached the point of carbide precipitation. I know it depends on the type of environment they will be used in. Let’s say they will be used in a Midwestern United States area subject to hot (170deg) cold (single digits) and humid environment, will not be subject to corrosive chemicals or gases.

Thanks,

By Lawrence (*****)

Date 12-13-2011 20:31

Very few codes speak to oxide color.

That silence is important.

Eggheads have spent countless hours and grant dollars defining exactly when service conditions merit treatment/passivation of that kind of thing.

Unless you are the egghead assigned to make the call, I would't worry too much :)

On the other hand it takes very little effort to add a wire brushing step into the WPS..

By Kix

Date 12-14-2011 01:05

I agree, but I'd think with this coating and the unlikelihood that it will corrode to the point of failure in the next 200 years that we should be able to let it go. These frame parts are very large and they take time to rig and flip so that the backside is exposed. There are intermittent stitch welds on a piece of 1/4"x 3" C channel that they want caulked in between the welds because of moisture coming in under the joint and causing corrosion in the heat tinted area on the inside. This all came up in a customer audit and was not in the project specification so I feel we should go after some money if they want this. I don't have a problem doing it if they want to pay for it ya know.

By jon20013 (*****)

Date 12-14-2011 02:16

As many of you know I'm a HUGE fan of Walt Sperko. Walt has an article titled "Rust on Stainless Steel - Oh my God." It's well worth a read, check his site at www.sperkoengineering.com

As Walt wisely points out there are many causes of "rust" on stainless steels and some of them were eye-openers to me. In this case, after reading Walt's paper, I don't believe your Customer is asking for anything extraordinary, but simple precautions (avoidances) as are pointed out in Walt's paper.

By Kix

Date 12-14-2011 16:24

I'll look into this read. It sounds very interesting. Thanks!

By 99205

Date 12-14-2011 02:15

I've had customer requirements similar to what your describing but my situation revolved around a waste water treatment plant. The link below has some insight into your concerns.

http://events.nace.org/library/corrosion/MatSelect/corrstainsteel.asp

By Kix

Date 12-14-2011 16:25

Thanks for the link. I will check this out for sure.

By ozniek

Date 12-14-2011 12:55

Hi Kix

Not sure what heresite coating you will be applying, but as a general rule you need a specific surface profile for the coating to adhere well. Chances are that the coating may not adhere that well on the oxidised surfaces. Not sure on the application, but words like "turbine inlet coil" makes it sound relatively critical. The type of thing you want to do once and leave for 30 years before having to mess around with again. If that is the case, then it is worth doing well. If this requires more than you quoted for, then obviously the client must pay for it, because he will be getting the benefit.

A couple of things to remember:

1) A coating that is not well applied can have a significantly reduced life. (Includes surface preperation.)
2) Coatings are often damaged, leaving the substrate exposed.
3) Stainless steel with a compromised surface can corrode substantially faster than ordinary carbon steel under many circumstances.

Put these together, and it can result in a very short service life, and excessive maintenance and down-time costs.

Just something else to consider: Power brushing of stainless is fraught with dangers. After a while the brush will start to "flare" and will scrape along the inside of the guard. This will result in iron contamination of the stainless part you are brushing. (Not to mention other sources of contamination of the wire brush.)

Regards
Niekie

By Kix

Date 12-14-2011 16:24

Thanks everyone for some really good info. The surface prep before the coating will tak care of most of the heat tint on the SS, but there are a couple spots covered by the tubes and fins that we will have to brush. I'm very interested in doing some reading on this now. Thanks again for everyone's input.